A Novel Hybrid Supercapacitor Electrode Utilizing Vertically Oriented Graphene Nanosheets Coated with Conformal Layer of Pseudocapacitive MnO2 Nanoparticles

Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities [1]. It is an excellent electrode material because of its outstanding material properties, including high specific surface area, excellent electrical conductivity, chemical and mechanical stability, and can be fabricated economically.

MnO₂ is a popular pseudocapacitive material that provides capacitance from the rapid, reversible faradaic reactions where the oxidation state of Mn varies between +3 and +4 in conjunction with the intercalation and deintercalation of the electrolyte cation, as represented by the following equation [2,3]: MnO₂ + X⁺ + e^- ↔ MnOOX (X= H, Li, Na, K). By incorporating the low-cost, environment friendly MnO₂into the high specific surface area graphene, we can combine the benefits of the two to develop advanced supercapacitors with high performance.

This paper reports a novel, hybrid supercapacitor achieved by incorporating a thin-film MnO₂ conformal coating on vertically oriented graphene nanosheets. Highly doped silicon substrate was used as the substrate to synthesize the vertically aligned graphene nanosheets by thermal CVD at 900°C in a gas mixture of methane and argon with a flow rate of 1:20. The height of the graphene was controlled by varying the CH₄ flow time. The Raman spectra in Figure 1 shows that the as deposited graphene film has the 2D peak at ~2700 cm^-1 with sharp FWHM of 60 cm^-1, the intensity ratio of 2D:G is larger than 4, and the D peak is minimal. Hence, confirming the as-grown film is single layer graphene. Figure 2(a) is a top view SEM image of the vertically aligned graphene nanosheets grown on the silicon substrate and the cross-section can be seen in figure 2(b). Electrochemical deposition was performed to direct deposition of MnO₂on the graphene nanosheets to achieve conformal coating and thus maximum utilization of the high specific surface area as evident from the SEM micrographs in figure 3.

Electrochemical characterization was performed in a flat cell in 3-electrode configuration using 0.1M KCl as the supporting electrolyte and a Ag/AgCl reference electrode. Cyclic voltammograms were recorded in a 0-1V potential window at different scan rates and MnO₂ film thicknesses. We observed an increase in capacitance with increase in MnO₂ thickness, evident by the increase in the currents at fixed scan rates, as seen in figure 4 where CVs recorded at 50mV/s have been overlaid. We recorded a high capacitance of 140mF/cm² or 42.4F/cm³at scan rate 2mV/s. This represents an enhancement of more than 50x over that obtained from just the graphene nanosheets. Detailed graphene synthesis, fabrication and characterization of the novel hybrid supercapacitor will be presented and discussed. 

References:

[1] A. K. Geim And K. S. Novoselov, Nature Materials, 6 (2007) 183-191.

[2] S. Wei, W. P. Kang, J. L. Davidson, B. R. Rogers, and J. H. Huang, ECS Transactions, 28 (8) 97-103 (2010).

[3] Wei Chen, Zhongli Fan, Lin Gu, Xinhe Bao and Chunlei Wang, Chem. Commun., 2010, 46, 3905–3907.