Permeation and diffusion of hydrogen in vanadium using deoxidation and glow-discharge surface cleaning techniques

Abstract

Vanadium (V) is an attractive hydrogen separation membrane material. However, its diffusion behavior under hydrogen permeation is not well understood mainly because of surface oxidation. Here, we have attempted to observe the permeation of hydrogen through V by subjecting V samples to different surface treatments e.g., electropolishing and plasma-induced cleaning using a glow-discharge technique. We have achieved a high flux of hydrogen controlled by long-range diffusion under a low hydrogen potential at 723–973 K. The diffusion coefficients of hydrogen in defect-free and high-purity samples analyzed from the permeation curves at 823–973 K are almost consistent with the literature data obtained by Gorsky effect and absorption measurements. However, the activation energy of diffusion (19 kJ mol−1) is 8–14 kJ mol−1 higher than the literature values. This difference in activation energy suggests that permeation through the active surface of V is highly sensitive to trapping by oxygen—a slight amount of oxygen dissolved from a very thin native oxide layer would segregate near the surface to trap hydrogen. Furthermore, we have found that residual strain, grain boundaries, and lattice defects, induced by mechanical deformation, reduce the flux and retard the diffusion considerably.