Southeast University professor Sun Zhengming Series team to make progress in the field of energy storage MXene

MXene is a class of layered material, a transition metal carbide or nitride, the interlayer sheet is mainly van der Waals force connector, having a series of excellent physical and chemical properties, e.g., hydrophilicity MXene having good, adjustable-pitch and the surface functional groups various other features. Structure, MXene the transition metal layer and a carbon layer composed of alternating, impart good electrical conductivity and MXene pseudocapacitive properties; component, compared single element two-dimensional material, M and X MXene double element comprising multiple elements (MXene solid solution), various types of covalent bond between the component and the MX gives MXene more extensive regulation of space. Rational use MXene structure and compositional characteristics, electrode material excellent performance can be prepared. Therefore, since the advent of so far, MXene outstanding performance in the field of energy storage, and high hopes. School of Materials Science and Engineering, Southeast University Professor Sun Zhengming team In MXene two-dimensional electrode material and its application in the field of energy storage carried out extensive research work, [ 123] made in super capacitor, secondary battery and flexible storage devices such as a series of studies , this year has been in the Advanced Functional Materials , published many papers on Nanoscale , 2D Materials high-impact journals force.

1, discloses a chemically modified electrode material MXene dimensional mechanism Chemical modification is an effective way to improve the electrochemical properties of two-dimensional material. Currently, for the chemical modification of graphene have carried out a lot of research work, nitrogen-doped, for example, experimental characterization and theoretical simulation results show that, mainly nitrogen pyrrole (pyrrolic), pyridine (pyridinic) and quaternary (Quaternary ) is present in three forms graphene structure, and the influence by the electronic structure of the material, to improve the wettability with the electrolytic solution, thereby improving the electrochemical properties of the electrode material. MXene electrode as a new two-dimensional material, because of its advantage of having a good conductivity, charge fast response characteristics and the like pseudo capacitors, supercapacitors with theExcellent prospects. In Ti3C2 an example, the two-dimensional material having a multilayer structure T-Ti-C-Ti-C-Ti-T, where, T is the surface functional groups introduced during etching, for example, -F, -OH and -O Wait. This particular structure gives excellent Ti3C2 regulation spatial structure and composition design. Chemical modification of the two-dimensional electrode material Ti3C2 there are a number of studies were reported, but the form of doping elements, especially on the mechanism of the electrochemical properties of the material there is a big controversy. To solve this problem, a method TF binding studies and experimental characterization of the first principle calculation,

discloses a nitrogen-doped successfully Ti3C2 mechanism and clarify the mechanism of doping elements on the electrochemical properties of the contribution for the chemical MXene modification provides a theoretical guidance.

(1) determine the doping N in the presence Ti3C2 site doped with nitrogen in order to reveal the possible presence Ti3C2 form, using first principles simulation the method of calculating the defect structure is formed can be doped all, the main consideration of the adsorption surface, and lattice-substituted functional group substituted three possibilities. The results show that, -O surface functional group having N atom certain adsorption, thereby forming a composite Ti-ON bonded to the corresponding formation energy of -2.87eV; -OH functional group of the surface may be substituted by N atom, thereby forming -N / -NH functional group, corresponding to the formation energy of -4.71 eV; lattice C atoms may also be substituted by N atom, can form a corresponding process is -1.31eV. Thus, in Ti3C2 structure, the nitrogen doping may exist in three forms: surface adsorption, substituted and unsubstituted functional group lattice.

东南大学孙正明教授团队在MXene储能领域取得系列进展 FIG 1. Ti3C2 nitrogen-doped first principles simulation:… A) may be doped with atomic structure schematic sites; b) forming energy calculation results; c) doped feasible energy Ti3C2 supercell schematic heteroaryl site; D) transition state energy.

(2) N doping illustrates the mechanism of the electrochemical properties Ti3C2 Electrochemical results found that there are three forms of nitrogen doping It can be higher than the capacity of the two-dimensional Ti3C2 electrode material. Analysis showed that the total capacity of the control surfaces andTwo diffusion portion of the control composition. Wherein the control surface of the electric double layer capacitor is determined by the microstructure of the material (layer spacing), the surface functional group by pseudocapacitive (-O / -N) surface adsorption or a group (N / NH) providing; diffusion control part by the outer valence of Ti atoms, i.e., affect the amount of space nuclear track.

东南大学孙正明教授团队在MXene储能领域取得系列进展 Figure 2. Analysis and electrochemical properties of N-Ti3C2 rails: a) rail element analysis N; track b) O elemental analysis; track c) Ti element analysis; d). N-Ti3C2 CV curve; E) capacitors – the contribution of diffusion behavior analysis; f) N-Ti3C2 impedance spectra.
The study, published in the

Advanced Functional Materials . After Lucheng Jie Bo Task Force and Dr. Yang Li as a co-first author, Zhang Wei, an associate professor and Professor Sun Zhengming for the common communication author. Original link: https: //onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202000852

2, multi-dimensional construct MXene hydrogel and applied to a flexible supercapacitor Development portable, compact electronic devices and wearable requires both flexibility and excellent electrochemical properties of the energy storage system. The conventional energy storage device, such as a super capacitor structure is designed to be flexible such powered wearable device is very attractive. The electrode material is an important part of the flexible supercapacitors, electrochemical activity which must have mechanical strength during use, even be stretchable properties. It will focus from conventional materials (e.g., metal oxide and carbon material), the steering intrinsic onto a flexible elastomeric polymer. Hydrogels are a set of conductive electrochemically active and advantages of a three-dimensional network molecular elastomeric polymer. The challenge that the mechanical flexibility and a conductive hydrogel electrochemical capacity to meet the requirements of the flexible electrode. The two-dimensional conductive sheet into the composite insulating polymer matrix is ​​considered to be one of the conductive hydrogel enhanced most effective strategy,At the same time as the reinforcing phase can also retain these inherent structural and functional advantages of the two-dimensional sheet. Graphene with excellent electrochemical and mechanical properties can be enhanced as a conductive hydrogel phase, but most graphene forming self-assembled hydrogels reduced graphene oxide by hydrothermal method, which can lead to water graphene hydrophilic gel deteriorates, thereby preventing infiltration of the electrolyte solution. The novel two-dimensional and three-dimensional network material can MXene synergy polymer, thereby enhancing the electrochemical activity and mechanical flexibility of the conductive hydrogel, is expected to be a candidate of the material of the flexible storage. Ti3C2 by the performance of a reasonable regulation, Ti3C2 two-dimensional and three-dimensional fiber nanoplatelet polyvinylalcohol (PVA) hydrogel matrix in combination with a one-dimensional conductive polypyrrole (of PPy), was prepared having excellent TF electrochemical capacity and flexibility ternary (1, 2 and 3D) conductive hydrogel electrode. Thanks to this unique hierarchical design, assembly is assembled into a multi-dimensional porous nanostructured interconnected, not only seriously inhibit stacked MXene nanosheet, and can promote diffusion of the electrolyte solution in the entire network, it exhibits excellent capacitance characteristic and mechanical properties.

(1) multi-dimensional design of hydrogel structure In order to realize this idea, freeze-thaw cycles take prepared MXene SYSTEM / PPy-PVA hydrogel composite, the specific preparation process in FIG. 3 shown in FIG. Ti3C2 (MXene) after peeling layer at least two-dimensional sheet size between 10 and 30 μm, can increase the contact area with the electrolyte solution, to achieve better ion transmission efficiency. MXene-PVA hydrogel having interconnected networks, the network consists of a thin outer wall of PVA composition, size from nanometers to hundreds of micrometers. After incorporation of PPy, a composite hydrogel retention macrostructure PPy nanofibers MXene nanosheet interleaved, and form a good conductive network. MXene / PPy-PVA hydrogel composite energy dispersive spectroscopy shows the relationship between the distribution of nitrogen element and titanium element, it was confirmed PPy nanofibers around MXene nanosheet growth MXene effectively suppressed in the hydrogel nanosheet It stacks again. Characteristic Raman peaks MXene-PVA hydrogel composite having both MXene and PVA, indicating that PVA can be MXene with good compatibility. MXene / PPy-PVATernary-level nanostructured hydrogels may provide a large available surface area, to promote ion diffusion and electron transfer, thereby enhancing the capacitance characteristic.

东南大学孙正明教授团队在MXene储能领域取得系列进展 FIG. 3 (a) MXene / PPy-PVA hydrogel prepared schematic; (bd) MXene, MXene-PVA hydrogels and MXene / PPy-PVA hydrogel microstructure; (e) Mxene composite hydrogel Raman spectrum.

is excellent (2) MXene gels the mechanical properties of the water Compared with the pure PVA hydrogels, MXene-PVA hydrogels since nanometers exhibit enhancement mechanical flexibility properties than the pure PVA hydrogel of high water. MXene-PVA hydrogels capable of various modifications (e.g. elongation, compression and knotting) to maintain and restore the original shape thereof. In the case of a fixed concentration of 10 wt% PVA, as MXene increased by 0.2 to a concentration of from 1 mgmL

-1 of the hydrogel MXene the tensile strength increased to 1.0 from 5.4 MPa, the elastic modulus 0.5 increased the amount of from 1.8 MPa, the deformation increase from 1.4 to 9.0 MJm -3 , while maintaining a similar strain at break of about 300%. Through a fiber dimensional and two-dimensional sheet of synergy, MXene / PPy-PVA hydrogel maximum tensile strength of 10.3 MPa, ultimate tensile strength than MXene-PVA hydrogel nearly doubled 5.4 MPa , with the intensity of the pure PVA hydrogels maximum tensile strength of 0.6 MPa contrast.

东南大学孙正明教授团队在MXene储能领域取得系列进展 FIG. 4 (a-c) MXene mechanical properties of the hydrogel; (d) MXene / crosslinked network reinforcing schematic and Mechanism of deformation in PPy-PVA.

(3) MXene hydrogel having a high specific capacitance and excellent cycle stability MXene / PPy-PVA hydrogel MXene-PVA hydrogel gum compared to higher electric double layer capacitor, a larger working voltage window. Introducing PPy can be widened space between the interlayer sheet MXene nanometers, effectively increasing distanceExchange interface area. MXene / PPy-PVA hydrogel ratio of capacitance at 1 A g

-1 The current density of 614 F g -1 . In addition, MXene / PPy hydrogel also showed a surprising ability to maintain capacitance (10000 cycles remain after 100% of its original capacitance) and high Coulomb efficiency (99.6%).

东南大学孙正明教授团队在MXene储能领域取得系列进展 FIG. 5 (ac) Electrochemical Characterization MXene hydrogel electrodes; (d) at 100 mV s-1 and a scan rate MXene-PVA and MXene / PPy-PVA hydrogel Comparative cyclic voltammograms; Comparative capacitance ratio (e) to a different current densities; Comparative (f) MXene / PPy mechanical and electrochemical properties of the hydrogel.
The study, published in Nanoscale

on , Zhang Wei, an associate professor and postgraduate research group Ma Jing students is co-first author, Zhang Wei, an associate professor and Professor Sun Zhengming as a co-corresponding author. Original link: https: //pubs.rsc.org/en/content/articlelanding/2020/nr/d0nr01414a# divAbstract

3, electrostatic self-assembly MXene / carbon composites as a ball having excellent lithium-sulfur battery! sulfur donor The lithium-sulfur battery having a high theoretical energy density (~ 2600 Wh Kg

-1 ) and a high theoretical specific capacity (1675 mAh g -1 ); while , elemental sulfur and low price, non-toxic, can meet the demand for new energy electric vehicles and large-scale renewable energy, it is considered one of the next generation of lithium secondary battery is most promising. However, the amount of the positive electrode adsorbing sulfur elemental sulfur, polysulfide shuttle intermediate effects triggered by the discharge of the final product low conductivity of lithium sulfide, up to 80% volume change problems limit the commercial process of lithium-sulfur battery. Based on the above requirements, a research group prepared by electrostatic self-assembly having sanmingRate capability and cyclability hollow porous carbon ball mounting structure (HPCSs) @MXene composite material (HPCSs @ d-Ti3C2), lithium-sulfur battery as a host sulfur positive electrode, a lithium-sulfur batteries is expected to increase. The main strategy: using two porous conductive material, and d-Ti3C2 HPCSs build a stable three-dimensional conductive network to achieve fast transfer of electrons, to enhance conductivity of the electrode; HPCSs using unique hollow structure, increasing the loading of elemental sulfur and provide space volume expansion while limiting polysulfide shuttle in the physical level; d-Ti3C2 the polarity of the surface, chemisorption polysulfide, effectively reducing the diffusion suppressing shuttle achieved physical and chemical adsorption confinement synergistic effect of improving the electrochemical performance of lithium-sulfur battery. Preparation of a sandwich structure

(1) electrostatic self-assembly HPCSs @ Ti3C2 In order to achieve the self-assembled MXene HPCSs and using modified polydimethyl diallyl ammonium chloride (of PDDA ) HPCSs adjusting the surface charge, the surface of the negatively charged by electrostatic d-Ti3C2 assembled to form a stable HPCSs-MXene-HPCSs stable sandwich structure, the specific preparation process shown in Figure 6. d-Ti3C2 nanosheets constituted a stable three-dimensional network skeleton crosslinked conductive, effectively increase the conductivity of the composite material. HPCSs uniformly and intimately fixed on both sides of the backbone, by electrostatic repulsion, there is a space between each of the clear HPCS by electrostatic attraction. This set of porous composite structure, the conductive network, in a polar surface, the adsorption of the chemical effect of discharge shuttle remission intermediate polysulfides may be joined by physical confinement improve polysulfide conversion kinetics, improve polarized electrodes, thereby improving the electrochemical performance. Preparation of

东南大学孙正明教授团队在MXene储能领域取得系列进展 The flowchart of FIG. 6 (a) HPCSs @ d-Ti3C2 of; (b-d) SEM and EDS elemental analysis FIGS energy spectrum; (e-g) TEM and HRTEM FIG.

(2) HPCSs @ d- Ti3C2 / S having excellent rate characteristics and cycle stability HPCSs @ d-Ti3C2 having a higher specific surface area and rich pore structure, to provide sufficient limiting receiving elemental sulfurSPATIAL, at 75% of the sulfur loading, HPCSs @ d-Ti3C2 / S sulfur as a positive electrode exhibits excellent electrochemical performance. Introducing MXene conductive network effectively reduces the voltage delay of the lithium-sulfur battery, reducing the degree of polarization of the electrode and the cathode to accelerate the oxidation-reduction of sulfur to enhance the reaction kinetics favor lithium-sulfur battery rate capability. Thus, HPCSs @ d-Ti3C2 / S exhibit better rate capability, when the current density of 0.1 C and back to 1 C, no significant drop in capacity (FIG. 7c). HPCSs @ d-Ti3C2 / S electrode ring 100 at 0.2 C and 1 C circulating loop coil 500, each having excellent capacity retention. At a current density of 1 C, average lap capacity fade rate of 0.069%. These results fully demonstrated, not only to build MXene conductive network to improve the electrochemical performance HPCSs / S of the electrode, the sandwich structural features also help to improve the stability of the electrode.

东南大学孙正明教授团队在MXene储能领域取得系列进展 Electrochemical Performance of FIG. 7 HPCSs @ d-Ti3C2 of: (a) Comparative 0.2mVs-1 at a scan rate of the CV curve; (b) the charge-discharge curve of voltage hysteresis comparison; (c) rate capability; constant at (d) 0.2C current density charge-discharge characteristics; (e) long cycle life at 1.0C current density curves; (f) damping rate vs. the cycle.
The research work published in international journals

on 2D Materials . Graduate research group Qi Qi students , PhD student Zhang Heng students as co-first authors, Bacon associate professor Zhang , Professor Min Zhou [123 ] and Professor Sun Zhengming as a co-corresponding author. Original link: https: //iopscience.iop.org/article/10.1088/2053-1583/ab79c1

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