In this work, we theoretically study the differential capacitance of an aqueous electrolyte in contact with a planar electrode, using classical density functional theory, and show how this measurable quantity can be used as a probe to better understand the structure and composition of the electric double layer at play. Specifically, we show how small trace amounts of divalent ions can influence the differential capacitance greatly and also how small ions dominate its behavior for high electrode potentials. In this study, we consider primitive model electrolytes and not only use the standard definition of the differential capacitance but also derive a new expression from mechanical equilibrium in a planar geometry. This expression reveals explicitly that the first layer of ions near the charged surface is key to its understanding. Our insights might be used as a guide in experiments to better understand the electrolyte–electrode interface as well as the (composition of the) bulk electrolyte.

中文翻译：

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差分电容作为双电层结构和电解质本体组成的探针

在这项工作中，我们使用经典密度泛函理论从理论上研究了与平面电极接触的水性电解质的微分电容，并展示了如何将此可测量量用作探针以更好地了解双电的结构和组成层在发挥。具体来说，我们展示了微量的二价离子如何极大地影响微分电容，以及小离子如何在高电极电位下主导其行为。在这项研究中，我们考虑原始模型电解质，不仅使用微分电容的标准定义，而且还从平面几何中的机械平衡中推导出新的表达式。该表达式明确表明，带电表面附近的第一层离子是理解的关键。