Anodic vs cathodic potentiostatic control of a methane producing microbial electrolysis cell aimed at biogas upgrading

A fully biological Microbial Electrolysis Cell (MEC) aimed at biogas upgrading has been operated under different operating conditions in order to enhance CO2 removal from a synthetic biogas. Specifically, CO2 reduction into CH4 occurred at the MEC biocathode with the oxidation of organic substrates in the anodic chamber partially sustaining the energy demand of the process. In the cathode chamber, methane formation was the main driver of current generation which, in turn, sustained alkalinity generation and related CO2 sorption. This study mainly focused on the minimization of the anodic and cathodic overpotentials to maximize the process efficiency. To accomplish this objective, an innovative strategy of MEC polarization was adopted, consisting in the shift of the potentiostatic control of the process from the anode (at +0.2 V vs SHE) to the cathode (at −0.65, −0.90 and −1.00 V vs SHE), along with the control of the fluid dynamic conditions of the anode chamber. An almost complete (99%) energy recovery was obtained by methane production with the cathode potential controlled at −0.65 V vs SHE. Finally, at the MEC cathode, current was utilized to reduce CO2 into CH4 (with a cathodic capture efficiency of about 70%) as well as to promote CO2 sorption into HCO3−. The latter represents the main CO2 removal mechanism that accounted for 85% of the CO2 removal.