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The ability to capture, store, and use CO2 is important for remediating greenhouse-gas emissions and combatting global warming. Herein, Au nanoparticles (Au-NPs) are synthesized for effective electrochemical CO2 reduction and syngas production, using polyethylenimine (PEI) as a ligand molecule. The PEI-assisted synthesis provides uniformly sized 3-nm Au NPs, whereas larger irregularly shaped NPs are formed in the absence of PEI in the synthesis solution. The Au-NPs synthesized with PEI (PEI-Au/C, average PEI Mw=2000) exhibit improved CO2 reduction activities compared to Au-NPs formed in the absence of PEI (bare Au NPs/C). PEI-Au/C displays a 34 % higher activity toward CO2 reduction than bare Au NPs/C; for example, PEI-Au/C exhibits a CO partial current density (j(CO)) of 28.6 mA cm(-2) at -1.13 V-RHE, while the value for bare Au NPs/C is 21.7 mA cm(-2); the enhanced j(CO) is mainly due to the larger surface area of PEI-Au/C. Furthermore, the PEI-Au/C electrode exhibits stable performance over 64 h, with an hourly current degradation rate of 0.25 %. The developed PEI-Au/C is employed in a CO2-reduction device coupled with an IrO2 water-oxidation catalyst and a proton-conducting perfluorinated membrane to form a PEI-Au/C vertical bar Nafion vertical bar IrO2 membrane-electrode assembly. The device using PEI-Au/C as the CO2-reduction catalyst exhibits a j(CO) of 4.47 mA/cm(2) at 2.0 V-cell. Importantly, the resulted PEI-Au/C is appropriate for efficient syngas production with a CO ratio of around 30-50 %.