In situ X-ray absorption spectroscopic studies of anodically deposited binary Mn–Fe mixed oxides with relevance to pseudocapacitance

Binary Mn–Fe oxides with different Mn/Fe content ratios were prepared by anodic deposition. The deposited oxides were studied by in situ X-ray absorption spectroscopy (XAS) in 2 M KCl solution during the charging–discharging process. The experimental results clearly confirmed that the oxidation states of both Mn and Fe changed back and forth with adjusting the applied potential, contributing to the pseudocapacitive characteristics of the binary oxides. It was also found that, within a potential range of 1 V, because of Fe oxide addition the variation in the Mn oxidation state was increased from 0.70 (+3.25 to +3.95 for plain Mn oxide) to 0.81 (+3.12 to +3.93 for Mn90Fe10 oxide or +3.10 to +3.91 for Mn75Fe25 oxide), while the Fe oxide itself demonstrated an oxidation state shift of only 0.55. Accordingly, the optimum pseudocapacitance of the binary Mn–Fe oxide could be only achieved when the amount of Fe oxide was properly controlled. The highest specific capacitance of 255 F g−1 was obtained with a Mn/Fe atomic ratio of 90/10, while plain Mn oxide revealed a capacitance of only 205 F g−1.