Mathematical Modeling of Oxygen Exchange and Transport in Air‐Perovskite‐YSZ Interface Regions: I. Reduction of Intermediately Adsorbed Oxygen

Abstract

The transport of oxygen in a porous perovskite solid oxide fuel cell cathode is modeled by use of the principles of porous electrode modeling, by taking into account exchange kinetics at the gas/electrode interface, bulk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interface. The mechanism for the latter is based on the assumption that intermediately adsorbed oxygen atoms are reduced at the cathode/electrolyte interface in favor of direct exchange of oxygen vacancies. The significance of concentration polarization is demonstrated even at very low overpotentials, especially if the adsorption process is slow. Under such conditions, the empirical correlation Rnormalpnormaleff∝pO2−normalm claimed to exist between the measured potential resistance and the partial pressure of oxygen cannot be justified on fundamental grounds. A limiting current is obtained at high cathodic overpotentials due to the depletion of intermediately adsorbed species at the cathode/electrolyte interface. The existence of a correlation inormallim∝pO2n is predicted, where the exponent n is determined by the kinetic and transport properties of the cathode for oxygen exchange and transport.

Nonlinear coupled equations for electrochemical cells as developed by the general equation for nonequilibrium reversible-irreversible coupling

“…19 Yttria stabilized zirconium oxide has a number of oxygen ion vacancies in the lattice (typically 60 mol/m 3 , 20 ) and oxygen ions can move in the lattice, jumping between vacancies. The distributions of Y 3 + over the Zr 2 + lattice matches the vacancy distribution, making the material electroneutral.…”

“…The material adsorbs oxygen gas and contains a large fraction of oxygen vacancies. 20 The perovskite has a certain thickness, say 10 μm, but we can deal with it as a two-dimensional system, and use as variables the integrals over the variable profiles in the material. Each layer with a finite thickness can be integrated and replaced by a surface in this manner.…”

“…The transfer of oxygen to the channel, and the transfer of the electron to the conduction band follows. According to the literature, 15,19,20 the chemical reaction (the second step) and the oxygen surface exchange (the third step) are slower than the other two. The second step is an activated electrochemical reaction.…”

Electrochemical Impedance Analysis of SOFC Cathode Reaction Using Evolutionary Programming

“…Svensson et al [19][20][21] developed a model that describes the polarization curves of a porous ABO 3 (P phase) electrode on YSZ (T phase). Two different models for the reaction path of oxygen in the cathode have been considered.…”

“…By separating the total response of the SCC to different processes, the activation energies as well as the dependence of the processes on the oxygen partial pressure can also be evaluated separately. The polarization curves obtained were analyzed using the Butler-Volmer (B-V) relations and the model proposed by Svensson [19][20][21] for the behavior of the examined cell.…”