Thermodynamics and electron transfer mechanisms of CO₂ bioelectroconversion to value-added chemicals: A state-of-the–art review

The bioelectroreduction of CO₂ to value-added chemical commodities, by bioeletrochemical system (BES), represents a promising approach for alleviating both energy crisis and global warming. Although a wide variety of studies have been undertaken to improve the efficiency of CO₂ bioelectroconversion and speculate electron pathways of electroactive microorganisms, the underlying mechanisms of CO₂ bioelectroconversion are still not fully understood. A much more comprehensive understanding of electron utilization mechanisms would be helpful to the practical applications of bioelectrosynthesis. Here, the working principles in bio-electrochemical CO₂ reduction were summarized, and the change of Gibbs free energy (ΔG) and dynamics for CO₂ bioelectroconversion were analyzed to evaluate the potential effects of external microenvironment changes (e.g., operational conditions, supply of mediators/conductive materials, electro-activity of key functional microbes, arious redox potentials of cellular redox compounds, etc.) on CO₂ bioelectroconversion and the electron transfer chains of bioelectrocatalysis. The potential electron transfer mechanisms for CO₂ bioelectroconversion to multi-carbon commodities were further summarized. Particularly, the functional proteins and genes associated with the metabolic pathway of CO₂ electroreduction were discussed. Finally, taking the CO₂-reducing pathways of methanogenic archaea as an example, the electron transfer pathways were illustrated in details, and the future perspectives for the possible improvement and research interests in bioelectrosynthesis of CO₂ capture and use were elaborated. This paper reviews the thermodynamics and electron transfer mechanisms of CO₂ bioelectroconversion, and provided the guidance for the practical implementations of employing and electrochemically manipulating electroactive functional microorganisms to electrocatalyze CO₂ to value-added low-carbon commodities.