Effects of ionomer-induced microenvironments in electrochemical nitrate reduction to ammonia Hojoong Choi 1 **, Sehun Seo 1 , Francesca M. Toma 1,2,3 * 1 Institute of Functional Materials for Sustainability, Helmholtz-Zentrum Hereon, Kantstraße 55, 14513 Teltow, Germany, 2 Liquid Sunlight Alliance and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States, 3 Faculty of Mechanical and Civil Engineering, Helmut Schmidt University, Holstenhofweg 85, 22043 Hamburg, Germany Presenteremail: **Hojoong.Choi@hereon.de *Corresponding author email: Francesca.Toma@hereon.de Ammonia (NH 3 ) is one of the essential chemicals for humanity, widely used in agricultural and industrial applications. Conventionally, NH 3 is primarily produced via the Haber–Bosch process, which marked a breakthrough in modern agriculture and industry. While this process has provided tremendous benefits through large-scale NH 3 production, it relies on high temperature and pressure conditions, leading to substantial energy consumption and CO 2 emissions. 1,2 This has created a need for a new paradigm in sustainable NH 3 production. In this context, the electrochemical nitrate (NO 3 − ) reduction reaction (NO 3 RR) presents a promising alternative for NH 3 production by utilizing NO 3 − , a ubiquitous pollutant commonly found in wastewater. The electrochemical NO 3 RR to NH 3 is recognized as a tandem catalytic process, comprising two major steps: NO 3 − to NO 2 − and NO 2 − to NH 3 . 2,3 Therefore, achieving high overall NH 3 conversion efficiency requires effective catalysis in both steps. While many researchers have focused on developing novel electrocatalysts for NO 3 RR, the influence of the underlying microenvironment on selective NH 3 production remains poorly understood. 2 Herein, we investigate the use of organic modifiers, specifically, four types of ionomers (Sustainion, PiperION, Nafion, and Aquivion), to enhance the electrochemical NO 3 RR to NH 3 on pure Cu electrocatalysts, which inherently exhibit a large energy barrier for hydrogenation due to limited H* availability. Our results show that Cu electrocatalysts modified with anion exchange ionomers (Sustainion and PiperION) exhibit low NH 3 conversion efficiencies. In contrast, those modified with cation exchange ionomers (Nafion and Aquivion) demonstrate significantly enhanced NH 3 conversion efficiencies, highlighting the critical role of cation- enriched microenvironments and H supply* in facilitating the electrochemical NO 3 RR to NH3. References 1. Y. Xiong, Y. Wang, J. Zhou, F. Liu, F. Hao and Z. Fan, Adv. Mater. , 2024, 36 , 2304021. 2. W. Wen, S. Fang, Y. Zhou, Y. Zhao, P. Li and X.-Y. Yu, Angew. Chem., Int. Ed. , 2024, 63 , e202408382. 3. W. He, J. Zhang, S. Dieckhöfer, S. Varhade, A. C. Brix, A. Lielpetere, S. Seisel, J. R. C. Junqueira and W. Schuhmann, Nat. Commun. , 2022, 13 , 1129.
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