Reverse Water Gas Shift versus Carbon Dioxide Electro-Reduction: The Reaction Pathway Responsible for Carbon Monoxide Production in Solid Oxide Co-Electrolysis Cells

Abstract

Solid oxide co-electrolysis cells can utilize renewable energy sources for the conversion of steam and carbon dioxide into valuable chemicals and feedstocks. An important challenge in the analysis of these devices is understanding the reaction pathway(s) that govern carbon monoxide generation. Studies in which co-electrolysis polarization lies between those of pure steam and pure carbon dioxide electrolysis suggest that carbon dioxide electro-reduction (CO2ER) and the reverse water gas shift (RWGS) reaction are both contributors to CO generation. However, experiments in which co-electrolysis polarization overlaps that of pure steam electrolysis propose that the RWGS reaction dominates CO production and CO2ER is negligible. Supported by dimensional analysis, thermodynamics, and reaction kinetics, this work elucidates the reasons for which the latter conclusion is infeasible, and provides evidence for why the observed overlap between co-electrolysis and pure steam electrolysis is a result of the slow kinetics of CO2ER in comparison to that of steam, with the RWGS reaction being inconsequential. For sufficiently thin cathode current collectors, we reveal that CO2ER is dominant over the RWGS reaction, while the rate of steam electro-reduction is much higher than that of carbon dioxide, which causes the co-electrolysis and pure steam electrolysis polarization curves to overlap. This is contrary to what has been proposed in previous experimental analyses. Ultimately, this work provides insight into how to design solid oxide co-electrolysis cells such that they can exploit a desired reaction pathway in order to improve their efficiency and product selectivity.