Extracellular electron uptake involved in CO2 valorization through microbial electrolysis cell: Mechanism study and reinforcement strategies for improved performance

The rapid expansion of renewable energy installation capacity is gaining significant attention and is projected to increase in the upcoming years due to combating climate change induced by excessive greenhouse gas (mainly CO2) emissions. However, the mismatch between renewable energy supply and consumption requires efficient electricity storage devices. As the global economy remains dependent on carbon-based chemicals, the application of electrosynthetic processes that convert excess electricity and CO2 into chemicals and energy-intensive fuels has attracted considerable attention. In this case, a microbial electrolysis cell (MEC) could leverage extracellular electron transfer to promote cellular metabolism, sustainably synthesizing diverse CO2-reducing products. Before fulfilling the potential of MEC, a comprehensive understanding of the biological mechanisms behind extracellular electron uptake is required to realize efficient channeling of cathode-derived electrons toward microbiomes for prominent CO2 valorization. Hence, this review starts with a comprehensive overview of typical electroactive microbiomes involving extracellular electron uptake and their associated genetic evidence. Then, viable methods, including spectroscopy, microscopy, electrochemical, and meta-omic methods, are introduced to facilitate the quantification and qualification of the electron uptake process. Finally, existing strategies for enhancing extracellular electron uptake for a prominent CO2 bioelectrosynthesis performance are discussed.