Resolving the double layer and hydration structure of competing ions at solid-liquid interfaces M. Olgiati,1 D. M. Arvelo,2 R. Garcia,2 M. Valtiner1 1TU Wien, Institute of Applied Physics, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria 2Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Madrid, Spain In the framework of clean energy technologies, electrochemical production of hydrogen from water and conversion of CO2 into valuable green fuels are of great interest, despite their kinetic and thermodynamic limitations. Overcoming these limitations requires an accurate molecular-level description of the transition states taking place at the solid-liquid interface. The chemistry of the supporting electrolyte was shown to affect the activities of catalytic reactions [1]. The weakly-hydrated cations, for example, seem to act as proton donors through water hydrolysis, which can alter the proton availability (i.e., kinetics) of HER [2] or buffer the interfacial pH to an ideal value for CO2 reduction in acidic media [3]. More recently, the effects of additions of organic cations derived from quaternary ammonium salts has been investigated [4]. For example, the size of the quaternary ammonium salt was found to provide selectivity towards CO2 reduction intermediates [5], thus affecting the reaction pathways. Additions of tetrabutylammonium ions to alkaline electrolytes were also shown to controversially promote the HER despite its physisorption and site-blocking effect [6]. This opens the question on how the organic ions are structuring and/or competing with (for example) alkali ions within the electric double layer under high surface charging regime. For this purpose, we investigate the structure of electric double layers consisting of ionic mixtures on the surface of muscovite mica. The mixtures consist of alkali ions (namely, Cs+) and quaternary ammonium salts with different hydrophobic character [tetraethylammonium (TEA+) or tetrabutylammonium (TBA+)]. Competition between the ions is investigated with high resolution AFM imaging and force spectroscopy, while the interfacial hydration structure is more deeply investigated with three-dimensional AFM (3D-AFM). Such understanding provides insights for tuning and optimizing the chemistry of electrocatalytic solutions. References: [1] D. Strmcnik et al., Nature Chemistry 1, 466-472 (2009) [2] X. Liu et al., Electrochimica Acta 507, 145068 (2024) [3] X. Liu et al., J. Am. Chem. Soc. 146, 5242-5251 (2024) [4] I. Ledezma-Yanez et al., ChemElectroChem 2, 1612-1622 (2015) [5] C. Deacon-Price et al., ACS Catal. 14, 12928-12939 (2024) [6] J. Fernandez-Vidal et al., ACS Catal. 14, 8130-8137 (2024)