Variants of Surface Charges and Capacitances in Electrocatalysis : Insights from Density-Potential Functional Theory Embedded with an Implicit Chemisorption Model

Prevalent electrolyte effects across a wide range of electrocatalytic reactions underscore the general importance of the local reaction conditions in the electrical double layer (EDL). Compared to traditional EDLs, the electrocatalytic siblings feature partially charged chemisorbates that could blur our long-held views of surface charge densities and differential capacitances—two interrelated quantities shaping the crucial local reaction conditions. Herein, five variants of surface charge density and three variants of differential capacitance in the presence of chemisorbates are defined and compared. A semiclassical model of electrocatalytic EDLs is developed for a quantitative analysis of the differences and interrelationships between these variants of surface charge densities and differential capacitances. It is revealed that the potential- and concentration-dependent net charge on these chemisorbates dramatically changes the surface charge densities and differential capacitances. Specifically, the free surface charge density could decrease as the electrode potential becomes more positive, implying that the corresponding differential capacitance is negative. The relationship between the free and total surface charge densities is analyzed with aid of the concept of electrosorption valency. By linking the electrosorption valency with the differential capacitance in the absence of chemisorbates, we explain the potential and concentration dependence of the former. The conceptual analysis presented in this work has important implications for experimental characterization and first-principles-based atomistic simulations of electrocatalysis and EDL effects. Particularly, we disclose a hidden yet potentially crucial disadvantage of widely employed atomistic simulation models that fix the coverage of chemisorbates. Proposing the self-consistent implicit model as an expedient remedy for this disadvantage, this work contributes to more realistic modeling of electrocatalytic EDLs.