Microbial electrosynthesis of methane in an up-scaled zero-gap cell

Microbial electrosynthesis (MES) offers a promising route for converting CO₂ into value-added products, yet low energy efficiency remains a major obstacle, especially during scale-up. To address this issue, an up-scaled zero-gap MES reactor with an extended 30-cm flow path was developed and operated under a range of applied voltages to assess energy conversion and methane production performances. Increasing the cell voltage from 2.3 V to 2.7-2.8 V boosted the current density by 131%, reaching 17.5 A m^-2. This improvement produced a corresponding rise in methane production from 1.4 to 6.9 L/L-d, achieving high coulombic efficiencies (>95%) and one of the highest energy efficiencies (45.2%) for methane synthesis at 30 °C. Simulations underscored the key role of hydrogen as the mediator of electron transfer, showing that sufficient in-situ hydrogen generation was essential for sustaining high methane production in the up-scaled reactor. Microbial community analysis revealed minimal spatial heterogeneity along the extended flow path, with the cathodic biofilms consistently dominated by hydrogenotrophic Methanobacterium (59.8% and 50.5% at the bottom and top), reflecting stable microbial functionality under scale-up conditions. Overall, the results demonstrate that with proper operational optimization, MES can be scaled up effectively without compromising energy efficiency or microbial-electrochemical synergy, offering a viable pathway for CO₂-to-methane conversion in up-scaled systems.