Mechanisms of adsorbate diffusion at electrochemical interfaces

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The diffusion of adsorbates on electrode surfaces in contact with aqueous electrolytes is a central elementary process in many electrochemical reactions but is still poorly understood. It is known that the high electric fields and the presence of coadsorbed species strongly influence surface transport at electrochemical interfaces. This results in a pronounced potential dependence of the diffusion rates and in a complex influence of the electrolyte composition. In particular chemisorbed anions, which are widespread in natural and technological systems, affect surface diffusion distinctly. As shown in our previous work, these effects are currently unpredictable and often surprising. In some cases, the anion species can even determine the sign of the potential dependence. In this project, we propose to address the role of anions in surface diffusion by combining in situ atomic-resolution microscopic studies with ab initio calculations. Experimentally, we will determine the diffusion rates quantitatively as a function of potential by direct observations of the adsorbates’ motion with high-speed scanning tunneling microscopy. Density functional theory will be used to calculate the energy barriers of different diffusion pathways and the associated surface dipole moment changes, which provide a measure for the influence of the electric field, i.e., for potential effects. We will apply these methods to well-defined electrochemical systems: sulfide and methyl thiolate diffusion on halide-covered (100) surfaces of gold, silver, and copper. With this approach, we aim to clarify the following specific questions: What is the influence of the halide coadsorbate coverage on the adsorbate’s surface mobility? Why does the diffusion rate increase with potential in the presence of a disordered mobile halide layer, whereas it decreases in the potential regime of ordered adlayers at saturation coverage. How does the type of structural order in the halide adlayer affects the potential-dependence and the mechanisms of surface diffusion? Can anisotropic diffusion be induced this way and tuned by the potential? What is the origin of the halide-dependent inversion of the potential dependence of sulfide diffusion on Cu(100)? Can this effect be also observed in other adsorbate systems and is it related to a halide-induced change in the diffusion mechanism? What is the role of water in the diffusion process?We will address these questions by studies of carefully selected systems, including studies of halide-free reference systems. The obtained results should resolve current mysteries in surface transport at electrochemical interfaces by providing insights into the microscopic diffusion mechanisms. This will contribute to a better fundamental understanding of these important elementary processes and thus provides a basis for tuning surface transport in electrochemical reactions by the potential and additives.