Design of High-Entropy Hydrides for Reversible Hydrogen Storage

Hydrogen is a promising energy carrier that can be produced with zero-carbon emissions and is expected to play a key role in future energy systems. However, storage and transportation of hydrogen in a safe and cost-effective way is challenging. One promising method is to store hydrogen in a solid metal hydride; compared to high-pressure hydrogen storage, the metal hydride technology offers potential safety advantages (due to low pressure operation), with the additional benefit of higher volumetric density. We developed a machine learning approach to design high entropy alloys (HEAs) with a high hydrogen-to-metal ratio and tunable equilibrium plateau pressures. We used a gradient boosting tree approach combined with SHapely Additive Predictions (SHAP) to elucidate simple physics-based design rules that dictate the thermodynamic properties of metal hydrides, then employed these models to screen HEA hydrides across a large compositional space (>21,000 distinct candidate compositions). We demonstrated how feature importance uncovers the strong dependence of the metal hydride equilibrium H₂ pressure on a volume-based descriptor that can be computed from just the elemental composition of the alloy material. This in turn permitted rational targeting of high-hydrogen capacity HEAs by their descriptor values. We used high-energy ball milling and arc melting approaches to synthesize dozens of HEA materials which cover a wide range of pressure and temperature regimes for reversible hydrogen uptake and release. The HEA materials were characterized by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron spectroscopy. Hydrogen storage properties were measured using the Sieverts techniques, and Pressure-Composition-Temperature measurements were used to determine the enthalpy and entropy of hydrogen uptake and release in the case of most promising HEAs. We show that frustrated chemical environments found in HEA materials composed of 4-, 5-, or 6- different atom types lead to important changes in the thermodynamics and kinetics of chemical processes, which can be leveraged for sustainable hydrogen storage applications..