Enhancing the selective synthesis of butyrate in microbial electrosynthesis system by gas diffusion membrane composite biocathode

The reduction of carbon dioxide (CO2) to high value-added multi-carbon compounds at the cathode is an emerging application of microbial electrosynthesis system (MES). In this study, a composite cathode consisting of hollow fiber membrane (HFM) and the carbon felt is designed to enhance the CO2 mass transfer of the cathode. The result shows that the main products are acetate and butyrate without other substances. The electrochemical performance of the electrode is significantly improved after biofilm becomes matures. The composite cathode significantly reduces the “threshold” for the synthesis of butyrate. Moreover, CO2 is dissolved and protons are consumed by synthesizing volatile fatty acids (VFAs) to maintain a stable pH inside the composite electrode. The synthesis mechanism of butyrate is that CO2 is converted sequentially into acetate and butyrate. The microenvironment of the composite electrode enriches Firmicute. This composite electrode provides a novel strategy for regulating the microenvironment. [Display omitted] •MES reactor with gas diffusion membrane dissolved CO2 only synthesized acetate and butyrate.•The composite biocathode lowered the threshold for synthesizing butyrate.•Dissolved CO2 in cooperation with synthesized VFAs controlled pH inside the electrode.•The composite electrode system could naturally enrich Firmicute (relative abundance: 89.60%).•The synthesis mechanism of butyrate was that CO2 was first converted to acetate and then to butyrate.