Bridging CO₂ electroreduction and microbial chain elongation toward C4-C6 carboxylic acids using formate and ethanol

High-value conversion of CO₂ through staged electrochemical-biological processes remains challenging because direct electrochemical synthesis of long-chain products involves complex reaction pathways and competing reactions. Microbial chain elongation provides an alternative route for upgrading low-carbon intermediates, but suitable bridging substrates are needed to connect upstream CO₂ conversion with downstream microbial metabolism. Here, we developed a staged electrochemical-biological platform using formate and ethanol as bridging substrates. A Cu-Ag gas-diffusion electrode integrated with a solid-state electrolyte reactor enabled electrolyte-free co-production of formate and ethanol from CO₂. At 300 mA cm^-2, the Faradaic efficiencies of formate and ethanol remained above 23 % and 17 %, respectively, over 30 h. A synthetic electrolyte-free formate-ethanol mixture was then used as a defined co-substrate for microbial upgrading. Compared with acetate-ethanol, formate-ethanol was associated with greater butyrate accumulation but lower caproate formation. Microbial community and metagenomic analyses revealed distinct community compositions and genetic potentials related to co-substrate conversion and chain-elongation functions. This study provides evidence that formate-derived carbon is incorporated during microbial chain elongation, linking CO₂ electroreduction with C4-C6 carboxylic acid production and offering a potential framework for coupling electrochemical carbon conversion with microbial product upgrading.