Fe/Co Zeolitic Imidazolate Framework Acted as an Electron Shuttle to Enhance Methanogenesis by Promoting Extracellular Electron Transport

In order to promote electron transfer rates in microbial electrolysis cell (MEC) methanogenesis reactions, an iron-doped zeolite imidazolate framework-67 (Fe/Co-ZIF) was synthesized and added to the solution of MEC to serve as an electron shuttle, improving the conversion of CO2 to CH4. Fe/Co-ZIF effectively promoted the expression of heme protein-related genes to accelerate direct electron transfer by cytochrome C while also stimulating the secretion of extracellular polymeric substances (EPS) and redox-active flavin to increase indirect electron transfer. Fe/Co-ZIF increased the expression of heme protein-related genes by 29.1% and the relative content of flavin by 53.2% at most. With the addition of Fe/Co-ZIF, methanoculleus became the dominant archaeal genera both on electrodes and in solution, while hydrogen-producing bacterial genera such as Sulfurospirillum also increased, benefiting interspecies coculture. Therefore, Fe/Co-ZIF increased the specific capacitance of the MEC system and enhanced its charge storing capability, which promoted CH4 yields by 1.62-fold. This study provides comprehensive insights into the role of Fe/Co-ZIF as an electron shuttle, facilitating extracellular direct and indirect electron transfer in microbial electrochemical systems.