Conductive MOFs

REVIEW

EnergyChem

www.journals.elsevier.com/energychem

Conductive MOFs

Wen-Hua Li, a Wei-Hua Deng, a , b Guan-E Wang, a and Gang Xu

∗ , a

a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), 155 Yangqiao Road West, Fuzhou, Fujian 350002, China b University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China ∗Correspondence: [email protected] (G. X.)

ABSTRACT: Metal-organic frameworks (MOFs) / porous coordination polymers (PCPs) have emerged as a new family of conductive solid materials with outstanding performances in a wide variety of applications, including fuel cells, batteries, supercapacitors, catalysts, sensors, electronics, thermoelectrics, and spintronics. Only in past 10 years, novel strategies have been

developed, which allowed for the rational design of both electronically and proton-conductive MOFs. In this review, the recent progress of MOF-based electronic and protic conductors, including materials preparations, conductivity measurements, conductive mechanisms, and applications is summarized and highlighted. Besides, the important breakthroughs of the MOF-based conductors for charge and proton transport, as well as the current status and challenges in this arena are elaborated.

KEYWORDS: Metal-organic frameworks, Porous coordination polymer, Conductive, Conducting, Applications

Received: January 1, 2020 Revised: February 20, 2020 Accepted: March 3, 2020

. INTRODUCTION

Metal-organic frameworks (MOFs), or porous coordinationpolymers (PCPs) are a class of porous, crystalline solid materials,which consist of metal nodes/clusters and organic linkers, have at-tracted much more attention and achieved considerable progressin past two decades. 1–4 MOFs with high-surface area crystallattices owned permanent porosity, high crystallinity, outstandingsurface area, tunable functionality and versatile frameworktopologies, show ing w idespread applications in energy storageand conversion, gas sorption and separation, catalysis, sensor,biomedicine and proton/ion conductors. 5–12 MOF materialsrepresent a true hybrid platform at the interface betweeninorganic (hard) and organic (soft) materials. Applications ofMOFs in electronic or protonic devices are relatively rare dueto lack of MOFs that display high electrical conductivity orproton conductivity. Although one of the earliest reports onelectronically conductive MOFs emerged in 2009 by Kitagawaand co-wokers. 13 only in the past five years have MOFs thatdisplay outstanding electrical conductivity ( > 10

−3 S cm

−1 ). Theexcellent charge transport coexisting with high surface area andporosity enable MOFs to be suitable in a wide variety of areas,such as electrocatalysis, chemiresistive sensors, supercapacitors,batteries, and electronics, and so on ( Scheme 1 ). 14–19 Bothhigh charge mobility and high charge density are prerequisitefor acquiring high electrical conductivity. For MOF materials,either the high-energy electrons of metal nodes, such as Cu

2 + ,or the redox-active linkers, such as benzoquinone-based ligands,can be the carrier sources. The charge transport in MOFs isdetermined by the spatial and energetic overlap between theorbitals: enhancing the orbital overlap can effectively improvethe charge mobility on the frameworks. So far, possible chargetransport modes in conductive MOFs can be described fromboth chemical and physical perspectives: (i) from a chemicaldesign principle, conductive MOFs can be sorted into categoriesof “through space” or “through bonds”; (ii) from a physical

© 2020 Elsevier Ltd. All rights reserved. 1

perspective, hopping or band theory is capable of reflectingthe intrinsic charge transport properties of conductive MOFs.Although numerous 2D and 3D electronically conductive MOFshave been reported recently with outstanding conductivities andversatile functions, a fundamental understanding of electron-lattice interactions and electrical transport pathways in MOFsare sti l l lacking in most cases. More experiment and precisetheoretical calculations are on demand in the future to elucidatethe electron conduction mechanism in PCP/MOF materials.

The earliest proton conductive MOFs have been reportedin 1979, whereas the proton conduction mechanism is uncleardue to lack of crystallinity in MOFs. 20 Until 2009, Kitagawa,Kitagawa, and Shimizu groups investigated proton conductionin crystalline MOFs. 21–26 After a decade of development,MOF proton conductors with the Grotthuss mechanism andthe vehicular mechanism have been well studied, displayingultrahigh suprotonic conductivity of 10

−2 S cm

−1 or higher( Scheme 2 ). 27–44 The study of proton-conducting MOFs canbe mainly divided into 3 categories: (a) proton-conductingMOFs under the aqueous condition ( < 100 °C), (b) proton-conducting MOFs under the anhydrous condition, and (c)proton-conducting MOFs in the inherent anhydrous and water-mediated condition. Most studies on proton-conducting MOFsare focused on its pellet samples or single crystals, which impededtheir further application in electrolyte materials as membranesin fuel cells. Thus, thin film proton conductors constructed frompristine MOFs or mixed-matrix membranes embedded in MOFshave also attracted considerable attention. Recently, glass-statePCPs/MOFs have been investigated for proton conduction withquite different properties compared with its crystalline states.

Published: 7 March 2020

DOI: 10.1016/j.enchem.2020.100029 EnergyChem 2 , 100029 (2020)

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EnergyChem REVIEW

Scheme 1. Timeline of important events in electronically conductive MOFs.

Scheme 2. Timeline of important events in proton-conductive MOFs.

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Most recently, a significant progress has been achieved inonductive MOFs. On the one hand, an impressive crop of new

OFs with remarkable conductivity has emerged. On the otherand, a variety of new electric-related applications in MOFsave been made a reality. In spite of this, the existing reviewsainly focus on either the early researches of conductive MOFs

r limited summary of their applications. In this review, theecent progress of conductive MOFs, including electronicallyonductive MOFs and proton-conducting MOFs are summa-ized and highlighted. Centered on the synthetic methods, chargeransport mechanisms, and applications, an overview on theecent progress of electronically conductive MOFs wi l l be made.or proton-conducting MOFs, the review focuses on the proton-onducting mechanism and representative types of MOF protononductors. Then, our scope excludes MOFs that exhibit iononductivity or proton-conducting MOF composites, as theseategories of materials have been reviewed elsewhere. Finally,he challenges and opportunities for further development of
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lectronically conductive and proton-conducting MOFs wi l l alsoe presented.

. ELEC TRONICALLY CONDUC TIVE MOFS

.1. Synthesis methods .1.1. Hydro-/solvothermal methods he hydro-/solvothermal methods have been widely adopted

n the synthesis of MOF materials. Teflon-lined stainless-steelutoclaves are the main tools performing the synthesis, which canroduce higher temperature and pressure in aqueous and non-queous solutions. Thus, the solubilit y and reactivit y of reactantsan be greatly increased, resulting in the uniformity regardinghe nucleation and fine-tuned growth process of MOF crystals. 45

rystal growth and design by adopting hydro-/solvothermalethods are studied very well in three dimensional (3D) MOFs. lot of π -conjugated two dimensional (2D) MOFs have also



EnergyChem REVIEW

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been synthesized with this method. For instance, Yaghi and co-workers synthesized a family of crystalline conductive MOFswith extended metal-catecholates, namely M-CAT (M = Co, Ni,Cu, the organic linker is 2,3,6,7,10,11-hexahydroxytriphenylene(HHTP)) with a facile hydrothermal method at 85 °C. 46 Dinc aand co-workers utilized hydrothermal methods to synthesizehexaiminotripheny(HITP)-based 2D conductive MOFs, such asNi 3 HITP 2

47 and Cu 3 HITP 2 , 48 and TTF-based 3D conductiveMOFs, including M 2 (TTFTB) (M = Mn, Co, Zn, and Cd). 49 , 50

Through similar strategies, a coronene-based semiconducting 2DMOF, PTC-Fe (PTC = 1,2,3,4,5,6,7,8,9,10,11,12-perthiolatedcoronene) with ferromagnetic behavior was successfully obtainedby Feng’s group. 51 The same group further reported a layeredMOF magnetic semiconductor, K 3 Fe 2 [(2,3,9,10,16,17,23,24-octahydroxy phthalocy aninato)Fe] (for short, K 3 Fe 2 [PcFe-O 8 ])by a facile hydrothermal method at 150 °C. 52 Recently, Lu and co-workers successfully obtained a conductive MOF single crystalwith a (-Cu-S-) n plane via a hydrothermal method, which showedthe electrical conductivity of 10.96 S cm

−1 , one of the highestvalues among MOF single crystals to date. 53

The simpleness and large scale of hydro/solvothermal synthe-sis methods make the synthesis of conductive MOF materialshigh-yield and low-cost. However, there are sti l l some shortagesin it: (i) all of the reactions are enclosed in a sealed autoclave andmake it difficult in real-time observation for the growth processof MOF materials; (ii) the hydro/solvothermal synthesis is quitesensitive to temperature, the solvents, surfactant additives and theconcentration of precursors; (iii) hydro/solvothermal processesare relatively heavy energy consuming compared with other wet-chemical methods.

2.1.2. Interface-assisted synthesis methods Although there has been remarkable progress in synthesis ofconductive CPs/MOFs, the most common morphology largelyrelies on single crystal or microcrystalline powders. In thiscase, it is challenging when concerning incorporating theminto highly efficient electronic devices. The interface-assistedsynthesis methods lay a solid foundation for fabricating single- orfew-layer conductive MOF thin films in controllable thickness inmacroscale (from μ2 to cm

2 ). 54–56 So far, numerous conductiveMOFs have been synthesized via interface-assisted synthesismethods, including gas-liquid and liquid-liquid interfacial synthe-sis, Langmuir-Blodgett method, solid-liquid interfacial synthesis,self-sacrificial templates, and self-assembly on the solid surface( Fig. 1 ).

In 2013, Nishihara and co-workers utilized the interface-assisted method to prepare a 2D conductive CP/MOF, Ni-

HT (BHT = Benzenehexathiol) comprising planar nickelbis(dithiolene) linkage. 57 A liquid–liquid interfacial reaction(water/dichloromethane (CH 2 Cl 2 ) interface) has been realizedto synthesize Ni-BHT films with a lateral size of ∼100 μmand a thickness of 1 −2 μm ( Fig. 2 (a)). The high-resolutiontransmission electron microscopy (TEM) images showed alayered structure at the edge of Ni-BHT film and high crystallinitywith a clear hexagonal diffraction pattern from selected-areaelectron diffraction (SAED) ( Fig. 2 (b)). PXRD analysis of Ni-

HT film in Fig. 2 (c) showed the diffraction peaks and simulatedcrystal structure with cell parameters of a = b = 14.1 A andc = 7.6 A. Furthermore, the single-layer Ni-BHT nanosheet witha thickness of 6 A was successfully prepared via a gas-liquidinterfacial reaction as shown in Fig. 2 (d) and (e). The samegroup further prepared electronically conducting Pd-BHT, Pt-

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HT, and Ni-TTB (TTB = triamino-trithiolbenzene) film viasimilar liquid-liquid interfacial synthesis methods, indicatingthe universality of the interfacial method in synthesis of 2Dconductive CP/MOF films. 58–60

Based on the same organic linker, Zhu and co-workersprepared Cu-BHT film with 2D layered structure on theCH 2 Cl 2 /H 2 O interface ( Fig. 3 (a)–(c)). 61 The highly crystallinethin film is piled up by plate-like nanosheets with variablethicknesses (in the range of 20–140 nm) ( Fig. 3 (d) and (e)).It is worth noting that this interface reaction is sensitive tothe air, and thus both the precursor solution and reactionshould proceed under the inert atmosphere. Furthermore, bycontrolling the solvent, temperature, reactant concentration andthe container size, the as-synthesized films showed significantlyimproved crystallinity. Synchrotron radiation grazing incidentX-ray diffraction (GIXRD) was performed for the structuralresolution. Cu-BHT displayed a 2D lattice with a perfect Kagomelattice (with unit cell of a = b = 8.76 A and c = 3.38 A), differentfrom the structure of Ni-BHT proposed by Nishihara et al. to agreat extent.

Using different metal centers, Zhu’s group further synthesizedconductive Ag-BHT nanowire film at the toluene/water interfacewith outstanding electrical conductivity of 250 S cm

−1 at300 K. 62 A free-standing film contains numerous closely-packedneed le-li ke nanocrystals which are featured with prism-structureand clear crystal lattice fringes w ith w idth of ∼100 nm and lengthof 1 μm ( Fig. 4 (a)–(d)). Determined by PXRD with Rietveldmethod, Ag-BHT showed the monoclinic space group I 2 /mwith the unit cell of a = 14.32 A, b = 9.32 A, c = 4.34 A, andβ = 94.5 °.

In 2017, Louie and co-workers developed a series of con-ductive MOF films based on hexaiminobenzene (HIB) ligandwith divalent transition metal centers (Co

2 + , Ni 2 + and Cu

2 + )by adopting interface-assisted synthesis strategy ( Fig. 4 (e)–(g)). 63 Both air–liquid and liquid–liquid interfacial methods havebeen utilized to prepare thin ( < 10 nm) and thick (1–2 μm)stacked layers of M-HIB film, respectively. However, the worsecrystallinity of these MOF thin films seriously damaged theirintrinsic charge transport properties, and showed an insulatingcharacter.

Apart from the interfacial reaction for preparing benzene-derived planar organic linkers based conductive MOFs,Melot’s group used a larger triphenylene-based linker HTTP(HTTP = 2, 3, 6, 7, 10, 11-hexathioltriphenylene) to preparehighly crystalline Co 3 HTTP 2 and Fe 3 HTTP 2 with layered2D structures via the liquid–liquid (ethyl acetate/water)interface. 64 , 65 It should be noticed that both MOFs demonstratedan unprecedented metallic transition with decreasingtemperature.

In 2017, Xu and colleagues proposed an air–liquid interfacialstrategy of preparing free-standing Ni 3 HITP 2 film with anultra-smooth surface and controllable thickness. 66 The highquality and the smooth film can be successfully achieved maybedue to the hydrophilic-to-hydrophobic transition of the ligandprecursor and then in-situ formed nanometer-thick Ni 3 HITP 2 nucleation on the water surface ( Fig. 5 (a)). A porous FETdevice was successfully fabricated based on conductive MOFfilm (with thickness of 105 nm) via a top-contact geometry( Fig. 5 (b)). They further successfully fabricated Ni 3 HITP 2 filmon polypropylene (PP) membrane as a separator for suppressingthe polysulfide shuttling in Li–S battery ( Fig. 5 (c)). 67 By utilizingthis facile air/liquid interfacial method, large-area and crack-free



EnergyChem REVIEW

Fig. 1. Schematic illustration of the interface-assisted synthesis method: (a) liquid–liquid and (b) gas–liquid interfacial synthesis; (c) Langmuir-–Blodgett method; (d) solid–liquid interfacial synthesis: (d)–(i) liquid-phase epitaxy layer-by-layer; (d-ii) Templated bottom-up self-assembly (e) Self- sacrificial templates. (f) Self-assembly on solid surface.

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rystal line MOF films with the dimension of 15.0 cm × 5.2 cman be obtained, and it is one of the largest MOF membraneseported. The conductive MOF membrane modified PP filmossessed noticeable rate capabilities and cycling stabilities.

As shown in Fig. 1 (c), the Langmuir–Blodgett (LB) methodnables the preparation of a single-layer film on the waterurface with controllable packing density via tailoring the surfaceressure. Thus, it offers an exciting opportunity to control

he reaction and structures of multifunctional materials at theolecular level. Since Kitagawa et al. reported the synthesis of

he MOF monolayer on the water surface in 2010, 68 the gas–iquid interface-assisted method has spread widely in MOF filmabrication. In 2015, Feng’s group reported the fabrication of single layer 2D MOF film with large areas, Ni 3 HTTP 2 , athe air–water interface via LB method. 69 Specifically, the HTTP

onomer as a sub-monolayer spreads over the water surface inhe LB groove ( Fig. 6 (a)). By increasing the surface pressurep to 10 mN m

−1 for packing the monomers into a denselm, the metal precursor solution was then injected into theater sub-phase. The coordination self-assembly occurred on

he surface along with the nickel ion diffusion. The final largerea film possessed square mi l limeter lateral size and single-layer

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eatured thickness of ∼0.7 nm ( Fig. 6 (b) and (c)). Using theame method, Marti-Gastaldo and co-workers completed theabrication of semiconductive ultrathin MOF (Cu-CAT-1) filmscross the mi l limeter-scale area with fine control over the desiredhickness. 70 Thus, it can be seen that due to this approach, it isossible to fabricate MOF-based devices and study the electric

ransport behaviors on the thinnest films (10 nm thick) reportedo far.

Recent research showed that the solid–liquid interface-assistedethod ushers in facile strategies of preparing conductive

D MOF nanostructures and directly integrating them intolectronic devices, when the following three types: (1) templateottom-up self-assembly, (2) liquid-phase epitaxy layer-by-layer,nd (3) self-sacrificial templates are involved. Among them, eachtrategy is featured with respective advantages and disadvantages,uch as various degrees of MOF structure controlling and theime of cost for preparation. However, these efforts offer effective

ethods to integrate various conductive MOF materials intounctional devices and thus allow for practical applications in theuture.

Mirica et al. utilized the templated bottom-up self-assemblyethod to in-situ grow conductive MOF nanowires on graphite



EnergyChem REVIEW

Fig. 2. (a) Photograph of synthesizing Ni-BHT film via liquid-liquid interfacial method. (b) Left: SAED pattern; right: image of HRTEM. (c) PXRD

and corresponding simulated structure. (d) Scheme of gas-liquid method for preparation of Ni-BHT single layer. (e) The topological images of Ni-BHT

single layer measured by STM. Reproduced with permission. 57 Copyright © 2013, American Chemical Society.

Fig. 3. (a) Photograph of synthesizing Cu-BHT film via liquid–liquid interfacial method. (b) The upside face (left) and downside (right) face of the as- prepared films. (c) SEM images of upside face with smooth surface. Inset: the enlarged scale. (d) SEM images of downside face with rough surfaces. inset: the enlarged scale. (e) The cross-section of Cu-BHT film. The scale bar in c is 200 nm, in d is 400 nm, and in e is 100 nm. Reproduced with permission. 61

Copyright © 2015, Springer Nature.

electrodes (for Cu 3 HHTP 2 and Ni-CAT) or soft textiles (for Ni-CAT and Ni 3 HITP 2 ) for chemiresistive sensors ( Fig. 7 (a)). 71 , 72

Xu et al. developed the templated bottom-up self-assemblymethod by in-situ growing Cu 3 HHTP 2 nanowires on hydrophiliccarbon fiber substrates which can be directly used as electrodesfor high performance supercapacitors ( Fig. 7 (b)). 73 In 2019,Xu and the colleagues reported the fabrication of conductiveMOF nanolayers ( ∼10 nm) on chemical treatment cellulosenanofibers (abbreviated as CNFs) to form nanofibri l lar core-shell structures via liquid-solid interfacial self-assembly. 74 Theformed CNF@MOF nanopaper showed excellent conductivityand flexibility.

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In 2017, adopting liquid-phase epitaxial method, Xu et al.fabricated the electronic conductive MOF (EC-MOF) film,Cu 3 HHTP 2 whose thickness can be precisely controlled with∼2 nm in each grow ing c ycle ( Fig. 7 (c)). 75 This highly crystallineand oriented EC-MOF film showed excellent sensing perfor-mance for 100 ppm NH 3 with 129% response, high selectivityand fast response at room temperature.

To synthesize high-oriented conductive MOF films, Medinaand Bein et al. put forward a vapor-assisted conversion strategyto grow highly oriented M-CAT-1 (M = Co

2 + , Ni 2 + , and Cu

2 + )thin films on various substrates, such as bare gold, ITO, quartz,and glasses ( Fig. 7 (d)). 76 The obtained M-CAT-1 films showed



EnergyChem REVIEW

Fig. 4. (a) Preparation of Ag-BHT film via liquid-liquid interfacial method. (b) The cross-section SEM images of Ag-BHT film. (c) Low-resolution TEM

image of Ag-BHT nanowire and corresponding elemental mapping. (d) High-resolution image of Ag-BHT with clear lattice fringe. Reproduced with permission. 62 Copyright © 2018, American Chemical Society. (e) Synthesis process of M 3 HIB 2 (M = Ni 2 + , Co 2 + , and Cu 2 + ). (f) Chemical structure of M 3 HIB 2 . (g) The photograph of liquid-liquid interfacial synthesis. Reproduced with permission. 63 Copyright © 2017, American Chemical Society.

Fig. 5. (a) The reversal of hydrophobic/hydrophilic nature of the HITP ligand during the interface reaction process. (b) Air/liquid interfacial method for preparation of Ni 3 HITP 2 film and “stamp” process for FET device fabrication. Reproduced with permission. 66 Copyright © 2017, American Chemical Society. (c) Air/liquid interfacial method to synthesis Ni 3 HITP 2 /PP separator used in Li–S battery. Reproduced with permission. 67 Copyright 2018, Wiley-VCH.

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ltrahigh crystallinity, preferred orientation along c -axis, andxcellent electrical conductivity higher than 10

−3 S cm

−1 . Thistrategy paves the way for utilizing 2D π -conjugated conductive

OFs as multifunctional active layers in related devices, includ-ng photovoltaic devices, FETs and chemiresistive gas sensors.

The self-sacrificial templates were also developed for con-uctive MOF synthesis by in-situ offering the reactant sources

or the targeted products via ion exchange at the liquid-solidnterface. In 2019, Zhang et al. employed this method and thenuccessfully prepared large area Cu-CAT-1 nanorod films (7 cm

2 )ith the Cu(OH) 2 nanowires as sacrificial templates, which canrovide the Cu

2 + source for the synthesis of final compounds. 77

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he conductive MOF-based hierarchical structures, featuredith a unique superhydrophilic and underwater superoleophobic

urface, displayed outstanding solar absorption. This strategy isonducive to development of a possible method for preparingarge-scale conductive MOF based multifunctional materials forractical applications.

In recent years, Lin et al. employed a self-assembly on-surfacetrategy to successf ully sy nthesize the single layer of conductive

OFs. 78 A new semiconducting 2D MOF, Cu 3 (C 6 O 6 ) wasynthesized on the Cu (111) surface with single layer thickness Fig. 8 (a) and (b)). 79 The high-resolution STM and DFTalculations confirmed the single layer structures ( Fig. 8 (c) and



EnergyChem REVIEW

Fig. 6. (a) Langmuir–Blodgett method to prepare Ni 3 HTTP 2 single layer on water surface. (b) TEM images showed the large-area Ni 3 HTTP 2 single layer on Cu grids. (c) AFM image of Ni 3 HTTP 2 single layer with thickness of 0.7 nm. Reproduced with permission. 69 Copyright 2015 John Wiley &

Sons, Inc.

Fig. 7. Templated bottom-up self-assembly method to grow Cu 3 HHTP 2 and Ni-CAT nanowires on graphite electrode (a) and grow Cu 3 HHTP 2 nanowires on carbon fibers (b). (c) The liquid-phase epitaxial method to prepare Cu 3 HHTP 2 film layer by layer. Reproduced with permission. 75

Copyright 2017, Wiley-VCH. (d) The vapor-assisted conversion method to prepare Cu 3 HHTP 2 , Co-CAT and Ni-CAT films between liquid-solid inter face. R eproduced with permission. 76 Copyright © 2019, American Chemical Society.

(d)). Interestingly, when adsorbed on Cu (111), this compoundperformed as a direct semiconductor with a band gap of 1.5 eV.Furthermore, an excellent charge mobility is expected due tothe highly dispersive conduction band with an effective massof 0.45 m e . In addition, the single layer of Ni 3 HITP 2 was alsosuccessfully obtained on the Au (111) substrate by the samegroup ( Fig. 8 (e) and (f)). 80 The single layer structure has beendissolved at sub-molecular resolution with STM technology.In addition to that, DFT calculations proved that the surface-adsorbed Ni 3 HITP 2 single layer was featured w ith a non-triv ialtopological gap, indicating a novel family of 2D materials withpotential quantum phases ( Fig. 8 (g) and (h)).

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2.2. Composition and crystal structures So far, plenty of conductive MOFs have been reported bytaking various synthetic approaches. Among them, π -conjugatedconductive MOFs based on planar multidentate ligands haveattracted significant attention due to their excellent electricalconductivity and outstanding functional diversity. In addition,conductive MOFs with π–π stacked pathways, with redoxactivity or mixed valence, or (-M-S-) n units, usually exhibitednoticeable conductivity and ultrahigh surface areas.

For π -conjugated conductive MOFs, the ligand and the metalcenter connect to each other in the plane by strong coordinationbonds, while the adjacent layers stack together through weak van



EnergyChem REVIEW

Fig. 8. (a) Cu 3 (C 6 O 6 ) single layer self-assembled on Cu (111). Inset is the dehydrated BHO -6H

molecule. (b) The corresponding high-resolution STM

image. (c) The simulated freestanding Cu 3 (C 6 O 6 ) layer (left) and its absorbed phase on Cu (111) substrate (right). Carbon, brow n; ox ygen, red ; copper adatom, blue; Cu substrate, yellow. (d) DFT-simulated STM image of Cu 3 (C 6 O 6 ) on Cu (111) substrate. Reproduced with permission. 79 Copyright 2020 John Wiley & Sons, Inc. (e) Ni 3 HITP 2 single layer self-assembled on Au (111). (f) The corresponding high-resolution STM images with Ni 3 HITP 2 single-layer network mode. (g) DFT-optimized single layer Ni 3 HITP 2 adsorbed on Au (111) substrate. (h) The illustration of charge transfer process between the single-layer Ni 3 HITP 2 and the Au (111) substrate at an isosurface level of 2.87 × 10 −4 e a −3 (Bohr radius). Reproduced with permission. 80

Copyright 2019 The Royal Society of Chemistry.

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er Walls interaction to form bulk crystals. Ni 3 HITP 2 is a typicalase of π -conjugated materials, in which the 2D layers extendedlong ab direction and weakly stacked along c axis, resulting in theulk crystal. 47 Besides Ni 3 HITP 2 , there are lots of other reported-conjugated materials based on benzene, triphenylene, phthalo-

yanine, and perthiolated coronene-derivatives with hydroxyl,hiol or amino groups linked via Fe, Co, Ni, or Cu metal centers. 17

heir π -conjugated nature makes them a special case of 2Donductive MOFs with the most electrical conductivities known.or conductive MOFs, the good spatial and energetic overlapetween orbitals of appropriate symmetry are required to form

acile charge transfer paths, giving bulk conductive crystals, likeu [Cu(pdt) 2 ] 13 and Zn 2 (TTFTB). 50 Because of the facile single

rystal growth condition and abundant structural tunability ofhis kind of MOFs, they are the ideal platform for the in-epth study of the conduction mechanism and structure-functionelationships. In this part, we summarized the crystal structuresnd the compositions of above widely explored conductive MOFaterials.

.2.1. Conductive MOFs based on planar multidentate igands

s the most conductive MOFs known, the π -conjugated conduc-ive MOFs are usually assembled by planar multidentate organicigands and planar metal-complex nodes with highly delocalized

-electrons, resulting in outstanding electrical conductivity.ig. 9 showed the reported organic monomers and metal centersf π -conjugated conductive MOFs to date. As the size extensionf the polycyclic aromatic hydrocarbons (PAHs), the π–π

nteraction between adjacent layers is expected to be enhanceds well as the improvement of the electron delocalization.owever, there are sti l l no regular relations that have been

bserved between the extension degree of PAHs and electricalransport properties. In fact, subtle changes in terminal groupsf the ligands, different metal centers, stacking modes betweendjacent layers, and crystal orientation all can dramatically

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nfluence the charge carries and mobility in conductive MOFs.or polycrystalline samples (pellets or films), the three factors,amely crystallinity, grain boundaries and structural defectsre indispensable and should be considered in conductivityeasurements. Benzene-derived linkers : Since Nishihara et al. reported the

-conjugated 2D Ni-BHT nanosheets adopting liquid-liquidnterfacial strategy, 57 the analogous compounds with various

-X 4 planar nodes have attracted great attention because thisind of 2D PCPs/MOFs usually shows outstanding electricalonductivity and the facile synthesis process. Although theingle crystals of these 2D conjugated MOFs are difficult to bebtained, the high quality films or microcrystals have already beenuccessfully prepared via sovolthermal or interfacial methods.

The benzene derivative based linkers includingexahydroxybenzene (HHB), 81 hexaiminobenzeneHIB), 57 , 63 , 82 , 83 hexathiolbenzene/BenzenehexathiolHTB/BHT), 58 , 59 , 61 , 62 , 84 triamino-trithiolbenzene (TTB), 60 , 85

nd hexaselenolatebenzene (HSB), 86 as shown in Fig. 10 andable 1 . Based on the linkers mentioned above, many kinds ofD π -conjugated conductive MOFs have been reported withransition metal notes. However, only Cu-based MOFs have beenynthesized for HHB and HSB whereas only Ni-based for TTB.here is plenty of room to develop more benzene-derived-linkerased 2D MOFs.

Until now, two main kinds of lattice structures have beeneported in benzene-derived linker based MOFs: one is theexagonal lattice with M-X 4 planar metal sites, and the other is agome str ucture with continuous connection along lateral plane Fig. 11 (a) and (c)). As shown in Fig. 11 (b), Cu 3 HHB 2 ownedexagonal layered structure along the ab plane and van der Wallslipped-stacking along the c -axis, forming 1D channels with theore size of ∼8 A. 81 However, Cu-BHT possessed the Kagome

attice extending along the ab plane with non-porous structures Fig. 11 (d)). 61



EnergyChem REVIEW

Fig. 9. Representative organic monomers and metal centers reported in conductive MOFs based on planar multidentate ligands.

Fig. 10. The benzene-derived linkers with hydroxyl 81 , animo 57 , 63 , 83 , 84 , thiol 58 , 59 , 61 , 62 , 84 and selenolate 86 groups utilized for fabrication of conductive PCPs/MOFs.

Fig. 11. (a) Chemical structure of benzene-derived-linker based 2D MOFs with hexagonal connection type. (b) Calculated crystal structures of Cu 3 HHB 2 w ith c y lindrical pores. R eproduced with permission. 81 Copyright © 2018, American Chemical Society. (c) Chemical str ucture of HTB -based 2D MOFs with Kagome-type connection. (d) A typical crystal structure with no porosity. Reproduced with permission. 61 Copyright © 2015, Springer Nature.

9 DOI: 10.1016/j.enchem.2020.100029 EnergyChem 2 , 100029 (2020)


EnergyChem REVIEW

Table 1. Summary of conductive PCPs/MOFs based on planar multidentate ligands reported to date.

Linker Formula Sample type Measure-ments σ /S cm

−1 μ/cm

2 V

−1 s −1 E a /eV BET/m

2 g −1 Ref.

HIB Cu 3 HIB 2 Pellet 4-probe 13 – – 114 82

HIB Ni 3 HIB 2 Pellet 4-probe 8 – – 152 82

HIB Cu 3 HIB 2 Pellet 4-probe 0.11 – – < 350 83

HIB Ni 3 HIB 2 Pellet 4-probe 0.7 – – < 350 83

HIB Co 3 HIB 2 – – 1.57 – – 240 113

HIB M 3 HIB 2 (Co/Ni/Cu) Film van der Pauw Low – – – 63

HTB Pb 3 HTB Pellet 2-probe 2 × 10 −6 – 0.37 – 84

HTB Pd 3 HTB 2 Film 4-probe 2.8 × 10 −2 – – – 58

HTB Cu 3 HTB Film 4-probe 1580 116 (e) [a] < 0.002 – 61

HTB Ni 3 HTB 2 Pellet 2-probe 0.15 99 (h) [a] – – 57

HTB Ni 3 HTB 2 Film van der Pauw 2.8 – 0.026 – 114

HTB Ni 3 HTB 2 Film (doped) van der Pauw 160 – 0.01 – 114

HTB Ag 5 HTB Film 4-probe 250 – < 0.016 – 62

HTB Pt 3 HTB 2 Film (doped) 4-probe 0.39 – – – 59

TTB Ni 3 TTB 2 Film van der Pauw 0.1 – 0.041 – 60

HHB Cu 3 HHB 2 Pellet van der Pauw 7.3 × 10 −8 – 0.46 158 81

HSB Cu 3 HSB Pellet 4-probe 110 – – – 86

HHTP Cu 3 HHTP 2 Crystal 2-probe 0.2 – – 540 46 , 115

HHTP Cu 3 HHTP 2 Rod 4-probe 1.5 – – – 89

HHTP Cu 3 HHTP 2 flake 2-probe 0.5 – – – 89

HHTP Fe 3 HHTP 2 Pellet 4-point 3 × 10 −3 – – – 116

HHTP Co-CAT Pellet 4-point 2.7 × 10 −6 – – – 116

HHTP Ni-CAT Pellet 4-point 0.1 – – – 116

HHTP La-HHTP Pellet 2-probe 8.2 × 10 −4 – 0.26 325 90

HHTP Nd-HHTP Pellet 2-probe 8.0 × 10 −4 – 0.24 513 90

HHTP Ho-HHTP Pellet 2-probe 5.3 × 10 −2 – 0.25 208 90

HHTP Yb-HHTP Pellet 2-probe 1.0 × 10 −2 – 0.25 452 90

HITP Ni 3 HITP 2 Pellet 2-probe 2 – – 690 47

HITP Ni 3 HITP 2 Film van der Pauw 40 – – 690 47

HITP Ni 3 HITP 2 crystal 4-probe 150 – – 690 89

HITP Cu 3 HITP 2 Pellet 2-probe 0.2 – – – 48

HTTP Pt 3 HTTP 2 pellet 2-probe 3.8 × 10 −6 – – – 87

HTTP Co 3 HTTP 2 Film van der Pauw 3.2 × 10 −2 – – – 64

HTTP Fe 3 HTTP 2 Film van der Pauw 0.2 – – – 65

HTTP Fe 3 HTTP 2 �3NH 4 Film 4-probe 3.4 × 10 −2 220 [b] 0.245 526 ±5 117

HTTP Cu 3 HTTP 2 Pellet 4-probe 2.4 × 10 −8 – – 171 118

HTTP Ni 3 HTTP 2 Pellet 4-probe 2.4 × 10 −4 – – 166 118

HTTP Co 3 HTTP 2 Pellet 4-probe 3.4 × 10 −9 – – 266 118

PcM-(NH 2 ) 8 PcNi-(NH) 8 –Ni 2 Pellet 4-probe 0.2 – – 593 91

PcM-OH 8 PcNi-O 8 –Ni 2 Pellet 4-probe 7.2 × 10 −4 – – 101 93

PcM-OH 8 PcNi-O 8 –Cu 2 Pellet 4-probe 1.4 × 10 −2 – – 284 93

PcM-OH 8 PcCu-O 8 -Zn 2 /I 2 Pellet AC impedance 6.9 × 10 −3 – – – 119

PcM-OH 8 PcCu-O 8 –Ni 2 /I 2 Pellet AC impedance 8.3 × 10 −3 – – – 119

PcM-OH 8 PcCu-O 8 -Fe 2 /I 2 Pellet AC impedance 9.7 × 10 −3 – – – 119

PcM-OH 8 PcCu-O 8 –Cu 2 Pellet 2-probe 10 −8 ∼10 −6 – – 358 92

PcM-OH 8 PcFe-O 8 -Fe 2 Pellet van der Pauw 2 × 10 −5 15 ±2 [b] 0.115 ∼0.261 206 52

NPcM-OH 8 NPcNi-O 8 –Ni 2 Pellet 4-probe 1.8 × 10 −2 – – 174 93

NPcM-OH 8 NPcNi-O 8 –Cu 2 Pellet 4-probe 3.1 × 10 −2 – – 267 93

PTC PTC-Fe Pellet 4-probe 10 – 0.2 210 ±5 51

DBC

–OH 8 DBC

–Cu Pellet 2-probe 0.01 – – 271 95

HHTP-THQ Cu 3 (HHTP)(THQ) Pellet 2-probe 2.5 × 10 −5 – 0.3 441.2 120

Note: σ , conductivity measured at room temperature; μ, charge mobility; E a , activation energy; BET, surface areas, as determined from 77 K N 2 adsorption isotherm. [a] Measured by FET devices; [b] Measured by TRTS technology. “–” Not reported.

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Triphenylene-derived linkers : The triphenylene-based MOFsroused significant scientific interest during the past 5 years,ecause of their facile preparation method, long-range orderrystal, and ultrahigh electrical conductivity. An emerged fam-ly of such MOFs consists of the triphenylene moiety withhelated groups of hydroxyl, amino, or thiol groups, suchs 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (HHTP), 46

, 3, 6, 7, 10, 11-hexaiminotriphenylene (HITP), 47 2, 3, 6,

10

, 10, 11-hexathioltriphenylene (HTTP), 87 and 2,3,6,7,10,11-exaselenoltriphenylene (HSTP) 88 ( Fig. 12 ).

The crystal structures prepared from triphenylene derivativeased linkers are quite sensitive to coordination chemical envi-onment so that subtle changes in the molecular substitution ofhe ligand or metal nodes could dramatically affect the crystaltructures. For example, the MOFs synthesized from HHTP



EnergyChem REVIEW

Fig. 12. The triphenylene-derivated linkers with hydroxyl 46 , animo 47 , thiol 87 and selenol 88 groups used for fabrication of conductive PCPs/MOFs.

Fig. 13. (a) HHTP-based MOFs with AA and AB stacking modes, the metal centers are Co 2 + , Ni 2 + , Cu 2 + , La 3 + , Nd 2 + , Ho 2 + , and Yb 2 + . (b) HITP- based MOFs with slipped AA stacking mode, the metal centers are Ni 2 + , Cu 2 + . (c) HTTP-based MOFs with eclipsed AA and staggered AA stacking modes, the metal centers are Fe 2 + , Co 2 + , and Pt 2 + .

N

and transition metal ions Ni 2 + , Co

2 + , and Cu

2 + are namedmetal-catecholates (M-CAT). 46 When HHTP coordinated witha Cu

2 + formed planar Cu 3 HHTP 2 , an AA stacking modes withstacking orientation tilted 15–23 ° from a direction perpendicularto the pore axis. This offset stacking in Cu 3 HHTP 2 can beascribed to the stacking faults or twin defects which wereproduced during the hydrothermal synthesis process ( Fig. 13 (a),upper). 89 However, when it coordinated with Co

2 + or Ni 2 + ,the AB stacking mode formed with the alternating stackedcontinuous layer of M 3 (HHTP) 2 (H 2 O) 6 and the discrete layer ofM 3 (HHTP)(H 2 O) 12 ( Fig. 13 (a), middle). Recently, Dinc a et al.have reported a novel coordination type between HHTP andLn

3 + metal ions with metal centers located between planes ofthe organic ligands, resulting in 3D frameworks. 90 These layeredlanthanide MOFs demonstrated efficient 1D charge transportwhich is perpendicular to the 2D sheets. Differently, when Cu

2 +

and Ni 2 + ions coordinated with HITP linker afforded 2D layersstacked in the slipped AA pattern ( Fig. 13 (b)). 47 , 48 When theterminal groups on the triphnylene core was changed from –

H 2 to –SH, two new stacking modes emerged ( Fig. 13 (c)). Theeclipsed AA pattern has appeared with Fe 2 + , and Co

2 + , while astaggered pattern was observed with the Pt 2 + metal center. 64 , 65 , 87

Phthalocyanine-derived linkers : One intriguing groupof π -conjugated conductive MOFs is composed of thephthalocyanine-based linkers, which can be atomic level tailored

11

either with hydroxyl- and amino-, or the change of center metals.Until now, only three kinds of linkers, namely 2, 3, 9, 10, 16, 17,23, 24-octaamino-phthalocyaninato M(II) (PcM-(NH 2 ) 8 ), 91

2, 3, 9, 10, 16, 17, 23, 24-octahydroxy-phthalocyaninato M(II)(PcM-(OH) 8 )., 92–94 and 3, 4, 12, 13, 21, 22, 30, 31-octahydroxy-naphthalocyaninato M(II) (NPcM-(OH) 8 ) 93 have been utilizedfor the preparation of 2D conductive MOFs ( Fig. 14 ).

To take NiPc-M and NiNPc-M as examples ( Fig. 15 ), thechemical structures of phthalocyanine-based MOFs possess the2D square lattice along the ab plane. 93 The pore size can beregulated by changing the catechol motif to naphthol with poresize increasing from 1.8 nm to 2.3 nm. Interestingly, the lateral2D layers of these MOFs prefer to stack in the eclipsed AApattern along the c -axis with high sy mmetry. The f ull conjugatedlayered structure, high degree of atomic modulation, and tunablepore structures made phthalocyanine-based MOFs attractivemultifunctional materials in gas sensing, batteries, magneticsemiconductors, etc .

Other fused-ring aromatic hydrocarbons-based linkers: It hasbeen proved that the π -conjugated MOF with planar layeredstructure is one of the most conductive MOFs. According to thetheoretical calculation based on optimized sing-layer structures,these MOFs were featured with highly dispersed valence and con-duction bands, resulting in band transport and excellent chargemobility within the lateral plane. Therefore, organic linkers based



EnergyChem REVIEW

Fig. 14. The phthalocyanine derivatives with animo 91 and hydroxyl 92–94 groups used for fabrication of π -conjugated 2D conductive MOFs.

Fig. 15. (a) Chemical structure of NiPc-M, M = Ni, Cu. (b) Simulated crystal structures of NiPc-M. (c) Chemical structure of NiNPc-M, M = Ni, Cu. (d) Simulated crystal structures of NiNPc-M. Reproduced with permission. 93 Copyright © 2019, American Chemical Society.

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n fused-ring aromatic hydrocarbons have become an effectivetrategy of synthesizing highly conductive MOFs. Recently, aerthiolated coronene based MOF, PTC-Fe (1, 2, 3, 4, 5, 6, 7, 8, 9,0, 11, 12-perthiolated coronene) has been successfully obtainedith outstanding electrical conductivity as high as 10 S cm

−1 . 51

nother fused-ring aromatic hydrocarbons-based linker, dibenzog, p]chrysene-2, 3, 6, 7, 10, 11, 14, 15-octaol (DBC

–OH8)as employed to synthesize a conductive MOF, Cu-DBC which

xhibited the noticeable electrical conductivity up to 0.01 S cm

−1

Fig. 16 ). 95

Fig. 17 (a) showed the chemical structure of PTC-Fe with alanar iron-bis(dithiolene) unit enabling strong π -d conjugationlong the ab plane. This MOF showed the hexagonal lattice alongateral direction with stacking layered structures along the c -axis Fig. 17 (b)). Despite similar M-X 4 linkage, Cu-DBC showed

12

4-fold interpenetration 3D structure with the conjugatedramework ( Fig. 17 (c)). DBC

–OH 8 is actually a twisted moleculeather than a planar one, thus leading to the interpenetratedorous structures ( Fig. 17 (d)).

.2.2. Conductive MOFs based on other representative igands

esigning 2D π conjugated structures has been proved to bene of the most successful strategies to obtain outstandingulk electrical conductivity in CPs/MOFs. However, this kindf conductive MOFs usually exhibit poor crystallinities and

imited surface areas which impeded their further studies andpplications. Recently, various kinds of new conductive MOFs aremerging with unique features, such as noticeable conductivity,uge surface areas and excellent crystallinities. To acquire



EnergyChem REVIEW

Fig. 16. The perthiolated coronene- (left) 51 and dibenzo[g, p]chrysene- (right) 95 based organic linkers.

Fig. 17. (a) Chemical structure of PTC-Fe. (b) Calculated corresponding crystal structures of P TC-Fe. R eproduced with permission. 51

Copyright©2018, Springer Nature. (c) Chemical structure of Cu-DBC. (d) Calculated corresponding crystal structure of Cu-DBC. Reproduced with permission. 95 Copyright 2019, Wiley-VCH.

effective pathways for long-range charge transport through theseentire MOF lattices, several strategies have been developed, suchas building effective π–π stacked pathways, 96 , 97 incorporatingredox-active ligands/mixed-valence metal nodes, 98–100 and in-troducing (-M-S-) n unit in the frameworks to facilitate chargedelocalization. 53 , 101 , 102 Adopting these strategies as well as de-signing suitable organic linkers ( Fig. 18 ), plenty of electronicallyconductive MOF single crystals have been synthesized withoutstanding bulk conductivities and large surface areas ( Table 2 ).

Conductive MOFs with π–π stacked pathways : As one ofthe non-covalent interactions, π–π stacking can effectivelygenerate facile charge-transport pathways, such as in organicsemiconductors or organic metals. Incorporating π–π stackinginto a rigid MOF structure wi l l endow the porous frameworkswith facile charge transpor t proper ties. In generally, a highefficiency of charge transport with π–π stacking units requiregood orbital overlap, such as very short distance between thepacking ligands.

13

In 2012, Dinc a et al. reported a TTF-based MOF,[Zn 2 TTFTB(H 2 O) 2 ] •H 2 O

•2DMF (Zn 2 (TTFTB) for short),with an outstanding intrinsic charge mobility of 0.2 cm

2 /V s. 49

Zn 2 (TTFTB) is comprised of infinite helical chains of corner-sharing pseudo-octahedra joined together by helical stacks ofTTFTB

4 − ligands ( Fig. 19 (a)). The shortest intermolecularS

•••S distance between adjacent ligand is 3.8 A ( Fig. 19 (b)). Thestudy showed that though only one relevant intermolecular S

•••Scontact, Zn 2 (TTFTB) is capable of partial charge delocalization,resulting in its high charge mobility. So far, plenty of conductiveMOFs have been successf ully sy nthesized based on TTF-basedorganic ligands with effective π–π stacking charge transportpathways.

In 2019, Ben and Medina, et al. reported another kind of π–π stacked conductive MOFs with anthracene-containing build-ing blocks. 103 The as-synthesized anthracene-based MOF-74(ANMOF-74) is composed of 4,4 -(anthracene-9,10-diyl)bis(2-hydroxybenzoic acid) and forms porous hexagonal frameworks



EnergyChem REVIEW

Fig. 18. Representative monomers reported in various kinds of conductive MOFs: (a) organic linkers utilized for building π–π stacked pathways 96 , 97 ; (b) organic linkers utilized for building conductive MOFs with redox activity or mixed valence 98–100 ; (c) (-M-S-) n unit used for building conductive MOFs with enhanced charge delocalization 53 , 101 , 102 . Notes: TTFTB = Tetrathiafulvalene-tetrabenzoate; TTF(py) 4 = Tetra(4- pyridyl)tetrathiafulvalene; AN

–C 2 H 2 -BDC = 4,4 ′ -(anthracene-9,10-diylbis(ethyne-2,1-diyl))- dibenzoic acid; AN-DOBDC = 4,4 -(anthracene-9,10- diyl)b is(2-hydroxybenzoic acid); BTDD = bis(1H-1,2,3-triazolo[4,5- b ], [4 ′ ,5 ′ -i] dibenzo [1,4] dioxin; H 2 BDT = 5,5 ′ -(1,4-phenylene)bis(1H- tetrazole)); BDP = 1,4-benzenedipyrazolate; DHBQ

2-/3 − = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone; TRI = 1,2,3-triazole; DTDN = 6,6 ′ -dithiodinicotinic acid; DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylate, DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylate; HT = Hydroxythiophenol; PDT = 2,3-pyrazinedithiolate.



EnergyChem REVIEW

Table 2. Summary of conductive MOFs based on other representative ligands reported to date.

Linker Formula Sample type Measure-ments σ/S cm

−1 μ/cm

2 V −1 s −1 E a /eV BET/m

2 g −1 Ref.

DOBDC Mn 2 (DOBDC) Pellet 2-probe 3.9 × 10 −13 – 0.54 287 105

DOBDC Fe 2 (DOBDC) Pellet 2-probe 3.2 × 10 −7 – 0.38 241 105

DSBDC Mn 2 (DSBDC) Pellet 2-probe 2.5 × 10 −12 0.01 [ a ] 0.02 [b] 0.81 232 102 , 105

DSBDC Fe 2 (DSBDC) Pellet 2-probe 3.9 × 10 −6 – 0.28 54 105

DHBQ (NBu 4 ) 2 Fe 2 (dhbq) 3 Pellet 2-probe 0.16 – 0.11 – 99

DHBQ Na 0.9 (NBu 4 ) 1.8 Fe 2 �(dhbq) 3 Pellet 2-probe 6.2 × 10 −3 – 0.18 – 99

DHBQ (H 2 NMe 2 ) 2 Fe 2 (Cl 2 dhbq) 3 Pellet 2-point 2.6 × 10 −3 – – – 104

DHBQ (H 2 NMe 2 ) 4 Fe 3 (Cl 2 dhbq) 3 (SO 4 ) 2 Pellet 2-point 8.4 × 10 −5 – – – 104

HT Cu-HT Pellet 4-probe 120 – – – 121

PDT Cu[Cu(PDT) 2 ] – – 6 × 10 −4 – 0.193 – 13

PDT Cu[Ni(PDT) 2 ] Film 2-probe 1 × 10 −8 – 0.49 385 101

PDT I 2 @Cu[Ni(PDT) 2 ] Film 2-probe 1 × 10 −4 – 0.18 – 101

Tri Fe(tri) 2 Pellet 4-probe 7.7 × 10 −5 – – 450 98

Tri Fe(tri) 2 (BF 4 ) 0 Pellet 2-probe 1 × 10 −10 – – 370 98

Tri Fe(tri) 2 (BF 4 ) 0.025 Pellet 2-probe 2 × 10 −5 – – – 98

Tri Fe(tri) 2 (BF 4 ) 0.05 Pellet 2-probe 2 × 10 −3 – – – 98

Tri Fe(tri) 2 (BF 4 ) 0.09 Pellet 2-probe 0.03(2) – – 230 98



BTDD Fe 2 Cl 2 (BTDD)(DMF) 2 Pellet 2-probe 10 −7 – – 365 122

TTFTB Mn 2 (TTFTB) Crystal 2-probe 8.6 × 10 −5 – – 594 50

TTFTB Co 2 (TTFTB) Crystal 2-probe 1.5 × 10 −5 – – 665 50

TTFTB Zn 2 (TTFTB) Crystal 2-probe 4.0 × 10 −6 0.2 [c] – 662 49 , 50

TTFTB Cd 2 (TTFTB) Crystal 2-probe 2.9 × 10 −4 – – 559 50

TTFTB La 4 (HTTFTB) 4 Pellet 2-probe 2.5 × 10 −6 0.28 596 97

TTFTB La(HTTFTB) 2 Pellet 2-probe 9 × 10 −7 0.20 454 97

TTFTB La 4 (TTFTB) 3 Pellet 2-probe 1.0 × 10 −9 0.44 362 97

TTF(py) 4 [Fe(dca) 2 ][TTF( py ) 4 ] 0.5 �0.5CH 2 Cl 2 (1) Pellet 2-probe 4.1 × 10 −9 – – – 106

TTF(py) 4 [Fe(dca)][TTF( py ) 4 ] �ClO 4 �CH 2 Cl 2 �2CH 3 OH (2) Pellet 2-probe 1.2 × 10 −7 – – – 106

TTF(py) 4 I 2 @1 Pellet 2-probe 1.3 × 10 −6 – – – 106

TTF(py) 4 I 2 @2 Pellet 2-probe 7.6 × 10 −5 – – – 106

AN

–C 2 H 2 -BDC NNU-27 Crystal 2-probe 1.3 × 10 −3 123

AN-DOBDC ANMOF-74(Zn) Pellet van der Pauw 6 × 10 −8 – – 1124 103

AN-DOBDC ANMOF-74(Mg) Pellet van der Pauw 5 × 10 −9 – – 1137 103

AN-DOBDC ANMOF-74(Ni) Pellet van der Pauw 4 × 10 −7 – – 1352 103

AN-DOBDC ANMOF-74(Co) Pellet van der Pauw 4 × 10 −8 – – 1213 103

AN-DOBDC ANMOF-74(Mn) Pellet van der Pauw 3 × 10 −8 – – 1748 103

DTDN [Cu2(6-Hmna)(6-mn)] •NH4 Crystal 4-probe 10.96 – 0.006 Low 53

BDP Fe 2 (BDP) 3 Crystal 2-probe 3.5 × 10 −7 ∼10 −3 [d] – 1230 107

BDP K 0.19 Fe 2 (BDP) 3 Crystal 2-probe 6.24 × 10 −5 ∼10 −2 – – 107



BDP K 0.98 Fe 2 (BDP) 3 Crystal 2-probe 2.52 × 10 −2 8.4 × 10 −1 – ∼610 107

BDP K 1.35 Fe 2 (BDP) 3 Crystal 2-probe 5.93 × 10 −2 6.6 × 10 −1 – < 430 107

BDP K 1.63 Fe 2 (BDP) 3 Crystal 2-probe 2.38 × 10 −3 3.4 × 10 −1 – – 107

BDP K 1.95 Fe 2 (BDP) 3 Crystal 2-probe 3.90 × 10 −3 5 × 10 −1 – – 107

BDT Fe 2 (BDT) 3 Crystal 2-probe 6(2) × 10 −5 – – 614 100

BDT Fe 2 (BDT) 3 /oxidized Crystal 2-probe 1.8 – 0.16 – 100

BTC

[e] TCNQ@Cu 3 BTC 2 Film 4-probe 0.07 – 0.041 214 109

H 4 TBAPy NiCB@NU-1000 Pellet EIS 2.7 × 10 −7 – – 1260 110

H 4 TBAPy C 60 @NU-901 Pellet 2-probe ∼10 −3 – – 1550 112

Note: σ , conductivity measured at room temperature; μ, charge mobility; E a , activation energy; BET, surface areas, as determined from 77 K N 2 adsorption isotherm. [a] Measured by TRMC; [b] Measured by TOF; [c] Measured by TRMC and TOF; [d] The charge mobility for BDP-based MOFs were all measured by FET devices. [e] BTC = 1,3,5-Benzenetricarboxylic acid. “–” Not reported.

F

with respective metal ions (Zn

2 + , Mg 2 + , Ni 2 +

, Co

2 + , Mn

2 + )( Fig. 19 (c)). What’s more, ANMOF-74 has infinite stacks of theanthracene core arranged along the c -direction with interlayerdistance of 5.7 A ( Fig. 19 (d)). Compared to regular MOF-74, theANMOF-74 showed 10

6 -fold enhanced electrical conductivitybecause of the incorporation of π–π stacked charge transportpathways.

Conductive MOFs with redox activity or mixed valence : Boththe organic linkers and metal centers can play a key role inthe charge carries transport of MOF materials. Among theligands, the redox-active ones are capable of improving the chargetransfer efficiency between metal centers in a MOF. Recently, the

15

benzoquinone-derived ligands have attracted significant attentiondue to the energetical ly simi lar frontier orbitals to the transitionmetals, such as 2,5-dihydroxybenzoquinone.

In 2015, Long et al. reported a semiquinoid 3D MOFcomposed of Fe III centers and semiquinoid linkers, (NBu 4 ) 2 -

e III 2 (dhbq) 3 (dhbq

2-/3 − = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone), which showed a conductivity of upto 0.16 ± 0.01 S cm

−1 at 298 K ( Fig. 20 (a) and (b)). 99 Thestudy showed that its ultrahigh conductivity derived from themixed-valence ligand w ith electron hopping w ithin the dhbq

2-/3 −redox manifold.



EnergyChem REVIEW

Fig. 19. (a) Crystal structure of [Zn 2 TTFTB(H 2 O) 2 ] •H 2 O

•2DMF viewed along c axis. Orange, yellow, red, and gray spheres represent Zn, S, O, and C atoms, respectively. (b) The helical TTF stack with the shortest intermolecular S •••S contact of 3.8 A. Reproduced with permission. 49 Copyright © 2012, American Chemical Society. (c) The crystal structure of ANMOF-74 with porous hexagonal frameworks viewed along c axis. (d) The helical metal-oxo chains (left) and the stacked anthracene moieties (right) along the crystallographic c -direction. Reproduced with permission. 103 Copyright 2019 The Royal Society of Chemistry.

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In addition to redox-active ligands, mixed-valence metalodes also can enable facile charge transfer in a rigid MOFtructures. In 2018, Dinc a et al. reported a 3D conductive

OF, Fe 2 (H 0.67 BDT) 3 �17(H 2 O) •0.5( i PrOH) (Fe 2 (BDT) 3 ),nd showed that its sing-crystal conductivity can be tunedver 5 orders of magnitude by varying the extent of Fe 2 + /3 +

ixed valence. 100 The structural analysis confirmed that thisompound possesses 1D (Fe-N-N-) ∞

chains extending in the100] direction ( Fig. 20 (c) and (d)). The study showed that theigh conductivity of the mixed-valence MOF derived from theartially oxidized metal nodes, resulting in Fe 3 + defect states.

To combine the redox process of both the metalenters and organic ligands in a MOF may generateynergistic effects on its conductivity as well as otherroperties. In 2020, Long et al. reported two Fe-semiquinoidonductive MOF materials, (H 2 NMe 2 ) 2 Fe 2 (Cl 2 dhbq) 3 nd (H 2 NMe 2 ) 4 Fe 3 (Cl 2 dhbq) 3 (SO 4 ) 2 (Cl 2 dhbq

n − =eprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone), ith conductiv ity of 2.6 × 10

−3 and 8.4 × 10

−5 S cm

−1 ,espectively. 104 This work showed that both compounds possesslectrochemical capacities of up to 195 mAh/g, which is muchigher than the isostructural frameworks containing redox-

nactive metal ions. The exceptional capacities arise from aombination of metal- and ligand-centered redox processes,hich is consistent with the electronic structure calculations. Conductive MOFs with (-M-S-)n units : An effective charge

ransport in a rigid MOF requires good energetic overlap ofhe orbitals between the metal ions and the coordinating atoms.he electronegativity of both chalcogen atoms and transition

16

etals are matched very well, resulting in facile charge transportathways and good electrical conductivity.

In 2015, Dinc a et al. studied the electrical transport prop-rties of 3D conductive MOFs, Fe 2 (DEBDC) (E = O, S)nd Mn 2 (DEBDC), with metal–oxygen (-M-O-) n and metal-

sulfur (-M-S-) n chains, respectively ( Fig. 21 (a) and (b)). 105

he Fe 2 (DSBDC) and Mn 2 (DSBDC) with (-M-S-) n chainshowed 1 order of magnitude higher conductivity than that ofe 2 (DOBDC) and Mn 2 (DOBDC), respectively. The electronictructure calculations showed that the enhanced orbital overlapith chalcogen atoms in above MOFs lower the charge hoppingarriers.

To expand the (-M-S-) n chain to (-M-S-) n planes can furthernhance the charge transfer along two directions. In 2019, Lut al. designed a highly conductive MOF [Cu 2 (6-Hmna)(6-n) •NH 4 ] n , composed of a 2D (-Cu-S-) n plane, showing an

ltrahigh single-crystal conductivity of 10.96 S cm

−1 ( Fig. 21 (c)nd (d)). 53 The electronic structure calculation showed that thealence band maximum (VBM) is mainly composed of Cu and, and the (-Cu-S-) n plane generates a highly dense pathway forharge transport through it.

.2.3. Conductive composite materials based on MOFs: oped MOFs and guest@MOFs oped MOFs : The doped strategy has been widely utilized in

norganic and organic semiconductors to control their chargeobility and charge carrier density. Recently, this strategy has

lso developed to regulate the electrical conductivities in MOFsy making use of its redox-active units. Long et al. reported



EnergyChem REVIEW

Fig. 20. (a) Crystal structure of (NBu 4 ) 2 Fe III 2 (dhbq) 3 with interpenetrated (10,3)-a nets forming 3D porous structures. (b) Top: Illustration of Fe III

center coordinated with two radical (dhbq 3 −) and one diamagnetic (dhbq 2 −) bridging ligand. Bottom: The part structure of (NBu 4 ) 2 Fe III 2 (dhbq) 3

showing the local coordination environment of two dhbq n −-bridged Fe III centers. Reproduced with permission. 99 Copyright © 2015, American Chemical Society. (c) Crystal structure of Fe 2 (BDT) 3 . (d) The proposed charge transport pathways along (Fe-N-N-) ∞

chains. Reproduced with permission. 100 Copyright © 2018, American Chemical Society.

a permanently porous semiconducting MOF, Cu [Ni(pdt) 2 ],which is an analogous MOF of Cu [Cu(pdt) 2 ] while showingpoor conductivity (1 × 10

−8 S cm

−1 ). 101 Considering the redox-active [Ni(pdt) 2 ] 2 − part of the framework, they introduced theoxidizing I 2 guest molecules into MOF pores and enhancedthe conductivity up to 1 × 10

−4 S cm

−1 . The enhancement inelectrical conductivity contributed to the partial oxidation of[Ni(pdt) 2 ] 2 − by I 2 , given loosely bound unpaired electrons ascharge carriers ( Fig. 22 (a)). Zuo et al. also utilized I 2 as guestmolecules to regulate the conductivity of the Fe(II)-based MOF,namely [Fe(dca) [TTF(py) 4 ] �ClO 4 �CH 2 Cl 2 �2CH 3 OH]. Itis interesting that, I 2 incorporation induced the crystal-to-crystal transformation, forming [Fe(dca) [TTF(py) 4 ] �0.5I 3 �ClO 4 �CH 2 Cl 2 �2CH 3 OH] w ith 1000-fold conductiv ityenhancement. 106 Furthermore, both the spin crossover andphoto-magnetic behaviors are modified by I 2 doping.

Long and co-workers prepared Fe 2 (BDP) 3 (BDP = 1,4-benzenedipyrazolate) which could be reduced to formK x Fe 2 (BDP) 3 (0 < x ≤ 2), when there is the 10,000-foldanisotropic enhancement in conductivity ( Fig. 22 (b)). 107 From

17

the study, it could be found that the doped complex possessedfully delocalized charge carriers within the original structures andnoticeable charge mobility comparable to the organic polymersand inorganic ceramics. The pristine phase Fe 2 (BDP) 3 showed arelatively low charge mobility of 0.02 cm

2 V

–1 s –1 which has beenincreased to a maximum value of 0.29 cm

2 V

–1 s –1 after reducingto K 0.8 Fe 2 (BDP) 3 .

Campbell et al. adopted ligand n-doping strategy toregulate the electrical conductivity in a 3D MOF, ZnNDI(NDI = naphthalene diimide), with a redox-active ligand. 108 Asshown in Fig. 22 (c), the ordered stacks of neutral ligands NDIcan be reversibly n-doping to NDI •−, forming a charge transportpathway that dramatically increase the framework’s conductivity.The study showed that the pellet conductivity of ZnNDI canbe increased from almost insulating ( < 10

14 S cm

−1 ) to 10

−7 Scm

−1 with stable redox cycling. The doped strategy has been utilized to regulate the conductiv-

ity in MOFs via reducing or oxidizing the metal notes or organicmoieties of the host frameworks. However, the precise controlof the doped level and the robustness of the doped systems are



EnergyChem REVIEW

Fig. 21. (a) Crystal structure of Fe 2 (DSBDC)(DMF) 2 •xDMF. (b) The infinite secondary building units in M 2 (DEBDC)(DMF) 2 •xDMF ( M = Fe, Mn; E = S , O). The purple line represents the (-M-E-) ∞

chains. Reproduced with permission. 105 Copyright © 2015, American Chemical Society. (c) Crystal structure of [Cu 2 (6-Hmna)(6-mn) •NH 4 ] n viewed along b -axis. (d) The 2D layer with Cu-S arrangement viewed long c -axis. Reproduced with permission. 53 Copyright © 2019, Springer Nature.

s

s

M

d

p

h

b

d

h

M

i

H

p

r

d

(

b

3

3e

c

c

t

b

M

i

p

d

a

a

tS

M

e

M

l

ba

C

i

2I

m

m

a

c

(

c

o

m

a

a

ti l l chal lenging in this field. More moderate and effective dopedtrategies are required in the future to obtain multifunctional

OF semiconductors. Guest@MOFs : Introducing redox-active guests or electron-

eficient/rich moieties into MOFs is proven to be anotherowerful way of controlling the electrical conductivity on theost frameworks. The formed strong donor-acceptor interactionsetween the inserted guest molecules and host frameworks canramatically enhance the conductivity of Guest@MOFs, whichave unique host-guest charge transfer different from the dopedOFs. As Allendorf and Talin et al. introduced TCNQ molecules

nto an almost insulating MOF, Cu 3 BTC 2 (HKUST-1, 3 BTC = 1,3,5-benzenetricarboxylic acid), new electronic

athways between TCNQ-terminal C

≡N group andedox-active Cu centers emerged, thus resulting in aramatically enhanced electrical conductivity (0.07 S cm

−1 ) Fig. 22 (d)). 109

Zuo et al. investigated the host-guest charge transfer propertiesased on a pair of redox-active MOF materials, namely MFM-00 (V

3 + ) and MFM-300 (V

4 + ). 111 Incorporating I 2 in MFM-00 (V

3 + ) to obtain I 2 @MFM-300 (V

3 + /4 + ) with 7 × 10

5

nhancement of conductivity was ascribed to the host-guestharge transfer via partial oxidation of the V metal centers. Inontrast, adsorption of I 2 in MFM-300 (V

IV ) without chargeransfer has no influence on the conductivity.

Farha et al. successfully incorporated nickel(IV)is(dicarbollide) (NiCB) into an insulating zirconium-basedOF, namely NU-1000, forming a mesoporous MOF with

18

mproved electrical conductivity (NiCB@NU-1000). 110 Theyrene-based linkers on the framework are known as electrononors whereas the bis(dicarbollide) units in NiCB acteds strong electron acceptors. As a result, the formed donor-cceptor interaction between NiCB and NU-1000 endowedhe MOF with high electrical conductivity up to 2.7 × 10

−7

cm

−1 ( Fig. 22 (e)). To render an electronically conductiveOF, Hupp et al. encapsulated C 60 , which has outstanding

lectron acceptable ability, within another zirconium-basedOF, NU-901 with 1,3,6,8-tetrakis( p -benzoate)pyrene as the

inker. 112 Surprisingly, the bulk conductivity of the MOF haseen improved from almost insulating to as high as 10

−3 S cm

−1

fter the fullerene incorporation. The conduction mechanism in 60 @NU-901 originated from strong electron donor-acceptor

nteraction, similar to NiCB@NU-1000.

.3. Electrical transport measurements t is a great challenge to conduct the precise electrical transport

easurements due to not only the intrinsic properties of targetaterials but also the quality of the device itself. Selecting

proper measurement technique according to the conductiveharacters is the key to success. Dinc a et al. presented a case studytaking Cd 2 (TTFTB) as an example) for measuring electricalonductivity in MOFs. The study exhibited the variabilityf conductivity values depending on different measurementethods. 124 Here we summarized the methods that have been

dopted to measure the electrical conductivity, activation energynd charge mobility of MOF materials.



EnergyChem REVIEW

Fig. 22. (a) Left: Crystal structure of Cu[Ni(pdt) 2 ] that can be oxidation by introducing I 2 molecules. Right: [Ni(pdt) 2 ] 2 − units of Cu[Ni(pdt) 2 ] with redox behavior. Reproduced with permission. 101 Copyright©2010, American Chemical Society. (b) Structure of K x Fe 2 (BDP) 3 , 0 < x ≤ 2 obtained by reducing Fe 2 (BDP) 3 with potassium naphthalenide. The green balls represent partially occupied K

+ ion sites. Reproduced with permission. 107

Copyright © 2018, Springer Nature. (c) The reversible ligand n-doping in MOF Zn-NDI with an increased electrical conductivity. Reproduced with permission. 108 Copy right © The Royal Society of Chemistry 2020. (d) Simulated structure of TCNQ@Cu 3 (BTC) 2 . The blue shading represents the possible charge-transport pathway via Cu-TCNQ chains. Reproduced with permission 14 Copyright 2016, Wiley-VCH. (e) Structure of NiCB@NU-1000 at 100 K. Reproduced with permission. 110 Copyright © 2018, American Chemical Society.

2.3.1. Conductivity measurements Ohm’s law is followed to describe the electrical conductance ( G )by measuring the specific value between the electrical current ( I )and voltage ( V ). Due to the geometrical parameter dependenceof G, electrical conductivity ( σ ) is defined as the magnitude, asshown below:

σ = G

l A

where l , the length between the voltage-drop; A , the cross-sectionarea. Electrical conductivity is usually measured in S cm

−1 or S

19

m

−1 (S = Siemens). It is important to note that not all conductorsobey Ohm’s law, especially the low dimensional conductors andsemiconductors which usual ly fol low the law in the limitedrange. Thus, the measurement current or voltage range has tobe noted for special conductors. The techniques for conductivitymeasurements include two-contact probe methods, four-contactprobe methods, four points methods and van der Pauw methods( Fig. 23 ). 125

When conductivity is moderate or extremely low, the sample’sresistance is larger enough (100–1000 times) to ignore contactresistance (in the range of 1–10 �), and then the two-contact



EnergyChem REVIEW

Fig. 23. Two contacts method (a) and four contacts method (b) used for measuring electrical conductivity of single crystal or pellets. In (a): left is for cuboid sample and right is for cylindrical sample. The four points (c) and van der Pauw method (d) used for measuring electrical conductivity in very thin samples. Here, F is a correction factor, f is the function related to the ratio between R AB and R AC , R AB = V CD /I AB , R AC = V BD /I AC . 126

m

s

t

i

p

r

m

h

a

r

m

d

c

p

b

s

fi

b

c

p

v

m

i

c

2C

v

c

b

b

t

E

w

d

c

m

o

s

t

A

σ

w

t

t

m

A

h

σ

w

t

1

r

t

ethod is the better choice for its facility and smaller limitingample size. However, when conductivity is significantly high,he four-contact method is more accurate because of the non-gnorable sample-conducting wire contact resistance. The fouroints and van der Pauw methods can eliminate the contactesistance and they are suitable to measure high conductive

aterials, especially for thin-film sample with irregular shapes. Another significant problem affecting conductivity is the

omogeneity and anisotropy of the sample. Up to now, therere four physical forms for MOF samples, and they are closelyelated to the electrical conductivity measurements, including

icrocrystalline pellets, polycrystalline films/membranes, singleomain films/membrans and single crystals. Because of poorrystallinity, the most conducting MOF measurements has beenerformed on pressed pellets. Nevertheless, the numerous grainoundaries and random orientation of crystallites mainly preventtudy of intrinsic charge transport properties. The polycrystallinelm possesses higher density than pellets, but suffers the draw-acks mentioned above. In fact, the single domain films and singlerystals are the best forms for studying intrinsic charge transportroperties (especially the anisotropic electrical conductivity). Iniew of the porosity and sensitive to the environment of MOFaterials, the ideal experimental limitations when studying their

ntrinsic charge transport behavior are in vacuum and dark and atontrollable temperature.

.3.2. Thermal variation of the conductivity ompared with room temperature conductivity, the thermal

ariation of conductivity is more crucial to acquire a clear pictureoncerning the electrical behavior of the material. According to

20

and theory, when the Fermi level ( E F ) crossed the conductionands, a metallic conductor obtained which usually gives rise

o high conductivity (10

1 –10

5 S cm

−1 ). In semiconductors, the F lies between the conduction band and the valence bandith a moderate band gap ( E g ). E g is defined as the energyifference between the maximum valence band and the minimumonduction band. 127 The fundamental E g can be experimentallyeasured utilizing ultraviolet photoelectron spectroscopy (UPS)

r inverse photoemission spectroscopy (IPES). For an intrinsicemiconductor (undoped), half of the band gap is called activa-ion energy ( E a ). For a classical semiconductor, E a follows therrhenius law:

( T ) = σ0 exp (

− E a

kT

)

here σ (T) is temperature dependent conductivity; σ 0 , a pre-exponential factor; k , the Boltzmann constant; and T , absoluteemperature. For amorphous conductive materials, the conduc-ion mechanism may change from a band model to a localized

odel whose charge transport obeys the hopping mechanism. modified exponential law has been developed to describe theopping model conductivity:

= σ0 exp [−

(T 0

T

)a ]

here exponent a is related to the dimensionality of theransport, when a = 1/(1 + d ), d is dimensionality. Hence, for, 2, and 3 dimensional transport, a is 1/2, 1/3, and 1/4,espectively. When a = 1/4, Mott’s law is obtained to describehree dimensional systems. σ 0 is a pre-exponential factor and T 0



EnergyChem REVIEW

T

is Mott temperature. 128 This variable range hopping model can beadopted to explain many conducting MOF materials.

2.3.3. Charge-mobility measurements Despite the rapid development of conductive MOFs in the last5 years, there are sti l l few repots about the measurements oftheir charge mobility. In 2012, Dinc a et al. reported the measure-ment of the intrinsic charge mobility in a permanently porousMOF Zn 2 (TTFTB) by flash photolysis-time-resolved microwaveconductivity (FP-TRMC) with 0.2 cm

2 V

−1 •s −1 . 49 In 2013, thesame group continued to measure the charge mobility of anotherconductive MOF Mn 2 (DSBDC) with the combination of FP-

RMC and direct-current time-of-flight (TOF) which revealedthe mobility of 0.01–0.02 cm

2 V

−1 s −1 . 102 In 2015, Zhu andco-workers reported a π -d conjugated MOF Cu-BHT whosefilm based field-effect transistor (FET) exhibited high mobilityof electron (116 cm

2 V

−1 s −1 ) and hole (99 cm

2 V

−1 s −1 ) at300 K. 61 Among the three methods mentioned above, FP-TRMCis a contactless and alternative current technique that gives short-range (several nanometers) charge transport behaviors under theelectric field lower than 10 V cm

−1 , while the contact and direct-current TOF and FET measurements reveal long range (tens ofmicrometers) transport behaviors under the higher electric field(10

4 –10

5 V cm

−1 ). 129 , 130 Hence, FP-TRMC technique is moreinformative to measure anisotropic crystalline materials due toits elimination of grain boundaries, impurities, defects or contactresistance. In 2018, a 2D conductive MOF Fe 3 (THT) 2 (NH 4 ) 3 was reported with a record mobility ( ∼220 cm

2 V

−1 s −1 )and band-like transport behavior by time-resolved teraherzspectroscopy (TRTS) and Hall effect measurements. 117 TRTSis an all-optical, contact-free method to explore intrinsic chargecarrier dynamics in semiconductors. Compared with TRMC(w ith frequenc y about GHz), TRTS provides higher frequency(0.6–1.6 THz) complex photoconductivity, being capable ofreflecting the intrinsic charge transport mechanisms. Hall effectmeasurements adopting van der Pauw sample configurationallow determination of the carrier type, density and mobility.However, for valid measurements, a few conditions are required:(i) flat samples with uniform thickness are needed for thevalid measurements; (ii) the sample must be complete withoutany isolated holes; (iii) the sample must be homogeneous andisotropic; (iv) all four contacts must be located at the edges of thesample; (v) each contact area should be at least 10-fold smallerthan the area of the entire sample; (vi) the sample thicknessshould be much less than the width and length of the sample. (vii)Symmetrical sample is the best choice. The hall bar is the idealmethod of measuring the hall effect. In addition, the space-chargelimited current (SCLC) strategies are also utilized to measurecharge mobility. 131

2.4. Mechanism of the electronically conducting in

MOFs Electrical conductivity of solid materials is dependent on thedensity ( n ) and mobility of the charge carriers (electrons orholes). To acquire high conductivity, both high charge mobilityand charge density are indispensable according to followingequation,

σ = ne μe + pe μh

where n or p is charge density of electrons or holes, e isthe elemental charge, μe / μh is mobility of electrons or holes.Charge density reflects the concentration of free charge carriers

21

or thermally activated carriers in metallic or semiconductingconductors, while charge mobility represents the charge transportefficienc y. A s known, MOFs are coordinately assembled by metalions and organic linkers, thus both of them can be the sources ofcharge carriers. The coordination chemistry provides designableorbital symmetries so as to acquire good spatial and energeticoverlap, which contributed to high charge mobility.

2.4.1. Electron transport modes in MOFs The reported charge transport modes in conductive MOFs canbe described from both chemical and physical perspectives.As shown in Fig. 24 (a), from a chemical design principle,conductive MOFs can be sorted into categories of “throughspace” or “through bonds”. Fig. 24 (b) exhibited the conductionmechanisms from a physical perspective with hopping or bandtheory. 17 , 132

Hopping transport depends on the charge hopping from thedonor to the acceptor sites. In general, the charge carries arelocalized at isolated sites with discrete energy levels and can hopfrom donor to neighboring acceptor under favorable conditions.The probability of hopping ( P ) between two states of spatialseparation R and energy separation W is given as following: 133

P ( R, W ) = exp

(−αR − W

kT

)

Here, κ is Boltzmann’s constant; α represent a constant. Thewhole conduction can be acquired by integrating all the distancesand energy states, working out Mott equations or Shklovskii andEfros equations when the dimensionality and state distributionare determined. The hopping mechanism has been identified inM 2 (DEBDC) (M = Mn, Fe; E = O, S), as shown in Fig. 25 (a). 102

For example, the charge in Mn 2 (DSBDC) hops through the 1Dinfinite (-Mn-S-) ∞

chains, resulting in intrinsic charge mobilityof 0.01 cm

2 V

−1 s −1 . However, due to its non-redox-active ligandsand low energy d

5 Mn

2 + ions, the compound showed lowcharge density, thus leading to the poor electrical conductivity of2.5 × 10

−12 S cm

−1 . Through-space charge transport relies on that of charge

carriers between acceptor and donor sites via non-covalenceinteractions ( π–π interactions), which could provide spatial andorbital overlap for transporting the charge through space. Thismechanism has been uncovered in tetrathiafulvalene (TTF)-based MOFs, M 2 (TTFTB) (M = Mn

2 + , Co

2 + , Zn

2 + , andCd

2 + ; TTFTB

4 − = tetrathiafulvalene tetrabenzoate). 49 , 50 TakeZn 2 (TTFTB) as an example. The TTF motif formed strong π–π stacked one dimensional helical columns with a relatively shortS ��S distance of ∼3.7 A between neighboring TTF molecules.In this case, noticeable intrinsic mobility of 0.2 cm

2 V

−1 s −1

for Zn 2 (TTFTB) was observed ( Fig. 25 (b)). Thus, the electricalconductivity can be effectively modulated by controlling the π–πstacking motifs.

The through-bond conduction relies on the charge trans-port through continuous valence bands in conductive MOFs,and it strongly depends on the orbital symmetry, and theenergy level matching between the metal node and the organicligand. This conduction mechanism has been realized in Cu[M( pdt) 2 ] ( pdt is 2,3-py razinedithiolate; M = Cu, Ni) withcontinuous Cu(pyrazine) two dimensional layered sheets withthe Cu bis(dithiolene) nodes as the charge transport networks( Fig. 25 (c)). 13 , 101

Conductive MOFs based on Fe centers usually showednoticeable electrical conductivity due to its smallest ionization



EnergyChem REVIEW

Fig. 24. The reported charge transport modes in conductive MOFs described from both chemical and physical perspectives. (a) From a chemical design principle: (i) “through space” transport; (ii) “through bond” transport. Reproduced with permission. 17 Copyright 2018, Royal Society of Chemistry. (b) From a physical perspective: (i) hopping. The charge has a small mobility in the form of infrequent hopping from one localized state to another. E T = Energy level of trap state. (ii) Band theory. Conduction occurs in the conduction band (for electrons). E C = Conduction band, E V = Valence band. The dashed line is a donor level that supplies the surplus of electrons but plays no further role. Reproduced with permission. 132 Copyright 2011, Wiley- VCH.

Fig. 25. (a) Hopping transport in Mn 2 (DSBDC). Reproduced with permission 102 Copyright©2013, American Chemical Society. (b) Charge transport through space in Zn 2 (TTFTB). Reproduced with permission 49 Copyright©2012, American Chemical Society. (c) Charge transport through band in Cu[M(pdt) 2 ] (M = Cu, Ni): left, the 3D crystal structure; right: the Cu(pyrazine) two dimensional layered sheet in Cu[Ni(pdt) 2 ]. Reproduced with permission 14 Copyright 2016, Wiley-VCH. (d) Conduction regulation in Fe(tri) 2 (BF 4 ) x . Reproduced with permission 98 Copyright©2018, American Chemical Society.



EnergyChem REVIEW

v

H

energy, redox-active Fe 2 + /3 + pair, and high energy electronsof Fe 2 + . 122 Besides, partially oxidizing the Fe 2 + centers couldproduce plenty of Fe 3 + defect states, leading to mixed-valencedoping in the materials with significantly enhanced electricalconductivity. Dinc a et al. showed the electrical conductivity ofFe 2 (BDT) 3 single crystal, which can be improved over 10

5 -fold by partial oxidation. 100 Long et al. synthesized the mixed-

alence MOF, namely Fe(tri) 2 (BF 4 ) x (tri − = 1,2,3-triazolate) bychemically oxidizing its poorly conductive framework Fe(tri) 2 .The study demonstrated that Fe(tri) 2 (BF 4 ) 0.33 displayed thehighest conductivity about 0.3 S cm

−1 at room temperaturewith 10

8 -fold improvement from that of the parent materials( Fig. 25 (d)). The enhanced electrical conductivity can beascribed to the highly delocalized charge between octahedral low-spin Fe 2 + and Fe 3 + centers. 98

The band transport mechanism depends on the delocalizationof the charge carriers through the conduction band or valenceband. The total charge mobility in the materials is determined bytwo parameters, the effective mass of the charge carrier and thecharge scattering frequency:

μ =

e τm ∗

where m

∗ is effective masses; e refers to the elemental charge, τrepresents the charge-scattering mean time. To achieve excellentmobility, a large τ and small m

∗ are required. A large τ canbe obtained with less charge-scattering sites that derived fromdisorders, defects, impurities, or boundaries ( Fig. 24 (b)). Toacquire a small effective mass, a well-dispersed band, highlysymmetric crystal lattice and a facile unit cell are prerequisiteconditions.

In 2018, the band-like charge transport in semiconductingMOFs was reported by Cánovas and Feng et al. 117 Theas-synthesized Fe 3 (THT) 2 (NH 4 ) 3 was composed of iron-bis(dithiolen) linkage and NH 4

+ as counter ions, showingDrude-type band-like transport with a record charge mobilityas high as 220 cm

2 V

−1 s −1 . The calculated band structure ofthis compound showed a bandgap of ∼350 meV indicatingthe semiconducting signature. TRTS was employed to studythe conductivity of MOFs in an optical and contact-freemanner, which can evaluate the intrinsic charge transport inpolycrystalline samples. Specifically, a femtosecond laser pulsewas utilized to optically inject the charge, then the change ofthe conductivity could be measured on ultrafast timescaleswith a ∼1 ps THz pulse. TRTS provided a time-dependentcomplex photoconductiv ity w ith a w ide frequenc y (0.6–1.6THz) dependent real and imaginary parts. The Drude modelcan fit the plot very well, which demonstrates band-like chargecarrier transport in Fe 3 (THT) 2 (NH 4 ) 3 .

2.4.2. Metallic states in MOFs The electronically conductive MOFs with high porosity can act asneat active electrodes or channels in various applications, such assupercapacitors, batteries, chemiresistive sensors, thermoelectricdevices and FETs. According to most reports, typically, conduc-tive MOFs are semiconductors with moderate bulk conductivityvalues ( < 1 S cm

−1 ). To explore conductive MOFs with metallicstates could considerably improve their performance and discovermore intriguing intrinsic physics in true hybrid solid materials.

A π -conjugated conductive Ni-BHT nanosheet was sucess-fully reported by Nishihara’s group. 57 The as-prepared pristinecompound showed the conductivity of 2.8 S cm

−1 at 300 K with aFermi edge measured by photoelectron spectroscopy, suggesting

23

the metallic nature in pristine Ni-BHT. In addition, conductivitycan be controlled during the redox process. The average oxidationnumber in pristine Ni-BHT is −3/4 which can be reduced to−1 and oxidized to 0 with tunable electrical conductivity. 114 Theoxidized Ni-BHT exhibited the ultrahigh electrical conductivityof 1.6 × 10

2 S cm

−1 at 300 K, which is 100-fold higher thanthe pristine compound. The band calculation suggested bothpristine Ni-BHT and oxidized Ni-BHT are metal lic whi le theexperimental measurements showed a semiconducting behaviordue to numerous grain boundaries and structural disorders. Infact, by precisely controlling the doping level in Ni-BHT, thiscompound can function as a 2D topological insulator (Tl) whichwi l l be discussed later.

The highest value reported in coordination polymers atambient conditions is Cu-BHT films with conductivity of 1,580 S cm

−1 . Cu-BHT possesses unique Kagome structuresalong the ab plane w ith a six fold axis of symmetr y. Ever ycopper atom coordinated with four sulfur atoms formed aplanar structure, forming a non-porous topological structures.The thermopower measurements with small values ( −4 to−10 μVK

−1 ) suggested the metallic nature of Cu-BHT. Inaddition, ultraviolet photoelectron spectroscopy (UPS) of thefilm with the Fermi edge and electronic structure calculationsof the Cu-BHT AA stacking mode further revealed its metallicbehavior. It is extremely interesting that its highly crystal line bul ksample showed superconductivity at about 0.25 K, realizing thesuperconductivity in coordination polymers. 134 The same groupalso reported an analogous compound, Cu-BHS with all six Satoms substituted by heavier Se atoms. This compound showedthe pellet conductivity of 110 S cm

−1 at 300 K which is oneof the highest values in conductive MOFs. The calculated bandstructure of Cu-BHS showed a highly dispersive band crossedby the Fermi level similar to Cu-BHT, together with Fermiedges revealed on UPS, indicating its intrinsic metallic nature. 86

Very recently, Zhu et al. further reported a metallic MOF,namely Ag-BHT with the infinite 2D silver-sulfur network, whichshowed high electrical conductivity of up to 250 S cm

−1 . 62 Thecompound is featured with a layered structure with alternativelystacked 2D Ag-S networks ( Fig. 26 (a)). The UPS of Ag-BHTdisplayed a clear Fermi edge which indicates its inherent featurefor metallic states. Although the calculated band structure showeda band gap of 0.8 eV, the Fermi level cut valence bands, in line withthe UPS results ( Fig. 26 (b)).

Due to the better overlap between the electronic wave func-tions of neighboring metal centers or organic linkers, metallic be-haviors are always discovered in smaller linker-based conductiveMOFs. However, recently, Marinescu and Melot et al. reportedthe metallic conductivity in 2D Co-HTTP with a periodic 2Dnetwork and larger size organic linkers ( Fig. 26 (c)). 64 The as-synthesized Co-HTTP showed the moderate conductivity of3.2 × 10

−2 S cm

−1 with a metallic transition behavior on cooling.The DFT calculation showed a semimetallic behavior resultingfrom the interlayer overlap of metal d and ligand p orbitals( Fig. 26 (d)). Recently, the same group synthesized an analogousFe-HTTP MOF via a liquid-liquid interfacial method, whichalso showed metallic behaviors similar to Co-HTTP. 65 Thedifference is that the as-prepared Fe-HTTP was easily oxidizedby air and resulted in the metallic-like feature at ambient orhigher temperature. According to the DFT calculations andexperimental results, the author proposed that both the intrinsicstacking faults and environmental factors play crucial roles inband structures and the semiconductor-to-metal transition of M-

TTP (M = Co, Fe).



EnergyChem REVIEW

Fig. 26. (a) Crystal structure of Ag-BHT along c-axis. C is in gray; S is in yellow; Ag is in blue. (b) Electronic band structure of Ag-BHT. Reproduced with permission. 62 Copyright©2018, American Chemical Society. (c) Structure of [Co 3 (THT) 2 ] 3 −. (d) Calculated electronic band structure and DOS of [Co 3 (THT) 2 ] 3 −. Reproduced with permission. 64 Copyright©2017, American Chemical Society.

Fig. 27. (a) Simulated crystal structures of Ni 3 HIB 2 and Cu 3 HIB 2 . (b) Calculated band structures of bulk Ni 3 HIB 2 and Cu 3 HIB 2 . Reproduced with permission. 82 Copyright©2017, American Chemical Society.

b

e

m

if

t

(

b

(

p

t

s

a

S

i

2

t

c

L

s

t

l

a

H

t

0

d

e

2

d

H

s

Although theoretical studies showed several 2D MOFs canehave as promising bulk metals, yet there is sti l l rare directxperimental evidence to reveal the metallic behaviors in theseaterials. Dinc a and co-workers reported the metallic behavior

n M 3 HIB 2 (M = Ni, Cu) with bulk conductivity of 8 S cm

−1

or Ni 3 HIB 2 and 13 S cm

−1 for Cu 3 HIB 2 at 300 K. 82 Both ofhem showed 2D layered structures with slipped stacking layers Fig. 27 (a)). According to the DFT electronic band structures,oth monolayer and bulk M 3 HIB 2 showed metallic behaviors Fig. 27 (b)). Interestingly, for the bulk states, the metallic featureresented in the ab plan whereas semiconducting nature along

he c direction. Recently, the same group fur ther repor ted aeries of MOFs constructed from Ln

3 + (Ln = La, Nd, Ho, Yb)nd HHTP with a noticeable electrical conductivity up to 0.05 cm

−1 measured by 2-probe method on pellet samples. Its worth noticing that, different from reported π -conjugatedD conductive MOFs with square-planar transition metal ions,

24

he Ln

3 + ions located between the planes of the linkers andonnect organic layers into a three dimensional structure vian–O chains. The electronic band structure calculated by DFT

uggested a metallic behavior along the c direction (A- Г) withhe bands crossing the Fermi level. Interestingly, the Fermi levelies inside a bandgap within the ab plane ( Г-K-M), indicating semiconductive behavior. The electrical conductivity of Ln-HTP are comparable to the most conductive MOFs reported

o date, ranging from 0.9 × 10

−4 S cm

−1 for La-HHTP to.05 S cm

−1 for Ho-HHTP at room temperature. The studyemonstrated a novel conduction mechanism in Ln-HHTP withfficient one dimensional charge transport perpendicular to theD sheets. 90

2D π -d conjugated MOFs attracted considerable attentionue to their highest electrical conductivities among MOFs.owever, their poor crystallinity and unsolved single-crystal

tructures impede the critical insight into their intrinsic transport



EnergyChem REVIEW

Fig. 28. (a) SEM image of single crystal Ni 3 HITP 2 based device. (scale bar: 1 μm). (b) I-V curves of single rod Ni 3 HITP 2 device at 295 and 1.4 K

with 4-probe and 2-probe methods. Inset: Normalized magnetoresistance at 1.4 K, 5 K, and 10 K. (c, d) Temperature dependent conductance (solid line) measured with 4-probe method on a single crystal device (c) and a polycrystalline film device (d) of Ni 3 HITP 2 . The dotted lines represented their corresponding Zabrodskii plots. Inset shows the temperature-dependent conductance plotted with linear axes. Reproduced with permission. 89

Copyright©2019, American Chemical Society.

properties. Recently, Dinc a et al. studied the single crystals ofNi 3 HITP 2 and Cu 3 HHTP 2 for their charge transpor t proper tiesand structural features. 89 Fig. 28 (a) and (b) displayed thesingle rod device of Ni 3 HITP 2 with the diameter of ∼200 nmand corresponding current-voltage plots at 1.4 and 295 K,respectively. The conductance G was measured through both 4-probe and 2-probe methods with 4-probe values of 1.3 mS at295 K and 0.7 mS at 1.4 K, and 2-probe conductance of 0.25mS at 295 K and 0.13 mS at 1.4 K. As Fig. 28 (c) shown, thetemperature dependent conductivity decreased to 0.3 K withnegligible decreasing. The Zabrodskii plot showed a positiveslope indicating the absence of the strong localization, thusleading to the metallicity in Ni 3 HITP 2 single crystals. Thedecreasing activation energy of Ni 3 HITP 2 with decreasing thetemperature indicated the non-zero conductance at T = 0 K.By contrast, the polycrystal line Ni 3 HITP 2 film displayed adramatically decreased conductance as temperature decreased ina small range ( Fig. 28 (d)). The Zabrodskii plot with a negativeslop indicated the signature of semiconductive behavior, resultingin zero conductance when T tends to be 0 K. They also measuredthe single crystalline rod and flake devices of Cu 3 HHTP 2 whichshowed the conductivity of 1.5 and 0.5 S cm

−1 , respectively at295 K. The similar conductivities in Cu 3 HHTP 2 rods and flakesindicated the comparable values between in-plane and out-of-plane conductivities. The temperature-dependent conductivityon Cu 3 HHTP 2 can only be conducted ranging from 300 to 200 Kwith a significant decrease in conductance which prevented lowtemperature characterization. These results indicated that theout-of-plane transport plays a key role in charge transport in2D MOFs. Furthermore, single crystal based charge transportcharacterization can intrinsically reveal the transport naturecompared with polycrystalline samples.

25

2.5. Application

Owing to their outstanding physical, chemical and electricalproperties, the family of conductive MOFs has received con-siderable attention as multifunctional materials to date. Theircharacteristics include the high degree of structural designability,high surface areas and good electrical conductivity, making themexcellent candidates of functional components in electrical andelectrochemical technologies, such as chemiresistive gas sensors,energy storage/conversion, catalysis, and electronics.

2.5.1. Sensing

Highly selective and sensitive chemiresisitive sensors are de-sirable in as the following fields, such as disease diagnosis,environmental monitoring, smart home equipment. 135–137 Todate, in spite of numerous chemiresistive sensors have beencommercialized, their performance sti l l needs further improve-ments with ppb-level sensitivity, long-term stability and efficientcross-selectivity. Due to the surface reaction dependent ofchemical sensors, MOF materials are a family of promisingcandidates for its ultrahigh surface areas and modifiab le porestructures. Meanwhile, MOFs with high electrical conductivitycan effectively transduce the sensing performance with electricalsignals.

3D conductive MOF-based chemiresistive sensing : Due to theirrecord-breaking large surface areas, MOFs are emerging aspromising sensing materials. In 2014, Zhang et al. reported thepure MOF (ZIF-67) based chemiresistor which can transducethe sensing signals only at 150 °C because of the large electronicband gap ( ∼1.98 eV). 138 The sensor showed low detection limitof 5 ppm and high humidity resistance (up to 70% RH). The samegroup further explored another 3D MOF (Co [(im) 2 ] n ) with se-



EnergyChem REVIEW

Fig. 29. (a) Single crystal structure of ZIF-67; (b) The resistance change of ZIF-67 based chemiresistor to 5–500 ppm formaldehyde at 150 °C. Reproduced with permission. 139 Copyright © 2014, American Chemical Society. (c) Crystal structure and the redox-active Ni(pdt) 2 subunit of Cu[Ni(pdt) 2 ]. (d) The gas-phase specific heat capacity dependent sensitivity of Cu[Ni(pdt) 2 ] toward various hydrocarbons. Reproduced with permission. 142 Copyright © 2019, American Chemical Society.

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ective chemiresistive sensing properties toward trimethyleamine2 ppm) at 75 °C ( Fig. 29 (a) and (b)). 139 Another famousD MOF (NH 2 -UiO-66) also exhibited chemiresistive sensingroperties to SO 2 (Responde = 21.6% to 10 ppm) at 150 °C

n Ar atmosphere. 140 Although above 3D MOFs exhibited theeasibility in chemiresistive materials, the challenges are obvious:

ue to the poor conductivity at room temperature (RT), theigher operation temperature brings high energy consumptionnd safety hazards. To enhance the electrical conductivity,upp et al. successf ully sy nthesized 3D conductive MOFs by

ntroducing the conductive agents in the pore ( σ = 1.8 × 10

−7

cm

−1 at RT) and showing chemiresistive properties to H 2 5%). 141 However, the filled void space in MOFs may impede thedsorption of adsorbates and hence decrease its chemiresistingerformance.

Long et al. demonstrated that the conductivity of 3D MOFu [Ni(pdt) 2 ] can be greatly changed by selectively adsorbing

he gaseous hydrocarbons, including ethane, ethane, ethylene,cetylene, propane, propylene, and cis-2-butene at room tem-erature ( Fig. 29 (c) and (d)). 142 The results showed that theensitivity of conductance in Cu [Ni(pdt) 2 ] guest insertion istrongly dependent on the gas-phase specific heat capacity of thedsorbate rather than its binding strength. It is new direction toevelop novel chemiresistive sensor materials focusing on bulkas adsorption properties instead of high temperature reactivityf surface modified composites.

2D conductive MOF-based chemiresistive sensing : 2D conductiveOFs developed very fast due to its highest conductivity

10

−3 –10

3 S cm

−1 ), ease of access (bottom-up solution-phaseynthesis), structural modularity, intrinsic columnar porosity andigh surface-to-volume ration of single/few layer thin films. In015, Dinc a and co-workers demonstrated 2D conductive MOFased chemiresistive sensors for detecting ammonia at room

26

emperature. 48 The as-synthesized Cu 3 HITP 2 showed a highlectrical conductiv ity (0.2 S cm

−1 ) and obv ious response to.5–10 ppm of ammonia. To extend the 2D conductive MOFased sensors, Dinc a’s group subsequently developed sensorrrays comprised of Cu 3 HHTP 2 , Cu 3 HITP 2 , and Ni 3 HITP 2 . 143

his sensor arrays can recognize and classify 16 volatile or-anic compounds (VOCs) into 5 categories (including alcohol,etones/ethers, aromatics, amines, and aliphatics) through therincipal component analysis (PCA) method ( Fig. 30 (a) andb)). A charge transfer mechanism may partially explain theensing performance considering its metal center dependentesponse (d

8 Ni 2 + versus d

9 Cu

2 + ). In addition, Cu 3 HITP 2 alsoxhibited a concentration dependent switch in response toward-butylamine which may be related to the hydrogen bonding

ormed between the linkers and the adsorbate. Besides, Gastaldond co-workers further uncovered the chemiresistive mechanismf 2D conductive MOFs Cu 3 HHTP 2 by DFT calculation. 144

ccording to the calculation and experiments, NH 3 and H 2 Ooleculars can coordinate to the Cu(II) open sites which

nduced the expansion of the lattice along c direction. Thexpansion of its unit cell increased the band gap and henceeduced the conductance of 2D MOF materials ( Fig. 30 (c) andd)). Very recently, Kitagawa et al. synthesized a new dual-igand 2D conductive MOF with modulated conductivity andorosity ( Fig. 30 (e)). 120 The Cu 3 (HHTP)(THQ) nanowire

hick film sensor showed excellent sensing performance towardow concentrations of NH 3 and an outstanding LOD (0.02–.35 ppm) ( Fig. 30 (f)). The sensing mechanism has been clearlyesolved by IR and PXRD characterization which demonstratedtrong NH 3 –Cu

+ /2 + interaction and lattice expansion along the -axis upon NH 3 adsorption.

In 2019, Mirica and co-workers further demonstrated a seriesf phthalcyanine-based 2D conductive MOFs as active materials



EnergyChem REVIEW

Fig. 30. (a) Structures of M 3 HHTP 2 and M 3 HITP 2 ( M = Cu, Ni). (b) PCA for 2D conductive MOF sensor arrays. Reproduced with permission. 143

Copyright © 2015, American Chemical Society. (c) Schematic representation of different coordination geometries of Cu(II) note and the corresponding simulated structures with different spacing distance. (d) DOS of Cu-CAT-1 in presence of H 2 O and NH 3 . Reproduced with permission. 144 Copyright 2018, Wiley-VCH. (e) Desired structure of Cu 3 (HHTP)(THQ). (f) Response-concentration log-log plots to NH 3 , acetone and ethanol. Reproduced with permission. 120 Copyright 2020, Wiley-VCH.

Fig. 31. (a) Synthesis and crystal structure of (left) NiPc-M and (right) NiNPc-M. (b) Gas sensing properties of NiPc-M and NiNPc-M MOFs of NH 3 (40 ppm), H 2 S (40 ppm), and NO (1 ppm) in dry and humidified conditions (under H 2 O with 5000 ppm). (Bottom right) PCA analysis of NiPc-M and NiNPc-M MOF-based sensors. Reproduced with permission. 93 Copyright©2019, American Chemical Society.

in chemiresistive sensors ( Fig. 31 (a)). 93 The as-synthsized NiPc-M and NiNPc-M (M = Ni, Cu) exhibited remarkable sensitivityat ppm to ppb detection limit toward NH 3 (0.31–0.33 ppm), H 2 S(19–32 ppb), and NO (1.0–1.1 ppb) and noticeable humidity-independent sensing performance ( Fig. 31 (b)). This bimetallicmolecular meshes enable modular control over selectivity andsensitivity in gas sensing via various ligands and metal notes.

Although 2D conductive MOFs are widely used in chemire-sistors, integration methods into electronic devices are sti l llimited. Most MOF based devices fabricated by drop casting ormechanical compression and abrasion which may damage theiractivity or conductivity. To facilitate the integration process of2D conductive MOF based chemiresistors, Mirica et al. directlygrew Cu 3 HHTP 2 and Ni-CAT nanowires on graphite electrodesdrawn on shrinkable polymer films with 4B pencils. 71 Thesensors exhibited a good response to 80 ppm of NH 3 (0.7% forCu 3 HHTP 2 ), NO (1.8% for Cu 3 HHTP 2 ), and H 2 S (4.2% forNi-CAT). Moreover, they successfully fabricated the Ni-basedconductive MOFs (Ni 3 HITP 2 and Ni-CAT) on textiles with

27

high flexibility and good mechanical stability ( Fig. 32 (a)). 72

In addition, they showed high response to 80 ppm of NOand H 2 S and humidity-resistance properties to 5000 ppm H 2 Ovapor ( ∼18% RH) ( Fig. 32 (b)). As mentioned in Fig. 13 , Ni-CAT showed different crystal structure or molecular formulacompared with Cu 3 HHTP 2 or Ni 3 HITP 2 . Therefore, thesestructure differences should be considered to draw the conclusionabout their structure-function relationships.

Meanwhile, Xu et al. synthesized 2D conductive MOFnanofilms via layer-by-layer liquid epitaxy method. 75 The as-synthesized crystal line Cu 3 HHTP 2 films have controlled thick-ness (20–100 nm, ∼2 nm per cycle) and preferred orientation(along c direction) ( Fig. 32 (c)). This 2D conductive MOF filmwith a thickness of 20 nm showed the best sensing performanceto 10 ppm NH 3 (Response = 45%) with a low detection limit(0.5 ppm) and good selectivity ( Fig. 32 (d)).

Materials with heterostructures always exhibited multifunc-tional properties beyond any of the single components. Xuet al. fabricated van der Waals hetero-structured MOF-on-MOF



EnergyChem REVIEW

Fig. 32. (a) Conductive MOF nanowires coated tex tile tex ture as the gas sensor. (b) Gas sensing performance of Ni-CAT and Ni 3 HITP 2 based devices toward 80 ppm NO and H 2 S in dry and humidified conditions (5000 ppm H 2 O). Reproduced with permission. 72 Copyright©2017, American Chemical Society (c) The chemiresistive NH 3 sensor based on Cu 3 HHTP 2 film. (d) Left: The NH 3 concentration-dependent response-recovery plot. Right: Response to various reducing gasses. Reproduced with permission. 75 Copyright 2017, Wiley-VCH.

Fig. 33. (a) Illustration of the preparation of MOF-on-MOF cascade structure via van der Waals integration method. AFM images of Cu-TCPP-0C

–Cu- HHTP-20C (b) and Cu-TCPP-10C-on-Cu-HHTP-20C (c). (d) long-term stability of Cu-TCPP-10C-on-Cu-HHTP-20C toward 100 ppm benzene gas. e. PCA for MOF-on-MOF sensor array’s response to five typical biomarkers. Reproduced with permission. 145 Copyright 2019, Wiley-VCH.

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hin films based 2D conductive MOFs to realize advancedhemiresistive sensing ( Fig. 33 (a)-(c)). 145 This van der Waalsntegration method deposited molecular sieving Cu-TCPP layernto semiconductive Cu 3 HHTP 2 layer and showed outstandingelectivity, stability and ultrahigh response to benzene at roomemperature ( Fig. 33 (d) and (e)). Interestingly, the heterostruc-ured films showed reversed selectivity toward benzene and NH 3 ompared with single Cu 3 HHTP 2 film.

Conductive MOFs have been developed as active materials inhemiresistive sensors. However, in spite of fast developments,he sensing mechanism and fundamental insight into host-guest

28

nteractions between the analytes and the host frameworks haveot been fully characterized. It is necessary to make further

mprovements from the following aspects: (i) improve theelective detections of specific gas molecules; (ii) the responsend sensitivity remain to be lower than metal oxide-basedhemiresistive sensors; (iii) the stability needs further improve-ent to meet the practical application in the future. As conductiveOFs developed faster, there is plenty of room for an in-depth

tudy of pure MOF-based chemiresistors. In consideration ofhe tunable pore size, intrinsic charge transport, and the atomicever control over molecular and supermolecular structures of



EnergyChem REVIEW

Fig. 34. (a) Chemical structure of Ni 3 HITP 2 . (b) Cyclic voltammetry (CV) curves of Ni 3 HITP 2 electrode with increased cell voltage (at 10 mV s − 1 ). (c) Comparison of area-normalized capacitances of Ni 3 HITP 2 with other EDLC active materials. Reproduced with permission. 157 Copyright © 2016, Springer Nature. (d) Chemical structure of Cu-HAB. (e) CV of Cu-HAB at scan rate from 1 to 100 mVs −1 . (f) Comparison of volumetric and area- normalized capacitances of Ni-HAB among various active materials. Reproduced with permission. 83 Copyright © 2018, Springer Nature.

D

MOFs, we expect robust and outstanding conductive MOFchemisensors to overcome the aforementioned challenges.

2.5.2. Energy storage Due to the energy and environmental crisis in modern society,renewable energy sources and sustainable storage technologiesattract considerable attention worldwide. With the boom oftransportation market (hybrid or full electric vehicles), highenergy/power density of electrical energy storage devices aredemanded. Among plenty of energy storage devices, batteries andsupercapacitors (SCs) are two major technologies. 146 , 147

Conductive MOFs for supercapacitors : Electrochemical doublelayer capacitors (EDLCs) have attracted considerable attentionbecause of their superior power density and excellent cyclabilitythan batteries and other capacitors. 148 , 149 The capacitance ofEDLC comes from the charge separation at the interface ofelectrode/electrolyte which is related with both the electricalconductivity and surface areas of active materials. However, theactive materials utilized so far in EDLCs are mainly carbon mate-rials (activated carbons, carbon nanotubes and graphene). 150–152

To develop novel categories of electrode materials that max-imize both conductivity and surface areas wi l l greatly drivethe transformative advances in EDLCs. Unfortunately, althoughconventional MOFs exhibit ultrahigh surface areas (up to 7000m

2 g −1 ). 153 The poor electrical conductivity made them onlyused in EDLCs as pyrolysis products or as hybrid composites withconductive additives which damaged its porous structures anddecreased its energy storage capacity. 12 , 154–156 Conductive MOFssimultaneously possess good charge transport and high surfacearea, and thus can act as sole electrode materials for EDLCs.

In 2017, the Dinc a group reported conductive MOF(Ni 3 HITP 2 ) served as the sole electrode materials in EDLCs( Fig. 34 (a)). 157 This is the earliest example of neat MOFs basedsupercapacitor, without any organic binders or conductiveadditives. The Ni 3 HITP 2 based device exhibited a ultrahigh

29

surface area-normalized capacitance of ∼18 μFcm

−2 whichis higher than most carbon materials and noticeable stability(10,000 c ycles w ith 90% retention) ( Fig. 34 (b) and (c)).Meanwhile, Xu et al. successfully grown conductive MOFnanowire arrays on carbon fiber substrates and they were directlyused as electrodes for solid-state supercapacitors. 115 Comparedwith the powder electrode, this nanostructured one showedthe ultrahigh area-normalized capacitance ( ∼22 μFcm

−2 ) andoutstanding rate performance (55% for 10 times current density)of all reported MOF materials for supercapacitors.

According to the energy storage mechanism, electrochemicalcapacitors can be divided into EDLCs and pseudocapacitors.Different from EDLCs whose capacitance stems from chargeseparation between the electrode and electrolyte, pseudocapac-itors store energy via reversible redox reactions. In 2018, Baoet al. demonstrated the potential of using redox-active conductiveMOFs in supercapacitors. 83 The as-synthesized Cu-HAB andNi-HAB exhibited pellet conductivity of 11 ±3 and 70 ±15 Sm

−1 , respectively, and they are capable of acting as conductiveadditive-free electrodes ( Fig. 34 (d)). The submi l limeter-thickpellets (thickness is 50 μm) of HAB MOF electrode showedexcellent volumetric capacitance ( ∼760 F cm

−3 ) and area-normalized capacitances ( > 20 F cm

−2 ). In addition, the HAB-based conductive MOF electrodes showed excellent reversibleredox behaviors as well as noticeable cycling stability (12,000c ycles w ith 90% retention) ( Fig. 34 (e) and (f)).

EDLCs possess higher power density but usually low capac-itance, while pseudocapacitors generally exhibit higher energydensity but suffer from poor stability. Chen et al. reported a novelconjugated Cu-catecholate based conductive MOF, namely Cu-

BC, with a high conductivity of ∼1.0 S m

−1 which merged bothadvance of EDLCs and pseudocapacitors ( Fig. 35 (a) and (b)). 95

As shown in Fig. 35 (c) and (d), the EDLC accounted for 62%while the rest 38% came from pseudocapacitance. As-synthesizedCu-DBC electrodes have ultrahigh gravimetric capacitance of 479



EnergyChem REVIEW

Fig. 35. (a) Schematic synthesis of Cu-DBC and its crystal structure. (b) CV curves of Cu-DBC based supercapacitor at different scan rates. (c) Plot of Cg against reciprocal of square root of scan rate ( ν−0.5). (d) Respective capacitance contribution of Cu-DBC at 10 mV s − 1 . (e) Comparison of energy and power density among other reported materials. Reproduced with permission. 95 Copyright 2019, Wiley-VCH.

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g −1 at 0.2 A g −1 , noticeable energy density and remarkableycling performance ( Fig. 35 (e)). Their work demonstratedhat the combination of abundant redox-active centers, highonductivity and large surface area in conductive MOFs canrovide a new possibility to realize the practical application innergy storage devices.

The miniaturized rechargeable energy storage system attractsuch attention because of its spread application in portable

lectronics, implantable microdevices, and the Internet of Things.ue to their excellent energy storage performance, Alshareef

eveloped microsupercapacitors based on conductive MOF,amely Ni-CAT. 158 This compound can selectively grew on 3D

aser scribed graphene (LSG) which acted as a matrix-currentollector ( Fig. 36 (a)). The integrated electrodes exhibited aide voltage window up to 1.4 V, outstanding area capacitancef 15.2 mF cm

−2 , noticeable energy and power density withhe values of 4.1 μWh cm

−2 and 7 mW cm

−2 , respectively Fig. 36 (b)). Furthermore, to improve the device flexibility,u et al. synthesized cellulose nanofiber @ conductive MOFs

ore-shell structure based free standing nanopapers with ul-rahigh electrical conductivity over 100 S cm

−1 , micro–me so-orosity, and remarkable mechanical property ( Fig. 36 (c)–(f)). 74

his nanopapers are directly utilized as electrodes for flexibleupercapacitors which endow fast charge transfer and robust

echanical flexibility, hence leading to noticeable volumetricapacitance (2800 mF cm

−3 ), relative high energy and powerensity of 185.7 mW h cm

−3 and 0.37 mW cm

−3 , respec-ively, and unchanged capacitance under different deformations Fig. 36 (g)–(i)).

Conductive MOFs are emerging as a new system of porouslectrode materials for supercapacitors. Up to now, both oflectric double layer capacitor and pseudocapacitors based ononductive MOFs are reported with outstanding performances.he tunable pore sizes, defined crystal structures, and the

oncentrated pore size distributions make conductive MOFs andeal platform to deeply study charge/discharge mechanism in su-ercapacitors. Though tremendous advances have been achieved

30

n this area, great challenges remain: (i) conductive MOFs withxcellent electrical conductivity values, which is comparable withctivated carbons and holey graphite ( ∼ 10

3 S m

−1 ), are sti l lare; (ii) the relative small surface areas bring new challengeso obtain highly accessible surface areas for electrolytes; (iii)he lack of robustness of MOF materials damage the stabilityf supercapacitors; (iv) molecular understanding of chargingynamics in conductive MOF as electrodes are desired. 159 In

he future, much efforts should focus on overcoming abovehallenges to develop high-performance conductive MOF basedupercapacitors with excellent rate capacity, higher energy/powerensity and longer cycling performance.

Electronically conductive MOFs for batteries : Among plentyf electrical energy storage devices, SCs and batteries are twoajor technologies. Compared to supercapacitors, the batteries

xhibited much higher energy densities which attracted muchttention, especially since the large-scale commercialization ofithium-ion batteries by Sony Inc. in the 1990s. 146 However,igher power and energy density are desired to satisfy the futureehicles which require design and synthesis of novel anode,athode and electrolyte materials with excellent electrochemicalerformance. Due to the tailored structures, tunable pore sizes,igh surface areas and controllable electrochemical properties,OFs have been widely employed in Li-based batteries, includ-

ng Li-ion, Li–S and Li–O 2 batteries. 160 Electronically conductiveOFs emerged as new classes of multifunctional materials

eveloped rapidly in last five years. The ultrahigh intrinsiconductivity coexisting with crystalline porous structures makehem a promising candidate materials utilized in batteries, such ascting as anode/cathode materials, host materials or separators.

Conductive MOF as anode/cathode materials Li-based batteries : In 2018, Nishihara et al. reported a 2D

onductive MOF, bis-(diimino)nickel framework (namely NiDI)ynthesized from hexaaminobenzene (HAB) and nickel salthich acted as a cathode material for Li-ion battery. 161 Thisnique compound has a special redox-active unit, Ni(L

isq ) 2 L = o-diiminobenzosemiquinonate) which undergoes both



EnergyChem REVIEW

Fig. 36. (a) Preparation of LSG/Ni-CAT MOF hybrid. (b) CV curves at different scan rates (upper), comparison of area-normalized capacitance (middle), cycling stability of LSG/Ni-CAT supercapacitor (bottom). Reproduced with permission. 158 Copyright 2019, Wiley-VCH. (c) Schematic illustration of preparation for CNF@c-MOF hybrid nanofibers. Low (d) and high (e) resolution SEM images of CNF@Ni-HITP nanofiber. (f) TEM image of single CNF@Ni-HITP fiber. (g) Photograph of CNF@Ni-HITP nanopaper folded by an origami. Electrochemical performance of CNF@Ni-HITP based EDLCs with CV curves at different potential windows (h) and under different folding angles (i). Reproduced with permission. 74

Copyright©2019, American Chemical Society.

Fig. 37. (a) Chemical structure and the 2-electron-reduction and 2-electron-oxidation of Ni(L

isq ) 2 . (b) Schematic structure of NiDI. (c) Schematic illustration of the redox states with ions-insertion/desertion for both cations and anions. Reproduced with permission. 161 Copyright 2018, Wiley- VCH. (d) Chemical structure of Mn-HAB. (e) The band structure of Mn-HAB monolayer along -K-M- directions and the associated Brillouin zone. Reproduced with permission. 164 Copyright 2018, Wiley-VCH.

two-electron-oxidation and two-electron-reduction with totalfour-electron transfer per metal site ( Fig. 37 (a)). Therefore,NiDI possessed a couple of redox states accompanied by ions-insertion/desertion for both cations and anions ( Fig. 37 (b) and(c)), which resulted in a high specific capacity of 155 mAh g −1 at10 mA g −1 during the range of 2.0 ∼4.5 V vs. Li/Li + and stablecycling performance.

Another conductive MOF, Cu 3 HHTP 2 with larger 1-D chan-nels ( ∼ 1.8 nm) and high conductivity was also utilized as cath-ode materials for high-rate performance Li-ion batteries. 162 Theas-synthesized Cu 3 HHTP 2 showed a reversible discharge-chargecapacity of ∼95 mAh g −1 at 1C during the range of 1.7–3.5 Vand outstanding C -rate capability. The study contributed theexcellent rate performance to the intrinsically high conductivityand open porous layered framework to provide the unimpededtunnels for Li ions intercalation and release. In addition toacting as the cathode material, Yuan et al. explored Cu 3 HHTP 2 nanowires utilized as competitive anodes for robust Li-ionbatteries. 163 The research uncovered the Li + storage mechanismof Cu 3 HHTP 2 nanowire that the reversible insertion/desertion

31

of Li + took place in the aromatic C 6 ring and 1-D channels of theframework. Owing to the outstanding structural merits, such as1-D channel, large pore size, and high electrical conductivity, theas-synthesized Cu 3 HHTP 2 nanowires have high Li-ion diffusioncoefficients and low charge transfer resistance, thus resulting ina noticeable rate-capacity of ∼631 mAh g −1 at current densityof 0.2 A g −1 and remained ∼381 mAh g −1 at 10-fold increasedcurrent density.

Wang et al. theoretically investigated a 2D conductive microp-orous MOF (Mn-HAB) as Li–S battery cathode via density func-tional theory calculations ( Fig. 37 (d) and (e)). 164 Consideringthe high atomic rations of metal/nitrogen, ultrahigh conductivityas well as permanent pore structures, Mn-HAB is expected toaddress following three challenges in Li–S batteries: (i) confiningthe dissolution of lithium polysulfides; (ii) facilitating the elec-tron conductivity; (iii) buffering the volumetric expansion duringthe lithiation process. Furthermore, the calculated theoreticalenergy density is 1395 Wh kg −1 , indicating that Mn-HAB can actas a high-performance cathode material applied in Li–S batteries.



EnergyChem REVIEW

Fig. 38. (a) Chemical structure of Co-HAB. (b) Proposed 3-electron reversible reaction in Co-HAB. (c) Rate capability of Co-HAB-D (D denote “the optimized product”) . (d) Comparison of areal capacity of Co-HAB-D with other reported anode materials. Reproduced with permission. 113

Copyright©2018, American Chemical Society. (e) Chemical structure of Fe 2 –O 8 -Pc-Cu. (f) Rate capability of the Fe 2 –O 8 -PcCu/I 2 electrodes. Reproduced with permission. 119 Copyright 2019, Wiley-VCH.

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Na-ion batteries : Compared with redox-active organic materialss electrodes in rechargeable batteries, redox-active ligand-basedonductive MOFs exhibited higher electronic conductivity andobust structures. Bao and co-workers reported a novel Co-ased 2D conductive MOF (namely Co-HAB) consisting of theedox-active linker, HAB, with high stability and conductivitypellet, 1.57 S cm

−1 ) ( Fig. 38 (a)). 113 They demonstrated thato-HAB was capable of storing three electrons and sodium ionser linker in organic electrolytes ( Fig. 38 (b)), leading to anltrahigh capacity of 291 mAh g −1 with outstanding stability Fig. 38 (c)). Additionally, Co-HAB exhibited remarkable rateapabilities (obtaining 152 mAh g −1 within 45 s) due to its highorosity and outstanding intrinsic conductivity ( Fig. 38 (d)).

To enhance the powder/energy density and stability of energytorage devices, the sodium-ion hybrid capacitors (SIHCs)ttracted much attention in recent years. In 2019, Liu and Lit al. reported the SIHC based on conductive MOFs withetal catechol structure, namely Cu-CAT or Cu 3 HHTP 2 , which

cted as an anode material. 165 The as-synthesized Cu-CATanowire/nickel foam anode showed a reversible capacity of260 mAh g −1 at current density of 0.1 A g −1 and noticeable

nergy/power density. The author demonstrated that the Cu-CAT nanowire anode stores sodium ions via the coordination

ith O in C

–O bonds accompanied by the reduction of Cu

2 + tou

+ . In 2019, Feng et al. reported a sodium-iodine battery based

n fully conjugated phthalocyanine Cu-MOF, M 2 –O 8 -PcCuM = Fe, Ni, Zn, PcCu = 2,3,9,10,16,17,23,24-octahydroxyhthalocyaninato-Cu), composited with I 2 serving as cathodeaterials ( Fig. 38 (e)). 119 Through the atomic level modulation,

e 2 –O 8 -PcCu/I 2 exhibited outstanding cycling durability with robust specific capacity of 150 mAh g −1 after more than 3000ycles, which can be ascribed to the Fe-O 4 planar nodes ine 2 –O 8 -PcCu mesh frameworks for the binding of polyiodide toestrain its dissolution into electrolytes ( Fig. 38 (f)).

32

Rechargeable aqueous zinc batteries (ZBs) : Although Li-ionatteries (LIBs) have been utilized extensively in cellular phonesnd laptops, their further application in large-scale (such as,lectric vehicles) is inhibited by high material cost and safetyoncerns. Rechargeable aqueous zinc batteries are an alternativeecause of their remarkable theoretical capacity (820 mAh g −1 ),elatively low toxicity and relatively low cost of zinc. In 2019,ao et al. reported a novel self-sacrificed synthesis of conductiveanadium-based 3D MOF (MIL-47) nanowire-bundle arraysn carbon nanotube fibers (CNTF) which can act as a binder-

free cathode for Zn-ion batteries ( Fig. 39 (a)). 166 Due to thebundant active sites, hierarchical porosity and high conductivity,his compound exhibited a noticeable volumetric capacity of01.8 mAh cm

−3 at current density of 0.1 A cm

−3 and a highate capability (64.3% retention after 50-fold current increase) inn aqueous electrolyte. The study showed that it is the organicoiety that would provide direct binding sites for Zn

2 + duringnergy storage/release process rather than the vanadium ions Fig. 39 (b)).

Very recently, Stoddart applied the conductive 2D MOF,u 3 HHTP 2 which has the 1-D channel and the redox-active π -

conjugated linker, into a zinc battery cathode ( Fig. 39 (c) andd)). 167 The hydrated Zn

2 + ions can insert directly into the hostD MOF structure with high diffusion rate and low interfacialesistance which enable Cu 3 HHTP 2 cathodes to follow anntercalation pseudocapacitance mechanism ( Fig. 39 (e)). Thetudy showed that this 2D conductive MOF exhibits a higheversible capacity of 228 mAh g −1 at 50 mA g −1 and a goodtability with 75.0% capacity retention after 500 cycles.

Conductive MOF as host materials : To suppress the polysulfidehuttling in Li–S batteries, Ni 3 HITP 2 , the graphene analogueonductive MOF, was utilized as the host material to trap andransform polysulfides during its charge/discharge process. 168

he cathode was further integrated with carbon nanotubes toonstruct the matrix conduction network for triggering the rate



EnergyChem REVIEW

Fig. 39. (a) Crystal structure of V-MOF (MIL-47) along a axis. (b) Probable Zn ions binding sites for coordination with the V-MOF@CNTF

166 . (c) Schematic representation of the rechargeable Zn-2D MOF cell. (d) Structure of Cu 3 (HHTP) 2 along c -axis. (e) Presentation of redox process in the coordination unit of Cu 3 (HHTP) 2 . Reproduced with permission. 167 Copyright © 2019, Springer Nature.

Fig. 40. (a, b) The cross-section SEM images of Ni 3 HITP 2 film modified PP film. (c) Photograph of the Li 2 S 6 solution treated with various barrier materials. (d) Rate performance of Ni 3 HITP 2 /PP, Carbon nanotube/PP, ZIF-8/PP, and the blank PP. (e) The cycling performance of Ni 3 HITP 2 /PP at 1C. Reproduced with permission. 67 Copyright 2018, Wiley-VCH.

and cycling performance of the Li–S battery. The as-synthesizedS@Ni 3 HITP 2 –CNTs with sulfur content of 65.5 wt% exhibitedan ultrahigh initial capacity of 1302.9 mAh g −1 and good capacityretention of 848.9 mAh g −1 after 100 cycles at 0.2 C. Thestudy showed that the pristine MOFs with abundant polar sitesand high conductivity can be promising host materials for highperformance Li–S batteries.

Conductive MOF as separators : In 2018, Xu et al. adopted aliquid-air interface assisted method to prepare a large-area crack-free conductive MOF membrane with high crystallinity andmicro-porosity. 67 This MOF membrane was in-situ fabricated onthe commercial polypropylene (PP) separator used as an fantasticlight weight barrier (with mass loading of only 0.066 mg cm

−2 )to suppress the polysulfide shuttling effect in Li–S batteries( Fig. 40 (a)–(c)). Contributing to the functional separator, thefabricated Li–S battery with sulfur-loading of 8.0 mg cm

−2

delivered the ultrahigh areal capacity of 7.24 mAh cm

−2 after 200cycles ( Fig. 40 (d) and (e)). In addition, a faci le filtration methodwas also developed to decorate the separator with the Ni 3 HITP 2 layer by Zheng and Li et al. 169 The designed separator effectivelymitigated the shuttle effect and enhanced the rate capability andcycling capability of the Li–S batteries.

Among electronically conductive MOFs, 2D π -conjugatedones with redox-active linkers, higher electrical conductivityand permanent porosity, have attracted considerable attentionas promising active materials in batteries. The outstanding

33

rate performance can be ascribed to their ultrahigh conduc-tivity and unimpeded channels, providing efficient and fastlithium/zinc/sodium ions intercalation and release. Further-more, the reversible multielectron-transfer of the ligands duringthe charge/discharge process resulted in a stable cycling perfor-mance. Though conductive MOFs have been successful ly uti lizedas host materials or separators in Li–S batteries, the pristine MOFfilms with low surface areas and fragile nature impeded theirfurther applications. In the future, the conductive MOF basedmixed-matrix membranes may be a promising strategy to developrobust and high performance components in batteries.

2.5.3. Energy conversion

The electrochemical energy conversion is one of the mostappealing technologies to store the clean and renewable energy,which is expected to reduce the environmental pollution andenergy shortage. 170 The active materials for electrochemicalcatalysis, including hydrogen evolution reaction (HER), oxygenevolution reaction (OER), oxygen reduction reaction (ORR),and carbon dioxide reduction reaction (CO 2 RR) are desirable. 171

Conductive MOFs are considered to be advanced electroactivematerials for energy transformations due to its unique features:(i) high intrinsic electron conductivity that facilitates the chargetransport; (ii) structural tenability with well-defined catalyticsites; (iii) tunable porosity and large surface that facilitates theelectrolyte diffusion; (iv) acting as host materials to incorporate



EnergyChem REVIEW

Fig. 41. (a) Chemical structure of [Co 3 (BHT) 2 ] 3 − and [Co 3 (THT) 2 ] 3 −. (b) Polarization curves of [Co 3 (BHT) 2 ] 3 − (red) and [Co 3 (THT) 2 ] 3 − (blue) in H 2 SO 4 solution (pH = 1.3) at 100 mV s − 1 . Reproduced with permission. 172 Copyright©2015, American Chemical Society (c) Langmuir–Blodgett method to synthesize 2D single-layer sheet of Ni-THT. (d) HER polarization plots in different electrolyte solutions: 0.5 M H 2 SO 4 (solid line), 0.025 M

H 2 SO 4 (dotted line), and 0.05 M KOH (dash-dotted line). Reproduced with permission. 69 Copyright 2015, Wiley-VCH. (e) Synthesis of Ni-AT with 2D

layered structure. (f) HER polarization curves of Pt, NiAT/GC, NiAT/GC after 500 cycles, and blank GC. Reproduced with permission. 60 Copyright 2017, The Royal Society of Chemistry.

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uest active catalysts for concerted catalysis. Recently, conductiveOFs, especially 2D conductive MOFs attracted considerable

ttention in electrochemical energy conversion applications. Hydrogen evolution reaction (HER) : In 2015, Marinescu et al.

roposed the bottom-up assembly of 2D MOFs, [Co 3 (BHT) 2 ] 3 −nd [Co 3 (THT) 2 ] 3 − with cobalt-dithioline catalytic sites,irectly onto HOPG substrate as effective HER catalysts Fig. 41 (a)). 172 In pH = 1.3, both of [Co 3 (BHT) 2 ] 3 − andCo 3 (THT) 2 ] 3 − showed noticeable overpotentials for HERith 0.34 V and 0.53 V at 10 mA cm

−2 , respectively ( Fig. 41 (b)).he mechanism of hydrogen evolution in both MOFs involvedrotonation of the sulfur sites on the dithiolene ligands after the

nitial reduction of Co

3 + /Co

2 + . According to Feng et al., the Langmuir–Blodgett method

as utilized to construct a free-standing, large-area, single-layerheet of Ni 3 HTTP 2 MOF and transferred directly onto thelassy carbon electrode as the electrocatalyst ( Fig. 41 (c)). 69 Theffective hydrogen evolution was performed at 0.5 M H 2 SO 4 ith an overpotential of 0.33 V at 10 mA cm

−2 ( Fig. 41 (d)).he outstanding catalytic properties were attributed to its high-

xposed electrocatalytic active sites during the HER process. Thetudy proposed that the mechanism is related to the protonationf sulfur sites in the metal bis(dithiolene).

In 2017, Nishihara and co-workers designed a novelonductive MOF, NiAT, which consists of Ni nodesnd 1,3,5-triaminobenzene-2,4,6-trithiol with a singleayer thickness (0.6 nm) ( Fig. 41 (e)). 60 Interestingly, theis(aminothiolato)nickel unit of NiAT can be interconvertible

o bis(iminothiolato)nickel of NiIT via the proton-couplededox reaction, which is accompanied by a drastically enhancedonductivity from 3 × 10

−6 to 1 × 10

−1 S cm

−1 . At pH = 1.3,iAT showed high HER performance with an operating potential

f −0.37 V at 10 mA cm

−2 and a Tafel slope of 128 mV dec −1

Fig. 41 (f)). Huang et al. further explored the morphologyffect of CuBHT (BHT = benzenehexathiolate) on HERerformance in H 2 SO 4 (pH = 1). 173 The study showed that

34

he nanoparticle morphology largely reduced the overpotentialor OER due to the Cu-edge site effect on the (100) plane andarger interfacial contact areas with the electrolyte comparedo films and nanocrystals. Furthermore, Marinescu et al.ynthesized a series of BHT-based MOFs with various metalotes (Fe/Co/Ni), and investigated the effect of coordinationetals on HER in H 2 SO 4 (pH = 1.3). 174 CoBHT displayed the

owest overpotential of 185 mV at 10 mA cm

−2 , much higher thanhat of NiBHT (331 mV) and FeBHT (473 mV), which resultedrom its lower charge transfer resistance and higher electrolyte-ccessible surface areas. In addition, these conductive MOF filmased electrocatalysts showed strong thickness-dependent HERctivity and a 244 nm CoBHT film performed the best. Fromhe results, it could be seen that a suitable thick film catalystith high charge transfer, fast proton diffusion and numerous

lectrochemically accessible active sites could dramaticallyromote the electrocatalytic activity.

Oxygen evolution reaction (OER) : The oxygen evolution re-ction (OER) is considered to be a core process for clean andenewable energy systems, including water splitting and metal-airatteries. 175 , 176 However, the OER process involves a multisteproton-coupled electron transfer and multi-phase reaction, which

mpede further development of high-performance electrocata-ysts 177–179 In general, high performance OER electrocatalystsossess high conductivity/mass transfer, rich abundant activeites, and faster interface reactions. Recently, conductive MOFsith robust porous structure and intrinsic electrical conductivityave emerged as candidates for efficient OER catalysts.

Zhao et al. prepared ultrathin nanosheet arrays of NiFe-MOF,onsisting of alternating 2,6-naphthalenedicarboxylic groups and

O 6 units, on nickel foams ( Fig. 42 (a)). 180 The as-synthesizediFe-MOF arrays, with hierarchical porosity, improved conduc-

ivity, and highly exposed active metal sites, and exhibited a smallverpotential of 240 mV for OER at 10 mA cm

−2 in 0.1 M KOH.nterestingly, the nanosheets also showed noticeable activity in

ER and when being utilized as both anode and cathode catalysts



EnergyChem REVIEW

Fig. 42. (a) Synthesis of NiFe-MOF nanosheet arrays with layered structures. (b) left: LSV plots of NiFe-MOF, Ni-MOF, bulk NiFe-MOF and IrO 2 for OER at 10 mVs −1 in0.1 M KOH; right: LSV curves of a full electrolytic cell utilizing two NiFe-MOF electrodes obtained at 10 mVs −1 in 0.1 M KOH. Reproduced with permission. 180 Copyright©2017, Springer Nature. (c) Eclipsed AA-stacking mode of NiPc-MOF. (d) pH-dependent LSV curves of the NiPc-MOF as the OER catalyst. Reproduced with permission. 91 Copyright 2018 The Royal Society of Chemistry.

H

in overall water splitting, the device delivered a voltage of only1.55 V at 10 mAcm

−2 and was superior to commercialized Pt/Ccathode and IrO 2 anode ( Fig. 42 (b)).

Recently, Du et al. reported a novel class of 2D conductivenickel phthalocyanine-based MOF (namely NiPc-MOF) withoutstanding catalytic activity for OER ( Fig. 42 (c)). 91 The NiPc-MOF film was successfully grown on FTO and directly used as thecatalyst, exhibiting an ultralow onset potential and overpotentialof 1.48 and 0.25 V, respectively, excellent mass activity (883.3 Ag −1 ), and noticeable catalytic stability ( Fig. 42 (d)).

Cao et al. also reported the conductive MOF thin-filmbased OER catalyst prepared via the Langmuir–Blodgettmethod. 181 The as-synthesized Co 3 HHTP 2 nanosheet showedlayer number-dependent activ ity w ith the best performance onthe four-layer Co 3 HHTP 2 film, which could produce ultrahighmass activity of 64.63 A mg −1 under 1.7 V vs. RHE. To furtherregulate the catalytic performance, Wang and Xu et al. adopteddoping strategy to synthesize Fe1Ni4-HHTP nanowire arrayswith enhanced OER results. 182 Compared with the origin Ni-

HTP nanowire arrays (overpotential = 380 mV), the dopedone, Fe1Ni4-HHTP showed ∼213 mV at 10 mA cm

−2 andoutstanding long-term stability in 1 M KOH.

Oxygen reduction reaction (ORR) : Oxygen reduction reaction(ORR) at the cathode in polymer electrolyte fuel cells ischallenging due to its sluggish kinetics. Pt and platinum groupmetals are sti l l major electrocatalysts for ORR nowadays. 183–186

However, their high cost, poor stability and methanol poisoningeffects limit the large-scale commercial application in fuel cells.Recently, conductive MOFs emerged as a new family of robustand active non-platinum group metal electrocatalysts for ORRs.Especially, 2D conductive MOFs with M-N x or M-O x unitsexhibited noticeable oxygen reduction activity and stability, sothat it may even compete with the most active non-platnum groupmetal electrocatalysts.

35

Dinc a et al. reported the use of 2D conductive MOFs withNi-N 4 sites for ORR. 187 The as-grown Ni 3 HITP 2 film on glassycarbon electrodes with a thickness of 120 nm was directly utilizedas the catalyst which showed 0.18 V overpotential for oxygenreduction in 0.10 M KOH ( Fig. 43 (a)). Being combined withthe experimental and computational data, they further exploredthe O 2 reduction mechanism. 188 The studies showed that theactive sites for catalytic O 2 reduction in Ni 3 HITP 2 is ligand-based rather than metal-based, which is significantly differentfrom most transition metal macrocycles ( Fig. 43 (b)). Then,the Dinc a’s group systematically explored the ORR perfor-mance in its analogous conductive MOFs, including Ni 3 HHTP 2 ,Cu 3 HHTP 2 , Co 3 HHTP 2 and Cu 3 HITP 2 . 189 The results provedthat materials constructed from identical ligands and bearingsimilar structure exhibited vastly different electronic states andchemical stability, resulting in distinct electrocatalytic response.The graphene analogue material, Ni 3 HITP 2 as a representativeexample, combines the crystalline porous structure of MOFs,the outstanding electrical conductivity and robust structure ofgraphitic materials, and the tailorable chemical structures at theatom level, so that it may enable the targeted design and synthesishigh performance ORR catalysts to be used in fuel cells andelectrolysers for clean energy applications.

After that, Hu and co-workers discovered a novel active siteM-O 6 in conductive MOFs, Ni/Co-CATs for ORR via a four-electron process. 190 To enhance their ORR performance, Ohet al. prepared bimetallic conductive 2D MOFs (Co x Ni y -CATs)with tunable ration of the two metal ions (Co

2 + and Ni 2 + ). 191

The obtained Co 0.27 Ni 0.73 –CAT possessed a diffusion-limitingcurrent density ( −5.68 mA cm

−2 ) comparable to that of Co-CAT with an onset potential (0.46 V) similar to Ni-CAT. Theauthor attributed the improved performance to the coexistenceof Co-O active moiety and highly conductive frameworks in Ni-CAT.



EnergyChem REVIEW

Fig. 43. (a) Polarization curves in ORR performance of Ni 3 HITP 2 and the glassy carbon electrode under O 2 and N 2 . (Scan rate is 5 mV s − 1 , rotation rate is 2, 000 r.p.m., electrolyte is 0.10 M aqueous KOH). Reproduced with permission. 187 Copyright©2016, Springer Nature. (b) Proposed mechanism

for 2e − O 2 electroreduction with Ni 3 HITP 2 . Reproduced with permission. 188 Copyright © 2017, American Chemical Society. (c) ORR polarization curves of PcCu-(OH) 8 –CNT, Pc-O 8 –Co/CNT and PcCu-O 8 –Co/CNT. (d) The possible ORR reaction mechanism. Reproduced with permission. 94

Copyright 2019, Wiley-VCH.

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Besides, Feng and Dong et al. developed a phthalocyanine-ased conductive 2D conjugated MOF as high-performanceRR catalyst with active Co-O 4 sites. 94 The as-

synthesized PcCu-O 8 –Co consisted of 2,3,9,10,16,17,23,24-ctahydroxyphthalocyaninato copper (PcCu-(OH) 8 ) and squarelanar Co-O 4 linkages. After hybridizing with carbon nanotubesPcCu-O 8 –Co/CNTs), the complex afforded ultrahigh ORRctiv ity w ith E 1/ 2 = 0.83 V vs. RHE due to the active Co-O 4 odes, intrinsic conductivity and periodic porosity ( Fig. 43 (c)nd (d)). Additionally, PcCu-O 8 –Co delivered a outstandingower density up to 94 mW cm

−2 as the cathode electrocatalystn zinc-air batteries, which outperformed the best Pt/C electrode.

.5.4. Conductive MOF-based electronics ield-effect transistors (FETs) : Field-effect transistors (FETs),nown as three-terminal semiconductor electronics, play a keyole in the field of modern electric devices. 192 Conductive MOFsith designable structure, high crystallinity, tunable electronicroperties and capable of membrane preparation, have emergeds novel porous channel materials in FETs with exciting potentialpplications in FET-based gas/ion sensors. In addition, FETevices also present a powerful strategy to study charge transportroperties in semiconductors, such as carrier type and carrierobility, which can promote the research of semiconduct-

ng/conducting MOF materials to a great extent. 131 , 193

In 2015, Zhu et al. fabricated a FET device based on Cu-BHThin film, which was prepared via a liquid–liquid interface reac-ion. 61 The Cu-BHT-based FET adopted bottom-gate bottom-

36

contact geometry and showed an ambipolar charge transportroperty, with the hole mobility of 99 cm

2 V

−1 s −1 and electronobility of 116 cm

2 V

−1 s −1 . Notably, the as-prepared film ownedulticrystalline nature with numerous grain boundaries. To

btain the intrinsic transport behavior in Cu-BHT, high qualityrystals or single domain monolayer are extremely necessary.

In 2017, Louie et al. synthesized a family of 2D conductiveOF films, M 3 HAB 2 (M = Co, Ni, and Cu) via liquid-liquid (for

hick films with 1–2 μm) or liquid-gas (for thin films < 10 nm)ethods. FET devices were fabricated on Ni-HAB thin films via

oth top-contact and bottom-contact geometry. 63 However, dueo the crystal defects and grain boundaries, the sample showedow conductivity with back gate dependent conductance.

Xu and co-workers successfully fabricated conductive MOFased porous FET that possessed top-contact device geometry Fig. 44 (a)). 66 The Ni 3 HITP 2 film (the thickness is 105 nm)cted as the active channel layer, show ing p-type behav ior w itholes as the majority carriers. The devices exhibited the currentn/off ratio of about 2 × 10

3 and outstanding hole mobilitys high as 48.6 cm

2 V

−1 s −1 ( Fig. 44 (b)). In addition to that,he porous features of conductive MOF-based channels couldreatly extend the applications of FET-based sensors and voltage-

gated ion channels. Recently, in order to improve the interfaceuality between MOF layers and substrates, Duan et al. developed solid-liquid interface method to grow Ni 3 HITP 2 film in situn the Si/SiO 2 wafer with bottom-contact device geometry Fig. 44 (c)). 194 The as-prepared MOF-based FET exhibitedutstanding mobility of 45.4 cm

2 V

−1 s −1 and was successfullypplied as a liquid-gated device for bio-sensing. The liquid-gated



EnergyChem REVIEW

Fig. 44. (a) Schematic representation of Ni 3 HITP 2 -based porous FETs. (b) Output curves of Ni 3 HITP 2 -based FETs. Reproduced with permission. 66

Copyright©2017, American Chemical Society. (c) Schematic illustration of liquid-gated Ni-MOF-FET. (d) I ds -V ds curves of Ni-MOF FET under gluconic acid varied from 10 −6 to 10 −3 g/mL. Reproduced with permission. 194 Copyright©2019, American Chemical Society.

FET showed noticeable response to gluconic acid varying from10

−6 to 10

−3 g/mL ( Fig. 44 (d)). Magnetic MOF semiconductors : To integrate long-range mag-

netic ordering and high electrical conductivity within a singlematerial is desirable in practical application of data operation andstorage. 195 , 196 As a typical example, ferromagnetic semiconduc-tors attract considerable attention in spintronics, which has beenwidely studied since the 1980s for the next generation deviceswith logic operations and memory functions. 197–200 Comparewith inorganic and organic materials as ferromagnetic semi-conductors, 201–209 magnetic MOF semiconductors have definedcrystal structures, highly tunable geometries and functions andare easy to be synthesized and processed.

So far, it remains challenging to acquire magnetic MOFsemiconductors due to most conventional 3D MOFs with poorelectrical conductivity and weak magnetic coupling. Recently,Long et al. made a breakthrough when concerning obtaininga 3D MOF (NBu 4 ) 2 Fe III

2 (dhbq) 3 w ith high conductiv ity of∼0.16 S cm

−1 at 298 K and magnetic ordering temperature of134 K. 99 A reduced material Na 0.9 (NBu 4 ) 1.8 Fe III

2 (dhbq) 3 wasalso synthesized with reduced conductivity and higher magneticordering temperature of 144 K ( Fig. 45 (a)). The study showedthat the immobilizing redox-active ligands with mixed-valencesinto the Fe III -based framework could form long-range charge-delocalization and strong magnetic exchange.

Later, Harris et al. reported a 2D MOF magnet utilizinga similar redox-active linker, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH 2 ) and Fe II center. 210 The as-synthesized(Me 2 NH 2 ) 2 [Fe 2 L 3 ] �2H 2 O �6DMF showed conductivity of1.4(7) × 10

−2 S cm

−1 and spontaneous magnetization below80 K, whereas its desolvated form (Me 2 NH 2 ) 2 [Fe 2 L 3 ] haslower conductivity of ∼1.0 × 10

−3 S cm

−1 and the decreasedordering temperature of 26 K. Furthermore, upon moderatereduction with Cp 2 Co in a DMF solution, a new crystal was

37

obtained via a single-crystal-to-single-crystal transformation,(Cp 2 Co) 1.43 (Me 2 NH 2 ) 1.57 [Fe 2 L 3 ] �4.9DMF which presentedan increased magnetic ordering temperature of up to T c = 105 Kwith electrical conductivity of up to 5.1(3) × 10

−4 S cm

−1

( Fig. 45 (b)). 211

In 2018, Feng et al. reported a π -d conjugated 2D semi-conducting MOFs composed of planar Fe–S 4 linkage and theperthiolated coronene-based linker, namely PTC-Fe ( Fig. 45 (c),left). 51 The compound exhibited ultrahigh electrical conductivityof ∼10 S cm

−1 at ambient temperature and a ferromagneticground state at low temperature ( < 20 K) ascribed to the hy-bridization of d and p orbitals from Fe, the coronene core, and theFe-S 4 nodes ( Fig. 45 (c), right). To further increase the magneticordering temperature in 2D conductive π -conjugated MOFs,the same group synthesized a novel conjugated MOF, K 3 Fe 2 [PcFe-O 8 ] that consisted of (2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)iron (PcFe-OH 8 ) as the ligand, square planarFe-O 4 as linkage, and K

+ as the counter ions. 52 This MOF wasfeatured with square lattices in the ab plane and layered structurestacking along the c -axis via van der Waals interaction ( Fig. 45 (d),left). K 3 Fe 2 [PcFe-O 8 ] showed moderate electrical conductivityof 2 × 10

−5 S cm

−1 at 350 K and a high charge mobility of ∼15cm

2 V

−1 s −1 at 300 K. Furthermore, even at 300 K and 350 K,this MOF sti l l showed magnetic hysteresis loops indicating themagnetic ordering temperature of up to 350 K ( Fig. 45 (d),right). According to DFT calculations, the ferromagnetic groundstate derived from orbital hybridization of d and p in Fe, thephthalocyanine core and Fe–O 4 nodes. According to Feng’swork, it was highlighted that conjugated 2D MOFs showed highpotential as the ferromagnetic semiconductor to be applied tospintronics.

Even though magnetic MOF semiconductors have made im-portant progress, many challenges remain, such as simultaneouslycombining high electrical conductivity and room temperature



EnergyChem REVIEW

Fig. 45. (a) left: Crystal structure of (NBu 4 ) 2 Fe III 2 (dhbq) 3 ; right: Temperature dependent conductivity of (NBu 4 ) 2 Fe III

2 (dhbq) 3 and Na 0.9 (NBu 4 ) 1.8 Fe III

2 (dhbq) 3 . Reproduced with permission. 99 Copyright©2015, American Chemical Society. (b) Reduction state in (Cp 2 Co) 1.43 (Me 2 NH 2 ) 1.57 [Fe 2 L 3 ] � 4.9DMF with tunable electrical conductivity and magnetic ordering temperature. Reproduced with permission. 211

Copyright©2017, American Chemical Societ y. (c) Left : Single layer of PTC-Fe with ferromagnetic ground state; right: hysteresis loop measured under different temperatures. Reproduced with permission. 51 Copyright©2018, Springer Nature. (d) Left: Crystal structure of K 3 Fe 2 [PcFe-O 8 ] in space filling modes. C = light cyan, N = blue, O = light pink, Fe 3 + in the phthalocyanine ring = orange, Fe 2 + in the linkage = green. H and K

+ are omitted for clarity. Right: Magnetic hysteresis loops of K 3 Fe 2 [PcFe-O 8 ] under different temperatures. Reproduced with permission. 52 Copyright © 2019, Springer Nature.

Fig. 46. (a) Schematic illustration of 2D conductive MOF film based vertical OSVs. (b) Magnetoresisitance loop for the LSMO/Cu 3 (HHTP) 2 (100 nm)/Co OSVs at 10 K. Reproduced with permission. 212 Copyright 2019, Wiley-VCH.

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agnetic ordering, chemical and physical stabi lity, faci le filmormation with tunable thickness, and high crystallinity and lessefects.

Semiconducting MOF layer in organic spin valves (OSVs) : In019, Chen et al. applied an oriented film of a 2D conductiveOF, Cu 3 HHTP 2 into organic spin valves as the nonmagnetic

rganic spacer layer ( Fig. 46 (a)). 212 Due to proper spinterfacesith the top metal FM electrodes, the as-fabricated OSVs,SMO/Cu 3 HHTP 2 /Co (LSMO = La 0.67 Sr 0.33 MnO 3 )xhibited a large negative magnetoresistance with MR = 25%t 10 K ( Fig. 46 (b)). Furthermore, the MR effect of this OSVould be regulated by the MOF film thickness varying from0 to 100 nm. Although the intrinsic physics of spin-polarizedransport was sti l l unclear, according to their findings, 2D π -donjugated MOFs acted as a promising platform for OSVs.onsidering the structure and function tunability, many defectseing reduced, and huge room for improvement of crystallinity

38

n 2D conductive MOF films, it is expected to develop higherformance spintronics in the future.

2D organic topological insulators (TIs) : Topological insulatorsTIs) are a family of materials with unique quantum transportroperties which can be applied in spintronics and quantumomputing. So far, almost all of the experimentally realized TIsome from inorganic materials. However, recently, the organic-ased 2D TI has been predicted theoretically by Liu’s groupnd experimentally made by Nishihara’s group, composed ofickel bis(dithiolen) planar nodes with a chemical formula ofi 3 BHT 2 ( Fig. 47 (a) and (b)). 57 , 114 , 213 Liu’s calculated results

howed that this compound exhibited nontrivial topologicaltates in both Dirac bands and flat bands ( Fig. 47 (c) and (d)).

owever, the experimental realization of the TI in Ni 3 BHT 2 equires extremely precise control of the oxidation state in theingle-layer film, which is definitely challenging. Nishihara ando-workers tried to control the redox state of this material (the



EnergyChem REVIEW

Fig. 47. (a) Schematic illustration of the structure of Ni-BHT. (b) Redox control of the average oxidation number of pristine Ni-BHT. Reproduced with permission. 114 Copyright © 2014, American Chemical Society. (c) Atomic structure of Ni-BHT used for theoretical calculations. The solid lines represent its unit cell and dashed lines showed the kagome lattice. (d) The semi-infinite Dirac edge states (with both spin-up and spin-down components) within the SOC gaps. Reproduced with permission. 213 Copyright © 2013, American Chemical Society.

original average oxidation number is 3/4) and only obtainedthe reduced state and the oxidized state with oxidation numbersof −1 and 0, respectively. Even though the oxidized Ni 3 BHT 2 showed enhanced electrical conductivity as high as 160 S cm

−1

at 300 K, it was a significant step into the realization of an organic2D-TI. 114

Recently, Nishihara et al. reported a palladium analogue,Pt 3 BHT 2 based on the same BHT organic linker via liquid–liquid interfacial reaction. 59 The calculated band structure of thiscompound showed a Dirac gap of 0.054 eV, wider than the nickelanalogue, suggesting the possibility of 2D topological insulationat room temperature. Interestingly, Pt 3 BHT 2 is insulating inthe pristine state which can be oxidized by I 2 with enhancedconductivity of up to 0.39 S cm

−1 . Conductive MOF based thermoelectrics : Thermoelectric devices

are attractive to a great extent because it has conversion abilitybetween electricity and heat, and can be applied to collectheat for power generation and cooling applications. 214 , 215 Adimensionless figure of merit ZT is used to determine theefficiency of thermoelectric energy conversion:

ZT =

S 2 σT

k Here, S is the Seebeck coefficient; σ refers to the electrical

conductivity, κ refers to the thermal conductivity, and T is the

39

absolute temperature. To improve the ZT value, much efforts havebeen focused on inorganic materials and organic materials. 216–218

However, considering the cost and not eco-friendly in inorganicmaterials and low charge mobility in organic materials, novelmaterials with outstanding electrical conductivity and ultralowthermal conductivity are appropriate for high performancethermoelectrics. Conductive MOFs have become promisingcandidates due to its long-range crystalline order, highly intrinsicconductivity and low thermal conductivity. Talin et al. reported athermoelectric effect in TCNQ@Cu 3 (BTC) 2 (TCNQ = 7,7,8,8-tetracyanoquinodimethane, BTC = 1,3,5-tricarboxylate) films,whose conductivity of 0.07 S cm

−1 results in a ZT value of7 × 10

−5 at 25 °C ( Fig. 48 (a) and (b)). 219 The relatively high ZTwas ascribed to a low κ of 0.27 W m

−1 K

−1 and the ultrahighSeebeck coefficient of 375 μV K

−1 ( Fig. 48 (c) and (d)). Theresults showed that MOFs can act as potential thermalelectricmaterials with tailored chemical structures and various post-synthetic strategies.

Even with the low κ value, the ZT of TCNQ@Cu 3 (BTC) 2 is limited by its moderate room temperature conductivity.Dinc a et al. applied Ni 3 HITP 2 , a microporous conductiveMOF to thermoelectric devices with ultralow thermal con-ductivity ( κ = 0.21 W m

−1 K

−1 ) and a record high ZT value(1.19 × 10

−3 ) at room temperature ( Fig. 48 (e)). 220 This MOFis featured with periodic microporous structure and natural



EnergyChem REVIEW

Fig. 48. (a) Schematic illustration of the measurements. (b) IR image during the measurements. (c) Temperature dependent Seebeck coefficient. (d) Temperature dependent ZT for TCNQ@Cu 3 (BTC) 2 film. Reproduced with permission. 219 Copyright 2015, Wiley-VCH. (e) Excellent charge transport and ultralow heat transport in microporous Ni 3 HITP 2 . The naturally nanostructures (f) and temperature dependent ZT (g) of Ni 3 HITP 2 . Reproduced with permission. 220 Copyright © 2017 Elsevier Inc.

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anostructures, which can effectively scatter phonons, thuseading to enhanced thermoelectric properties ( Fig. 48 (f)). Moremportantly, Ni 3 HITP 2 showed electrical conductivity as high as0 S cm

−1 , which caused the 17-fold enhancement of the ZTompared with TCNQ@Cu 3 BTC 2 ( Fig. 48 (g)).

.5.5. Other applications based on conductive MOFs onductive MOFs with potential catalytic open sites and tunable

onfined environment can act as the new generation of platformsor catalyzing reactions with high size selectivity and orderedistribution of accessible active-sites on the MOF framework. Re-ently, Mirica et al. applied conductive PCPs/MOFs M 3 HTTP 2 M = Co, Ni, and Cu) with reactive metal bis(dithiolene)ctive sites into ethylene capture and release by electrochemicalontrol. 118 The ethylene was captured by applying a positiveias of + 2 V to porous M 3 HTTP 2 and released by a negativeotential of −2 V. The as-prepared MOF thin film deviceshowed noticeable and electrochemically-captured ethylene with–6 mg g −1 . It was interesting to mention that these MOFs arelso resistant to poisoning by common interfering gases (such asO and H 2 S), indicating its robust properties. The mechanism of

thylene capture and release is sti l l unclear, and it was proposedhat ethylene interacts with the metal bis(dithiolene) moietyithin the framework by a cycloaddition reaction, which can be

ontrolled by the bias.

. PROTON

–CONDUCTIVE MOFS

he enormous energy consumption and worsening of urban en-ironment pollution makes fuel cell technology attract increasingttention recently. 221 , 222 Fuel cell is capable to convert chemicalnergy into electrical energy without the emission of CO 2 , whichomprised of a fuel component (hydrogen, methanol, naturalas etc. ) and an oxidant (oxygen, air or hydrogen peroxide). 223

herefore, the technologies based on polymer electrolyte mem-rane fuel cells (PEMFCs) has drawn a great deal of attentions well. A variety of important PEMFC features are determinedy the properties of the polymer electrolyte membrane (PEM),he core of which is electrolyte materials. At present, the mostommonly-used membrane separator in fuel cells is Nafion-basedroton-conducting membranes which consist of a perfluorinated

40

olyethylene backbone and grafted side chains with sulfoniccid groups. 224–227 The remarkable success achieved by Nafions attributed to its high proton conductivity (in the order of0

−1 ∼10

−2 S cm

−1 at T < 85 °C with humidification), robusttructure and excellent reusability. 228 However, its large-scalepplication is restricted by the loss of conductivity due to dehy-ration at elevated temperatures and high cost. Therefore, it isonsidered necessary but challenging to develop novel materialshat possess excellent proton-conducting performances. Provid-ng a truly hybrid approach, PCP/MOF materials have emergeds a new class of proton-conducting materials due to the ease oftructural determination and tunability of components. 27–44 Inddition, the crystal structure of MOFs creates an opportunity toonduct an in-depth study of the proton conduction mechanism,hich is beneficial to guide the future synthesis of novel proton-

conducting materials.

.1. Proton exchange membrane fuel cells he fuel cells technologies based on polymer electrolyte mem-ranes have been applied in various fields. The development ofEMFCs is conducive to promoting the design and preparation ofovel proton conductive materials. A better electrolyte material,s the core of the PEM, must satisfy the following require-ents: 229 (a) ultrahigh proton conductivity ( > 10

−2 S cm

−1 )nd low electronic conductivity, (b) highly thermal and chemicaltability under fuel cell operating conditions, (c) moderateensity and porosity to prevent the crossover of fuel gases, (d)

ow cost, (e) thin-film processability, (f) compatibility with otheruel cell components, etc. Such organic conductive polymers as

afion, polybenzimidazole and sulphonated polyether ketonesave been applied as electrolytes in fuel cells, among whichafion is the most successful one. However, its widespread

pplication is constrained by high cost, necessary active humid-fication, and physical degradation from hydration/dehydrationrocess. Although high proton-conducting materials based in-rganic or composite materials have been extensively explored

o overcome above drawbacks, 230–235 however, the relative lowerformance sti l l limit their application, hence a sort of high per-

ormance proton-conducting materials are desirable to overcomehe limitation faced by polymer membranes.



EnergyChem REVIEW

Fig. 49. Model of proton conduction. Grotthuss Mechanism, the protons are passed along the hydrogen bonds (upper). Vehicle Mechanism, the movement of the solvated protons take place with the aid of a moving “vehicle”, e.g., H 2 O or NH 3 as complex ion (H 3 O

+ or NH 4 + ) (bottom).

3.2. Mechanism of proton conducting in MOFs There are two principal mechanisms related to proton transport,which include the Grotthuss mechanism and the vehicular mech-anism ( Fig. 49 ). 27–29 The Grotthuss mechanism, also known asproton-hopping mechanism, establishes the proton conductionpath within an infinite network of hydrogen bonds. The resultingH 3 O

+ species within a water cluster transfer the proton withconcurrent severing of hydrogen-bonds, and subsequent rear-rangement between nearly H 2 O molecules. Along the conduc-tion pathway, protons “hop” through protonation/deprotonationbetween water molecules.

The vehicular mechanism involves the transport of protonthrough self-diffusion of proton carriers, H 3 O

+ , NH 4 + , etc.,

bonded to such “vehicle” as H 2 O, NH 3 , etc. The “unladen” vehi-cles move along opposite direction. The vehicle shows a diffusioncoefficient corresponding to the proton conduction and acts asa Bronsted base (proton acceptor) towards its crystallographicenvironment. The activation energy ( E a ) calculated from the acimpedance data has been utilized to identify the two types ofprocesses. Due to the hydrogen-bond cleavage energy within therange of 2–3 kcal mol −1 (0.09–0.13 eV), Grotthus mechanismrequires lower activation energy ( E a < 0.4 eV). In the presenceof vehicular mechanism, the transport of larger ionic species(possessing larger mass) need a larger energy ( E a > 0.4 eV).

The ionic conductivity of MOFs can be measured by thealternating current (AC) impedance measurement of a pellet,thin film or single crystal in a wide frequency range. The ionicconductivity ( σ ) can be calculated as in Eq. (1) ,

σ = zenμ (1)

where z refers to the valence of the ion; e , elementary charge; n ,charge carrier concentration; and μ represents carrier mobility. Inaddition, Arrhenius equation for ionic conductivity derived fromthe Nernst-Einstein relation can be express in Eq. (2) to evaluatethe relationship of conductivity and temperature:

σ =

n e 2 D 0 exp

(�S m

k

)kT

exp

(−E a

kT

)→ σ =

σ0

kT

exp

(−E a

kT

)

41

(2)

where σ refers to ionic conductivity (S cm

−1 ), n is the numberof charge carriers, e is the charge on the mobile ion, D 0 is aconstant which is related to the mechanism of ionic conductivity,k is the Boltzann constant, T is the temperature (K), �S m

is themotional entropy and E a is enthalpy for ion transport (that is theactivation energy). For proton conductivity, the charge remainsconstant ( + 1) and proton mobility is affected by kinetic factorswith variable concentration of protons. Therefore, to enhancethe proton conductivity of MOF materials, more charge carriers,greater motion entropy, three dimensional conducting pathwaysand lower activation energy are prerequisite conditions.

3.3. Proton sources in MOFs In general, there are three types of proton sources in MOFs asshown in Fig. 50

35 , 44 : Type I is located in pores as counterions, forexample, H 3 O

+ , NH 4 + , NH 2 (Me) 2 + , H 2 PO 4

− and OH

− whichare introduced into MOF pores during the synthesis processto maintain the neutrality of the framework. 236 , 237 Type II isthe modified the organic strut of frameworks with dangling acidfunctional groups, such as -(SO 3 H), -(COOH), and –(PO 3 H 2 ),which is capable to act as additional proton sources in thepore environment. 238–240 Both type I and type II are regardedas intrinsic sources that can form H-bonding networks withadditional water molecules and framework, resulting in enhancedproton conductivity.

Type III refers to the protic organic molecules, as representedby imidazole and histamine, or charge-neutral nonvolatile acids,such as H 2 SO 4 and H 3 PO 4 , which are encapsulated into MOFpores using a simple inclusion method. 241–243 Recently, ionicliquid was also acting as guest molecules to be incorporatedinto MOFs, resulting in effective H

+ conduction in the hostframework. 244 In general, the protic guest molecules are incor-porated into the pores for the formation of H-bonding network.Therefore, a high proton conductivity depends on a potenthigh-proton donation of the guest molecules w hich should ow n



EnergyChem REVIEW

Fig. 50. Three types of proton sources in MOFs: (a) type I: counterions located in pores; (b) type II: dangling acid functional groups of the organic linker; (c) type III: protic organic molecules or charge-neutral nonvolatile acids in the pores. Reproduced with permission. 44 Copyright2019 the Royal Society of Chemistry.

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relative low acid dissociation constant ( pK a ) at moderateemperature.

.4. Types of proton-conductive MOFs n 1979, Kanda et al. reported the proton conducting co-rdination polymer, R 2 -dtoa-Cu ( R = HOCH 2 - and CH 3 -,toa = dithiooxamide). 20 Subsequently, Kitagawa et al. reported hydrogen-doped (HOC 2 H 4 ) 2 dtoaCu that could reach a pro-on conductivity value of 2.2 × 10

−6 S cm

−1 (27 °C and00%RH). 245 Despite the relative low conductivity exhibitedy early proton-conducting coordination polymers, they pavedhe way for further exploration of high performance proton-onducting MOFs. Until 2009, Kitagawa, Kitagawa, and Shimizuroups took the lead in performing study on the protononduction in crystalline MOFs, based on which the protononduction mechanism was revealed. 21–26 Since then, variety ofifferent crystalline proton conductive PCPs/MOFs have beenystematically and extensively studied.

.4.1. Proton-conductive MOFs under aqueous condition

urrently, most reported proton-conducting MOFs operate at T 100 °C with the assistance of water molecules and hydrogen-

ond networks. Water molecules were utilized to form hydrogenond or to act as proton carriers. In this section, water-mediatedOFs are classified into oxalate-, carboxylate-, sulphonate-

arboxylate-, phosphonate-, and mixed-linkers-based framework,espectively.

Proton-conductive oxalate-based MOFs: Kitagawa et al. con-ucted a study on the proton conductivity of a couple of oxalate-ased MOFs. Among them, Fe(ox) �2H 2 O (ox = oxalate) is anique example due to the formation of a 1-D arrays of waterolecules, and reaches a proton conductivity of 1.3 × 10

−3 Sm

−1 (25 °C, 98% RH). 25 Owing to the outstanding conductiv-ty and low activation energy (0.37 eV), Fe(ox) �2H 2 O was iden-ified as a super-protonic conductor. Meanwhile, they reportednother highly proton-conductive oxalate-based framework withormula of (NH 4 ) 2 (adp) [Zn 2 (ox) 3 ] �3H 2 O, (adp refers to adipiccid) which was based on a rational design. 23 To incorporatehe proton carriers into this MOF, protonated counter ions,hich modify the framework with acidic groups and fil l the

42

cidic molecules in the pore spaces, are jointly used to generatewo dimensional H-bonding networks, resulting in the protononductivity up to 8 × 10

−3 S cm

−1 at 25 °C and 85% RH.dditionally, H. Kitagawa and colleagues reported the example of

ontrollable proton conductivity in MOFs through a proton con-uction pathway mediated by guest adsorption/desorption. 246

NH 4 ) 2 (adp) [Zn 2 (ox) 3 ] �nH 2 O ( n = 0, 2, 3) involves three re-ersibly transferable phases, which incude the anhydrate ( n = 0),ehydrate ( n = 2), and trihydrate ( n = 3) phase. As revealed by

he mechanism study, the water adsorption/desorption processaused the hydrogen-bonding networks to be rearranged, thusegulating the proton conductivity from 10

−12 S cm

−1 to0

−2 S cm

−1 ( Fig. 51 (a) and (b)). Subsequently, a cation-substituted MOF with formula of K 2 (H 2 adp) [Zn 2 (ox) 3 ] �3H 2 O

as reported which possesses the same crystal structure asNH 4 ) 2 (adp) [Zn 2 (ox) 3 ] �3H 2 O. 247 However, this potassium-on-substituted MOF showed significantly lower proton conduc-ivity less than 1.2 × 10

−4 S cm

−1 (25 °C and 98% RH), whichndicates that the inorganic cations could mediate the protononduction.

Bimetallic complexes linked via oxalate anions werelso explored by Okawa and colleagues. 22 The as-

synthesized NH(prol) 3 [M

II Cr III (ox) 3 ] (prol = tri(3-ydroxy lpropy l)ammonium, M

II = Mn

2 + , Fe 2 + , Co

2 + ) formed 2-D honeycomb layer and a pore with size of 7.96 A × 9.16 A.n ammonium counter-ion was used to modify the pore

paces thus generating hydrophilic layers composed of hydroxylroups for ultrahigh proton conductivity ( Fig. 51 (c) and (d)).urthermore, these MOFs demonstrated both high protononductivity and ferromagnetism ( T C = 5–10 K) within aingle material. As indicated by another study, the tri(3-ydroxy lpropy l)ammonium cations in the aforementionedaterial can be replaced by trialkyl(carboxymethyl) ammonium

ons, to obtain a new bimetallic oxalate-based compound,R 3 (CH 2 COOH) [M a

II Cr b III (ox) 3 ] (R = ethy l (Et), n–buty lBu); M a M b = MnCr, FeCr, FeFe). 248 The carboxyl of therganic cations remained in the honeycomb channel of theimetallic layers and acted as a proton carrier. Moreover, theydrophilicity of the organic cations ( [NR 3 (CH 2 COOH)] + )an affect the interlayer hydrophilicity, hence further influencests proton-conducting properties. In this compound, Et-based



EnergyChem REVIEW

Fig. 51. (a) Crystal structure and (b) representation of the hydrogen-bonding networks of (NH 4 ) 2 (adp)[Zn 2 (ox) 3 ] �n H 2 O ( n = 0, 2, 3). Reproduced with permission. 246 Copyright © 2014, American Chemical Society. (c) View of NH(prol) 3 [Mn II Cr III (ox) 3 ] �2H 2 O along a -axis; (d) Representation of the bimetallic layer in the ab -plane with cations in the pore. Reproduced with permission. 22 Copyright © 2009, American Chemical Society.

cations exhibited higher proton conductivity than the Bu-basedone. Again, of significance are the M a M b = FeFe MOFs, whichshowed both proton conductivity and Náel N-type ferrimagnetbehavior ( T c = 42–44 K).

Train and Verdaguer et al. synthesized a high proton-conducting chiral metal-organic quartz-like framework withferromagnetic behavior. 249 The as synthesized (NH 4 ) 4 [MnCr 2 (ox) 6 ] �4H 2 O contained two-types of one dimensionalchannels formed by the oxalated anions and Mn, Cr cationsalong c-axis ( Fig. 52 (a) and (b)). Channel A and channel Bhave diameters of 5.23 A and 7.52 A, respectively. Besides, onlychannel A is shown to line with terminal oxalates on the innersurface, thus forming a hydrophilic layer fil led with ordered guestwater molecules. Derived from the H- bonded species within theA channels, this compound showed a high proton conductivityup to 1.1 × 10

−3 S cm

−1 at 23 °C and 96% RH with a lowactivation energy of 0.23 eV.

In addition, the networks of oxalate-based MOFs can affecttheir proton conductivity. Kitagawa et al. reported a seriesof LaM(ox) 3 �10H 2 O (M = Cr, Co, Ru, La; ox = oxalate)with network dependent proton conductivity. 250 Amongthem, LaCr(ox) 3 �10H 2 O and LaCo(ox) 3 �10H 2 O possessedladder structure, which gave rise to a channel network fil ledwith water molecules. In comparison, LaRu(ox) 3 �10H 2 O andLaLa(ox) 3 �10H 2 O possess honeycomb sheet structure, whichcould lead to the formation of a layer network through hydrogen-bonds ( Fig. 52 (c)). The ladder networks showed remarkablyhigh proton conductivity (10

−6 –10

−5 S cm

−1 at 40–95% RH)than the layer networks (10

−8 S cm

−1 at 40–95% RH). Huang et al. reported a water-stable Lanthanide-oxalate

MOFs with ultrahigh proton conductivity and noticeable lumi-

43

nescence. 251 The as-synthesized (N 2 H 5 ) [M a M b (ox) 4 (N 2 H 5 )]�4H 2 O (M a M b = CeEu, Nd 2 ) incorporated hydrazinium asproton source and lanthanide ions to realize luminescence( Fig. 53 ). The dense hydrogen bond network contributed totheir high proton conduction. Specifically, CeEu- and Nd 2 -basedframeworks reach a room temperature proton conductivity of3.42 × 10

−3 and 2.70 × 10

−3 S cm

−1 at 100% RH, respectively.Besides, it was found out that both the proton conductivity andfluorescence emission of these compounds are determined byhumidity.

Among the reported oxalated-based proton conductingMOFs, a lanthanide one [Eu 2 (CO 3 )(ox) 2 (H 2 O) 2 ] �4H 2 O(ox = oxalate) is extraordinary due to its humidity-independentproton conduction that increases with the working temperature(2.08 × 10

−3 S cm

−1 at 150 °C). 252 The study showed that thewell-ordered one-dimensional H-bonding pathways within thechannels of the structure is significant to proton conduction( Fig. 54 (a)). As shown in Fig. 54 (b), a jump in conductivityis observed in the proximity of 100 °C, where is supposed tocause a decrease because of thermal loss of water. The unusualphenomenon can be interpreted as that the remaining aqualigands, together with the oxalate ligands formed new hydrogen-bonded arrays which mediated the proton transport ( Fig. 54 (c)).The positive correlation between proton conductivity andtemperature may derive from the thermal facilitation of protontransfer within the hydrogen-bonded arrays.

Oxalate-based MOFs are considered to be one of the mostwidely studied proton-conductive MOFs under hydrous con-ditions because of its high crystallinity, structural diversityand ultrahigh stability for water. Furthermore, oxalate-basedMOFs were considered as one of the earliest proton-conductive



EnergyChem REVIEW

Fig. 52. (a) Crystal structure of the bimetallic (NH 4 ) 4 [MnCr 2 (ox) 6 ] �4H 2 O along c -ax is with A-ty pe channel and B-type channel (Mn polyhedral in purple, Cr polyhedral in green). (b) The corresponding quartz-like assembly of the MnCr 4 tetrahedra along the c -axis. Reproduced with permission. 249

Copyright © 2011, American Chemical Society. (c) Representation of crystal structure and the network-based proton conductivity of LaM(ox) 3 �10H 2 O

( M = Cr, Co, Ru, La; ox = oxalate). Reproduced with permission. 250 Copyright © 2015, American Chemical Society.

Fig. 53. (a) 3D pillared-layer-type framework (along a-axis) of (N 2 H 5 )[CeEu(ox) 4 (N 2 H 5 )] �4H 2 O; (b) a bilayer assembly along c-axis; (c) the humidity-dependent room temperature proton conductivity (green line) and humidity-dependent luminescence intensity of Eu 3 + at 612 nm. (d) Digital photos of under different RHs. Reproduced with permission. 251 Copyright 2017, Wiley-VCH.



EnergyChem REVIEW

Fig. 54. (a) Single crystal structures of [Eu 2 (CO 3 )(ox) 2 (H 2 O) 2 ] �4H 2 O exhibiting 1-D channels along a -axis filled with guest water molecules insides. (b) Arrhenius plots of conductivity measured from 25 to 150 °C and without humidity. � represent the heating cycle, � represent the cooling cycle. Inset: � represent cooling cycle, ◦ represent reheating cycle. (c) Representation of the 1-D H-bonded array of aqua ligands and neighboring ox group (the green bonds) along a-axis. Reproduced with permission. 252 Copyright © 2014, American Chemical Society.

MOFs with rational design. Despite the tremendous advancesin this kind of conductive MOFs, most of them showed protonconductivity less than 10

−3 S cm

−1 . Proton-conducting carboxylate-based MOFs : Among various

proton-conducting MOF materials, carboxylate-based one hasattracted a great deal of attention because of its unique charac-teristics: (i) carboxylate organic linkers possess versatile coor-dination modes and relatively strong coordination ability whichis conducive to the construction of robust MOF materials; (ii)the O

− or OH group of carboxylate unit is in favor of forminghydrogen bonds with guest molecules in MOF pores, resulting inefficiency proton transfer. (iii) The dangling –COOH of MOFswi l l donate protons as well as make up plentiful hydrogen bondnetworks in the framework. 23 , 253 , 254

Recently, Li et al. reported the development of carboxylate-based proton conducting MOFs before 2019, including aliphaticcarboxylate-based, pheny l carboxy late-based, pheny l carboxy latecontaining other groups-based, N-/S-/O-heterocycliccarboxylate-based, and mixed carboxylate-based MOFs. Here weintend to focus on the representative carboxylate-based MOFswith robust structures or high proton conductivity over 10

−3 Scm

−1 . 41

In 2013, Banerjee et al. synthesized two indium isophthalate-based MOFs, In-IA-2D-1 and In-IA-2D-2 with isomeric struc-tures and successfully incorporated different cations in thepores. 255 Tetrahedral In(III) secondary building units (SBUs)coordinated with isophthalic acid linkers to form a 2-D layeredstructures. In-IA-2D-1 incorporated H 2 O and [(CH 3 ) 2 NH 2 ] +

cations in the rectangular channel, whereas In-IA-2D-2 incor-porates DMF molecules and [(CH 3 ) 2 NH 2 ] + cations inside( Fig. 55 (a)). As a result, both of In-IA-2D-1 and In-IA-2D-2 exhibited remarkable proton conductivity of 3.4 × 10

−3 and4.2 × 10

−4 S cm

−1 at 27 °C and 98% RH, respectively. Thedifference can be attributed to that the DMF guest moleculerestricted the effective formation of hydrogen-bonding networkbetween water molecules and [(CH 3 ) 2 NH 2 ] + cations.

45

Another important proton conducting MOFs possessthe imidazolium functionalized carboxylate ligands.Sen et al. successf ully sy nthesized such kind MOF,[(Zn 0.25 ) 8 (O)Zn 6 (L) 12 (H 2 O) 29 (DMF) 69 (NO 3 ) 2 ] n (H 2 L = 1,3-bis(4-carboxyphenyl)imidazolium) with a uniqueZn 8 O cluster connected with 6 macrocycles ( Fig. 55 (b)). 256 Theunique feature of the structure is the imidazolium moieties withmethylene groups aligned inside the channels ( Fig. 55 (c)) whichcan be exposed to any solvent molecules entering the channels.The study showed that its proton conductivity is highly humiditydependent and reached maximum value 2.3 × 10

−3 S cm

−1

at ambient temperature and 95% RH. The water adsorptionexperiments further confirmed that number of hydrated watermolecules play a key role in the transport of protons. Thecalculated activation energy is 0.22 eV which indicates theGrotthuss-type proton conduction derived from the formedhydrogen-bonds between the imidazolium groups and waterguest molecules. In addition to introduce the imidazoliummoieties, polyoxometalates also can be incorporated in MOFstructure to enhance the proton conductivity. Wei and co-workerssuccessfully introduced Keggin anions [PM 12 O 40 ]. 3 − (M = W,Mo) into a 3-D MOF which showed proton conductivity of∼3.0 × 10

−7 S cm

−1 at room temperature and 98% RH. 257

However, the conductivity can be significantly enhanced to1.25 × 10

−3 S cm

−1 (Mo) and 1.56 × 10

−3 S cm

−1 (W) at 98%RH when the temperature was increased to 100 °C.

Hupp et al. reported another strategy in which protonconductivity can be regulated by altering the coordinated solventmolecules on the metal site. 258 The 3-D MOF, HKUST-1possesses Cu

II center and 1,3,5-benzenetricarboxylate (BTC)linkers whose coordination solvent molecules can be fullychanged as H 2 O, EtOH, MeOH and MeCN. The protonconductivity studies showed that H 2 O

–HKUST-1 showed thehighest conductivity of 1.5 × 10

−5 S cm

−1 which is ∼75 timeslarger than those of EtOH

–HKUST-1 and MeCN

–HKUST-1. Later, Chen et al. developed a one-step straightforward



EnergyChem REVIEW

Fig. 55. (a) Crystal structure of In-IA-2D-1 with [(CH 3 ) 2 NH 2 ] + cations (left) and In-IA-2D-2 with [(CH 3 ) 2 NH 2 ] + cations and DMF molecules (right). Reproduced with permission. 255 Copyright 2013 The Royal Society of Chemistry. (b) Zn 8 O cluster connected to six metallomacrocycles; (c) Space-filling model of [(Zn 0.25 ) 8 (O)Zn 6 (L) 12 (H 2 O) 29 (DMF) 69 (NO 3 ) 2 ] n along a -axis with aligned imidazolium moieties. Reproduced with permission. 256 Copyright © 2012, American Chemical Society.

Fig. 56. (a) Comparison of one-step and two-step methods to synthesis Im@(NENU-3) and Im-Cu@(NENU-3a). (b) Upper: view of the proposed proton-conducting pathways in Im@(NENU-3). Bottom: View of the proposed proton-conducting pathways in Im-Cu@(NENU-3a). Reproduced with permission. 259 Copyright © 2017, American Chemical Society.

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trategy and two-step strategy to incorporate imidazole guestolecules into a highly stable MOFs [Cu 12 (BTC) 8 (H 2 O) 12 ]

HPW 12 O 40 ] (namely NENU-3) to obtain Im@NENU-3 andm-Cu@NENU-3a, respectively ( Fig. 56 (a)). 259 Single crystaltructure reveals that imidazole molecules coordinated withopper notes in Im-Cu@NENU-3a which block proton transportathway and as guest molecules in Im@NENU-3. As a result,m@NENU-3 exhibits a ultrahigh conductivity of 1.82 × 10

−2 Sm

−1 , which is significantly higher than that of Im-Cu@NENU-a (3.16 × 10

−4 S cm

−1 ) at 70 °C and 90%RH ( Fig. 56 (b)). In general, the phosphonate functionalized carboxylate-

ased MOFs exhibited noticeable proton conductivity becausef the incorporating proton source and hydrophilicity ofhosphonate group. Cabeza et al. reported such kind of MOFs Ca(HO 3 PC 6 H 3 COOH) 2 ] 2 [(HO 3 PC 6 H 3 (COO) 2 H)(H 2 O) 2 ]5H 2 O, its dehydrated one Ca [(HO 3 PC 6 H 3 COOH) 2 ] 2 (HO 3 PC 6 H 3 (COO) 2 H)(H 2 O) 2 ] and ammonia-xposed one NH 3 @Ca [(HO 3 PC 6 H 3 COOH) 2 ] 2 (HO 3 PC 6 H 3 (COO) 2 H)(H 2 O) 2 ]. 260 This main frameworkas pi l lared layered structure with 1-D hydrophilic channel

46

long b axis. Among these MOFs, ammonia exposed onehowed the best proton conductivity with 6.6 × 10

−3 Sm

−1 at 24 °C and 98% RH. The enhanced conductivityay due to the rupture of interlayer phosphonate/Ca 2 + or

arboxylate/Ca 2 + bonds in NH 3 @Ca [(HO 3 PC 6 H 3 COOH) 2 ] 2 (HO 3 PC 6 H 3 (COO) 2 H)(H 2 O) 2 ] ( Fig. 57 (a)). The proton-ransfer activation energy, E a , in the aforementioned MOFsange from 0.24 to 0.4 eV, indicating a Grotthuss H

+ transferechanism ( Fig. 57 (b)). The results indicate the importance

f internal H-bonding network and post-synthetic cavityderivatization” with selected guest molecules.

In 2017, Chen et al. reported a flexible and robust MOF,r 3 ( μ3 –O)(H 2 O) 3 (NDC(SO 3 ) 2 ) 3 �(NH 2 (CH 3 ) 2 + ) 5 (BUT-(Cr), BUT represents Beijing University of Technology)onstructed from a naphthalene-2,6-dicarboxylate organicigands consisting of abundant sulfonic acid (-SO 3 H) sites

hich showed a superprotonic conductivity of 4.63 × 10

−2 Sm

−1 at 80 °C and100% RH ( Fig. 57 (c)). 261 Through exchangeith H 2 SO 4 , the counter cations were replaced with proton so

s to obtain Cr 3 ( μ3 –O)(H 2 O) 3 (NDC(SO 3 H 5/6 ) 2 ) 3 ((BUT-



EnergyChem REVIEW

Fig. 57. (a) Illustration of the rupture of the 3-D structure to form 2-D intercalated derivatives; (b) Arrhenius plots with temperature ranging from 10 to 24 °C. Reproduced with permission. 260 Copyright © 2014, American Chemical Society. (c) Crystal structure of BUT-8(M) ( M = Cr, Al) and the ion exchange in BUT-8(Cr). (d) Arrhenius plots of BUT-8(Cr)A, BUT-8(Cr) and MIL-101-SO 3 H. Reproduced with permission. 261 Copyright © 2017, Springer Nature.

8(Cr) A), which exhibited an enhanced proton conductivityup to 1.27 × 10

−1 S cm

−1 at 80 °C and100% RH ( Fig. 57 (d)).Furthermore, this MOF was shown to underwent water-content-dependent structural transformation under variable humidity(“self-adaption” framework) that contributes to the wide rangeRH dependent high proton conductivity.

In order to improve proton conductivity, Das et al. proposedtemplate assisted approach to synthesize MOF protonconductors. 262 Templating poly carboxylate or carboxylicacid into coordination frameworks is considered to be a powerfulstrategy to construct high proton conducting materials. Theas synthesized [Co-(bpy)(H 2 O) 4 ](fdc) •(H 2 O) 1.5 (namelyCo-fdc), [Co(bpy)(H 2 O) 4 ](btec) 0.5 •H 2 O (namely Co-tetra),and [Co(bpy)(H 2 O) 4 ](Hbtc) •(H 2 O) 1.5 (namely Co-tri)showed proton conductivity of 4.85 × 10

−3 , 4.15 × 10

−2 ,and 1.49 × 10

−1 S cm

−1 , respectively at 80 °C and100% RH( Fig. 58 (a)). The structural analysis revealed that Co-fdc andCo-tetra possessed complete deprotonation while Co-tri waspartially protonated. Additionally, Co-tri and Co-tetra exhibitedhigh conductivity of 2.92 × 10

−2 and 1.38 × 10

−2 S cm

−1 ,respectively, at relative low temperature (40 °C) and 98% RH( Fig. 58 (b)).

In 2018, Serre et al. reported an aliphatic carboxylate-basedMOF, MIP-202 (Zr) with ultrahigh proton conductivity androbust framework. 263 MIP-202 (Zr) is assembled by a 12-connected Zr 6 ( μ3 –O) 4 ( μ3 –OH) 4 (COO

−) 12 cluster SBU andthe aspartic acid linker to form a 3-D structure ( Fig. 58 (c)). The–NH 2 group on the linker skeleton is protonated and coexistswith remaining Cl − to form –NH 3

+ /Cl − pairs which interactwith water molecules forming a complicated 3D Hydrogen bondnetwork ( Fig. 58 (d)). As a result, it exhibited excellent protonconductivity of 0.011 S cm

−1 at 90 °C and 95% RH.

47

Proton-conductive phosphonate-based MOFs : In addition tocarboxylates, proton-conducting studies have also been carriedout earlier on various phosphonate-based MOFs. 264–268 Zhenget al. summarized the progress of proton conductive metal phos-phonate framework before 2019, including main group metal,transition metal, lanthanide and actinide metal, and mixed metalphosphonates. 42 Here we intend to focus on the representativephosphonate -based MOFs with high proton conductivity ( >10

−3 S cm

−1 ) or unique proton conduction mechanism. Thephosphonate groups own unique advantages, including highchemical and thermal stability, acidity, and high versatility toform multidimensional frameworks. The phosphonate-basedMOFs are considered as promising candidates of high protonconductors.

Shimizu group from University of Calgary did lotsof pioneering works in proton-conducting phosphonateframeworks. In 2010, Taylor et al. synthesized a metalphosphonate framework Zn 3 (L)(H 2 O) 2 •2H 2 O (namelyPCMOF-3, H 6 L = 1, 3, 5-benzenetriphosphonic acid) withproton conductivity of 3.5 × 10

−5 S cm

−1 at room temperatureand 98% RH. 269 The phosphonate oxygen atoms, coordinationwater molecules and ordered uncoordinated water molecules(forming hydrogen bonds with Zn) in PCMOF-3 togethergenerated a hydrophilic interlayer which accounts for themoderate conductivity and an ultralow activation energy of0.17 eV. Taylor then further reported [La(H 5 L)(H 2 O) 4 ] (namelyPCMOF-5, L = 1, 2, 4, 5-tetrakisphosphonomethylbenzene)with enhanced proton conductivity of 2.5 × 10

−3 S cm

−1 at60 °C and 98% RH ( Fig. 59 (a)). 270 PCMOF-5 used a flexible,tetrapodal linker to form a 3 dimaneional structures with 1dimensional channels of 5.81 A. A facile one-dimensional H-bonding array along a -axis has been established between freephosphonic acid groups and lattice water molecules which



EnergyChem REVIEW

Fig. 58. (a) Template-assisted method in the existing [Co(bpy)(H 2 O) 4 ] 2 + chain with a different degree of protonation and ionic carrier concentration to obtain enhanced proton conductivity. (b) Arrhenius plots for Co-fdc, Co-tri, and Co-tetra at ambient temperature and 98% RH. Reproduced with permission. 262 Copyright 2018, Wiley-VCH. (c) The 12-connected Zr 6 ( μ3 –O) 4 ( μ3 –OH) 4 (COO

−) 12 cluster SBU, the aspartic acid linker and the crystal structure of MIP-202 (Zr) viewed along a -axis. (d) left: a presentative H-bonded water-bridge network formed by the ammonia and water molecules. Right: hydrogenbonding with the possible involvement of Cl − ions. N atom is in blue, O atom is in red, Cl − is in green. Reproduced with permission. 263 Copyright © 2018, Springer Nature.

Fig. 59. (a) View of PCMOF-5 along a -axis. (b) The 1-D hydrogen-bonded arrays formed along a -axis between phosphonic acid groups and lattice water molecules. Reproduced with permission. 270 Copyright © 2013, American Chemical Society. (c) The schematic representation of the synthesis of PCMOF-2.5 and PCMOF2.5(Pz/Tz) from β-PCMOF-2. (d) Proton conductivity data (90% RH) for various compounds. Reproduced with permission. 272 Copyright © 2013, American Chemical Society.



EnergyChem REVIEW

Fig. 60. (a) Structure of PCMOF10 with zigzag ladders forming a 2-D grid. (b) H-bonding array constructed from O

–H

•••O type hydrogen-bond between the lattice water molecules and intra-layer oxygen atoms. Reproduced with permission. 273 Copyright © 2015, American Chemical Society. (c) Crystal structure of [La 3 L 4

8 (H 2 O) 6 ]Cl •xH 2 O along [001] with the enlarged 1-D channel (1.9 nm in diameter) filled with coordinated (red) and non-coordinated (yellow) water molecules; phosphonate free oxygen atoms are shown in light blue. Reproduced with permission. 274 Copyright 2014, Wiley-VCH.

T

suggests an efficient Grotthuss mechanism with low activationenergy ( E a = 0.16 eV) ( Fig. 59 (b)). What’s important, thiscompound can tolerate boiled water for as long as 1 week, whichindicates a highly robust material.

To further enhance the proton transport in phosphonateframeworks, Kim et al. utilized “isomorphous ligand replace-ment” strategy to design PCMOF-2.5 hybridizing two PC-MOFs: β-PCMOF-2 and PCMOF-3 ( Fig. 59 (c)). 271 Specifically,combine the linker 2,4,6-trihydroxy-1,3,5-benzene trisulfonatefrom β-PCMOF-2 with the linker benzene-1,3,5-triphosphonatefrom PCMOF-3 into one structure to form 1-D channelsand 3-D crystal structures. The one dimensional channels ofPCMOF-2.5 were decorated with sulfonate groups and hydrogenphosphonate to construct effective proton-conducting pathwaysand substantially improve the conductivity up to 2.1 × 10

−2 Scm

−1 at 85 °C and 98% RH. Interestingly, this mixed linkersystems could only be obtained through a solid state syntheticroute in situ formed during the impedance measurements. Theconductivity value was further enhanced by 1 order after thesuccessful loading of two different heterocycles, PCMOF2.5(Pz) and PCMOF2.5 (Tz) (Pz = 1 H -pyrazole, Tz = 1 H -1,2,4-

riazole) exhibit ultrahigh conductivity values over 1 × 10

−1 Scm

−1 (85 °C and 90% RH), while keeping the original MOFstructure( Fig. 59 (d)). 272

Shimizu et al. also reported a magnesium phosphonate frame-work, Mg 2 (H 2 O) 4 (H 2 L

−1 ) •H 2 O (PCMOF-10) (H 6 L

1 = 2,5-dicarboxy-1,4-benzenediphosphonic acid), namely PCMOF-10which possesses a 2-D layered architecture. 273 The MgO 6 octahedra and PO 3 C tetrahedral formed zigzag ladders whichwere pi l lared by H 2 L

1 anions ( Fig. 60 (a)). An extensive H-bond network is formed among the coordination water, thelattice water, carboxylate and phosphonate groups which re-sulted in an ultrahigh proton conductivity over 3.55 × 10

−2

S cm

−1 (70 °C and 95% RH) ( Fig. 60 (b)). Begum et al.found that the incorporated triazole group into the phospho-nate linker can largely enhance the operating temperature. 274

The as-synthesized [La 3 L 4 8 (H 2 O) 6 ]Cl •xH 2 O (H 2 L

8 = 4-(4H-1,2,4-triazol-4-yl)phenylphosphonic acid) consists of La III ionsand μ4 -, μ5 - and μ6 -bridging triazolylphenylphosphonate lig-ands which showed robust structures ( Fig. 60 (c)). This com-

49

pound showed the proton conductivity of 1.7 × 10

−4 S cm

−1 at110 °C and 98% RH. Importantly, this material is a rare exampleof a phosphonate-based MOF with large hydrophilic channels of1.9 nm in diameter.

In 2015, Zheng et al. reported a layered Co-Caphosphonate [Co

III Ca II (notpH 2 )(H 2 O) 2 ]ClO 4 •nH 2 O (namelyCoCa •nH 2 O, notpH 6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris-(methylenephosphonic acid)) which undergoes a single-crystal-to-single-crystal transformation between CoCa •2H 2 O andCoCa •4H 2 O induced by humidity at ambient temperature. 275

Compare to CoCa •4H 2 O (95% RH), the continuous H-bond network is interrupted in CoCa •2H 2 O (40% RH) whichresulted in ∼5 order of magnitude lower proton conductivity.Anisotropic proton conductivity measurements on single crystalof CoCa •4H 2 O revealed that [010] direction of H-bond exten-sion is the preferred proton transfer pathway w ith conductiv ity of1.0 × 10

−3 S cm

−1 at 25 °C and 95% RH ( Fig. 61 ). Understanding the proton-conducting mechanism at

molecular level is crucial for developing novel materialswith enhanced proton conductivity. Schr ӧder and Yangutilized quasi-elastic neutron scattering (QENS) toexplore the proton transfer mechanism in a phosphonate-MOF, [M 3 (H 3 L) 2 (H 2 O) 9 (C 2 H 6 SO) 3 ] (M = Ni, Co;H 6 H = benzene-1,3,5-p-phenylphosphonic acid), namelyMFM-500(Ni) and MFM-500(Co), respectively. 276 Thismaterials showed proton conductivity of 4.5 × 10

−4 and4.4 × 10

−5 S cm

−1 for MFM-500(Ni) and MFM-500(Co),respectively, at 25 °C and 98% RH. QENS indicated that theproton conduction of MFM-500(Ni) is mediated by a newmechanism defined as “free diffusion inside a sphere”.

3.4.2. Proton-conductive MOFs under anhydrous condition

Most reported proton-conducting MOFs are based on water-mediated process and operating at low temperature whichseverely limit their practical application. However, it’s desiredto study the proton-conducting MOFs under high temperature(greater than 80 °C), anhydrous conditions, in order to be analternative to Nafion. Recently, the loading carrier molecules inMOFs has been considered as an efficient method to improve theproton conduction. Heterocyclic organic molecules (imidazole,



EnergyChem REVIEW

Fig. 61. (a) and (b) Schematic illustration of H-bond network viewed along [202] direction in CoCa •4H 2 O and CoCa •2H 2 O, respectively. Color codes: pale pueple tetrahedral is (PO 3 C), blue and yellow octahedral is (CoO 3 N 3 ) and (CaO 6 ), respectively. (c) Hydrogen bond network and anisotropic proton conductivity in CoCa •4H 2 O. Reproduced with permission. 275 Copyright © 2015, American Chemical Society.

Fig. 62. (a) Crystal structure of β-PCMOF2 along the c-axis. (b) A space-fil ling mode of pore structure in β-PCMOF2 with high degree sulfonation. (c) The Arrhenius plots of β-PCMOF2, [ β-PCMOF2(Tz) 0.3 ], [ β-PCMOF2(Tz) 0.45 ], and [ β-PCMOF2(Tz) 0.6 ] evaluated under anhydrous conditions. Reproduced with permission. 26 Copyright © 2009, Springer Nature.

1

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H-1,2,4-triazole or benzeylimidazole) have attracted consider-ble attention due to their unique features: (i) facile protonransfer property and amphiprotic nature; (ii) higher meltingoints; (iii) be capable of forming hydrogen bond network with

he host framework; (iv) smaller size enables fast transport. Thushe incorporation of heterocyclic organic molecules in MOFsould generate a promising proton conductive materials that canork under anhydrous and high temperature conditions. In 2009, Hurd et al. prepared a sulfonate MOF Na 3 (2,4,6-

rihydroxy-1,3,5-benzenenetrisulfonate), namely β-PCMOF2hich possesses a 3-D honeycomb structure and a 1-D channelsith a diameter of 5.65(2) −5.91(2) A ( Fig. 62 (a) and (b)). 26

he anhydrous proton conductivity was enhanced to 2 × 10

−4

cm

−1 , 5 × 10

−4 S cm

−1 , and 4 × 10

−4 S cm

−1 at 150 °C, atriazole loadings of 0.3, 0.45 and 0.6 respectively ( Fig. 62 (c)).dditionally, β-PCMOF2(Tz) 0.45 was used to construct aembrane-electrode assembly (MEA) in fuel cells which

xhibited good stability for 72 h at 100 °C. Another example was reported by Kitagawa et al., wherein

he imidazole molecules act as the guest proton carriers andhe MOFs act as the host framework with variant affinities Fig. 63 (a) and (b)). 24 The as-synthesized two kinds hostrameworks including Al( μ2 –OH)(1,4-ndc) n (1,4-ndc = 1,4-aphthalenedicarboxylate) and Al( μ2 –OH)(1,4-bdc) n 1,4-bdc = benzenedicarbox ylate) w hich have similar poretructures but own different shapes and surface potentials Fig. 63 (c) and (d)). There are 14% weight imidazole loading inl( μ2 –OH)(1,4-ndc) n while 30% loading in Al( μ2 –OH)(1,4-dc) n , however, the anhydrous proton conductivity of the former

M

50

omplex (2.2 × 10

−5 S cm

−1 at 120 °C) is higher than that of theatter (1.0 × 10

−7 S cm

−1 at 120 °C). The difference is ascribed to the interactions between the

midazole and the channel surfaces: the ndc-based host frame-ork with non-polar channels allows the polar imidazole pass

hrough freely, while the bdc-based host framework with polarhannels interact strongly with the imidazole, hence impedeheir transport. Then the same group continuously incorporatedistamine into the ndc-based host framework. 241 The loading his-

amine in Al( μ2 –OH)(1,4-ndc) n is calculated as one histamineolecule per Al 3 + ion. This material showed proton conductivity

f 1.7 × 10

−3 S cm

−1 at 150 °C. The group also reported a two-dimensional coordination poly-

er, [Zn(H 2 PO 4 ) 2 (C 2 N 3 H 3 ) 2 ] n (labeled 1)with anhydrousroton conductivity at 150 °C ( Fig. 64 (a) and (b)). 237 Asnown, the defects in MOFs play a key role on their proton-

conducting properties. 277–280 To enhance the proton conduction,onodentate H 2 PO 4

− defects and uncoordinated H 3 PO 4 wereuccessfully incorporated into the framework. ZnO, HTz andxcess phosphoric acid solution reacted in water by a liquid-ssisted mechanochemical method. According to the amountsf H 3 PO 4 reacted, a series compounds labelled 2, 3, 4, 5 werebtained. The Arrhenius curves for the proton conductivitiesf 2–4 are showed in Fig. 64 (c) with temperature range of 30–50 °C under anhydrous conditions. The proton conductivity

ncreased and activation energy decreased with the increasingmounts of H 3 PO 4 , which indicates the key role of H 3 PO 4 innhydrous proton conduction.

The proton conduction can be easily realized in hydrophilicOFs under hydrous conditions because of its permanent poros-



EnergyChem REVIEW

Fig. 63. Imidazole guest molecules located in the nanochannel of the host framework with a high affinity (a) and low affinity (b). 3D structures of ndc-based aluminum porous framework (c) and bdc-based aluminum porous framework (d) acting as host frameworks. Reproduced with permission. Reproduced with permission. 24 Copyright © 2009, Springer Nature.

Fig. 64. The H-bonding network of [Zn(H 2 PO 4 ) 2 (C 2 N 3 H 3 ) 2 ] n viewed along a -axis (a) and b -axis(b). The hydrogen-bonds between adjacent H 2 PO 4 −

ions represented in green dotted lines, and between H 2 PO 4 − ions and HTz molecules are in blue dotted lines. [Zn in purple, P in yellow, O in red, N in

blue, C in black, and H in light pink.]. c. Arrhenius plot of the proton conductivities of 2 (black circle), 3(red circle), 4(blue circle), and 5(green circle) under dry nitrogen. Reproduced with permission. 237 Copyright © 2016, American Chemical Society.

ity and polar features, providing much spaces to form continuoushydrogen-bond networks. However, the water molecules cannotact as proton carries at T ≥ 100 °C. The proton-conductiveMOFs under high temperature ( > 150 °C) and anhydrousconditions in MOFs depending on loading of nonvolatile carriermolecules, such as heterocyclic organic molecules and inorganicacid. Unfortunately, owing to the lack of thermal stability ofMOFs or the volatilization of proton-carriers, there are relativefew reports on high-temperature proton-conductive MOFs.Up to now, most reported proton-conductive MOFs underanhydrous conditions were below 200 °C, and their protonconductivity value is less than 0.01 S cm

−1 . In the future, itis significant to develop high performance proton-conductiveMOFs ( > 0.01 S cm

−1 ) with high tolerance temperature ( > 200°C) for fuel cell technology, especially as an alternative to Nafion.

3.4.3. Proton-conductive MOFs under both anhydrous and

humidified conditions Various strategies have been utilized to construct high proton-conducting materials, among which, incorporating proton car-riers into pores of host frameworks and introducing proton

51

carrier groups into MOF structure are considered to be effectiveapproaches. To construct a framework with proton-conductingligands and proton carrier available pores afford a single MOFmaterial with dual functionality.

An oxalate-based MOF material, [(Me 2 NH 2 ) 3 (SO 4 )] 2 [M 2 (ox) 3 ] was found to conduct protons under anhydrous aswell as humidified conditions. 281 This compound exhibitedremarkable proton conductivity up to 1 × 10

−4 S cm

−1 at150 °C and a ultrahigh water-assited proton conductivityof 4.2 × 10

−2 S cm

−1 at 98% RH. The anhydrous and aqueousproton conductivity demonstrated activation energy of 0.129 and0.130 eV, respectively which indicate the Grotthuss mechanismof the proton transformation.

Horike and Kitagawa et al. reported a flexible structurewith hydrated form [Zn 3 (H 2 PO 4 ) 6 (H 2 O) 3 ](Hbim)(Hbim = benzimidazole) and dehydrated form[Zn 3 (H 2 PO 4 ) 6 ](Hbim) which showed different protonconducting behavior. 282 The hydrated one has acidic protonsin the structure and exhibited low conductivity of 1.4 × 10

−7 Scm

−1 at 30 °C and 6.1 × 10

−7 S cm

−1 at 60 °C. However, thedehydrated one obtained high proton conductivity of 1.3 × 10

−3



EnergyChem REVIEW

Fig. 65. (a) Views of the crystal structure of (Me 2 NH 2 )[Eu(PiPhtA)] along a -axis (the red spheres represent the uncoordinated O atoms of phosphonate); (b) The sandwich structure along c -axis with (Me 2 NH 2 ) + as counter cations. (c) The cations interact with the uncoordinated phosphonate oxygen atoms to form N

–H

••••O hydrogen bond chains along c -axis. Reproduced with permission. 283 Copyright © 2017, American Chemical Society.

S

p

p

M

(

s

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c

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c

(

a

l

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M

2

cm

−1 at 120 °C due to the rearrangement of the conductionath and benzimidazole molecules facilitates the transport ofrotons.

In 2017, Zang and co-workers reported a phosphonateOF, (Me 2 NH 2 ) [Eu(PiPhtA)] (H 4 PiPhtA = 5-

phosphonomethyl)isophthalic) which has an anionic layeredtructure ( Fig. 65 (a) and (b)). 283 The Me 2 NH 2

+ cations locatedn the interlayer space and interact with the oxygen atoms of thencoordinated phosphonate to form N

–H

••••O hydrogen bondhains along c -axis ( Fig. 65 (c)). The thermally stable MOF (upo 300 °C) crystal showed a anisotropic proton conductivity of.23 × 10

−3 S cm

−1 along c -axis but no conductivity along otherirections at 150 °C. Interestingly, a pellet of its microcrystallineample showed humidity-dependent proton conductivity andeached 1.25 × 10

−3 S cm

−1 at 100 °C and 98% RH. PCP/MOF-based proton-conducting materials have made

reat progress over the past decade, however, it is sti l l chal lengingo find a PCP/MOF with robust structure and excellent anhy-rous proton conductivity which can be directly utilized as aractical electrolyte in a fuel cells. In 2018, Wang et al. reported novel proton-conducting material, (NH 4 ) 3 [Zr(H 2/3 PO 4 ) 3 ]ZrP), which is composed of 1-D zirconium phosphate anionichains and fully ordered NH 4

+ acting as the balance chargeations ( Fig. 66 (a) and (b)) 284 The proton conductivity underumidified condition was evaluated of ZrP which showed anltrahigh proton conductivity of 1.21 × 10

−2 S cm

−1 at 90 °Cnd 95% RH, suggesting an important role of water moleculesn the proton transport ( Fig. 66 (c)). The studies also showedhat protons are disordered within an inherent H-bonded infinitehains of acid-base pairs (N

–H

•••O-P), resulting in a robustnhydrous proton conductivity up to 1.45 × 10

−3 S cm

−1 at80 °C ( Fig. 66 (d)). In view of the excellent intermediate-

S

52

emperature proton conductivity of ZrP, they further assembly itnto a H 2 /O 2 fuel cell, which exhibited an electrical power densitys high as 12 mW cm

−2 at 180 °C, a record value among othereported cells fabricated from crystalline solid electrolytes.

.4.4. Proton-conductive MOF films t remains challenging to apply proton-conductive MOF mate-ials as electrolyte materials in fuel cells due to the formationf membrane from their crystalline materials. Up to now, mosteported proton conducting MOFs are either pellet samples, orn single crystals. To develop facile membrane formation strate-ies in proton-conductive MOFs are desirable for their practicalpplications in fuel cells. Some early efforts in film/membraneormation have yielded exciting results. 6 , 43 , 285–289

An effective strategy is to prepare nanofilms of MOFs withigh proton-conducting performance. Kitagawa et al. demon-trated the proton-conducting behavior in a highly oriented

OF nanofilms, Cu-TCPP, which is composed of 5,10,15,20-etrakis(4-carboxyphenyl)porphyrin (TCPP) and Cu metalotes. 290 The compound was assembled by TCPP units with Cuenters, which was then linked to Cu 2 (COO) 4 paddle-wheelso produce a 3-D layered structure, with 1-D channels alonghe c -axis ( Fig. 67 (a)–(c)). This MOF nanofilm-based deviceas fabricated by a facile “stamp” method with high orientation direction. Synchrotron grazing incidence X-ray diffractionGIXRD) results demonstrated the in-plane hk0 diffraction peaksnd out-of-plane 001 diffraction peaks, indicating that the abayer of Cu-TCPP nanosheet is parallel to the bottom substrate Fig. 67 (d)). The impedance measurements showed that this

OF nanofilm reached a conductivity of 3.2 × 10

−8 S cm

−1 at5 °C and 40% RH, which increased to as high as 3.9 × 10

−3

cm

−1 at 98% RH ( Fig. 67 (e)). GIXRD measurements under



EnergyChem REVIEW

Fig. 66. (a) Crystal structure of ZrP. (b) View of the counter-ions NH

4 + arranged between the adjacent chains. (c) Impedance plot of ZrP at 90 °C and 95% RH. (d) Impedance plot of ZrP at 180 °C under anhydrous conditions. Reproduced with permission. 284 Copyright © 2018, American Chemical Society.

Fig. 67. (a) Illustration of MOF nanofilm based device for electrical measurement. (b) and (c) Simulated crystal structure of MOF nanofilm. (d) Out- of-plane synchrotron GIXRD of MOF nanofilm under different humidity. (e) Arrhenius plots of the proton conductivity of the MOF nanofilm. Inset: Impedance plot of Cu-TCPP nanofilm at ambient temperature (98%RH). Reproduced with permission. 290 Copyright © 2013, American Chemical Society.

humid conditions further confirmed the water stability of theMOF. The enhanced proton conductivity of Cu-TCPP nanofilmoriginated from the dangling carboxylic acid groups on its surface,which acted as Lewis acids, as well as effective proton donors.

3.4.5. Glass-state MOFs for proton conductivity Glass-state CPs/MOFs prepared by melt-quenching are regardedas one of the most attractive species considering their abilityto form porous materials and multifunctional properties. 291–298

Kitagawa et al. developed a family of CPs with intriguing glassstates, which exhibited outstanding proton conductivity. 299–301

53

In 2016, they synthesized a 2D CP crystal, [Cd(H 2 PO 4 ) 2 (1,2,4-triazole) 2 ] (namely CdTz), composed of octahedrally coordi-nated Cd

2 + ions, H 2 PO 4 − and 1,2,4-triazole ligands, resulting

in 2D layered structures ( Fig. 68 (a), upper). 299 a-CdTz-40, a-CdTz-240, and a-CdTz-500 represent products with 40, 240,and 500 min mi l ling times from original CdTz crystals. ThePXRD and DSC characterized the glass-states in the mi l ledcompounds with amorphous phases, a glass transitions ( T g )and exothermic crystallizations ( T c ) ( Fig. 68 (a), bottom). Thelow proton conductivity of CdTz measured in Ar atmosphereshowed 4 × 10

−10 S cm

−1 at 50 °C and 8 × 10

−7 S cm

−1 at



EnergyChem REVIEW

Fig. 68. (a) Crystal structure of CdTz at 20 °C. PXRD pattern (bottom left) and DSC (bottom right) in the first heating process of CdTz (black), a-CdTz-40 (brown), a-CdTz-240 (green), a-CdTz-500 (purple). The arrows indicted the glass transition temperature. (b) The corresponding proton conductivity at various temperatures. Reproduced with permission. 299 Copyright 2016, Wiley-VCH. (c) Crystal structure of [Zn(HPO 4 )(H 2 PO 4 ) 2 ](ImH 2 ) 2 consisting of 1D anionic [Zn(HPO 4 ) −(H 2 PO 4 ) 2 ] 2 − chains. Zn, purple; C, light gray; O, pink; N, blue; P, gray; HPO 4

2 −, yellow; H 2 PO 4 −, gray. (d) Nyquist plots of the melt-quenched glass (doped 5 mol% pyranine) under light with increasing irradiation time.

Reproduced with permission. 300 Copyright 2017, Wiley-VCH.

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50 °C, ascribed to the strong interaction between H 2 PO 4 −

roups limiting the dynamic movements of protons. By contrast,he proton conductivity of a-CdTz-40, a-CdTz-240, and a-CdTz-00 below their T c are over two orders of magnitude higher

mprovement with 1.0 × 10

−6 , 5.4 × 10

−5 , and 1.0 × 10

−4 Sm

−1 , respectively ( Fig. 68 (b)). The study showed that thecidity of H 2 PO 4

− in the structure of a-CdTz-x is enhanced byhe glass formation, resulting in higher proton conductivity.

The same group further utilized the melting behavior of CPso regulate its proton conductivity. 300 The as-synthesized dopingZn(HPO 4 ) 2 (H 2 PO 4 ) 2 ](ImH 2 ) 2 (ImH 2 = monoprotonatedmidazole) coordination polymer melt with strong acids,rifluoromethanesulfonic acid acting as proton carries withnhanced proton conductivity ( Fig. 68 (c)). Interestingly, whenxtended the doping strategy to organic photo acid molecule,hey obtained optically switchable proton conductivity in the

elting coordination polymers ( Fig. 68 (d)).

. CHALLENGES AND PERSPECTIVES

OFs/PCPs have been rapidly developed as significant mul-ifunctional materials over the last two decades. More than0,000 different MOFs, with outstanding structural flexibilitynd chemical diversity, have been reported and explored toate. Furthermore, the ultrahigh porosity/surface areas, tunable

unctionality and high thermal/chemical stability of MOFsake them excellent candidates for a variety of energy-related

pplications. Unfortunately, most MOFs are electrical or proticnsulators, thus restricting their applications in electronic devices.

he design and synthesis of electronically conductive or proton-onducting MOF materials, to render the coexistence betweenacile charge transport or proton conduction and high porosity,

54

ould be conducive to developing the next generation ofechnologies. 302

As a new family of semiconductors/conductors, featuredith defined crystal structures, unique charge-spin-lattice inter-

ctions, and tunable electronic band structures, electronicallyonductive MOFs/PCPs have aroused extensive attention inondensed matter physics (such as magnetic semiconductor,uperconductor, and topological insulator) recently. Considereds a true hybrid material with unique electronic properties,onductive MOFs are different from inorganic materials (suchs, graphene) with uneasy functionalization, or organic polymershich suffer from low-charge mobility due to lack of long-

ange order. Electronically conductive MOFs have undergoneremendous growth over the past five years, and several strategiesave been developed to render the electrical conduction in

hem: (i) modulating π–π stacking within 3D MOFs; (ii)esigning 2D π -conjugated MOFs with delocalized chargearriers; (iii) incorporating redox-active organic linkers or mixed-alence metal centers; and (iv) introducing guest molecules toender long-range charge transport on the host frameworks. Morepecifically, synthetic methods, charge transport mechanism andelated applications on electronically conductive CPs/MOFsere summarized in this rev iew, try ing our best to refer all related

eports. However, we should say sorry for the missed referencesn this review article because of the impossibility to exhaust allelated publications on this topic. Overall, the emerged electron-cally conductive MOFs have unique advantages, including: (i)tructural flexibility and tunability; (ii) outstanding electricalonductivity capable of facilitating the use of MOFs in electronicnd electrochemical devices; (iii) high surface area and porosityenefiting size sieving and active sites exposing; (iv) facileynthesis methods to facilitate the integration of the active MOF



EnergyChem REVIEW

layer/film into devices; (v) the defined and versatile topologicalstructures provide ideal platforms for studying electron-lattice-spin interactions. Despite significant advances have been achievedover recent years, electronically conductive MOFs are sti l l intheir infanc y, w ith great challenges in: (i) the lack of robustnessmakes them sensitive to high temperature and pressure, as wellas strong acid/base conditions, thus hampering their futureapplications. (ii) Synthesizing and characterizing single-crystalsof conductive MOFs remain to be challenging, especially for2D conductive MOFs. The intrinsic charge transport studies ofconductive MOFs, due to lack of high-quality crystal samples,are sti l l rare. The defects within the structures and their effectson electric transport properties in MOFs remain to be unclear.(iii) The computational calculations should be in concert withthe experimental studies to uncover the intrinsic propertiesof conductive MOF materials. (iv) Despite the conductivityvalues in MOFs are enough high and sufficient for variouselectric-based applications, their relatively small surface area(lower than 1000 m

2 g −1 ) brings new challenges to realizemultifunctional materials with both outstanding conductivityand highly accessible surface areas.

MOFs/PCPs, as proton-conducting materials, have attractedsignificant interest and have been considered as ideal platforms foracquiring deeper insight into proton transport process. Thoughtremendous advances have been achieved recently, MOFs asproton conductors are sti l l faced with several key hurdles, such asadequately high proton conductivity over 0.01 S cm

−1 , chemicaland thermal stability, mechanical strength, and processability inthe form of membranes. Up to now, plenty of MOFs based onproton conducting platforms show super-protonic conductivitieswithin the range of 0.01 to 0.1 S cm

−1 . The robustness ofthe emerging proton-conducting MOFs must be consideredfor future industrial applications. From commercial perspectiveof fuel cells, the proton-conducting component are expectedto tolerate repeating hydration-dehydration process and hightemperature. Furthermore, their processability is an importantpoint and cannot be overlooked. This challenge, with thedevelopment of film/membrane preparation and mixed matrixmembrane in MOFs, wi l l be overcome in the future. MOFs arecapable of designing the proton-transfer motifs in a long-rangeordered matrix, whose defined crystal structures serve as an idealplatform to study the proton-conduction mechanism by a varietyof characterization techniques. From the perspective of design,the high proton-conducting MOFs should be endowed withfollowing features: (i) proton-conducting pathways mediated byordered alignment of protic sites; (ii) acidic groups anchoringon the backbones of the frameworks; (iii) high density ofcarrier concentration; (iv) matched pK a values in the protontransfer pathways. Although great progress has been made onproton-conducting MOFs, there is sti l l a long way to go for thepractical application in PEMFC. With the deep understanding ofstructure-property relationships in MOFs as proton conductors,continued advancement in this field is expected.

Overall, the field of conductive MOFs has yielded tremen-dous development over the past decade. Extensive efforts havebeen dedicated to promoting MOF-based semiconductors andproton conductors into a new era with both challenges andopportunities. With close collaborations among experimentalists,theoreticians and computational chemists in this field, it isexpected to make a more significant breakthrough regardingconductive MOFs as a new generation of functional materials in
the near future.
55

ACKNOWLEDGMENTS

We gratefully acknowledge the financial support from KeyResearch Program of Frontier Science, CAS (QYZDB-SSW-SLH023), NSFC (21805276, 21773245, 21822109, 21801243,51602311, 21850410462), Project Funded by China Postdoc-toral Science Foundation ( 2019M662254 ) and NSF of FujianProvince (2017J05094, 2019J01129), International PartnershipProgram of CAS (121835KYSB201800).

AUTHOR CONTRIBUTIONS

Wen-Hua Li: Writing - original draft, Writing - review & editing.Wei-Hua Deng: Writing - original draft. Guan-E Wang: Formalanalysis. Gang Xu: Conceptualization, Supervision, Writing -review & editing.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

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AUTHOR BIOGRAPHIES

Wen-Hua Li earned his Ph.D. in 2019 from Fujian Insti- tute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences, University of Chinese Academy of Sciences, Beijing. He is currently a postdoc under the guidance of Prof. Gang Xu at FJIRSM. His research work is focused on the development of conductive MOF-based multifunctional materials.

Wei-Hua Deng earned her BS degree in 2015 from

Fuzhou University. She is currently a Ph.D. student under the supervision of Prof. Gang Xu at FJIRSM. Her research work is focused on conductive MOF-based chemeresistive sensors.

Guan-E Wang earned her Ph.D. in 2015 from Fu- jian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences. She is currently an associate professor in FJIRSM. Her research work is focused on porous conductive inorganic-organic hybrids.

Gang Xu received his Ph.D. in 2008 at FJIRSM, CAS. Then he worked as postdoctoral fellows at COSDAF in Cit y Universit y of Hong Kong (Hong Kong), University of Regensburg (Alexander von Humboldt fellowship, Germany) and Kyoto University ( JSPS fellowship, Japan) in sequence. In 2013, he joined FJIRSM and undertook his independent research as a PI. His research interests focus on thesurface/interfacial coordination chemistry of materials and the relatedelectrical devices.








































































































































































































https://doi.org/10.1080/02603594.2019.1704738


Conductive MOFs

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