High-Temperature Electrocatalysis

Perovskite oxides have high catalytic activities for oxygen electrocatalysis competitive to platinum at elevated temperatures and thus they are promising candidates for many clean-energy technologies such as solid oxide fuel cells (SOFC) or oxygen separation membranes (OSM). We depict the working principle of both high temperature solid state devices. SOFCs convert chemical energy into electrical energy . At the cathode (blue) oxygen is reduced (O2 + 4 e- → 2 O2-), oxygen ions (O2–) move through the solid electrolyte (orange) towards the anode (green) where they react with the fuel (e.g. with hydrogen: 2 O2– + 2 H2 → 2 H2O + 4 e–). The electrons (e–) pass the outer circuit. Comparable processes occur in OSMs oxygen is reduced at one side, oxygen ions pass through the gas tight membrane (other ions cannot pass), and are oxidized at the other side. In contrast to the electrolyte of an SOFC which is a pure oxygen ion conductor (the electrons should pass through the outer circuit), the membrane is composed of a mixed ionic and electronic conductor (MIEC).

The main barrier to achieving acceptable chemical-to-electrical conversion efficiency in SOFCs is the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Similarly, the aim to realize high oxygen fluxes in OSMs requires fast oxygen surface exchange as well as fast oxygen ion diffusion inside the bulk material. Today, a lack of fundamental understanding of the detailed oxygen surface exchange mechanism limits the development of highly active catalysts to enhance the device efficiency. In addition, achieving long term stability of highly active materials at elevated temperature is an important challenge.