Detailed characterisation of solid-electrolyte interfaces is essential for the better design of energy storage systems, particularly those such as supercapacitors that undergo rapid charge/discharge cycles. Computational simulations using molecular dynamics (MD) offer powerful tools to resolve the microscopic structural features and dynamics of solution species at interfaces. Conventional simulations, however, are usually carried out in thermodynamically closed systems, where significant ion adsorption at the interface can lead to unphysical depletion of the bulk solution. Here we demonstrate a novel pseudo-open boundary technique, called constant chemical potential MD (CμMD), and apply it to simulate the graphite basal plane–NaCl(aq) interface over a wide range of bulk solution concentrations. Our model was validated through calculation of potential of zero charge (PZC) and specific capacitance values, which we compared to experiments at freshly cleaved HOPG surfaces. Our results show that the spatial extent of the double layer region is surprisingly insensitive to bulk ion concentrations; increasing the bulk concentration only led to a denser, more ordered interfacial region in the solution phase. This affected the screening of charges throughout the double layer region, which in turn affected the electrochemical properties of the interface. Charge ordering in layers led to fluctuations in the measured electric potential, which represents a significant departure from the simple Poisson-Boltzmann-based models usually used to predict these properties. With ion concentrations in the double layer region in some cases up to three times greater than in the bulk, we discuss the effect of the observed spatially dependent potential on salt precipitation. Consistent with experimental measurements, a negative change in the PZC with increasing bulk concentration was observed. Introducing surface charges to the graphite basal plane allowed us to compute the capacitance in the double layer. In good agreement with experiments, we observed a propensity to store greater amounts of ionic charge when negative surface potentials were applied. We demonstrate that both of these effects can be completely explained mathematically by considering the over-screening of charges in the double layer. Our approach offers a new paradigm to explore a host of phenomena at complex interfaces with atomic resolution.

Characterising the structure and electrochemical properties of a graphite-solution interface using constant chemical potential molecular dynamics

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.