HSE Physicists Propose Unified Theory for Describing Electric Double Layer
To develop more efficient batteries and catalysts, it is essential to understand the processes occurring at the metal–solution interface in the electric double layer (EDL). Physicists at HSE MIEM have proposed a unified theoretical model of the EDL that simultaneously accounts for selective adsorption of ions on the surface and partial charge transfer between ions and the metal—phenomena that had previously been described separately. The model’s predictions are consistent with experimental data. In the future, it may be used in the development of batteries, supercapacitors, and catalysts. The study has been published in Electrochimica Acta.
All electrochemical devices have a surface through which electric charge is transferred. In batteries and supercapacitors, these surfaces are electrodes—usually metal plates—with the space between them filled with an electrolyte containing ions. At the interface between the metal and the electrolyte, the ions approach the surface, arrange themselves near it, and partially redistribute their electron density. This process leads to the formation of the electric double layer (EDL)—the region at the interface between the metal and the electrolyte where charge accumulates. The properties of the EDL determine how much charge an electrode can store, how quickly it can release that charge, and how efficiently reactions occur on its surface. Therefore, it is crucial to have an accurate theoretical description of this region.
For a long time, descriptions of the EDL were fragmented, requiring several models to be combined. Researchers at the HSE MIEM Laboratory for Computational Physics have developed a theoretical model in which both layers are described by a single system of equations. The model accounts for two key processes: specific adsorption—the 'sticking' of ions to the surface due to chemical interactions—and partial charge transfer, in which an ion and the electrode exchange a fraction of an electron’s charge.
To develop the description using quantum chemical calculations and computer simulations, the researchers studied how an ion behaves in solution near a metal surface: the distance at which it is typically located, the strength of its attraction to the metal, and the fraction of charge it can transfer. The resulting values were used as model parameters and incorporated into a more general theoretical framework describing the entire electric double layer.
The model was tested on systems involving silver electrodes and solutions of KPF₆ (potassium hexafluorophosphate), NaF (sodium fluoride), as well as a mixture of the two, and the results were compared with available experimental data. The model accurately reproduced capacitance measurements and explained the behaviour of the sodium fluoride–potassium hexafluorophosphate mixture within the EDL. In this mixture, one type of ion is preferentially adsorbed while the other is nearly absent, and the model correctly describes the displacement of one type of ion by the other.
Yury Budkov
'To develop efficient batteries and catalysts, it is essential to understand what happens at the metal–solution interface within the EDL,' explains Prof. Yury Budkov of HSE MIEM. 'Our model accounts for both how strongly ions adhere to the surface and how they exchange charge with the metal. In the future, we plan to adapt the model to systems in which these effects are more pronounced, such as electrocatalysis. This will allow calculations to provide deeper insight into how the choice of metal and electrolyte influences EDL properties and to apply this knowledge in the design of new electrochemical devices.'
The study was conducted within the framework of the Mirror Laboratories project 'Predicting the Properties of Molecular Systems: Combining Machine Learning Methods and Classical Modelling Methods.'
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