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Nano Energy

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Review

Recent advance in new-generation integrated devices for energy harvestingand storage

Sining Yuna,∗, Yongwei Zhanga, Qi Xub, Jinmei Liub, Yong Qinb

a Functional Materials Laboratory (FML), School of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, Chinab School of Advanced Materials and Nanotechnology, Xidian University, Xi'an, Shaanxi, 710126, China

A R T I C L E I N F O

Keywords:Integrated devicesLithium-ion batteriesSupercapacitorsNanogeneratorsBiofuel cellsSolar cells

A B S T R A C T

Energy harvesting and storage devices, including lithium-ion batteries (LIBs), supercapacitors (SCs), nanogen-erators (NGs), biofuel cells (BFCs), photodetectors (PDs), and solar cells, play a vital role in human daily life dueto the possibility of replacing conventional energy from fossil fuels. However, these isolated devices only havelimited performance and/or sole applicability, and cannot provide enough energy for application with long-termrun and the ever-changing working positions. This suggests that it is urgent to develop the fine self-poweredsystems to meet the growing demand of energy for long-term use in different environment scenes. Developingintegrated power pack, combining energy harvesting and storage, is an effective path to obtain a small size, lightweight, high density and high reliability energy system. In this review, eight types of multifunctional integrateddevices, such as LIB&SC, LIB&NG, BFC&NG, PD&BFC, SC&PD, SC&solar cells, NG&SC&solar cell, and LIB&solarcells, for energy harvesting and storage are reviewed in a broad sense, and a comprehensive summary of therecent development trends and highlights in the integrated device fields is given. Finally, the challenges andfuture outlooks for their successful commercialization are featured based on the recent advances and importantfindings.

1. Introduction

Due to the limited capacity, greenhouse gases emission of the tra-ditional fossil fuel, and the increasing energy requirement, developingnew and highly efficient technologies for harvesting energy from theenvironment has become a matter of great urgency. Harvesting theunused and wasted environmental green energy, such as solar energy,wind energy, microbe energy, and kinetic energy, and converting theminto a more useable form is a promising way for the long-term energyneeds and environmental sustainability. Up to date, a large number ofenergy conversion technologies, such as solar cells [1–4], piezoelectricnanogenerators (PENGs) [5–9], triboelectric nanogenerators (TENGs)[10–12], and biofuel cells (BFCs) [13–15], have been developed toconvert the diverse environmental energy into electricity. However,these environmental energies are highly dependent on when and wherethey are available, so the harvested energy could not provide con-tinuous power supply which is always not in good alignment with theactual demand.

One promising solution is to integrate different kinds of energyharvesters into one unit, which can harvest diverse ambient energiessimultaneously, and thus enhance the environmental adaptability of

energy harvesters. Taking the implantable device as an example, byintegrating a PENG and a BFC based on a simple RC high pass filter[16], the hybrid energy scavenging device can convert both the glucosefrom the biofluid and the kinetic energy from breathing into electricity.The two energy harvesting approaches can work simultaneously orindividually, thereby boosting output energy and service lifetime of theoriginal devices. In addition, integrating different devices together,through the synergistic effect between the devices having differentoperation mechanisms, one could obtain much larger power output ascompared with its two individual power output components [17],which facilitates more effective multi-type energies harvesting.

The other solution is to develop an energy conversion and storagesystem, through which the electrical energy, harvested from the en-vironment, can be stored high-efficiently into energy storage devices forfuture energy requirements. A large number of energy storage devices,such as lithium-ion batteries (LIBs) [18–20], lithium-sulfur batteries[21–23], and supercapacitors (SCs) [24–26], can be the appropriatecandidates. For example, under sunlight illumination, a photo-chargingprocess in the semiconductor will convert the solar energy into elec-tricity and store it by an electrochemical way in the lithium battery; thestored electrochemical energy can then be delivered to the electronics.

https://doi.org/10.1016/j.nanoen.2019.03.074Received 23 January 2019; Received in revised form 1 March 2019; Accepted 21 March 2019

∗ Corresponding author.E-mail addresses: alexsyun1974@aliyun.com, yunsining@xauat.edu.cn (S. Yun).

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Depending on the principle of each harvesting technology, the amountof energy output varies significantly to meet different needs.

Based on above-mentioned two solutions, in recent years, manyintegrated power packs have been widely developed through com-bining different devices into one unit for different application purposes.The sharp increase of the research passion in the new energy fields(solar cells, LIBs, SCs, and fuel cells) results in a giant increase of re-search literatures on the integrated devices. This means that there is alarge room for a Review related with new-generation integrated devicesfor energy harvesting and storage. Therefore, recent advances from theworldwide research groups have to be highly addressed to stimulategreat progress in the integrated device fields. In this regard, the purposeof this review is to cover the integrated device research in a broad senseand provide an overview of trend in new-generation integrated devicesfor energy harvesting and storage applications. Different energy har-vesting and storage technologies (such as solar cells, NGs, SCs, BFCs,and LIBs) are reviewed. Each technology is discussed both in workingprinciples and the main findings from recent literatures. The superiorityand limitation of each technology are identified and compared.Different devices are interconnected and assembled into multi-functional integrated devices, as shown in Fig. 1. Indeed, the develop-ment of integrated device technologies for energy harvesting and sto-rage is of importance for meeting the special demands in some quarters.It must be stated in this Review that some devices is linked devices, butnot integrated devices with “back-to-back” structure or common elec-trode. We call them as integrated device only based on the originalliteratures. In addition, this Review firstly introduces the energy har-vesting and storage devices (NG, LIB, SC and BFC), and then focuses onintegrated devices (LIB&SC, LIB&NG, BFC&NG, PD&BFC, SC&PD, SC&solar cells, SC&PD&Si solar cells, LIB&DSSC, and SC&DSSC&NG), whichwill be discussed in the Part Three. More integrated devices based onsolar cells have been review in our previous review paper [27].

2. Energy harvesting and storage devices

2.1. NG devices for energy harvesting

Modern industry requires novel clean energy sources as an

alternative to the common power stations based on combustion ofpetrol or gas as well as new technologies associated with energy con-version and storage. Nanogenerators (NGs) are developed to harvestsmall-scale energies and convert them into electricity [5,28,29]. AfterPENG was pioneered by Professor Zhong-Lin Wang at the Georgia In-stitute of Technology in 2006, TENG and pyroelectric nanogenerator(PyENG) were proposed in 2012 [30–33]. PENG and TENG are twomain members in the family of NGs for harvesting mechanical energywhich is ubiquitously abundant in the ambient environment [34–42].The working mechanism of the PENG is based on the piezoelectricproperty of piezoelectric materials. When the PENG device is deformedby an external force, a piezoelectric potential will be introduced insidethe device. As a result, a potential difference will be generated acrossthe two electrodes of PENG due to the surface bound charge, and it willdrive the electrons to flow in the external load until equilibrium. Whenthe external force is released and the PENG recovers to its originalshape, the piezopotential vanishes and the accumulated electrons willflow back in the opposite direction. Utilizing the strain-induced pie-zoelectric polarization, lots of advanced materials, such as PVDF(2.6 V/0.6 μA) [43], NaNbO3 (3.2 V/72 nA) [44], BaTiO3 (5.5 V/350 nA) [45], PZT (6 V/45 nA, 3.2 V/50 nA) [46,47], BZT-BCT (3.25 V/55 nA) [48], PMN-PT (7.8 V/2.29 μA) [49], and novel integrationstructures, such as position-controlled vertical ZnO nanowire (NW)arrays (58 V/134 μA) [50], three dimensional integrated ultralong PZTNW (209 V/53 μA) [9], and horizontal integrated flexible PZT film(200 V/150 μA) [51], have been developed for continuously enhancedenergy harvesting performance and applications [52–68]. TENG hasbeen rapidly developed with four operation modes through the coupledtriboelectrification and electrostatic induction effects, and it has shownexcellent capability to harvest all the types of mechanical energies andconverted into electric energy [69–85], such as water waves (354 V/270 μA/3.33Wm−2) [86], human motions (428 V/1.395mA) [82],vibrations (287.4 V/76.8 μA) [69], wind (334 V/67 μA) [87], sound(232 V/2.1 mA) [88], etc. Wasted heat is one of the most abundant andwidely available energy sources in our living environment and in-dustrial activities. PyENG is emerging as a powerful tool to scavengewasted heat. Based on the change of spontaneous polarization due tothe temperature fluctuation, a number of pyroelectric materials havebeen adopted to fabricate PyENGs, such as ZnO (20 mV/0.5 nA) [33],PZT (22 V/430 nA) [89], and PVDF (42 V/2.5 μA) [90] to convertthermal energy into electricity for powering small electronics[89,91–94]. In order to enhance scavenging efficiency of NG, re-searchers also have demonstrated many kinds of hybrid NGs, such aselectromagnetic-triboelectric NGs, piezoelectric-triboelectric NGs,pyroelectric-piezoelectric NGs, to harvesting energy complementally[95–99]. In general, lots of researches have been done to promote theperformance of the NGs through new materials, new or more flexiblestructures, new physical effects, etc., which could lead to breakthroughsin the applications of NGs [100–103].

The first NG has been invented for more than 10 years, and manyprogresses have been made. Both the output voltage and output currenthave been greatly enhanced, so one of the development tendency ishow to use NGs in every kinds of important applications, especially howto find some killer applications for them. The in vivo and implantablebiomedical devices, the portable personal electronic devices and largescale sensor network such as internet of things will be the importantapplication directions of NG [104–108]. Another development ten-dency is to continuously improve the output of NG to push forward itsapplications in large-scale energy such as “blue energy”. The third de-velopment tendency will be how to effectively integrate NG with otherenergy harvesting technologies or functional sensors to develop a morepowerful energy harvesting module and self-powered nanosystem forthe advancement of the total conversion efficiency [109–115].

Fig. 1. Integrated power packs (LIB&SC, LIB&NG, BFC&NG, PD&BFC, SC&PD,SC&solar cells, SC&PD&Si solar cells, LIB&DSSC, and SC&DSSC&NG) based onenergy harvesting and storage devices. LIB: lithium-ion battery; NG: nanogen-erator; SC: supercapacitor; BFC: biofuel cell; PD: photodetector. Noted that thePD is a functional device that delivers information by electrical signals, whichcannot be regarded as energy harvesting device on firm sense. In addition, wecannot find the integrated device based on BFC and solar cell in the pre-lit-erature research.

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2.2. LIB devices for energy storage

As a transducer that converts chemical energy into electrical energy,LIB has been widely applied in portable digital products since its suc-cessful commercialization in 1990s [116–122]. It consists of a positiveand a negative electrode separated by an electrolyte solution containingdissociated salts [123–126]. To meet the demand for high energydensity, low cost, and reliable safety, various cathode compound ma-terials with hierarchical structures have been reported, such as high-capacity layered transition-metal oxides [127–133], high-potential Mn-based spinels [134–139], and highly stable polyanion-type compounds[140–147]. Zhang et al. demonstrated LiNi0.915Co0.075Al0.01O2 (NCA)with Zr(OH)4 coating as cathode material for LIBs, and 0.50 wt% Zr(OH)4 coated NCA delivered a capacity of 197.6 mAh g−1 at the firstcycle and 154.3 mAh g−1 after 100 cycles with a capacity retention of78.1% at 1 C rate [133]. Zhang et al. reported one-dimensional porousnanostructures of LiNi0.5Mn1.5O4 with ordered P4332 phase cathodematerial for LIB, which delivered specific capacities of 140 and 109 mAh g−1 at 1 and 20 C rates, respectively, and, a capacity retention of 91%was sustained after 500 cycles at a 5 C cycling rate [138]. Wei et al.presented a design of hierarchical Li3V2(PO4)3/C mesoporous NWs ascathode materials in LIB, which showed outstanding high-rate and ul-tralong-life performance with capacity retention of 80.0% after 3000cycles at 5 C in 3-4.3 V, and even at 10 C, it still delivered 88.0% of itstheoretical capacity [146].

Anode materials can be divided into three categories depending onthe reaction mechanism that occurs in the charge/discharge process:insertion reaction [148–155], alloying reaction [156–164], and con-version reaction [165–172]. Most of them have higher capacity thancathode materials. Jo et al. reported Li4Ti5O12-coated Fe/Fe3O4 hybridas safe anode material to reduce the lithium-ion diffusion length andimprove strain tolerance during Li ion insertion/extraction withmaintaining 72% of the initial capacity after 700 cycles [152]. Li et al.investigated GeP5 as an anode material for LIBs, and GeP5/C exhibitedsuperior cycle stability and excellent high-rate performance with a ca-pacity of 2127mA h g−1 at 5 A g−1 [159]. Chen et al. demonstrated thehierarchical CNT/Co3O4 microtubes with high reversible capacity of1281 mA h g−1 at 0.1 A g−1 and long cycle life over 200 cycles as anodematerial for LIB [165]. The electrolyte, which commonly refers to asolution comprising the salts and solvents, constitutes the third keycomponent of a battery. Suitable non-aqueous electrolytes can beroughly divided into three groups: liquid, solid, and polymer [167].Sufficiently conductive polymer electrolytes can be obtained by trap-ping liquid solutions of organic electrolytes in polymer matrices such aspolyacrylonitrile (PAN) and poly (vinylidene fluoride) (PVDF). Jianget al. reported polymer electrolyte membranes comprised of PVDF,ethylene carbonate (EC), propylene carbonate (PC) and a Li salt (LiX =LiSO3CF3, LiPF6 or LiN(SO2F3)2) with anodic stability up to 4.0 V on Al,4.2 V on Ni and 4.5 V on stainless steel, as well as cathodic stabilitydown to 0 V on both Ni and stainless steel [173]. Although the materialswith hierarchical structure have made great progress in LIBs, there arestill some challenges. Novel preparation routes with fewer steps andlower cost should be developed to realize commercialization. In addi-tion, in-situ observation of changes in hierarchical structures during Li+

intercalation/deintercalation is expected to be reported to achievelarge-scale production and deeply reveal the interaction mechanisms[126].

Recently, high cost and scarcity of Li pose challenge for LIB large-scale application [174,175]. Other metal-ion batteries, such as Na (270mA h g−1) [176], K (340 mA h g−1) [177], Zn (352 mA h g−1) [178],and Mg (220 mA h g−1) [179], offer significant advantages over lithiumin terms of the natural abundance, large-scale distribution, easy pro-cessing, and nontoxicity.

2.3. SC devices for energy storage

In recent years, SCs have attracted significant attention, mainly dueto their high power density, and long lifecycle [180–185]. Further de-velopments have led to the recognition that the SC can play an im-portant role in complementing batteries or fuel cells in their energystorage functions by providing back-up power supplies to protectagainst power disruptions [186–190]. The SC, also called electro-chemical capacitors, stores energy using either ion adsorption (elec-trochemical double layer capacitors) or fast surface redox reactions(pseudo-capacitors), which consists of two electrodes, an electrolyte,and a separator that electrically isolates the two electrodes [191–193].A notable improvement in performance (such as overall voltage, en-ergy, and power densities) has been achieved through recent advancesin understanding charge storage mechanisms and the development ofadvanced nanostructured materials [194–205]. Pang et al. reported akind of heterogenous Co3O4-nanocube/Co(OH)2-nanosheet hybrid forapplications as flexible SC electrode, which offered a large capacitanceof 1164 F g−1 at 1.2 A g−1, a maximum energy density of 9.4mW hcm−3, and over 5000 cycles with 97.4% retention of its original specificcapacitance [200]. Yang et al. designed a solid-state flexible asym-metric SC with alpha-MnO2 NWs and amorphous Fe2O3 nanotubesgrown on flexible carbon fabric with an extended operating voltagewindow of 1.6 V and high energy density of 0.55mW h cm−3 [202].

The most important component in SC is the electrode materials.Large surface area, appropriate pore-size distribution, and high con-ductivity are essential properties of the electrode materials to achievelarge capacitance [206–218]. Sheberla et al. showed that conductiveporous MOF Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2)can serve as the sole electrode material in the electrochemical doublelayer capacitor with a very high surface area-normalized capacitance of18 μF cm−2, and capacity retention greater than 90% over 10000 cycles[214]. Yang et al. prepared ZnFe2O4 nanoparticles/active carbon fibercomposites electrode with a cotton derived active carbon fiber withhierarchical porous architecture as template, which exhibited highspecific capacitance of 192 F g−1 [217].

The discovery that ion desolvation occurs in pores smaller than thesolvated ions has led to higher capacitance for electrochemical doublelayer capacitors using carbon electrodes with subnanometre pores,which opened the door to design high-energy density devices using avariety of electrolytes [24,219–226]. El-Kady et al. demonstrated ascalable process of graphene micro-SCs over large areas by direct laserwriting on graphite oxide films using a standard Light Scribe DVDburner with a power density close to 200W cm−3 [222]. Advancedapproaches to increase the energy density are to hybridize the electrodematerials by adding electrochemically active materials and combiningpseudo-capacitive nanomaterials, including oxides, nitrides and poly-mers [227–235]. Sekhar et al. prepared core-shell-like architecturesconsisting of NiO nanosheet arrays grafted Co3O4eNiO fish thorns-likenanostructures for positive electrode in hybrid SCs. At a current densityof 2mA cm−2, the SC provided high areal capacitance of623.5 mF cm−2 with a maximum energy density of 216.1 μW h cm−2

and power density of 27.7mW cm−2 [231]. The use of carbon nano-tubes (CNTs) has further advanced micro-electrochemical capacitors,enabling flexible and adaptable devices to be made [236–241]. Salu-nkhe et al. reported an asymmetric SCs using coaxial CNT/Ni(OH)2composites as positive electrode and reduced graphene oxide as nega-tive electrode, whose operation voltage was expanded to 1.8 V inaqueous electrolyte and energy density of 35W h kg−1 at a powerdensity of 1.8 kW kg−1 [239]. Mathematical modeling and simulationwill be the key to succeed in designing high-energy and high-powerdevices, more details please consult previous literatures [242–244].

2.4. BFC devices for energy conversion

The BFCs have been paid considerable attention because they are

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recognized as a new kind of energy conversion technology that pos-sesses striking properties, such as operation in mild conditions andpotential to be used as in vivo power sources for bioelectronics in-cluding micropumps, pacemakers, and so forth [245–257]. Anothermain reason for attractions of BFC is that it can not only generateelectrical power but also consumes a wide range of organic wastes (e.g.corn husks, whey, or noxious waste such as animal or human sewage)[253,258–260]. Represented by microbial [261,262] and enzyme-based[263,264] bioelectrochemical devices, BFC aims at production ofelectrical energy through direct conversion of chemical energy. Theconversion is achieved by coupling an oxidation reaction supplyingelectrons at the anode with a reduction reaction utilizing electrons atthe cathode. These two reactions are electronically separated inside thesystem to force electrons to flow through an external circuit, while ionmovement inside the system maintains charge balance and completesthe electrical circuit [265–269]. Several strategies have been applied toenhance the power output, stability, and investigate the mechanisms ofbioelectrochemical processes involved in their operations [270–294].

Kim et al. reported a simple dispersion of intact multi-walled carbonnanotubes (MWCNTs) by adding them directly into an aqueous solutionof glucose oxidase, resulting in a 7.5-times higher power output pro-duced by the anode [278]. Plumere et al. showed that the integration ofan O2-sensitive hydrogenase into a specifically designed viologen-basedredox polymer protects the enzyme from O2 damage and high-potentialdeactivation, and the open-circuit voltage (Voc) of 947 ± 3mV, short-circuit current density of 323 ± 52 μA cm−2 and maximum powerdensity of 178 ± 19 μWcm−2 values were determined [288]. So et al.reported a one-compartment direct electron transfer type bioelec-trochemical devices and the maximum power density of 2.6 mW cm−2

was achieved [291]. Wu et al. showed the formation of electrostaticchanneling upon protein-protein association, which is the first struc-tural evidence of substrate channeling in the Krebs cycle metabolon byinvivo cross-linking and mass spectrometry [294]. Despite the BFCshave many advantages over traditional fuel cells, their practical ap-plications still require to overcome many difficult engineering pro-blems, particularly related to their short lifetime, poor power densitiesand energy efficiencies.

Aside from above-mentioned energy harvesting and storage devices,another important type of energy harvesting device is solar cell, in-cluding dye-sensitized solar cell and perovskite solar cell [295–302],which has already been reviewed in details in recent review paper [27].Here, we will not introduce solar cells any longer.

3. Integrated devices for energy harvesting and storage

3.1. LIB and SC integrated devices

LIBs and SCs are two mainstream energy storage devices widelyused in almost every appliance of daily life [303]. However, on onehand, the LIBs commonly can deliver high energy density(150–200W h kg−1) but at the expense of low power density and poorcycling stability; on the other hand, SCs can provide high power density(2–5 kW kg−1) and longer cycle life (over 1×105 cycles) while sufferfrom low energy density. To combine the advantages of both LIBs andSCs, the integrated system has emerged, which can be considered as oneof the promising candidates to bridge the gap between LIBs and SCs.

Usually, a LIB&SC integrated device consists of a capacitor-typecathode with fast charge/discharge capability, a LIB-type anode withlarge capacity, and a nonaqueous lithium containing electrolyte thatprovides wide working voltage window [304]. As presented in Fig. 2a,the discharge process is characterized by electric double-layer forma-tion on the activated carbon surface of the cathode. Hence, the charge isstored on the surface of activated carbon cathode; on the other hand,the charge process is characterized by the intercalation process in thehard carbon anode [305]. However, how to realize high-performanceLIB&SC with both high energy density and power density remains a

great challenge. For this end, many LIB&SC based on various carbonelectrode materials were explored.

Activated carbon, graphite, CNT, and graphene-based materialsshow higher effective specific surface area, better control of channels,and higher conductivity, which makes them better potential candidatesfor LIB&SC electrodes. In this case, Zheng et al. [306] used activatedcarbon anode and hard carbon/lithium to stabilize metal powercathode to assemble a LIB&SC integrated device, and achieved as highas the specific energy and energy density of 30W h kg−1 and 39W hL−1, respectively. In addition, the capacitor was cycled over 1× 104

cycles with capacitance degradation of less than 20%. Wu et al. [307]designed a LIB&SC based on Li3VO4/carbon nanofibers and electro-chemically-exfoliated graphene sheets, which exhibited an energydensity of 110W h kg−1 and good cycling performance. Ma et al. [308]employed Fe3O4/graphene composite anode and activated carboncathode to assemble soft-packaging LIB&SC integrated device with aenergy density of 120W h kg−1, a power density of 45.4 kW kg−1

(achieved at 60.5W h kg−1), and an excellent capacity retention up to94.1% after 1.0× 103 cycles and 81.4% after 1.0× 104 cycles. Dubal etal. [309] demonstrated a LIB&SC integrated device with BiVO4 as ananode and partially reduced graphene oxide (PRGO) as a cathodeelectrode, delivering energy density of 152W h kg−1 and a powerdensity of 9.6 kW kg−1. This integrated device exhibited good cy-clability of 81% over 6.0× 103 cycles.

Although these LIB&SC integrated devices with carbon-based elec-trode materials have achieved the higher energy density, the powerdensity has to be sacrificed at most cases. In pursuing higher energydensity with no sacrifice of power density, Xia et al. [244] applied Band N dual-doped carbon nanofibers as both capacitor-type cathode andbattery-type anode for LIB&SC, obtained a high energy density (220Wh kg−1 at 225W kg−1) and a high power density (2.25×104W kg−1 at104W h kg−1). Main reasons are that B and N dual doping results inincreased surface area, expanded interlayer distance, improved charge-storage active sites, and enhanced electrical conductivity of the carbonnanofiber networks. The proposed charge-storage mechanism is sche-matically shown in Fig. 2b [244]. During the charge process, PF6− ionsare absorbed at the porous surface with B and N functional groups andthe defects moieties for the cathode, whereas Li+ ions are either in-tercalated into the graphite layer or absorbed at the surface for theanode. The discharge process is the reverse of the charge process.

Considering traditional energy storage systems including SCs andLIBs typically appear in a rigid plate that is unfavorable for many ap-plications, especially in the fields of portable and highly integratedequipments, which require small size, light weight, and high flexibility,the integration of active materials on unusual current collectors/sub-strates enables electrodes and devices own new physical or chemicalfunctionalities, which cannot be achieved using common current col-lectors/substrates. In this regard, a flexible LIB&SC integrated devicewas designed either by directly using flexible free-standing active ma-terial films-based electrodes or growing active materials on flexiblesubstrates. In 2013, Peng et al. [310] proposed a LIB&SC integrateddevice based on wire-shaped micro-SCs and micro-batteries with highcapacitive performances by using aligned MWCNT fibers as electrodes.In this system, the energy and power densities in both SCs and batteriescould be further greatly improved by incorporation of MnO2 or otheroxides nanoparticles into MWCNT fibers. The integrated LIB&SC hashigh energy density compared to conventional electric double-layercapacitors, and high power density compared to batteries as well ashaving the long cycle life. In 2015, Liu et al. [311] designed a flexibleLIB&SC by using MWCNT network film and Li4Ti5O12 NW array, whichwere both grown directly on carbon cloth (Fig. 2c-d). The device ex-hibited 3 V output voltage and impressive cycling stability up to3×103 cycles. Maximum energy and power density of 4.38mWh cm−3

and 560mW cm−3 were obtained, respectively. The energy densityvalue was superior to those of previous thin-film SCs fabricated directlyon carbon cloth and even comparable to the commercial thin-film

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lithium battery; the power density also approached to that of thecommercial 5.5 V/100mF SC (can be charged within 3s; Fig. 2e).

In general, these combined features make the integrated LIB&SCexhibit a remarkable energy density, much higher than the commercialSCs, and can even compare with commercial LIBs with improved powerdensity. However, the output energy density and power density of theseLIB&SC integrated devices still maintained at a low level, and their

stability also needs to be further improved. It should be noted that theoperating voltage of a LIB is generally constant while the voltage of a SCmostly change during the charge and discharge process, which can adddifficulty when improving the efficiency of the whole device. Therefore,the revised strategy needs to be targeted towards circuit and device toachieve constant voltage charging and discharging for the SC unit.Moreover, the energy storage components are not limited to SC and LIB,

Fig. 2. (a) Schematic illustration of SC&LIBwith a battery intercalation type stabilizedlithium metal powder (SLMP) loaded hardcarbon anode and an electric double-layercapacitor type activated carbon cathode.Reproduced with permission from Ref.[305]. Copyright 2016, ACS. (b) Schematicillustration for charge-storage mechanism ofSC&LIB based on B and N dual-doped 3Dcarbon nanofibers. Reproduced with per-mission from Ref. [244]. Copyright 2017,Wiley-VCH. (c) Schematic illustration ofbinder-free SC&LIB structure. (d) SEMimage of Li4Ti5O12 NW array anode, theinset is digital photograph of the flexible LIB&SC. (e) Ragone plot of the device. Re-produced with permission from Ref. [311].Copyright 2015, Nature Publishing Group.

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and other exciting types of energy storage devices, such as sodium-ionbatteries, zinc–air batteries, etc., are heavily researched in the in-tegrated solar cell systems [27].

3.2. LIB and NG integrated devices

Considering the variable frequency and irregular amplitude of thepulsed AC output, the electricity generated by the NGs cannot be useddirectly to power most of the electronic devices. Therefore, an energystorage unit is needed to harvest the electricity generated by the NGsand supply a regulated output for the electronic devices. LIBs, as theconventional energy storage unit, are often used for the storage of en-ergy harvested by the NGs. Usually, the electricity generation and en-ergy storage are two separate parts, Xue et al. [312] hybridized thesetwo parts into one. In this work, the researchers replaced a conventionalPE separator with a separator with piezoelectric property. When excitedby the external stimulus, the electric field generated by the piezo-electric separator will contribute to providing the chemical energystorage for the LIB. During working, the integrated device as a wholewill suffer a dynamical external stress. Therefore, the devices shouldpossess a rather good mechanical robustness tolerance for the futureuse. The structure of hybrid LIB&NG power cell is depicted in Fig. 3.Compared with a conventional LIB, the polyethylene separator wasreplaced with a polarized PVDF film. When a compressive force wasapplied on the cell, the created piezoelectric potential by the PVDF filmwill drive the Li+ ions to migrate from the cathode to the anode untilthe two electrodes possess the same chemical potential, in which

process the LIB is charged. After the force is released, a portion of theLi+ ions will diffuse back to the cathode. When a compressive force wasexerted on the hybrid LIB&NG power cell at a frequency of 2.3 Hz, thevoltage of the cell increased from 327mV to 395mV within 4min.

Aiming at supplying energy for flexible devices, Pu et al. [313]developed a wearable self-charging power system by integrating aflexible LIB belt with a whole-textile TENG-cloth. In this work, the LIBand the TENG share a same T-shirt as the common substrate, and theyare connected via a bridge rectifier to regulate the TENG's output. Theenergy produced by the TENG can be stored in the LIB for powering thepersonal electronics. The TENG-cloth aims to harvest the mechanicalenergy during a man's daily activity such as arm swing, elbow bending.As for the LIB belt, the Ni-cloth was used as the flexible current col-lectors in both cathode and anode to form a pouch cell as shown inFig. 4a-d. The active materials can maintain well after completelyfolding the textile electrode (Fig. 4b-c), which demonstrated an in-timate adhesion between the active materials and the current collectors.The as-designed LIB has a discharge capacity of 81mA h g−1 normal-ized by the weight of cathode LiFePO4. After 30 times of 180° folding,the LIB belt possesses 80% capacity retention. As shown in Fig. 4e, belt-type Ni-cloth, and parylene-cloth were used as building blocks to woventhe TENG-cloth. The TENG can work either at a vertical contact-se-paration mode or horizontal sliding mode, therefore, the TENG-clothcan be worn at anywhere as long as there is a relative motion. Finally,the TENG-cloth was integrated with the LIB belt via a bridge rectifier toform the integrated LIB&NG device to supply power for a heartbeatmeter strap, as shown in Fig. 4f. When the TENG-cloth works in the

Fig. 3. (a) Schematic illustration of the integrated LIB&NG device. (b) Photograph of the integrated LIB&NG power cell on the bottom of a shoe. (c) Cross-sectionalSEM of integrated LIB&NG power cell. (d) Enlarged view of the TiO2 nanotubes. Reproduced with permission from Ref. [312]. Copyright 2012, ACS.

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contact separation mode at a frequency of 0.7 Hz, the charging time ofthe LIB belt for the first, second, and third cycle is 4, 9, and 14 h, re-spectively, and the corresponding discharge capacities are 1.3, 2.8, and4.4 mA h m−2, respectively.

The LIB and NG integrated devices can not only harvest the irre-gular mechanical energy in the human body and ambient, but alsoprovide the regulated and manageable output for most of the electronicdevices. Though great progress has been made, the charging time of theLIB is still too long for practical use. Improving the power density ofNGs and the energy-storage efficiency may be a future developmenttrend for the LIB and NG integrated devices. It should be noted thatthere is no control experiment about the combined devices and the twoseparate devices in above-mentioned two classical LIB&NG integratedsystems. Therefore, it is difficult to compare the output or efficiencybetween the hybrid and the separate devices. In Ref. [313], the re-searchers did not hybrid the NG and LIB into one unit, these two deviceswere integrated at a level that they share a common T-shirt. The NG andLIB are separate devices and connected via a bridge rectifier.

3.3. BFC and NG integrated devices

With the aim of building self-powered nanodevices/nanosystems forin vivo biomedical and other applications, in 2010, Hansen et al. [314]integrated a BFC and NG to form an integrated BFC&NG device forharvesting both biofuel energy and mechanical energy, as shown inFig. 5. In this work, the separate BFC and NG units were connected via aRC high filter in parallel, which aims to increase the integrated devices’lifetime and output. The biofuel energy and mechanical energy wereharvested by the BFC and NG, separately. The integrated device was

designed to convert the biochemical energy in biofluid (via BFC), me-chanical energy during breathing or from the heart beating (via NG)into electricity. As for the PENG, electrospun PVDF nanofibers werealigned across the patterned Au electrodes, then a layer of poly-dimethylsiloxane (PDMS) was deposited on it for dielectric protectionand biocompatibility, followed by poling the device on the paraffin oil.Under cyclic mechanical loading, Voc of 20mV and short circuit currentof 0.3 nA were generated. As for the BFC, MWCNTs were coated on thepatterned electrode, then glucose oxidase and laccase were immobilizedon the MWCNT coated electrodes to form the anode and cathode of theBFC, respectively. Besides used for anchoring the enzymes on theelectrodes, the MWCNTs can also facilitate the electron transfer be-tween the enzymes and the electrodes. When the device is in contactwith fluid that contains glucose, the glucose is electro-oxidized to glu-conolactone at the anode, and the released electron will go around theexternal circuit to arrive the cathode to reduce dissolved O2 to water atthe cathode. By placing the BFC into a phosphate buffer solution (PBS,pH=7.0) with glucose of 5mM, Voc of 50mV and short circuit currentof 11 nA were generated (Fig. 5a). The PENG was connected with BFCvia a RC high pass filter (Fig. 5b-c) which can effectively block thedirect current (DC) voltage generated by the BFC in one direction andpassing the alternative current (AC) voltage generated by PENG. Afterthe integration, the peak voltage in integrated BFC&NG system wasimproved from ∼50mV to ∼95mV. The average peak output power ofthe integrated device is equal to the sum of two separated devices. Thebiofuel energy and mechanical energy are harvested by the BFC andNG, separately. In order to form a compact structure, these two cells canbe conceptually integrated back-to-back (Fig. 5d). Indeed, authors didnot maximize the output of BFC to clearly illustrate the integration of

Fig. 4. (a) Photograph of the Ni-cloth substrate, LiFePO4-coated cathode, and Li4Ti5O12-coated anode. (b) Photograph of a bended anode. (c) SEM image of an anodefilm after a complete folding. (d) Photograph of a fabricated LIB belt. (e) Schematic illustration of fabricating a TENG-cloth. (f) The equivalent circuit and photographof the integrated LIB&NG self-charging power system. Reproduced with permission from Ref. [313]. Copyright 2015, Wiley-VCH.

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the BFC and NG.In order to realize a more compact integration and reduce the in-

tegrated cell's volume, Pan et al. [315] developed a fiber based in-tegrated cell as shown in Fig. 6. In this research, a carbon fiber was usedas the common substrate for the PENG and BFC to realize a morecompact integration. The biofuel energy and mechanical energy are

harvested by the individual components BFC and NG, separately, in theintegrated device. Placing the integrated device in a biofluid and ex-citing the common fiber, the biofuel and mechanical energy can beharvested simultaneously. As for the PENG, a densely ZnO textured filmwas grown on the carbon fiber as working component. The carbon fiberwas used both as substrate and core electrode. Then, by etching the ZnO

Fig. 5. (a) Open circuit voltage (Voc) of the BFC, PENG, and hybrid BFC&NG device. (b) Circuit diagram used for integrated BFC&NG device. (c) Schematic ofintegrated BFC&NG device. (d) Conceptual design of an integrated BFC&NG device. Reproduced with permission from Ref. [314]. Copyright 2010, ACS.

Fig. 6. (a) Schematic 3D representation of theintegrated BFC&NG device. The inserts in theupper left are SEM images of a textured ZnO NWfilm grown around a carbon fiber. An opticalphotograph of the device is shown at the lowercorner. (b) Working principle of the fiber NG. (c)I–V characteristic of the fiber NG. Reproducedwith permission from Ref. [315]. Copyright2011, Wiley-VCH.

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NW films at one end of the fiber to expose the carbon fiber as coreelectrode and contacting the top surface of ZnO NW film using silverpaste and tape as top electrode. As for the BFC, a layer of soft epoxypolymer was coated on the carbon fiber as an insulator at the other endof the carbon fiber, two Au electrodes were then patterned onto it andcoated with CNTs. Finally, glucose oxidase (GOx) and laccase wereimmobilized on the Au electrodes to form the anode and cathode, re-spectively. Because the separator membrane and mediator have beeneliminated, the size of the BFC has reduced a lot. Immersing the BFCinto a bio-liquid, the BFC part will generate an output voltage of100mV and output current of 100 nA. When a pressure was cyclicallyexerted on the bio-liquid, the PENG can output a Voc of 3.0 V, and ashort-circuit current of 200 nA (Fig. 6b-c). Interestingly, after in-tegrating the AC NG and DC BFC together, the output of the hybrid BFC&NG cell is close to the sum of PENG and BFC. A fiber NG and a fiberBFC are designed onto a carbon fiber, which is an important finding inintegrated device fields.

3.4. PD and BFC integrated devices

Considering environmental monitoring and medical therapy treat-ment, weak light detection is highly desired. In this regard, an in-tegrated PD&BFC system of a self-powered ultrasensitive PD driven by a

high-efficiency microscale BFC was designed by using a carbon fiber-ZnO NW hybridized structure [316]. In this BFC shown in Fig. 7a, thecarbon fiber acted as the substrate onto which the about 250 nm-longZnO NWs were firstly grown by a physical vapor deposition method. Athin layer of Nafion was subsequently spin-coated on the fiber-NWhybrid structure as the anion exchange membrane. A solution micro-pool was then constructed by squeezing poly (methyl methacrylate)(PMMA) polymer solution onto the Nafion film in a ring dam shape.Yeast and glucose were mixed in solution and finally dripped into themicropool. The BFC was connected to the outer circuit by two elec-trodes. The principle of the BFC is also shown in Fig. 7a. Glucose wasdecomposed by yeast in the micropool to form CO2 and electron-protonpairs. The produced electrons transfer from the anode to the cathodethrough an external circuit whereas the produced protons or phosphateanions migrate from anode to the cathode through the separatormembrane. At the cathode, H+ and electron recombine with an oxygenmolecule to form H2O. A porous Nafion anion exchange membraneallows various charged or neutral species to pass through efficiently,and the permeation of the oxygen adsorbed and immobilized on theZnO NW surface through the porous membrane is reduced, which resultin a higher output in the integrated device.

Once a drop of yeast solution was dripped into the micropool, theBFC started to generate a DC output, as shown in Fig. 7b-c. The short-

Fig. 7. (a) Schematic principle of a BFC basedon carbon fiber and ZnO NW hybrid structure.(b) Circuit diagram used for integrated PD&BFCsystem based on a BFC and a PD. EVS is externalvoltage source; blue is BFC. The diodes at thetwo ends of the device stand for the localSchottky contacts between CdS and metal elec-trodes. (c) Open-circuit voltage (Voc, green) andshort-circuit current (Isc, red) of the PD&BFCintegrated device and bare carbon fiber basedBFC. (d) Schematic illustration of a self-poweredintegrated PD&BFC device consisting of a BFCand a PD, E stands for electrode. MFC: microbialfuel cell or BFC. Reproduced with permissionfrom Ref. [316]. Copyright 2012, Wiley-VCH.

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circuit current (Isc) of about 56 nA, the Voc of about 295mV, and thecorresponding power density of about 30Wm−2 of electrode geometryarea are achieved. The Voc of about 295mV in the BFC was high enoughto drive a CdS NW ultrasensitive PD with a responsivity of more than300 AW−1. The noise-equivalent power of the self-powered PD fordetection of UV, blue, and green light at 10 Hz is about 5.0×10−18,1.5× 10−17, and 2.8× 10−17WHz−1/2, respectively. This indicatesthe self-powered integrated nanosystem based on ZnO NWs has ex-tremely high sensitivity that provides more opportunities for self-powered detectors. To fully understand this type of PD&BFC integrateddevice, a schematic illustration of the self-powered integrated nano-system consisting of a single-fiber NW hybrid-structured BFC with asingle CdS naowire PD in series is also shown in Fig. 7d. This self-powered PD&BFC integrated system may be expected to find moreextensive application fields, such as medical therapy, environmentalmonitoring, defense technology, and personal electronics.

3.5. SC and PD and Si solar cell integrated devices

Integrated devices with the capability of storing different types ofenergies have been investigated extensively. Recently, an increasinginterest for integrated photodetecting system based on SCs and PDs hasattracted considerable attention [317–320]. An all-solid-state asym-metric SC can simultaneously realize energy storage and optoelectronicdetection by combining a SC and a PD on a single fiber [317], in whichthe Co3O4 NWs grown on nickel fibers act as the positive electrodewhereas the 2D graphene layers serve as both the negative electrodeand light-sensitive material. The as-assembled integrated SC&PDsystem exhibits an increased potential window from 0.6 V to 1.5 V,enhanced stored energy and delivered power by 1860% as comparedwith that of the SC with a potential window of 0–0.6 V. In addition, thisself-powered integrated SC&PD system shows excellent flexibility thatis expected to be used in smart, wearable and portable electronics[317].

To optimize maximum functionality within a minimized sized chip,a flexible on-chip micro-SC based on graphene-based hybrid nanos-tructure has been designed by using a microelectronic photo-litho-graphy technology combined with plasma etching technique. As-fabri-cated flexible on-chip micro-SC can present superior flexibility andstability even after repetition of charge/discharge cycles under differentbending statuses. The designed flexible multi-functional nano/micro-systems with integrated energy units and functional detecting units on asingle chip exhibit comparable self-powered working performance toconventional devices driven by external energy storage units, which arepromising for the highly stable integrated applications in miniaturizedportable electronic devices [319,320].

To further demonstrate potential applications of these integratedsystems in the future sustainable self-sufficient sensor networks. Wanget al. [318] used a silicon solar cell, a fiber SC and an ultraviolet (UV)PD to integrate a self-powered SC&PD photodetecting system, as illu-strated in Fig. 8a. The solar cells generated a voltage of approximately0.7 V under the illumination of a household fluorescent lamp, andcharged for fiber SCs connected in parallel to about 0.5 V. This in-tegrated SC&solar cells energy harvesting and storage device can pro-vide a stable 0.3 V bias for the PD based on TiO2 NWs. The currentincreases nonlinearly in both forward and reverse bias directions in thedark and under light, indicating a Schottky contact between Au andTiO2 NWs (Fig. 8b). The intensity of UV light can be quantified by usingthe photocurrent resulted from the charged fiber SCs by the conven-tional solar cell. The photocurrent (Iph) follows a fitted power-law re-lationship (Fig. 8c). The photocurrent production can maintain a stablerepeatability, and the voltage periodically decreased slightly from0.305 V to 0.293 V, then recovered to 0.305 V, by switching the “ON”state to the “OFF” state in periodically turning the UV light on and offfor more than 1 h (Fig. 8d). This integrated system (solar cell&SC&PD)based on the solar cell charged fiber SCs as power source can provide

steady and adequate energy for the PD. The development of the in-tegrated system (based on solar cells, SC and PD) faces immense chal-lenges ahead for high specific capacitance, superior flexibility and ex-cellent conductivity.

3.6. SC and solar cell integrated devices

The integrated devices based on DSSCs and PSCs (such as tandemdevices and dye-sensitized photoelectrosynthesis cells) for energy har-vesting and conversion have been reviewed in several review papers[27,321–326]. In this section, we will put much attention on the self-charging power packs, integrating both SCs and solar cells into a singledevice, and several important integrated systems are highlighted. Theschematic illustration of integrated devices based on SC and differentsolar cells is depicted in Fig. 9.

3.6.1. SC and DSSC integrated devicesTo store generated electrical power from photovoltaic devices,

Miyasaka et al. [327] designed an efficient self-charging capacitor fordirect storage of solar energy into a single cell. The SC&DSSC integrateddevice consists of a Ru complex dye-sensitized TiO2 film, a LiI hole-trapping layer, and an activated carbon counter electrode in contactwith an (CH3CH2)4NBF4/propylene carbonate electrolyte solution, inwhich photogenerated charges are stored at the electric double layer. Itachieved a capacitance of 0.69 F cm−2 when repeated charge-dischargecycles with a charging voltage of 0.45 V. Despite the obtained 0.45 V ismuch behind the theoretical value (1.5 V), optimization of the hole-transferring material at the dye/activated carbon interfaces promisesfurther enhancement of the photovoltage. For developing a highly ef-ficient, lightweight, flexible, and portable energy package, Zou et al.[328] proposed a fiber-shaped SC&DSSC integrated device (Fig. 10a).The electrode of fiber DSSC (FDSSC) and fiber SC (FSC) both using astainless steel wire coated with polyaniline via anodic deposition. Thisintegrated device exhibited an overall energy conversion efficiency of2.1%. Lou et al. [329] designed a flexible printable SC&DSSC integratedenergy device with charging potential reaching up to 1.8 V, and itshowed stable performances under various extreme mechanical loadingconditions in outdoor testing. Compared with traditional integratedpower systems, these flexible integrated devices are capable of in-tegrating with cloth. For more details on DSSCs, please consult ourrecent review papers [3,27,330–333].

3.6.2. SC and PSC integrated devicesPeng et al. [338] developed an all-solid-state and flexible SC&PSC

“energy fiber” that has efficiently integrated the functions of photo-voltaic conversion and energy storage. The photovoltaic conversionpart and the energy storage part were adopted “energy fiber” coaxialstructure, which contributes to the charge transport rapidly. Whenthese two parts work independently, which get highest PCE of 1.01%,specific capacitance in length of 0.077 mF cm−1, and specific capaci-tance in area of 1.61× 10−7W h cm−2, respectively. The highestoverall efficiency of 0.82% is achieved as the integrated device works.To meet dramatically increased energy demand, Lee et al. [334] de-signed a compact and monolithically stacked SC&PSC integrated deviceby combining a polyvinyl alcohol (PVA)/phosphoric acid (H3PO4)-based solid-state SC and a polymer-based solar cell, which exhibited avery high storage efficiency (ηstorage) and overall efficiency (ηoverall) of64.59% and 5.07%, respectively. Fig. 10b sketched a schematic of thefabrication process and device architecture of the SC&PSC devicecombining the solid-state electrochemical capacitor and the large-areaphotovoltaic cell. Notably, the silver epoxy played a crucial role in thesuccessful integration of the SC and the PSC due to its strong adhesionand high electric conductivity. Lechêne and Arias et al. [339] combinedorganic solar cells with fully printed SCs to form an integrated system.Under simulated indoor light, the system yields a total energy conver-sion and storage efficiency (ECSE) of 2.9%. Those energy and power

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levels would be sufficient to power low-consumption electronic deviceswith low duty cycles.

3.6.3. SC and QDSC integrated devicesFor energy conversion, quantum dot solar cells (QDSC) have several

attributes such as size dependent electronic structure, good environ-mental stability, low cost and solution processability. Recently,Narayanan and Deepa et al. [335] demonstrated a novel SC&QDSCintegrated device by combining a plasmonic QDSC and a carbon-basedSC (Fig. 10c-d), in which MWCNTs were employed as a counter elec-trode for the QDSC and also as one of the electrodes for a symmetricalSC. Au fibers are integrated into a TiO2/CdS electrode to form a TiO2/CdS/Au photoanode. The photocurrent generated by the plasmonicQDSC part was efficiently directed to the SC part, and yielded a capa-citance of 150 F g−1 without the aid of any external electrical pulse.The innovative design offers a new paradigm for combining low costphotovoltaics with energy storage.

3.6.4. SC and PeSC integrated devicesWang et al. [340] developed a power pack combining a

CH3NH3PbI3 perovskite solar cell (PeSC) with a polypyrrole SC thatconnected in series through external wires, which can be used for solarenergy conversion and storage purposes. The SC&PeSC device attainedan output voltage of 1.45 V under AM 1.5G illumination, and theoverall output efficiency of 20% with an output voltage of 1.46 V whenthe voltage of ploypyrrole-based SC was set at 0.6 V. These results de-monstrate that the integrated self-powered system has great potentialfor its application in electric vehicles and mobile telephones. Fan et al.[336] designed an integrated device by using poly (3,4-ethylenediox-ythiophene) (PEDOT)-carbon electrode that bridges the SC part and theprintable PeSC part. As shown in Fig. 10e, the photogenerated electronsand holes from the excited perovskite were transferred and electro-chemically stored in the PEDOT-carbon cathode and anode. This SC&PeSC device exhibited a maximum overall efficiency up to 4.70% with ahigh energy storage efficiency of 73.77%. Impressively, Lee et al. [334]designed an integrated devices combined with polyvinyl alcohol (PVA)/H3PO4-based solid-state SC and PeSC, which exhibited a very high

Fig. 8. (a) Schematic representation of an integrated system based on the solar cell, fibered SC and PD. (b) Typical photocurrent-voltage characteristics of the TiO2

NW PD in the dark and under light. (c) Photocurrent (Iph) of the PD as a function of the incident light intensity at 0.305 V bias. (d) A transient current response of thePD and the corresponding bias change during the testing. Reproduced with permission from Ref. [318]. Copyright 2016, Elsevier.

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storage efficiency and overall efficiency of 80.31% and 10.97%, re-spectively.

3.6.5. SC and Si solar cell integrated devicesIn light of high cost of thin-film silicon solar, Gu et al. [337] de-

monstrated an on-chip concept of the SC integrated with silicon solarcells using a laser scribed graphene oxide film in an area of 3×3 cm2,which can lead to the miniaturization in size and in cost of optoelec-tronic devices. As presented in Fig. 10f, the integrated device achieveda coulombic efficiency of 62%, which is directly written on the reverseside of solar cell without any loss in the solar cell performance. Theenergy density (5 μW h cm−3) and power density (4.6 W cm−2) of theintegrated devices are comparable to those of electrolytic capacitorseven after a number of charging-discharging measurements. In addi-tion, Westover and Pint et al. [341] integrated a SC into the backside ofa silicon solar cell, yielding a coulombic efficiency of 84%. These in-tegrated devices demonstrated that the photovoltaic device can chargethe SC under an external load, and a constant current load can bemaintained through periods of intermittent illumination.

3.6.6. SC and hybrid solar cell integrated devicesCompared with the intensive research on improving the PCE of

different solar cells or energy storage devices, integrated systemscombing energy conversion and storage functions are still far to be wellinvestigated. In 2017, Wang et al. [234] developed an integrated deviceby combining a hybrid silicon NW/polymer heterojunction solar cellwith a polypyrrole-based SC (Fig. 10g). The integrated system yielded atotal photoelectric conversion to storage efficiency of 10.5%. In addi-tion, an integrated device based on ultrathin Si substrate is demon-strated to expand its feasibility and potential application in flexibleenergy conversion and storage devices. Pei et al. [342] designed a newintegrated device consisting of a silicon NW array/poly (3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) hybrid solar celland a laser-scribed graphene (LSG) SC. The gold film serves as theshared common electrode, combining a solar energy harvesting unit onthe top and a SC energy storage unit at the bottom. The hybrid solar cellexhibited a power conversion efficiency (PCE) of 12.37%. In addition,the LSG demonstrated excellent energy storage capability for the powerpack, with the specific capacitance of 350.66 F g−1, and the overallefficiency of the power unit is 2.92%. This integrated system is veryuseful for versatile and practical applications.

Table 1 summarized working parameters of integrated device basedon SC and solar cells. The highest overall total efficiency of 10.97% is

achieved in the reported integrated SC&PeSC system, which can becontributed to the relatively high efficiency of the solar energy cap-turing part (16.1%). The higher PCE (13.39%) of hybrid solar cellscauses the integrated devices based on SC and hybrid solar cells to gethigher overall efficiency of 10.5%. By contrast, the SC&DSSC deviceachieves overall total efficiency of 0.82–5.07% whereas the SC&PSCdevice obtains overall total efficiency of 1.57%. Therefore, it is an ef-fective way to improve the overall efficiency of integrated devices byimproving the PCEs of solar cells. Actually, the overall efficiencies ofintegrated devices are lower than the PCEs of solar cells in Table 1, itmay be limited by the capacity of SC and loss at the series resistance.

In terms of above mentioned integrated systems based on SCs andsolar cells, future research on the integrated devices based on SC andsolar cells should be concentrated on improving the performance of theintegrated units and optimizing the device architecture, materials de-sign, compatibility, stability, process cost, scalable manufacturingprocess, encapsulation, etc, which are highly desirable for integrateddevices based on solar cells to increase PCEs beyond current levels toreal applications [27]. It should be noted that these integrated systemsfor energy storage based on DSSC or PeSCs have a high potential forefficient and direct storage of solar energy due to their exceptional low-light performance which enables their use for indoor applications, andtunability of color and transparency which allows for the tailoring ofthe integrated device design to the customers’ individual demand.

3.7. SC and DSSC and NG integrated devices

Many self-powered integrated devices capture only limited energyin the environment, therefore, it is essential to develop an integrateddevice that can simultaneously utilize multi-forms of energy within theenvironment, such as solar energy and mechanical energy. Wang et al.[353] proposed a kind of fiber-based integrated NG&SC&DSSC device,as shown in Fig. 11. Three kinds of devices (NG, SC, and DSSC) wereintegrated into one micro-size fiber. In this research, two power unitswhich can harvest mechanical energy and solar energy separately, andone energy storage unit are integrated compactly into one micro-sizefiber. This is the first prototype of integrating hybrid power units andenergy storage unit in a single micro-size fiber. The SC in the fiber canbe charged by either exposing the fiber to sunlight or exciting the fiberwith external stimulus. Notably, NG is the first device built on the fiberwhich converts mechanical energy into electricity with the use of pie-zoelectric property of ZnO. The second device fabricated around thefiber is a DSSC with the use of high quality graphene with goodtransparency. The SC part consists of two electrodes, which employedZnO NWs grown on the plastic wire and graphene grown on Cu mesh,with polymer gel electrolyte (PVA/H3PO4) filled between. Furthermore,the vertically grown ZnO NWs are the active units for the NG thatharvests mechanical energy and the core of DSSC as well as SC withlarge surface area. Graphene is used as the cylindrical electrodes forNG, DSSC, and SC. The as-prepared graphene film with fine smoothstructure is of high quality without critical defects (Fig. 11a), and thedistinct G- and 2D-peaks are observed in the Raman spectrum (Fig. 11b)for the graphene film at approximately 1600 cm−1 and 2700 cm−1,respectively. A plastic wire covered with ZnO NWs can be seen clearlyin Fig. 11c. Accordingly, the NG in integrated NG&SC&DSSC devicedelivered maximum output current of ∼2 nA, maximum output voltageof 7mV, the DSSC in integrated NG&SC&DSSC device achieves a short-circuit current density of 0.35mA cm−2, Voc of 0.17 V, fill factor of0.39, and PCE of 0.02%, and the SC in integrated NG&SC&DSSC devicedemonstrated an area specific capacitance of 0.4 mF cm−2. Im-pressively, there is no output characterization about the integrated NG&SC&DSSC devices. Although the performance of integrated device isrelatively low (a specific capacitance of 0.4 mF cm−2), this architectureof integrated NG&SC&DSSC device open a new avenue for designingflexible and stretchable fiber-based electronic circuits or power-shirts.Very recently, Xu et al. [108] developed a new kind of all-textile energy

Fig. 9. Schematic illustration of integrated devices based on SC and solar cells(SC&DSSC, SC&PSC, SC&QDSC, SC&PeSC, SC&Si solar cell, and SC&hybridsolar cell). DSSC: dye-sensitized solar cell; QDSC: quantum dot-sensitized solarcell; PeSC: perovskite solar cell; PSC: polymer/organic solar cell.

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harvesters (A-TEHs) with a 3D fabric structural integrity that can har-vest and convert mechanical energy into electricity directly, and the A-TEHs was used to connect with a 200 μF commercial capacitor with abridge rectifier, its charging capacity reached to 11.3 mV−1 at 1400 N.Benefiting from the full structural integrity of fiber materials, the ex-cellent textile properties, that all wearable electronics try to achieve,were truly realized. It should be noted that several classical works onthe integrated NG&DSSC reported by Professor of Zhong-Lin Wang havebeen reviewed in two recent papers [27,325].

3.8. LIB and solar cell integrated devices

LIB and solar cells are superior energy storage and conversion de-vices, but they always have been used as an independent device. Inorder to simultaneously accomplish energy conversion and storage, apossible way is to integrate LIB and solar cells into a single device. Inthis regard, Wang et al. [354] fabricated a single integrated power packof DSSC and LIB, as shown in Fig. 12a. This LIB&DSSC integrated deviceis composed of two parts, the upper part is the series-wound DSSC,whereas lower part is the TiO2-based LIB. LIB and DSSC used a commonTi foil substrate on which double-sided TiO2 nanotubes (TNTs) arrayswere grown. Top-side arrays act as photoanode of top DSSC and

Fig. 10. (a) Schematic illustration and photograph of an integrated SC&DSSC consisting of FDSSC and FSC. Reproduced with permission from Ref. [328]. Copyright2013, RSC. (b) Device architecture of monolithically stacked SC&PSC integrated device. Reproduced with permission from Ref. [334]. Copyright 2016 RSC. (c)Energy band diagram of QDSC and (d) schematic illustration of integrated SC&QDSC. Reproduced with permission from Ref. [335]. Copyright 2012, Elsevier. (e)Schematic illustration and work mechanism of SC&PeSC device. Reproduced with permission from Ref. [336]. Copyright 2016, Wiley-VCH. (f) Schematic illustrationof the solar-charging in integrated device with c-Si solar cell. Reproduced with permission from Ref. [337]. Copyright 2015, AIP. (g) Schematic illustration of flexibleintegrated device containing SiNW-based heterojunction hybrid solar cell and polypyrrole-based supercapacitor. Reproduced with permission from Ref. [234].Copyright 2017, RSC.

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Table 1Comparison of reported integrated devices based on SC and solar cells for energy conversion and storage.

Devices PCE Power density Energy density Capacitance Columbic efficiency Overall efficiency Ref.

DSSC part 4.56% —— —— —— —— —— [343]SC part —— 4.2mW cm−2 0.95 μW h cm−3 19mF cm−2 —— —— [343]SC&DSSC —— —— —— —— —— 2.1% [343]PSC part 9.00% —— —— —— —— —— [344]SC part —— —— —— 155 F g−1 —— —— [344]SC&DSSC —— —— —— —— —— 5.07% [344]PSC part 1.01% … … … … … [345]SC part … … … 0.077mF cm−1 … … [345]SC&DSSC … … … … … 0.82% [345]PSC part 6.2% … … … … … [339]SC part … … … 130mF cm−2 … … [339]SC&PSC … … … … … 1.57% [339]PeSC part 13.6% … … … … … [346]SC part … … … 572mF cm−2 … … [346]SC&PeSC … … … … 10% [346]PeSC part 0.46% … … … … … [347]SC part … … … 150mF cm−2 … … [347]SC&PeSC … … … 30mF cm−2 … … [347]PeSC part 6.37% … … … … … [348]SC part … 7.4mW cm−2 0.783 μW h cm−3 … … … [348]SC&PeSC … … … … … 4.70% [348]PeSC part 16.1% … … … … … [344]SC part … … … 150 F g−1 … … [344]SC&PeSC … … … … … 10.97% [344]Si solar cell 14.9% … … … … … [349]SC part … 4.6 W cm−2 5 μW h cm−3 … … … [349]SC&Si solar cells … … … … 62% … [349]Si solar cell 14.8% … … … … … [350]SC&Si solar cells … … … 0.14 F m−2 84% … [350]hybrid cell 13.39% … … … … … [351]SC par … … … 234mF cm−2 … … [351]SC&hybrid cell … … … … … 10.5% [351]hybrid cell 12.37% … … … … … [352]SC part … … … 16.37mF cm−2 … … [352]SC&hybrid cell … … … … … 2.92% [352]

Fig. 11. Schematic illustration of a fiber-basedintegrated NG&SC&DSSC device. (a) SEM imageof a graphene film with fine smooth structure.(b) Raman spectra of the graphene film. (c) SEMimage of Au-coated plastic wire covered withZnO NW arrays. The inset is SEM image of ZnONW arrays grown along the radial direction ofthe plastic wire. Reproduced with permissionfrom Ref. [353]. Copyright 2011, Wiley.

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another side acts as anode of bottom LIB, respectively. The semi-transparent Pt is used as counter electrode of DSSCs, N719 and N749are used as sensitizers to realize co-sensitization. For the LIB part, theLiCoO2/conductive carbon/binder mixture on an aluminum foil wasused as the cathode, and polyethylene was adopted to separate thecathode and the anode. Under sunlight irradiation, the photoanode ofDSSCs generates electrons and injects into the anode of LIB, while thegenerated holes accumulate at the Pt electrode. Meanwhile, when theelectrons flow to the LIB and realize charging process, cathode willrelease electrons and flow to the DSSC via an external circuit to com-bine with the holes in the Pt electrode. In this LIB&DSSC integrateddevice, the voltage is increased to 2.996 V in less than 8min, and thedischarge capacity is about 38.89 μA h under a discharge current den-sity of 100 μA. The total energy and storage efficiency of the system isup to 0.82%.

Indeed, as for this LIB&DSSC integrated device with “back-to-back”structure, an important issue is its instability due to the poor adhesionof the electrode thin films to the substrate that limits practical appli-cations. Frankly, some integrated devices with the “back-to-back”planar structure face same problem, that is, electrode thin films crack orpeel off from the substrate, resulting in poor adhesion of the electrodefilms to the substrate. An effective method has been proposed to solvethis issue in previous literature [355–358].

Aiming at supplying energy for flexible electronics and wearable

applications, Peng et al. [359] designed an integrated device with core-sheath structure via integrating solar cells and LIB into a flexible fiber,as shown in Fig. 12b. In this LIB&DSSC integrated device, LIB acted ascore structure while DSSC served as sheath structure. For the LIB part,LiMn2O4 (LMO) and Li4Ti5O12 (LTO) nanoparticles were incorporatedaligned MWCNT fiber as positive electrode (MWCNT/LMO) and nega-tive electrode (MWCNT/LTO), respectively. The MWCNT wrapped onthe wire shaped LIB part was cut into segments that act as counterelectrode of DSSCs. TiO2 nanotube arrays grown on the surface of thespring-shaped titanium wire were employed as DSSC photoanode thatwas sensitized by the N719 dye. The photoanode and CE of the DSSCpart was connected with the MWCNT/LTO and MWCNT/LMO elec-trodes of the LIB part, respectively. LIB and DSSC were separated byrubber fiber and incorporated a switch to realize the photocharging anddischarging process. Under illumination, the switch will turn on, andthe photoanode of DSSCs will generate electrons that inject into the LIBto realize charging process. During the discharging process, the switchwill turn off and LIB begins to discharge to the external circuit. Thisdevice not only obtains lightweight, flexible and wearable advantagesbut also achieve both high photovoltage of 5.12 V and high outputvoltage of 2.6 V. In general, two examples of integrated LIB&DSSC de-vices provide a wonderful model for guiding the design and develop-ment of future energy harvesting and storage devices. For optimizingLIB structure, selecting appropriate electrode materials can improve

Fig. 12. Schematic illustration of an integrated LIB&DSSC device. (a) An integrated LIB&DSSC based on double-sided TiO2 nanotube arrays. Reproduced withpermission from Ref. [354]. Copyright 2012, ACS. (b) An integrated LIB&DSSC based on a flexible fiber. Reproduced with permission from Ref. [359]. Copyright2016, RSC.

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battery performance. In this respect, Xu and Mai et al. [172] summar-ized recent advance in structure and property optimizations of batteryelectrode materials, and provided the deep understanding of structure-property correlations that will bring new insight into the design of idealbattery materials.

4. Summary and future outlook

In summary, we have reviewed the recent advances in the new-generation integrated energy harvesting and storage devices. Eighttypes of integrated devices, such as LIB&SC, LIB&NG, BFC&NG, PD&BFC, SC&PD, SC&solar cells, NG&SC&solar cell, and LIB&solar cells,have been highlighted. In addition, we reviewed the structure-activityrelationship between the device performance and structure composi-tions, and revealed how the nanomaterials and nanotechnologies pro-mote the isolated/integrated devices’ performance, which might beuseful for designing energy harvesters, energy storage devices, and self-powered systems.

Compared with isolated devices, the new-generation integrateddevices for energy harvesting and storage possess several advantages.(i) A variety of energy harvesting or/and storage devices integrated intoone self-powered system can significantly narrow the devices’ size andweight, in which partial components commonly play a dual role in self-powered pack. Based on this feature, high performance self-poweredintegrated devices can be designed by using multi-functional nanoma-terials, such as graphene, CNT, carbon fiber, and activated carbon. (ii)The self-powered integrated devices effectively avoid the disadvantagesof isolated devices for energy harvesting or/and storage. For instance,self-powered integrated devices can simultaneously achieve high en-ergy density and high power density. In addition, the self-powered in-tegrated devices have much more important advantages than the iso-lated devices, such as lower series resistance, more compact packaging,lower cost of materials when using a common electrode, etc. (iii) Theself-powered integrated devices can provide high reliability and im-proved stability in contrast to those isolated devices. The integrateddevices, such as LIB&SC, LIB&NG, BFC&NG, SC&PD, PD&BFC, SC&solarcells, and LIB&DSSC, have exhibited potential application advantages.

Although the emerging integrated energy harvesting and storagedevices provide opportunities for utilizing clean energy efficiently,some problems still need to be solved. (i) The energy-densities orpower-densities of the integrated devices are generally not high, andtheir performance will deteriorate with the increasing operation time.In addition, the lifecycle cost of the energy storage system is still stayingin a high position. With respect to the use of integrated devices in futureenergy storage systems, further investigations should focus on an in-creased fundamental understanding of not only the discovered in-tegrated devices behavior, but also the interactions between the in-tegrated devices and other components in these hybrid systems. For theemerged integrated devices, the energy densities, energy conversionefficiencies, and usage stability should be further enhanced by opti-mizing the structures and compositions of the devices, and the cost ofdevices should also be reduced. It is apparent that the total workingperformance of an integrated device will be limited by every unit.Therefore, how to make them cooperatively work should be the keyproblem. (ii) The practical application of each integrated unit hardlyachieves if they are out of the ideal working conditions. Further in-vestigations on property optimizations, structure designs, and systemintegrations are urgently needed to make the integrated systems ap-plicable for scaled-up fabrication and practical applications. In addi-tion, looking for other high-quality integrated devices is highly desiredto overcome the current obstacles. In the long term, any widespread useof integrated energy devices should be guided by environmental re-sponsibility that calls for finding the most efficient preparation andrecycling techniques. (iii) The mechanisms of present integrated sys-tems mainly depend on isolated devices, thus limiting the developmentof integrated systems. Further investigations based on first-principle

density functional theory calculations may explore integrated systemswith new integrating mechanisms [3,360–365], rather than combiningisolated devices into one unit through a simple way. The optimizationon the performance or the efficiency of integrated devices will provide agreat opportunity to realize industrial manufacture in future. (iv) Themismatch of impedance between individual power unit should be ad-dressed. The NG has much larger internal impedance than that of theDSSC and BFC. A mismatch of the impedance would result in a largecurrent leakage, which will seriously deteriorate the final output powerof the hybrid devices. Utilizing some electronic components for theinner connection among various parts of the hybrid device to suppressthe impedance mismatch is highly desired. (v) The output of NGs ingenerally is pulsed signals with variable frequency and irregular am-plitude. The conventional energy storage units such as SC or LIB aredesigned for storage of the power unit with constant amplitude DCsignal. Therefore, apart from the current energy storage units, designingnew energy storage units for the output of NGs is greatly desired formore efficient energy storage for the integrated devices. (vi) It is im-portant to consider the power and density matching of energy genera-tion and storage for the solar cell devices integrated with energy storagedevices in future investigations.

Author contributions

S. Yun designed the outline of this review manuscript. Q. Xu, Y. Qinand S. Yun wrote Section 1. J. Liu, Y. Qin and S. Yun wrote Section 2. Y.Zhang and S. Yun wrote Section 3.1. Q. Xu, Y. Qin and S. Yun wroteSection 3.2-3.3. S. Yun wrote Section 3.4-3.5. Y. Zhang and S. Yunwrote Section 3.6-3.8. S. Yun wrote Abstract, Summary and FutureOutlook, Q. Xu and J. Liu gave a revision. S. Yun gave a final revisionfor all sections. All authors reviewed the manuscript and gave a finalproof-reading.

Acknowledgments

Authors thank casual assistant X. Zhou for collecting literatures andediting text. Financial support from NSFC (51672208), National Science& Technology Pillar Program during the Twelfth Five-year Plan Period(2012BAD47B02), Sci-Tech R&D Program of Shaanxi Province(2010K01-120, 2011JM6010 and 2015JM5183), and ShaanxiProvincial Department of Education (2013JK0927) is greatly ac-knowledged. The project was partly sponsored by SRF ((2012)940) forROCS, SEM.

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411–416.

Sining Yun is a Professor in School of Materials Scienceand Engineering, Xi'an University of Architecture andTechnology, China. He obtained his PhD at Xi'an JiaotongUniversity in 2007 and was a postdoctoral fellow in YonseiUniversity (Korea). He is a visiting professor at State KeyLab. of Fine Chemicals (China), University of Reading (UK),Prof. Michael Grätzel and Prof. Anders Hagfeldt's labora-tory at EPFL (Switzerland). He is currently a ResearchDirector and Group Leader of Functional MaterialsLaboratory and Key Lab of Nanomaterials andNanotechnology of Shaanxi Province (China). His researchfocuses on the solar energy and biomass energy. Details canbe found at: http://xy.xauat.edu.cn/gnclyjs/listyjsgk.asp?

id=262&bh=2080

Yongwei Zhang received his B.S. and M.S degree fromQinghai University (QHU), China, and Kunming Universityof Science and Technology (KMUST), China, in 2013 and2018, respectively. He is currently a Ph.D. student in Schoolof Materials Science and Engineering at Xi'an University ofArchitecture and Technology (XAUAT) in China under thesupervision of Professor Sining Yun. His research mainlyfocuses on synthesis catalytic materials, especially singleatom catalysts, for solar energy and biomass energy.

Qi Xu received his B.S and Ph.D from the School of PhysicalScience and Technology of Lanzhou University in 2010 and2017, respectively. Currently, he is a assistant professor inthe School of Advanced Materials and Nanotechnology,Xidian University, China. His research focused on designingand fabricating self-cleaning materials, and high perfor-mance UV sensor.

Jinmei Liu received her B.S and PhD from the School ofPhysical Science and Technology of Lanzhou University in2011 and 2016, respectively. She is currently a assistantprofessor in School of Advanced Materials andNanotechnology, Xidian University, China. Her researchfocused on the design and synthesis of new materials forenergy harvesting devices, flexible self-powered system.She has abundant experiences on synthesizing inorganicoxide nanowire arrays and nanofibers, and fabricatingflexible functional nanodevices.

Yong Qin received his B.S. (1999) in Material Physics andPh.D. (2004) in Material Physics and Chemistry fromLanzhou University. From 2007 to 2009, he worked as avisiting scholar and Postdoc in Professor Zhong Lin Wang'sgroup at Georgia Institute of Technology. Currently, he is aprofessor at the Institute of Nanoscience andNanotechnology, Lanzhou University, where he holds aCheung Kong Chair Professorship. His research interestsinclude nanoenergy technology, functional nanodevice andself-powered nanosystem. Details can be found at: http://www.yqin.lzu.edu.cn.

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