Mxene is a kind of layered material of transition metal carbides or nitrides. The lamellae are mainly connected by van der Waals force. It has a series of excellent physical and chemical properties. For example, mxene has the characteristics of good hydrophilicity, adjustable layer spacing and various surface functional groups. In terms of structure, mxene is composed of carbon layer and transition metal layer alternately, which endows mxene with good conductivity and pseudo capacitance characteristics; in terms of composition, mxene contains M and X double element multi-element (mxene solid solution) compared with single element two-dimensional materials, and multiple types of valence bond components between M-X endow mxene with more abundant regulation space. The electrode materials with excellent performance can be prepared by using the structure and composition characteristics of mxene reasonably. Therefore, since its inception, mxene has been outstanding in the field of energy storage and has been placed high hopes. Professor Sun Zhengming's team from the school of materials science and engineering of Southeast University has carried out a lot of research work in mxene two-dimensional electrode materials and its application in the field of energy storage, and has made a series of research results in super capacitors, secondary batteries and flexible energy storage devices. This year, they have been working in advanced functional materials, nanoscale and 2D Many papers have been published in high impact journals such as materials. 1. The mechanism of chemical modification of mxene two-dimensional electrode materials is revealed. Chemical modification is an effective way to improve the electrochemical properties of two-dimensional materials. At present, a lot of research work has been carried out on the chemical modification of graphene. Taking nitrogen doping as an example, experimental characterization and theoretical simulation results show that nitrogen mainly exists in the graphene structure in the form of pyrrolic, pyridinic and Quaternary, and improves the wettability with electrolyte by affecting the electronic structure of the material, so as to improve the electrical properties Electrochemical properties of polar materials. Mxene, as a new type of two-dimensional electrode material, has an excellent application prospect in the field of supercapacitors due to its advantages of good conductivity, fast charge response and pseudo capacitance. Taking Ti3C2 as an example, the two-dimensional material has a multilayer structure of t-ti-c-ti-c-ti-t, where t is the surface functional group introduced in the etching process, such as - F, - oh and - O. This special structure gives Ti3C2 excellent composition design and structure control space. The chemical modification of Ti3C2 two-dimensional electrode materials has been reported, but the existence of doping elements, especially the mechanism of the influence on the electrochemical properties of the materials, is still controversial. In order to solve this problem, the research team successfully revealed the nitrogen doping mechanism of Ti3C2, and clarified the contribution mechanism of doping elements to the electrochemical performance, which provided theoretical guidance for the chemical modification of mxene. (1) The existence site of doped N in Ti3C2 is determined to reveal the possible existence form of doped N in Ti3C2. The defect formation energy of all doped structures is calculated by the first principle simulation method, which mainly considers three kinds of energy: surface adsorption, functional group substitution and lattice substitution. The results show that the surface of - O functional group has a certain adsorption on N atom, thus forming Ti-O-N complex bonding, the corresponding formation energy is - 2.87ev; the surface of - Oh functional group may be replaced by N atom, and then form - N / - NH functional group, the corresponding formation energy is - 4.71ev; the lattice of C atom may also be replaced by N atom, the corresponding process of formation energy is - 1.31eV. Therefore, there are three possible forms of nitrogen doping in the Ti3C2 structure: surface adsorption, functional group substitution and lattice substitution. Fig. 1. First principle simulation of Ti3C2 nitrogen doping: a) schematic diagram of atomic structure of possible doping sites; b) calculation results of formation energy; c) schematic diagram of Ti3C2 supercell of feasible doping sites; d) transition state energy. (2) The influence mechanism of N-doping on the electrochemical performance of Ti3C2 was explained. The results of electrochemical performance test showed that the specific capacity of two-dimensional Ti3C2 electrode materials could be improved by three forms of nitrogen doping. The analysis shows that the total capacity is composed of two parts: surface control and diffusion control. Among them, the surface controlled double-layer capacitance is determined by the microstructure (layer spacing), the surface pseudo capacitance is provided by the functional group (- O / - n) or the surface adsorbed group (n / NH), and the diffusion control part is affected by the valence of the outer Ti atom, that is, the number of its nuclear and outer space orbits. Figure 2. Orbit analysis and electrochemical performance of n-ti3c2: a) orbit analysis of N element; b) orbit analysis of O element; c) orbit analysis of Ti element; d) CV Curve of n-ti3c2; E) contribution analysis of capacitance diffusion behavior; F) impedance spectrum of n-ti3c2. The research results are published on advanced functional materials. Lu chengjiebo and Yang Li of the research group are the co first authors, and Zhang Wei and sun Zhengming are the co correspondents. Original link: https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202000852 2, multi-dimensional construction of MXene hydrogel and applied to flexible super capacitor development of portable, miniaturized and wearable electronic devices requiring flexible and excellent electrochemical performance of energy storage system. It is very attractive to design the flexible structure of traditional energy storage devices, such as super capacitors, to power these wearable devices. Electrode material is an important part of flexible supercapacitor, which must have electrochemical activity, mechanical strength and even extensibility in the process of use. People will turn their attention from traditional materials (such as metal oxides and carbon materials) to intrinsic flexible elastomer polymers. Conductive hydrogel is an elastomer polymer with the advantages of electrochemical activity and polymer three-dimensional network. The challenge is that the mechanical flexibility and electrochemical capacity of conductive hydrogel should satisfy the requirements of flexible electrodes simultaneously. The incorporation of conductive two-dimensional nanosheets into polymer insulating matrix is considered as one of the most effective strategies for enhancing the conductivity of hydrogels. As an enhancement phase, it can also retain the inherent structural advantages and functions of these two-dimensional nanosheets. Graphene can be used as an enhanced phase of conductive hydrogel by virtue of its excellent electrochemical and mechanical properties. However, most graphene hydrogels are self assembled by hydrothermal reduction of graphene oxide, which leads to poor hydrophilicity of graphene hydrogels and thus impedes electrolyte infiltration. The new two-dimensional material MXene can interact with three-dimensional polymer network, thereby enhancing the electrochemical activity and mechanical flexibility of conductive hydrogel. It is expected to become a candidate for flexible energy storage. By adjusting the properties of Ti3C2 reasonably, the two-dimensional Ti3C2 nanosheets were combined with one dimensional conductive polypyrrole (PPy) nanofibers and three dimensional polyvinyl alcohol (PVA) hydrogel matrix. A group of three (1, 2 and 3D) conductive hydrogel electrodes with excellent flexibility and electrochemical capacity were prepared. Thanks to this unique hierarchical design, multi-dimensional components are assembled into interconnected porous nanostructures, which not only effectively inhibit the serious stacking of mxene nano chips, but also promote the diffusion of electrolyte solution in the whole network, showing excellent capacitance and mechanical properties. (1) to achieve multi-dimensional hydrogel structure design, in order to achieve this assumption, MXene/PPy-PVA composite hydrogels were prepared by freezing and thawing cycle. The specific preparation process is shown in Figure 3. The size of exfoliated Ti3C2 (mxene) is between 10 and 30 μ m, which can improve the contact area with electrolyte solution and achieve better ion transfer efficiency. MXene-PVA hydrogel has an interconnected network consisting of thin PVA outer walls, ranging from nanometer to several hundred microns. After incorporation of PPy, the composite hydrogel retained the macro structure of PPy nanofibers interwoven with MXene nanosheets and formed a good conductive network. The energy dispersive spectrum of MXene/PPy-PVA composite hydrogel shows the distribution relationship between nitrogen and titanium. It is confirmed that PPy nanofibers grow around the MXene nanosheets and effectively inhibit the re stacking of MXene nanosheets in hydrogels. The characteristic Raman peaks of MXene and PVA in MXene-PVA composite hydrogel show that MXene and PVA can be well compatible. The three element multilevel nanostructure of MXene/PPy-PVA hydrogel can provide larger specific surface area to promote ionic diffusion and electron transport, thereby enhancing the capacitance characteristics. Fig. 3 Schematic diagram of preparation of (a) MXene/PPy-PVA hydrogel; (B-D) microstructure of MXene, MXene-PVA hydrogel and MXene/PPy-PVA hydrogel; (E) Raman spectra of Mxene composite hydrogels.
(2) MXene hydrogel has excellent mechanical properties compared with pure PVA hydrogel. MXene-PVA hydrogel shows higher mechanical flexibility than pure PVA hydrogel because of nano enhancement. MXene-PVA hydrogel can maintain and restore its original shape under various deformations such as elongation, compression and knotting. With the fixed PVA concentration of 10 wt%, with the increase of MXene concentration from 0.2 to 1 mgmL -1, the tensile strength of MXene hydrogel increased from 1 to 5.4 MPa, and the elastic modulus increased from 0.5 to 1.8 MPa, and the deformation energy increased from 1.4 to 9 MJm -3, while similar 300% rupture was maintained. The synergistic effect of one-dimensional nanofibers and two-dimensional nanosheets showed that the maximum tensile strength of MXene/PPy-PVA hydrogel was 10.3 MPa, which was nearly twice the maximum tensile strength of MXene-PVA hydrogel 5.4 MPa, which was in sharp contrast to the strength of pure PVA hydrogel with a maximum tensile strength of 0.6 MPa. Fig. 4 mechanical properties of (A-C) MXene hydrogel; (d) schematic diagram of MXene/PPy-PVA cross-linking network and its enhancement mechanism under deformation.
(3) MXene hydrogel has high specific capacitance and excellent cycling stability. Compared with MXene-PVA hydrogel, MXene/PPy-PVA hydrogel has higher electrical capacitance and larger working voltage window. The introduction of PPy can broaden the interlayer space between mxene nano chips and effectively increase the interface area of ion exchange. The specific capacitance of MXene/PPy-PVA hydrogel is 614 F g-1 at the current density of 1 A g-1. In addition, the MXene/PPy hydrogel also exhibited a surprisingly capacitive retention (100% of the original capacitance after 10000 cycles) and a high coulombic efficiency (99.6%). Figure 5 (A-C) MXene hydrogel electrode electrochemical characterization; (d) cyclic voltammetry curve comparison with MXene-PVA and MXene/PPy-PVA hydrogel at 100 mV S-1 scanning rate; (E) comparison of specific capacitance under different current densities; (f) comparison of mechanical and electrochemical properties of MXene/PPy hydrogels. The research results were published in nanoscale. Associate Professor Zhang Wei and Ma Jing, a graduate student of the research group, are the co first authors, and Associate Professor Zhang Wei and Professor Sun Zhengming are the co correspondents. Original link: https://pubs.rsc.org/en/content/article landing/2020/nr/d0nr01414a ා! Divabstract3. Electrostatic self-assembled mxene / carbon sphere composite system as an excellent sulfur carrier of lithium sulfur battery lithium sulfur battery has high theoretical energy density (~ 2600 wh kg-1) and high theoretical specific capacity (1675 Mah At the same time, elemental sulfur is cheap, non-toxic and harmless, which can meet the needs of new energy electric vehicles and large-scale renewable energy, and is considered to be one of the most potential next generation lithium secondary batteries. However, the problems such as the adsorption of elemental sulfur in the sulfur positive electrode, the shuttle effect caused by polysulfide, the low conductivity of lithium sulfide, and the volume change of up to 80% limit the commercialization process of lithium sulfur battery. Based on the above requirements, the research group prepared hollow porous carbon spheres (HPCSS) @ mxene composite (HPCSS @ d-ti3c2) with sandwich structure by electrostatic self-assembly method,