Producing High Concentrations of Hydrogen in Palladium via Electrochemical Insertion from Aqueous and Solid Electrolytes

TL;DR

This study demonstrates the electrochemical synthesis of near-stoichiometric palladium hydride (PdHx) at ambient conditions using liquid and solid electrolytes, revealing the dynamics and limitations of hydrogen insertion.

Contribution

It introduces a combined electrochemical and X-ray diffraction method with a new calibration to accurately measure PdHx composition during electrochemical insertion.

Findings

Achieved H:Pd ratios near unity at ambient pressure.

Identified electrochemomechanical damage as a limiting factor.

Provided new design rules for high hydrogen concentration in metal hydrides.

Abstract

Metal hydrides are critical materials in numerous technologies including hydrogen storage, gas separation, and electrocatalysis. Here, using Pd-H as a model metal hydride, we perform electrochemical insertion studies of hydrogen via liquid and solid state electrolytes at 1 atm ambient pressure, and achieve H:Pd ratios near unity, the theoretical solubility limit. We show that the compositions achieved result from a dynamic balance between the rate of hydrogen insertion and evolution from the Pd lattice, the combined kinetics of which are sufficiently rapid that operando experiments are necessary to characterize instantaneous PdHx composition. We use simultaneous electrochemical insertion and X-ray diffraction measurements, combined with a new calibration of lattice parameter versus hydrogen concentration, to enable accurate quantification of the composition of electrochemically synthesized PdHx. Furthermore, we show that the achievable hydrogen concentration is severely limited by electrochemomechanical damage to the palladium and/or substrate. The understanding embodied in these results helps to establish new design rules for achieving high hydrogen concentrations in metal hydrides.