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.