NiFeSn alloy-enhanced microbial electrosynthesis for carbon dioxide conversion to butyric and caproic acids

A mixed anaerobic microbial consortium producing short- and medium-chain fatty acids from CO₂ and H₂ was developed through mixed culture cultivation under a CO₂/H₂ atmosphere combined with the serial transfer technique to promote the growth of acetogenic and chain-elongating bacteria. Consequently, this enrichment was used to inoculate laboratory-scale microbial electrosynthesis (MES) cells that were continuously fed with CO₂. To improve MES performance, a novel three-dimensional conductive polylactate cathode coated with a NiFeSn alloy was employed in one MES cell, while another MES cell was equipped with a carbon felt (CF) cathode. CO₂ conversion and chain elongation were substantially increased in the NiFeSn cell. In this cell, a steady-state volumetric CO₂ conversion rate of 2.4 L (L_c d)^-1 was achieved, with corresponding butyrate and caproate production rates of 1 g (L_c d)^-1 and 0.07 g (L_c d)^-1, respectively, at a current density of up to 4 mA cm^-2. The formation of C16-C18 fatty acids was also observed. 16S rRNA amplicon sequencing of the catholytes and cathodic biofilms revealed a significant shift in microbial populations from Clostridium-driven acetogenesis and chain elongation in the enrichment to one most likely driven by the genera Eubacterium, Megasphaera and Caproiciproducens in MES cells.