Product‐Reshaped Gas‐Phase Hydrodynamics Govern CO 2 Electrolysis in Gas Diffusion Electrodes

High‐rate 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 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, consumption, CO formation and consumption, and gaseous product evolution progressively dilute the ‐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 delivery, CO generation, controlled CO consumption and limited dilution of the reacting gas phase.