ELECTROCHEMICAL CHARACTERISTICS OF HYDROGEN STORAGE ALLOYS MODIFIED BY ELECTROLESS NICKEL COATINGS

Multicomponent hydrogen storage alloy with the composition MmNi3.31Mn0.37Al0.28Co0.64 (Mm = misch metal consisting of 24.87% La, 52.56% Ce, 5.57% Pr, 16.86% Nd and 0.14% Sm) was used as a negative electrode material. To improve the performance of the negative electrode, the alloy particles were modified with various electroless Ni-P and Ni-B coatings. Electrochemical properties such as discharge capacity and high-rate dischargeability of the negative electrode were generally improved by the surface modifications. However, in the case of a Ni-P (10 wt% P) coating, the discharge capacity and high-rate dischargeability decreased with increasing thickness, due to the low hydrogen permeability and high electric resistivity of the coating. Consequently, both kinds of coating and their thickness are critical factors in determining the performance of such a negative electrode.