Modeling of Multi-Physics Phenomena for High-Temperature Co-Electrolysis
Authors: Nohl Markus , Mazumder Jenifar , Vinke I. C. , Eichel Rüdiger-A. , de Haart L. G. J. 
Published: July 2021

Co-electrolysis as value chain process can contribute to various sectors like the chemical industry [1] or the energy sector [2] due to its high flexibility [3]. With its product syngas (a mixture of hydrogen (H2) and carbon monoxide (CO)), co-electrolysis acts as an upstream process to meet changing demands within the respective downstream supply chains [3]. Additionally, when 100% of renewable energy sources are used, green syngas can be produced thus coupling and connecting renewables into the existing value chains [4,5]. In order to fulfill industrial needs, the process of co-electrolysis has to be examined in detail. This work presents a 3D model of a simplified Solid Oxide Electrolysis Cell (SOEC) operating at high temperatures in the co-electrolysis mode. In this model, we evaluate the boundaries of co-electrolysis, the steam and CO2 electrolysis, as well as a detailed description of the co-electrolysis process with the help of a proposed reaction scheme. The model is fully coupled including electrochemical, chemical, heat and transport phenomena occurring at single cell level. We consider the heat of the reaction(s), accompanying reactions including possible surface reactions in co-electrolysis operation mode, Maxwell-Stefan interaction and electrochemical charge transfer reactions of the Butler-Volmer type. With calculating experimental characteristics like i-V curves (see Figure) or electrochemical impedance spectroscopy (EIS) curves, the influence of every parameter considered in each reaction step can be evaluated according to its value range. The model is in first instance validated with the pure electrolysis types. With the model, we can visualize the particle distribution within the porous cell, see the development of overpotential (see Figure) and obtain experimental characteristics which allow us to identify the contributions of each mechanism to them. This is an important step towards fundamental understanding of the co-electrolysis process on a small scale which can then be transferred to the large scale applications on stack level. Effects of degradation are envisioned to be incorporated into the model as well.

Figure caption: Figure 1: Plot of the overpotential distribution at 0.6 A∙cm-2 within the porous cell electrodes with appropriate dimensions (left) and corresponding polarization curve with a current sweep from 0 to 3 A∙cm-2 in steps of 0.2 A∙cm-2 (right) both for steam electrolysis at 800°C.

References

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[5] Li, W.; Wang, H.; Shi, Y.; Cai, N. (2013): Performance and methane production characteristics of H2O–CO2 co-electrolysis in solid oxide electrolysis cells. In: International Journal of Hydrogen Energy, vol. 38, no. 25, pp. 11104–11109

Figure 1