Entry Date:
July 17, 2013

In Situ Investigation of the Surface Activity of Model Cathodes for SOFC / SOEC

Principal Investigator Bilge Yildiz

Surfaces of SOFC /SOEC electrodes, comprising complex oxides, are not static, evolve with the functional conditions, and correlate with the activity to oxygen reduction and water splitting. We are focusing on the dynamics of cathode surface chemistry and electronic structure, which govern oxygen reduction activity by affecting the oxygen adsorption strength, and the barriers to oxygen dissociation and incorporation into sub-surface layers of the cathode. The high-temperature electronic structure of these surfaces was never measured prior to our work. The potential discovery of the fundamental correlation of electronic structure to surface activity on cathodes can enable the tailoring of material compositions with superior surface activities that lead to more efficient SOFC / SOEC systems. The electronic d-band structure is a well established descriptor of the oxygen reduction activity on transition metals. However, such a simple activity descriptor for transition metal perovskite oxides used as fuel cell cathodes does not yet exist given the complexity of their surface chemistry at high temperatures. To indentify key characteristics of the surface electronic structure, we are taking advantage of our capability to probe the surface chemistry and electronic structure consistently and at elevated temperatures, in connection with our first principles-based calculations. We are performing novel in situ studies of how the chemistry and electronic structure on cathode surfaces evolve with temperature and oxygen pressure, using scanning tunneling microscopy/spectroscopy and photoelectron spectroscopy studies. This information is then related to the electrochemical activity of the cathode surface. The model cathode materials that we investigate are LaxSr1-xMnO3 and Sr (Ti1-xFex)O3 thin films with controlled crystallography.

Sr(Ti1-xFex)O3 (STF) solid state systems have been found to be a promising cathode material, stable over a wide range of temperatures and oxygen pressures (PO2). The capability of adjusting its ionic and electronic conductivity over wide limits via variations in the Fe content make it an ideal model material for the study of correlation between surface chemical and electronic structure and its catalytic activity. We synthesize STF oxide films with controlled defect chemistries by pulsed laser deposition and investigate the in-situ evolution of local electronic and chemical properties under extreme conditions (i.e., high temperature and high oxygen pressure) to understand the mechanisms of oxygen reduction and its correlation to the surface electronic structure.