Electrochemical oxidation of Mn²⁺ on boron-doped diamond electrodes with Bi³⁺ used as an electron transfer mediator

Electrochemical oxidation of Mn²⁺ with and without the presence of Bi³⁺ was studied using voltammetric and in situ spectro-electrochemical techniques at boron-doped diamond (BDD) electrodes in 1.0 M HClO₄. Electrochemical oxidation of only Mn²⁺ resulted in the formation of mostly MnO₂ with MnO₄ ^- produced as a minor product. The MnO₂ film formed on the electrode surface, which is an inevitable part of Mn²⁺ oxidation, shows a blocking effect on the formation of MnO₄^-, and reduces the overall current efficiency of MnO₄^- production. Higher Mn²⁺ concentrations result in less MnO ₄^- production due to the formation of more MnO ₂. The addition of Bi³⁺ increased the current efficiency of MnO₄^- production. The Bi³⁺ is oxidized to Bi(V), which acts as an electrocatalyst in MnO₄^- production. The Bi(V) oxidizes MnO₂, formed on the electrode surface, to MnO₄^-. This increases the production of MnO ₄^- by removing the blocking film to provide an active electrode (bare BDD) surface, which is available for further Mn²⁺ oxidation.