Open Submitted: 02 July 2013 Accepted: 12 August 2013 Published Online: 21 August 2013
AIP Advances 3, 082118 (2013); https://doi.org/10.1063/1.4819270
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• Abdulhakeem Bello
• Omobosede O. Fashedemi
• Joel N. Lekitima
• Mopeli Fabiane
• David Dodoo-Arhin
• Kenneth I. Ozoemena
• Yury Gogotsi
• Alan T. Charlie Johnson
• Ncholu Manyala

We have fabricated a symmetric electrochemical capacitor with high energy and power densities based on a composite of graphene foam (GF) with ∼80 wt% of manganese oxide (MnO2) deposited by hydrothermal synthesis. Raman spectroscopy and X-ray diffraction measurements showed the presence of nanocrystalline MnO2 on the GF, while scanning and transmission electron microscopies showed needle-like manganese oxide coated and anchored onto the surface of graphene. Electrochemical measurements of the composite electrode gave a specific capacitance of 240 Fg−1 at a current density of 0.1 Ag−1 for symmetric supercapacitors using a two-electrode configuration. A maximum energy density of 8.3 Whkg−1 was obtained, with power density of 20 kWkg−1 and no capacitance loss after 1000 cycles. GF is an excellent support for pseudo-capacitive oxide materials such as MnO2, and the composite electrode provided a high energy density due to a combination of double-layer and redox capacitance mechanisms.
Electric double-layer capacitors (EDLCs) are charge-storage devices with a high power density, long cyclic life and low maintenance cost.1–41. B. E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications 1999. 2. P. Simon and Y. Gogotsi, “Capacitive Energy Storage in Nanostructured Carbon–Electrolyte Systems,” Acc. Chem. Res. 46, 1094–1103 (2012). https://doi.org/10.1021/ar200306b 3. J. Miller and A. Burke, “Electrochemical capacitors: challenges and opportunities for real-world applications,” Electrochem. Soc. Inter. 17, 53–57 (2008). 4. A. S. Aricò, P. Bruce, B. Scrosati, J.-M. Tarascon, and W. Van Schalkwijk, “Nanostructured materials for advanced energy conversion and storage devices,” Nat. Mater. 4, 366–77 (2005). https://doi.org/10.1038/nmat1368 They fill the gap between batteries (high energy density) and electrolytic capacitors (high power density).5,65. R. Kötz, “Principles and applications of electrochemical capacitors,” Electrochim. Acta 45, 2483–2498 (2000). https://doi.org/10.1016/S0013-4686(00)00354-6 6. X. Zhao, B. M. Sánchez, P. J. Dobson, and P. S. Grant, “The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices,” Nanoscale 3, 839–55 (2011). https://doi.org/10.1039/c0nr00594k Such properties make them potentially useful for a wide range of applications such as hybrid vehicles, cordless electric tools, memory back-up, cellular phones, medical devices, military and consumer electronics.77. P. J Hall, M. Mirzaeian, S. I. Fletcher, F. B. Sillars, A. J. R. Rennie, G. O. Shitta-Bey, G. Wilson, A. Crude, and R. Carter, “Energy storage in electrochemical capacitors: Designing functional materials to improve performance,” Energy Environ. Sci. 3, 1238–1251 (2010). https://doi.org/10.1039/c0ee00004c
Nevertheless, the energy stored in supercapacitors (<10 Whkg−1) is low compared to lithium ion batteries (>100 Whkg−1). This has imposed significant challenges in employing supercapacitors as primary power source for battery replacement.8,98. A. Burke, “R&D considerations for the performance and application of electrochemical capacitors,” Electrochim. Acta 53, 1083–1091 (2007). https://doi.org/10.1016/j.electacta.2007.01.011 9. E. Frackowiak and F. Béguin, “Carbon materials for the electrochemical storage of energy in capacitors,” Carbon 39, 937–950 (2001). https://doi.org/10.1016/S0008-6223(00)00183-4 However, the performance of these devices depends on the type, properties and morphologies of the materials being used. Therefore much attention has been given to developing novel nanostructured materials with high specific surface area (SSA), controlled pore size distribution, increased operating voltage and comprehensive understanding of electrode/electrolyte interfaces at the nanoscale.2,10–122. P. Simon and Y. Gogotsi, “Capacitive Energy Storage in Nanostructured Carbon–Electrolyte Systems,” Acc. Chem. Res. 46, 1094–1103 (2012). https://doi.org/10.1021/ar200306b 10. S. W Lee, B. M. Gallant, H. R. Byon, P. T Hammond, and Y. Shao-Horn, “Nanostructured carbon-based electrodes: bridging the gap between thin-film lithium-ion batteries and electrochemical capacitors,” Energy Environ. Sci. 4, 1972–1985 (2011). https://doi.org/10.1039/c0ee00642d 11. L. L. Zhang, R. Zhou, and X. S. Zhao, “Graphene-based materials as supercapacitor electrodes,” J. Mater. Chem. 20, 5983–5992 (2010). https://doi.org/10.1039/c000417k 12. F. Du, D. Yu, L. Dai, S. Ganguli, V. Varshney, and A. K. Roy, “Preparation of Tunable 3D Pillared Carbon NanotubeÀGraphene Networks for High-Performance Capacitance,” Chem. Mater. 23, 4810–4816 (2011). https://doi.org/10.1021/cm2021214 This led to many research activities focused on developing electrode materials for supercapacitors that are capable of storing more energy without sacrificing cyclic life and power density. Pseudocapacitors are energy storage devices that undergo electron transfer reactions.1313. J. R. Millerand and P. Simon, “Materials science. Electrochemical capacitors for energy management,” Science 321, 651–2 (2008). https://doi.org/10.1126/science.1158736 An example of such pseudocapacitive materials is manganese oxide (MnO2). MnO2 has attracted much attention as an electrode material because of its multiple reversible electrochemical reactions, natural abundance, low cost, and environmental compatibility.14–1614. W. Wei, X. Cui, W. Chen, and D. G. Ivey, “Manganese oxide-based materials as electrochemical supercapacitor electrodes,” Chem. Soc. rev. 40, 1697–721 (2011). https://doi.org/10.1039/c0cs00127a 15. J. Liu, J. Essner, and J. Li, “Hybrid Supercapacitor Based on Coaxially Coated Manganese Oxide on Vertically Aligned Carbon Nanofiber Arrays,” Chem. Mater. 22, 5022–5030 (2010). https://doi.org/10.1021/cm101591p 16. C.-Y. Chen, C.-Y. Fan, M.-T. Lee, and J.-K. Chang, “Tightly connected MnO2–graphene with tunable energy density and power density for supercapacitor applications,” J. Mater. Chem. 22, 7697–7700 (2012). https://doi.org/10.1039/c2jm16707g However, due to the low electrical conductivity of MnO2, the rate capability and performance of MnO2 electrodes is limited, which has hindered its potential application. The redox activity, capacitive performance and utilization of MnO2 can be increased by adding conductive materials, such as carbon nanotubes,17,1817. M. Sang-Bok, N. Kyung-Wan, Y. Won-Sub, Y. Xiao-Qing, A. Kyun-Young, O. Ki-Hwan, and K. Kwang-Bum, “Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications,” J. Power Sources 178, 483–489 (2008). https://doi.org/10.1016/j.jpowsour.2007.12.027 18. A. E. Fischer, K. A. Pettigrew, D. R. Rolison, R. M. Stroud, and J. W Long, “Incorporation of homogeneous, nanoscale MnO2 within ultraporous carbon structures via self-limiting electroless deposition: Implications for electrochemical capacitors,” Nano lett. 7, 281–286 (2007). https://doi.org/10.1021/nl062263i conducting polymers19,2019. R. I. Jafri, A. K Mishra, and S. Ramaprabhu, “Polyaniline–MnO2 nanotube hybrid nanocomposite as supercapacitor electrode material in acidic electrolyte,” J. Mater. Chem. 21, 17601–17605 (2011). https://doi.org/10.1039/c1jm13191e 20. J. Kim, K. H. Lee, L. J. Overzet, and G. S. Lee, “Synthesis and Electrochemical Properties of Spin-Capable Carbon Nanotube Sheet/MnO,” Nano Lett. 11, 2611–2617 (2011). https://doi.org/10.1021/nl200513a or graphene.21,2221. H. Wang, C. Li-Feng, Y. Yang, H. S. Casalongue, J. T. Robinson, Y. Liang, Y. Cui, and H. Dai, “Mn3O4 -Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries,” J. Am. Chem. Soc. 132, 13978–13980 (2010). https://doi.org/10.1021/ja105296a 22. J. Zhu and J He, “Facile synthesis of graphene-wrapped honeycomb MnO2 nanospheres and their application in supercapacitors,” ACS appl. mater. interfaces 4, 1770–1776 (2012). https://doi.org/10.1021/am3000165 Different morphologies and shapes of the MnO2 particles have also been reported; for example, nano-whiskers have been shown to have excellent electrochemical properties because of their large surface area.2323. L. Li, Z.-Y. Qin, L.-F. Wang, H.-J. Liu, and M.-F Zhu, “Anchoring alpha-manganese oxide nanocrystallites on multi-walled carbon nanotubes as electrode materials for supercapacitor,” J. Nanoparticle Res. 12, 2349–2353 (2010). https://doi.org/10.1007/s11051-010-9980-8
Graphene has been explored for electrochemical storage applications due to its high specific surface area, high electrical conductivity, chemical stability and excellent mechanical properties.24,2524. M. D. Stoller, S. Park, Y. Zhu, J An, and R. S. Ruoff, “Graphene-based ultracapacitors,” Nano lett. 8, 3498–3502 (2008). https://doi.org/10.1021/nl802558y 25. Y. Wang, Z. Q. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, and Y Chen, “Supercapacitor Devices Based on Graphene Materials,” J. Phy. Chem. C 113, 13103–13107 (2009). https://doi.org/10.1021/jp902214f Graphene also offers a suitable platform for accommodating metal oxides. This is attributed to its large surface area, which allows for uniform loading and binding sites for incorporation of the metal oxide materials such as MnO2. Also, the high conductivity of the graphene significantly improves the conductivity of the composite electrode.
Recently, Chen et al.2626. Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, and H-M Cheng, “Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition,” Nat. mater. 10, 424–428 (2011). https://doi.org/10.1038/nmat3001 reported a three-dimensional (3D) flexible and conducting interconnected graphene network formed using chemical vapour deposition onto a catalytic nickel foam template. Graphene foam provides a desirable combination of a highly conductive network27,2827. Y. Chen, X. Zhang, P. Yu, and Y Ma, “Electrophoretic deposition of graphene nanosheets on nickel foams for electrochemical capacitors,” J. Power Sources 195, 3031–3035 (2010). https://doi.org/10.1016/j.jpowsour.2009.11.057 28. H. Ji, L. Zhang, M. Pettes, H Li, and S. Chen, “Ultrathin graphite foam: A three-dimensional conductive network for battery electrodes,” Nano lett. 12, 2446−2451 (2012). https://doi.org/10.1021/nl300528p and a high porosity support structure that is suitable for incorporation of nanoparticles or other fillers to occupy the pores for electrochemical applications.29,3029. X. Huang, X Qi, F. Boey, and H. Zhang, “Graphene-based composites,” Chem. Soc. rev. 41, 666–686 (2012). https://doi.org/10.1039/c1cs15078b 30. Y. Xu, X. Huang, Z. Lin, X. Zhong, Y. Huang, and X. Duan, “One-step strategy to graphene/Ni(OH)2 composite hydrogels as advanced three-dimensional supercapacitor electrode materials,” Nano Res. 6, 65–76 (2012). https://doi.org/10.1007/s12274-012-0284-4
Several studies have reported on synthesis of various graphene foam/MnO2 (GF/MnO2) nanocomposites using chemical synthesis,3131. S. Chen, J. Zhu, X. Wu, Q. Han, and X. Wang, “Graphene oxide−MnO2 nanocomposites for supercapacitors,” Acs Nano 4, 2822–2830 (2010). https://doi.org/10.1021/nn901311t microwave irradiation,3232. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, and F. Wei, “Fast and reversible surface redox reaction of graphene–MnO2 composites as supercapacitor electrodes,” Carbon 48, 3825–3833 (2010). https://doi.org/10.1016/j.carbon.2010.06.047 electrodeposition,3333. Q. Cheng, J. Tang, J. Maa, H. Zhanga, N. Shinyaa, and L-C. Qinc, “Graphene and nanostructured MnO2 composite electrodes for supercapacitors,” Carbon 49, 2917–2925 (2011). https://doi.org/10.1016/j.carbon.2011.02.068 redox deposition3434. Z. Li, J. Wang, S. Liu, X. Liu, and S. Yang, “Synthesis of hydrothermally reduced graphene/MnO2 composites and their electrochemical properties as supercapacitors,” J. Power Sources 196, 8160–8165 (2011). https://doi.org/10.1016/j.jpowsour.2011.05.036 and polymer-assisted chemical reduction.3535. Y. Qian, S. Lu, and F. Gao, “Preparation of MnO2/graphene composite as electrode material for supercapacitors,” J. Mater. Sci. 46, 3517–3522 (2011). https://doi.org/10.1007/s10853-011-5260-y Recently, Dong et al.3636. X. Dong, X. Wang, J. Wang, H. Song, X. Li, L. Wang, M. B. Chan-Park, C. M. Li, and P. Chen, “Synthesis of a MnO2–graphene foam hybrid with controlled MnO2 particle shape and its use as a supercapacitor electrode,” Carbon 50, 4865–4870 (2012). https://doi.org/10.1016/j.carbon.2012.06.014 reported a simple synthesis for 3D hybrid structures of MnO2 on graphene foam in which the morphology of the MnO2 nanostructures was controlled by the acidity of the solution. The large specific surface area of GF ensured a large loading capacity for MnO2 nanostructures and a large active surface area for rapid charge transfer and a large double layer capacitance. They also showed that graphene-metal oxides composites offered good adhesion for the metal oxide particles, which prevented detachment and agglomeration, thereby leading to improved capacitance in the three electrode configuration.
In this study, we report on a symmetric electrochemical capacitor fabricated using GF/MnO2 nanostructure composite electrodes based on a nickel foam current collector. Hydrothermal synthesis of MnO2 on graphene was performed with the goal of combining double-layer and redox capacitance for achieving high energy and power densities in one device.
A. Synthesis of graphene foam/MnO2 composite
Graphene foam (GF) was synthesised by chemical vapour deposition (CVD) onto a catalytic nickel foam (Alantum Innovations in Alloy Foam Munich, Germany). The detailed procedure is described in our previous report.3737. A. Bello, K. Makgopa, M Fabiane, D. Dodoo-Ahrin, K. I. Ozoemena, and N. Manyala, “Chemical adsorption of NiO nanostructures on nickel foam-graphene for supercapacitor applications,” J. Mat. Sci. (2013). https://doi.org/10.1007/s10853-013-7471-x To provide mechanical support for the GF during etching of nickel, polymethylmethacrylate (PMMA) was drop coated on the sample and baked at 180 °C for 30 minutes. The samples were then placed in 3 M HCl solution at 80 °C for 5 hours to ensure that nickel was completely etched away. The resulting GF sample was placed in acetone at 50 °C for 30 minutes to remove the PMMA. The samples were then rinsed in deionised water and dried.
GF/MnO2 composites were prepared using a hydrothermal reduction technique. Typically, 15 mg of GF was dispersed in 50 mL of 0.02 M KMnO4 (Merck), and the mixture was refluxed at 150 °C for 48 hours with continuous magnetic stirring. The resultant dispersion was then centrifuged and washed several times with deionized water, and finally dried at 60 °C in an oven. All chemicals used were of analytical grade and used as received. Deionized water was used throughout the synthesis process.
B. Physical characterization of composite
Raman spectra of GF/MnO2 composites were recorded using a WiTec-alpha 300R+ confocal Raman spectrometer (WiTec GmbH) with the laser power of 1.5 mW in order to minimize heating effects. The excitation source was a 532-nm laser through a numerical aperture of 0.9 and 100× magnification. The GF/MnO2 composite crystal structure was characterized using powder X-ray diffraction (XRD). An XPERT-PRO diffractometer PANalytical BV, Netherlands with theta/theta geometry, operating a cobalt tube at 35 kV and 50 mA was used. The XRD patterns of all specimens were recorded in the 20.0°-80.0° 2θ range with a step size of 0.017° and a counting time of 15.240 seconds per step. Qualitative phase analysis of samples was conducted using the X’pert Highscore search match software. The surface morphology and microstructure of the composite were investigated using a Zeiss Ultra Plus 55 field emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 2.0 kV, nitrogen gas sorption analysis at 77 K (Micromeritics ASAP 2020) and transmission electron microscopy (TEM) JEOL JEM-2100F microscope operated at 200 kV. Typically, TEM samples were prepared by dispersing the samples in ethanol and dropped on lacey carbon grid for analysis.
C. Electrochemical characterization
The working electrodes for electrochemical evaluation were prepared by mixing 80 wt.% of GF/MnO2 with 10 wt. % carbon black and 10 wt. % polyvinylidene difluoride (PVdF) binder in an agate mortar. The mixture was then dissolved in 1-methyl-2-pyrrolidinone (NMP) to form a paste. The paste was coated on the Ni foam as a current collector with a diameter of 16 mm and dried at 60 °C in an oven for 8 hours to ensure complete evaporation of the NMP.
The electrochemical test of the GF/MnO2 electrode was performed in a three electrode configuration using glassy carbon plate as counter electrode and Ag/AgCl (3 M KCl) as the reference electrode. The symmetric GF/MnO2/GF/MnO2 system was investigated in a two-electrode configuration, with a glass microfiber filter paper as the separator and 1 M Na2SO4 solution serving as the electrolyte in both configurations. The electrochemical properties of the supercapacitor were studied using cyclic voltammetry (CV), galvanostatic charge-discharge (GV) and electrochemical impedance spectroscopy (EIS) using an Autolab PGSTAT workstation 302 (ECO-CHEMIE) driven by the general purpose electrochemical system (GPES) software.
The CV tests were carried out in the potential range of 0 to 1 V at different scan rates ranging from 5 mV/s to 100 mV/s. The galvanostatic charge-discharge measurements were also carried out at different current densities from 0.1 Ag−1 to 0.5 Ag−1 and the EIS measurements were performed in the frequency range of 100 kHz-10 mHz. The EIS data were analyzed using Nyquist plots.11. B. E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications 1999.
The cell capacitance, Ccell, was then calculated from the slope of galvanostatic charge-discharge curves according to the following equation
 $Ccell=iΔtΔv$ (1)
where i is the constant current for charge-discharge, Δt is the discharge time and Δv is the discharge voltage. The specific capacitance (per unit mass) was then calculated according to the following equation3838. M. D. Stoller and Rodney S. Ruoff, “Best practice methods for determining an electrode material's performance for ultracapacitors,” Energy Environ. Sci. 3, 1294–1301 (2010). https://doi.org/10.1039/c0ee00074d
 $Csp=4Ccellm$ (2)
where m is the mass of a single electrodes in the cell.
Fig. 1(a) shows the Raman spectrum of the graphene foam, with prominent G and 2D peaks at 1582.2 cm−1 and 2705.7 cm−1, respectively. The relative intensity 2D/G ratio of 1.2 and FWHM of 38.8 cm−1 were attributed to few layer graphene. The absence of the D-peak (disorder) at 1350 cm−1 showed that the graphene foam was of good quality, with a low defect density.4040. A. C. Ferrari, “Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects,” Solid State Comm. 143, 47–57 (2007). https://doi.org/10.1016/j.ssc.2007.03.052 The Raman spectrum of the GF/MnO2 composite (Fig. 1(b)) showed an additional sharp peak at 649.9 cm−1, which was ascribed to the Ag mode arising from breathing vibrations of MnO6 octahedral double chains. It also corresponds to the Mn-O stretching vibration mode in the basal plane of MnO6 octahedral chains and symmetric stretching vibration Mn-O of MnO6 groups.4141. T. Gao, H. Fjellvåg, and P. A Norby, “comparison study on Raman scattering properties of α- and β-MnO2,” Analytica Chim. Acta 648, 235–239 (2009). https://doi.org/10.1016/j.aca.2009.06.059 The well-defined Raman spectrum reflects the good crystallinity of the MnO2 in the composite. The observed reduction of the 2D/G intensity ratio after the formation MnO2 may be due to the introduction of disorder during hydrothermal synthesis or electronic interactions between MnO2 and GF.4242. Z. Ni, T. Yu, Z. Luo, Y. Wang, and L. Liu, “Probing charged impurities in suspended graphene using Raman spectroscopy,” ACS Nano 3, 569–574 (2009). https://doi.org/10.1021/nn900130g
Both XRD patterns had narrow diffraction peaks at 2θ = 31° and 64.5°, which correspond to the (002) and (004) reflections of hexagonal graphite, respectively. They show that the crystalline order of the CVD-grown multi-layer graphene foam was preserved after formation of the GF/MnO2 composite. The data for the GF/MnO2 composite (Fig. 2(b)) showed additional diffraction peaks at 2θ = 14.2°, 28.8°, 42.8°, and 77.7°, which are representative of α and γ phases of MnO2. The diffraction peaks at 14.2° and 28.8° were assigned to the (110) and, (310) reflections of the α-MnO2 phase in accordance with the ICDD PDF card No. 44-0141, while the peaks at 42.8 and 77.7 were assigned to (131) and, (421) reflections of γ-MnO2,4343. L. Chen, L-J. Sun, F. Luan, Y. Liang, Y. Li, and X-X Liu, “Synthesis and pseudocapacitive studies of composite films of polyaniline and manganese oxide nanoparticles,” J. Power Sources 195, 3742–3747 (2010). https://doi.org/10.1016/j.jpowsour.2009.12.036 in accordance with the ICDD PDF card No. 14-0644 respectively.
The surface area of both GF and GF/MnO2 samples were measured using nitrogen gas sorption analysis at 77 K. Typically, the nitrogen adsorption and desorption show type ll/lV isotherm as shown in Fig. 3 and the corresponding specific surface area values of 42 m2g−1 and 98 m2g−1 respectively.
SEM analysis showed that the nickel foam's three-dimensional (3D) porous network structure makes it an excellent template for CVD synthesis of a 3D graphene network. The SEM image of GF is shown in Fig. 4(a) after the nickel template was etched away. The GF retained the porous nature of the underlying Ni template, and therefore provided abundant surface binding sites for MnO2 deposition. Fig. 4(b)-4(c) are SEM images of the final GF/MnO2 composite at different magnifications. Fig. 4(c) reveals that most of the MnO2 nanostructures took on a needle-like structure, which was further confirmed by transmission electron microscopy Fig. 4(d). TEM micrographs also revealed a porous, interconnected network of graphene flakes that was expected to improve the capacitive performance of the composite by enabling easy access of ions and high ionic mobility at the electrode/electrolyte interface.
Cyclic voltammetry (CV) of GF/MnO2 electrode at different scan rates in a three electrode configuration is shown in Fig. 5(a), the CVs of the GF/MnO2 exhibit square-like shape in a potential window of (0-1 V vs Ag/AgCl), indicating ideal capacitive performance. The galvanostatic charge-discharge curves measured at three different current densities are shown in Fig. 5(b). From these curves, the specific capacitance was calculated using equation (1). At current densities of 0.2 Ag−1, 0.5 Ag−1 and 0.7 Ag−1 the calculated specific capacitance values of GF/MnO2 electrode are 478, 341, and 266 Fg−1 respectively. The good capacitive performance can be attributed to the good contact made between the MnO2 and GF, which enhances the low-conductivity of MnO2 needle-like structure, thus enhancing the utilization of the composite in pseudocapacitive reaction leading to rapid charge transport mechanism which results in fast dynamics and easy access to ions in electrolyte thereby reducing the diffusion resistance.
Fig. 6(a) shows the Symmetric CV of the GF at a scan rate of 10 mV/s and of the GF/MnO2 composite at scan rates of 10 mV/s, 25 mV/s and 50 mV/s. The CV data for the GF appears as a straight line in the plot, indicating that it had a very low capacitance compared to the GF/MnO2 composite. In contrast, the CV loops for GF/MnO2 composite supercapacitor are nearly rectangular in shape, which is an indication of good capacitive behaviour and low contact resistance.11. B. E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications 1999. The enhanced capacitive result is attributed to the electrochemical capacitance from MnO2 since it is obvious that the electric double-layer capacitance from GF is very small as seen from the CV. Thus, >90% of capacitance comes from MnO2, at least at low scan rates. CV loops expand with increasing scan rate and maintain the rectangular shape, which is a characteristic of increasing current response of the device, indicating a capacitive performance of the MnO2 needle-like structure on GF up to 50 mV/s.
Cyclic performance of electrodes material is very important for practical applications. Galvanostatic charge-discharge measurements were performed on GF/MnO2 composite at different current densities in the potential range from 0 to 1V and the data are presented in Fig. 6(b). It was observed that the GF/MnO2 composite curves deviated from a linear shape, showing a contribution from redox processes. The capacitance of the cell was calculate using equation (1) while the maximum specific capacitance of the electrode material obtained using equations (2) was 240 Fg−1 at a current density of 0.1 Ag−1, compared to 17 Fg−1 for the GF at the same current density. The high specific capacitance was attributed to the presence of needle-like MnO2. The small voltage (IR) drop observed (from charge discharge curves) indicates a low internal resistance of the device, consistent with a good adhesion between the nanostructured oxide and GF surface that improved the conductivity of electrodes.
The capacitance retention as a function of cycle number is presented in Fig. 6(c). The figure shows that after 1000 cycles at a current density of 0.5 Ag−1, the GF/MnO2 composite symmetric supercapacitor device retained its initial specific capacitance. The energy storage (E) per unit mass of the GF/MnO2 electrode and the maximum energy density of the coin cell was calculated using the equation (3) for C corresponding to Csp and Ccell respectively. The values were found to be 34 Whkg−1 and 8.3 Whkg−1.
 $E=12CV2M,$ (3)
where V is the potential window and M is the total mass of electrode material.
Fig. 6(d) shows the Nyquist plot of the GF/MnO2 composite electrode. It is a representation of the real and imaginary parts of the impedance in the sample. The plot is divided into two regions; the high-frequency region, which is a characteristic of the charge transfer process taking place at the electrode\electrolyte interface and a straight line in the low-frequency region, which represents the electron-transfer diffusion process. The intercept at the high frequency region on the x-axis corresponds to the resistance of the electrolyte solution (Rs), and is also referred to as the equivalent series resistance (ESR) which comprises of the resistance of aqueous electrolyte, the intrinsic resistance of the composite material and the contact resistance at the electrode interface. The inset to Fig. 6(d) shows that there is no loop at high frequency that might correspond to contact resistance and the ESR value for the GF/MnO2 composite electrode is found to be 2.9 Ω. This is a good value for composite containing 80% of poorly conducting metal oxide. This can be attributed to the good conductivity of GF.
The maximum power density (Pmax) of the GF/MnO2 electrodes was calculated using equation (4):
 $Pmax=V24MRs$ (4)
where V is voltage applied (1 V), Rs (2.9 Ω) is the ESR, and M (4.5 mg) is the total mass of active material in the two electrodes. The maximum power density obtained for the GF/MnO2 composite is 20 kWkg−1.
Our symmetric (same anode and cathode) GF/MnO2 composite supercapacitor exhibited a specific capacitance, energy density of electrode material and power density values of 240 Fg−1, 34 Whkg−1 and 20 kWkg−1 respectively, which are comparable to the values reported by Rakhi et al.3939. R. B. Rakhi, W. Chen, D Cha, and H. N. Alshareef, “Nanostructured Ternary Electrodes for Energy-Storage Applications,” Adv. Energy Mater. 2, 381–389 (2012). https://doi.org/10.1002/aenm.201100609 for symmetric supercapacitors with graphene nanoplatelets-(γ-MnO2/CNT)-nanocomposite electrodes in 1 M KOH. At the maximum voltage of 1 V they reported an electrode energy density of 43 Whkg−1 and a power density of 26 kWkg−1. Higher energy and power density values were reported for asymmetric devices, the voltage window for which can be extended to 2 V. For example Fan et al. have reported an energy density of 51.1 Wh kg−1 and power density of 102.2 Wkg−1 for an asymmetric supercapacitor using a graphene/MnO2 composite as a positive electrode and activated carbon nanofibers as a negative electrode in a neutral aqueous Na2SO4 electrolyte,4545. B. G. Choi, M. Yang, W. H. Hong, J. W. Choi, and Y. S. Huh, “3D macroporous graphene frameworks for supercapacitors with high energy and power densities,” ACS nano 6, 4020–8 (2012). https://doi.org/10.1021/nn3003345 while Choi et al. reported values of 44 Whkg−1 and 25 kWkg−1 for energy and power densities of asymmetric supercapacitor device of chemically modified graphene e-CMG/e-CMG-MnO2 composite.4444. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Q. Ning, T. Y. Li, and F. Wei, “Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density,” Adv. Funct. Mater. 21, 2366–2375 (2011). https://doi.org/10.1002/adfm.201100058 Our results in the symmetric configuration with the maximum voltage window of 1 V give values that are comparable to those obtained in the asymmetric configuration. Therefore, GF with hydrothermally deposited MnO2 needles-like structure is a promising candidate for capacitive energy storage.
Graphene foam/MnO2 composite electrodes have been fabricated by hydrothermal deposition of MnO2 from KMnO4, and their electrochemical properties have been tested in a symmetric device with Na2SO4 as the electrolyte. The 3D porous network structure of GF allows for a uniform coating and high loading of α- and γ-MnO2 needle-like structures. The composite exhibits a specific capacitance of 240 Fg−1 at a current density of 0.1 Ag−1 and no change in capacitance was observed after 1000 cycles at a current density of 0.5 Ag−1. The composite electrodes exhibited energy and power densities of 34 Whkg−1 and 20 kWkg−1 respectively, which are among the highest values reported so far for symmetric two-electrode cells. The results obtained here indicate that graphene foam/MnO2 composites are promising for electrochemical applications.
This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF). Any opinion, findings and conclusions or recommendations expressed in this work are those of authors and therefore the NRF and DST do not accept any liability with regard thereto. A.T.C.J acknowledges support from the LRSM, through the U.S. National Science Foundation MRSEC, Grant No. DMR-1120901. Y.G. was supported by the US Department of Energy, Energy Storage Systems Research Program through Sandia National Laboratory. A. B. acknowledges the financial support from University of Pretoria for his study from both Energy Institutional research theme (IRT) and PhD bursary scheme.
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