Hydrogen transport through V–Fe alloy membranes: Permeation, diffusion, effects of deviation from Sieverts’ law

Abstract

Vanadium alloy membranes are the most promising alternative to commercial membranes of palladium alloy for producing ultrapure hydrogen, while Fe is the most suitable alloying element due to its high ability to reduce the excess hydrogen solubility. Membrane samples from bcc alloys V- κFe (0 ≤ κ ≤ 13.1 at.%) were fabricated in the form of thin-walled tubes with Pd-coated inner and outer sides. The hydrogen permeation and diffusion were studied at inlet pressure of 0.1–0.8 MPa and temperature of 300–450 °C. The hydrogen permeation through the membranes of all the studied alloys turned out significantly higher than through a similar Pd membrane. The hydrogen diffusivity in V- κFe alloys decreased with increasing the alloying degree κ and in any case was somewhat lower than in pure V. The permeation flux continued to increase in proportion to square root of hydrogen pressure even in the region where concentration of dissolved hydrogen approached the solubility limit remaining almost unchanged. This non-Fick behavior was caused by an increase in hydrogen diffusivity, the more significant the greater degree of deviation from Sieverts' law due to approaching the saturation.