Product-Reshaped Gas-Phase Hydrodynamics Govern CO₂ Electrolysis in Gas Diffusion Electrodes

High-rate ${\mathrm{CO}}_2$ electrolysis is commonly assessed using current density, single-pass conversion and Faradaic efficiency, but these global metrics do not indicate whether the local reaction environment required for selective multicarbon formation is preserved during scale-up. Using an experimentally validated, flow-direction-resolved continuum model of a Cu-based gas-diffusion-electrode (GDE) flow-cell system, we demonstrate that gas-fed ${\mathrm{CO}}_2$ electrolysers behave as reacting-flow electrochemical reactors in which electrochemical conversion transforms the gas phase in the cathode compartment and makes it part of the reactor response. As cathode potential becomes more negative, ${\mathrm{CO}}_2$ consumption, CO formation and consumption, and gaseous product evolution progressively dilute the ${\mathrm{CO}}_2$ -rich reacting gas phase, altering gas composition, density and velocity and feeding back into reactant delivery and intermediate availability. This reaction-induced hydrodynamic response creates an operating window favoring ethylene formation and reveals a conservation-driven feedback that becomes unavoidable during scale-up: as electrode length, conversion and gaseous product formation increase, extended electrodes must preserve the spatial overlap between ${\mathrm{CO}}_2$ delivery, CO generation, controlled CO consumption and limited dilution of the reacting gas phase.