Nanoscale Insights Into Mechanism Of Graphene Based Supercapacitor And Experimental Optimization

The detrimental long-term effects of greenhouse gase missions and rapid depletion of fossil fuels underscore the urgency of exploring and scaling up renewable power resources and related energy generation,storage as well as conservation technologies.In this regard,electric double-layer capacitors(EDLCs),a type of novel electrochemical devices that store energy through reversible adsorption of ionic species at highly porous electrode surfaces,have garnered substantial attentions.Compared with conventional batteries,EDLCs experience some extraordinary characteristics,such as high power density(>10 kW kg-1),ultrafast charging/discharging rate(<30 s),superior lifespan(>105 cycles)and wide operating temperature window(-40?85?)?To advance the EDLC performances,the active materials have been developed from conventional porous structures(e.g.,activated carbons)to low-dimensional nanomaterials,e.g.,two-dimensional graphene.The electrode nanostructures have stimulated many abnormal phenomena at the nanoscale,e.g.,edge effects and size effects.Under the strong electric field induced by surface charges(i.e.,electrons),the charge storage capability of nanostructure based EDLCs are predominately determined by the microscopic ion distributions and transport behaviors within the nanoscale pores.As new nanomaterials are developed,new challenges emerge.The abnormal effects,e.g.,size and edge effects,could not be predicted by the traditional Goup-Chapman-Stern theory.Finite element method(FEM),based on solving the partial differential equations or a continuum model(e.g.,Navier-Stokes equations)fails to capture the behaviors of molecular or ions at the nanoscale.Classic molecular mechanics(e.g.,Monte Carlo method),based on random motion,can only describe the static properties of systems.Besides,density functional theory(DFT),based on quantum mechanical calculation,are usually employed to predict the electronic properties of tens of atoms owing to the huge computational costs,while molecular dynamics simulation(MD)fail to describe the quantum effects.In this work,a combined DFT&MD multi-scale simulation is proposed to explore the charge storage mechanisms of grapheme based EDLCs,aiming to elaborate the unusual edge effects and size effects from electron and atomic level.Besides,we propose a plasma based method to prepare new nanomaterials to overcome the great challenges stemmed from traditional chemical reduction methods,e.g.,easy agglomeration,low surface area,and uncontrollable pore structure/size of grapheme materials.The as-obtained mechanisms will be instructive in experimentally designing the morphology/structure of graphene and optimization of electrolytes for high performances of EDLCs.A multi-scale simulation is proposed to investigate the underlying mechanisms of edge effects.First of all,graphenes with different lengths(i.e.,edge ratios)are built.DFT and Bader method are employed to explore the realistic surface charge distributions of graphenes,in which obvious edge effects are observed.The as-obtained initial charge distributions are subsequently applied to the MD systems.A comprehensive evaluation of the EDLC structures(e.g.,number density,free energy barriers,EDLC thickness and ion separation degree)was further carried out to unveil the charge storage mechanisms.As for multilayer graphene edges,the influences of interlayer spacing(from 3.4 A to 5.0 A)on the capacitive and dynamic behaviors are explored.Especially,the contributions of ions and solvents to the achieved capacitance are quantified individually by decomposing the total electric potential distributions.Different with monolayer graphene edge and traditional views,a novel mechanism dominated by solvent effects is proposed for the first time.The as-obtained novel perspectives can provide instructive strategies for preparing active materials with rich edges to further advance the optimization of EDLCs and meanwhile highlight the crucial role of solvents in determining capacitive performances.On the other hand,the effects of graphene channel width on the charge storage behaviors are investigated.For nonconfined space(planar graphene),the ion size,valences and mixtures manifest a negligible effect on the electrode capacitance,going beyond traditional Helmholtz theory.Thus,a kinetic-dominated charging mechanism within representative aqueous electrolytes is proposed,highlighting that regulating the ion-solvent interactions could be a new avenue to manipulate the electrolyte dynamic behaviors.As for confined graphene space,an abnormal capacitance enhancement is observed,well corroborating the experimental results.Besides,we point out that ion accumulating in a single-file structure(i.e.,monolayer)could maximize the charge storage capability,in which the optimized channel width for energy storage is obtained:Crystal diameter dc<Channel width d<Solvation diameter dhExcept the edge and size effects,the influences of surface wettability on the charging mechanisms of graphene nanometer channels are investigated.By means of varying the Lennard-Jones parameters of electrolyte-electrode interactions,the interfacial wettability of graphenes(quantified by contact angle ?)is transformed from hydrophobic(?=142.5°)to hydrophilic state(?=48.3°).With integrating the Poisson equation,the charge storage capability of nonconfined and confined space is obtained.In stark contradiction with the long-held notions(i.e.,monotonic trend),an asymmetric bell-shaped capacitance-wettability curve(i.e.,nonmonotonic pattern)is demonstrated for the first time,which yields anomalous capacitance enhancement at an intermediate wetting state.Similar results are also observed on the solvent-free ionic liquids([BMI][PF6]).We highlight that neither super-hydrophobic nor super-hydrophilic materials are beneficial for reinforcing the EDLC's performances,while moderate surface wettability is highly preferable for aqueous solutions.The as-obtained molecular-level insights demonstrate that wettability property of electrode materials should be precisely controlled to achieve optimal energy storage of EDLCs.The as-obtained mechanisms from DFT&MD simulations provide instructive strategies for designing the electrode morphology and optimizing the electrolyte properties experimentally.In this work,new graphene nanostructures(i.e.,vertically-oriented graphenes,VGs)are prepared with plasma-based approaches,exhibiting many unique features,such as vertical orientation on the substrate,non-agglomerated three-dimensional inter-networked morphology,controlled inter-sheet connectivity,as well as exposed ultra-thin edges.Through adjusting the growth time,an obvious capacitance enhancement of-4.3 fold is demonstrated.With regulating the plasma sources,the electrode capacitance is improved by-2.3 times.Besides,a novel method of solvent addition is proposed to great challenges of room temperature ionic liquids(e.g.,high viscosity,ultra-low conductivity and dynamics),which significantly enhance the capacitance by almost 100 times.Besides,an optimum concentration of ionic liquids for energy storage is also obtained.Together with metal oxide(i.e.,MnO2),VGs are also employed for improving the performances of Pseudo-capacitors.The capacitive electrodes are fabricated by an in situ growth of VGs on the buckypaper(made of carbon nanotubes)followed by electrochemical deposition of MnO2.VG growth could prominently increase the accessible surface area and charge transfer pathways,demonstrating an excellent energy density of 41.3 Wh kg-1 and high power density of 31.3 kWkg-1.High-throughput,scalable fabrication of graphene is an essential step for industrial-scale applications.With the customized microwave plasma-enhanced chemical vapor deposition system,large-scale fabrication of VGs is achieved.After short time growth,high-quality graphene samples(size up to 15×15 cm2)could be obtained,which may be applied to a wide range of applications in energy storage and conversion(e.g.,Li-ion battery and flow battery),water purification/transportation,and biomedical systems.