High electron flux facilitates carbon chain elongation in microbial electrosynthesis of butyrate and caproate from carbon dioxide.

Microbial electrosynthesis (MES) converts carbon dioxide (CO2) into valuable chemicals, yet the yield of multi-carbon chemicals is often hindered by inefficient electron supply and competing conversion routes. In this study, we demonstrated that high electron flux, facilitated by supplemented neutral red (NR), could promote carbon chain elongation during CO2-to-C4/C6 electrosynthesis. NR enabled an additional electron-transfer route, enhancing electron partitioning and stabilizing butyrate and caproate yields at 1317 and 765 mg·L-1, respectively. The faradaic efficiencies of multi-carbon products attained 89.42%, among which butyrate, caproate, and acetate accounted for 30.44%, 21.19%, and 37.79%, which possessed considerable advantages over the control and literature results. The high electron flux was found to reduce the charge-transfer resistance of bioelectrode, increase biofilm viability, and elevate contents of NADH/NAD+, ATP, and c-type cytochromes. Furthermore, the superior intracellular reducing equivalents, the inhibited methanogenesis, and the functional complementarity between Acetobacterium and Thiomonas contributed to the observed electron partitioning and C4/C6 production under NR supplementation. Finally, techno-economic analysis demonstrated that the total cost and annual revenue of the MES system with NR were 0.18- and 2.11-fold, respectively, relative to the control. These findings proved the practical applicability of MES of multi-carbon chemicals from CO2 at high electron flux.