A. Kas, P. Izadi, I. Dinges, M. Stockl, F. Harnisch
Journal of CO2 Utilization 2025:
A circular economy demands efficient conversion of carbon dioxide (CO2) into valuable chemicals including C1-compounds like formate as building blocks for chemical production. The electrochemical CO2 reduction reaction (eCO2RR) in aqueous solutions is a promising approach, being limited by low CO2 solubility that restricts reaction rates and energy efficiency. In this study, we systematically investigated eCO2RR 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 eCO2RR 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 eCO2RR, offering a path towards large-scale carbon capture and utilization.
