Effect of the surface chemistry of activated carbon on its electrochemical properties in electric double layer capacitors

Petroleum coke-based activated carbons (ACs) were modified through H₂ reduction and HNO₃ oxidation. Pore structures and surface chemistry of the ACs were characterized by N₂ adsorption and X-ray photoelectron spectroscopy. The Brunauer-Emmett-Teller surface areas of ACs were slightly decreased after such treatment. Although surface oxygen groups (especially carbonyl groups) were generated when ACs were treated by HNO₃, electric double layer capacitance, internal resistance (IR), equivalent series resistance (ESR), and selfdischarge of an AC electrode exhibited a remarkable increase. At the same time, the number of surface oxygen groups of ACs modified by H₂ remarkably decreased and the capacitance was markedly increased , while IR, ESR, and self-discharge decreased. This reduction modification of ACs is beneficial in improving electrochemical properties and probably provides suitable pores for electrolyte to penetrate electrochemically.