Mechanisms of adsorbate diffusion at electrochemical interfaces

在许多电化学反应中，吸附物在与电解质水溶液接触的电极表面上的扩散是一个核心的基本过程，但仍然知之甚少。已知高电场和共吸附物种的存在强烈影响电化学界面处的表面输运。这导致扩散速率的明显的电势依赖性和电解质组成的复杂影响。特别是化学吸附的阴离子，这是广泛存在于自然和技术系统，影响表面扩散明显。正如我们以前的工作所示，这些影响目前是不可预测的，而且往往令人惊讶。在某些情况下，阴离子种类甚至可以确定电位依赖性的符号。在这个项目中，我们建议解决的作用，阴离子在表面扩散结合原位原子分辨率的微观研究从头计算。在实验上，我们将通过高速扫描隧道显微镜直接观察吸附物的运动来定量地确定扩散速率作为电势的函数。密度泛函理论将用于计算不同扩散途径的能垒和相关的表面偶极矩变化，这为电场的影响提供了度量，即，潜在的影响。我们将把这些方法应用于定义明确的电化学系统：硫化物和甲基硫醇盐在金、银和铜的卤化物覆盖的（100）表面上的扩散。通过这种方法，我们的目标是澄清以下具体问题：卤化物coadsorbate覆盖吸附物的表面流动性的影响是什么？为什么在无序的移动的卤化物层的存在下，扩散速率随电位增加而增加，而在饱和覆盖度的有序吸附层的电位区域中，扩散速率却降低。卤化物吸附层的结构顺序如何影响电位依赖性和表面扩散机制？各向异性扩散能以这种方式诱导并由势调整吗？硫化物扩散对Cu（100）的势依赖性的卤化物依赖性反转的起源是什么？这种效应是否也可以在其他吸附物系统中观察到，它是否与卤化物引起的扩散机制变化有关？水在扩散过程中的作用是什么？我们将通过对精心挑选的系统进行研究来解决这些问题，包括对无卤化物参比系统的研究。所获得的结果应解决目前的奥秘，在电化学界面的表面运输提供深入的微观扩散机制。这将有助于更好地理解这些重要的基本过程，从而提供了一个基础，调整表面传输的电位和添加剂的电化学反应。

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.