Non-ideal hydrogen permeation through V-alloy membranes

In the present work, an experimental and theoretical investigation on hydrogen permeation through different three-element V/Al-alloy membranes – V85Cr10Al5, V88Co4Al8 and V85Ni10Al5 (alloy composition expressed in atom %) – is carried out at 400 °C under conditions of moderate values of hydrogen pressure (up to 5 bar). To analyse the membrane behaviour in these conditions, at which system works far from the infinite dilution (non-ideal conditions), an approach already available in the literature is extended to allow splitting ideal and non-ideal contributions on the overall permeability. Furthermore, such an extension allows the non-ideal contribution of H-diffusion and solubility to be recognised and quantified, thus giving the possibility of describing the curvature of the flux trend with hydrogen pressure more precisely with respect to the original form. The permeation tests, performed keeping the atmospheric pressure in the permeate side, first reveal that the V85Cr10Al5-alloy gives the greatest Sieverts-like permeability – here referred to as apparent permeability, which is constant at a certain temperature value in the pressure range considered for its calculation – followed by V88Co4Al8 and V85Ni10Al5. Afterwards, the same permeation data are used to calculate the so-called intrinsic (ideal) permeability as well as the non-ideal terms, which describe diffusion and solubility effect. The same analysis is performed also on some data taken from the literature for comparison. As results, we found that the intrinsic permeability shows this trend: V85Cr10Al5 > V85Ni10Al5 > V88Co4Al8, whereas the extent of non-ideal contribution, expressed in terms of a Non-Ideality Index, is found to follow this order: V88Co4Al8 > V85Cr10Al5 > V85Ni10Al5. This leads to a situation where the overall permeability show a maximum for Co and Cr at around 350 kPa, whereas it is monotonically increasing for the V/Ni-membranes in the pressure range considered. The presence of this maximum is explained by considering the contrasting effect of diffusion and solubility at progressively higher hydrogen pressure. The presented approach allows a precise characterisation of metal membranes in conditions of relatively high pressure, which are of interest for practical industrial applications.