Hydrogen dissociation and diffusion through molten metal alloy membranes

Existing dense metallic hydrogen separation membranes deactivate above 823 K (550 °C). Recently, high-temperature molten gallium membranes were reported to have hydrogen diffusion coefficients 10 times greater than solid palladium; however, the overall hydrogen flux falls short of state-of-the-art palladium-based membranes due to slow dissociative adsorption of hydrogen. To increase this rate, we investigate molten alloys of transition metals for the first time. Rates of hydrogen dissociation on 15 top candidate molten metal alloys were quantified using the H 2 -D 2 isotopic exchange reaction. Alloys exhibited higher dissociative adsorption rates than pure metals. For instance, the experimentally determined apparent activation energy for hydrogen dissociation significantly decreased from 187 kJ/mol for pure molten bismuth to 91 kJ/mol for molten Cu 0.03 Bi 0.97 . Density functional theory (DFT) calculations corroborated these findings, indicating considerably lower barriers for H 2 dissociation on Cu 0.03 Bi 0.97 versus pure bismuth. Experimentally determined hydrogen diffusion, obtained using a Sievert's apparatus, were similar for Bi, Cu 0.03 Bi 0.97 , and Ni 0.03 Bi 0.97 . This suggests that the primary benefit of alloying transition metals with low-melting metals is to increase the rate of dissociative adsorption rather than diffusion. Ab initio molecular dynamics (AIMD) calculations indicated that Cu atoms prefer to be in the bulk over the surface of Cu 0.03 Bi 0.97 . Copper atoms solvated by bismuth take electrons from bismuth to become negatively charged. We propose this electronic modification of bismuth by sub-surface copper leads to bismuth acting as the active sites for homolytic hydrogen dissociation, thereby improving performance. • Alloying transition metals with low melting metals increases H 2 dissociation rate. • Alloying transition metals improves hydrogen diffusion. • Homolytic H 2 dissociation occurs on electronically modified Bi atoms on Cu-Bi.