Fig. 1.

CVs of a 1-cm² FTO electrode in 0.5 mM Mn²⁺ and 0.9 M KNO₃ solution at 100-mV/s scan rate showing the first (red line) and second (blue line) cycles for (A) the as-deposited δ-MnO₂ and (B) the as-deposited film activated via the incorporation of Mn³⁺ ions by the comproportionation reaction (Eq. 4). A–D indicate the waves discussed in the text.