REDOX PROCESSES AT THE MANGANESE-DIOXIDE ELECTRODE .3. DETECTION OF SOLUBLE AND SOLID INTERMEDIATES DURING REDUCTION

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 we re investigated using a stationary detector electrode (soluble interme diates) and x-ray diffraction (solid-state intermediates). Reduction o f each electrode type can be divided into a homogeneous stage followed by a heterogeneous stage. For all electrode types, homogeneous reduct ion was a solid-state process involving proton and electron insertion into the manganese dioxide structure, causing a lattice expansion. Tow ard the end of homogeneous EMD reduction, soluble species were detecte d, presumably due to an equilibrium shift between solid and solution p hase Mn3+ species. The homogeneous/heterogeneous transition was also e lectrode dependent; i.e., similar to MnO1.55 for EMD and Bi-CMEMD, sim ilar to MnO1.83 for birnessite, and similar to MnO1.73 for Bi-birnessi te. Heterogeneous electrochemical behavior was also electrode dependen t. Initial heterogeneous reduction of EMD, Bi-birnessite, and Bi-CMEMD proceeded through a soluble Mn3+ intermediate to form Mn(OH)(2). Elec trolyte concentration effects were more pronounced in this stage, sinc e more concentrated KOH electrolytes lead to greater Mn3+ solubility. The composition at which Mn(OH)(2) was first detected in the Bi-birnes site electrode suggested that the Mn(IV) to Mn(III) and Mn(III) to Mn( II) reduction processes occurred simultaneously. Heterogeneous reducti on of birnessite was a solid-state process that resulted in Mn3O4, whi ch is electrochemically inactive. Mn(OH)(2) oxidation resulted in form ation of birnessite, the exact nature of which depended on the presenc e or absence of Bi3+ ions. Under these deep discharge cycling conditio ns, the EMD electrode behaved poorly due to the eventual formation of Mn3O4. However, the Bi-birnessite and Bi-CMEMD electrodes are recharge able due to the presence of Bi3+ ions, which prevent Mn3O4 formation.