A modelling study on how AC:DC mode-switching can improve co-electrolysis operations on solid oxide cells

The production of syngas by co-electrolysis in solid oxide cells (SOCs) is limited at industrial scale by the operational difficulties of high thermoneutral potential and the shortened lifetime due to carbon deposition. This work analyses the effects of pulsed electrical current, named “AC:DC operation”, on a single SOC during co-electrolysis. The model is built in Python, and it represents an adiabatic dynamic simulation over a two-dimensional domain. The model parameters related to mass transport and overpotentials were fitted from experimental tests. It was shown that the AC:DC operation can be tuned to homogenize the temperature across the cell, reducing the temperature decrease from inlet to outlet (<100°C) while operating at average voltage values below the nominal thermoneutral potential. In comparison to the regular DC mode, such AC:DC operation could be used to raise the outlet temperature of the cell up to +76°C, with a consequent increase in carbon dioxide conversion from 64% to 73%, when the nominal reactant conversion was fixed at 80%. Moreover, it was also shown that increasing the current density increases the risk of carbon deposition when operating in DC, entering the carbon formation region at −0.4 A/cm². We demonstrate via model-predictive simulations that the use of AC:DC, thanks to the increase in the cell outlet temperature, moves the operation out of the carbon formation threshold zone, allowing for coke-free operation in an expanded current density regime between −0.4 and −0.6 A/cm². These results provide useful insight into experimental co-electrolysis and advancing the commercial pathway of the technology.