Effect of synthesis routes on the performance of hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitors

Abstract

MnO2 · nH2O-EG composites for electrochemical capacitors were prepared using commercially available low cost exfoliated graphite (EG) as a conductive substrate, and (a) potassium permanganate and (b) manganese(II) acetate solutions by two different routes. Method (1) was addition of EG to (a), followed by 1 h stirring and then slow addition of (b), and in Method (2) the solutions (a) and (b) were swapped. Using Method (1) submicron or smaller sized MnO2 · nH2O particles having mesopores were formed, whereas Method (2) produced lumps of aggregated particles of several tens microns without mesopores, though specific surface areas were not very different and both were similar by XRD. Although EG alone showed only about 2 F g−1, the capacitance per net amount of MnO2 in 1 mol L−1 Na2SO4 solution increased proportionally with EG content and was always larger by Method (1) than Method (2), that is, the utilization ratio of MnO2 increased with EG content and the effect was more prominent in the case of Method (1). The results indicated that EG is a good conductive material for MnO2 · nH2O electrochemical capacitors if appropriate synthesis processes are used. The performance of the composites strongly depends on synthesis method, even using the same raw materials. It was suggested that the morphology of the products was a primary factor leading to different performance rather than composition.