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

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.

Hydrogen trapping at surface and subsurface vacancies of low-index surfaces of Pd

“…The barriers for H atoms segregated in different sites at the surface is quite low, lower than 0.1 eV and the barrier for escaping from the subsurface vacancy into the bulk is less than from the surface. Therefore the nH-V s and nH-V ss clusters can alter kinetics of Pd charging with H. This conclusion is in line with experimental findings in electrochemical charging of Pd, accelerated when the surface vacancies are created at a high charging current and in H-Pd codeposition which produces a very high effective H pressure [11].…”

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications

“…Very recently, Chiang et al have systematically investigated the electrochemical insertion of H into Pd via three different electrolyte media. 46 When combined with kinetic overpotentials presenting in typical electrochemical cells (Fig. 5), the total cell voltage remained in the order of 1-10 V, which was easily accessible in most experimental designs.…”

“…Previous studies suggested that electrochemical H intercalation in Pd nanomaterials was a multi-step process in an acidic liquid electrolyte 41,45 or solid electrolyte. 46 Firstly, H adatoms were formed by the reduction of H + or dissociation of H 2 absorbed onto Pd. Secondly, surface H adatoms crossed the Pd surface to reach the sub-surface regions and then bulk regions in interstitial sites of the Pd lattice.…”

“…Reproduced with permission. 46 Copyright 2019, American Chemical Society. 11 In the typical one-step synthesis, the polycrystal β-PdH 0.43 showing an fcc structure could be obtained after heating palladium(II) acetylacetonate (Pd(acac) 2 ) in DMF at 160 °C for 4 h. In the two-step approach, Pd NCs with different shapes were firstly synthesized, which were then respectively treated in DMF at 160 °C for 16 h to give β-PdH 0.43 while maintaining the corresponding morphology.…”

Thermodynamic analysis of the electrochemical synthesis of ammonia in solid-state proton-conducting electrochemical reactors considering interfacial potential steps

“…The aforementioned studies were all conducted in oxygen-ion conducting electrolyte materials. In terms of proton-conducting materials, Bench et al 29 have performed an experimental study of electrochemical insertion of hydrogen into palladium, suggesting the generation of high-pressure hydrogen at the interface. However, the above theories about interfacial potential steps haven't been considered and discussed in the thermodynamic analysis of electrochemical synthesis.…”