Bulk Thermodynamics Determines Surface Hydrogen Concentration in Membranes

Abstract

The functioning of hydrogen selective metal membranes relies on the fine-tuned interaction of their surfaces with the inner bulk. The link between hydrogen permeation and surface concentration is unveiled using a novel combination of thin film preparation and surface science analysis to probe and intentionally modify the surface properties of palladium vanadium composite membranes. The in situ preparation allows for observation of ultra-clean hydrogenation and the quantification of the permeation as a function of temperature and pressure. The sub-surface hydrogen concentration is determined by operando reflecting electron energy loss spectroscopy (REELS) and surface elemental characterization by X-ray photoelectron spectroscopy and Auger electron spectroscopy under oxygen-free conditions as a function of pressure and temperature, from which the surface hydrogen pressure-composition isotherms are derived. The energy dependence of REELS reveals that the VHx layer exhibits a hydrogen concentration gradient. Modeling of the permeation behavior and surface hydrogen concentration yields activation energies in good agreement with similar hydrogen absorbing thin film systems for the overall process, the dissociation at the Pd surface, the diffusion in the membrane, and the dissociation at the V surface. The bulk thermodynamic properties of the metal hydride govern the desorption process.