DOI: 10.1002/chem.201303483

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& Graphene

Manganese Oxide/Graphene Aerogel Composites as an Outstanding Supercapacitor Electrode Material Chun-Chieh Wang, Hsuan-Ching Chen, and Shih-Yuan Lu*[a]

Abstract: Graphene aerogels (GA), prepared with an organic sol–gel process, possessing a high specific surface area of 793 m2 g1, a high pore volume of 3 cm3 g1, and a large average pore size of 17 nm, were applied as a support for manganese oxide for supercapacitor applications. The manganese oxide was electrochemically deposited into the highly porous GA to form MnO2/GA composites. The composites, at a high manganese oxide loading of 61 wt. %, exhibited a high specific capacitance of 410 F g1 at 2 mV s1. More importantly, the high rate specific capacitances measured at 1000 mV s1 for these composites were two-fold higher than those obtained with samples prepared in the absence of the GA support. The specific capacitance retention ratio, based on the specific capacitance obtained at 25 mV s1, was main-

tained high, at 85 %, even at the high scan rate of 1000 mV s1, in contrast with the significantly lower value of 67 % for the plain manganese oxide sample. For the cycling stability, the specific capacitance of the composite electrode decayed by only 5 % after 50,000 cycles at 1000 mV s1. The success of this MnO2/GA composite may be attributed to the structural advantages of high specific surface areas, high pore volumes, large pore sizes, and three-dimensionally well-connected network of the GA support. These structural advantages made possible the high mass loading of the active material, manganese oxide, large amounts of electroactive surfaces for the superficial redox events, fast masstransfer within the porous structure, and well-connected conductive paths for the involved charge transport.

Introduction

participate in the relevant redox reaction, which is practically impossible. One way to approach the theoretical limit is to use ultrathin MnOx nanostructure deposits, often tens to hundreds of nanometers thick.[8–10] This approach, although achieving relatively higher specific capacitances and being valuable for mechanistic study, is not practically useful because the extremely low active material mass of microgram levels cannot store/deliver technologically meaningful amounts of electric energy. When prepared as deposits of micrometer thickness with an active material mass of milligram level, the specific capacitance of the MnOx can only achieve a level of less than 200 F g1 and often fails to show the necessary capacitance retention at high scan (charging/discharging) rates mainly because of the poor electrical conductivity of the MnOx and low material utilization of the thick structure involved.[11, 12] To improve on the abovementioned shortcomings for better performing supercapacitive electrodes, mesoporous carbonaceous supports of high electrical conductivities and high surface areas to host the deposition of MnOx have been extensively investigated, including carbon nanotubes (CNT),[13] graphene sheets (GS),[14] carbon nanofoam/aerogels (CA),[15] and exfoliated graphite.[16] At high MnOx loadings, for example, over 45 wt. %, a condition necessary for practical applications, the specific capacitance of such MnOx/C composites can only achieve a level of 250 F g1.[13, 15] Apparently, there are still rooms for improvement for the types of carbonaceous support and the way to incorporate the MnOx into the support.

Supercapacitors, a new and promising energy-storage device, exhibiting high power densities, outstanding cycling stability, short charging/discharging times, and excellent electrochemical reversibility, have attracted tremendous research attention in recent years.[1] Currently, the main research efforts in this area are devoted to electrode material development. Promising electrode materials include transition-metal oxides, carbonaceous materials, and conducting polymers,[2–4] offering either plain electrochemical double-layer capacitances as in carbonaceous materials or extra pseudo-capacitances generated from superficial Faradaic redox reactions as in transition-metal oxides and conducting polymers.[5] Manganese oxides (MnOx), because of their Earth-abundance, low cost, good capacitive properties, and environmental friendliness, have been an excellent electrode material candidate for supercapacitor applications.[6] Additionally, MnOx functions in neutral aqueous electrolytes, a further advantage in terms of environmental friendliness.[7] Under the assumption of redox reactions involving one electron-transfer per manganese atom, the theoretical specific capacitance of MnOx can be estimated to be as high as 1370 F g1.[8] The theoretical limit however can only be achieved if all manganese atoms of MnOx [a] C.-C. Wang, H.-C. Chen, Prof. Dr. S.-Y. Lu Department of Chemical Engineering National Tsing-Hua University, No. 101 Section 2, Kuang-Fu Road, Hsinchu 30013 (Taiwan R.o.C.) E-mail: [email protected] Chem. Eur. J. 2014, 20, 517 – 523

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Full Paper Results and Discussion

In this work, we have developed a new type of carbonaceous support for MnOx. It is a three-dimensionally well-connected mesoporous carbonaceous template of high porosities, high specific surface areas, and high electric conductivities, constructed from chemically linked graphene sheets, the socalled graphene aerogels (GA).[17] In contrast to physically linked 3D structures of graphene sheets through p–p stacking,[18, 19] GAs possess much higher electrical conductivities because the chemical linkages at the graphene sheet junctions, formed through sol–gel polymerization of resorcinol and formaldehyde, offer much lower charge-transport resistances.[17] The high specific surface area and high porosity of the GA provide an advantageous platform to host the deposition of MnOx as a thin nanostructure, which is critical for high active material utilization, and to offer fast mass-transfer paths for electrolytes of involved electrochemical reactions. Consequently, GAs possess the essential structural and electric characteristics to serve as an excellent support for MnOx for supercapacitor electrode applications. GAs were prepared with a sol–gel process involving a graphene oxide (GO) suspension. After calcination, the electric conductivity of the monolithic GA was measured with a fourprobe method to be as high as 147 S m1. Its apparent density, as determined from the volume and mass of the GA monolith, was as low as 0.055 g cm3. A wide range of processes have been developed for the incorporation of MnOx into porous supports, including solution chemistry,[15] the electrophoretic method,[20] hydrothermal,[14] electrostatics deposition,[21] electrodeposition,[22] and microwave-assisted deposition.[23] We developed a cathodic reduction process to deposit MnO2 into GAs from an electrolyte containing KMnO4 and K2SO4 to prepare the MnO2/GA composite electrode for supercapacitor applications. For capacitive performance tests, cyclic voltammetry (CV) was conducted in a three-electrode system in a neutral aqueous electrolyte, Na2SO4. This electrolyte system possesses the advantages of high ionic conductivities, low-cost, non-flammability, and good safety. At a high MnO2 loading of 61 wt. %, a high specific capacitance of 410 F g1 was achieved for the MnO2/GAs composite electrode at a scan rate of 2 mV s1. The high rate capability and long-term cycling stability were investigated with both CV measurements and the galvanostatic charging/discharging technique. A high capacitance retention of 85 % from 25 to 1000 mV s1 was achieved for the MnO2/GA composite electrode, which is significantly higher than that of the MnO2 electrode without the GA support (67 %). Furthermore, the specific capacitance decayed by only 5 % after 50,000 cycles at 1000 mV s1, revealing the excellent long-term stability of the electrode. Accordingly, GAs are proven an excellent support for hosting active materials for supercapacitor electrodes, and MnO2/GA composites are a promising supercapacitor electrode material for the next-generation energy-storage purposes.

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The crystallographic structures of the raw, intermediate, and product materials of GA are shown in Figure 1 a for comparison. The pronounced characteristic diffraction peak of 26 8 is

Figure 1. (a) XRD patterns for graphite, graphite oxide, GA, and MnOx/GA; (b) XPS spectra for C and O in graphite oxide and GA; (c) XPS spectrum of C1s for graphite oxide; d) XPS spectrum of C1s for GA.

evident for graphite. When graphite is converted into graphite oxides, the inclusion of oxygen-containing functional groups, such as epoxide, hydroxyl, and carboxyl groups, at the basal planes of the graphite layers significantly expands the layer spacing, leading to a drastic left-shift in 2q from 26 to around 9 8. The material was then reduced to remove most of the oxygen-containing groups, exfoliated into monolayer graphene or few-layer graphite sheets, and three-dimensionally linked to form 3D mesoporous GA. The GA became basically amorphous, lacking large-scale atomically ordered domains, and exhibited only a broad, weak diffraction peak centering around 25 8.[24] Also included in Figure 1 a is the XRD pattern for the MnOx/GA composite. No significant diffraction peaks can be identified except for the diffraction peak located at 26 8 contributed by the graphite substrate, indicating the amorphous nature of the deposited MnOx. Figure 1 b shows the XPS spectra of the graphite oxide and GA. Evidently, the intensity of O1s and thus the oxygen content were drastically reduced, from 33 to 6 %, when the graphite oxide was thermally reduced to form the building block of GA: the graphene sheet. The C1s signals were locally enlarged and shown in Figure 1 c and d for further comparison. The C1s signal of the graphite oxide showed an evident double peak appearance with a pronounced shoulder on the right wing of the second peak. However, this double peak simplified to a single peak with two shoulders on its right wing for the GAs. These signals can be de-convoluted into a combination of four constituent peaks located at 284.7, 286.8, 287.8, and 288.6 eV, attributable to the CC and C=C of aromatic rings, epoxide or hydroxyl groups, carbonyl, and carboxyl groups, respective518

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Full Paper ly.[25, 26] From the comparison of Figure 1 c and d, it is evident again that the oxygen-containing groups were significantly reduced because of the thermal reduction treatment on the graphite oxide. Also worth noting is the slight shift in binding energy of the CC and C=C of aromatic rings to lower values because of the removal of the strong electron-drawing element, O, from the basal plane. The GA appeared as a black monolith (see in Figure 2 a). The size of the monolith decreased after the calcination treatment

IUPAC classifications.[27] The specific surface area, determined based on the Brunauer–Emmett–Teller (BET) model, was 793 m2 g1. This value is far less than the theoretical surface area of 2600 m2 g1 that is estimated based on a single graphene sheet, possibly because of the layering and random heaping of graphene sheets within the assembly.[28] The pore structure of the GA was characterized with the Barrett–Joyner– Halenda (BJH) model. The pore volume thus determined was as high as 3 cm3 g1 and the pore size ranged from 2 to 100 nm with an average of 17 nm. With the above, one can conclude that the GA possessed a mesoporous structure of large pore volumes, high specific surface areas, and suitable pore sizes, and was highly conductive. These structural features and electric property are well-suited for applications as a hosting support for active materials of supercapacitor electrodes. The active material, MnO2, was electrochemically deposited into the GA to form the MnO2/GA composite electrode. Before the deposition, the GA was ground into a fine powder and drop-cast onto a graphite substrate for the deposition. The electrodeposition was carried out at a potentiostatic mode of 0.8 V, referenced to Pt coil, in an electrolyte mixture of 0.1 m KMnO4 and 0.1 m K2SO4. MnO2 was expected to form through the following cathodic reduction reaction [Eq. (1)] in a neutral aqueous solution:[29] MnO4  þ 2 H2 O þ 3 e ! MnO2 þ 4 OH

The morphologies of the GA and MnO2/GA (at 61 wt. % loading) composite were shown in Figure 3 a and b as SEM images, respectively. Evidently, the GA appeared as a 3D porous structure of well-connected thin sheets. As for the MnO2/GA composite, it can be observed from Figure 3 b that the MnO2 nanostructure was grown within the GA support. If examined closely, these MnO2 nanostructures preferred to grow along/near the sheet edges. Normally, the sheet edges are chemically more active and also charges tend to reside at sharp locations, both contributing to the preferential growth of MnO2 along/ near the sheet edges. In fact, such growth preference is beneficial for not blocking the pore openings, maintaining fast masstransfers within the composite. As evident from the comparison of Figure 3 a and b, the pore space was well-retained, instead of being filled up, after the MnO2 deposition.

Figure 2. (a) Photographs of GA monolith before and after calcination treatment; (b) Nitrogen adsorption/desorption isotherm of GA. Inset is corresponding pore-size distribution. (*) = desorption curve; (&) = adsorption curve.

Figure 3 c and d show the TEM images of the GA and MnO2/ GA, respectively. The thinness of the graphene entities of the GA was manifested by the high transparency of the TEM image. Upon deposition of MnO2 into the GA, the transparency of the TEM image was greatly reduced because of the existence of MnO2 deposits. The MnO2 domain appeared to be amorphous from the corresponding HRTEM image (inset of Figure 3 d). Figure 3 e shows the EDX result of the MnO2/GA composite. Evidently, the stoichiometry of MnO2 was confirmed from the atomic ratio of Mn/O being close to 1:2. The K signals came from the remaining precursor KMnO4 as an adsorbate, which can be rinsed away with deionized water. The XPS spectrum of the MnO2/GA composite is shown in Figure 4. As expected, signals of C, O, and Mn can all be identi-

because of the removal of the residual chemicals and oxygencontaining functional groups. Despite this size decrease, the shape and monolithicity of the GA however remained intact, indicating the good structural stability of the GA attributable to the 3D chemical linkages. The electrical conductivity was measured with the four-probe method to be 147 S m1, which was two orders of magnitude higher than those of physically linked 3D graphene structures.[18] The microstructural properties of the GA were characterized with N2 adsorption/desorption analyses conducted at 77 K. The isotherm and pore-size distribution are shown in Figure 2 b. The isotherm can be classified as a type IV isotherm with a pronounced type H3 hysteresis loop, typical for mesoporous materials, according to Chem. Eur. J. 2014, 20, 517 – 523

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ð1Þ

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Full Paper The peaks of Mn2p3/2 and Mn2p1/2 were located at 642.4 and 654.1 eV, respectively, with a binding energy separation of 11.7 eV, in good agreement with the values reported in literature for MnIV.[30] This further confirms the deposit composition of MnO2. The deposition rate of MnO2 was investigated to reveal an extra advantage of using GAs as the hosting support. As a comparison, 0.25 mg of MnO2 was deposited into the GA with a deposition time of 2 h, whereas it took 4 h to deposit the same amount of MnO2 without the GA support. For a further comparison, the precursor, KMnO4, can be deposited onto the surface of carbonaceous materials through an electroless reduction process, and it took 5 h to achieve a mass loading of 0.3 mg with the present GA support.[15] The enhanced deposition rate may be attributed to the advantageous structural features and electric properties of the GA support. With the GA support, its large surface area allows more accommodation of the MnO2 deposits; its high pore volume and large pore size enable fast mass-transfer of involved reactants; and its high electric conductivity improves the charge transfer and charge transport necessary for relevant electrochemical depositions. The capacitive performances of the samples were characterized with the CV measurements. Figure 5 a shows the CV loops obtained for the MnO2/GA (at 61 wt. % loading) composite at scan rates of 2 and 25 mV s1 in 0.5 m Na2SO4. The currents were normalized by the mass of the active material and the working area of the electrode. Both CV loops were rectangular and symmetric, implying excellent electrochemical reversibility and negligible iR drops for the charging/discharging process, an essential characteristic for supercapacitors. The specific capacitances determined from the CV loops were 410 (corresponding to 21 F cm3) and 312 F g1 for scan rates of 2 and 25 mV s1, respectively. Also interesting to note is the pronounced reduction peak located at 0.5 V. It can be attributed to the superficial adsorption of NaI of the electrolyte as follows [Eq. (2)]:[31, 32]

Figure 3. SEM images of (a) GA and (b) MnO2/GA composite. TEM images of (c) GA and (d) MnO2/GA composite. Inset shows an HRTEM image for MnO2/ GA composite. (e) Elemental analysis of MnO2/GA composite.

MnO2 þ Naþ þ e $ MnOONa

ð2Þ

The corresponding reverse anodic peak was located at around 0.55 V as can be seen from the locally enlarged CV curve presented in the inset of Figure 5 a. Figure 5 b compares the CV loops obtained at 25 mV s1 for electrodes containing the same amount of MnO2 deposits, one with and the other without the composition with the GA support. The CV loop recorded for the GA support is included as the inset of the Figure for comparison. Evidently, the GA support alone contributed only a negligible amount of capacitance as compared with those of the plain MnO2 (deposition of MnO2 without the GA support) and the MnO2/GA composite as can be judged from the relative magnitudes of the vertical axis. The contribution of the graphite substrate to the currents of the product was negligible, at least two orders of magnitude smaller. In addition, the contribution of the GA support to the currents of the product was far less than that by the active material, about one order of magnitude smaller. Gener-

Figure 4. XPS spectrum of MnO2/GA composite. Inset shows locally enlarged spectrum for Mn2p peaks.

fied from the spectrum. If compared with the O1s signal of the GA shown in Figure 1 b, the O1s signal from the MnO2/GA composite is much higher in intensity because of the presence of the MnO2 deposits. As for the Mn element, its characteristic signals of Mn2p were locally enlarged as shown in the inset. Chem. Eur. J. 2014, 20, 517 – 523

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Figure 6. CV loops for (a) MnO2/GA composite electrode and (b) plain MnO2 electrode at increasing scan rates from 25 to 1000 mV s1 in 0.5 m Na2SO4 ; (c) Specific capacitance versus scan rate plot for MnO2/GA composite and plain MnO2 electrodes; (d) Capacitance retention of MnO2/GA composite and plain MnO2 electrodes as functions of scan rate; (e) Galvanostatic charging/discharging curves of MnO2/GA composite electrode at three different current densities, 1, 10, and 50 A g1. Inset shows locally enlarged charging/ discharging curves for the 50 A g1 case; (f) Specific capacitance as a function of cycle number for plain MnO2 and MnO2/GA composite electrodes at 1000 mV s1.

Figure 5. (a) CV loops of MnO2/GA composite electrode recorded at 2 and 25 mV s1 in 0.5 m Na2SO4. Inset is locally enlarged CV loop of MnO2/GA composite electrode recorded at 2 mV s1; (b) CV loops of plain MnO2 and MnO2/ GA composite electrodes recorded at 25 mV s1 in 0.5 m Na2SO4. Inset: CV loop of GA electrode recorded at 25 mV s1 in 0.5 m Na2SO4.

ally speaking, carbonaceous materials can offer non-negligible amounts of capacitances in acidic or basic electrolytes, but they perform poorly in neutral electrolytes. The GA support offered only 25 F g1 at 25 mV s1. In this work, the contribution of the GA support in specific capacitance was subtracted from that of the MnO2/GA composite to obtain the specific capacitance data for MnO2 only. It is clear from Figure 5 that the plain MnO2 electrode delivered significantly less currents than the MnO2/GA composite electrode, and the specific capacitance acquired for the plain MnO2 electrode was 184 F g1, much less than 312 F g1 for the MnO2/GA composite electrode. This shows again the benefit of using GA as a support for the active material. With the GA support, its large surface area allows more dispersed and thus thinner MnO2 deposits giving better material utilization for capacitance generation; its high pore volume and large pore size enable fast mass-transfer of involved electrolytes; and its high electric conductivity improves the charge transfer and charge transport necessary for relevant redox reactions. For a further comparison, the MnO2/ GA composite prepared with the electroless reduction method showed a specific capacitance of 162 F g1 at 25 mV s1, which Chem. Eur. J. 2014, 20, 517 – 523

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is far less than the value of 312 F g1 obtained by the MnO2/GA composite prepared with the electrochemical deposition method. The high rate capability of the plain MnO2 and MnO2/GA composite electrodes is next investigated by recording the CV loops at increasing scan rates, from 25 to 1000 mV s1. The results were shown in Figure 6 a and b. The differences between the two sets of data became evident at high scan rates. For the MnO2/GA composite electrode, the rectangularity and symmetry of the loops were well-maintained, even at the extremely high scan rate of 1000 mV s1, indicating its excellent high rate capability. On the contrary, the rectangularity and symmetry of the CV loops of the plain MnO2 electrode deteriorated with increasing scan rates. At 1000 mV s1 the CV loop appeared inclined, implying development of significant electrochemical irreversibility and internal electrical resistance (iR drops). The success of the present MnO2/GA composite electrode in terms of high rate capability may be attributed to the fast mass-transfer within the mesoporous support enabled by the large pore volume and pore size of the GA, and to the fast 521

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Full Paper Experimental Section

charge-transfer and transport provided by the well-connected 3D conductive structure of the GA. The specific capacitances as determined from the corresponding CV loops were plotted against the scan rate for both samples for further comparison. The results were shown in Figure 6 c. Evidently, the MnO2/GA composite electrode consistently outperformed the plain MnO2 electrode over the entire scan rate range in terms of the specific capacitance. At 1000 mV s1, a good specific capacitance of 264 F g1 was achieved for the MnO2/GA composite electrode, more than two times that for the plain MnO2 electrode of 123 F g1. In fact, the specific capacitance of the plain MnO2 electrode decayed more rapidly than that of the MnO2/GA composite electrode. This can be more clearly observed by comparing the retention ratio of the specific capacitance, with respect to that obtained at 25 mV s1, with increasing scan rates. Figure 6 d shows that the MnO2/GA composite electrode performed much better than the plain MnO2 electrode in this regard, with 85 versus 67 %, another advantage of using the GA as a support attributable again to the advantageous structural features and high electric conductivity of the GA. The galvanostatic charging/discharging characteristics of the MnO2/GA composite electrode were also studied, with the results shown in Figure 6 e. The charging/discharging curves appeared linear and symmetric even at the high current density of 50 A g1, indicating excellent electrochemical reversibility, negligible iR drops, and outstanding Columbic efficiencies. Finally, the cycling stabilities of the plain MnO2 and MnO2/ GAs composite electrodes were tested up to 50,000 cycles at 1000 mV s1. The capacitance retention was excellent for both electrodes. Except for the initial slight drop, the specific capacitance remained almost constant during the entire cycling tests. The cycling stability of the MnO2/GA was outstanding with a decay of only 5 % in specific capacitance at the end of the 50,000 cycles.

Synthesis of graphene aerogels Graphene aerogels were prepared through sol–gel chemistry from graphene oxide (GO) suspensions mixed with organic carbonaceous precursors: resorcinol and formaldehyde.[33, 34] The graphene oxide was first prepared from natural graphite (Alfa Aesar, 325 mesh, 99.8 %) with a modified Hummers method.[35, 36] Briefly, graphite (0.5 g), sodium nitrate (0.5 g), and concentrated sulfuric acid (23 mL) were stirred in an ice bath to form a graphite suspension. Potassium permanganate (3 g) and deionized water (DI, 140 mL) was then added to this suspension, followed by a slow addition of 35 % H2O2, leading to a color change from brown to yellow. The suspension was filtered and washed with diluted HCl solution. The filter cake was dispersed in DI water and was removed from the larger-sized unreacted graphite with a low-speed centrifugation at 1000 rpm. The collected supernatant was further centrifuged at 8000 rpm to remove water-soluble byproduct. The final sediment was stored in a vacuum oven of 40 8C for 2 days to afford the graphite oxide powders. Graphene oxide suspensions were obtained through exfoliation of graphite oxide powders dispersed in DI water. The exfoliation was achieved with a tip sonicator (Misonix, XL-2000). The dark graphene oxide suspension was then mixed with resorcinol (0.684 g, Alfa Aesar, 99 %), formaldehyde (0.724 mL, ECHO, 36.6 %), and Na2CO3 (3.23 mg, Showa, 99.5 %). The mixture was stirred vigorously until all chemicals were completely dissolved and the suspension turned dark-brown. Here, Na2CO3 served as the catalyst to promote the gelation of the mixture. The resulting wet gels were dried with supercritical CO2 followed by calcination in a N2 atmosphere at 1050 8C for 3 h to afford the monolithic GAs.

Preparation of MnO2/GA composite electrodes All electrode samples used in this work were prepared on graphite substrates of 10  10  3 mm. These graphite substrates were first polished with SiC papers followed by cleaning with 0.1 m HCl and DI water three times and then dried for later use. An exposed area of 1  1 cm2 was constructed by using a poly-tetrafluorene ethylene (PTFE) tape. Prior to MnO2 deposition, the GA powders were ground and mixed with a binder, poly-vinylidene fluoride (PVDF, Alfa Aesar), at a mass ratio of 4:1. The above solid mixture (0.2 mg) was dispersed into 1-methyl-2-pyrrolidone (50 mL NMP, TEDIA, 95 %), a solvent for the binder. The suspension was then drop cast onto the graphite substrate and dried at 80 8C in an oven overnight. A GA layer of around 50–60 mm thick was formed on top of the graphite substrate. The MnO2/GA composite electrode was prepared with an electrochemical deposition process by using the mixture of 0.1 m KMnO4 (J. T. Baker, 99 %) and 0.1 m K2SO4 (Riedelde-Haen, 99 %) as the electrolyte and Pt coil as the reference electrode. The deposition of MnO2 was operated at 0.8 V with the system kept at 25 8C by using a circulatory water bath (MODELB402 L, FIRSTEK). Typically, an amount of 0.25 mg MnO2 can be deposited into the GA template in 2 h. For plain MnO2 electrodes, GAs were not used and MnO2 was electrochemically deposited onto the graphite substrate. The mass of MnO2 deposit was determined by the weight difference before and after the deposition as measured by a microbalance with an accuracy of 0.01 mg (AUW120D, Shimadzu).

Conclusion A new type of carbonaceous support, graphene aerogels, was developed for MnO2 in supercapacitor applications. The advantageous structural features, including well-connected 3D mesoporous network, high specific surface areas, high pore volumes, suitably large pore sizes, and high electric conductivities of the GA support make it an excellent host for MnO2. The MnO2/GA composite electrode thus obtained exhibited outstanding supercapacitive characteristics, such as excellent electrochemical reversibility, high rate capability, cycling stability, and Columbic efficiency, and negligible iR drops, far superior to the plain MnO2 electrode. At a high mass loading of 61 wt. %, a specific capacitance as high as 410 F g1 was achieved at 2 mV s1. In addition, the specific capacitance decayed by only 5 % after 50,000 cycles at 1000 mV s1. The present MnO2/GA composite electrode proves to be a promising candidate for next-generation supercapacitor electrodes.

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Full Paper Electrochemical measurements

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The electrochemical characterizations were carried out using a three-electrode system. Both CV and galvanostatic charge/discharge measurements were conducted by using an electrochemical workstation (CHI6275D, CH Instruments Inc). The MnO2/GA- or MnO2-coated graphite substrate served as the working electrode, with Ag/AgCl and Pt coil for the reference electrode and counter electrode, respectively. The CV and galvanostatic charge/discharge curves were recorded in a potential window of 0 to 0.9 V at different scan rates (25, 100, 500, and 1000 mV s1) and current densities (1, 10, and 50 A g1), respectively in an aqueous electrolyte of 0.5 m Na2SO4 (Showa, 99 %). The specific capacitance, Cs, was determined from the CV loops [Eq. (3)]:[37]

Cs ¼ ð

Z

IdVÞ ðumDVÞ1

ð3Þ

in which I is the response current density (A cm2), DV is the width of the potential window (in V), u is the scan rate (in V s1), and m is the mass of the active material, MnO2, (in g). The integration was performed over the negative scans.

Characterizations An X-ray diffractometer (UltimaIV, Rigaku), equipped with CuKa (l = 1.5406 ) and operated at a voltage and current of 40 kV and 40 mA, respectively, was used to determine the crystalline structure of graphite, graphite oxide, GA, and MnO2/GA. The O, C, and Mn contents of graphite oxide, GA, and MnO2/GA were characterized with a high-resolution X-ray photoelectron spectrometer (XPS, PHI Quantera SXM, ULVAC-PHI). Energy dispersive X-ray (EDX, Oxford 6587, Oxford instruments) spectroscopy and scanning electron microscopy (SEM, JSM-5600, JEOL) were conducted to determine the elemental composition and to observe the morphology of the MnO2/GA composites, respectively. The microstructure of the samples was further investigated with a transmission electron microscope (TEM, Philips Technai G2, FEI) operated at 200 kV. The overall microstructural characteristics of the samples were measured with the N2 adsorption/desorption analyses conducted at 77 K (NOVA 2000e, Quantachrome). The samples were degassed at 100 8C for 8 h before measurements and the specific surface area and poresize distribution were determined with the BET and BJH models, respectively. The conductivity of the GA was measured with a fourprobe detector connected to a source meter (Keithley 2400, Keithley).

Acknowledgements This work was financially supported by the National Science Council of Taiwan under grant NSC-101–2221-E-007–111-MY3 and by the Low Carbon Energy Research Center of the National Tsing-Hua University. Keywords: composite electrode · electrochemical deposition · graphene aerogel · manganese oxide · supercapacitor

Chem. Eur. J. 2014, 20, 517 – 523

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Received: September 4, 2013 Published online on December 10, 2013

523

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

graphene aerogel composites as an outstanding supercapacitor electrode material.

Graphene aerogels (GA), prepared with an organic sol-gel process, possessing a high specific surface area of 793 m(2)  g(-1) , a high pore volume of 3...
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