Role of electrolytes composition, ionic crossover and CO2 availability in bioelectrochemical methanation

Bioelectrochemical methanation (BEM) is a promising carbon capture and utilization (CCU) technology that converts CO₂ and renewable electricity into synthetic methane under mild operating conditions. However, the performance and stability of BEM stacks are strongly influenced by electrolyte composition, ionic crossover and carbon availability, factors that remain poorly understood beyond single-cell systems. In this work, a three-cell BEM stack was operated under galvanostatic control for 83 days to systematically investigate the combined effects of electrolyte composition, applied current density and dissolved CO₂ concentration on methane production, purity and electrochemical efficiency. The stack was operated at 6 and 12 A m⁻², while dissolved CO₂ in the catholyte was progressively reduced from near saturation to < 0.22 g L⁻¹. Results show that excessive ionic crossover led to salt accumulation and partial inhibition of methanogenesis, which was mitigated by electrolyte replacement that restored microbial activity and stabilized conductivity below inhibitory thresholds. Lower dissolved CO₂ concentrations significantly improved CH₄ purity (up to 88%) while sustaining high production rates (1.1 ± 0.3 L-CH₄ L⁻¹_cathode d⁻¹) and cathodic Coulombic efficiencies up to 99%. Ion balance analysis revealed that charge transport was dominated by cations and phosphate species rather than protons, contributing to increased ohmic and pH-related losses. Overall, this study demonstrates that controlling dissolved CO₂ availability and managing ionic crossover in BEM stacks are critical levers for optimizing their performances. These findings provide practical guidance for electrolyte design and CO₂ delivery strategies in BEM systems, supporting the development of scalable bioelectrochemical routes for CO₂ utilization.