Enhanced Performance of H₂-Mediated Acetate-Producing Microbial Electrosynthesis at a Mildly Alkaline pH of 7.5

Optimizing catholyte pH is pivotal for intensifying H₂-mediated microbial electrosynthesis (MES) of acetate from CO₂ because pH co-governs gas-liquid transfer, cathodic overpotential, and acetogen's activity, yet their coupled effects on reactor performance remain poorly resolved. Here, we systematically investigated acetate production from CO₂ across pH 7.0-9.0 by integrating H₂ transfer, cell voltage, productivity, and microbial community shifts. Increasing pH enhanced H₂ availability through higher mass-transfer rates, but also increased cathodic overpotential and cell voltage, establishing a trade-off between substrate supply and electrical energy demand. The optimum pH of 7.5 delivered the shortest lag phase (2.2 ± 0.3 d), the highest acetate titer (18.43 ± 0.92 g·L^-1), near-complete H₂ uptake, and peak coulombic efficiency up to 79%, yielding minimal electricity intensity (~26.95 kWh·kg^-1). Further pH elevation to 8.5-9.0 disrupted this balance, leading to reduced productivity and sharply increased energy consumption (136.77-236.96 kWh·kg^-1). Notably, the specific NaOH addition for pH control at pH 7.5 remained comparable to that at pH 7.0 due to greater productivity. Microbial community analysis revealed acetogen enrichment near pH 7.5 and diversification at pH 9.0. These results define a pH-centered design strategy that aligns H₂ transfer with microbial kinetics while minimizing energy and base usage, offering guidance for process intensification and scale-up.