The structure of the electrical double layer (EDL) at the electrode-electrolyte interface is paramount in determining the selectivity and kinetics of numerous electrochemical reactions. Traditional models like the Gouy-Chapman-Stern (GCS) model, while useful, lack the atomic-level detail necessary to fully explain ion-specific effects, overcharging, and water orientational asymmetry. Alkali metal cations, in particular, significantly influence the interfacial solvation environment and electrochemically active sites, impacting the proton-coupled electron-transfer (PCET) barrier in HER/HOR and mass transport in oxidation reactions. Existing techniques like vibrational spectroscopy and diffraction methods offer limited spatial resolution or suffer from spectral assignment challenges. Recent advancements in noncontact atomic force microscopy (AFM) with CO-terminated tips have enabled atomic-scale visualization of interfacial ion-water interactions, offering a new avenue for investigation. This study aims to leverage these advanced techniques to gain a comprehensive understanding of the atomic structure of the EDL and its impact on HER kinetics, focusing specifically on the ion-specific effects of alkali metal cations (Li⁺, K⁺, and Cs⁺) on a well-defined Au(111) surface. The detailed atomic-level understanding gained from this research will contribute significantly to the design and optimization of electrochemical catalysts and devices.

Extensive research has explored the EDL's importance in electrochemical processes. The GCS model provides a general framework, but its mean-field approach fails to capture molecular-level details. Ion-specific effects, a key observation in many electrochemical systems, are not adequately explained by this model. Studies using vibrational spectroscopy, such as surface-enhanced Raman spectroscopy (SERS) and SEIRAS, have provided insights into water orientation and hydrogen bonding near electrified interfaces, but spatial resolution remains a limitation. The recent development of AFM with CO-terminated tips has improved the resolution, enabling visualization of individual hydrated ions and water structures. Several studies have highlighted the role of alkali metal cations in modifying interfacial solvation and impacting HER kinetics, suggesting a strong correlation between cation type and reaction efficiency. However, a comprehensive, atomic-level understanding of how these cations affect interfacial water structure and HER activity is still lacking.

This study employed a combination of experimental techniques and computational modeling to investigate the structure of cation-water networks on a charged Au(111) surface. High-resolution STM and AFM, using CO-functionalized tips, were used to visualize the atomic arrangements of alkali metal cations (Li⁺, K⁺, Cs⁺) and water molecules at cryogenic temperatures (5 K). The Au(111) surface was negatively charged by depositing alkali metal atoms, which transfer electrons to the substrate. AFM images were acquired at varying tip heights to distinguish between electrostatic and Pauli repulsion forces, providing information on the spatial arrangement of both cations and water molecules. In situ SEIRAS measurements were performed on polycrystalline Au electrodes in alkaline, neutral, and acidic electrolytes containing different alkali metal cations to probe the interfacial water structure under ambient conditions. The experiments spanned potentials from the potential of zero charge (PZC) to the HER-relevant potential region where the Au surface is negatively charged. The OH stretching modes of water molecules were analyzed to determine the degree of hydrogen bonding. Density Functional Theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) to investigate the structural stability of the proposed atomic models and simulate AFM frequency shift images. Electrochemical measurements using a three-electrode system were carried out on a polycrystalline Au rotating disk electrode (RDE) to determine HER kinetics in different electrolytes at varied pH values. The Marcus-Hush-Chidsey (MHC) formalism and the Born model were applied to analyze the relationship between HER kinetics and interfacial solvation environment.

High-resolution STM and AFM revealed distinct cation-specific water structures on the Au(111) surface. The Li⁺-water layer showed a well-ordered hexagonal ice-like structure with Li⁺ cations elevated from the surface and hydrated by three water molecules, showing a structure-making effect on the interfacial water network. In contrast, Cs⁺ cations adsorbed directly onto the surface, interacting directly with the Au substrate. This led to a distorted, less ordered water network with Cs⁺ hydrated by five water molecules exhibiting a structure-breaking effect. The K⁺-water layer demonstrated intermediate behavior, with K⁺ cations hydrated by four water molecules forming short chains interspersed within a more disordered hydrogen-bonded water network. The number of water molecules in the first hydration shell increases from Li⁺ to Cs⁺ (3, 4, and 5 respectively). The cation-surface separation decreases from Li⁺ to Cs⁺ due to direct surface adsorption for K⁺ and Cs⁺, potentially blocking active sites. In situ SEIRAS experiments under ambient conditions confirmed these cation-dependent water structures. SEIRAS data showed that Cs⁺ promotes weakly H-bonded (isolated) water molecules, while Li⁺ promotes symmetrically H-bonded (ice-like) water molecules at the negatively charged Au/electrolyte interface. HER experiments indicated that exchange current density decreases in the order Li⁺ > Na⁺ > K⁺ > Cs⁺, across acidic, neutral, and alkaline electrolytes. The MHC formalism and Born model analysis showed that reorganization energy and interfacial dielectric constant increase with increasing cation size (Li⁺ to Cs⁺), impacting the HER kinetics. This is attributed to the disruption of the interfacial H-bonding network by structure-breaking cations, resulting in higher reorganization energy and a larger interfacial dielectric constant, which hinders proton transfer.

The findings directly address the research question regarding the impact of ion-specific water structures on HER kinetics. The observed cation-dependent water structures provide a microscopic explanation for the variations in HER activity. Structure-making cations like Li⁺ maintain a well-structured interfacial water layer, facilitating PCET and leading to faster HER kinetics. Conversely, structure-breaking cations like Cs⁺ disrupt the water network, increasing the reorganization energy and the interfacial dielectric constant, thereby impeding the proton transfer processes during HER. The agreement between low-temperature STM/AFM data and ambient-condition SEIRAS results suggests that some intrinsic aspects of these ion-specific water structures persist under more realistic electrochemical conditions, despite the thermal fluctuations that may affect their stability and lifetime. This study provides atomic-level insights into the complex interplay between interfacial water structure, ion-specific effects, and electrochemical reaction kinetics.

This research provides a comprehensive understanding of the atomic-level structure of cation-water networks at electrified Au interfaces and their profound influence on HER kinetics. The combination of high-resolution STM/AFM, SEIRAS, and electrochemical measurements reveals a clear correlation between the structure-making/breaking ability of alkali metal cations and HER activity. Future work could extend these studies to other cations and anions and explore the dynamic aspects of interfacial water structure using advanced techniques to fully capture liquid-like states and their dynamics.

The STM/AFM experiments were conducted at cryogenic temperatures (5 K) and under high vacuum. While some aspects of the observed structures likely persist under ambient conditions, the effects of thermal fluctuations and the presence of a bulk electrolyte on the long-range ordering should be considered. The use of a well-defined Au(111) single crystal as a model system may not perfectly reflect the behavior of polycrystalline Au electrodes used in the electrochemical experiments. The low solubility of CsClO4 limited the range of concentrations investigated for some experiments.