Application of gas diffusion biocathode in microbial electrosynthesis from carbon dioxide

<p>Microbial catalysis of carbon dioxide (CO₂) reduction to multi-carbon compounds at the cathode is a highly attractive application of microbial electrosynthesis (MES). The microbes reduce CO₂ by either taking the electrons or reducing the equivalents produced at the cathode. While using gaseous CO₂ as the carbon source, the biological reduction process depends on the dissolution and mass transfer of CO₂ in the electrolyte. In order to deal with this issue, a gas diffusion electrode (GDE) was investigated by feeding CO₂ through the GDE into the MES reactor for its reduction at the biocathode. A combination of the catalyst layer (porous activated carbon and Teflon binder) and the hydrophobic gas diffusion layer (GDL) creates a three-phase interface at the electrode. So, CO₂ and reducing equivalents will be available to the biocatalyst on the cathode surface. An enriched inoculum consisting of acetogenic bacteria, prepared from an anaerobic sludge, was used as a biocatalyst. The cathode potential was maintained at −1.1 V vs Ag/AgCl to facilitate direct and/or hydrogen-mediated CO₂ reduction. Bioelectrochemical CO₂ reduction mainly produced acetate but also extended the products to ethanol and butyrate. Average acetate production rates of 32 and 61 mg/L/day, respectively, with 20 and 80 % CO₂ gas mixture feed were achieved with 10 cm² of GDE. The maximum acetate production rate remained 238 mg/L/day for 20 % CO₂ gas mixture. In conclusion, a gas diffusion biocathode supported bioelectrochemical CO₂ reduction with enhanced mass transfer rate at continuous supply of gaseous CO₂. [Figure not available: see fulltext.]