DOI: 10.1002/elan.201800782 Polyethylenimine-assisted Synthesis of Au Nanoparticles for Efficient Syngas Production Young-Hoon Chung + , [a] Min Gwan Ha + , [a, d] Youngseung Na, [e] Hee-Young Park, [a] Hyoung-Juhn Kim, [a, b] Dirk Henkensmeier, [a, b, c] Sung Jong Yoo, [a, b] Jin Young Kim, [a, b, c] So Young Lee, [a] Seung Woo Lee, [f] Hyun S. Park,* [a, b] Yong-Tae Kim,* [d] and Jong Hyun Jang* [a, b, c] Abstract: The ability to capture, store, and use CO 2 is important for remediating greenhouse-gas emissions and combatting global warming. Herein, Au nanoparticles (Au-NPs) are synthesized for effective electrochemical CO 2 reduction and syngas production, using polyethyleni- mine (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 CO 2 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 CO 2 reduction than bare Au NPs/C; for example, PEI  Au/C exhibits a CO partial current density (j CO ) of 28.6 mAcm 2 at 1.13 V RHE , while the value for bare Au NPs/C is 21.7 mAcm 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 CO 2 -reduction device coupled with an IrO 2 water-oxidation catalyst and a proton- conducting perfluorinated membrane to form a PEI  Au/ C j Nafion j IrO 2 membrane-electrode assembly. The de- vice using PEI  Au/C as the CO 2 -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%. Keywords: CO 2 reduction · Electrocatalyst · Au nanoparticles · Polyethylenimine · CO 2 electrolyzer 1 Introduction The realization of practical and efficient CO 2 capture, storage, and use is imperative for mitigating the impact of global warming caused by soaring concentrations of greenhouse gases in the atmosphere [1,2]. CO 2 is a major product of fossil fuel combustion. The level of CO 2 in the atmosphere was 405 ppm as of March 2017, which is approximately 40% above pre-industrial levels; this value is accompanied by a record increase in the worldwide consumption of fossil fuels [2–4]. Soaring CO 2 concen- trations have resulted in an increase the global temper- ature of approximately 1 °C, and a sea-level rise of approximately 88 mm compared to the pre-industrial era [3]. As the consequences of global warming begin to appear, including extreme weather events, heavy precip- itation, and rising sea levels, the development of efficient and massive processes that capture and use CO 2 are urgently required [5]. In order to avoid the catastrophic results of global warming, the global mean temperature increase needs to be limited to 3 °C; consequently, the maximum allowable global CO 2 emission is 1,700 billion metric tons by 2050, with a CO 2 -emission rate that needs to remain constant from 2013 onwards [6]. Electrochemical CO 2 reduction is considered to be an attractive route for storing renewable electrical energy in the form of chemical bonds, while CO 2 electrolysis decreases the level of CO 2 in the atmosphere. The capture and use of CO 2 is a means of converting intermittent [a] Y.-H. Chung, + M. G. Ha, + H.-Y. Park, H.-J. Kim, D. Henkensmeier, S. J. Yoo, J. Y. Kim, S. Y. Lee, H. S. Park, J. H. Jang Center for Hydrogen Fuel Cell Research, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Tel.: + 82-2-958-5250 Tel.: + 82-2-958-5287 Fax: + 82-2-958-5199 E-mail: hspark@kist.re.kr jhjang@kist.re.kr [b] H.-J. Kim, D. Henkensmeier, S. J. Yoo, J. Y. Kim, H. S. Park, J. H. Jang Division of Energy & Environment Technology, KIST School, University of Science and Technology (UST), Seoul 02792, Republic of Korea [c] D. Henkensmeier, J. Y. Kim, J. H. Jang Green School, Korea University, Seoul 02841, Republic of Korea [d] M. G. Ha, + Y.-T. Kim School of Mechanical Engineering, Pusan National University, Busan 46241, Republic of Korea Tel.: + 82-51-510-1012 Fax: + 82-51-514-0685 E-mail: yongtae@pusan.ac.kr [e] Y. Na Department of Mechanical and Information Engineering, University of Seoul, Seoul 02504, Republic of Korea [f] S. W. Lee George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA [ + ] These authors contributed equally to this work Full Paper www.electroanalysis.wiley-vch.de © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2019, 31, 1401 – 1408 1401