A Model for Extracting Fundamental Kinetic Rates of SOFC Cathode Materials from Oxygen Isotope Exchange Experiments

Losses from cathode polarization in solid oxide fuel cells (SOFC) limit their performance at intermediate temperatures. A fundamental understanding of the oxygen reduction mechanisms on SOFC cathodes is essential for the development of lower temperature SOFCs with high performance. In order to separate the various contributions to cathode polarization, different techniques and experiments have been carried out to gain an understanding of the kinetics occurring at the gas-solid interface. Isotope exchange is a powerful tool for analyzing the oxygen exchange process (1, 2, 3, 4, 5), however, most isotope tracer experiments are limited in their scope. Experiments typically take one of two approaches, a focus on the kinetics of oxygen adsorption on the surface (SSIKTA), or the diffusion of oxygen through the lattice (SIMS). For SOFC cathodes, we are concerned with both the dissociation of oxygen on the surface and the ability of oxygen atoms to incorporate into and diffuse through the lattice. 

In this study, we attempt to determine the ORR mechanism and extract fundamental kinetics rates for La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3-x (LSM) using isothermal isotope exchange (IIE) (6,7). Studies of isotope exchange can greatly contribute to the elucidation of the interactions between molecular oxygen and the cathode surface. Isotope exchange can help characterize the dissociation of oxygen molecules at the oxide surface as well as the conduction of oxygen ions within the oxide. IIE experiments will be conducted at various temperatures and oxygen partial pressures.

To extract fundamental kinetic rates from in-situ isotope exchange experiments on LSM and LSCF, a model has been developed based on a two-step reaction mechanism across the heterogeneous gas-solid interface. The coupled reactions can be shown in two elementary steps: dissociative adsorption and incorporation. The relation between the fundamental kinetics rates and surface exchange coefficient (k_ex) (8, 9) will also be linked. 

Acknowledgments 

This work was supported by the Department of Energy under contract DEFE0009084. The authors would like to thank Dr. Dongxia Liu for reviewing our model.

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