Cycling stability of RNi5 (RÂ =Â La, La+Ce) hydrides during the operation of metal hydride hydrogen compressor

This work presents results of the experimental studies (XRD, SEM, PCT) of hydride forming intermetallides used in the first (LaNi5) and the second (La0.5Ce0.5Ni5) stages of industrial-scale metal hydride hydrogen compressor providing H2 compression from 3.5 to 150 atm with the productivity about 10 Nm3/h. During the operation, both materials underwent 18,180 hydrogenation/dehydrogenation (h/d) cycles which included H2 absorption at the pressure of 3.5 atm (LaNi5) and 35-38 atm (La0.5Ce0.5Ni5) at T = 15-20 °C followed by H2 desorption at the pressure of 35-38 atm (LaNi5) and 150 atm (La0.5Ce0.5Ni5) at T = 150-160 °C. It was found that the observed ∼30% drop of the productivity of the compressor by the end of its operation is associated with a degradation of the first stage hydride material (LaNi5) under conditions specified above. The cycling resulted in the appearance of Ni and LaH2+x phases in addition to the parent intermetallide. In turn, the cycled LaNi5 exhibited more than 20% lower hydrogen storage capacity than the alloy at the beginning of the cycling; the cycling was also found to result in a noticeable sloping of initially flat plateau. Conversely, the degradation effects in La0.5Ce0.5Ni5 were found to be much less pronounced, in spite of the higher operating H2 pressures. The observed effect was associated with the decrease of thermodynamic driving force (TDF) of AB5 disproportionation in H2 when substituting La with Ce.