The hydrogen transport through the metal alloy membranes with a spatial variation of the alloy composition: Potential diffusion and enhanced permeation

It is feasible to make hydrogen separation membranes of an alloy whose elemental or phase composition controllably varies at least in one direction. However the problem of hydrogen transport through such membranes is found not to be solved with the standard equation of diffusion in heterogeneous media. That is because the effect of heterogeneity on diffusion phenomena is considered to be caused by only spatial variations of diffusion coefficient while the spatial difference in the potential energy of diffusing particles due to their interactions with the inhomogeneous medium is not taken into consideration. The corrected equation shows the existence of an additional driving force making possible the diffusion in the direction opposite to that prescribed by Fick's law even in the isothermal medium when there is no any external field of force. The solution for the hydrogen transport through the membrane made of the alloy of variable composition indicates the possibility of great increase in the hydrogen permeation flux due to the optimization of spatial distribution of the alloy composition.