Bioelectrochemical CO2 Reduction to Acetic Acid and Ethanol: Improved Microbial Electrosynthesis Using Gas Diffusion Electrodes

Abstract

Microbial Electrosynthesis (MES) comprises electro-reduction of carbon dioxide (CO2) to multi-carbon organic compounds by chemolithotrophs using electrons from a cathode. Reduction of CO2 to chemicals through microbial electrocatalysis was investigated by using a mixed culture of acetogenic and carboxydotrophic bacteria forming a microbial biofilm supported on a carbon based electrode, as biocathode, in a two chamber reactor. The biofilm was developed after a start-up phase with fructose and later on, growing on bicarbonate as substrate at sufficiently negative cathode potential (hydrogen evolution) in a couple of subsequent fed-batch operations. CO2 reduction could occur via direct electron transfer from the electrode or indirectly via mediators or via hydrogen at more reductive potential. Predominantly, Acetic acid was produced along with other volatile fatty acids (VFAs) while applying − 1.1 V/Ag/AgCl cathode potential, along with hydrogen evolution. At the initial stage of fed-batch operation, higher carbon recovery up to 60% was observed from bicarbonate (dissolved CO2) to acetic acid while after accumulation of acetate, the recovery rate went down to 12% as acetate degradation/conversion started or other unmeasured products formed. Maximum acetate production rate achieved during the operation was 40 g m− 2 day− 1 corresponding to coulumbic efficiency of 41%. Microbial analysis of catholyte at the end of the experiment showed that the bacterial community was dominated by Cellulomonas, Stappia and Pseudomonas spp. These results suggest that the mixed culture enriched with acetogenic bacteria can catalyze the electro-reduction of CO2 into a number of chemicals like VFAs through direct or indirect electron transfer mechanisms. While using gaseous CO2 as carbon source, the dissolution and mass transfer of CO2 to the biocatalyst limit the biological reduction process. In addition, the bacterial attachment and retention of reducing equivalent specially hydrogen also restrict the process at the cathode. In order to deal with these issues, a gas diffusion cathode (GDC) (VITO Core™) and a flow-through porous carbon felt cathode were separately tested in MES for CO2 reduction. In principal, the porous activated carbon with hydrophobic binder layer in GDC creates a three-phase interface that makes CO2 and reducing equivalents available to the bacteria. Flow-through graphite felt cathode retains the suspended biomass and electrochemically produced hydrogen when the catholyte is forced to flow through it. An enriched inoculum of acetogenic bacteria, isolated from wastewater sludge was used as biocatalyst. The cathode potentials were maintained at − 0.9 to − 1.1 V vs Ag/AgCl to facilitate CO2 reduction also via the hydrogen evolved at the cathode. On average, CO2 reduction to acetate was achieved with the production rate ∼35 to 43 mg/L/d supplying 20% (v/v) CO2 gas mixture in both the reactors. In the reactors without GDCs or modified cathode, CO2 reduction was never steady for a long period of operation. Acetate was the primary product of CO2 reduction but ethanol and butyrate were also produced concurrently at pH lower than 6. The highest acetate production rate reached in GDC reactor was ∼550 mg/L/d supplying 80% (v/v) CO2 mixture over the GDC. In conclusion, gas diffusion and flow-through cathodes were useful to develop stable CO2 reducing biocathodes and also to operate in continuous mode. Keywords: Microbial electrosynthesis, CO2 reduction, Gas diffusion cathode, Flow-through biocathode, Biocathode, Autotrophic Bioproduction.