Study Of Energy Storage Mechanism In Supercapacitors By Constant-Potential Molecular Dynamics Simulations

Energy storage technology can help achieve a balance between power production and consumption.It plays a crucial role in pushing forward the revolution of the energy sector and promoting sustainable energy development.As an advanced electrical energy storage device,electrical double layer capacitors,also known as supercapacitors,have advantages of their high-power density,fast charging process,wide operating temperature range and ultralong cycle life.Over the past decade,the performance of supercapacitors has greatly improved,as new electrode and electrolyte materials have been implemented and gained an active role.Thus,understanding the mechanisms underlying charge storage in the electrode/electrolyte interfaces is important for further development of supercapacitors with high energy and power density.Molecular dynamics simulation is an effective theoretical tool to study the energy storage mechanism in supercapacitors.In this dissertation,we introduce the electrical potential coupling algorithm in molecular dynamics to achieve the real-time update of the charge distribution on electrode surfaces during simulation processes,thus being able to deliver accurate molecular insights of the solid/liquid interfaces and their charging dynamics.Using this constant potential method,molecular dynamics simulations are performed to pursue the optimization of common supercapacitors and investigate the charge storage mechanisms of new electrode/electrolyte materials.The main results of the dissertation are summarized as follows.(1)To address the demand for theoretical studies on electrified solid-liquid interfaces,the constant potential method,which corresponds to reality,has been introduced and implemented in the open source molecular dynamics software.Results show that the traditional constant charge method cannot provide reliable results in the complex systems containing small electrolyte ions or nanoconfinements.This study demonstrates the importance of using constant potential method in the molecular dynamics simulations of supercapacitors.(2)Using the developed constant potential molecular dynamics simulations,the capacitive performance and charging dynamics of the microporous carbon electrode in aqueous electrolytes has been investigated.Simulation results reveal that the subnanometer carbon pores can provide high energy densities when using as electrodes.Moreover,the charging pattern of microporous carbon transitions from solely counter-ion adsorption to ion-exchange and the charging process speeds up as the pore size increases.This study also exhibits the promising potential of the subnanometer carbons in capacitive deionization as they in nature are permselective.(3)A nanoscale-to-macroscale simulation framework combining constant potential molecular dynamics simulations,transmission line theoretical model as well as circuit simulations has been developed.This multiscale simulation method is implemented to study the energy storage and power delivery of conductive metal-organic framework(MOF)electrodes in supercapacitors with room-temperature ionic liquids(RTILs)as electrolytes.Such a multiscale method can deliver simulation results that are quantitatively in agreement with experimental data.The study not only demonstrates that using conductive MOFs as electrodes can provide exceptional performance,but also points out a blueprint of seeking the optimal combination of MOFs and RTILs via high throughput screening.(4)In the last part of the essay,we use constant potential simulations to investigate a practical challenge in the use of room-temperature ionic liquids in supercapacitors– the electrosorption of impurity water on the electrode surfaces.The simulations disclose that the hydrophilic ionic liquids can maintain their electrochemical chemical windows even if the impurity water exists.On the contrary,water in hydrophobic ionic liquids tends to accumulate at the charged electrode surface,thus limiting the working voltage of supercapacitors.This study can help facilitate the realistic use of ionic liquids in the electrochemical applications.