ELECTROCHEMICAL AND SURFACE-PROPERTIES OF LOW-COST, COBALT-FREE LANI5-TYPE HYDROGEN STORAGE ALLOYS

For many applications AB5 type hydrogen storage alloys are still too expensive. We report on the results of studies aimed at lowering the alloy costs by partial substitutions of A and B elements respectively. We have improved the capacity of silicon-containing cobalt-free AB5-type alloys by optimization of the La:Ce ratio in the A component and by the substitution of aluminium by manganese in the B component. The alloy Lm0.5Mm0.5Ni4.2Mn0.2Al0.3Si0.3 (Mm, rare earth mischmetal; Lm, lanthanum-rich mischmetal) showed a maximum capacity of 270 mA h g-1 and good cycling stability, i.e. the optimum in price/performance ratio. Substitution of larger amounts of nickel with copper could further lower the price of such alloys; however, these copper-containing alloys showed low kinetics and a slow activation. Electrochemical measurements were carried out on ten different single-phase AB5 alloys and on a two-phase AB5.36-type alloy. Up to 500 charge-discharge cycles were performed. Samples were characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis and photoelectron spectroscopy. Photoelectron spectroscopic analysis of Lm0.5Mm0.5Ni2.4Cu2Mn0.25Si0.35 after 30 electrochemical charge-discharge cycles points to a thick nickel oxide top surface layer, no enrichment of metallic nickel in near surface layers and to a slightly enhanced metallic copper subsurface layer. No passivating silicon surface oxide film could be found. We attribute the low electrochemical kinetics of this alloy to the absence of metallic nickel at the electrode-electrolyte interface.