Design Of MXene Based Composite Porous Materials For Energy Storage And Their Capacitive Performance

The increasing environmental problems caused by the transition away from fossil fuel use have created an urgent need for clean,renewable energy.Among them,the design and development of energy storage devices is crucial to storing new energy sources.And with their high-power density and long cycle life,supercapacitor can be used as energy storage devices in many industries,including portable and smart wearable electronic devices.However,most current supercapacitors have significantly sacrificed their multiplicity performance and power density in the development of energy density,so one of the directions to advance the development of next-generation electrochemical energy storage devices is to increase their energy density in the development of high-multiplicity supercapacitor.One of the important steps is the design of efficient electrode materials.MXene,a two-dimensional transition metal carbide and nitride,is considered an ideal functional material in the field of electrochemical energy storage due to its high surface area and polar solvent dispersion.As a typical representative,Ti₃C₂T_x has been widely used as an electrode material for supercapacitors due to its ultra-high electrical conductivity(≈9800 S cm^-1),ultra-high bulk capacitance(≈1500 F cm^-3)and unique mechanical properties.However,similar to other 2D materials,the inevitable collapse and restacking problems caused by van der Waals forces will reduce the MXene surface active sites and slow down the ion transport rate.In this paper,the porous electrode materials for high-magnification energy storage are constructed from the perspective of accelerating ion transport and electrochemical reaction rate by using MXene based composite porous materials as the main research object to prepare supercapacitors with high energy density,and the main content and innovation points are as follows:Inspired by the spider web structure,an innovative electrospray technique with a self-assembled spider web structure was proposed to obtain polyacrylonitrile/MXene（PAN/MXene）nanofiber structure,which was finally carbonised at high temperatures to obtain PAN/MXene with a two-dimensional topological spider web-like morphology.For PAN/MXene nanofiber,the defects are reduced or even eliminated;the atoms on the crystalline surface are densely and compactly arranged;and the degree of crystalline surface arrangement is further improved.When the carbonization temperature is 800℃,the grain size can reach 0.95 nm,the grain surface spacing is0.385 nm,and the surface crystallinity is 51.11%.The electrochemical test results showed that the mass specific capacitance of PAN/Mxene-800 could reach 150 F g^-1 at a current density of 2.0 A g^-1 in a 1 mol L^-1 H₂SO₄ electrolyte,and when the current density was increased to 10.0 A g^-1,the mass specific capacitance could still maintain 122 F g^-1 with a capacitance retention rate of 81.33%.The assembled PAN/MXene-800//PAN/MXene-800 symmetrical supercapacitor can reach the maximum energy density and power density of 14.09 Wh kg^-1 and 1795.32 W kg^-1,and its a capacity retention rate was 95.6%after 5000 charge/discharge cycles at a current density of 2.0 A g^-1.MXene/reduced graphene oxide composite（MXene/r GO-Fe）gels were successfully prepared by an Fe²⁺assisted gelation strategy at room temperature.Benefiting from the interaction between Fe²⁺ions and the oxygen-containing groups on Ti₃C₂T_x MXene and r GO,the resulting MXene/r GO-Fe samples exhibited a 3D porous structure with a high specific surface area of 59.5 m² g^-1 and a more mesoporous pore size（9.3 nm）distribution,which improved the local stacking problem of MXene and r GO nanosheets and expanded the layer spacing between MXene sheet layers.The effect of the ratio of MXene to r GO on the performance of the MXene/r GO composite electrode was also investigated.The results showed that the prepared MXene/r GO composite electrode has optimal electrochemical performance when the mass ratio of MXene to r GO is 4:1 and its area specific capacitance is 26.6 m F cm^-2.The MXene/r GO-Fe micro supercapacitor has an area specific capacitance of 26.6 m F cm^-2 at a current density of 0.2 m A cm^-2.It also exhibits a maximum energy density of 2.22μWh cm^-2 at a power density of 120μW cm^-2,and has 87.30%capacitance retention after 5,000 cycles,providing excellent cycling stability.By using r GO and polypyrrole（PPy）particles as"double spacers",functionalized Ti₃C₂T_xMXene composite electrodes were fabricated through an interfacial self-polymerization process.The self-stacking of r GO and MXene nanosheets is effectively suppressed.The layer spacing between MXene sheets is expanded,and a unique three-dimensional porous conductive network structure is achieved.The test results showed that the prepared r GO/MXene-PPy electrode has a high specific surface area of 52.3 m² g^-1 and a smaller mesoporous pore size（11.0 nm）distribution.In addition,the conducting polymer PPy molecules were uniformly covered between the r GO and MXene nanosheets,which resulted in the excellent stability of the prepared r GO/MXene-PPy electrodes,with no significant changes in surface morphology and compositional structure when placed in humid air for 21 days.The electrochemical test results showed that the area specific capacitance of the r GO/MXene-PPy micro-supercapacitor was 82.1 m F cm^-2 at a current density of0.1 m A cm^-2,and when the current density was increased to 1.0 m A cm^-2,the specific capacitance retention was 62.12%,exhibiting excellent multiplicative performance.r GO/MXene PPy devices exhibited excellent multiplicative performance at a power density of 57.35μW cm^-2,exhibiting a maximum energy density of 4.11μWh cm^-2 and a capacitance retention of 91.2%even after 5000cycles of testing,showing excellent cycling stability.An ion-induced densification strategy was used to prepare an aqueous MXene/SA-Fe ink with a large ion-accessible active surface and high densification.SA was attached to MXene via hydrogen bonding,which prevents the restacking of MXene nanosheets and improves the MXene-nanosheet alignment.Subsequently,metal ions are tightly bonded to MXene surface oxygen groups to promote densification of MXene films and reduce macroporous pores,thus optimising the mechanical properties and oxidation resistance of MXene films,which can be stored at room temperature for 21days.The synergistic effect of hydrogen bonding and ionic bonding resulted in the formation of dense 3D porous MXene complexes with high specific surface area(49.58 m² g^-1).Among the divalent metal ions,Fe²⁺resulted in MXene sheets with the shortest interlayer distance,and the surface morphology exhibited more folds and ridge-like elevations.The electrochemical test results show that the MXene/SA-Fe micro-supercapacitor has a high area specific capacitance of 123.8 m F cm^-2 at scan rate of 5 m V s^-1;and still achieves a capacitance value of 76.8 m F cm^-2 with a capacitance retention rate of 62.1%when the scan rate is increased to 100 m V s^-1,exhibiting excellent multiplicative performance.At a power density of 33.7μW cm^-2,the maximum energy density of 8.44μWh cm^-2 is demonstrated;the capacitance retention rate is 91.4%after 10,000cycles of testing,showing excellent cycling stability.In addition,at a bending strain of 5%,the capacitance retention rate after 10,000 bending cycles is 90.0%,demonstrating excellent mechanical stability.An interfacial self-assembly technique produced three-dimensional（3D）MXene composite aerogels.The interlayer spacing of MXene nanosheets was expanded,and the random stacking of MXene nanosheets was effectively prevented by using MXene nanosheets as the backbone and artificial humic acid（A-HA）as the reinforcing phase under room temperature conditions.And Fe²⁺as a cross-linking agent induced the orderly arrangement between MXene and A-HA,forming a unique 3D ordered porous Fe-MXene/A-HA（FMA）aerogel.The electrochemical test results showed that the mass specific capacitance of FMA could reach 607.88 A g^-1 at a current density of1 A g^-1 in a 1 mol L^-1 H₂SO₄ electrolyte,and even at a high current density of 8 A g^-1,the mass specific capacitance could still be maintained at 509.16 A g^-1 with a capacitance retention rate of83.76%.The assembled FMA-2//FMA-2 symmetric supercapacitor has the maximum mass energy density and power density were 10.67 Wh kg^-1 and 539.05 W kg^-1,respectively.By introducing polydimethylsiloxane（PDMS）microspheres,the surface of MXene nanosheets was enriched with hydrophobic groups,which resulted in more active hydrophobic sites in the prepared aerogel.And the PDMS-Fe-MXene/A-HA（PFMA）aerogels still exhibit a high specific surface area(59.56 m² g^-1),and also have excellent properties such as high hydrophobicity,superelasticity（stress value remains constant after 50 compression-release cycles）and good thermal stability.Moreover,PFMA aerogels exhibit efficient adsorption capacity(64-121 g^-1)for a range of organic pollutants.In particular,the prepared PFMA aerogels exhibited excellent cyclic recyclability during cyclic absorption-extrusion and absorption-combustion processes.In addition,the PFMA aerogels were used as skimmers for continuous collection of oil/water mixtures in a separation system,exhibiting a high flow flux of 23,478 L h^-1m².