EFFECT OF MANGANESE HYDROXIDE LAYER ON THE ELECTROCHEMISTRY OF NICKEL-HYDROXIDE THIN-FILMS

The cyclic voltammetry behavior of a binary electrode obtained by sequential electrochemical deposition of hydrous manganese oxide and nickel hydroxide onto nickel substrate is described. The effect of repetitive potential steps and potential sweeps on the cyclic voltammetry of this electrode, Ni/Mn (OH)2 and Ni/Ni(OH)2 electrodes is described as well. The cyclic voltammogram of a freshly prepared Ni/Mn(OH)2/Ni(OH)2 electrode consists in the individual redox waves of Mn(OH)2 and Ni(OH)2 which are shifted towards more and less positive material, respectively. After repetitive potential steps between −0.55V vs. SCE and a positive limit that depended upon the pH of the solution, the cyclic voltammogram is characterized by a well-defined anodic wave at about −0.1V in 0.5M KOH that is attributed to a new manganese-nickel hydroxide phase. The redox waves of Ni(OH)2 are still observed and the oxygen overpotential is similar to that of the Ni/Ni(OH)2 electrode. The pH-dependence of the voltammetric waves for the Ni/Mn(OH)2/Ni(OH)2 electrode was also studied. Anodic waves corresponding to oxidation of manganese-nickel hydroxide and nickel hydroxide phase shifted by 70–95 mV/pH unit. The charge associated with the oxidation of the manganese-nickel hydroxide phase is independent of pH in contrast to that of the Ni(OH)2 oxidation which decreased as the pH is lowered (in solution containing only KOH). However, in solution of ionic strength equal or higher than 0.5M the size of the Ni(OH)2 anodic wave is almost pH independent, at low sweep rate. © 1990, The Electrochemical Society, Inc. All rights reserved.