Facile synthesis of palladium hydride via ionic gate-driven protonation using a deep eutectic solvent

Developing novel protocols for hydrogen (H) loading is crucial for furthering the investigation of hydrides as potential high-temperature superconductors at lower pressures compared to recent discoveries. Ionic gating-induced protonation (IGP) has emerged as a promising technique for H loading due to its inherent simplicity, but it can be limited in the maximum density of injected H when ionic liquids are used as a gating medium. Additionally, most ionic liquids are limited by high production costs, complex synthesis, and potential toxicity. Here, we demonstrate that large H concentrations can be successfully injected in both palladium (Pd) bulk foils and thin films (up to a stoichiometry PdH0.89) by using a deep eutectic solvent (DES) choline chloride:glycerol 1:3 as gate electrolyte and applying gate voltages in excess of the cathodic stability limit. The attained H concentrations are large enough to induce superconductivity in Pd, albeit with an incomplete resistive transition which suggests a strongly inhomogeneous H incorporation in the Pd matrix. This DES-based IGP protocol can be used as a guideline for maximizing H loading in different materials, although specific details of the applied voltage profile might require adjustments based on the material under investigation.