LARGE DISCHARGE CAPACITY FROM CARBON ELECTRODES IN SULFURIC-ACID WITH OXIDANT

The discharge performance of the graphite intercalation compounds in sulfuric acid containing nitric acid (H2SO4-GICs) was studied by focusing on the effects of oxidant and carbon nanotexture. A large discharge capacity from H2SO4-GICs synthesized by using an excess amount of HNO3, more than 150 times of the theoretical value (93 mAh/g carbon), was obtained depending on the amount of oxidant added, the discharge current, and the nanotexture of carbon electrode. The experimental results are explained in terms of competition between the de-intercalation of sulfuric acid due to galvanostatic reduction and the re-intercalation due to chemical oxidation by HNO3 during discharging. However, a subsidiary reaction decreases the effective amount of HNO3 on the discharge by a small current and also on the cycle of chemical charging and electrochemical discharging. The oxidant KMnO4 gave only a little larger capacity for discharge than the theoretical one, because it was reduced to the manganese oxide precipitates during the oxidation of the carbon electrode.