Mechanism Of Energy And Mass Transfer Enhancement On Edges Of Vertically Oriented Graphenes And Energy-related Applications

Efficient energy storage and conversion technology is regarded as a fundamental technical support to promote the large-scale application of renewable energy.In recent years,carbon-based energy nanomaterials have triggered enormous attention due to their abundance in resource,low cost and controllable morphology.Energy and mass transfer processes within and at the interface of energy materials are the key physical mechanisms that determine the performance of energy storage and conversion.Previous studies have considered the nanoscale energy and mass transfer of carbon-based energy nanomaterials conform to the typical structure-property relationship.Edge structures widely exist in graphene quantum dots,carbon nanotubes,graphene,molybdenum disulfide and other energy nanomaterials.However,the exceptional properties derived from the edge structure are indistinctly branded as"edge effect",which require further investigation.In this work,the edge-enhanced energy and mass transfer mechanisms of oriented carbon-based nanomaterials are systematically investigated.The researches mainly focus on the following two aspects.For mechanism study,the light-induced field enhance effect near graphene edge sites is revealed for the first time.And the correlation between edge structure and electron polarization behavior is depicted by combining advanced experimental methods such as near-field nano-imaging technology,in-situ electrochemical quartz crystal microbalance(EQCM)measurement,and theoretical calculation methods such as density functional theory(DFT)calculation and molecular dynamics(MD)simulation.Furthermore,the ion distribution and transport near graphene edges are estimated,and the edge-enhanced mechanism of the solid-liquid interfacial phase equilibrium was quantitively analyzed.For application research,based on the as-synthesized vertically oriented graphenes(VGs)with adjustable edges,high-performance photocatalytic hydrogen production,capacitive deionization and supercapacitor energy storage are proposed.VGs prepared by plasma-enhanced chemical vapor deposition(PECVD)have the merit of controllable edge morphology.As a result,the edge density of VGs has been effectively manipulated by argon plasma bombardment.The distribution of electron density at the closed edges of graphene is studied using DFT calculation,revealing the spontaneous aggregation of electrons at the edge sites.Dark-field scanning Kelvin probe microscopy(SKPM)measurements are consistent with the DFT calculation results,which further confirm the non-uniform charge distribution of graphene surface and its strong dependence on the surface nanomorphology,i.e.,the electron redistribution at the curved edges of the VGs.In this thesis,the photoelectric response up to 92 m A cm^-2 of VGs has been demonstrated for the first time.These edge-induced photocurrent responses may originate from the directional migration and accumulation of photoexcited hot electrons,which are distinct from the photocurrent responses in semiconductor materials.Photo-induced force microscopy(Pi FM)measurements validated the significant near-field photoinduced force gradient on the nano-edges of VGs,which is caused by electron redistribution.It is also found that the electric field enhancement near the edges is related to the wavelength of the incident light.Under infrared(IR)excitation,distinct"hot spot"Pi FM signals can be observed at the nano-edges.Further theoretical analyses indicate that the electric field enhancement effect at the nano-edges can be decoupled into two factors,i.e.,the non-resonance enhancement effect and resonance enhancement effect.The latter dominates the total edge electric field enhancement and can be controlled by the shape(aspect ratio)of nano-edges,Fermi level of graphene nano-edges and the specific excitation wavelengthThe nano-edges can serve as highly active reaction sites due to the photoinduced electric field enhancement(PEFE)effect.Specifically,a built-in electric field would form at VG edges and interfaces after loading the semiconductor photocatalysts,promoting the separation of photogenerated electron-hole pairs.Based on this mechanism,highly dispersed mesoporous graphite carbon nitride/vertically oriented graphene photocatalysts(GVN/NVG)are prepared by nano-confined synthesis.DFT calculations indicate the localized surface charge distribution and reduced bandgap of GVN/NVG samples compared with bulk graphite carbon nitride(g-C₃N₄)samples.The photocatalytic hydrogen evolution activity of GVN/NVG samples reaches 41.7?mol h^-1 cm^-2(equivalent to 225 L m^-2 per 24 hours,STP)under full-spectrum illumination and without co-catalyst,which is one order of magnitude higher than the bulk g-C₃N₄(2.5?mol h^-1 cm^-2).The PEFE effect is further extended to the concept of solar nanoionics(SNI),which directly feeds solar energy into the common ionic system and achieves enhanced and controllable ion transport by the solar-excited photocarriers and electric fields localized near the VG edges.The utility of the SNI concept is demonstrated in a typical and critically important nano-ionic system–capacitive deionization(CDI).The localized photo-electric fields control and restructure the nano-ionic flows leading to selective transport of ions with different mobility and dramatically enhanced kinetics,as validated by in situ electrochemical quartz crystal microbalance measurements and molecular dynamics simulations.With the photo-enhanced ionic transport kinetics,an average adsorption capacity of 33 mg g^–1 at 200 mg L^–1(165 mg g^–1 at 5000 mg L^–1),a fast adsorption/desorption response and impressive efficiency are achieved.Benefiting from the high electrochemical activity of the plasma-made edges and extensive adaptability to the growth substrates,a vertically-oriented foam(VGF)supercapacitor using mixed room temperature ionic liquid(RTILs)to realize AC line filtering application with fast frequency response and ultrahigh energy density is demonstrated.VGF supercapacitors using mixed RTILs could realize ultrahigh areal energy density of 1.23?Wh cm^–2at 120 Hz and fast frequency response(?_RC=?1.3ms).The work proposes an effective solution to the contradiction between energy density and power density.On the one hand,RTILs with high electrochemical stability are used as electrolytes to achieve high cell voltage.On the other hand,the hierarchical structural design principles of electrodes are followed to accommodate the high viscosity of RTILs.As a result,the advantageous electrochemical activity of graphene edges is fully utilized to achieve high-frequency response performance.