Candidate alloys (four AB 5 s and one AB 2 ) were cycled under conditions emulating service in a metal-hydride hydrogen compressor, with a fresh charge of 99.98%-pure hydrogen taken in at the start of each isochoric heating part-cycle. The samples therefore suffered both intrinsic (temperature-induced) and extrinsic (impurity-induced) degradation. Comparing the results with those from a previous study using extremely pure hydrogen allowed the two kinds of degradation to be distinguished. While all alloys retained their parent crystal structures, their resistance to impurity-induced degradation varied widely, and differently from intrinsic-only degradation, which is not a good predictor of extrinsic degradation, nor vice-versa. Among the alloys studied, only Mm(NiCoMnAl) 4.82 is a good candidate for compressor service with impure hydrogen. Impurities accelerated the structural degradation of all the AB 5 -type alloys except MmNi 4.2 Co 0.8 , along with causing changes to the pressure–composition isotherms, showing that their effect is internal, as well as at the surface. The findings highlight the complex nature of degradation. This work establishes a methodology for systematically assessing impurity-induced degradation and points to the need for fundamental studies to better understand degradation phenomena so that such results can be understood within a unified atomic-level picture. • Systematic comparison of five hydrogen storage alloys (4 AB 5 s plus 1 AB 2 ) under isochoric heating • New charge of 99.98% hydrogen each cycle emulates a metal-hydride hydrogen compressor • Comparison with cycling under extremely pure hydrogen allows delineation of extrinsic degradation • Extrinsic degradation ranged from negligible (MmNi 4.2 Co 0.8 ) to severe (Hydralloy C5) • Susceptibility to intrinsic degradation did not predict susceptibility to extrinsic degradation
Zohra et al. (Sun,) studied this question.