Oxygen-Deficient Ruddlesden-Popper-Type Lanthanum Strontium Cuprate Doped with Bismuth as a Cathode for Solid Oxide Fuel Cells.

Abstract

Ruddlesden-Popper-type strontium-doped lanthanum cuprates are unique in oxygen defects because of the oxygen-deficient composition. This work increases the oxygen vacancy concentration through bismuth-doping and thus promotes the electrochemical performance for oxygen reduction reaction (ORR) in solid oxide fuel cells. X-ray diffraction shows that up to 10% A-site elements can be doped with bismuth. The doping improves the catalytic activity through (1) increasing oxygen vacancy concentration by 87.5 and 65.5% at room temperature and 800 °C, respectively, as demonstrated by iodometric titration and thermogravimetric analysis, (2) greatly reducing the energy for oxygen vacancy formation as shown by density functional theory calculation, (3) forming additional reactive oxygen species at the near surface region as suggested with X-ray photoelectron spectroscopy, and (4) enhancing the oxygen transport properties as exhibited with electrical conductivity relaxation. In addition, bismuth doping reduces the thermal expansion coefficient to a level that could exactly match the thermal expansion behavior to the electrolytes. Consequently, the interfacial polarization resistance for ORR is decreased by 43% at 800 °C for the cuprate-based composite electrodes. The decrease is greatly attributed to the enhancement in the charge-transfer process, the rate-limiting step. Further, the peak power density for a model cell is increased from 530 to 630 mW·cm-2 at 800 °C. Bismuth-doping is a promising strategy to modify the catalytic properties of unique cuprates toward ORR.