Electroreduction of CO₂/CO to C₂ Products: Process Modeling, Downstream Separation, System Integration, and Economic Analysis

Abstract

Direct electrochemical reduction of CO 2 to C 2 products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO 2 is first electrochemically reduced to CO and subsequently converted to desired C 2 products has the potential to overcome the limitations posed by direct CO 2 electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO 2 conversion routes to C 2 products. For the indirect route, CO 2 to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO 2 conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO 2 conversion pathways are presented, which includes CO 2 capture, CO 2 (and CO) conversion, CO 2 (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m 2 , and an electricity price of <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO 2 /CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO 2 electrolyzers and possible solutions are discussed as well.

Solubilities and Transport Properties of CO2, Oxalic Acid, and Formic Acid in Mixed Solvents Composed of Deep Eutectic Solvents, Methanol, and Propylene Carbonate

“… 4 − 6 CO 2 can be electrochemically converted to a number of valuable materials and fuels, spanning polymers, acids, alcohols, and hydrocarbons. 3 , 7 Valuable CO 2 electroreduction products include formic and oxalic acid, which are the simplest forms of monocarboxylic and dicarboxylic acids, respectively. 8 , 9 The CO 2 electoreduction to these acids require only two moles of electrons per mole of product and have a high market price.…”

“… 8 , 9 The CO 2 electoreduction to these acids require only two moles of electrons per mole of product and have a high market price. 7 , 10 Formic acid can be produced with high Faraday efficiencies (>95%) and current densities (150 mA cm –2 ) using gas diffusion electrodes. 10 In 2018, formic acid was reported to have a total market value of $756.5 MM, with a market price of approximately $400/tonne.…”

Surface Coverage as an Important Parameter for Predicting Selectivity Trends in Electrochemical CO2 Reduction

“… 4 − 6 Moreover, the two-electron reduction products carbon monoxide and formate/formic acid are of interest as well since there are several sustainable use cases for both. The case for carbon monoxide involves the further processing to produce other chemicals, 3 , 7 , 8 and the case for formate/formic acid is based on the current market for the chemical and future ideas for a bioeconomy or as a liquid reservoir of H 2 or CO. 9 − 11 With these multifaceted applications for the CO 2 RR, understanding the trends in the electrocatalytic performance of different electrode materials toward the CO 2 RR is crucial for catalyst development and optimization for the reaction. 1 , 12 Initial experimental studies by Hori et al mapped the activity and selectivity of many monometallic electrocatalysts for the CO 2 RR.…”

Reduced Graphene Oxide Overlayer on Copper Nanocube Electrodes Steers the Selectivity Towards Ethanol in Electrochemical Reduction of Carbon Dioxide

“…The renewable energy‐powered CO 2 electroreduction to multi‐carbon products is a promising approach to valorizing CO 2 to generate fuels and platform chemicals [1,2] . There is a lot of progress in the selective electrochemical reduction of CO 2 to products like [3,4] carbon monoxide, [5,6] methane, [7] and ethylene, [8] with a Faradaic efficiency (FE) value greater than 70 %.…”

“…The renewable energy-powered CO 2 electroreduction to multicarbon products is a promising approach to valorizing CO 2 to generate fuels and platform chemicals. [1,2] There is a lot of progress in the selective electrochemical reduction of CO 2 to products like [3,4] carbon monoxide, [5,6] methane, [7] and ethylene, [8] with a Faradaic efficiency (FE) value greater than 70 %. However, there are a handful of reports on selective electrochemical CO 2 reduction to C 2 + oxygenates because of the complexity in their post CÀ C coupling reaction pathways resulting from the competition between multi-carbon gaseous and liquid product formation.…”