Microbial electrosynthesis (MES) enables the electrochemical reduction of CO₂. This study demonstrates a biofilm-based microbial porous cathode in a directed flow-through electrochemical system that continuously reduces CO₂ to even-chain C2-C6 carboxylic acids over 248 days. The system achieves a 3-fold higher biofilm concentration, volumetric current density, and productivity than the state of the art, reaching a new record of -35 kA m⁻³ cathode and 69 kg C m⁻³ cathode day⁻¹. These results are comparable to those achieved in syngas and chain elongation fermentations. The study highlights key design parameters for efficient electricity-driven CO₂ reduction. The directed-flow-through bioelectrochemical reactor (DFBR) allows three times denser biofilm than previous state-of-the-art, resulting in 5-fold higher volumetric current density and productivity. The DFBR also achieves higher carbon selectivity and faradaic efficiency into C4 and C6. The study shows that the DFBR can produce carboxylic acids with higher value than acetate. The results demonstrate that MES can achieve volumetric productivity comparable to syngas fermentation. The study also identifies Clostridium luticellarii and Eubacterium limosum as dominant species in the microbial community. The findings suggest that MES has the potential to become a competitive technology for efficient CO₂ valorization.Microbial electrosynthesis (MES) enables the electrochemical reduction of CO₂. This study demonstrates a biofilm-based microbial porous cathode in a directed flow-through electrochemical system that continuously reduces CO₂ to even-chain C2-C6 carboxylic acids over 248 days. The system achieves a 3-fold higher biofilm concentration, volumetric current density, and productivity than the state of the art, reaching a new record of -35 kA m⁻³ cathode and 69 kg C m⁻³ cathode day⁻¹. These results are comparable to those achieved in syngas and chain elongation fermentations. The study highlights key design parameters for efficient electricity-driven CO₂ reduction. The directed-flow-through bioelectrochemical reactor (DFBR) allows three times denser biofilm than previous state-of-the-art, resulting in 5-fold higher volumetric current density and productivity. The DFBR also achieves higher carbon selectivity and faradaic efficiency into C4 and C6. The study shows that the DFBR can produce carboxylic acids with higher value than acetate. The results demonstrate that MES can achieve volumetric productivity comparable to syngas fermentation. The study also identifies Clostridium luticellarii and Eubacterium limosum as dominant species in the microbial community. The findings suggest that MES has the potential to become a competitive technology for efficient CO₂ valorization.