(360ad) Electrical Double Layer Capacitance and Polarizability Modeled Using Classical Molecular Dynamics.

Electrical double layers (EDL), which form in response to surface charge on an electrode in contact with electrolyte solution, influence the thermodynamics and kinetics of electrochemical processes. Atomistic modeling plays a key role in elucidating the physical characteristics of the EDL, which in turn can inform rational design and optimization for various important electrochemical systems such as electrocatalytic energy storage, capacitors, and fuel cells. In this poster presentation, we will present a classical molecular dynamics (MD) model for the EDL at the metal electrode and aqueous electrolyte interface. More specifically, we measure: 1) the double layer polarizability by calculating the dielectric constant and 2) the double layer capacitances at varying electrode potentials. In performing these measurements, we introduce two novel “add-ons” to the MD simulations: first is the description of the metal polarizability that we termed “QDyn,” second is an alternating electric field (AC field) approach for measuring the dielectric response of water. For in-plane dielectric constant, we found the first water layer (z < 0.5nm) to exhibit a larger-than-bulk value (ε≈110). In contrast, the out-of-plane dielectric constant of this first layer is abnormally low (ε≈3). Next, we measure the double layer capacitance by varying the surface charge and assessing the corresponding electrostatic potential at the surface. Our MD-measured capacitance gives excellent agreement with the classical Gouy-Chapman-Stern (GCS) theory yet deviates from experimental values. We attribute this experimental deviation to the lack of charge spill-over from the electrode, which is a quantum effect that is missing from our classical model. Regardless, the dielectric constant and capacitance measurements made from our classical MD model are self-consistent through the GCS theory. Overall, our model provides a fully atomistic description of the EDL at adequate time and length scales, a feat that other state-of-the-art modeling tools still struggle with. As future work, this classical MD model will be combined with ab initio description of charge transfer processes to paint a more complete picture of the effect of EDL in electrochemistry.