Reversible redox reaction on the oxygen-containing functional groups of an electrochemically modified graphite electrode for the pseudo-capacitance

Reversible redox reactions on the oxygen-containing functional groups of an electrochemically modified graphite electrode (GE) by recurrent galvanic pulses in 2.3 M H2SO4 solution are investigated. The crystal morphology, surface composition and electrochemical performance of electrochemically modified GE are monitored by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption, Fourier transform infrared spectroscopy, thermogravimetry, Boehm's titration, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry (CV) tests. After the electrochemical modification, the material shows a turbostratic structure with many small domains consisting of multi-layered graphene sheets in parallel, and presents a high specific capacitance of 179.7 F g(-1), as well as a good stability after 10000 CV cycles. The high specific capacitance is mainly attributed to the continuous reversible redox reactions of the active groups among the hydroxyl (reduced state), carbonyl (half-oxidized state) and carboxyl (fully oxidized state) groups; a redox mechanism is obtained consequently. The amount of active groups on the modified GE surface is about 848-875 mu mol g(-1) by the Boehm titration. Their contribution to the pseudo-capacitance according to the redox mechanism is about 130-140 F g(-1), which is consistent with the results of electrochemical measurements.