Synthesis And Supercapacitive Behaviors Of Two-dimensional Layered Composite Electrode Materials

Being confronted with the energy crisis and environmental problems, the explorations of clean and renewable energy materials as well as their devices are urgently demanded. Two-dimensional （2D） materials have received great attention after the discovery of graphene’s unique electronic properties. Other 2D materials such as metal hydroxides, transition metal chalcogenides have also been explored as electrode materials for supercapacitor due to their unique physicochemical properties such as large surface area, sizable bandgaps and high electrical conductivity. In order to get high performance electrode materials with high specific capacitance and superior rate performance and long cycling stability, we prepared a variety of two-dimensional materials and their composites, including graphene, layered double hydroxides （LDH）, layered metaldichalcogenides （TMDs） and investigated their electrochemical propertiesin the present work.The phase and morphology of as-obtained samples are characterized by XRD, SEM, XPS, FT-IR and electrochemical properties are measured by cycle voltammetry （CV）, galvanostatic charge-discharge （GCD） andelectrochemical impedance spectroscopy （EIS）. The main results are as follows:Chemical growth of mixed nickel-cobalt hydroxides （Ni-Co LDHs） with reduced graphene oxide （rGO） on the Ni foam substrate as electrode and explores its application in supercapacitors.The nanostructured （Ni-Co LDHs）/rGO film with different ratios of LDHs/rGO on Ni foam are prepared by using hydrothermal synthesis method. The XRD and XPS results of the electrodes confirm the crystal structure consists of Ni-Co LDHs. The morphological properties reveal that nanoflowers of Ni-Co LDHs and nanosheets of rGO. The electrochemical performance reveals that the （Ni-Co LDHs）/rGO electrode has a much higher specific capacitances than that of bare Ni-Co LDHs nanocrystals. The （Ni-Co LDHs）/rGO electrode exhibits maximum specific capacitance of 2270 F/gin 6 M KOH at scan rate of 5 mV/s, asuperb rate capability （42% capacity retention at 100 mV/s） and a good electrochemical stability with 91.5% of the initial capacitance over 1000 consecutive cycles at 20 mV/s. The superior pseudoelectrochemical properties of Ni-Co LDHs are synergistically reinforced withhigh surface area offered by a conducting graphene network, which stimulates effective utilization of redoxcharacteristics and improves electrochemical energystorage performance.Mesoporous a-NiS, Co₃S₄ and CoNi2S4 electrode materials were synthesized by a simple one-step hydrothermal method, and their electrochemical properties was investigated. X-ray diffraction and electron microscopy results show that as-obtained mesoporous sulfide electrode materials are assembled by single-phase sulfide nanoparticles and the CoNi2S4 electrode materials have the unique structure of ruffle-like microspheres consisting of 2D nanoplates. The binary CoNi2S4 electrode exhibits much higher specific capacitance of 1678.3 F/g than that of either Co3S4（1532.7 F/g）or a-NiS（787.4 F/g）in 6 mol/L KOH at a scan rate of 5 mV/s, and shows excellent capability retention of 45.8% from 5 mV/s to 100 mV/s, which is～15% higher than that of a-NiS and C03S4. The excellent cycling stability of binary CoNi2S4 electrode is achieved with 96.3% of the initial capacitance over 900 consecutive cycles at 15 A/g. Moreover, the columbic efficiency consistently remains above 94.3% within 900 cycles. The fine specific capacitance and good cycling stability demonstrate that the mesoporous CoN12S4 electrode has potential applications in supercapacitors.Ni-MOFs@NiSx composite materials were synthesized via a stepwise hydrothermal method involving precipitation and in-situ sulfurization of Ni-MOF. By controlling Ostwald ripening reaction times during the sulfurization process, Ni-MOFs@NiSx with tailored NiSx nanostructure have been fabricated as electrode materials for electrochemical capacitor application. This unique three-dimensional structure promoted the charge transfer and the diffusion of the electrolyte ions, thus improving the efficiency of the material. The experimental results show that the Ni-MOFs@NiSx materials exhibits much higher specific capacitance of 716 F/g than that of Ni-MOFs（124.5 F/g） in 6 mol/L KOH at scan rate of 5 mV/s, The excellent cycling stability of Ni-MOFs@NiSx electrode is achieved with 86.3% of the initial capacitance over 1000 consecutive cycles at 10 A/g, which is higher than that of Ni-MOFs （-61.4%）. The high specific capacitance and good cycling stability demonstrate that the Ni-MOFs@NiSx electrode has potential application in supercapacitors.