Fluidized Bed Cathodes Enable High-Rate CO ₂ to Formate Production and In Situ Microbial Growth

Electrochemical CO ₂ Reduction Reaction (eCO ₂ RR) to formate offers a sustainable and promising route for storing renewable electricity in a microbial energy substrate. Conventional CO ₂ electrolyzers rely on flat electrodes with limited catalytic surface area, mass-transfer constraints, and electrolytes that are incompatible with microbial growth, preventing direct coupling of electrochemical formate production with biological conversion. Here, we developed a biocompatible CO ₂ electrolysis configuration that uses fluidized particles of Sn, Bi, and In as electrocatalysts to enhance CO□-to-formate conversion while supporting microbial utilization in the same chamber. The fluidized configuration enabled high formate production rates up to 0.24 mmol h□¹ cm□² with Faradaic efficiencies of 86–92%, representing a two-fold improvement over static electrodes. The system also achieved a carbon conversion efficiency of 42%, approaching the theoretical limit for co-generation of formate and bicarbonate. Importantly, electrolysis was performed in a biocompatible medium that supported direct in situ cultivation of Cupriavidus necator within the cathode chamber using only electricity and CO ₂ /O ₂ as fed substrates. C. necator continuously consumed electro-generated formate for 5-6 days under aerobic conditions, demonstrating integrated CO ₂ -to-biomass conversion. This work establishes a fluidized electrocatalytic platform for simultaneous electrosynthesis and microbial upgrading in a single reactor, providing a scalable strategy for renewable carbon fixation and sustainable biomanufacturing.