Identifying Drivers of catalytic activity through systematic surface modification of cathode materials

To systematically study the effect of surface composition on the catalytic activity for oxygen exchange, (La Sr) Mn3 (LSM) powder was infiltrated with various metal nitrate solutions (La, Sr, Co, Fe) and then calcined to deposit the resultant metal oxide on the surface of the powder. The elements chosen for LSM infiltration were selected because they are the metal constituents of La Sr Co Fe O3-δ (LSCF), which is initially a much more active cathode material. Results from LSM show a stronger effect from the addition of B site elements; thus, LSCF was correspondingly infiltrated with manganese. By comparing the different surfaces, a better understanding of the underlying drivers of catalytic activity is gained. X-ray photoelectron spectroscopy (XPS) spectra of the infiltrated powders were compared to those of LSM and LSCF to detect differences in surface properties. The catalytic activity was examined using temperature-programmed isotope exchange, where labeled oxygen was used to determine the temperature at which the material begins to exchange oxygen with the gas phase. Cobalt infiltration improved the performance of LSM the most, while iron reduced the apparent activity. This behavior is attributed to the oxygen-surface bond strength of the oxides. Overall, LSCF remains the most active material studied, even though it contains more iron than cobalt on the B site. According to XPS, the iron on the surface of the infiltrated LSM and that on the surface of LSCF are electronically, different, while the cobalt on both surfaces are similar. Therefore, the surface composition cannot be the only consideration in predicting catalytic properties. Furthermore, surface modification did not change the apparent rate-limiting step for oxygen reduction in the infiltrated materials. © 2009 The Electrochemical Society.