Degradation of intermetallic hydrogen storage alloys under isochoric thermal cycling: heat-pump-like conditions

A systematic investigation is reported of intrinsic degradation in six representative intermetallic hydrogen storage alloys (LaNi 5 , Mm(NiCoMnAl) 4.82 , MmNi 4.2 Co 0.8 , MmNi 4.35 Co 0.8 Al 0.05, Mm 0.8 Ca 0.2 Ni 5 and Hydralloy C5) subjected to >2000 isochoric thermal cycles between 0 and 90 °C under extremely pure hydrogen, emulating service in a closed metal-hydride heat pump. Reversible hydrogen capacity losses ranging from negligible to >50 % were observed. X-ray diffraction revealed the formation of Ni (or Ni–Co) precipitates in the degraded AB 5 alloys, accompanied by increased diffuse scattering and anisotropic lattice strain, indicating progressive microstructural disorder, consistent with disproportionation of AB 5 to B metal and amorphous A hydride without ruling out other possibilities. The results underscore the strong dependence on alloy composition of intrinsic degradation under isochoric thermal cycling and highlight the importance of evaluating metal hydrides under operating conditions representative of service conditions as well as the need for further investigations to understand degradation mechanisms at the atomic level. • Systematic comparison of 6H 2 storage alloys (5 AB 5 s, 1 AB 2 ) under isochoric cycling. • Metal-hydride heat pump emulation with a single charge of extremely pure hydrogen. • Degradation ranged from negligible (Hydralloy C5) to severe (MmNi 4.35 Co 0.8 Al 0.05 ). • Correlations found between degradation, lattice strain and increased XRD background. • XRD on AB 5 s consistent with disproportionation to amorphous A-hydride and B metal.