Key Role of Interfacial Cobalt Segregation in Stable Low-Resistance Composite Oxygen-Reducing Electrodes

The development of efficient and stable oxygen-reducing electrodes is challenging but vital for the production of efficient electrochemical cells. Composite electrodes composed of mixed ionic-electronic conducting La_1-SrCo_1-FeO_3-δ and ionic conducting doped CeO₂ are considered promising components for solid oxide fuel cells. However, no consensus has been reached regarding the reasons of the good electrode performance, and inconsistent performance has been reported among various research groups. To mitigate the difficulties related to analyzing composite electrodes, this study applied three-terminal cathodic polarization to dense and nanoscale La_0.6Sr_0.4CoO_3-δ-Ce_0.8Sm_0.2O_1.9 (LSC-SDC) model electrodes. The critical factors determining the performance of the composite electrodes are the segregation of catalytic cobalt oxides to the electrolyte interfaces and the oxide-ion conducting paths provided by SDC. The addition of Co₃O₄ to the LSC-SDC electrode resulted in reduced LSC decomposition; thus, the interfacial and electrode resistances were low and stable. In the Co₃O₄-added LSC-SDC electrode under cathodic polarization, Co₃O₄ turned wurtzite-type CoO, which suggested that the Co₃O₄ addition suppressed the decomposition of LSC and, thus, the cathodic bias was maintained from the electrode surface to electrode-electrolyte interface. This study shows that cobalt oxide segregation behavior must be considered when discussing the performance of composite electrodes. Furthermore, by controlling the segregation process, microstructure, and phase evolution, stable low-resistance composite oxygen-reducing electrodes can be fabricated.