Effects of thickness and electrolytes on the capacitive characteristics of anodically deposited hydrous manganese oxide coatings

An amorphous nanostructured hydrous manganese oxide (MnO2 · nH2O) coating was anodically deposited onto a graphite substrate from an aqueous MnSO4 solution. Various deposition charges were employed to achieve different coating thickness. The capacitive characteristics of the as-grown MnO2 coatings were evaluated in various neutral alkali metal chloride solutions by cyclic voltammetry and chronopotentiometry. The as-grown MnO2 coatings exhibited a capacitance of 275 F g−1 in a 0.5 M KCl solution. Dependence of coating thickness and electrolytes on the capacitive properties of MnO2 coatings was demonstrated. The relationship of Ca with the deposition charge exhibits linearity in the range of 4.0 C cm−2, and there is a peak at around 4.5 C cm−2, however, the value of Cm keeps diminished. With the increase of the discharge current density, more rapid decrease of the specific capacitance was found when measured in the electrolyte with larger hydrated alkali metal cation in the order of Li+ > Na+ > K+. This was especially noted for thicker MnO2 coatings.