Which dominates CO2 to CO electroreduction from low to industrial current density: Catalyst activity or CO2 availability?

Electrochemical reduction of CO 2 (CO 2 RR) offers a sustainable route for converting renewable electricity into value-added chemicals. However, scaling CO 2 RR to industrially relevant current densities remains challenging due to intrinsic kinetics barriers, competing side reactions such as the hydrogen evolution reaction (HER), and limitations in CO 2 mass transport. To elucidate whether catalyst activity or CO 2 availability dominates CO 2 RR performance across current densities, we engineered flow-through hollow fiber gas diffusion electrodes (HFGDEs) integrated with in-situ grown, defect-rich silver nanosheets. At low current densities, catalytic activity is the primary determinant. The developed HFGDE with defect-rich silver nanosheets yields a high Faradaic efficiency of CO of 93.5 % at −0.8 (V vs. RHE), attributed to enhanced adsorption of *COOH intermediate on silver defect sites, as confirmed by in-situ Raman spectroscopy and density functional theory (DFT) calculations. However, as the system scales to industrially relevant current densities (up to 500 mA/cm 2 ), CO 2 availability becomes the dominant limitation due to the intensified fight between CO 2 and H 2 O for electrons. The HFGDE architecture addresses this challenge by continuously supplying convective CO 2 flow to the active sites, enabling a CO partial current density of 381.8 mA/cm 2 under a total current density of 500 mA/cm 2 . These findings highlight that while catalyst activity governs CO 2 RR performance at lower current densities, adequate CO 2 supply is essential to maintain high selectivity and suppress HER under industrial conditions. While catalyst activity dominates CO 2 to CO electroreduction at low current densities, CO 2 availability becomes the limiting factor under high current densities. Under these conditions, the rapid electron transfer accelerates competition between CO 2 and H 2 O, favoring the undesired HER when CO 2 is insufficient. The continuous and localized CO 2 delivery enabled by flow-through hollow fiber gas-diffusion electrodes is critical for maintaining high CO 2 availability at the reaction interface, thereby effectively suppressing the HER and sustaining CO 2 RR performance under industrially relevant conditions. • The HFGDE serves as a versatile platform for probing CO 2 RR performance across current densities up to 500 mA/cm 2 . • The abundant defect sites in silver nanosheets facilitate the stabilization of *COOH intermediates. • The continuous and localized CO 2 delivery enabled by flow-through HFGDE is critical for maintaining high CO 2 availability. • CO 2 availability dominates CO 2 to CO electroreduction under industrial-level current densities.