Study On Preparation And Supercapacitor Performance Of BCN-based Composites

With the rapid development of modern society,the fourth industrial revolution has been changing the societal landscape and our life-style,together with higher requirements raised for new energy storage devices.In the existing relatively mature electrochemical energy storage systems,supercapacitors(SCs)are highly anticipated technical devices,which can stabilize the output capacitance under the impact of large currents and have an ultra-long cycle life as well.However,compared with lithium-ion batteries and other energy storage devices,the much lower energy density of supercapacitors has always been the bottleneck for the industrial application.According to the formula of energy density E=CV2/2,the energy density of supercapacitor devices can be improved by increasing the specific capacitance of electrode materials and broadening their voltage window.In recent years,2D material boroncarbonitride(BCN)has emerged in many frontier fields.BCN shows many advantages while being used as electrode material for supercapacitor,such as excellent conductivity,abundant surface defects and additional capacitance contribution from heteroatoms.In order to further enhance its energy density,we selected BCN as the base material,then optimized and modified the substrate by loading different electroactive materials to design and synthesize a series of BCN-based composite materials.The main research works are as follows:(1)The ultrathin BCN nanosheets with large size were prepared by the "ultrasonic-ball milling" method in which Al2O3 abrasive was added in the liquid-phase ultrasonic exfoliation process.And the electrochemical properties of BCNNS before and after exfoliation were compared,then the general applicability of this method for preparing large size 2D materials was also studied.The morphology and structure of BCN nanosheets were characterized by SEM,TEM,AFM,XRD,Raman and XPS tests.The results indicate that the thickness and size of BCN nanosheets are 2.1 nm and 5-20 ?m,respectively,which prove its feature of an ideal 2D material In electrochemical tests,the BCN nanosheets exhibit higher specific capacitance(457 F g-1)and longer cycle life(104%capacitance retention after 20000 cycles)than previously reports.In this method,ball milling and liquid-phase ultrasonic are combined to achieve effective exfoliating of 2D materials by using the synergistic effect of shear force and friction force.The results show that ultrasonic-ball milling is a universal method to prepare high-quality 2D materials,and this strategy also provides a novel path for obtaining a variety of large-size ultrathin 2D materials.(2)BCN-PANI composite was prepared by electrodeposition on BCN nanosheets,then the electrochemical properties,together with the electrochemical kinetics of the sample were further studied.The structure and morphology characterization confirmed the successful synthesis of BCN-PANI sample,together with strong ?-? stacking effect between PANI and BCN.The electrochemical performance test showed that polyaniline(PANI)modification was a feasible way to enhance the capacitive reaction of BCN nanosheets.Electrochemical characterization confirmed that the sample can reach high specific capacitance(1360.5 F g-1 at a current density of 0.15 A g-1),excellent cycle stability(89.6%capacitance retention after 10000 cycles)and ultrahigh energy density(up to 67.1 Wh kg-1 in 1 M Et4N·BF4 electrolyte).Moreover,quantitative analysis of electrochemical kinetics process revealed 72.6%of the total capacitance is attributed to the surface capacitance behavior under CV scan rate of 0.8 mV s-1,which may be one of the main reasons for the increase of the specific capacitance.The results show that the ?-? stacking effect between PANI and BCN network can accelerate the electron transfer and reduce the internal resistance of the material,thereby improving the kinetic process and enhancing the electrochemical performance.Thus,our method provides a feasible strategy for improving the electrochemical performance of carbon-based supercapacitors.(3)BCN-MnO2/Mn3O4 self-supporting material was prepared by solid-phase calcination combining with electrodeposition process.This method optimized the preparation technology of the working electrode and successfully introduced a large number of oxygen defects in the electrode material.Then,the structural characteristics and supercapacitor performance of the material were further studied.SEM and TEM proved that manganese oxides have been uniformly grown on the surface of BCN structure with various pore diameter,and structural characterization such as EPR and Raman confirmed the existence of oxygen defects and the high defect density of the material.Subsequently,oxygen defects-riched manganese oxides were grown on the surface of self-supported BCN materials by electrodeposition followed with air calcination process.The morphology and structural characterization show that the phase of the material is BCN-MnO2/Mn3O4.A large number of oxygen defects and the coexistence of various valences of manganese elements in the material can effectively improve the specific capacitance of the material(1471.1 mF cm-2 at current density of 1 mA cm-2).The results show that the double exchange interaction between Mn elements with high and low valence can effectively improve the conductivity of the electrode materials,supplemented by the existence of a large number of oxygen defects,then their synergistic effect can effectively improve the electrochemical kinetics process and enhance the overall specific capacitance of the materials.Our strategy provides a new technology for the preparation of working electrodes,together with a simple and effective way for the introduction of oxygen defects in transition metal oxide materials.(4)The self-supported MnO/MnS@BCN material was prepared through a hydrothermal and high-temperature annealing process,and the influence of the synergistic effect between BCN and MnO/MnS heterostructure on the voltage window of supercapacitor was studied.Morphological characterization confirmed the uniform growth of MnO/MnS particles on the surface of BCN,while structural characterization demonstrated the successful preparation of MnO/MnS@BCN structure and the existence of built-in electric field in the heterogeneous interface of MnO/MnS.The results of electrochemical characterization strongly corroborated that the voltage window of symmetric supercapacitor devices based on MnO/MnS@BCN material can be successfully extended to twice of that of single component material,and its energy density can also be increased to 75.0 Wh kg-1.Then results of first-principles calculation reveal the mechanism of the increased working voltage,for that the built-in electric field spontaneously formed in the reasonably constructed heterostructure can increase the adsorption energy of Li+,which is conducive to the storage and migration of Li+at the electrode interface,thus effectively inhibiting the electrochemical decomposition of H2O.And the synergistic effect between BCN and MnO/MnS heterostructure can further widen the working voltage of supercapacitor devices.Therefore,our design method opens up a new path to improve the voltage window of supercapacitor devices.