Operando Evaluation of the Electrochemically Active Area in a Solid Oxide Fuel Cell Porous Electrode by Micro X-ray Absorption Spectroscopy.

An operando X-ray absorption spectroscopic technique, which enables us to measure X-ray absorption spectra with a position resolution of submicrometers at increased temperatures while controlling atmospheres and passing an electrical current through the specimen, was developed. By applying this technique, the electrochemically active area in a porous La0.6Sr0.4CoO3-δ electrode for a solid oxide fuel cell (SOFC) was experimentally and directly evaluated for the first time. The characteristic length of the active area was approximately 1 μm from the electrode-electrolyte interface under a cathodic overpotential of 140 mV at 873 K under 10-2 bar of P(O2), although the investigated electrode was thicker than 50 μm. This demonstrated that a reaction in an SOFC electrode inhomogeneously proceeds in a very limited area near the electrode-electrolyte interface, even when a mixed ionic and electronic conducting oxide is used as the electrode, and that direct evaluation of the active area can provide guidelines for high-performance electrode design.