(Invited) Triple Phase Boundary Reaction in a Mixed-Conducting SOFC Cathode

Abstract

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.