The Technical and Energetic Challenges of Separating (Photo)Electrochemical Carbon Dioxide Reduction Products

Known catalysts for (photo)electrochemical carbon dioxide (CO2) reduction typically generate multiple products, including hydrogen, carbon monoxide, hydrocarbons, and oxygenates, making product separation a ubiquitous, yet often overlooked, challenge. Here, we review CO2 reduction products using available catalysts and discuss approaches for product separation along with estimates of separation energy requirements. We illustrate potential complexities and discuss opportunities to minimize separations by utilizing product mixtures. We also examine potential CO2 sources, their energy requirements, and net CO2 emissions. Finally, we discuss use of waste energy sources and integrate this information into an overall energy balance assessment. Using a common sustainability metric, energy return on energy investment (EROEI), we find that an EROEI of similar to 2.0 may be possible, before including separation and CO2 production energy. For EROEI to remain above one (the break-even point), these additional energy requirements, including embodied energy of equipment, must be no greater than half of the product energy.