The functioning mechanism of Al valid substitution for Co in improving the cycling performance of Zr–Co–Al based hydrogen isotope storage alloys

Abstract

In order to optimize the hydrogen isotope storage properties of ZrCo alloy and promote its application in the storage and delivery system (SDS), ZrCo alloy is modified by partial substitution of Co with Al and subsequent melt spinning process. The microstructure and hydrogen isotope storage properties, especially the cycling performance, of ZrCo1-xAlx (x = 0–0.15) samples were thoroughly investigated. It is found that all as-cast Al-substituted alloys are consisted of a main phase of ZrCo, while the segregation of Al element at grain boundary is more serious with the increase of Al doping content, which restrains the valid substitution content of Al for Co in ZrCo phase. The initial activation performances of the Al-substituted alloys are enhanced and excellent hydriding kinetics are maintained. With the increase of Al substitution content in ZrCo phase of ZrCo1-xAlx (x = 0–0.15) alloys, the initial saturated hydrogen capacities of α solid-solution phase are improved significantly, which is attributed to the decline of valence electron concentrations (VEC) for Al-substituted alloys. Further analysis shows that the hydrogenation-dehydrogenation process of ZrCo1-xAlx (x = 0–0.15) alloys after multiple cycles has been altered as intercalation and de-intercalation of hydrogen atoms between ZrCo phase and α phase, leading to cycling capacity stabilization. More importantly, the improved terminal saturated hydrogen capacities of α phase in the Al-substituted alloys, which represents enhanced final cycling stable capacities, is linked with the increase of initial saturated hydrogen capacities of α phase after Al substitution for Co in ZrCo phase. In addition, the valid substitution content of Al for Co in ZrCo phase is further increased without any element segregation for ZrCo0.9Al0.1 alloy processed by melt spinning, which behaves further optimized final cycling stable capacity of 0.93 wt%, and its variation of the hydrogenation and dehydrogenation paths is similar to those of the above samples. In this work, the mechanism of cycling capacity stabilization has been explored in detail, which provides guidance to improve the cycling stability of ZrCo-based hydrogen isotope storage alloys from point of phase structure of ZrCo alloy.