Electrochemical study of the graphite-HNO3 system

The anodic oxidation of highly oriented pyrolytic graphite in 50 to 98% HNO 3 was studied in a galvanostatic mode at a current of 1.5 mA. At HNO3 conce ntrations from 55 to 98%, the potential-vs.-charge curves showed steplike b ehavior, and the number of steps increased with decreasing acid concentrati on, reflecting the successive formation of stage III, II, and I graphite ni trate. Electrochemical synthesis of stage I graphite nitrate was shown to b e possible in 75 to 98% HNO3. The anodic oxidation in 60 to 70% HNO3 result ed in a stage I + II mixture, and that in 55% HNO3 yielded a mixture of sta ge II and a defect graphite phase. In 50% HNO3, no intercalation compounds were formed. The specific charge required for the formation of stage I grap hite nitrate was determined as a function of HNO3 concentration. The assump tion is made that, in dilute HNO3 solution containing significant amounts o f water, rapid electrochemical oxidation leads to the formation of graphite oxides.