An electrolytic-hydrogen-fed moving bed biofilm reactor for efficient microbial electrosynthesis of methane from CO2

Microbial electrosynthesis (MES) is an emerging technology for CO2 fixation and renewable energy storage. Currently, the low current density hinders the practical application of this technology. In situ coupling of electrochemical hydrogen production, and hydrogen gas fermentation has been proposed as a promising way to enhance the current density of MES. However, due to the low solubility of hydrogen, how to ensure a high coulombic efficiency (i.e. hydrogen utilization efficiency) under high current density is the main challenge for this type of reactor. Here, we report a novel electrolytic-hydrogen-fed moving bed biofilm reactor (electroMBBR) reactor for methane production. The reactor consists of an electrochemical cell at the bottom and an MBBR column at the top. The MBBR column prolongs the retention time of the electrolytically-produced hydrogen and enhances the mass transfer of hydrogen. Consequently, the methanogens in the reactor could efficiently convert the electrolytically-produced hydrogen and the externally-supplied CO2 into methane. The 4.5 L reactor achieved a maximum methane production rate of 1.42 L center dot L-1 center dot d-1 (141.5 L center dot m- 2cat center dot d- 1) at 5 A (111.1 A center dot m- 2cat). The CH4 production rate is more than two times higher than the maximum reported value based on biofilm-driven MES (0.54 L center dot L-1 center dot d-1, 65 L center dot m- 2cat center dot d-1). Microbial community analysis showed that the Methanobrevibacter was the dominant methane-producing archaea. These results demonstrated under high current density high coulombic efficiency could be achieved if proper hydrogen retention time and good hydrogen mass transfer were ensured in hydrogen-mediated MES reactors. The electro-MBBR appears to be a promising MES setup for scaling up and practical application.