Modular multi-cathode microbial electrosynthesis enabling predictable scale-out conversion of CO₂ to acetate

Microbial electrosynthesis (MES) uses electrosynthetic microorganisms to convert CO₂ into valuable intermediate chemicals, enabling sustainable carbon-negative CO₂ capture and utilization (CCU). Nevertheless, the scaling up of MES reactors remains a challenge because increasing the electrode area and reactor volume often deteriorates the electrochemical and biological uniformity, leading to performance losses in a scaled-up system. This paper proposes a modular multi-cathode chamber MES configuration in which multiple cathodes in separated chambers share a single anode, allowing modular expansion of the MES while maintaining a homogeneous bioelectrochemical reaction. The multi-cathode MES was sequentially scaled from 0.25 to 2 L by increasing the number of cathode chambers. Acetate production during MES operation showed a linear correlation with the cumulative reactor volume and cathode projected area (R² = 0.99) over an eightfold increase in reactor scale. Specifically, the surface-normalized biofilm density, faradaic efficiency, and carbon recovery to acetate were consistent across different scales without compromising the bioelectrochemical performance during scale-out. These results show that the multi-cathode configuration enables linear and reliable scaling of CO₂-to-acetate electrosynthesis toward the practical applications of MES.