Enhancement of Bioelectrochemical CO<sub>2</sub> Reduction with a Carbon Brush Electrode via Direct Electron Transfer
Chuanqi Liu, Xin Yuan, Yuyi Gu, Haoqiang Chen, Dezhi Sun, Pengsong Li, Min Li, Yan Dang, Jessica A. Smith, Dawn E. Holmes
Abstract
Bioelectrochemical CO2 reduction is a promising method for biogas upgrading. However, the CO2 reduction efficiency in these bioelectrical systems is always relatively low and limits their application in real projects. In this study, biogas upgrading performance was compared in two different microbial electrolysis cells (MECs) constructed with different biocathode materials: carbon brush or a graphite plate. Both CH4 content and CH4 yield were higher in the carbon brush reactor than in the graphite plate reactor (91.2% vs 89.8%, 5.2 mmol/day vs 4.3 mmol/day). A higher proportion of CO2 was reduced to CH4 in the carbon brush reactor (22.7%) than of the graphite plate reactor (6.5%). In addition, the bioelectrochemical CO2 reduction efficiency of the carbon brush reactor reached as high as 602 mol/day/m3. Microbial community analysis revealed that Methanothrix, a genus of acetoclastic methanogens that can directly accept electrons for CO2 reduction, dominated the archaeal community in both reactors. Transcriptomic studies also revealed that Methanothrix species colonizing the carbon brush surface were actively expressing genes that code for enzymes required for electromethanogenesis but not for acetoclastic methanogenesis. These results suggest that Methanothrix species were participating in direct electron transfer (DET) in the carbon brush reactor and that carbon brush is a better biocathode material in bioelectrochemical biogas upgrading systems.