Electrolyte Ion Transport during Charging and Discharging in a Graphene -- Potassium Chloride (KCl) Electric Double Layer Capacitor (EDLC): A Molecular Dynamics Modeling Investigation

Supercapacitors have emerged as an important class of energy storage devices that can provide rapid release of energy within short time duration. Compared to conventional batteries, these have high power densities. The critical process of charging and discharging in a supercapacitor is based on the transport of associated electrolyte ions. In this thesis, ionic transport at the molecular level is investigated via molecular dynamics (MD) modeling. Specifically, the charging and discharging process in a graphene - potassium chloride (KCl) electric double layer capacitor (EDLC) is modeled and studied. A systematic preliminary study was performed to establish the molecular modeling configuration and parameters for the Graphene - KCl EDLC. Transient dynamics analysis of the charging process was performed by the application of a potential difference across the two graphene electrodes. MD analysis results showed transient progression of K+ and Cl- ions forming a double layer configuration with this applied potential difference. The surface ion concentration of K+ and Cl- ions in the double layer region was employed to obtain predicted representative capacitance values for this graphene - KCl EDLC system. The discharging process in the MD analysis was emulated through a quasi-static discharging model by decrementing the charge on the graphene electrodes. A series of MD simulations were completed starting from the charged graphene - KCl EDLC system subjected to discrete reduced electrode charges. MD analysis results showed a systematic disappearance of the formed double layer during charging. Discharging showed a reversal of the ion transport that occurred during charging with a decrease in the electrolyte ion concentration in the double layer region along with a simultaneous increase in the bulk ion concentrations. The distance between average centers of masses of K+ and Cl- ions for each quasi-static simulation was computed representing an equivalent voltage change behavior during discharge. This center of mass distance exhibited a decreasing characteristic that is consistent with typical behavior observed in an EDLC. Further studies were performed to understand variations in double layer formation with changes in applied voltage, electrolyte concentration and temperature during charging. All results showed formation of double layer in all cases, and effects of these parameters on electrolyte ion transport and predicted representative capacitance values are presented.