Experimental assessment of different reactor configuration approaches for direct CO2 electroreduction to formic acid

Electrochemical CO₂ conversion is a promising technology for reducing industrial CO₂ emissions. The conversion of CO2 to formic acid (HCOOH) typically requires an intermediate acidolysis step when conventional CO₂ electrolyzers produce formate. Developing reactors capable of directly producing HCOOH production could significantly enhance the scalability of CO₂ electroreduction. This study evaluates and compares two reactor configurations: (i) a three-compartment reactor and (ii) a two-compartment electrolyzer with a bipolar membrane. In the three-compartment reactor, the effects of current density, CO₂ flow rate, and water content in the CO2 gas feed are analyzed, thus they are scarcely explored yet. Under optimal conditions, a current density of 200 mA cm⁻², a CO₂ flow rate of 20 mL min⁻¹, 0.5 g h⁻¹ of water content, HCOOH concentration of 125 g L⁻¹ with 57 % Faradaic Efficiency, and an energy consumption of 368 kWh kmol⁻¹ are achieved. In the two-compartment electrolyzer, various catholyte solutions (0.1 M KCl and 0.5 M KHCO₃) are tested to assess the impact of current density and flow rate. The best results are obtained with 0.1 M KCl at a current density of 90 mA cm⁻² and a flow rate of 0.15 mL min⁻¹ cm⁻², producing 5.28 g L⁻¹ of HCOOH with a Faradaic Efficiency of 62 %. However, energy consumption is higher at 572 kWh kmol⁻¹ due to the overpotential required for water dissociation in the bipolar membrane. These findings demonstrate the potential of both reactor designs for advancing industrial CO₂ electroreduction to HCOOH, with unique trade-offs between efficiency and energy consumption in each configuration.

The authors fully acknowledge the financial support received from the Spanish State Research Agency (AEI) through the projects PID2022–138491OB-C31 (MICIU/AEI /10.13039/501100011033 and ERDF/EU), and PLEC2022–009398 (MCIN/AEI/10.13039/501100011033 and Union Europea Next Generation EU/PRTR). The financial support from the “Complementary Plan in the area of Energy and Renewable Hydrogen” funded by the Autonomous Community of Cantabria, Spain, and the European Union Next Generation EU/PRTR, is also gratefully acknowledged. The present work is related to CAPTUS Project. This project has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No 101118265. The authors gratefully acknowledge the support of the Society for the Regional Development of Cantabria (SODERCAN Group) through Grants SB100513774 and SB100513775 (Convocatoria para el año 2023 de las subvenciones del programa de fomento de la economía circular) and Grant SB80988064 (Convocatoria para el año 2022 de las subvenciones correspondientes al programa de fomento de la transferencia tecnológica (INVESNOVA). Jose Antonio Abarca gratefully acknowledges the predoctoral research grant (FPI) PRE2021–097200.