In a conventional SOFC cathode using mixed ionic and electronic conductor (MIEC), the electrochemical reaction proceeds not only at triple-phase boundaries (TPB: gas/electrode/electrolyte) but also on double-phase boundaries (DPB: gas/electrode). In the MIEC cathode, electrochemical reaction generally occurs within a limited area from electrode/electrolyte interface because the resistance for oxide ion conductivity becomes significant compared to that for surface reaction as the diffusion length of oxide ion becomes longer.

Our research group has experimentally clarified the reaction area in a porous MIEC cathode by operando micro X-ray absorption spectroscopy (XAS) measurements (1). In a porous La_0.6Sr_0.4CoO_3-δ electrode under a constant cathodic overpotential of 140 mV at 873 K in 10^-2 bar of oxygen partial pressure P(O₂), it was found the electrochemical reaction proceeded within 2 µm from the electrode/electrolyte interface. In the higher P(O₂), the reaction area would become even shorter. It is in general believed that the total electrochemical reaction in a porous MIEC cathode is dominated by the reaction on DPB and the contribution of the reaction at TPB is insignificant. However, when the electrochemical reaction proceeds only in the vicinity of the electrode/electrolyte interface, as mentioned above, it is considered that the electrochemical reaction at TPB cannot be ignored.

In this study, for investigating the contribution of the TPB reaction, patterned thin film electrodes with/without TPB, which were kinds of columnar electrodes, were fabricated. The distributions of electrochemical reaction in the model electrodes were evaluated by operando micro XAS measurements.

In a La_0.6Sr_0.4CoO_3-δ patterned electrode without TPB, the electrochemical reaction proceeded within 60 µm from the electrode/electrolyte interface under a constant cathodic overpotential of 190 mV at 973 K in 10^-1 bar of P(O₂). On the other hand, in the patterned electrode with TPB, the electrochemical reaction proceeded within 40 µm from the interface under the similar experimental conditions. The distribution of electrochemical reaction was decreased by the introduction of TPB, which demonstrated the contribution of the TPB reaction even in the MIEC cathode.

The distribution of electrochemical reaction was also evaluated by isotopic exchange experiments and following secondary ion mass spectrometry (SIMS) measurements (2). ¹⁸O/¹⁶O isotopic exchanges were performed with/without cathodic overpotential, and the isotope exchange profiles were analyzed by SIMS measurements. The electrochemically incorporated ¹⁸O was evaluated as the difference between the isotope exchange profiles with/without cathodic overpotential. In the presentation, the contribution of the TPB reaction in the MIEC cathode will be discussed based on the results obtained by isotopic exchange measurements.

1. Y. Fujimaki, H. Watanabe, Y. Terada, T. Nakamura, K. Yashiro, S. Hashimoto, T. Kawada and K. Amezawa, ECS Trans., 2013, 57 (1), 1925-1932.

2. Y. Fujimaki, T. Nakamura, K. Develos-Bagarinao, K. Yamaji, K. Yashiro, T. Kawada, F. Iguchi, H. Yugami and K. Amezawa, ECS Trans., 2015, 68 (1), 623-630.