Engineering catalyst microenvironments with organic modifiers to enhance selectivity in CO 2 electroreduction on Cu and Ag surfaces Phannaro Nhem 1 **, Maria Balk 1 , 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 Faculty of Mechanical and Civil Engineering, Helmut Schmidt University, Holstenhofweg 85, 22043 Hamburg, Germany, 3 Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States **Presenteremail: Phannaro.Nhem@hereon.de *Corresponding author Email: Francesca.Toma@hereon.de Electrochemical CO 2 reduction (CO2R) to valuable products is often limited by competing reactions and poor selectivity. In this work, we report a molecular strategy to enhance CO 2 R selectivity on non-precious metal catalysts, specifically Cu and Ag, through the incorporation of organic surface modifiers. On Cu electrodes, a series of polymeric and molecular modifiers with varying hydrophilicity, proticity, and charge reveals clear structure–reactivity trends: protic modifiers favor the H 2 evolution reaction (HER), hydrophilic species promote formic acid formation, and cationic hydrophobic modifiers enhance CO production. ReaxFF reactive molecular dynamics simulations suggest that these modifiers influence surface hydride formation and interfacial water structuring, both critical factors in determining product selectivity. On Ag electrodes, once a quaternary ammonium compound (dihexadecyldimethylammonium bromide) was introduced, the product selectivity was shifted drastically with an increase of CO Faradaic efficiency from 25% to 97%, while strongly suppressing H 2 generation. Simulations attribute a defined structure of hydrophobic microenvironment that enriches CO 2 availability near the catalyst sites to be a factor of such behavior, while maintaining adequate proton supply from water for CO 2 activation. These findings highlight the importance of microenvironment engineering in steering CO 2 R pathways and offer a molecular-level design framework for next-generation CO 2 reduction systems, including tandem architectures for solar-driven hydrocarbon production. The demonstrated link between organic modifiers and CO 2 R selectivity also suggests promising opportunities to use synthetic derivatives with bespoke structural features as tunable modifiers. For example, peptoids can be suitable candidates due to their intrinsic stability. By systematically varying their side-chain residues, one can precisely tailor the local catalytic environment. References 1. Buckley, A. K.; Cheng, T.; Oh, M. H.; Su, G. M.; Garrison, J.; Utan, S. W.; Zhu, C.; Toste, F. D.; Goddard, W. A.; Toma, F. M. Approaching 100% Selectivity at Low Potential on Ag for Electrochemical CO2 Reduction to CO Using a Surface Additive. ACS Catal. 2021, 11, 9034–9042. 2. Buckley, A. K.; Lee, M.; Cheng, T.; Kazantsev, R. V.; Larson, D. M.; Goddard, W. A.; Toste, F. D.; Toma, F. M. Electrocatalysis at Organic–Metal Interfaces: Identification of Structure–Reactivity Relationships for CO2 Reduction at Modified Cu Surfaces. J. Am. Chem. Soc. 2019, 141, 18, 7355–7364.
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