Modeling of Overpotentials in an Anode-Supported Planar SOFC Using a Detailed Simulation Model

Abstract

In this paper results from a parameter study of an anode-supported solid oxide fuel cell (SOFC) are presented. The effects on performance, current-voltage (I-V) characteristics, polarization voltages, and diffusion coefficients are modeled for different temperatures, electrolyte thickness, porosities, and pore sizes. The analysis is carried out for a planar SOFC with YSZ electrolyte, LSM cathode, and Ni-YSZ anode, with thicknesses 20, 50, and 500 μm respectively, and with co-flow geometry. The predicted performance is validated with measured data found in the literature with good agreement. Standard equations for binary and Knudsen diffusion in porous media, concentration overpotentials, and Ohm’s law are used in the modeling. Activation overpotential is predicted by use of temperature dependent linear equations at both the anode and cathode sides. It is found that both ohmic and activation overpotentials decrease considerably with increasing temperature, while concentration overpotentials increase moderately with increasing temperature. The effect on concentration overpotentials can be explained by the reduced gas density with increased temperature, despite the increasing diffusion coefficient. Furthermore, it was found that increasing the pore sizes decreases concentration overpotentials. At low pore size the Knudsen diffusion coefficient is a bottleneck for the diffusion coefficient since it is much lower than the binary diffusion coefficient. It has been demonstrated that by increasing the pore size the Knudsen diffusion coefficient is improved. The effect of porosity has much in common with the effect of pore size; increasing porosity leads to decreased concentration overpotential due to the improved diffusion coefficient. As a natural phenomenon for anode-supported cells, most of the concentration overpotentials take place at the anode side due to its thick structure despite the high diffusion coefficient of hydrogen. It must be underlined that in this study the effect of different porosities and pore sizes is modeled at the anode and cathode substrates only without taking into account the length of the three phase boundary (LTPB) near the electrolyte interface. However, since these microstructural parameters can have an impact on the LTPB, they can also have an impact on the activation overpotentials. This is not considered here and will be taken into account in future work.