Gas diffusion electrodes enable enhanced energy efficiency of electrochemical CO2 reduction in natural brine-inspired electrolytes

A circular economy demands efficient conversion of carbon dioxide (CO 2 ) into valuable chemicals including C 1 -compounds like formate as building blocks for chemical production. The electrochemical CO 2 reduction reaction (eCO 2 RR) in aqueous solutions is a promising approach, being limited by low CO 2 solubility that restricts reaction rates and energy efficiency. In this study, we systematically investigated eCO 2 RR to formate using gas diffusion electrodes (GDEs) in electrolyte solutions with moderate (3 % w/v), high (10 % w/v), and hypersaline (17 % w/v) NaCl concentrations, representing natural saline water bodies. Notably, the presence of NaCl did not affect eCO 2 RR performance showing stable formate production rates of 1.30 ± 0.13 mmol L −1 h −1 cm −2 at a current density of 50 mA cm −2 across all salinities. Coulombic efficiencies ( CE ) for formate were similar across salinities starting at 80–90 % at 30 min and decreasing to ∼70 % after 120 min. Despite an expected ∼50 % decrease in CO₂ solubility with increasing salinity, GDEs ensured efficient CO₂ supply, preventing major performance losses. High salt electrolytes improved performance mainly by increasing electrolytic conductivity; however, benefits may also originate from an alternative anodic reaction, namely the chlorine evolution reaction (CER) instead of the oxygen evolution reaction (OER). At 17 % w/v NaCl, cell voltage decreased by 50.0 % and energy efficiency improved by up to 194.6 % when compared to sodium phosphate buffer, assuming CER was dominant. These findings indicate that the selection of anodic reaction is decisively influencing the energy efficiency of the eCO₂RR in saline electrolytes. Thus, we suggest that saline or brackish water can be sourced as electrolyte solutions for eCO 2 RR, offering a path towards large-scale carbon capture and utilization. • Selectivity of eCO 2 RR to formate was unaffected across 3–17 % NaCl salinity. • High salinity did not limit CO 2 availability due to effective gas handling by GDEs. • Cell voltage was reduced by 50.0 %, and energy efficiency increased by 194.6 % under 17 % NaCl salinity. • Chlorine evolution outperformed oxygen evolution due to higher solubility and reactive absorption.