Redox Processes at the Manganese Dioxide Electrode: III. Detection of Soluble and Solid Intermediates during Reduction

Abstract

The soluble and solid‐state intermediates formed during redox cycling of electrodeposited manganese dioxide (EMD), birnessite and chemically modified EMD (Bi‐CMEMD), and birnessite (Bi‐birnessite) electrodes were investigated using a stationary detector electrode (soluble intermediates) and x‐ray diffraction (solid‐state intermediates). Reduction of each electrode type can be divided into a homogeneous stage followed by a heterogeneous stage. For all electrode types, homogeneous reduction was a solid‐state process involving proton and electron insertion into the manganese dioxide structure, causing a lattice expansion. Toward the end of homogeneous EMD reduction, soluble species were detected, presumably due to an equilibrium shift between solid and solution phase species. The homogeneous/heterogeneous transition was also electrode dependent; i.e., for EMD and Bi‐CMEMD, for birnessite, and for Bi‐birnessite. Heterogeneous electrochemical behavior was also electrode dependent. Initial heterogeneous reduction of EMD, Bi‐birnessite, and Bi‐CMEMD proceeded through a soluble intermediate to form . Electrolyte concentration effects were more pronounced in this stage, since more concentrated KOH electrolytes lead to greater solubility. The composition at which was first detected in the Bi‐birnessite electrode suggested that the Mn(IV) to Mn(III) and Mn(III) to Mn(II) reduction processes occurred simultaneously. Heterogeneous reduction of birnessite was a solid‐state process that resulted in , which is electrochemically inactive. oxidation resulted in formation of birnessite, the exact nature of which depended on the presence or absence of ions. Under these deep discharge cycling conditions, the EMD electrode behaved poorly due to the eventual formation of . However, the Bi‐birnessite and Bi‐CMEMD electrodes are rechargeable due to the presence of ions, which prevent formation.