An integrated electrochemical and biochemical system for sequential reduction of CO2 to methane

Converting CO2 into commodity chemicals including fuels has emerged as a worldwide focus for science and engineering communities. However, the current state of art in CO2 fixation has low energy efficiency with slow kinetics in the process of CO2 reduction. Here, we propose a new “two-step” approach for CO2 reduction to methane. An integrated reduction system (IRS) was demonstrated to combine the advantages of both electrochemical and biological CO2 reduction reactions. A three-chambered electrochemical cell was designed, in which CO2 was first reduced to formate electrochemically with a decent selectivity. The produced formate was separated by electrodialysis, and subsequently consumed by a strain of methanogen, Methanococcus maripaludis. Through this IRS, CO2 was eventually converted into CH4, allowing easy product separation. In a 100-h test, the total electron recovery efficiency of CH4 was around 37%, with an average CH4 production rate of 0.01 mmole d−1 mL−1. Five volts of voltage was applied to the system, while the cathodic potential slightly decreased from −1.5 V to −1.4 V (vs. Ag/AgCl), showing an excellent stability of the electrode. The energy efficiency of CO2 conversion in terms of electricity consumption was calculated to be 16.9 g per kW h. The results show the promise of IRS presented here in CO2 reduction. Since the electrochemical step and the biochemical step proceeds in separated chambers, independent operation and maintenance of either step become possible. Furthermore, the proposed CO2 IRS offers possibilities of producing a variety of chemicals by introducing appropriate microbial communities using formate as a substrate.