Electric double layers (EDLs), occurring ubiquitously at solid–liquid interfaces, are critical for electrochemical energy conversion and storage processes such as capacitive charging and redox reactions. However, to date the molecular-scale structure of EDLs remains elusive. Here we report an advanced technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular-scale EDL structure of an ionic liquid under different electrode potentials. We observe not only multiple discrete ionic layers in the EDL on a graphite electrode but also a quasi-periodic molecular density distribution within each layer. Furthermore, we find pronounced 3D reconfiguration of the EDL at different voltages, especially in the first layer. Combining the experimental results with molecular dynamics simulations, we find potential-dependent molecular redistribution and reorientation in the innermost EDL layer, both of which are critical to EDL capacitive charging. We expect this mechanistic understanding to have profound impacts on the rational design of electrode–electrolyte interfaces for energy conversion and storage.

中文翻译：

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离子液体在电界面上的三维分子映射

双电层（EDL）普遍存在于固液界面，对于电化学能量转换和存储过程（例如电容性充电和氧化还原反应）至关重要。但是，迄今为止，EDL的分子规模结构仍然难以捉摸。在这里，我们报告了一种先进的技术，电化学三维原子力显微镜（EC-3D-AFM），并用它来直接成像在不同电极电位下离子液体的分子级EDL结构。我们不仅在石墨电极上的EDL中观察到多个离散的离子层，而且还观察到了每一层中的准周期性分子密度分布。此外，我们发现在不同电压下，特别是在第一层中，EDL的3D重新配置明显。将实验结果与分子动力学模拟相结合，我们在最内层的EDL层中发现了与电位有关的分子重新分布和重新取向，这两者对于EDL电容充电至关重要。我们希望这种机械理解对能量转换和存储的电极-电解质界面的合理设计有深远的影响。