Kinetics of hydrogen evolution reaction at nonstoichiometric hydrogen storage alloys for nickel-hydrogen batteries

Abstract

The kinetics of the hydrogen evolution reaction at Mm(B5)x electrodes (Mm = misch metal, B5 = Ni3.6Mn0.4Al0.3Co0.7 x = 0.88, 1.00 and 1.12) was investigated by measuring the potential decay immediately after the interruption of an applied cathodic current. The total overvoltage (η) for the hydrogen evolution reaction was divided into two components (η1 and η2) corresponding to the Volmer and the Tafel reactions. The exchange current densities of the elementary reactions (iOV and iOT) were then evaluated by extrapolating the Tafel lines for η1 and η2. The values of η1 and η2 at the Mm(B5)0.88 electrode were found to be almost the same as those at the Mm(B5)1.00 electrode. At the Mm(B5)1.12 electrode, however, the values of η2, i0V/i0T and activation energy for the Tafel reaction were very large, compared with those at the other two electrodes, indicating, that the Tafel reaction was fairly depressed at the Mm(B5)1.12 electrode. The Mm(B5)1.12 electrode gave a high-rate chargeability owing to the high overvoltage for the Tafel reaction.