Impact of Cathode Surface Area on Gas–Liquid Mass Transfer and Acetate Production Efficiency in H2-Mediated Microbial Electrosynthesis from CO2

Hydrogen-mediated microbial electrosynthesis (MES) of chemicals from CO2 relies on effective gas–liquid transfer at the cathode interface, yet the extent to which cathode surface area regulates acetate productivity remains insufficiently quantified. In this study, three identical MES reactors equipped with stainless-steel cathodes of different geometric areas (8 × 1, 8 × 4, and 8 × 16 cm2) were operated at a constant electric current of 0.3 A. The largest cathode significantly accelerated hydrogen mass transfer (kLa = 0.592 h−1), reaching dissolution equilibrium within 3 min, which was nearly twice as fast as the smallest electrode. Upon inoculation with enriched acetate-producing microbial consortia, the 8 × 16 cm2cathode reactor fed with CO2 achieved the highest steady-state acetate concentration of 32 g·L−1 produced at a rate of 2.12 g·L−1·d−1, with 94% hydrogen utilization, and 59% coulombic efficiency. In contrast, smaller electrodes exhibited rapid bubble detachment and reduced residence time, thereby limiting microbial gas uptake, and resulting in low acetate productivity. These findings demonstrate that cathode surface area is a key engineering lever controlling both hydrogen availability and electron recovery efficiency in H2-driven MES. The results provide practical guidance for electrode design and scale-up of CO2-to-acetate bioconversion via the MES process.