Downstream of the CO₂ Electrolyzer: Assessing the Energy Intensity of Product Separation

Abstract

The electrochemical reduction of carbon dioxide (CO 2 RR) to chemical feedstocks, such as ethylene (C 2 H 4 ), is an attractive means to mitigate emissions and store intermittent renewable electricity. Much research has focused on improving CO 2 electrolysis cell efficiency; less attention has been paid to the downstream purification of outlet product streams. In this work, we model the use of mature downstream separation technologies as part of the overall production of polymer-grade C 2 H 4 from CO 2 . We find that CO 2 removal is the most energy-intensive downstream separation step. We identify opportunities to reduce separation energies to ∼22 GJ/tonne C 2 H 4 through necessary improvements in C 2 H 4 selectivity (>57%), cathodic CO 2 conversion (>80%), and CO 2 crossover (0 mol CO 2 /mol e − ). This work highlights the influence of cell performance parameters on downstream separation costs and motivates the development of new, efficient separation processes better suited to the distinctive outlet streams of CO 2 electrolyzers.

Carbon Catalysts for Electrochemical CO2 Reduction toward Multicarbon Products

“…It is almost impossible to convert CO 2 fully, and the current cryogenic distillation technology for the separation of gaseous products is energy‐intensive. [ 59 ] Thus, developing low‐cost separation approaches is necessary to enable downstream applications.…”

“…The current cryogenic distillation technology to separate gases suffers from high energy consumption. [59] As a result, developing energy-efficient gas separation technology with high compatibility with CO 2 electrolyzers is urgently needed. Two promising solutions could be adsorption-based gas separation over porous materials (e.g., porous coordination polymers) [187,188] and membrane-based technology, [189] which can separate gases in a highly selective and energy-saving manner.…”

Steering the Selectivity of Electrochemical CO2 Reduction in Acidic Media

“…The techno‐economic analysis (TEA) by Sargent et al. suggested that CO 2 regeneration from carbonate takes up to 23 % of the capital cost for CO 2 electrolysers [6,9] …”

Sequential vs integrated CO2 capture and electrochemical conversion: An energy comparison

“…electrolyte recovery, product separation). 11,12 Given the eventual need to combine CO2 capture and electrochemical conversion processes, and the diminishing energy efficiency returns from optimizing each process separately, researchers have considered the techno-economic and energy benefits of combining capture and conversion. 13,14 For chemical processes, the discussion of whether 'to integrate, or not to integrate' CO2 capture processes with conversion has been proposed to reduce overall energy requirements.…”

“…If fulfilled, the integrated process may save > 230 kJ/molCO2 to regenerate (bi)carbonates to hydroxide. 4,19 Finally, an additional 30 -79.2 kJ/molCO2 for product separation 12,25 can also be avoided because gas products can separate from the capture media spontaneously as a result of their low solubility (e.g., 1 mM for CO and 5 mM for C2H4 at 20 °C and 1 atm) 26 . Such a high-level analysis indicates that an ideal integrated route could save a total energy benefit of about 500 kJ/molCO2 converted to CO versus the sequential route.…”