Article pubs.acs.org/JPCC
Theoretical Study of EMIM+ Adsorption on Silver Electrode Surfaces Makoto Urushihara,†,‡ Karen Chan,§ Chuan Shi,§ and Jens K. Nørskov*,†,§ †
SUNCAT Center for Catalysis and Interface Science, SLAC National Accelerator Laboratory, 2675 Sand Hill Road, Menlo Park, California 94025, United States ‡ Central Research Institute, Mitsubishi Materials Corporation, 1002-14 Mukohyama, Naka-shi, Ibaraki 311-0102, Japan § SUNCAT Center for Catalysis and Interface Science, Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States ABSTRACT: Imidazolium ionic liquid additives improve the activity and selectivity for electrochemical CO2 reduction reaction on a variety of catalyst materials. To date, there is no consensus about the mechanism by which it does so. As a first step, we determined the Pourbaix diagram for EMIM+ at the Ag(111)|water interface using density functional theory calculations. The obtained surface Pourbaix diagram shows that adsorbed EMIM+ densely covers the entire silver surface under experimental conditions; we suggest this has important implications for CO2 reduction.
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INTRODUCTION Electrochemical CO2 reduction has the potential to mitigate increasing atmospheric CO2 levels and to provide storage of electricity from intermittent renewable resources. While the reaction has been studied over the past few decades, existing catalysts are far from satisfying reasonable criteria for reaction activity and selectivity.1 Toward this end, a variety of materials have recently been examined for CO2 reduction, such as nanostructured transition metal catalysts,2−4 as well as nonprecious metal materials such as molybdenum sulfide5,6 and nitrogen-doped carbon nanofibers and nanotubes.7−9 Pyridinium (co)catalysts have also been shown to reduce CO2 to methanol on transition metals10,11 and semiconductor p-GaP12 electrodes, and there is currently no consensus about its mechanism.10−16 Imidazolium ionic liquid additives provide another promising avenue to improve the reactivity and selectivity, as they have been shown to enhance both the activity and selectivity for CO2 electrochemical reduction on various catalysts.6,7,17−21 On polycrystalline silver in an aqueous electrolyte, the addition of 1-methyl-3-ethylimidazolium tetrafluoroborate (EMIM-BF4) decreases the onset potential for CO production from −0.8 to −0.2 V and at the same time suppresses the hydrogen evolution reaction (HER).17 On molybdenum disulfide, the addition of EMIM-BF4 also decreased the overpotential to
