Evaluation of Exchange Current Density for LSM Porous Cathode Based on Measurement of Three-Phase Boundary Length

The microstructure of porous electrodes has a strong impact on a power generation performance of solid oxide fuel cells. Sufficient gas diffusion, electron transport and oxide-ion transport to the reaction sites is necessary for active electrochemical reactions. Recent advancement of 3-D microstructure observation by a focused ion beam and scanning electron microscope (FIB-SEM) or micro X-ray computed tomography enabled quantification of electrode microstructure. The three dimensional datasets of porous electrodes are also applied to numerical simulations of the electrode's polarization characteristics. Understanding the correlation between the electrode performance and microstructure is important to obtain guidelines for optimal microstructure. The numerical simulation requires a rate equation for the electrochemical reaction occurring at the reaction sites. It is often expressed by Butler-Volmer like form that includes the density of three-phase or two-phase boundary and the exchange current density per unit reaction site, i.e. unit triple phase boundary length for three phase boundary (TPB) reactions and unit area for two phase boundary reactions. Generally, the exchange current density per unit TPB length was derived from experiments using flat and dense patterned electrodes since there is an advantage that TPB length can be determined from its geometric shapes. However, we often observe a discrepancy between the polarization characteristics obtained from the simulation and the experimental measurements. One of the possible reasons for the discrepancy is the rate equation evaluated from the patterned electrodes which structure is very different from the real porous electrodes. This study focuses on LSM (Lanthanum Strontium Manganite) porous cathode. Although the number is still limited, we can find reports on the exchange current density per unit TPB length obtained from patterned LSM electrodes typically with a thickness of several hundred nanometers. It is also reported in literature that the thickness of patterned LSM electrodes affects activation overpotential. Since LSM has small oxide ion conductivity, oxide ion can be transferred through LSM phase, and the charge transfer at interface of LSM phase and pore phase (two phase boundary) can occur. This phenomenon may bring overestimation of the exchange current density per unit TPB length in the experiment with patterned LSM. In this study, we derive a model of exchange current density per unit TPB length considering its temperature and oxygen partial pressure dependency from experiments using LSM porous cathode with a particle diameter of approximately 3 micrometers. We assume that the charge transfer occurs only at TPB in this large particle. We first conducted power generation experiments using LSM/YSZ/Ni-YSZ button cells. For the reference electrode, a platinum wire was attached so as to surround the side edge of the electrolyte disk. Therefore the potential difference between the cathode and the reference electrode can be obtained. Activation overpotentials were obtained by impedance spectroscopy varying temperature (1073 - 1223 K) and oxygen partial pressure (5 - 21 %). After the power generation experiment, the cathode microstructure was observed by FIB-SEM and three-dimensional microstructure dataset with dimensions of 18 μm / 25 μm / 12 μm was obtained. From the geometry of its microstructure, TPB length was extracted and quantified. The exchange current densities per unit TPB length were calculated by substituting current density, activation overpotential and TPB density into Butler-Volmer like form. Eventually, obtained exchange current densities per unit TPB length are fitted into a power-law equation that has temperature and oxygen partial pressure term. As a result, obtained exchange current densities per unit TPB length in this study is found to be smaller than the ones estimated from patterned electrode at any condition. The discrepancy between them increased with decreasing temperature and/or with increasing oxygen partial pressure. For instance, the exchange current density derived in this study was less than half of that derived by Radhakrishnan et al. from patterned electrode at 1023 K for 21 % of oxygen. For actual LSM or LSM/YSZ composite cathodes using relatively large LSM particles, the equation of exchange current density per unit TPB length derived in this study is appropriate.