Thermodynamics of the (Sm2Fe17-xGax+H2) system

Some hypo-stoichiometric Zr-based Laves phase alloys were prepared and studied from a viewpoint of discharge capacity for electrochemical application. After careful alloy design of ZrMn2-based hydrogen storage alloys through changing their stoichiometry while substituting or adding some alloying elements, the Zr(Mn0.2V0.2Ni0.6)1.8 alloy reveals relatively good properties with regard to hydrogen storage capacity, hydrogen equilibrium pressure and electrochemical discharge capacity. In order to improve the discharge capacity and ratecapability, Zr is partially replaced by Ti. The discharge capacity of Zr1-xTix(Mn0.2V0.2Ni0.6)1.8 (x=0.0, 0.2, 0.3, 0.4, 0.6) alloy electrodes at 30°C reaches a maximum value and decreases as the Ti fraction increases. In view of electrochemical and thermodynamic characteristics, the occurrence of a maximal phenomenon of the electrochemical discharge capacity of the alloy is attributed to a competition between decreasing hydrogen storage capacity and increasing rate-capability with Ti fraction. However, as the Ti fraction increases, the discharge capacity decreases drastically with repeated electrochemical cycling. Judging from the analysis of surface composition by Auger electron spectroscopy (AES), the rapid degradation with increasing Ti fraction in Zr-based alloy is ascribed to the fast growth of the oxygen-penetrated layer with cycling. Therefore, it is assured that the stoichiometry and Ti fraction should be optimized to obtain a good cycle life of the electrode maintaining high discharge capacity. On the basis of above results, the hydrogen storage capacity of the alloy with optimized composition (Zr0.65Ti0.35(Mn0.3V0.14Cr0.11Ni0.65)1.76) is about 1.68 wt% under 10 atm of equilibrium hydrogen pressure and the discharge capacity of the alloy is about 421 mAh/g at a discharge rate of 50 mA/g, which shows the highest level in performance of the Zr-based alloy ever developed.