The differential capacitance as a probe for the electric double layer structure and the electrolyte bulk composition

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. Published under an exclusive license by AIP Publishing.

Automated summary

This theoretical study investigates the differential capacitance of an aqueous electrolyte in contact with a planar electrode, demonstrating that small trace amounts of divalent ions can greatly influence the differential capacitance, while small ions dominate its behavior for high electrode potentials. A new differential capacitance expression is derived from mechanical equilibrium in a planar geometry, emphasizing the importance of the first layer of ions near the charged surface for understanding. These insights can serve as a guide in experiments to better understand the electrolyte-electrode interface and the composition of the bulk electrolyte.