On-line current control for continuous conversion of CO2 to CH4 in a microbial electrosynthesis cell

This study demonstrates the continuous conversion of carbon dioxide (CO2) to methane (CH4) in a microbial electrosynthesis (MES) cell with on-line CO2 concentration measurements in the cathode off-gas and feedback control of the current. First, a dynamic non-linear model describing CO2 conversion to acetate and CH4 was used to evaluate the impact of MES cell current on electricity consumption and cathode off-gas composition, and to select proportional-integral-derivative (PID) controller parameters. Next, on-line current control based on CO2 measurements was demonstrated in laboratory-scale 0.5 L and 1 L (cathode volume) MES cells fed with pure CO2 or synthetic biogas (40% CO2 and 60% CH4). In all tests, feedback control resulted in stable long-term CH4 production at a volumetric rate of 0.8-1.4 L (LR d)-1, near-constant CO2 content in the cathode off-gas, and Coulombic efficiency of 65-80%. Acetate (500 - 1000 mg L-1) was observed in cathodic liquid in all tests, suggesting relatively low activity of acetoclastic methanogens in the cathodic biofilm. The model and the pro-posed approach for current control can be also applied for MES of products other than CH4, such as carboxylic acids.