Investigation on Reaction Mechanism in an SOFC Composite Cathode by Using Patterned Thin Film Electrodes

Abstract

Introduction For the further commercial use of solid oxide fuel cells (SOFCs), operating at intermediate temperatures is demanded. When reducing the operating temperature, the degradation of cathodic activity occupies the largest part of degradation of cell performance. Therefore, to improve the performance of SOFCs at intermediate temperature, the activity of cathodic reaction has to be improved.The cathodic reaction of SOFCs proceeds not only at triple phase boundaries (TPBs) of electrode, electrolyte and gas but also at double phase boundaries (DPBs) of electrode and gas, when a mixed ionic and electronic conductor (MIEC) is used. Thus, increasing the DPB area is considered as the best way to improve the cathodic performance. However, it was reported that composite cathodes of MIEC and electrolyte particles showed better performance than simple MIEC cathodes, despite the DPB area decreased [1]. Possible reasons for such performance enhancement can be considered as (i) promotion of oxide ion diffusion and/or (ii) the increase of TPB concentration. However, the reason for the performance enhancement by making a composite cathode are not-well understood so far.In this study, we aimed to understand the cathodic reaction in a composite cathode of MIEC and electrolyte. For this purpose, we proposed new types of original model electrodes to simulate the reaction in actual porous composite electrodes. From electrochemical and the operando X-ray absorption spectroscopic measurements of the model electrodes, we tried to separately evaluate the contribution of ion diffusion and TPB reactions on the performance improvement of composite cathode. Methods When using a porous electrode, it is difficult to investigate the reaction mechanism because of its complicated microstructures. Our research group proposed so-called “patterned thin film model electrode” like Figure 1(a) as a model electrode to reproduce the reaction in a porous electrode while removing the influence of microstructure [2]. In this model electrode, the contact area of electrode and electrolyte was limited by partially inserting an insulating layer, and thus the reaction distribution was formed in the electrode as a function of the distance from the electrode/electrolyte interface.In this study, we proposed two new types of model electrodes as Figure 1(b) and 1(c) to simulate the porous composite electrode. In the patterned composite electrode (b), an electrolyte layer was inserted between the electrode and insulator. On the other hand, in the patterned composite electrode (c), a part of electrode was removed to introduce TPBs. From the difference between the model electrodes (a) and (b), the contribution of oxide ion diffusion in composite electrode was estimated. From the difference between the model electrodes (b) and (c) the contribution of the TPB reaction was estimated. In this study, we selected La0.6Sr0.4CoO3-δ (LSC64) as an MIEC cathode and Gd0.1Ce0.9O2-δ (GDC10) as an electrolyte material. Operando micro X-ray absorption spectroscopy (μ-XAS) measurement was conducted with the model electrodes to investigate the effective reaction area of the model electrode. In this measurement, a change in the oxygen chemical potential can be detected as a change of X-ray absorbance at the fixed energy near the Co K-edge accompanying a change of the oxidation state of Co in the LSC64 [3]. Thus, the reaction distribution could be evaluated by measuring the change of the X-ray absorbance due to the polarization as a function of the distance from the electrode/electrolyte interface. Results and Discussion Figure 2 shows the results of μ-XAS measurement at 973 K under 1 bar O2 with cathodic polarization. The area where the ratio of absorbance is larger than 1 corresponds to the area where the oxygen chemical potential is changed due to the polarization, i.e. the effective reaction area. The reaction area of the model electrode (b) was longer than that of the model electrode (a). This suggested oxide ion diffusion was improved by inserting the GDC layer. On the other hand, the reaction area of the model electrode (c) was longer than that of the model electrode (b). Generally, the reaction on the mixed conducting LSC64 electrode is supposed to proceed DPB reaction mainly. However, the longer reaction area of model electrode (c) indicated that the contribution of TPB reaction was also significant in the LSC64 composite electrode. As a conclusion, both promotion of oxide diffusion and the increase of TPB effective for the enhancement of the cathodic reaction in MIEC composite electrode.[1] Y. T. Kim, et al., Solid State Ionics, 309, 77-85 (2017).[2] K. Amezawa, et al., ECS transactions, 62(2), 129-135 (2015).[3] Y. Orikasa, et al., Journal of Physical Chemistry C, 115, 16433–16438 (2011). Figure 1