Numerical Study on Progress of Cr Poisoning in LSM-YSZ Cathode of a Planar Solid Oxide Fuel Cell
Authors: Iwai Hiroshi , Yamaguchi Yuji , Kishimoto Masashi , Saito Motohiro , Yoshida Hideo 
Published: February 2020

Performance degradation of solid oxide fuel cells (SOFCs) due to impurities in the supplied gasses is an important issue that has to be overcome to maintain a sufficient efficiency in long-term operations. Chromium, which is contained in the metal separators and the walls of flow passages in the systems, causes severe damage on the cathode performance. There is a number of experimental studies and thermodynamic analyses on this issue; one of the important reactions related to the chromium poisoning of LSM-YSZ cathodes is reported to be an electrochemical reaction of CrO2(OH)2 at triple-phase boundaries (TPBs). As a result of this reaction, solid Cr2O3 forms at the TPBs blocking the reduction of oxygen. Because the activity of this reaction depends on the local temperature, gas species concentration, and electrochemical potentials, the degradation of LSM-YSZ by the above chromium poisoning is expected to progress non-uniformly in a cell. The aim of this study is to understand how the chromium poisoning progresses in the LSM-YSZ cathode and influences the overall performance of the cells.

In this study we perform a 2D CFD simulation considering chromium poisoning on an LSM-YSZ cathode. The computational domain covers a single planar-type anode supported SOFC with a streamwise length of 80 mm. The model consists of seven sub-domains: upper separator, fuel channel, Ni-YSZ porous anode, YSZ electrolyte, LSM-YSZ porous cathode, air channel, and lower separator.

Numerical simulations are conducted by coupling the thermo-fluid models and the electric/ionic current field models. Both anode and cathode electrodes are treated as porous materials. A volume-averaging method, in which all the physical values are locally averaged in a representative elementary volume, is applied. Forchheimer-extended Darcy model is applied to the momentum equations so that the same equations are to be used both for the gas channels and for the porous regions. For the energy equation, a local thermal non-equilibrium model is adopted in the porous regions. The microstructural parameters of the electordes, such as volume fraction, TPB density and the tortuosity factors, are required to evaluate the effective transport coefficients and the reaction rate in the porous electrodes; they are obtained from the FIB-SEM analysis of the electrodes of an in-house button cell. The electrochemical potential fields of the electrons and oxygen ions are solved to obtain electric/ionic current distributions. The thermo-fluid models and the electric/ionic potential field models are coupled by the electrochemical reaction model expressed by Butler-Volmer-like equations. The chromium poisoning model proposed by Nakajo et al. is applied to predict the progress of the degradation phenomena occurring in the LSM-YSZ cathode. The inlet temperature of the gasses and the terminal voltage of the cell are kept constant at 750 °C and 0.75 V, respectively.

The calculation results show that the Cr poisoning is more prominent near the upstream region of the cells in the vicinity of the cathode-electrolyte interface at the early stage of the operation. It is ascribed to the relatively low local temperature which also accompanies the high activation overpotential. Owing to the damage to the cathode in the upstream region, the active reaction zone of the electrochemical reaction shifts towards the downstream of the cell. The damaged region gradually expands to the downstream affecting the distributions of the local current density, activation overpotential and ohmic loss in the cathode.