A continuum modeling study on the electrochemical performance and stability of solid oxide cells and oxygen permeation membranes

Abstract

The structure, materials and transport kinetics of solid oxide cells (SOCs) and oxygen permeation membranes (OPMs) are quite similar, but a unified and explicit model is lacking. Here, a continuum model for evaluating the electrochemical performance and stability of SOCs and OPMs is developed based on linear irreversible thermodynamics. The oxygen partial pressure at the electrode/electrolyte interface is used to evaluate the stability of SOCs and OPMs, and is calculated by considering the gas exchange at the electrode/electrolyte interface and the mixed ionic and electronic conduction in the electrolyte. The calculation results are verified by the literature data. The effect of fundamental material properties on the SOC's and OPM’s performance under certain operating conditions is studied via parametric scanning. In addition, the stabilities of the SOCs and OPMs are also discussed for various operating conditions. The results show that the open circuit voltage (OCV) and stability of SOCs under open-circuit is not only influenced by the magnitude of resistances of the electrolyte, and the air and fuel electrodes, but also the matching of them. Since the OPM can be regarded as the SOC under the open-circuit condition, the present model is also applicable for calculating the oxygen permeation flux. It is shown that the oxygen permeability is increased by lowering the OCV of the OPM. Optimization of SOFC’s performance should consider the matching of the electrode and the electrolyte resistances and the effect of resistance on stability. The evaluation of the performance of SOECs should consider the achieved electrolytic current density combing with the Faradic efficiency. Promotion of the ionic and/or electronic conductivity of electrolytes can enhance the stability of SOECs, but should be treated carefully due to the sacrifice of Faradic efficiency.