Elucidating Primary Degradation Mechanisms in High-Cycling-Capacity, Compositionally Tunable High-Entropy Hydrides
Abstract
The hydrogen sorption propertiesof single-phase bcc(TiVNb)(100-x )Cr (x) alloys (x = 0-35) are reported.All alloysabsorb hydrogen quickly at 25 & DEG;C, forming fcc hydrides with storagecapacity depending on the Cr content. A thermodynamic destabilizationof the fcc hydride is observed with increasing Cr concentration, whichagrees well with previous compositional machine learning models formetal hydride thermodynamics.
The steric effect or repulsive interactionsbetween Cr-H might be responsible for this behavior. The cyclingperformances of the TiVNbCr alloy show an initial decrease in capacity,which cannot be explained by a structural change.
Pair distributionfunction analysis of the total X-ray scattering on the first and lastcycled hydrides demonstrated an average random fcc structure withoutlattice distortion at short-range order. If the as-cast alloy containsa very low density of defects, the first hydrogen absorption introducesdislocations and vacancies that cumulate into small vacancy clusters,as revealed by positron annihilation spectroscopy.
Finally, the mainreason for the capacity drop seems to be due to dislocations formedduring cycling, while the presence of vacancy clusters might be relatedto the lattice relaxation. Having identified the major contributionto the capacity loss, compositional modifications to the TiVNbCr systemcan now be explored that minimize defect formation and maximize materialcycling performance.