Substantially Improved Microbial Electrosynthesis of Methane Achieved by Improving Hydrogen Retention and Flow...

Efficient hydrogen utilization by microorganisms is crucial for improving the energy-to-chemical efficiency in microbial electrosynthesis (MES). We therefore developed a new rectangular zero-gap cell design featuring an extended flow path to improve hydrogen retention and conversion to biomethane. Multiphase flow modeling within porous carbon felt cathodes revealed the new configuration with a trapezoidal inlet substantially reduced flow dead zones and tripled hydrogen retention time versus circular cells. At -1 V vs Ag/AgCl, increasing catholyte flow rate from 0.8 to 2.5 mL/min raised current densities from 19 to 24 A/m² (30 °C), reaching a peak Coulombic efficiency (CE) of 82% for methane production (7.0 L/L-d). Further increasing the flow rate to 7.5 mL/min or temperature to 37 °C slightly improved methane production (7.2-7.7 L/L-d) but reduced hydrogen retention in cells based on modeling results, lowering CEs and energy efficiencies due to unreacted hydrogen. Matching cathode potential to flow rates and temperatures could balance H₂ production and retention, significantly improving CE to 96% toward 7.5 L/L-d methane production with a high energy efficiency of 36% (-0.95 V vs Ag/AgCl, 37 °C). These findings underscore the importance of improving flow distribution and hydrogen retention within zero-gap MES cells to enhance energy and Coulombic efficiencies.