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The performance of an SOC depends on the testing environment. Regarding single cell tests with ideal contacting (gold and nickel meshes) and inert flow fields (Al2O3), the performance is limited by the intrinsic losses in the cell as contact losses and any poisoning effects are minimized. In an SOC-stack exhibiting metallic interconnectors, the performance is furthermore affected by contact resistances and limitations in gas supply. In this contribution single cell testing is extended to a stack-like contacting applying three different metallic interconnectors (Crofer 22 APU, ASTM 441 and UNS S44330) with and without a cerium-cobalt PVD-coating. Cell performance and losses are analyzed by IV-characteristics, impedance spectroscopy and DRT analysis. For all uncoated interconnectors, significant performance losses due to increased contact losses and air electrode polarization were observed. With the CeCo-coating losses could be drastically reduced leading to a cell performance close to the ideal case for all three steel grades.

Modelling of the co-electrolysis process requires understanding of the underlying reaction pathways. These include the electrochemical steam reduction, the electrochemical CO2 reduction and their coupling via the catalytic (reverse-)water-gas-shift reaction (RWGS). The assumption of a very fast water-gas-shift reaction and therefore neglectable CO2 electro-reduction is commonly used to model the co-electrolysis process. In contrast to that, it was proposed from previous studies, that the electrochemical conversion of CO / CO2 can be present under H2 / H2O / CO / CO2 gas mixtures on Ni/GDC fuel electrodes. We present results from a complex variation of operating parameters for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times (DRT). Four peaks are identified, including one yet undocumented suspected gas diffusion process. A possible influence of the electrochemical conversion of CO/CO2 starting from pCO2 / pH2O > 1 is shown.

Angewandte Chemie / International edition e202218850 (2023). doi:10.1002/anie.202218850 special issue: "Hot Topic: Water Splitting", Published by Wiley-VCH, Weinheim

The deconvolution of physicochemical processes in impedance spectra of SOCs with nickel/ceria fuel electrodes is challenging as gas diffusion strongly overlaps with the electrochemical processes at fuel and air electrode. To overcome this issue, symmetrical cells were applied and the gas diffusion process at the fuel electrode was quantified by altering the inert component (nitrogen/helium) in a ternary fuel gas mixture. An effective gas transport parameter considering microstructural and geometrical features was derived, enabling a precise quantification of polarization resistances related to gas diffusion and hydrogen electrooxidation. The obtained values were applied to parameterize an existing dc cell model. The model validation in fuel cell and electrolyzer mode showed an excellent agreement between measured and simulated current/voltage characteristics over a wide range of technically meaningful gas compositions and operating temperatures.