Anion vs cation exchange membrane strongly affect mechanisms and yield of CO2 fixation in a microbial electrolysis cell

The CO2 removal from a concentrated gas stream (simulating biogas) has been investigated by using two identical fully bio-catalyzed microbial electrolysis cell (MEC), equipped with either a proton exchange membrane (PEM-MEC) or an anion exchange membrane (AEM-MEC). The equivalents deriving from the anodic oxidation of the organic matter were mainly converted into current, with an average coulombic efficiency between 53 ± 9% and 85 ± 15%, resulting in a little microbial growth (with an observed growth yield between 0.17 and 0.18 gCOD/gCOD). The cathode compartment was continuously bubbled with a gas mixture containing CO2 (30% v/v, N2 balance) and the presence of an hydrogenophilic autotrophic culture allowed for CO2 reduction into CH4, with a cathode capture efficiency between 47 ± 2% and 80 ± 1%, respectively. In both systems, the first mechanisms of CO2 removal was its sorption as bicarbonate ion at high concentration in the MEC cathode, which was supported by alkalinity generation, needed by electroneutrality maintenance. However, in the AEM-MEC, 5.4 g/Ld of CO2 were removed by crossing the membrane (which was due to both molecular diffusion and ionic transport) whereas in the PEM-MEC only 3.2 g/Ld of CO2 were removed (through the osmotic overflow which was spilled from the cathodic liquid phase). Moreover, PEM-MEC showed higher COD removal efficiency (78 ± 7%) and methane production rate (83 ± 24 meq/Ld) than AEM-MEC but showed a higher energy demand per unit of removed CO2 (2.36 vs 0.78 vs kWh/Nm3 CO2removed). It is noteworthy that AEM-MEC energy demand was lower than full scale processes for biogas upgrading such as water scrubbing.