Use of oxygen isotopic exchange to explore catalytic activity and the mechanism of oxygen reduction on oxides

The electrochemical performance of intermediate temperature solid oxide fuel cells is limited by high polarization losses incurred from the oxygen reduction reaction at the cathode. The mechanism of oxygen reduction as well as the key characteristics responsible for high oxygen reactivity are currently not well understood and represent a hurdle in the targeted development of electro-catalytically active cathode materials. These properties were studied using heterogeneous catalysis techniques coupled with labeled oxygen. The primary materials studied were (La0.8Sr0.2)0.98 MnO3+/-delta (LSM) and La0.6Sr0.4 Co0.2Fe0.8O3-delta (LSCF).;Temperature programmed isotopic exchange indicated that LSM was less active for surface exchange than LSCF, in agreement with previous results from the literature. This confirms the validity of isotopic exchange as a means to gauge the activity for oxygen reduction. Samples of LSM and LSCF were infiltrated with metal oxides to modify the surface properties and tested with this technique to identify trends in catalytic activity. Of the samples examined, LSCF was the most active. Addition of cobalt to LSM improved the activity while iron infiltration was detrimental. According to XPS, the iron on the surface of LSCF and LSM were of different oxidation states, which could explain why it caused a negative effect in LSM. These results indicate that optimization of catalytic activity is possible through surface modification; however, it is not only the surface composition, but also the electronic properties of the surface which determine activity.;A two-step mechanism consisting of dissociative adsorption of molecular oxygen followed by incorporation into the lattice was proposed and used to model the behavior obtained from isothermal isotopic switching. Effective rate constants for each step were obtained by fitting the model to the gas phase isotopologue distribution. These parameter values indicate which step is rate-limiting. LSCF was shown to be limited by dissociative adsorption. The reaction for LSM was slower than predicted by the model, suggesting that the diffusion of oxygen from the particle core to the surface is the actual rate limiting step. Based off these results, LSCF would be a good candidate for surface modification to improve dissociative adsorption and LSM could be improved by increasing the oxygen diffusivity.