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.