A Simplified Model for Reversible Solid Oxide Fuel Cells

Abstract

: Butler-Volmer Equations based on a first order approximation of global redox reaction kinetics have been used in the past successfully for design analysis of solid oxide fuel cells. Such a model forms the backbone of the widely used multidimensional code, DREAM-SOFC originally developed at West Virginia University and currently being used by the fuel cell research team at the US DOE lab, National Energy Technology Laboratory (NETL). Recent interest in using solid oxide cells (SOCs) for hydrogen production has prompted the question as to whether the Butler-Vomer Model (BVM) empirically calibrated and used in DREAM-SOFC would be appropriate for simulation of reversible fuel cells as well. To this end we have simulated anode supported Ni/YSZ/LSCF button cells in reversible mode and found that the BVM is not capable of reproducing the trends observed in experiments performed at various applied steam, hydrogen and oxygen concentrations, even if some key parameters such as effective bulk diffusivity and exchange current density are treated as calibration parameters and adjusted freely. To bridge this gap, we modified the oxygen electrode model using a simplified version of the single pathway oxygen reduction reaction equations originally put forth by Svensson et al. [1]. In the simplified model, it is assumed that the adsorbed oxygen concentration remains close to its equilibrium value in most of the oxygen electrode active layer. This assumption eliminates the need to solve the more difficult, stiff surface coverage transport equation leaving only the oxygen ion vacancy concentration to be solved for, thus simplifying the model greatly. This paper presents the mathematical model, and an in-depth analysis of the results from this model applied to button cells with LSCF and LSM electrodes. Figure 1 shows the comparison between the simulated and experimental polarization curves. As shown, good agreement is achieved between simulations and experiments for fuel cell and electrolysis modes. We conclude that by extending the BVM only by adding the vacancy concentration transport equation, leads to a simple yet sufficient model with which one can perform very fast calibration and design analysis of reversible SOCs with minimal computation time.Reference:[1] A.M. Svensson, et al., J. Electrochem. Soc., 145(4), 1390 (1998) doi: 10.1149/1.1838471 Figure 1