THIN-FILM PREPARATION OF HYDROGEN STORAGE ALLOYS AND THEIR CHARACTERISTICS AS METAL HYDRIDE ELECTRODES

Thin films of hydrogen storage alloys (LaNi5, LaNi2.5Co2.5) were prepared by RF sputtering under argon and hydrogen atmospheres. Crystallinity (amorphous or crstalline) and electrical capacity of the thin films depended on the types of targets and the kinds of substrates and the conditions for RF sputtering such as RF power, temperature, and atmosphere. Both crystalline-oriented and amorphous films were obtained. In the former film, the c-axis was parallel to the substrate plane. Hydrogen was absorbed as a solid solution, causing no new phase. Electrode properties such as discharge capacity charge-discharge cycle life, discharge capability, temperature dependence, and self-discharge rate were examined and discussed in comparison with those for the bulk materials. These alloy films had no pressure plateau on electrochemical pressure-composition isotherms. The maximum capacities were 160 mAh/g for LaNi5 crystalline film, 80 mAh/g for LaNi5-H film prepared under Ar-H2 atmosphere, and 80 mAh/g for the LaNi2.5Co2.5 amorphous film. The capacity of LaNi5 films was reduced to half after 100 cycles, while the LaNi5-H and LaNi2.5Co2.5 films showed only small capacity decay even after 500 cycles (about 10-25%). The LaNi2.5Co2.5 film had better discharge capability at low temperatures than the LaNi5 film. Self-discharge rates of these electrodes were so high that the storage capacities were completely lost within one week.