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.
Keywords: CO2 conversion; energy efficiency; hydrogen retention time; microbial electrosynthesis; multiphase flow; zero-gap flow cell.