| The electrical double layer structures(EDLs)is widely available in the charge separation at the solid-liquid interface which is common in micro/nano devices,supercapacitors and many important processes of microfluidic and energy transformation.It plays an important role on the performance of the relevant microfluidic and energy-storage devices.Especially,in the micro field,many different phenomenon appear,such as the pronounced viscous dissipation effect and the quantization of mass and energy.All of these bring the unique features for the mass and energy transformation of EDLs at the solid-liquid interface compared with those in the macro scale,which has already attracted lots of attentions.Therefore,the work launched in this field shows not only great scientific significances for the development and improvement of mass and energy transformation theories,but also high values of practical applications for the control and optimization of mass and energy transformation relevant to EDLs.The size effect appears in the micro wold,surface morphology shows more significantly influence on the microscale mass transportation and energy storage processes in EDLs.While,the mechanisms of surface roughness effects on the mass transportation and energy storage of EDLs at the solid-liquid interface are not completely understood.Especially,the micro structures of EDLs at the rough solid-liquid interface still need further study.The effective control of microscale mass transportation and energy storage still need further exploration.Therefore,this work carried out the seriously study on the mechanisms of the mass transportation and energy storage in EDLs revelant to the electroosmotic flow and supercapacitors by molecular level simulation and experimental methods.We constructed the molecular models of solid-liquid interfaces for electroosmotic flow and supercapacitors,explored the distribution and motion details of micro particles,illustrated the mechanisms of the mass transportation and energy storage in EDLs,demonstrated the influences of solid roughness and roughness dimension.Qn the EDL structure and further on the electroosmotic flow properties,analyzed the migration of electrolyte ions at electrode surface with different surface roughness and charges,and showed the relationship of surface roughness and electrolyte species on the control of mass and energy transformation.In addition,the experimental characterization and test of the graphene-based supercapacitors is conducted in our work,for further study the influence of the electrode roughness on the capacitance.Major results and conclusions are listed as follows:(1)The molecular models of electroosmotic flow with rough nanochannels are constructed,and the molecular dynamics simulations are carried out to explore the movement of fluid in nanochannels,the mechanisms of the mass transportation at solid-liquid interface are explored,the movement of NaCl aqueous solution driven by external electric field in the nanochannels with smooth bottom wall,rectangular rough bottom wall and fractal cantor rough bottom wall are compared,the distribution and motion details of micro particles near different rough surfaces,as well as the relationship between electroosmotic velocity,flow potential and the roughness height and dimension,are analyzed.It shows that there is a layering distribution of water molecules and Cl-ions in the near wall region;For the rectangular rough nanochannel,the anions are concentrated near the wall surface and even trapped into the concave region because of the positive charged wall surface,while the cations gather in the bulk solution,the existence of roughness reduces the oscillation amplitude of the water molecules and Cl-ions at the first and second layer adjacent to the solid wall;No matter whether the surface is rough or smooth,when the external electric field is added,a plug-like velocity profile is observed in electroosmotic flow,and the presence of surface roughness reduces the electroosmotic velocity and zeta potential in nanochannel,the trend is pronounced for the channel wall with higher roughness height;In addition,for the cases with fractal cantor rough bottom wall,with the same roughness height,the fractal dimension is considered for the electroosmotic flow,the results suggest that with the increasing of fractal dimension of roughness bottom wall,both the electroosmotic velocity and the zeta potential decrease.(2)The solid-liquid interface model of graphene-ionic liquid and the graphene double layer model are constructed,which are numerical analyzed by the molecular dynamics method.The mechanisms of the energy storage at solid-liquid interface are explored,the influences of charge density,electrode morphology and channel width on the distributions of ions and molecules are illustrated,the capacitance values under different rough electrodes are compared,the interaction among cations,anions,and electrode are also analyzed.In addition,the adsorption and migration of Li+ cation on the graphene surface are finally calculated by first-principles method.The study shows that the traditional compact-diffusion EDL structure seems not tenable any more for ionic liquid.Because of the strong electrostatic interaction among cations and anions,ionic liquid shows more significant layering distribution than inorganic electrolyte solution outside the electrode,the ions layers can be extended deeper into the bulk electrolyte solution by the larger interaction from the rough electrode;For big[EMIM]+ cations,the significant steric effect dramatically reduces its migration ability and thus weakens the electrostatic repulsion and attraction,especially on the rough electrode surface;For the small[BF4]-anions,they usually stack in the space among cations,the limitation of electrode grooves on the anion are not pronounced,thus they are sensitive to the change of electrode charge;For graphene-ionic liquid capacitance,due to the greater effective contact area between the ions and electrode,the rough electrode possesses a larger capacitance than the non-rough one;What's more,the camel-shape C-V curve for the rough electrode is asymmetric,i.e.the capacitance increases more significantly when the electrode is positively charged,which is attribute to the densely stacked[BF4]-anions on the positively charged electrode surface;For the LiClO4 in methyl acrylate solvent,the ions form the clusters because of the electrostatic attraction.It is necessary for one ion to strip the counterions around it when the ion burst into the narrow graphene gap,which needs to overcome high energy barrier.It is benefit for the trapping of ion pairs in the wider graphene gap;In aspect of diffusion,the diffusion rate of ions in the graphene gap is lower than that in the bulk solution due to the limitation of graphene.In addition,the first-principle calculation shows the co-adsorption of Li+ ions on the graphene surface,when the Li+ ion migrates along the graphene plane,the periodic oscillated potential energy is appeared for the hexagonal structure of graphene.(3)In experiment,the graphene-based supercapacitor is developed and the corresponding test and characterization are carried out,the electrode roughness is introduced by adding the foam Ni in the supercapacitor,the influences of electrode roughness on capacitance are characterized from the aspects of C-V curve,specific capacitance and so on.It shows that the electrode roughness can indeed affect the mass and energy transformation of EDLs at the solid-liquid interface,the addition of foam Ni could increase the electrode roughness and reduce the electrochemical impedance,which is benefit for the ion diffusion and transportation.Thus,higher specific capacitance and rectangular C-V curve is observed for the graphene-based supercapacitors with foam Ni.The reduce of specific capacitance is smaller with the increasing of sweeping rate.These results mean that the graphene-based supercapacitors with foam Ni behave well electrochemical stability and high energy density.In summary,the distribution and motion details of micro particles at the EDLs in electroosmotic flow processes and supercapacitor charge/discharge processes are illustrated,respectively,from molecular/atomic level study in our work,specially,the mechanisms of surface roughness effects on the mass transportation and energy storage of EDLs at the solid-liquid interface are futther illustrated.This work could provide a strong theoretical support for the development of electroosmotic flow and the energy storage devices,and would also be valuable for relevant research. |
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