Elucidating the Reaction Pathways Responsible for Carbon Monoxide Production in Solid Oxide Co-Electrolysis Cells: A Theoretical Framework

Abstract

The continuous emission of anthropogenic greenhouse gases, in tandem with the demand for renewable and intermittent energy storage systems, has given rise to the development of technologies which can convert carbon dioxide into useful chemicals and feedstocks. Solid oxide co-electrolysis cells have the potential to help address these demands, as they can efficiently convert carbon dioxide and steam into synthesis gas (carbon monoxide and hydrogen) utilizing clean electricity, which can then be supplied downstream to Fischer-Tropsch reactors in order to produce synthetic liquid fuels. Advantages of solid oxide co-electrolysis cells are mainly attributed to their capacity to operate at high temperatures, which in turn enhances the catalytic performance of electrode materials and reduces the amount of energy required to facilitate the electrochemical reduction of carbon dioxide and steam. The development of these devices has consequently drawn significant attention from designers and researchers, and have emerged as a promising alternative for renewable energy storage and synthesis gas production. One important challenge that has arisen in the analysis of solid oxide co-electrolysis cells is determining the reaction pathways responsible for carbon monoxide production. Studies have suggested that the reverse water gas shift reaction is the primary mechanism [1,2], since the co-electrolysis polarization curves have been shown to be identical to those obtained in pure steam electrolysis, while others have found that such curves lie between the results produced by pure steam and pure carbon dioxide electrolysis, thus indicating the electrochemical reduction of carbon dioxide and reverse water gas shift reaction are both contributors [3,4]. Given the varying operating conditions and cell designs that have been implemented in these studies, it is difficult to draw meaningful comparisons between these results and to deduce the reaction pathways by which carbon monoxide is generated. The objective of the current work is to, therefore, develop a theoretical framework to elucidate the reaction pathways which govern carbon monoxide production for a broad range of operating conditions and design specifications. An in-house 1-Plus-1D transport model is developed to quantify how each transport mechanism affects the rate of production and consumption of carbon monoxide, as well as the operating potential of a solid oxide co-electrolysis cell. This analysis is intended to cultivate an improved understanding of the reaction pathways responsible for carbon monoxide production and to reveal how they can be exploited in order to enhance the performance of solid oxide co-electrolysis cells.[1] P. Kim-Lohsoontorn and J. Bae, J. Power Sources, 196, 7161 (2011).[2] C. Stoots, J. O’Brien and J. Hartvigsen, Int. J. Hydrogen Energy, 34, 4208 (2009).[3] C. Graves, S. D. Ebbesen and M. Mogensen, Solid State Ion., 192, 398 (2011).[4] W. Li, H. Wang, Y. Shi and N. Cai, Int. J. Hydrogen Energy, 38, 11104 (2013).