Continuous conversion of CO2 to e-methane by bioelectrochemical methanation: influence of dissolved CO₂ and ion crossover

i) Optimization of electrolytes Maria Vega-Paredesa*, Simone Colantonia, Marina Martín-Sandovala, Radu Ghemisa,Daniele Molognonia, Eduard Borràsa aLeitat technological center, C/ de la Innovació 2, 08225 Terrassa, Spain, [email protected] Continuous conversion of CO2to e-methane by bioelectrochemical methanation: influence of dissolved CO₂and ion crossover ACKNOWLEDGEMENTS. The authors thank the European Union’s Horizon Europe research and innovation program for financial support under the project “NET-Fuels” (Grant No.101083780). ●●●INTRODUCTION ●●● Bioelectrochemical methanation (BEM), also known as electromethanogenesis,is an emerging power-to-gas technology for renewable energy storage converting carbon dioxide (CO2)and electricity into synthetic methane (e-methane). Main technological challenges addressed in this study: •CO₂supply and availability ▪Most studies use bicarbonate as C source which is unrealistic for industrial applications. ▪Low CO₂solubility in water limits CH4production rate and quality. •Electrolytes and overpotential optimization ▪High electrolyte conductivity reduces BEM stack overpotential. ▪Non-specific ion crossover leads to salinity buildup at the cathode, harming methanogenic activity. BEM stack 3 double-chamber cells CATHODE: stainless-steel wool (100 cm2) ANODE:titanium mesh coated with IrO2-MMO (9 cm2) SEPARATOR: Cation Exchange Membrane (100 cm2) ELECTROLYTES: potassium phosphate monobasic solution (0.1 M, PPMS), highconductivity electrolyte-A (15 mScm⁻¹) and low-conductivity electrolyte-B (4 mScm⁻¹) RESULTS i) Optimization of electrolytes ii) Electrochemical characterization of the BEM stack iii) Influence of CO2injection strategy on e-methane production OBJECTIVES ii) Electrochemical characterization of the BEM stack Materials and Methods Time (days) Phase Current density (A m-2) Dissolved CO₂ ranges (%) Anolyte Catholyte 0 -14 Start-up 6100 -50 PPMS Electrolyte –A 15 - 21 12 100 -50 PPMS Electrolyte –A 21 - 44 Operation 12 80 -50 PPMS Electrolyte –A 45 - 63 12 80 -50 PPMS Electrolyte –B 64 - 76 12 60 -20 Electrolyte - B Electrolyte –B 76 - 83 12 20 - 5 Electrolyte - B Electrolyte –B Experimental design highlights Experimental Setup Cation crossover increased catholyte conductivity and affected BEM performance. Switching to low-conductivity catholyte reduced salinity stress and improved microbial growth and CH4production. pH and ionic overpotentials were the main contributors to cell voltage, highlighting the importance of electrolyte buffering and membrane ion transport in minimizing energy losses. Low concentration of dissolved CO₂(<20% saturation) increased e-methane purity to ~90% and production rate up to 0.96 ±0.33 L L⁻¹ d⁻¹. Ahydrogenotrophic methanogenic community established successfully on stainless steel-based BEM system. (20 mL min-1) •Catholyte conductivity increased from 9.5 to 22 mS cm⁻¹by day 45 →attributed to cation crossover (mainly Na+, K+) from anode to cathode. •High salinity represents a risk for BEM performance due to inhibition of methanogenic biomass. •Day 44: 30% of Electrolyte-A was replaced with low-conductivity Electrolyte-B →conductivity decreased below 19 mS cm⁻¹. •Switching to low-conductivity catholyte (Electrolyte-B) improved microbial growth rate and CH4production. iii) Influence of CO2injection strategy on e-methane production •Regulating CO2injection is an effective strategy to enhance both e-methane production rate and quality in BEM stacks. •Days 76 –83: Maintaining dissolved CO₂below 20%in the catholyte (approx. 300 mg L-1) via more frequent gas supply events resulted in: ✓0.96 ±0.33 L-CH4L⁻¹ day⁻¹. ✓~ 90% CH4 ✓< 6% unreacted CO2 •Acidic anode (pH ~2) resulted in a large pH split between electrolytes. •E losses linked to pH gradient (ηpH) were the main contributor to total cell overpotential. •Days 0 –14 and 15 –44: Cell overpotential rose from 0.15 V (@ 6 A m⁻²) to 0.3 V(@ 12 A m⁻²). oGreater ion fluxes and electrochemical driving forces →higher ionic resistance (ηionic) . •Day 44 –Catholyte exchange: slight rise in ηionic due to lower electrolyte conductivity. •Day 63 –Anolyte exchange: temporary spike in cell overpotential ≈1 V; mostly due to ηunknown (membrane resistance + activation/electron transfer losses). Taxonomic composition, based on 16S rRNA, revealed a mixed culture enriched with hydrogenotrophic methanogens Microbiological analysis KEY CONCLUSIONS Catholyte exchange Anolyte exchange Anolyte exchangeCatholyte exchange M79NET-FUELSFE102032