Stability and Structure of Binary Metal Hydrides under Pressure, Electrochemical Potential and Combined Pressure-Electrochemistry

TL;DR

The paper tackles the challenge of controllably synthesizing binary metal hydrides by introducing the $P^2$ framework, which couples pressure and electrochemical potential to map stable MH$_x$ phases via density functional theory. By constructing Gibbs free-energy-based convex hulls at fixed $P,T$ and $U_{\text{RHE}}$, the authors predict phase diagrams for Sc, Y, La, V, and Cr, and demonstrate that many predictions align with experimental observations, while also forecasting new hydrides accessible through pressure–electrochemistry. A key finding is that hydrogen transfer and structural diversity increase with reduced metal ionization potential and larger ionic radii, enabling stabilization of higher hydrides at practical conditions when pressure and potential are combined. The results underscore the potential of pressure–electrochemistry as a practical route to novel hydrides with implications for hydrogen storage and superconductivity, while highlighting the need to account for entropy and nuclear quantum effects in future work.

Abstract

Metal hydrides can be tuned to have a diverse range of properties and find applications in hydrogen storage and superconductivity. Finding methods to control the synthesis of hydrides can open up new pathways to unlock novel hydride compounds with desired properties. We introduced the idea of utilizing electrochemistry as an additional tuning knob and in this work, we study the synthesis of binary metal hydrides using high pressure, electrochemistry and combined pressure-electrochemistry. Using density functional theory calculations, we predict the phase diagrams of selected transition metal hydrides under combined pressure and electrochemical conditions and demonstrate that the approach agrees well with experimental observations for most phases. We use the phase diagrams to determine trends in the stability of binary metal hydrides of scandium, yttrium and lanthanum as well as discuss the hydrogen-metal charge transfer at different pressures. Furthermore, we predict a diverse range of vanadium and chromium hydrides that could potentially be synthesized using pressure electrochemistry. These predictions highlight the value of exploring pressure-electrochemistry as a pathway to novel hydride synthesis.

Figures (7)

• Figure 1: Demonstrating the effect of a) pressure and b) electrocehmical potential on the stability of yttrium hydrides using the convex hull approach. Varying the conditions determines which phases appear on the convex hull and are therefore stable.
• Figure 2: $P^2$ phase diagrams for a) scandium b) yttrium and c) lanthanum hydrides. The dashed line is a guide to the eye for $U_{\text{RHE}}=0$. The dash-dotted line shows the potential below which HER is expected to dominate as calculated by Guan et al.guan_combining_2021
• Figure 3:
• Figure 4: $P^2$ phase diagrams for a) vanadium and b) chromium hydrides. The dashed line is a guide to the eye for $U_{\text{RHE}}=0$
• Figure S1: $P^2$ diagram for Y-H system up to 200GPa. The experimentally synthesized YH4 amd YH6 are found stable.