Probing dynamics and impedance of inhomogeneous electrolytes with alternating-current non-equilibrium molecular dynamics (AC-NEMD) Simulations

Abstract

The alternating current non-equilibrium molecular dynamics (AC-NEMD) simulation technique is applied to several cases of inhomogeneous electrolytes. A sinusoidal external electric field is applied during the molecular dynamics simulation of an electrolyte, the ionic current response varies in magnitude and phase shift depending on the frequency and magnitude applied. The frequency dependent current response reveals the relaxation dynamics of the electrolyte. The electrolytes studied include aqueous electrolyte confined in a cylindrical nanopore with a smooth wall and yttria-stabilized zirconia (YSZ). The aqueous electrolyte was modeled by both a primitive model and a solvent primitive model. The YSZ electrolyte was modeled by a Born-Meyer-Buckingham (BMB) model. While the two cases are very different in geometry and molecular details, both show similar features in the AC-NEMD current response with a maximum conductivity at an optimum frequency and resistance-capacitance-inductance (RLC) circuit behavior. For ions confined in a cylindrical channel, the capacitance behavior can be explained by confinement effect and single-file motion of ions. On the other hand, the barriers along the passage between oxygen vacancy sites in YSZ may provide a similar effect of “confinement”. In the AC-NEMD, the generated ohmic heat is effectively removed, and a constant temperature is maintained by either an Anderson thermostat or a Nose-Hoover thermostat.