DIFFUSION-REACTION IN MIXED IONIC-ELECTRONIC SOLID OXIDE MEMBRANES WITH POROUS-ELECTRODES

We solve the transport equation for diffusion-reaction in the limit of small pressure gradient in a mixed ionic-electronic conductor (MIEC) solid oxide membrane consisting of a porous anode and cathode separated by a thin dense layer. The maximum enhancement in the ion current in the porous membrane, compared to the non-porous membrane, is root L(d)S(1-phi)/tau(s)+phi, where L(d)=D-IE/K is the length scale that determines the transition from diffusion limited to surface exchange limited transport in the non-porous membrane, K is the surface exchange coefficient, D-IE the effective ionic diffusion coefficient, S the pore surface area per unit volume, phi the porosity and tau(s) the tortuosity of the solid phase. Maximum enhancement is achieved when the thickness of the membrane L >> L(p), and when L(g) >> L(p), where L(p)=root L(d)(1-phi S tau(s) is the width of the active region of the membrane and L(g) is the length scale for variation of the chemical potential of the gas in the pores of the membrane.