POTENTIAL OSCILLATIONS DURING GALVANOSTATIC OVEROXIDATION OF GRAPHITE IN AQUEOUS SULFURIC ACIDS

Potential oscillations of a graphite electrode during galvanostatic overoxidation in aqueous solutions of sulphuric acid to yield electrochemical graphite oxide (EGO) were studied experimentally. Oscillatory behaviour was observed only in a well-defined range of medium acid concentrations and current densities. Typical time constants Delta t were about 1 h. A zone model is introduced to rationalize the phenomenon. It is proposed that the formation of C-OH groups proceeds in the presence of co-intercalated and on-transported water molecules at the most positive potentials. Protons are liberated, and some of them combine with the anions to form acid molecules. Their off-transport is severely hindered by the hydrogen bond structure between the C-OH groups. However, further oxidation to C=O groups or dehydration of vicinal C-OH to epoxy groups in the limited zones yields a more open structure, H2O, H+ and HA are transported more easily, and a depolarization effect is established. This sequence is repeated during electrochemical conversion in the next zone. The depth L of the zone is estimated to be about 2-3 mu m, and the diffusion coefficient D is of the order of 10(-11) cm(2) s(-1). The virtual stoichiometry in the zones (and of the whole EGO crystal at the end of the charge) is C25OH.