Boundaries of High-Temperature Co-Electrolysis Towards Direct CO2-Electrolysis

Abstract

High-temperature co-electrolysis of CO2 and H2O at elevated temperatures between 700 °C and 900 °C valorises CO2 to produce a mixture of carbon monoxide (CO) and hydrogen (H2), called syngas. Co-electrolysis has the great advantage over conventional processes, that the desired syngas ratios of downstream processes can be realized by varying process parameters such as temperature and feed gas composition accordingly in a one step process. Co-electrolysis can also play a vital role in counteracting power fluctuations of renewable energy sources by storing temporarily unused electricity through conversion to other energy resources like chemicals or heat for later use.The underlying processes in co-electrolysis for CO production are direct electrochemical CO2 reduction and reverse water gas shift equilibrium (RWGS). Their specific significance has not been clarified in detail yet and was controversially discussed in literature up to this day [1,2].The impact of the equilibrium partial pressure of H2O on the physical processes in the transition boundary of co-electrolysis towards direct CO2-electrolysis was investigated by AC and DC measurements for various gas compositions. The analysis led to identifying the role of the underlying electrochemical processes during co-electrolysis, in particular the electrochemical CO2 reduction compared to the conversion of CO2 in the reverse water-gas shift reaction and the electrochemical H2O reduction. The area specific resistance (ASR) was, amongst others, taken as an indicator to determine, which of the reduction reactions (H2O or CO2 reduction) is dominant depending on the gas composition. The experiments were conducted using commercially available cathode-supported full cells (Elcogen) made of Ni-8YSZ/8YSZ/CGO/LSC.Results as seen in Figure 1 show that the ASR for an equilibrium concentration of 5 % H2O is considerably larger than for higher H2O contents. Above 15 % H2O, the ASR shows no dependency on the gas composition and is comparable to pure H2O-electrolysis [3]. These observations underline the hypothesis that CO2-electrolysis becomes pre-dominant compared to H2O-electrolysis for low H2O content during co-electrolysis. With increasing H2O content, CO2-electrolysis becomes less significant and carbon dioxide is converted in the reverse water gas shift equilibrium. The origin of the discrepancy in literature was found to be the different operating H2O concentrations. A threshold has been established for the perception of CO2-electrolysis during co-electrolysis experiments.figure caption: Arrhenius plot of ASROCV for different steam concentrations at 6 l·h-1.[1] C. Stoots, J. O'Brien, J. Hartvigsen, Int. J. Hydrogen Energy 2009, 34, 4208.[2] S. D. Ebbesen, R. Knibbe, M. Mogensen, J. Electrochem. Soc. 2012, 159, F482-F489.[3] L. Dittrich, M. Nohl, E. E. Jaekel, S. Foit, L.G.J. (Bert) de Haart, R.-A. Eichel J. Electrochem. Soc. 2019, 166, F971-F975. Figure 1