On assessing the energy efficiency of bioelectrochemical systems for cathodic synthesis and remediation

Technologies integrating microbiology and electrochemistry are advancing toward technical scales, necessitating early efficiency assessments to guide engineering. Energy efficiency—calculated by balancing energy input and output and linked to economic viability—is an important benchmark. This work emphasizes the need to evaluate energy efficiency in microbial-electrochemical technologies, focusing on examples like cathodic synthesis and bioremediation. Two methodologies are highlighted: voltage efficiency and product-related energy efficiency. Voltage efficiency, is ideal for comparing different experimental setups and electrochemical processes to produce energy carriers and represents the state-of-the-art approach in the field. However, an energy efficiency based on voltage efficiency might exhibit limitations in some cases, and in scenarios where side-products like those at counter electrodes and the evolved heat are not utilized. In addition, we discuss the product-related energy efficiency as an alternative, which evaluates the energy input relative to the useful work provided by oxidation of working electrode products. For systems lacking tangible products, normalizing energy consumption to treated entities, such as the volume or amount of contaminant removed, is recommended. These concepts are illustrated using the electrochemical CO 2 reduction reaction and bioelectrochemical denitrification, showcasing how energy efficiency assessments can aid in optimizing experimental designs and operations for microbial-electrochemical technologies.