Controllable Synthesis And Capacitive Properties Of Molybdenum Nitride-based Nanobelt Arrays Grown On Carbon Cloth

With the development of portable and flexible wearable electronics,functional high-performance energy storage devices are highly desired.Supercapacitors(SCs)are promising electrochemical energy storage devices due to fast charge-discharge rate,high power density and long life.Transition metal nitrides(TMNs),especially molybdenum nitrides have attracted much attention as electrode material for SCs due to their metal-like conductivity and large pseudocapacitance.However,the practical applications of molybdenum nitrides in high-performance SCs are restrained:(1)Molybdenum nitride is a ceramic material that is brittle and difficult to utilize in flexible devices.(2)Molybdenum nitrides are commonly prepared by nitriding molybdenum oxide precursors and the products generally contain residual oxygen species.However,the relationship between the chemical composition(N,O contents)and capacitive performance of materials is still unclear.(3)Pseudocapacitance is generated though the surface redox reaction,which is highly dependent on the specific surface area(SSA)and the number of electrochemical active sites of the materials.However,bulk molybdenum nitrides generally exhibit small SSA leading to low electrochemical utilization and small specific capacitance.In this thesis,we synthesize MoO_xN_y nanobelts(NBs)array with different nitrogen-oxygen ratio(N/O)on the surface of the carbon cloth(CC),and reveal the influences of different N/O on capacitive properties of materials.Additionally,the high-capacitances C-MoO_xN_y NBs/CC-700 electrode materials are prepared and the intrinsic mechanism is also proposed.The main content and results are summarized as follows:1.Synthesis of MoO_xN_y NBs/CC electrodes and their N/O dependent capacitive properties.Moprecursor NBs on the carbon cloth are prepared by a facile hydrothermal synthesis,which is further converted into MoO₃ NBs/CC by calcination under Air at 400?.The MoO_xN_yNBs/CC with different N/O was prepared via nitriding MoO₃ NBs/CC under NH₃ atmosphere at different nitridation temperature.The N/O of MoO_xN_y NBs/CC increase with the nitridation temperatures from 500 to 800? and the corresponding materials exhibit better rate capability and cycling stability,however,their specific capacitance decreases.And the MoO_xN_y NBs/CC-700 exhibit the best comprehensive capacitance performance in terms of specific capacitance,rate capability and cyclability.At 700?,X-ray diffraction reveals that MoO₃ is transformed into?-MoN phase and the N/O is measured to be 1.39 by the X-ray photoelectron spectroscopy.MoO_xN_y NBs/CC-700 electrode has an areal specific capacity of 187.79 m F cm^-2 at 1 m A cm^-2,and the retention rate is 52.87%when the current density increases from 1 to 50 m A cm^-2.Moreover,the MoO_xN_y NBs/CC-700 electrode exhibits extraordinary cycling stability with91.32%capacitance retention after 10,000 cycles at the current density of 20 m A cm^-2.This work reveals the influence of different N/O on capacitive properties in MoO_xN_y NBs/CC,and provides guidance for the design of high-performance TMNs electrode materials.2.Synthesis and capacitive properties of carbon hybrid C-MoO_xN_y NBs/CC-700.Mo(O,C)NBs/CC are fabricated from pre-carbonization Moprecursor NBs at 500? in Ar.And further converted into C-MoO_xN_y NBs/CC-700 by nitriding at 700? under NH₃ atmosphere.The C-MoO_xN_y NBs/CC-700 has an areal specific capacity of 353.37 m F cm^-2 at 1 m A cm^-2,which is an increase of 88.17%compared to MoO_xN_y NBs/CC-700.The capacitance retention of C-MoO_xN_y NBs/CC-700 is 51.54%when the current density increases from 1 to 50 m A cm^-2.Moreover,the areal specific capacity of C-MoO_xN_y NBs/CC-700 electrode is 180 m F cm^-2 after10,000 cycles at 20 m A cm^-2 and the decay rate is only 0.0016%per cycle.The C-MoO_xN_yNBs/CC-700 have small MoO_xN_y nanoparticles compared to MoO_xN_y NBs/CC-700,which provide more electrochemical active sites and improve MoO_xN_y utilization,resulting in enhanced capacitive properties.Additionally,the carbon provides fast ion diffusion pathway and have with high SSA,offers electric double layer capacitance.