Transport in solid oxide porous electrodes: Effect of gas diffusion

Abstract

We extend our previous treatment of a mixed ionic electronic conductor membrane, consisting of a porous cathode and anode separated by a thin non-porous layer, to the case where mass transport of molecules in the porous electrodes can be the rate-limiting step. The linearized transport equations for the ion-hole pairs in the solid and of the gas molecules in the pores are characterized by the length scales L P = √ L d (1 − φ) Sτ s and L g = 2L p √[τ s φ/τ(1 − φ)][ D g c g D IE c i ] respectively, 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 electrodes, K is the surface exchange coefficient, D IE and D g are the diffusion coefficients of the ion-hole pairs and of the molecules, c i and c g are the concentrations of the ions and molecules, S is the pore surface area per unit volume, φ the porosity and τ s and τ the tortuosities of the solid and pore phases respectively. When L g ⪢ L p, which is the case treated previously, the rate-limiting step in the transport is ionic diffusion and surface exchange. Enhancements in oxygen ion current of two orders in magnitude, over non-porous electrodes, are in principle achievable with porous perovskite MIEC having surface area s = 10 6 cm −1. When L g⪡ L p the rate-limiting step is mass transport in the pores and the enhancement in ion current is substantially reduced.