Aqueous Intercalation of Graphite at a Near-Neutral pH

Graphite has been extensively studied as a battery electrode, but few investigations have explored its behavior in water at neutral pH. Here, we investigate graphite intercalation in an aqueous ammonium sulfate electrolyte with a pH of 6. We identified potentials at which bisulfate intercalation and deintercalation compete with water oxidation. In situ X-ray diffraction revealed that the interlayer space of graphite expands during intercalation, reaching a stage-two graphite intercalation compound. Irreversible changes to the graphite occurred during intercalation, as demonstrated by the appearance of sp3-type carbon, hydroxyl, and carbonyl groups in Raman and X-ray photoemission spectra. Our findings indicate that intercalation of graphite in aqueous media at a near-neutral pH and modest potentials is possible, but improved chemical and structural stability of the graphite and electrolyte are required to achieve long-term operation.