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Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO 2 . To optimize electromethanogenesis process, this study physically prepared and examined several carbon-based cathode materials: carbon stick (CS), CS twined by Ti wire (CS-Ti) or covered with carbon fiber (CS-CF), graphite felt (CS-GF) and carbon cloth (CS-CC). CS-GF electrode had constantly stable methane production (75.8 mL/L/d at −0.9 V vs. Ag/AgCl) while CS-CC showed a suppressed performance over time caused by the desposition of inorganic shell. Electrode material properties affected biofilms growth, cell-electrode contact behaviors and electron exchange. Methane formation with CS-CC biocathode was H 2 -concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H 2 route. This study re-interpreted the application potentials of carbon-based materials in CO 2 electroreduction and electrofuel recovery, providing valuable guidance for materials’ selection. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of anolyte circulation in microbial electrolysis cell was investigated. • Semi-continuous anolyte circulation increased CH 4 yield by 5.4 times. • PEM fouling impeding the transfer of proton led to low performance. • Semi-continuous anolyte circulation enhanced the transfer of electron and proton. • Enrichment of electroactive and methanogenic microorganism occurred. CO 2 bioelectromethanosynthesis represents a promising strategy for the capture and utilization of CO 2. In such process, the continuous generation of electron (e-) and proton (H+) in anodic oxidation are of prime importance for the efficient cathodic CO 2 electroreduction and process stability. Proton transfer, however, is very easy to be hindered due to the fouling of proton exchange membrane (PEM). In this study, an artificial channel in microbial electrolysis cell (MEC) was proposed to strengthen the transport of protons from anodic to cathodic compartment, and H+-rich anolyte was semi-continuously circulated to the cathodic chamber to provide protons for CO 2 electroreduction. The results indicated that the daily CH 4 yield in cathode with anolyte circulation (18.5 mL/d·L-reactor) was 5.4-fold higher than that without circulation (2.9 mL/d·L-reactor). Meanwhile, efficient anodic biodegradation of organic components was observed with COD, proteins and polysaccharides removal of up to 95.6 ± 1.9%, 96.3 ± 3.7% and 99.1 ± 0.2% respectively, which supplied a continuous e-/H+ donor for CO 2 electroconversion. 16S rRNA gene pyrosequencing analysis identified a large number of proteins-utilizing Bacteroidetes (14.11%) and polysaccharides-consuming Thermotogae (18.49%) in anodic biofilm, conductive to the biodegradation of organic components. Moreover, a high abundance of Methanobacterium (81.07%) was detected to prevail in cathodic biofilm, demonstrating the occurrence of highly enhanced CH 4 bioelectrosysthesis. The syntrophic and symbiotic relationship was established in the dual-bioelectrode system, creating a beneficial environment and an energy-efficient approach for biowaste refinery and CO 2 electromethanosynthesis. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Current status of MECs for CO 2 electromethanogenesis was summarized. • The working principles of CO 2 electromethanogenesis were summarized. • Several test methods was proposed to analyze the properties. • The critical factors effecting CH 4 production have been discussed. Abstract Given the aggravated greenhouse effect caused by CO 2 and the current energy shortage, CO 2 capture and reuse has been gaining ever-increasing concerns. Microbial Electrolysis Cells (MECs) has been considered to be a promising alternative to recycle CO 2 bioelectrochemically to low-carbon electrofuels such as CH 4 by combining electroactive microorganisms with electrochemical stimulation, enabling both CO 2 fixation and energy recovery. In spite of the numerous efforts dedicated in this field in recent years, there are still many problems that hinder CO 2 bioelectroconversion technique from the scaling-up and potential industrialization. This review comprehensively summarized the working principles, extracellular electron transfers behaviors, and the critical factors limiting the wide-spread utilization of CO 2 electromethanogenesis. Various characterization and electrochemical testing methods for helping to uncover the underlying mechanisms in CO 2 electromethanogenesis have been introduced. In addition, future research needs for pushing forward the development of MECs technology in real-world CO 2 fixation and recycling were elaborated. [ABSTRACT FROM AUTHOR]