Unraveling microstructure effects in Ni-YSZ anodes by 3D-analysis, FE-simulation and experimental characterization

摘要

Microstructure has an important influence on the performance of SOFC electrodes. In recent years considerable progress has been achieved in describing microstructure parameters (e.g. TPB, tortuosity, particle size distributions) on a quantitative level based on high-resolution tomography. However, the performance of fuel cell electrodes is based on a complex interplay of various transport and electrochemical processes. Hence, in order to unravel the influence of microstructure (and microstructure degradation) on the electrode performance, it is not sufficient to just quantify the critical microstructure parameters, but also to incorporate these parameters into models that allow simulation of electrode reaction mechanism including the complex interplay of various physico-chemical processes. In this study we first present the recent progress in the elaboration of the relationship between effective transport properties with the transport relevant parameters (i.e. percolating phase volume fraction, tortuosity, constrictivity, size distributions of particle bulges and bottlenecks). Furthermore a model was developed to simulate the complex reaction mechanism of Ni-YSZ anodes. This model is capable to incorporate all relevant microstructure parameters that influence charge transport (ionic, electric) and charge transfer (fuel oxidation). The model allows distinguishing between different components of the ASR, which are related either to limitations of charge transport (ionic, electric) or charge transfer (electrochemistry) within the anode. In literature the influence of active reaction sites (i.e. TPB) is strongly emphasized. In the present paper we also focus on limitations in charge transport due to microstructure effects. Examples are presented which highlight the effects of grain size on the effective electric and ionic conductivity and corresponding anode performance. The data are compared with experimental data from EIS. The presented methodology gives new insight on the effects of microstructure variation, because it links critical microstructure parameters with anode performance and with the associated ASR components from different rate limiting processes.