Abstract
Electric double-layer supercapacitors (EDLCs) are electrochemical energy storage devices that play a major role in today’s energy landscape. Compared with the mainstream lithium batteries, EDLCs offer faster charge/discharge rates and improved safety because of their energy storage mechanism. Moreover, EDLCs using solid polymer electrolytes offer the possibility to develop multifunctional structural energy storage devices. However, it has been experimentally observed that capacitive performance dramatically decreases when we switch liquid electrolytes to solid polymer electrolytes in EDLCs. The fundamental mechanism behind this phenomenon is difficult to characterize experimentally. In this work, the electrode-electrolyte interface in EDLCs is studied using molecular dynamics (MD) simulations which can reveal atomic behaviors that are not experimentally measurable. Two EDLC models are constructed with graphene electrodes and two different electrolytes, including an aqueous solution-based liquid electrolyte and a polymer-based solid electrolyte. Both systems are configured to contain the same salt concentration. The modeled systems are simulated via the LAMMPS package using the NPT-NVT (equilibration and production respectively) ensemble. Molecular phenomena such as the trajectories of atoms, Inner and Outer Helmholtz Layers (IHL and OHL), ion distributions, and electrode-ion screening effect are characterized at various applied charge levels. The results show that the polymer electrolyte system exhibits an IHL similar to that observed in liquid electrolyte EDLC systems that have been studied in literature. Its also observed that the concentration of ions at the electrode-electrolyte interface is lower for the polymer electrolyte than for the aqueous electrolyte. In addition, the ion-electrode distance in the polymer electrolyte is greater compared to that in the aqueous one. Ionic conductivities of the two systems are determined by both experimental measurements and simulations to validate the feasibility of MD simulations. These observations give us new molecular insights to the interface of polymer electrolyte-based EDLCs.