Hydrogen transport through the tubular membranes of V-Pd alloys: Permeation, diffusion, surface processes and WGS mixture test of membrane assembly

The membranes of vanadium alloys can be a more productive and less expensive alternative to the membranes of palladium alloys for the extraction of ultrapure hydrogen. Alloying of V with Pd is very effective for the reduction of excessive hydrogen solubility inherent in pure V while the ductility of substitutional V-Pd alloys remains sufficient for making thin walled tubular membranes for practical applications. The hydrogen transport through the tubular membranes of substitutional V-κPd alloys with Pd coating of inner and outer sides was systematically studied with variation of alloy composition (0 ≤ κ ≤ 13.3 at%), temperature (300 - 500 °C) and pressure (0.001 - 1.2 MPa). The permeation flux density was almost independent of temperature for all investigated alloys. It was found that the main reason for this is the surface processes whose role dramatically increased with decreasing temperature, getting crucial at 300 °C. The record hydrogen diffusivity inherent in pure V reduced by alloying with Pd no more than 3 times even at the maximum degree of alloying (13.3%) and lowest temperature (300 °C). An assembly of 18 tubular membranes of V-5.2at%Pd with 145 µm wall thickness was designed for feeding 1 kW PEMFC and demonstrated a highly productive extraction of ultrapure hydrogen from WGS mixture.