Helmholtz Capacitance of Aqueous NaCl Solutions at the Au(100) Electrode from Polarizable and Nonpolarizable Molecular Dynamics Simulations

Aqueous NaCl solutions at gold electrodes serve as prototypical electrochemical interfaces. A fundamental property of these interfaces is the differential capacitance, from which structural information and double-layer response to external voltage are often inferred. However, many physical effects contribute to the capacitance including solvent and metal structural rearrange-ment, specific ion adsorption, and electron spillover, and separating these contributions is difficult. In this regard, computer simulations can play an important role to separately characterize these different contributions and aid in the interpretation of electrochemical capacitance data. In this work, we utilize fixed voltage molecular dynamics simulations to characterize the structure and capacitance of the Au(100) and NaCl aqueous electrolyte interface at various applied voltages, comparing predictions from both polarizable and nonpolarizable force fields. We explicitly compute the double-layer contribution to the capacitance in the absence of specific ion adsorption, surface reconstruction, or electronic effects. We find that due to the reorientation of water molecules at the electrode surface, the Helmholtz/inner layer capacitance increases at negative potentials relative to the potential of zero charge, while decreasing at positive potentials. Interestingly, while polarizable and nonpolarizable force fields predict similar capacitance profiles, they differ in their prediction of microscopic structural response. For example, chloride contact ion pairs are predicted at positive polarization with the nonpolarizable force field but not with the polarizable force field. Overall, we find that the inner layer capacitance for all salt concentrations is well-described by a generalized Helmholtz model that incorporates a voltage-dependent, dielectric constant for the Stern layer.

Automated summary

Aqueous NaCl solutions at gold electrodes serve as prototypical electrochemical interfaces. Computer simulations can help interpret electrochemical capacitance data, and our study found that the inner layer capacitance can be described by a generalized Helmholtz model.