Monte Carlo simulations of Electric Double Layer Capacitors with aqueous electrolytes

Abstract

Electric double layer capacitors (EDLCs) also known as supercapacitors are a category of energy storage devices that are known for their remarkable power delivery (upto 1000KW/kg). The energy storage is through physical adsorption of ions on the surfaces or inside the pores of electrode forming electric double layers. This simple charge storage mechanism leads to a lifetime of > 1 million cycles and cyclic efficiency of >90%. However, they are limited by their energy densities which don't exceed 10Wh/Kg. The specific energy can be improved by increasing the capacitance of the EDLC. The differential capacitance of an EDLC holds complex relation with properties of electrode, electrolyte, and operating conditions. The Molecular simulations studies on EDLCs help uncover these microscopic phenomena that govern the capacitance in EDLCs.

The molecular simulations of EDLC requires simulation of positive, negative electrodes and electrolyte, which is usually done in single simulation box. This restricts the size of the electrode in one of the dimensions making it finite in that direction and limits the simulated EDLC system sizes to nano meters. The problem with system sizes can be alleviated using Gibbs Ensemble Monte Carlo simulation technique where, both the electrodes are simulated in separate boxes and each box is periodically repeated in three dimensions eliminating any system size discrepancies. Furthermore, Grand canonical ensemble is used to avoid the simulation of bulk electrolyte phase. The electrodes are subjected to constant external potential difference to replicate the experimental conditions.

In this work, Gibbs ensemble-based Monte carlo simulations on EDLC are performed with aqueous NaCl solution as electrolyte and Graphene as electrode for a set of operating conditions. Graphene is modelled as slit-pore type electrode . The water molecules are explicitly simulated using Continuous Fractional component Monte Carlo method to include the hydration effects of ions by water. Differential capacitance is computed within voltage of 0-2V for pore widths of 0.65nm, 0.79nm and 0.9nm at temperatures of 416K and 350K for electrolyte concentration of 2.1M. The simulations help understand the effect of pore width, temperature, and voltage on the performance of EDLCs.