(Invited) Designing SOC for Power-to-X Applications: A Multi-Physical Modelling Approach
Authors: Ivers-Tiffée Ellen , Dierickx Sebastian , Russner Niklas , Weber André 
Published: May 2020

In the current global energy transition, the Solid Oxide Cell (SOC) represents a promising technology for stationary energy generation and storage, since it facilitates highly efficient and low-emission energy conversion. Besides converting hydrogen or hydrocarbon fuels directly into electricity in fuel cell mode, the SOC can also be operated in a reverse mode as a Solid Oxide Electrolyzer Cell (SOEC). By operating the SOC as an electrolyzer, any surplus electrical energy produced from fluctuating renewable energy sources (e.g. solar or wind energy) can be utilized to synthesize syngas and subsequently hydrocarbon fuels. The outlet syngas composition is directly correlated to the applied operating conditions as H2O/CO2-ratio, temperature and current. Therefore, it is of utmost importance to gain a deep understanding of these relationships in order to establish optimal operational strategies for different applications.
During electrolyzer operation the key component of the SOC is the fuel electrode (anode), since it facilitates the (i) electrochemical charge transfer reaction and (ii) the chemical reaction kinetics (cf. Fig. 1(c)). In state-of-the-art, anode-supported SOCs the anode relies on two porous, Ni / stabilized zirconia layers with specified phase compositions and microstructural characteristics: (i) the highly porous and catalytically active anode substrate (AS) and (ii) the electrochemically active anode functional layer (AFL) (cf. Fig. 1(a)). To raise fuel conversion efficiency, we must first gain a profound understanding of the optimal double-layer design for the desired application. Numerical simulations enable an efficient variation of geometrical, microstructural and material parameters over wide ranges and provide correlations between anode design and SOEC performance.
In this contribution, we demonstrate a multi-physical modelling approach to predict the SOC performance in both fuel cell and electrolyzer operation. To do so, we combine two of our recently published modelling approaches: (i) the physically meaningful, three-channeled transmission line model for double-layered cermet anodes [1,2] and (ii) the non-isothermal FEM stacklayer model [3,4]. For the first time, a powerful mode framework is realized (cf. Fig. 1(b)), which is capable of spatially resolving all of the above mentioned processes, in both anode layers, within a complete stacklayer. The model considers the complex interplay of (i) multi-component gas transport coupled with (ii) catalytic reforming reactions, (iii) charge carrier transport, (iv) electrochemical charge transfer, (v) coupled heat release and consumption as well as (vi) heat transport in the individual stack components.
We demonstrate a comprehensive, model-based optimization of Ni-based cermet anodes in a SOC stack layer with respect to (i) the selection of the electrolyte material, (ii) the modification of the microstructure and (iii) adapting of the layer thicknesses. Furthermore, the impact of the operating conditions on the optimal cell configuration will be discussed. Finally, we derive operational strategies for efficient SOEC operation by investigating the impact of (i) the inlet gases, (ii) the operating temperature and (iii) the applied current on the produced syngas composition.
[1] S. Dierickx, J. Joos, A. Weber, E. Ivers-Tiffée, Advanced impedance modelling of Ni/8YSZ cermet anodes, Electrochim. Acta. 265 (2018) 736–750.
[2] S. Dierickx, T. Mundloch, A. Weber, E. Ivers-Tiffée, Advanced impedance model for double-layered solid oxide fuel cell cermet anodes, J. Power Sources. 415 (2019) 69–82.
[3] H. Geisler, A. Kromp, A. Weber, E. Ivers-Tiffée, Stationary FEM Model for Performance Evaluation of Planar Solid Oxide Fuel Cells Connected by Metal Interconnectors, J. Electrochem. Soc. 161 (2014) F778–F788.
[4] N. Russner, S. Dierickx, A. Weber, R. Reimert, E. Ivers-Tiffée, Multiphysical modelling of planar Solid oxide fuel cell stack layers, J. Power Sources (in press).

Figure 1