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

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 studies of electrochemical insertion of hydrogen into palladium via three different electrolyte media, aqueous liquid, polymeric solid, and ceramic solid electrolytes, from a gas phase source at 1 atm pressure. We show that the compositions achieved result from a dynamic balance between the rates of hydrogen insertion and evolution from the Pd lattice, the combined kinetics of which are sufficiently rapid that operando experiments are necessary to characterize the instantaneous PdHx composition. Therefore, we use simultaneous electrochemical insertion and X-ray diffraction measurements, combined with a new calibration of lattice parameter versus hydrogen concentration, to accurately quantify the PdHx composition. Furthermore, we show that the achievable hydrogen concentration is severely limited by electrochemomechanical damage to palladium and/or the substrate and that such damage is minimized by a combination of thin palladium films and compliant substrates. Under optimal conditions, we reach a maximum hydrogen concentration of x = 0.96 +/- 0.02. The understanding embodied in these results helps to establish new design rules for achieving high hydrogen concentrations in metal hydrides.