Surface effects on deuterium permeation through vanadium membranes

Dense vanadium-based membranes offer high permeability and perfect selectivity to hydrogen isotopes, maintain favorable neutronic properties, and are compatible with liquid metals such as PbLi. These properties make vanadium membranes a promising fusion fuel cycle technology for processes such as tritium extraction from PbLi and exhaust processing. Surface contamination has a deleterious effect on the gas-phase hydrogen permeation through vanadium, and the reported permeabilities range from 10^-14 to 10^-7 mol m^-1 s^-1 Pa^-0.5. Thin dense films of palladium applied to clean vanadium surfaces enable a consistently high hydrogen permeability. In this study, uncoated vanadium resulted in deuterium permeabilities ranging from 2.8 × 10^-11 to 6.4 × 10^-9 mol m^-1 s^-1 Pa^-0.5 at 300 °C–700 °C, respectively. Post-test analysis revealed a VO_x surface layer and VC_x subsurface layer formed on the feed side, while the as-received surface oxide dissolved leaving a submonolayer oxide on the permeate surface. The Pd-coated V resulted in a maximum deuterium permeability of 2.1 × 10^-7 mol m^-1 s^-1 Pa^-0.5 at 375 °C upon activation of the Pd surface by oxidation and reduction. The deuterium permeation declined upon heating to 500 °C due to intermetallic diffusion between the Pd and V. The Mo₂C-coated V resulted in deuterium permeabilities ranging from 2.7 × 10^-10 to 1.8 × 10^-9 at 500 °C–700 °C, respectively, and a post-test analysis found the carbon in the Mo₂C layer had dissolved into the V near the interface.