(Invited) Contribution of Triple/Double Phase Boundary Reactions in Mixed Conducting Oxide Cathodes in SOFCs and PCFCs

Abstract

Development of high performance cathode is one of largest challenges for the full-scale commercialization of solid oxide fuel cells (SOFCs) using an oxide ion conducting electrolyte and proton ceramics fuel cells (PCFCs) using a proton conducting electrolyte. The cathodic reactions in SOFCs and PCFCs are the electrochemical reduction of oxygen gas. Since ions (oxide ion or proton), electrons, and oxygen gas molcules exist in the electrolyte, the electrode, and the gas phase, respectively, the cathodic reaciton basically takes place only at triple phase boundaries (TPBs). On the other hand, the reaction site can be expanded from TPBs to double phase boundaries (DPBs), when a mixed ionic-electronic conductor (MIEC) is used as the electrode material, because ions can exist not only in the electrolyte but also in the electrode. In such an MIEC cathode, it is generally believed that the dominant reaction sites are DPBs, because the DPB area is much larger than the TPB area. However, as far as we know, there are no clear experimental evidences which quantitatively showed how significantly the TPB reaction contributes to the total reaction in an MIEC fuel cell cathode. It is important to know the contribution of TPB/DPB reactions separately for the material and structure designs of high performance fuel cell cahtodes.In practical SOFCs and PCFCs, porous electrodes are conventionally used. But for evaluating the contributions of TPB/DPB reactions, experiments using porous electrodes are not appropriate because of complicated microstructures and inhomogeneous electrode reaction distribution in porous electrodes [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 separately [2]. Instead, our group proposed new and original model electrodes, which is so-called « patterned thin film electrodes », as shown in Figs. 1(A) and (B) [2]. These electrodes are sorts 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 one type of the model electrodes (Fig. 1(B)), a part of the electrode film was removed to introduce TPBs. By applying these novel model electrodes, the contribution of the DPB reaction can be evaluated from the model electrode (A), while 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 and pulsed laser deposition techniques.By applying the above-mentioned model electrodes, we first investigate the dominant reaction pathway in SOFC MIEC cathodes, for instance an La0.6Sr0.4CoO3-δ or La0.6Sr0.4Co0.2Fe0.8O3-δ cathode on a Ce0.9Gd0.1O1.95 electrolyte. Consequently, it was found (i) DPBs are the dominant reaction sites at operation temperatures of conventional SOFCs (i.e. above 973 K), and (ii) Contribution of TPB reactions becomes significant with decreasing temperature below 873 K, increasing amibient p(O2), and decrasing the applied DC bias. These results suggested that the contribution of TPB reactions should be taken into account for desigining the cathode in intermediate tempeature SOFCs. Similar measurements were applied to investigated the cathodic reactions in PCFCs. Here La0.6Sr0.4CoO3- δ and BaGd0.3La0.7Co2O6-δ were chosen as cathode materials. It was clearly shown that the dominant reaciton site is stongly affected by electrode material, possibly depending on protonic conductivity in the electrode materials. Moreover, our moedel electrodes can be employed to investigate the degradation mechanism of SOFC or PCFC cathodes. For instance, Cr poisoning phenomena in SOFC cathodes was examined, and, as a results, it was found that several degradation modes exist depending on the position in the electrodes. In the presentation, details of these our research activities on SOFC and PCFC cathodic reacions by using patterned thin film model electrodes will be introduced. Reference s [1] S. B. Adler, Chem. Rev., 104, 4791 (2004). [2] K. Mizuno, et al., to be submitted. Figure 1