Comprehensive model for CGO based anodes

References: 1. A novel approach for analyzing electrochemical properties of mixed conducting solid oxide fuel cell anode materials by impedance spectroscopy, A. Nenning, A. K. Opitz, T. M. Huber, and J. Fleig, Phys. Chem. Chem. Phys., vol. 16, no. 40, pp. 22321–22336, 2014. 2. Modeling the impedance response of mixed-conducting thin film electrodes, C. Chen, D. Chen, W. C. Chueh, and F. Ciucci, Phys. Chem. Chem. Phys., vol. 16, no. 23, pp. 11573–11583, 2014.Gadolinium doped Ceria (CGO) is a promising material for SOFC anodes because of its mixed ionic electronic conductivity (MIEC), its high catalytic activity for the hydrogen oxida-tion reaction (HOR) and its robustness against degradation. However, many processes in CGO based anodes are still poorly understood and there is no consensus yet on how to inter-pret the corresponding electrochemical impedance spectroscopy (EIS) data. In the literature, especially the low frequency arc is often either depicted as gas impedance or as chemical capacitance process, without conclusive evidence. Moreover, with EIS, charge transport re-sistances in CGO are often not distinguishable from other overlapping processes. For dense thin film MIEC electrodes many sophisticated models are available using transmission line (e.g. [1]) and finite element method (FEM) models (e.g. [2]). In contrast, for porous high per-formance MIEC anodes there are hardly any modelling approaches published, which are ca-pable to capture the complex physico-chemical processes. This is, however, a crucial pre-requisite for a systematic materials optimization. In this contribution we present a model, which enables physical insight into the complex processes involved in a porous CGO based anode on the button cell level. The FEM model is implemented in 1D in the commercial software package Comsol Multiphysics. In this model, the full Nernst-Planck-Poisson equations are implemented for the transport of Ce3+-ions and oxygen ion vacancies. Steady state and linear perturbation simulations are performed in order to compute the DC-behaviour and the EIS-spectra, respectively. The model captures the spa-tial distribution of the reaction zone and associated transport pathways of the charge carriers. Thereby, parameter sets that result in transport limited and surface reaction limited cases are studied. Moreover, the model enables to distinguish the impact of chemical capacitance, HOR resistance and gas impedance on the EIS spectra for button cell conditions. Furthermore, the appropriate material laws, as e.g. the dependency of the HOR re-sistance on the operating conditions (pO2, pH2O), can be deduced from EIS-measurements. By linking bulk material properties, fabrication parameters, microstructure effects and operat-ing conditions with the cell performance, we present a way towards a systematic materials optimization for CGO based anodes