(Invited) Mechanism of Cathodic Reaction in Proton-Conducting Ceramic Fuel Cells Investigated By Patterned Model Electrodes

Abstract

Proton-conducting ceramic fuel cell (PCFCs) is one type of solid oxide fuel cell using a solid-state proton-conductor, typically BaZrO3-based and BaCeO3-based ceramics, as an electrolyte. PCFC is expected to operate at intermediate temperatures with high energy conversion efficiency. One of technological challenges for realization of PCFC is the development of high performace cathode. Cathodic reaction in PCFC is electrochemical reduction of oxygen gas foriming water vapor. Since protons, electrons, and oxygen gas molcules exist in the electrolyte, electrode and gas phase, respectively, the electrode reaciton takes place at triple phase boundaries (TPBs) in principle. 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. In such an MIEC electrode, the dominant reaction sites are believed to be DPBs because of their large area. So far, some oxides are reported to exhibit mixed protonic-electronic conductor, but no clear experimental evidences have been provided to show how significantly TPB/DPB reactions contribute to the total reaction.Recently we proposed new and original model electrodes, which can be schematically illustrated in Fig. 1, to investigate a gas reaction on a solid electrolyte. These so-called "patterned thin film electrodes" are kinds of thin film electrodes. However, in contrast to a conventional thin film electrode, the electrode/electrolyte contact area was limitted by inserting a slitted insulating layer between the electrode and the electrolyte, and threfore our model electrodes can behave as a columar electrode. In the electrode of Fig. 1(a), the electrode reaction proceeds only through DPBs, and the reaction current gradually decreased with increasing the distance from the electrode/electrolyte interface depending on proton diffusion in the electrode layer. On the other hand, in the electrode of Fig. 1(b), a part of the electrode film was removed from the electrode of Fig.1(a) to introduce TPBs. In this case, the electrode reaction takes place not only through DPBs but also through TPBs. Then, by comparing the electrode performances between these novel model electrodes, the contribution of TPB/DPB reactions can be separately evaluated.We applied these model electrodes to investigate the mechanism of the PCFC cathodic reactions. (La,Sr)CoO3-d was chosen as a cathode material, and the model electrodes were fabricated on a Ba(Zr,Yb)O3-d electrolyte. On the lateral and opposite sides of the electrolyte, porous Pt and Pd electrode was set as a reference and a counter electrodes. DC polarization and electrochemical impedance spectroscopy measurements were performed with the model electrodes at 773-973 K. As results, it was found TPBs are the dominant reaction site for (La,Sr)CoO3-d PCFC cathode, although DPB reaction slightly contributes to the total reaction. More detailed disucssion will be given in the presentation based on the results by operando X-ray absorption measurements. Figure 1