Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor

Commercially available low cost exfoliated graphite (EG, nominal diameter 130 μm) was used as a conductive substrate for electrochemical capacitor of hydrated Mn(IV) oxide, MnO2·nH2O. The MnO2·nH2O–EG composites were prepared by addition of EG to potassium permanganate solution, followed by 1 h stirring and then slow addition of manganese(II) acetate solution. By this procedure submicrometer or smaller sized MnO2·nH2O particles having mesopores of 6–12 nm in diameter were formed on the graphite sheets of EG. Although EG alone showed only about 2 F g−1, the composites showed good rectangular cyclic voltammograms at 2–20 mV s−1 in 1 mol L−1 Na2SO4. The capacitance per net amount of MnO2 increased proportionally with EG content, that is, utilization ratio of MnO2 increased with EG content. The composites of MnO2·nH2O and smaller diameter of EG (nominal diameter 45 μm) or artificial graphite powder (average diameter 3.7 μm) showed fairly good performance at 2 mV s−1, but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors.