Self-discharge of the nickel electrode in presence of hydrogen II. Electrochemical approach

The Ni/H-2 self-discharge kinetics were studied by means of Ni/H-2 cells eq uipped with a pressure sensor to determine the time-dependence of cell pres sure. The cell pressure variation was found to depend strongly upon cell st ate of charge (e.g., whether the nickel oxyhydroxide was of beta- or gamma- type). For overcharged Ni/H-2 cells (e.g., having a mixture of beta- and ga mma-NiOOH as the nickel electrode) two different rate processes were observ ed for the self-discharge rate as a function of time, For longer times the curve could be well fitted by assuming a single self-discharge mechanism co rresponding to the direct reduction of beta(III) by hydrogen. In contrast, for shorter times the first part of the P(H-2) = f(t) experimental curve ex hibited a higher tangential slope, suggesting most likely a combination of two self-discharge mechanisms, (i) the direct reduction of gamma-phase by h ydrogen and (ii) electrochemical reduction involving the catalytic oxidatio n of hydrogen at the sintered matrix. (C) 1999 The Electrochemical Society. S0013-4651(98)10-017-4. All rights reserved.