An Analysis of Capacitive Contributions of the Cell Impedance during High-Temperature CO₂ and Co-Electrolysis

High-temperature electrolysis is often studied by electrochemical impedance spectroscopy (EIS). The EIS spectra are analyzed using equivalent circuit models (ECM). In many cases, these models use basic elements such as resistor, capacitances or inductances. The discussion of the modelling results is often limited to resistances [e.g. 1-4]. Only some rare papers discuss capacitances as well [e.g. 5-7]. A detailed analysis of all capacitive contributions of the overall cell impedance in electrolysis mode in dependence on temperature, current and composition is missing in literature. The present study provides this detailed analysis of the capacitive contributions, which in the models are usually represented by constant phase elements (CPEs). Impedance spectroscopy measurements of high-temperature CO 2 and co-electrolysis on conventional fuel electrode supported cells were performed. The cells consisted of a Ni/YSZ fuel electrode, a YSZ electrolyte, a GDC barrier layer and a (La,Sr)CoO 3 air electrode. The spectra were analyzed by an ECM consisting of an inductance, a serial resistance and four RQ elements (resistance and constant phase element in parallel). The dependences of the resistances on operating conditions were already reported elsewhere [1,8]. The variation of the parameters temperature, current and composition showed different trends for the single elements. For example, the capacitive contribution of processes 1 – 3 (high to middle frequency range) increases with temperature while it decreases for process 4 (low frequency range). The low frequency process had been assigned to a gas phase diffusion process according to the dependence of the corresponding resistance [1]. The CPE element of process 4 also shows a prominent dependence on the current density. This might correlate with the changing ratio of CO 2 to CO in the gas phase and thus support the assignment. Further results of the dependencies of the elements will be presented and discussed. [1] S. Foit, L. Dittrich, T. Duyster, I. Vinke, R.-A. Eichel, L.G.J. de Haart, Processes 2020 , 8 , 1390. [2] S.E. Wolf, L. Dittrich, M. Nohl, T. Duyster, I.C. Vinke, R.-A. Eichel, L.G.J. de Haart, J. Electrochem. Soc. 2022 , 169 , 034531. [3] S.D. Ebbesen, X. Sun, M. B. Mogensen, Faraday Discuss. 2015 , 182 , 393–422. [4] A. Leonide, V. Sonn, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc. 2008 , 155 , B36. [5] S. Primdahl, M. Mogensen, J. Electrochem. Soc. 1997 , 144 , 3409. [6] E.-C. Shin, J. Ma, P.-A. Ahn, H.-H. Seo, D. T. Nguyen, J. S. Lee, Electrochim. Acta 2016 , 188 , 240-253. [7] X. Ge, C. Fu, S. H. Chan, Phys. Chem. Chem. Phys. 2011 , 13 , 15134-15142. [8] L. Dittrich, Tailoring of the Synthesis Gas Composition during High-Temperature Co-Electrolysis (Aachen, Germany) ( 2021 ).