Non-Monotonic Variation of Potential-Dependent Surface Diffusion at Electrochemical Interfaces in the Presence of Coadsorbates

The influence of coadsorbed ions on adsorbate diffusion, an inherent effect at solid–liquid interfaces, was studied for adsorbed sulfur on Ag(100) electrodes in the presence of bromide or iodide. Quantitative in situ high-speed scanning tunnelling microscopy (video-STM) measurements were performed both in the potential regime of the c(2×2) halide adlayer at its saturation coverage and in the regime of a disordered adlayer where the halide coverage increases with potential. These studies reveal a surprising non-monotonic potential dependence of S_ad diffusion with an initial increase with halide coverage, followed by a decrease upon halide adlayer ordering into the c(2×2) structure. Density functional theory (DFT) and Monte Carlo (MC) simulations only qualitatively reproduce the rise in S_ad mobility with halide coverage, suggesting that many-adsorbate interactions and the presence of the electrolyte need to be considered.