Evaluation of Reaction Mechanism of PCFC Composite Cathodes by Utilizing Patterned Thin Film Model Electrodes

Abstract

Proton-conducting ceramic-electrolyte fuel cell (PCFC) is expected as a fuel cell operating at intermediate temperatures (400-600℃) with high efficiency. Aiming to commercial application of PCFC, the decrease of polarization resistance at the cathode is a major challenge because the voltage loss in the cathode occupies the largest part of the total loss [1]. The fuel cell cathodic reaction is thought to basically occur only at the triple phase boundaries (TPBs) of gases, electrode, and electrolyte. To enlarge the electrochemical active site, mixed ionic and electronic conductors (MIECs) are often used, since the cathodic reaction can then take place also on the double phase boundaries (DPBs) of gases and electrode in this case. Based on this idea, recently, PCFC cathodes, which are supposed to show mixed protonic and electronic conduction, have been reported to achieve decent performance. On the other hand, it is experimentally shown that some cathode materials typically utilized in solid oxide fuel cells (SOFCs), such as La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), also exhibited same-level performance, even though their protonic conductivities are believed small. These results suggested that the kinetics of TPB reaction in an LSC or LSCF cathode may be quite fast [2, 3], and increasing TPBs, for instance by making a composite, is supposed as one of the most effective ways to promote the electrode performance.From the background mentioned above, in this work, we aimed to quantitatively clarify the effect of making a composite on the performance of PCFC cathode. In practical porous electrodes, the complex geometry and microstructures limit quantitative analysis of reaction mechanism including dominant reaction site, active reaction area, the rate limiting process and so on. In previous works, we proposed model electrodes, so-called “patterned thin film electrodes”, which simplify the microstructures while reproduce the reaction in a porous electrode [2]. Figures 1 (a) and (b) show the schematics of patterned thin film electrodes without and with TPBs, respectively. Utilizing these model electrodes, contributions of TPB and DPB reactions to the total reaction in PCFC cathodes could be quantitatively and separately evaluated [3].In this work, we further proposed other two types of patterned thin film electrodes, which can model reactions in a composite PCFC cathode. Figures 1 (c) and (d) show the schematics of “the patterned thin film composite electrodes” without and with TPBs, respectively. In these model electrodes, an electrolyte layer was inserted between the insulator and the electrode layers, which corresponds to electrolyte particles in a porous composite electrode. In the model composite electrode in Figure 1 (d), a part of electrode layer is removed to expose a part of electrolyte layer to gases, namely to introduce TPBs. By utilizing the model electrode in Figure (c), we can investigate the effect of the promotion of ionic diffusion due to the addition of electrolyte particles in a composite electrode. On the other hand, the effect of the increase in TPBs can be evaluated from the difference of the electrode performance between the model electrodes in Figures (c) and (d).In this work, La0.6Sr0.4CoO3-δ (LSC) and BaZr0.8Yb0.2O3-δ (BZYb) were selected as the cathode and electrolyte materials. The patterned thin film model composite electrodes were fabricated by applying photolithography, pulse laser deposition and ion milling techniques. The total thickness of the patterned thin film model composite electrode (electrode and electrolyte layers) was set close to that of the patterned thin film model electrode in previous works. A three-terminal electrochemical cell was fabricated together with a counter and a reference electrodes of porous palladium, and DC polarization measurements were conducted. In the presentation, the effect of the addition of the electrolyte in a composite LSC/BZYb cathode will be discussed in detail based on the experimental results.Acknowledgements: A part of this work was supported by NEDO, Japan.[1] C.Duan, et al., Appl. Phys. Rev., 7, 011314 (2020).[2] K. Amezawa, et al., ECS Trans., 77, 41-47 (2017).[3] K. Nishidate, et al., PACRIM-13, Okinawa, Japan, 29-P-S02-27 (2019). Figure 1