Abstract: 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 cm², a CO2 flow rate of 20 mL min¹, 0.5 g h¹ of water content, HCOOH concentration of 125 g L¹ with 57 % Faradaic Efficiency, and an energy consumption of 368 kWh kmol¹ are achieved. In the two-compartment electrolyzer, various catholyte solutions (0.1 M KCl and 0.5 M KHCO) 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 cm² and a flow rate of 0.15 mL min¹ cm², producing 5.28 g L¹ of HCOOH with a Faradaic Efficiency of 62 %. However, energy consumption is higher at 572 kWh kmol¹ 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.

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