Contribution of Triple/Double Phase Boundary Reactions in Mixed Conducting Oxide Cathodes in SOFCs and PCFCs
Authors: Amezawa Koji , Mizuno Keita , Nishidate Katsuya , Kimura Yuta , Nakamura Takashi , Strandbakke Ragnar , Szpunar Iga , Mikami Yuichi , Kuroha Tomohiro , Yashiro Keiji , Iguchi Fumitada , Kawada Tatsuya 
Published: July 2021

Improvement of cathodic performance is one of challenges for full-scale commercialization of solid oxide fuel cells (SOFCs) and protonic ceramic fuel cells (PCFCs). At cathodes in SOFCs and PCFCs, oxygen gas is electrochemically reduced. The cathodic reacitons basically takes place only at triple phase boundaries (TPBs) of the electrolyte, the electrode, and the gas phases, because ions (oxide ion or proton), electrons, and oxygen gas molcules exist in the different phases. On the other hand, when a mixed ionic-electronic conductor (MIEC) is used as the electrode material, the reaction site can be expanded from TPBs to double phase boundaries (DPBs) of the electrode and the gas phases, since ions can exist not only in the electrolyte but also in the electrode in this case. Thus, it is generally believed that DPBs are the dominant reaction sites in an MIEC cathode, because the DPB area is much larger than the TPB area. However, as far as we know, no one could experimentally succeed to show how significantly the TPB reaction contributes to the total reaction in an MIEC cathode. In order to properly design a high pefromance cathode, it is important to understand the contribution of TPB/DPB reactions separately and quantitatively.
In practical SOFCs and PCFCs, porous electrodes are conventionally used. However, it is not easy to quantitatively evaluate the contributions of TPB/DPB reactions from experiments using porous electrodes because of their complicated microstructures and inhomogeneous electrode reaction distribution [1]. In order to avoid such experimental difficulties with porous electrodes, many researchers applied the model electrodes, such as a dense thin film electrode and a microelectrode. However, these conventional model electrodes tend to overestimate the contribution of the DPB reaction and thus are still not appropriate to investigate the contributions of the TPB/DPB reactions [2]. Instead, our group proposed new and original model electrodes, which is so-called "patterned thin film electrodes", as schematically illustrated in Figs. 1(A) and (B) [2]. These model electrodes are a kind of thin film electrodes, but the area of the electrode/electrolyte contact was limitted by inserting the slitted insulating layer between the electrode and the electrolyte. In the model electrodes of Fig. 1(B), a part of the electrode film was removed to introduce TPBs. By utilizing these model electrodes, the contribution of the DPB reaction can be evaluated from the model electrode (A), whereas the contribution of the TPB reaction from the difference between the model electrodes (A) and (B). These model electrode can be fabricated by helps of photolithography, pulsed laser deposition, and gas spputtering techniques.
By applying the aforementioned model electrodes, we first investigated the contributions of TPB/DPB reactions in SOFC MIEC cathodes, for instance an La0.6Sr0.4CoO3-d or La0.6Sr0.4Co0.2Fe0.8O3-d cathode on a Ce0.9Gd0.1O1.95 electrolyte. We consequently found: (i) DPBs are the dominant reaction sites at operation temperatures of conventional SOFCs (i.e. above 973 K), but (ii) contribution of TPB reactions becomes significant as temperature decreases below 873 K, amibient p(O2) increases, and the applied DC bias decrases. These results tell us that the contribution of TPB reactions should be taken into account for desigining the cathode in intermediate tempeature SOFCs, even when MIEC is used as electrode materials. Similar measurements were performed to investigate the cathodic reactions in PCFCs. Here La0.6Sr0.4CoO3-d and BaGd0.3La0.7Co2O6-δ were chosen as cathode materials. As results, it was found that the dominant reaciton site is stongly affected by electrode material, possibly depending on protonic conductivity in the electrode materials. More details of our researches on SOFC and PCFC cathodic reacions by using patterned thin film model electrodes will be reported in the presentation.
Acknowledgements : A part of this work was supported by NEDO, Japan.

Reference
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[1] S. B. Adler, Chem. Rev., 104, 4791 (2004).
[2] K. Mizuno, et al., to be submitted.

Figure 1