We investigated the electrochemical oxidation of HOPG (highly oriented pyrolytic graphite) in sulfuric acid solution at less than 1.0 V vs. NHE to clarify the oxidation mechanism of carbon materials in the electrode layer in polymer electrolyte fuel cells (PEFCs). It was found by surface analysis with X-ray photoelectron spectroscopy that several kinds of surface functional groups were formed dependently on the potential. In addition, C-Br peak was detected on the surface of HOPG after immersion in HBr solution following holding at 1.0 V for 15 h, which suggests that lactones were formed in the course of electrochemical oxidation. Based on these results, it was reasonable to consider that HOPG was electrochemically oxidized through formation and oxidation of surface functional groups such as alcohol, carbonyl, carboxyl and lactones to CO_2.
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