Research On Electromagnetic Protection And Energy Storage Application Of MXene Composites

Transition metal carbon/nitride（MXene）is an emerging two-dimensional（2D）nanomaterial with unique layered structure,large surface area,metal-like properties,high conductivity,abundant surface functional groups（-O,-OH,and-F）,and ease of processing.Among them,the high conductivity and large surface area of MXene make it highly desirable in the field of electromagnetic protection.Electromagnetic protection mainly protects users or equipment from electromagnetic wave radiation interference and can be divided into two aspects:electromagnetic wave absorption（EMA）and electromagnetic interference（EMI）shielding.Currently,widely used electronic devices cause electromagnetic wave radiation,which not only reduces the lifespan of the device itself,but also harms the health of human being.At the same time,electromagnetic countermeasure is an important area of modern warfare,and electromagnetic interference weapons are becoming increasingly sophisticated,making the research demand for electromagnetic shielding increasingly strong.Devices used for electromagnetic protection require high mechanical strength,strong absorption or high shielding effectiveness（SE）,but MXene has the problems of easy self-accumulation,poor mechanical strength,poor impedance matching due to high dielectric constant,and single loss mechanism,which pose severe challenges to structural regulation and improvement of shielding performance.An effective method to solve the above problems is to construct MXene into a three-dimensional（3D）structure.This thesis studied the relationship between composite composition and properties by constructing 3D porous foam and sandwiched structures based on MXene composites,providing new research ideas for the preparation of high-performance EMA and EMI shielding materials.Meanwhile,the high conductivity and rich surface functional groups of MXene also enable its excellent electrochemical performances in various electrolytes,making it a suitable electrode material for supercapacitors.Supercapacitors are a type of electrochemical energy storage device with characteristics of the high power density and excellent cycling stability,which have important application prospects energy storage.The properties of electrode materials are extremely critical to the performance of supercapacitors.However,the self-stacking characteristic of MXene leads to insufficient exposure of active surface area and long electrolyte ion transport pathway,thereby affecting the rate performance and cycle life of MXene-based supercapacitors.This study revealed that designing the electrolyte ion transport channel by combining MXene with graphene derivatives through hydrogen bonding is an effective research solution for obtaining high-performance supercapacitors.The specific research content in the fields of electromagnetic protection and supercapacitors mentioned above can be divided into three parts:（1）Study on the EMA performance of MXene/Graphene oxide（GO）/Co₃O₄ nanorod3D porous aerogel:In order to address the issues of self-stacking,single loss mechanism,and poor gelation properties of MXene,a 3D porous MXene/GO was constructed with GO as a skeleton and MXene attached to it.Based on the electric-magnetic synergistic effect for EMA,one-dimensional anisotropic Co₃O₄ nanorods were introduced to provide magnetic loss in the MXene/GO aerogel.The loss mechanism of absorbing materials was systematically discussed.The resulting MGCR（MXene/GO/Co₃O₄ nanorod）aerogel had a minimum reflection loss of-71.87 d B（effective absorption rate of 99.999991%）at a thickness of 2.07 mm and maximum effective absorption bandwidth of 6.88 GHz.By adjusting the thickness of the aerogel from 2-6 mm,effective absorption within frequencies ranging from 2-18 GHz could be achieved.Additionally,this aerogel also exhibited excellent hydrophobicity which ensured its stability when used in humid environments.The attenuation characteristics of the far field radar cross section（RCS）of the aerogel were also studied,indicating its applicability under practical conditions.（2）Study on the EMI performance of MXene/HG（MXene/Holey graphene）sandwich films:To address the issues of easy self-stacking and weak mechanical strength of MXene,a sandwich MXene/HG composite film was prepared by introducing holey graphene between the MXene flakes.HG can provide more dipole polarization loss,and the heterogeneous interfaces between HG and MXene can also provide interface polarization loss.Finally,this film achieved an EMI SE of 56.15 d B（shielding efficiency of 99.9996%）at a thickness of 5μm.The hydrogen bonds between MXene and HG provide greater mechanical strength,allowing the film to withstand a stress of 120 MPa under the strain of3.5%.The EMI shielding film also has good Joule heating and excellent photothermal conversion ability.The above multifunctional characteristics make it possible for EMI shielding films to be used in more scenarios.（3）Study on the supercapacitors performance of MXene/HG electrode:To address the problems of long electrolyte ion transport pathway and the slow reaction rate for MXene-based supercapacitor,we studied the effect of MXene flake size on its capacitance performances,and designed electrolyte ion transport channels are regulated by controlling the mass ratio of MXene to HG.The composite structure exposes more active sites of MXene,which is favorable for infiltration of electrolyte.Porous structure of HG facilitates the construction of a shorter electrolyte ion transport path,thereby increasing the reaction rate.The sandwich film was assembled into a symmetrical supercapacitor with a gravimetric capacitance of 106.86 F g^-1 at a current density of 0.5 A g^-1 and 86.85 F g^-1 at a current density up to 10 A g^-1,showing excellent rate capability with capacitance retention rate of81.10%.The capacity retention is 96%after 10000 cycles,indicating its superior cyclic stability.