UC Berkeley

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 ∼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. To limit climate change, we must meet our energy needs without increasing atmospheric carbon dioxide (CO2). Although still at an early stage, (photo)electrochemical CO2 reduction (“solar fuels”) offers potentially scalable and transformative technology to make fuels and chemicals. However, product separation and its energy requirements are a critical and often overlooked challenge. While proven and effective, distillation is energy intensive; more efficient methods (e.g., solvent extraction, membranes, and other approaches) are useful but often less selective and not universally applicable. With currently known technology, the energy return on energy investment sustainability metric could be low or even below one. Identifying major contributing factors may help focus research on critical areas, such as trade-offs between product selectivity and overall efficiency, and facilitate interaction between catalysis and separations researchers to help solve the challenges of making solar fuels. Unlike electricity, there are fewer options available to decarbonize fuels (e.g., for building heat or transportation). “Solar fuels” are a promising alternative to fossil-derived fuels, as they are not limited by land constraints associated with biomass fuels and can have low net CO2 emissions. However, catalysts used to convert CO2 and water into fuels using sunlight typically yield a mixture of products, making product separation a ubiquitous, yet often overlooked, challenge. We explore possible approaches, CO2 considerations, and energy implications.