Degradation of metal hydrides in hydrogen-based thermodynamic machines: A review

Degradation of performance over lifetime is a challenge for interstitial metal-hydride hydrogen storage systems, especially those employed in thermodynamic machines such as compressors and heat pumps, since these must execute many thousands of cycles of absorption and desorption with minimal loss of throughput. Degradation manifests typically as diminished reversible hydrogen capacity, sometimes accompanied by undesirable changes in the shape of the absorption/desorption isotherm or slower kinetics. Understanding the origin and evolution of degradation during absorption/desorption cycling is crucial to designing for long service life with minimal maintenance and replacement costs. This review examines the degradation mechanisms observed in interstitial metal hydrides, focussing on those relevant to thermodynamic machines. Based on the reviewed literature, identifying standard measures for evaluating the degradation of metal hydrides in thermodynamic machines proves challenging. This challenge stems from the variety of reported alloy compositions, and from the widely differing operational configurations employed in published research studies. Furthermore, the degradation mechanisms that manifest during extended absorption/desorption cycling (decrepitation, sintering, hydrogen trapping, loss of crystallinity, disproportionation) are intertwined, sometimes acting sequentially and sometimes concurrently. Time spent in the hydride phase at elevated temperature is a key controlling factor. This review offers insights to aid the selection of alloys for service in hydrogen storage generally and thermodynamic machines in particular. Additionally, an in-depth exploration of the degradation of LaNi5, by far the most studied hydrogen storage alloy, is presented, highlighting the intertwined nature of the acting degradation mechanisms.