Flow electrolyzers based on gas-diffusion electrodes (GDEs) have been increasingly employed to advance toward industry-relevant electrochemical CO2 reduction reaction (CO2RR) performance, though fundamental understanding of the GDE system is still lacking. Here, we propose that regulating local CO2 concentration on copper (Cu) surfaces is an effective and general strategy to promote C-C coupling in CO2yRR. LocalCO(2) concentration could influence the surface coverage of *CO2, *H, and *CO, which affects the reaction pathways toward multi-carbon (C2+) products. Guided by mass-transport modeling, we have identified three approaches to modulate the local CO2 concentration in GDE-based electrolyzers: (1) catalyst layer structure, (2) feed CO2 concentration, and (3) feed flow rate. Utilizing Cu2O nanoparticles as the model catalysts, modulation of local CO2 concentration enabled an optimized faradaic efficiency toward C2+ products of up to 75.5% at 300 mA cm(-2) and C2+ partial current density of up to 342 mA cm(-2) in 1.0 M KHCO3.