Electrochemical quartz crystal microbalance and rotating ring disk electrode analysis of manganese dioxide electrodeposition for thin film electrochemical capacitors

This work compares the electrodeposition behavior of thin films of manganese dioxide produced from solutions of four commonly available Mn(II) salts; namely, sulfate, chloride, nitrate and acetate, as well as their corresponding acids. The deposition behavior is investigated in terms of the masses of the thin films produced, calculated by integration of charge current during deposition, and by the electrochemical quartz crystal microbalance (EQCM). Generally, it was shown that EQCM masses deviated from the mass expected from current integration, with increases ranging from 14-40%, depending on the anion. The most notable deviations from this trend were the high-acid (0.1 M) and low-Mn(II) (0.01-0.001 M) solutions where MnCl₂ and HCl were used. This difference is discussed in terms of the deposition mechanism, and in particular, complexation of the Mn(III) intermediate with excess chloride in solution, accounting for any anomalous behavior. This claim is further corroborated through rotating ring disk electrode (RRDE) data, which was used to detect any Mn(III) that would be lost to the solution after an initial oxidation. Their behavior as supercapacitor electrode materials has also been compared.