Duality of Theories for the Electrical Double Layer in Concentrated Electrolytes

Abstract

Understanding the electrical double layer (EDL), i.e, the distribution of electrolyte at an electrified interface, in concentrated electrolytes is important for various technologies, such as supercapacitors, batteries and electrocatalysis. Atomistic approaches offer unprecedented detail, but are too computationally expensive to exhaustively investigate the EDL of concentrated electrolytes, motivating the development of continuum theories. In these concentrated electrolytes, correlations between ions and solvents are strong, through electrostatic and specific interactions, as well as significant excluded volume effects of the complicated molecular species, making the development of theories challenging. Thus far, there are mainly two distinct \textit{simple} theoretical approaches to understand the EDL of concentrated electrolytes, with account of these correlations beyond mean-field. One is a local-density approximation (LDA) based on treating electrostatic and specific interactions beyond mean-field through the ionic aggregation and solvation; where a simple conceptual understanding can be gained and reasonable agreement with experiments in terms of integrated quantities, but poor agreement for ion profiles and Debye capacitance. The other approach is to treat electrostatic correlations and excluded volume effects more rigorously with beyond LDA approaches, but at the cost of simplifying the chemical interactions between species; where excellent agreement can be obtained for ion profiles, differential capacitance, etc., but mainly for the simplified hard-sphere systems that the theories are based on. Here, we describe the merits and downfalls of these two approaches, how they have contributed to understanding anomalous underscreening, and outline future directions for these theoretical approaches.