[Background introduction]
      With the depletion of traditional fossil energy in the world, the development of renewable energy and the application of new technologies have attracted widespread attention. As an energy storage device, water-based hybrid capacitors have broad application potential in the field of renewable energy. Because of their combined advantages of high power density of supercapacitors and high energy density of batteries. In general, supercapacitors can be classified into electrochemical double layer capacitors (EDLC) and tantalum capacitors according to the charge storage mechanism of the electrode material. The double-layer capacitor is caused by the reversible adsorption/desorption of ions on the surface of the electrode in the electrolyte, and the charge is rapidly accumulated. The two-layer electrode material is mainly a porous carbon material having a high specific surface area and a porosity. Tantalum capacitors are a Faraday process that stores energy from a redox reaction between an electrolyte ion and an active material. Common electrode materials are transition metal oxides and conductive polymers such as MnO2, RuO2, polythiophene, and polypyrrole. Tantalum capacitors have high power density and good cycle stability, but the energy density is relatively low. The energy storage of the battery comes from the redox reaction in the electrode, and its electrochemical reaction is controlled by diffusion. Therefore, the battery always exhibits a higher energy density than the supercapacitor. The high power density of the combined capacitor type electrode and the high energy density of the battery type electrode are effective methods for constructing a hybrid capacitor.

[Introduction]
       Recently, Professor Xionghong Ji of South China University of Technology reported the preparation of CuD nanoparticles modified 2D Ti3C2 material and the corresponding electrochemical performance test. Two-dimensional (2D) layered materials become suitable electrode materials for electrochemical energy storage devices due to their unique properties. CuS nanoparticles are hydrothermally distributed on Ti3C2 nanosheets obtained by selective etching of Ti3AlC2 to form a sandwich. Ti3C2 / CuS composite. Based on the standard three-electrode system, all Ti3C2 /CuS composite electrodes have enhanced electrochemical performance and severe redox reactions compared to Ti3C2 electrodes. The optimum specific capacity of the Ti3C2/CuS composite electrode is as high as 169.5 C g-1 at a current density of 1 A g-1, which is about 5 times that of Ti3C2. The increase in the specific capacity of the composite electrode is attributed to the excellent electronic conductivity of Ti3C2 and the synergistic effect of the excellent electrochemical reactivity of CuS. In addition, a typical asymmetric supercapacitor device with Ti3C2/CuS composite as the positive electrode and Ti3C2 MXene as the negative electrode has a high energy density of 15.4 W h kg-1 at a power density of 750.2 W kg-1, and at 2 A g- At a current density of 1, the cycle retention of 5,000 cycles is 82.4% of the initial capacitance. The strategy in this work can be extended to the synthesis of other two-dimensional layered materials.

       The results were published online in the Journal of Materials Chemisty A:
    A facile method for synthesizing CuS decorated Ti3C2 MXene with enhanced performance for asymmetric supercapacitors

[Graphic introduction]

Figure 1 Schematic diagram of the synthesis of Ti3C2 / CuS composite

Figure 2 (a) XRD patterns of CuS, Ti3C2, TC-6, TC-9 and TC-12 (b) Corresponding XRD magnification of Ti3C2 (002) peak

Figure 3 (a) SEM images of Ti3C2, (b)TC-6, (c) TC-9, and (d)TC-12

Figure 4 (a) LRTEM image of Ti3C2, (b) HRTEM image and (c) corresponding SAED pattern; (d) LRTEM image of TC-9, (e) HRTEM image, and (f) corresponding SAED pattern

Figure 5 High-resolution XPS spectra of TC-9: (a) Ti 2p, (b) C 1s, (c) Cu 2p, (d) S 2p, (e) O 1s and (f) F 1s

Figure 6 (a) CV curves of Ti3C2, CuS, TC-6, TC-9 and TC-12 electrodes at a scan rate of 10 mV s-1 (b) Ti3C2 and (c) TC-9 electrodes at different scan rates CV curve

Figure 7(a) shows the GCD curves of Ti3C2, CuS, TC-6, TC-9 and TC-12 electrodes at current density 1 A g-1; (b) Ti3C2 and (c) TC-9 electrodes at different currents GCD curve at density; (d) relationship between specific capacity and current density of Ti3C2, CuS, TC-6, TC-9 and TC-12 electrodes; (e) Cyclic performance of TC-9 electrode; GCD curve of TC-9 electrode before and after 5000 cycles of g -1; (f) EIS curve of Ti3C2, CuS and TC-9 electrodes, and amplification curve in the high frequency region of the inner graph.

Figure 8. Electrochemical performance of TC-9 // Ti3C2 asymmetric supercapacitors (a) CV curves at different scan rates (b) GCD curves at different current densities (c) Specific capacitance vs. various current densities ( d) Comparison of average power density and energy density (e) Cyclic stability at current density of 2 A g-1 after 5000 cycles (f) TC-9 // Ti3C2 asymmetric supercapacitor device illuminates red LED

[Summary of this article]
      A Ti3C2/CuS composite with a sandwich structure was prepared by a simple hydrothermal method. In addition to excellent conductivity of Ti3C2 nanosheets and excellent electrochemical reactivity of CuS, intercalated CuS nanoparticles also extend the distance between Ti3C2 nanosheets, thereby increasing the ion-accessible surface area and accelerating ions/ Electron transport improves the electrochemical performance of Ti3C2 /CuS composites. At a current density of 1 A g-1, the composite is used as a positive electrode for alkaline hybrid capacitors with a capacity of up to 169.5 C g -1 and its capacity is maintained after 5000 charge/discharge cycles at a current density of 5 A g-1. The rate is 90.5%. The TC-9// Ti3C2 asymmetric supercapacitor device exhibits a high energy density of 15.4 Wh kg-1 at a power density of 750.2 W kg-1 and remains after 5000 cycles at a current density of 2 A g-1 Maintain 82.4% of the initial capacitance. This work shows that the prepared Ti3C2/CuS composite is a promising electrode material that promotes the development of high performance electrochemical energy storage.

Literature link:
DOI: 10.1039/c9ta00085b

Source: WeChat public account MXene Frontier