In this study Ag nanoparticles supported on carbon black (Ag/C) were studied as catalysts for the electrochemical reduction of CO2to CO. The nanoparticles were synthesized on three carbon supports, namely Super P, Vulcan and Ketjenblack with surface areas from 50 to 800 m2g−1using cysteamine as a linker as proposed by Kimet al., J. Am. Chem. Soc., 2015,137, 13844.Gas diffusion electrodes were fabricated with all three Ag/Cs and then characterized in a zero-gap electrolyzer. All three supported catalysts achieve high voltage efficiencies, mass activities, and faradaic efficiencies above 80% up to 200 mA cm−2with Ag loadings of ∼0.07 mg cm−2. Using an IrO2anode, a partial CO current density of 196 mA cm−2at 2.95 V and a mass activity of 3920 mA mg−1at a cell voltage of 3.2 V was achieved. When changing the electrolyte from 0.1 M KOH to 0.1 M CsOH, it is possible to achieve 90% FECOat 300 mA cm−2. This results in a mass activity up to 5400 mA mg−1. Moreover, long-term tests at 300 mA cm−2with 0.1 M CsOH resulted in FECOremaining above 80% over 11 h. The electrochemical performance did not show a dependence on the carbon support, indicating that mass transport is limiting the cathode, rather than catalyst kinetics. It is worth noting that this may only apply to electrodes with PTFE binders as used in this study, and electrodes with ionomer binders may show a dependence on the catalyst support.