Abstract: The scale-up of gas-phase CO2 electroreduction to formate is crucial for its industrial application but remains largely unexplored. This work presents the design and validation of a 100 cm2 electrolyzer prototype featuring a zero-gap configuration and a serpentine flow field to ensure uniform CO2 distribution. Scaling up a CO2 electrolyzer requires optimized flow field design, in this case, a serpentine geometry enhances CO2 transport and minimizes mass transfer limitations, thereby improving overall performance. Experimental prototype testing is conducted to evaluate the effects of current density and water content in the CO2 feed. Optimal performance is achieved at 200 mA cm-2 and a water content of 15 g h-1, yielding a formate concentration of 760 gL-1, a Faradaic efficiency of 67%, a production rate of 7 mmol m-2 s-1, and an energy consumption of 507 kWh kmol-1. Comparisons with a 10 cm2 lab-scale reactor reveal improved CO2 conversion and production rate, validating the benefits of optimized flow field design and scale-up approach. While energy efficiency is somewhat reduced to increased Ohmic losses, the overall results support the technical feasibility of scaling gas-phase CO2-to-formate electrolysis. Further improvements in design and energy management are still needed to advance toward industrial implementation.

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