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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 (jCO) of 28.6 mA cm−2 at −1.13 VRHE, while the value for bare Au NPs/C is 21.7 mA cm−2; the enhanced jCO 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|Nafion|IrO2 membrane‐electrode assembly. The device using PEI−Au/C as the CO2‐reduction catalyst exhibits a jCO of 4.47 mA/cm2 at 2.0 Vcell. Importantly, the resulted PEI−Au/C is appropriate for efficient syngas production with a CO ratio of around 30–50 %. [ABSTRACT FROM AUTHOR]

For the improvement for the commercialization of electrochemical carbon dioxide (CO2) conversion technology, it is important to develop a large area Ag gas diffusion electrode (GDE), that exhibits a high electrochemical CO2 conversion efficiency and high cell performance in a membrane electrode assembly (MEA)-type CO2 electrolyzer. In this study, the electrodeposition of Ag on a carbon-paper gas diffusion layer was performed to fabricate a large area (25.5 and 136 cm2) Ag GDE for application to an MEA-type CO2 electrolyzer. To achieve uniformity throughout this large area, an optimization of the electrodeposition variables, such as the electrodes system, electrodes arrangement, deposition current and deposition time was performed with respect to the total electrolysis current, CO production current, Faradaic efficiency (FE), and deposition morphology. The optimal conditions, that is, galvanostatic deposition at 0.83 mA/cm2 for 50 min in a horizontal, two-electrode system with a working-counter electrode distance of 4 cm, did ensure a uniform performance throughout the electrode. The position-averaged CO current densities of 2.72 and 2.76 mA/cm2 and FEs of 83.78% (with a variation of 3.25%) and 82.78% (with a variation of 8.68%) were obtained for 25.5 and 136 cm2 Ag GDEs, respectively. The fabricated 136 cm2 Ag GDE was further used in MEA-type CO2 electrolyzers having an active geometric area of 107.44 cm2, giving potential-dependent CO conversion efficiencies of 41.99%–57.75% at Vcell = 2.2–2.6 V. [ABSTRACT FROM AUTHOR]