Covalently-Functionalized Graphene for Supercapacitor Application

Due to the power requirements for electric vehicles, tremendous work has been devoted to energy storage devices with high power density, energy density, and rate performance. Electrochemical supercapacitors are very attractive because of their better cycling performance and higher power density as compared to Lithium-ion batteries.¹ Carbon materials have been considered as promising materials as supercapacitors due to their high surface area, excellent electrical conductivity, and good stability.² The double-layer capacitance of a carbon supercapacitor is proportional to the surface area of its electrode material accessible by the electrolyte ions.³ However, commercial supercapacitors based on activated carbon electrodes only deliver an specific capacitance of about 100 F/g, which is significantly lower than that of Li-ion batteries.⁴ Therefore, preparing materials with high surface area and suitable pore size distribution is very critical in achieve high-performance supercapacitors. Recently, graphene, a two-dimensional single-atom-thick layer of sp2-bonded carbon, has drawn much attention due to its exotic properties, such as high surface area, electric conductivity, and mechanical strength.⁵ These properties make graphene a very promising candidate for supercapacitor application.⁶ However, due to the strong van de Waals force between neighboring layers, graphene sheets are inclined to restack into aggregates a few layers thick, resulting in a large decrease in the surface area. Here, we report facile fabrication of covalently-functionalized graphene as supercapacitor electrodes. One example is covalently-grafted polyaniline on graphene oxide, and the other example is phenylbenzimidazole-functionalized graphene.

A facile three-step synthesis to prepare covalently-grafted PANI/GO nanocomposites at room temperatures. The covalently-grafted composites formed a uniform hierarchical morphology of nanorod-like PANI grown on planar GO sheets, in contrast to a nonuniform morphology of nongrafted composites. The covalently-grafted PANI/GO composites exhibited higher surface area and larger pore volume compared with PANI and and noncovalently-grafted PANI/GO composites. These features allow the increased exposure of PANI to the electrolyte ions, resulting in a more accessible PANI surface for redox reaction species and faster ion transport. The covalently-grafted PANI/GO composites prepared with an aniline/GO ratio of 6:1 showed the highest capacitance of 422 F/g at a current density of 1 A/g. The capacitance can be retained at about 83% after 2000 cycles at 2 A/g. The excellent electrochemical performance can be attributed to the maximized synergistic effect between GO and PANI in the PANI/GO composites prepared via this covalent grafting approach.

Phenylbenzimidazole covalently-functionalized graphene was prepared by hyderthermal treatment of benzoic acid-functionalized graphene oxide and o-phenyldiamine in a teflon-lined autoclave for 16 h at 160 °C. Difference from pristine graphene oxide, carboxyl groups are present on both edge and basel plane of graphene oxide sheets in the benzoic acid-functionalized graphene oxide, which was prepared from diazonium reaction. The phenylbenzimidazole group can effectively act as the spacers to partially prevent the restacking/aggregation of the graphene sheets. The phenylbenzimidazole groups are also electrochemical active and can contribute to psedocapacitance. When used as supercapacitor electrodes, the functionalized graphene materials exhibit much better electrochemical performances (more than twice as much capacitance) than pure graphene, implying their potential for energy storage applications.

Figure 1. (a) CV curves of polyaniline and and covalently-grafted PANI/GO composites in 1 M H₂SO₄ at a scan rate of 10 mV/s; (b) CV curves of graphene and and Phenylbenzimidazole covalently-functionalized graphene in 1 M Na₂SO₄at a scan rate of 10 mV/s.

Reference

1.          Conway, B. E., Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Springer: 1999.

2.          Zhang, L. L.; Zhao, X. S., Chemical Society Reviews 2009, 38(9), 2520-2531.

3.          Wang, L.; Fujita, M.; Inagaki, M., Electrochimica Acta 2006, 51(19), 4096-4102.

4.          Pandolfo, A. G.; Hollenkamp, A. F., Journal of Power Sources 2006, 157(1), 11-27.

5.          Allen, M. J.; Tung, V. C.; Kaner, R. B., Chemical Reviews 2009, 110(1), 132-145.

6.          Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S., Nano Letters 2008, 8 (10), 3498-3502.