Influence of interphase boundary coherency in high-entropy alloys on their hydrogen storage performance

TL;DR

This paper investigates how the coherence of interphase boundaries in high-entropy alloys affects their ability to store hydrogen, revealing that incoherent boundaries with sufficient fraction enhance activation and storage performance.

Contribution

It demonstrates that incoherent interphase boundaries with adequate fraction improve hydrogen activation in HEAs, providing a strategy for designing better hydrogen storage materials.

Findings

Incoherent boundaries facilitate hydrogen absorption without activation.

High fraction of incoherent boundaries enhances storage performance.

Coherent boundaries are less effective due to lower free volume and boundary energy.

Abstract

High-entropy alloys (HEAs) have potential for storing hydrogen reversibly at room temperature due to their tunable thermodynamics; however, they usually suffer from the issue of difficult activation. This study shows that while interphase boundaries are effective in activating some HEAs, some other dual-phase HEAs still require extra high-temperature activation. To understand why interphase boundaries are not always effective for activation, microstructural features and hydrogen storage performance of six HEAs with dual phases are compared. Detailed analysis confirms that interphase boundaries are effective for hydrogen absorption without the need for activation treatment, provided that: (i) their fraction is high enough, and (ii) they are not coherent. These findings are discussed in terms of free volume and boundary energy. Coherent interphase boundaries are associated with lower free volume and thus do not act as fast hydrogen diffusion paths. Moreover, the boundary energy of coherent boundaries is lower than incoherent boundaries, making them less favorable sites for heterogeneous hydride nucleation. This research thus suggests that the introduction of incoherent interphase boundaries with a proper fraction is a solution for activating hydrogen storage materials.