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Solid State Sciences 12 (2010) 1677e1682

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Solid State Sciences

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Microstructure control of MnO2/CNT hybrids under in-situ hydrothermalconditions

Fei Teng a,b, Sunand Santhanagopalan a, Dennis Desheng Meng a,*

aMulti-Scale Energy System (MuSES) Laboratory, Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, MI 49931, USAb School of Environmental Science and Engineering, Nanjing University of Information Science and Technology Nanjing 210044, P.R. China

a r t i c l e i n f o

Article history:Received 3 December 2009Received in revised form5 July 2010Accepted 21 July 2010Available online 27 July 2010

Keywords:MnO2/CNTsHybridin situHydrothermalCapacitive behavior

* Corresponding author. Tel./fax: þ1 906 487 3551.E-mail address: dmeng@mtu.edu (D.D. Meng).

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.07.026

a b s t r a c t

In this work, single-crystalline MnO2 nanoparticles were directly grown on the surface of multi-walledcarbon nanotubes (CNTs) homogeneously under in-situ hydrothermal conditions, during which the CNTswere well dispersed in aqueous solution with the aid of dodecyl benzene sulphonic acid sodium (SDBS).This stable suspension ensures the continuous deposition of the MnO2 nanocrystals. It was found that theMnO2/CNTs nanocomposites formed in the presence of CNTs, but the MnO2 nanowires formed withoutCNTs under the same hydrothermal conditions. Moreover, the as-synthesized MnO2/CNTs sampleshowed a high specific capacity and cycling stability, which was ascribed to its highly-homogeneoushybrid nanostructure. This homogeneous MnO2/CNTs nanocomposite is shown to be able to take fulladvantages of both the high capacity of MnO2 and the high electron conductivity of CNTs by integratingthem homogeneously. This homogeneous hybrid nanostructure is a promising electrode material forenergy storage/conversion devices with excellent performances.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Currently, supercapacitors and lithium-ion batteries havegained great attention owing to their potential applications inenergy storage/conversion devices from portable electronics toelectric vehicles (EV). Various transition metal oxides have beeninvestigated as electrode materials [1e3]. Among them, MnO2 isintensively exploited as one of the most promising electrodematerials due to its high theoretical capacity, favorable pseudoca-pacitive behavior, environmental benign nature, natural abundanceand low cost [4e21]. However, a major drawback of MnO2 is itspoor electrical conductivity, which severely limits its charge storagebehavior. One of the most effective approaches to overcome thisdisadvantage is to introduce electronically conductive materials(e.g., carbon black and CNTs). Because of this, various MnO2/CNTsnanocomposites have been researched [22e31]. For example,MnO2 nanowires on CNT paper by electrodeposition [22], highly-dispersed g-MnO2 on CNT arrays by electrochemically-induceddeposition [23], coaxial MnO2/CNT array electrode [24], MnO2nanoflowers on vertically-aligned CNT arrays [25], and MnO2 onthree-dimensional CNTs by electrodeposition [26]. Theseresearches have demonstrated that the electrochemical

son SAS. All rights reserved.

performances of electrodes or devices can be significantlyenhanced by these hybrid structures. However, the capacitivevalues (usually less than 450 F g�1) of these MnO2-basedcomposites are still far lower than the theory value (1100 F g�1) ofMnO2, since the usability and homogeneity of MnO2 on CNTs bythese methods are still low. Moreover, although considerableefforts have been made to achieve controllable deposition on CNTs,it is still challenging to simply synthesize highly-homogeneousnanocomposites for further improvement in electrochemistryperformances [27,28]. Hence, the fundamental research on thephysical and chemical structure of MnO2/CNTs hybrids and theirformation mechanism is still necessary to achieve high usabilityand homogeneity of MnO2.

In this work, the MnO2/CNTs hybrid nanostructures wereprepared by an in-situ hydrothermal method. This homogeneoushybrid nanostructure will take full advantage of the multiplefunctionalities by integrating the high storage capacity of MnO2nanoparticles and the high conductivity of CNTs. The microstruc-ture of the MnO2/CNTs hybrid sample was revealed by field-emis-sion electron microscopy (FE-SEM), high-resolution transmissionelectron microscopy (HRTEM), electron diffraction (ED), X-rayenergy dispersive spectroscopy (EDX) and X-ray diffractometry(XRD). The capacitive behavior of the MnO2/CNTs hybrid electrodewas measured and compared with that of the mechanically-mixedMnO2 and CNTs electrode. The highly-homogeneous hybrid elec-trode will have (i) the improved electrical conductivity due to the

F. Teng et al. / Solid State Sciences 12 (2010) 1677e16821678

presence of the CNTs conductive network; and (ii) the enhancedelectrochemical accessibility of electrolyte due to the reducedtendency for agglomeration of nanoparticles and the mesoporousstructure by the entangled one-dimensional (1-D) MnO2/CNThybrid nanostructures.

2. Experimental

2.1. Chemicals

In this work, deionized water (DI) water was used in all exper-iments. All chemicals were of analytical grade and used as received.MnSO4$H2O, KMnO4, dodecyl benzene sulphonic acid sodium(SDBS, C18H29SO3Na), and Na2SO4 were purchased from Fisher.Polytetrafluoroethylene (PTFE) and N-methyl-2-pyrrolidone werepurchased from Aldrich. Multi-wall carbon nanotubes (CNTs) werepurchased from MER Corporation.

2.2. Sample preparation

The MnO2/CNTs hybrids were prepared by a simple in-situhydrothermal method. Specifically, 0.5 g of SDBS and 0.3001 g ofCNTs were added to 40 mL aqueous solution containing 0.005 MMnSO4$H2O and 0.005 M KMnO4. The mixture was stirred for 2 h toform a stable suspension, and then loaded into a Teflon-linedstainless-steel autoclave (capacity: 50mL). The autoclavewas heatedto 160 �C and kept at 160 �C for 12 h. The autoclave was coolednaturally to room temperature. The black precipitate was separatedby centrifugation, and thenwashed with water and ethanol for threetimes. After that, the recovered product was vacuum-dried at 60 �Covernight. According to the amount of CNTs added and the weight ofthe as-obtained sample, it was calculated that 90 wt% of CNTs wascontained in the MnO2/CNT composites.

For comparison, a-MnO2 nanowires were also prepared bya hydrothermal method as previously reported [29].

2.3. Characterization

Scanning electron microscopy (SEM) images were taken witha Hitachi S-4700 field-emission scanning electron microscope(FE-SEM). The FE-SEM samples were coated with a 5 nm thin plat-inum/chromium coating using DC sputtering. The accelerationvoltage was 10 keV and the acceleration current was 1.2 nA. Themorphology, crystalline structure, surface structure, and elementcomposition of the samples were determined using high-resolutiontransmission electron microscopy (HRTEM) on a JEOL JEM-4000FXequipped with electron diffraction an energy dispersive X-rayspectroscopy (EDX) with an acceleration voltage of 200 kV. Thepowders were ultrasonically dispersed in ethanol, and thendeposited on a thin amorphous carbon film supported by a coppergrid. The crystal structures of the samples were characterized byX-ray powder diffractometer (XRD, Rigaku D/MAX-RB), usinggraphite monochromatized Cu Ka radiation (l ¼ 0.154 nm), oper-ating at 40 kV and 50 mA. The XRD patterns were obtained in therange of 10e70� (2q) at a scanning rate of 5� min�1.

2.4. Electrode fabrication and electrochemical properties test

The fabrication processes of the electrodes are described asbelow: for the MnO2/CNTs hybrid electrode, MnO2/CNTs hybridpowder (95 wt%) was mixed with PTFE (5 wt%) as a binder in anagate mortar. The mixture was then ground sufficiently to ensuregood dispersion, wherein N-methyl-2-pyrrolidone was used assolvent. The resultant slurry was cast onto the pre-cleaned stain-less-steel plate. The electrode was air-dried overnight and then

dried under vacuum at 80 �C for 24 h. The as-fabricatedMnO2/CNTshybrid electrode was designated as HTMC. The loading was deter-mined by a weighing method. For comparison, a-MnO2 powder(10wt%) was mechanically mixed with CNTs (90wt%) in an agatemortar, and the mixture was then similarly ground to ensure gooddispersion. Finally, the mixture (95 wt%) of both MnO2 and CNTspowders was mixed with PTFE (5 wt%). The electrode was preparedwith the same procedure as mentioned above, which was desig-nated as MMMC electrode.

Cyclic voltammetry (CV) and galvanostatic chargeedischargemeasurements were performed using a potentiostat/galvanostat(Princeton Applied Research P/G, V4) with a three-electrodeconfiguration. Pt wire and Ag/AgCl (saturated KCl, 0.222 V vs.standard hydrogen electrode (S.H.E)) were employed as the counterand reference electrodes, respectively. Prior to use, the electrolytesolutions were purged with N2 gas for 0.5 h. CVs were measuredbetween 0.1 and 0.8 V in a 0.5MNa2SO4 electrolyte at different scanrates. Galvanostatic chargeedischarge measurements were per-formed at a constant current of 50 mA between 0.1 and 0.8 V.

3. Results and discussion

3.1. Microstructure and crystal structures of the samples

Fig. 1 shows the FE-SEM images of the samples obtained. Thedense and entangled MnO2/CNTs hybrid nanostructures forma three-dimensional network structure. This three-dimensionalnanoporous structure is expected to provide a large oxideeelectrolyte interface for facile ion transportation as well asa conductive network for enhanced electronic conductivity whichwill improve the specific capacitance and high-rate capability ofMnO2 [30]. Observed at high magnifications (Fig. 1b), the MnO2nanoparticles are observed to be uniformly and exclusively grownon the surface of CNTs. Although nanorods or nanowires are thecommon morphology of MnO2 under hydrothermal conditions[29], the as-obtained sample by us is found to be mainly composedof the MnO2/CNTs nanocomposites. As shown in Fig. 1c, the MnO2nanowires have been obtained without adding CNTs, which havea length of about 2 mm and a diameter of about 30 nm. It seems thatthe nucleation and growth processes of MnO2 were significantlyinfluenced by the introduction of CNTs, which have altered thecrystallization behavior of MnO2. We herein conjecture that theMnO4

� or Mn2� ions may get selectively adsorbed onto the surfaceof CNTs, and the redox reaction (MnO4

�withMn2� or C) maymainlytake place on the surface of CNT. As a result, the MnO2/CNT nano-composites form under hydrothermal conditions, the redox reac-tion between Mn2þ and MnO4

� will take place on the surface ofCNTs. Consequently, MnO2 nucleated heterogeneously via hydro-thermal deposition because CNTs have prevented MnO2 fromgrowing into nanorods. This heterogeneous nucleation process hasbeen reported by Ma et al. [32] and Yue et al. [33].

TEM was further performed to characterize the microstructuresand crystal structures of the MnO2/CNTs hybrids. It is clear that theMnO2 nanoparticles of 30e40 nm in size were uniformly depositedon the surface of CNTs (Fig. 2a,b). Here, SDBS plays an importantrole in the formation of MnO2/CNTs hybrid nanostructures. First, itis employed to effectively disperse CNTs in aqueous solution toform a stable suspension, which favors continuous deposition ofMnO2 nanocrystals. Second, SDBS may non-covalently function-alize CNT surface through a surfactant-wrapping mechanism,which also favors to anchor MnO2 nanoparticles. The surfacestructure and crystalline nature of these MnO2 nanoparticles hasbeen confirmed by HRTEM observation and ED analysis. Fig. 2cshows the lattice fringes of the MnO2 nanoparticle with a d-spacingof 0.50 nm, corresponding to the (200) plane of the tetragonal

Fig. 1. FE-SEM micrographs of the samples: (a) low-magnification and (b) high-magnification views of the MnO2/CNTs hybrids; (c) MnO2 nanorods.

F. Teng et al. / Solid State Sciences 12 (2010) 1677e1682 1679

MnO2 crystal. The diffraction spots in Fig. 2d can be assigned to the(200), (400) and (101) planes of MnO2 crystal, revealing the single-crystalline nature. The ED and HRTEM analysis reveal that theMnO2 crystal may grow along the [001] direction. In Fig. 2e, thed-spacing of the lattice fringe is measured to be 0.34 nm corre-sponding to the (002) planes of the CNTs. The wavy structure ofgraphitic sheets in a short range may indicate the CNT has moredefects and amorphous carbon, lower graphitization degree andmore disordered structure. Fig. 2f shows the typical ED patterns ofCNTs. It is found in our experiment that the interaction between theMnO2 nanoparticles and the CNTs was strong, which has been

Fig. 2. TEM micrographs of MnO2/CNTs hybrid: (a) low-magnification view; (b) high-magniCNT; (f) ED pattern of CNT.

confirmed by the fact that the nanoparticles could not be removedunder intensive sonication. All the results above firmly confirm theformation of the homogeneous MnO2/CNTs nanocomposites.

Fig. 3a shows the XRD patterns of the CNTs, the diffraction peaksat 2q values of 26�, 42� and 54� were ascribed to the (002), (100)and (004) reflections of CNTs, respectively [21,34,35]. Fig. 3b showsthe XRD patterns of the resultant MnO2/CNTs sample. In addition tothe (002) diffraction peaks of CNTs, all the other peaks can beindexed to the tetragonal phase of a-MnO2 (JCPDS No. 44-0141)with lattice constants of a ¼ 0.982 nm and c¼ 0.286 nm, indicatingthat the a-MnO2 crystal was formed in the as-prepared MnO2/CNTs

fication view; (c) lattice fringes of MnO2; (d) ED pattern of MnO2; (e) lattice fringes of

10 20 30 40 50 60 70

(100

)

(220

)

(400

)

(510

)

(541

)

(002

)

(521

)

(600

)

(411

)

(301

)

(310

)

(200

)

(110

)

(004

)

(c)

(b)

Inte

nsity

(a.u

.)

2 Theta (degree)

(a)

(002

)

(211

)

Fig. 3. X-ray diffraction patterns of (a) CNTs, (b) MnO2/CNTs hybrid and (c) MnO2

nanorods.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-0.016

-0.008

0.000

0.008

0.016100 mV s-1

80 mV s-1

50 mV s-1

Cur

rent

/ A c

m-2

20 mV s-1

a

F. Teng et al. / Solid State Sciences 12 (2010) 1677e16821680

hybrid sample. Fig. 3c shows the XRD patterns of the as-preparedMnO2 sample without adding CNTs, confirming the formation ofpure a-MnO2 crystals referring to the JCPDS No. 44-0141.

3.2. Electrochemical properties of the samples

Cyclic voltammetry was performed to investigate the capacitiveproperties of the HTMC and MMMC electrodes in 0.5 M Na2SO4aqueous solution. Unlike other studies that use extended potentialranges of �0.3 to 1.0 V [36,37], we only consider the range of0.1e0.8 V (vs. Ag/AgCl) because MnO2 is highly reversible andstable in this potential range [38]. The extension of the potentialrange may lead to the occurrence of irreversible reactions such assome transformations of Mn4þ to Mn7þ in the anodic region andMn3þ to Mn2þ in the cathodic region. Mn7þ and Mn2þ are water-soluble species that can cause the deterioration of the electrodematerial upon cycling. In the potential range of 0.1e0.8 V, manga-nese ion species undergo a highly reversible transformationbetween Mn3þ and Mn4þ. Fig. 4 shows the typical cyclic voltam-mograms (CVs) of the samples at a scanning rate of 50 mV s�1 ina potential range from 0.1 to 0.8 V vs. Ag/AgCl. It is observed that the

0.0 0.2 0.4 0.6 0.8-0.010

-0.005

0.000

0.005

0.010

(c)

Cur

rent

/ A

cm-2

Potential/ V

(a) (b)

Fig. 4. Cyclic voltammetrigrams of the samples at a scanning rate of 50 mV s�1: (a)CNTs; (b) MMMC; (c) HTMC, MMMC mechanically-mixed MnO2 þ CNTs, HTMC MnO2/CNTs hybrid.

shapes of the voltammograms are close to rectangular for bothMnO2-based electrodes, showing the characteristic CV of a near-ideal supercapacitor. By contrast, the current response obtained onthe CNT electrode is much smaller, although it also shows the near-rectangle shape of a near-ideal supercapacitor. Therefore, webelieve that the high capacitive current responses of both MnO2-containing electrodes mainly result from the redox reaction ofMnO2. It is well known that the power characteristic of an electrodematerial is strongly dependent upon the electrochemical kinetics ofthe redox transition. Moreover, it is clearly observed that the HTMCelectrode has a higher capacity than the MMMC electrode. Fig. 5ashows the CVs of the HTMC at different scanning rates. It can beobserved that the CV current density increases gradually with theincrease of the scanning rate and all CV curves show nearly-rectangular shape, even at a high scan rate of 100 mV s�1. This mayindicate that the electrolyte ions can effectively contact the activeelectrode materials. We believe that the three-dimensional (3-D)porous structure of the HTMC electrode can significantly facilitatethe penetration of protons or other cations into thewhole electrodematrix. Similar results have been reported previously [16,26].Fig. 5b shows the CVs of the MMMC electrode at different scanrates. At low scan rates of 20 and 50 mV s�1, the CVs show nearly-rectangular shape reflecting the redox characteristics of a nearly-ideal supercapacitor. However, at higher scan rates of 80 and100 mV s�1, the redox patterns deviate from each other with

Potential/ V

0.0 0.2 0.4 0.6 0.8

-0.016

-0.008

0.000

0.008

0.016100 mV s-1

80 mV s-1

50 mV s-1

Cur

rent

/ A c

m-2

Potential/ V

20 mV s-1

b

Fig. 5. Cyclic voltammetrigrams of the samples at different scanning rates: (a) HTMC;(b) MMMC.

0 50 100 150 200 250 300 3500.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pote

ntia

l/ V

Time/ s

1st cycle

500th cycle

1st cycle

500th cycle

Fig. 7. Chronopotentiograms of the electrodes in 0.5 M Na2SO4 aqueous solution ata constant current of 50 mA: Black line, HMMC; red line, MMMC; Solid line, 500thcycle; Dot line, 1st cycle. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

F. Teng et al. / Solid State Sciences 12 (2010) 1677e1682 1681

respect to the voltammogram shape and current value. Increasingscan rate has a direct impact on the diffusion of Naþ into the MnO2matrix. At high scan rates, the Naþ ion will only approach the outersurface of the electrode. Since the inner part of the electrodematerial has little contribution to pseudocapacitance, a decrease inpseudocapacitance is expected. The current response at thereversal of potential is high for the HTMC electrode. The CVs of theMMMC electrode become slightly distorted and display delayedcurrent reversals at both ends of the potential limits, because theelectrode severely suffers from a high polarization resistance. TheHTMC electrode has far better electrochemical reversibility thanthe MMMC one. The desirable high-rate capacitive behavior of theHTMC electrode is mainly attributed to the high electron conduc-tivity due to its highly-homogeneous hybrid nanostructure. Itappears that the higher electrical conductivity of the CNTs in thehybrid electrode facilitates the energy extraction fromMnO2, whichresults in a higher capacitance [10]. For the MMMC electrode, themixing homogeneity of MnO2 with CNTs is poor, resulting ina relatively low conductivity. The cycling stability of the HTMCelectrode was further investigated at a scanning rate of 50 mV s�1

(Fig. 6). The current response values are fairly stable over the 1000cycles. The excellent cycle performance of the HTMC electrode isattributed to its homogeneous hybrid nanostructure. The presenceof CNTs greatly alleviates the stress caused by the cycling, becauseof its excellent mechanical and resiliency properties [39]. Bycontrast, it have been reported that pure MnO2 shows a rapiddecline in its capacitance value upon cycling for even 500 cycles orso, mainly caused by the cyclic volumetric variations of the oxideparticles upon cycling [8,39,40].

Fig. 7 gives the galvanostatic chargeedischarge curves of bothelectrodes, which show almost symmetric profile in the potentialregion. The discharge capacitance of the electrode can also beevaluated from the chargeedischarge curve according to thefollowing equation [41]:

Cs ¼ ði� DtÞ=ðDV �mÞ (1)

where i is the discharge current (mA); t is the discharge time (s); DVis the potential window (mV);m is the mass of the active materials(g). The Cs values for the HMMC and MMMC electrodes are calcu-lated as 125 and 86 F g�1, respectively. The low Cs value is attributedto the low content (10 wt%) of MnO2 in the composites. After 500cycling, the HMMC electrode almost maintained its initial Cs value(117 F g�1, 94%), while the Cs value of the MMMC electrode

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

Cur

rent

/ A c

m-1

Potential/ V

Fig. 6. Cyclic voltammetrigrams of the HTMC electrode running for1000 cycles ata scanning rate of 50 mV s�1.

significantly decreased (74 F g�1, 85%). The good cycle stability ofthe HMMC electrode may be related to its hybrid nanostructure.CNTs not only increase the electrode conductivity, but also providemore active sites for the redox reaction of metal oxide by increasingthe specific surface area [31]. In the mechanical mixed MMMCsample, CNTs and MnO2 are weakly bound by the binder. Duringthe electrochemical cycling, the MnO2 nanoparticles may beseparated from CNTs, leading to the obvious decrease of itscapacitance. In contrast, MnO2 are strongly adhered to CNTsthrough the stronger chemical (e.g., covalent) bond in the HMMCcomposite sample. Therefore, the HMMC sample can maintain itsstructural homogeneity and stability and thus achieve much bettercapacity performance.

Two mechanisms have been proposed to explain the chargestorage behavior of MnO2. The first mechanism involves the interca-lation of protons (Hþ) or alkali metal cations (Cþ) into active materialupon reduction and their deintercalation upon oxidation [42].

MnO2 þ Hþ þ e�/MnOOH (2)

or

MnO2 þ Cþ þ e�/MnOOC (3)

The second mechanism is based on the surface adsorption ofelectrolyte cations (Cþ) on MnO2 [43,44].

ðMnO2Þsurfaceþ�Cþ

�þ e�/

�MnO�

2 Cþ�surface

(4)

where Cþ ¼ Naþ, Kþ, Liþ.We believe that bothmechanisms are effective and contribute to

the excellent capacitive behavior of the HTMC electrode. First, theMnO2 nanoparticles are highly dispersed on the CNTs and theMnO2aggregation has been effectively reduced for the HTMC sample. Ahigh active surface area of MnO2 can be provided favoring to theintercalation/deintercalation reaction or surface adsorption of Cþ.Secondly, the electron conductivity of the homogeneous hybridnanostructure is significantly enhanced due to the highly uniformdistribution of MnO2 nanoparticles on the CNTs. The hybrid archi-tecture can enhance the energy extraction from MnO2, whichresults in a high capacitance [18,45]. The electrical conductivities ofHTMC and MMMC electrodes are determined to be 1.1 � 10�2 and7.0 � 10�2 S cm�1, respectively, much higher than that of pureMnO2 (10�6e10�3 S cm�1). The reported mechanical grinding

F. Teng et al. / Solid State Sciences 12 (2010) 1677e16821682

method is not considered enough to achieve the nanoscalehomogeneity for highly-tangled CNTs and MnO2 nanowires. Thepoor intrinsic electronic conductivity of MnO2 thus leads to highpolarization of the electrode. The high polarization resistanceresults in non-perfect rectangular CV curves [10]. Reddy et al. [24]have reported that the excellent cyclic stability and capacity of theMnO2/CNTs coaxial nanotube electrodes result from the hybridnature of the electrodes with improved electronic conductivity andthe dual mechanism of storage. They believe that the hybridstructure formed by the highly conductive CNT core offersenhanced electronic transport to the MnO2 shell, and it acts asa buffer to alleviate the volume expansion, and that the CNTsprovide additional sites for (Cþ) storage with dual mechanism,which leads to an enhanced reversible capacity for the HTMCelectrode. Besides these factors mentioned above, we also believedthat the 3-D nanoporous hybrid structure of the HTMC electrodecan provide a much larger interfacial area between MnO2 andelectrolyte, which improves electron conduction through the CNTnetwork and facile ionic transfer within the nanopores [11]. Withthe 3-D nanoporous structure, the specific capacitance can besignificantly improved. An obvious advantage of our technique isthat the CNTs and MnO2 can be well mixed and intimately con-nected with each other by this in situ hydrothermal method. Byemploying this homogeneous hybrid nanostructure, the function-ality of different materials can be integrated together to take fulladvantage of them. The reported hybrid electrode with 3-D nano-porous structure is expected to offer a promising route to signifi-cantly improve the specific capacitance of supercapacitors.

4. Conclusions

A simple in-situ method has been demonstrated to grow single-crystallineMnO2 nanoparticles uniformly on the surface of CNTs. Thehydrothermally-synthesized MnO2/CNTs hybrid electrode showedbetter capacitive performance and cycle stability than the mechan-ically-mixed counterpart, which has been ascribed to the highhomogeneity and 3-D nanoporous network structure of the former.The hybrid MnO2/CNTs nanostructure effectively integrated the highcapacity of MnO2 with the high electron conductivity of CNTs.

Acknowledgments

This work is financially supported by the Michigan Tech FacultyStartup Fund (N4901928249) and National Science Foundation ofChina (NSFC20944004).

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