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Electron‐transfer pathways occurring in biocathodes are still unknown. We demonstrate here that high rates of acetate production by microbial electrosynthesis are mainly driven by an electron flux from the electrode to carbon dioxide, occurring via biologically induced hydrogen, with (99±1) % electron recovery into acetate. Nevertheless, acetate production is shown to occur exclusively within the biofilm. The acetate producers, putatively Acetoanaerobium, showed the remarkable ability to consume a high H₂ flux before it could escape from the biofilm. At zero wastage of H₂ gas, it allows superior production rates and lesser technical bottlenecks over technologies that rely on mass transfer of H₂ to microorganisms suspended in aqueous solution. This study suggests that bacterial modification of the electrode surface (possibly via synthesis of Cu nanoparticles) is directly involved in the significant enhancement of …