Structure Design And Performance Research Of Three-dimensional MXene Electrodes Of Supercapacitors

Two-dimensional transition metal carbons/nitrides(MXenes)have become candidate materials for high-power and long-life supercapacitors due to their controllable surface functional groups and pseudocapacitive energy storage behavior.However,the intrinsic stacking problem of two-dimensional materials limits the utilization of the active sites of MXene,hinders the transport of ions and electrons,further affecting the electrochemical performance of MXene.The three-dimensional structure of MXene can significantly alleviate the stacking problem and improve its electrochemical performance.In order to obtain high performance MXene-based electrode materials for supercapacitors,3D MXene@MFC foam electrode materials and MXene/CAC film electrode materials were prepared by three-dimensional skeleton loading method and derived carbon intercalation method,respectively.Moreover,the structure of MXene was regulated by ice template method,and the VA-MXene electrode was obtained.(1)3D MXene@MFC foam electrode materials were prepared by loading MXene on melamine formaldehyde foam carbonization skeleton(MFC)and then freeze-dried.The 3D structure not only effectively alleviated the stacking problem of MXene layers,increased the layer spacing and active surface of MXene,but also facilitated the diffusion of ions,thus displaying excellent electrochemical performance as supercapacitor electrode.3D MXene@MFC foam electrode exhibited a specific capacitance of 293.2 F g-1 at 1 A g-1,which was significantly higher than that of pure MXene electrode.Even at 500 A g-1,it could maintain the capacitance of 144.6 F g-1,showing excellent rate performance.In addition,after 30000 at 100 A g-1,the capacitance of 3D MXene@MFC foam electrode only decayed 4.9%,indicating great cycling stability.Further research showed that the 3D MXene@MFC foam electrode could achieve the best capacitance performance at 1.8 mg cm-2.(2)The liquid MXene/alginate composites film were immersed in calcium chloride solution,then the MXene/CAC composites film with large spacing and high density were prepared by the gel reaction and subsequent hightemperature carbonization treatment.As intercalation particles,CAC between MXene layers not only provided partial capacitance,but also enlarged the interlayer spacing of mxene,increased the active sites and promoted the ion/electron transport,thus greatly improving the electrochemical performance of MXene/CAC.In addition,the binding water and hydroxyl groups between the MXene layers were removed by high temperature treatment,which improved the density of the film and further achieved high volume specific capacitance.The specific capacitance of MXene/CAC-5%composite film reached 372.6 F g-1(1270.2 F cm-3)at 1 A g-1.At 1000 A g-1,the high mass specific capacitance of 198.3 F g-1 was still maintained,and the rate performance was significantly improved compared with pure MXene.After 30000 cycles at 10 A g-1,the capacitance retention of MXene/CAC-5%was 93.5%.Besides,when the mass loading of MXene/CAC-5%composite film was 10 mg cm-2,it showed a specific capacitance of 273.1 F g-1 at 1 A g-1.Furthermore,MXene/CAC-5%composite film was assembled into symmetrical supercapacitor.When the power density was 783.1 W Kg-1,the energy density of the symmetrical supercapacitor reached 14.9 Wh Kg-1;when the power density was 31.0 KW Kg-1,the corresponding energy density reached 6.7 Wh Kg-1.(3)3D VA-MXene electrode was constructed on a copper foil substrate by vacuum freeze-drying method to transform the water between the MXene layers into ice templates.Compared with the MXene films prepared by traditional vacuum suction filtration and scraping drying methods,the array structure not only effectively alleviated the problem of tight stacking of MXene layers,but also greatly shortened the ion transport path,thus improving the specific capacitance and rate performance of the VA-MXene electrode.3D VAMXene exhibited a specific capacitance of 376.4 F g-1 at 1 A g-1,while at 500 Ag-1,it maintained a specific capacitance of 277.3 F g-1.Moreover,the capacitance loss ratio of 3D VA-MXene was only 2%at 100 A g-1 after 30000 cycles.The ice template method is simple and does not need to introduce other materials,which provides a new idea for the construction of 3D MXene.