In Situ Experimental Measurement And Molecular Dynamics Simulation On Energy And Mass Transfer During Electrostatic Adsorption Process For Various Electrolytes Inside Graphene Two-dimensional Nanochannels

Electrostatic adsorption at the solid-liquid interface is a basic physical chemistry phenomenon.A typical application is supercapacitor,which has the advantages of ultra-fast charge-discharge rate,ultra-high power density and ultra-long cycle life.Because of these advantages,it has broad application prospects in the fields of new energy power generation system,electric power transportation,national defense and military affairs.With the development of new nanoelectrode such as graphene and new electrolytes such as gel polymer electrolyte,room temperature ionic liquids electrolyte and“water in salt”electrolyte,the unusual behaviors of electrolyte ions are observed during the process of electrostatic adsorption due to the uniquely crowded intermolecular forces under severe nano-confinement.In this case,the conventional theory based on infinite plate space and the mean field method can't explain these abnormal phenomena effectively and accurately.Thus,it is urgent to understand the energy and mass transfer process of the electrostatic adsorption,which will help provide theoretical guidance for the design of high-performance supercapacitors for various applications.Here,the In Situ electrochemical quartz crystal microbalance?EQCM?technique and molecular dynamics?MD?simulation are used for revealing the mechanisms of ion transport and ion distribution.We focus on the energy and mass transfer process during the electrostatic adsorption at the solid-liquid interface for the system of“various kinds of electrolytes/two-dimensional nanochannel structures”and develop the related theories.As for the aqueous electrolyte/graphene nanochannel system,the ion behaviors during the energy and mass transfer of electrostatic adsorption for activated carbon??1.13 nm?and graphene??1.07 nm?with similar pore size/interlayer spacing are measured by EQCM measurements.It is found that the graphene involves single-ion dominated ion exchange,which is different from the counter-ion adsorption of activated carbon.Such difference can be attributed to the various ion dynamics properties inside activated carbon and graphene.Specifically,the tortuosity of ion diffusion path arising from the disordered arrangement of pores in activated carbon decreases the diffusion coefficients of ions by over two orders of magnitude compared with bulk electrolyte.As such,the movements of ions in bulk electrolyte dominate the process of electrostatic adsorption?i.e.,counter-ion adsorption?.On the contrary,a short ion diffusion length with a well-defined layered structure of graphene allows a fast ion diffusion and the ions can move freely in and out the nanochannels.Thus,the ions with a high diffusion coefficient can dominate the process of electrostatic adsorption.Further ion behaviors and ion diffusion coefficients for graphene with various interlayer spacings from 0.4nm to 2.0 nm are examined by EQCM measurements and MD simulation,we first reveal that the difference of diffusion coefficient between anions and cations determines the ion behaviors during process of electrostatic adsorption.Such conclusion is also verified in various aqueous electrolytes and provides a new effective strategy for modulating ion behaviors in the process of electrostatic adsorption.As for the gel polymer electrolyte/graphene nanochannel system,we establish the interfacial ion transport model based on EQCM measurements and thermodynamics theory.The lithium chloride?Li Cl?and Polyvinyl Alcohol?PVA?are used as typical electrolyte and polymer.During the process of electrostatic adsorption,the real-time fluxes of cation and anion in and out of graphene nanochannels are quantitatively obtained.It is found that the hindrance of PVA to cation transport is more obvious than that of anion owing to the strong interaction between cation and-OH groups in PVA.The distributions of gel polymer electrolyte?including Li Cl,PVA and H₂O?are also studied by MD simulation.It is found that PVA exhibits“size effect”and“electrostatic shielding effect”.The“size effect”means that PVA occupies the interface space and impedes the ions,which can reduce the capacity of energy storage.The“electrostatic shielding effect”means that PVA increases the interface permittivity and separates the anions and cations effectively,which can enhance the capacity of energy storage.As such,the energy storage performance first increases and then decreases with the increase of PVA concentration.As for the room temperature ionic liquids electrolyte/graphene nanochannel system,we combine the MD simulation and transmission line model to calculate the transport impedances of ions inside graphene nanochannels during the process of electrostatic adsorption.It is found that the ion impedances oscillate significant with the increase of graphene potential.Such phenomenon is related to the relative strength between solid-liquid interface interaction and ion interaction at different stages of electrostatic adsorption.Besides,the interfacial ion transport model based on MD simulation and thermodynamics theory are established to examine the effect of charging rate on the ion behaviors.At a low charging-discharging rate,the response of charges stored on graphene electrode and ion flux in and out of the graphene electrode?i.e.,counter-ion adsorption and co-ion desorption?to the time?potential?is very close to that under thermodynamic equilibrium state.At a high charging-discharging rate,however,the co-ion desorption deviates from the thermodynamic equilibrium state significantly.Thus,the main factor for the decrease of the energy storage capacity during the process of electrostatic adsorption at a high charge-discharge rate is the hindrance of co-ion movement inside confined spaces.As for the“water in salt”electrolyte/graphene nanochannel system,we use MD simulation to conduct a comprehensive comparison of the distribution of ions and solvents between"water in salt"and"salt in water"electrolytes inside graphene nanochannel with lithium ditrifluoromethane sulfonamide?Li TFSI?as the electrolyte.A significant difference of cation distribution inside negatively charged nanochannels is observed for these two electrolytes,which is mainly attributed to the different number of free water molecules.In addition,the effect of nitrogen-doped graphene on the distribution of ions and solvents is negligible due to the strong interaction between electrolyte and graphene inside confined spaces.