Multiscale and Multiphysical Numerical Simulations of Solid Oxide Cell (SOC)
Authors: Zhang Shidong , Wang Kai , Yu Shangzhe , Kruse Nicolas , Peters Roland , Kunz Felix , Eichel Rudiger-A 
Published: September 2023

Electrochemical applications play a key role for the topic of “green hydrogen” for the de-carbonization of the energy and mobility sectors. Electrochemical systems and processes, including fuel cells and electrolysers, have witnessed several benefits over conventional combustion-based technologies currently being widely used in power plants and vehicles. The ceramic high-temperature technologies by means of SOC exhibits high efficiencies, with a thermoelectric conversion efficiency as high as 60% and a total efficiency of up to 90% in fuel cell operation and even higher in electrolysis mode. The SOC technology, therefore, sees a promising future in the production of green hydrogen and electricity.
Providing high operating temperature, over 600 oC, the SOC system shows the capability to operate with diverse types of gas mixtures, for example, hydrogen, ammonia, and carbon-containing mixtures such as methane (CH4), carbon monoxide (CO) in fuel cell operation (SOFC) and steam and/or carbon dioxide (CO2) in electrolysis operation (SOEC). The design of the SOC stack enables a reversible operation (rSOC) between fuel cell and electrolysis modes. It indicates the SOC system can perform with high efficiencies in both operating modes, which also widens the scope of possible applications.
Challenges remain when it comes to commercialization of the SOC technology, in both the investment costs (CAPEX) and operating costs (OPEX) aspects. From the technological and scientific point of view, the physical transport phenomena in SOCs need to be understood which can be done by the help of experimental and numerical investigations. Cheaper and long-lasting material alternatives may be found for the cell, stack and system development afterwards.
Detailed experimental investigations usually require a lot of effort in time and data analysis. To promote the scientific and technological studies on the SOC technology, numerical investigations by using multiscale and multiphysical models are carried out in this work. The models include an in-house designed/written phase field model (PFM) 1, and an open-source based computational fluid dynamics (CFD) model, openFuelCell2 2 (based on OpenFOAM). The former accounts for the microstructure evolution of the Ni/YSZ composition. The latter addresses the multiphysical transport processes in different phases, i.e., ionic transfer in YSZ, electronic transfer in Ni, and the gas diffusion in the gas phase.
The figure below shows the computational domain and different phases in the numerical simulations. The evolution of Ni/YSZ composition can be predicted by the PFM. It is supposed to reproduce the Ni agglomeration that has been observed in SOFC long-term experiments 3. The simulation result shown at the left-most is obtained by performing the PFM simulation (with 96 x 96 x 96 voxels) representing the microstructure change for a certain time duration. The computational domain (192 x 192 x 192 voxels) consists of three phases, namely, YSZ, Ni, and gas, as shown in the middle and on the right side. It is refined in each direction to better capture the triple phase regions (lower right) shared by the three phases, which refers to the active sites that enable the electrochemical reaction to be conducted. By applying different governing equations on these phases, the CFD model, openFuelCell2, can describe the transport phenomena numerically. Hence, the performance degradation of a SOFC due to Ni agglomeration can be captured by carrying out the simulations for different time durations. The effective properties may be derived as well so that they can be used in numerical simulations with larger scales.
Acknowledgement
The authors would like to thank their colleagues at Forschungszentrum Jülich GmbH for their great support and the Helmholtz Society, the German Federal Ministry of Education and Research as well as the Ministry of Culture and Science of the Federal State of North Rhine-Westphalia for financing these activities as part of the Living Lab Energy Campus.
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Figure 1