Preparation Of NiCo LDH@MXene/biocarbon Composite Electrode And Study Of Supercapacitor Performanc

The massive consumption of fossil fuels and the deterioration of the global environment are seriously affecting the living environment of human beings.In addition,massive emissions of carbon dioxide have led to global warming.In 2020,China has set the world the goal of achieving carbon peaking by 2030 and carbon neutrality by 2060.The development of sustainable energy sources is an effective solution to the problems of fossil energy consumption and environmental pollution.But how to store and convert sustainable energy efficiently is an important challenge that needs to be addressed in the world today.Supercapacitors are an important class of electrochemical energy conversion and storage devices,with the advantages of high power densities,fast charging-discharging rates,and long cycle lives.However,their low energy densities limit their widespread use in practice.The construction of high performance electrode materials is one of the key methods to increase the energy density of supercapacitors.NiCo LDH has the advantages of high theoretical specific capacitance,easily adjustable components and good anion insertion,making it one of the most promising electrode materials for current applications.However,NiCo LDH has poor self-conductivity and is prone to self-stacking in the process of energy storage,leading to the degradation of capacity and cycling performance.Therefore,how to solve the problem of NiCo LDH conductivity and self-aggregation is a key issue that needs to be solved for the widespread use of NiCo LDH in practice.In this paper,NiCo LDH@MXene and NiCo LDH@BC composites were designed and prepared by compounding NiCo LDH with the two-dimensional material MXene or low-cost biomass-derived activated carbon as a starting point to solve the problems of NiCo LDH conductivity as well as self-aggregation.Constructing a conductive network and dispersing NiCo LDH at the same time can effectively solve the problems of poor electrical conductivity and easy stacking of NiCo LDH,so as to effectively improve the electrochemical performance of NiCo LDH.The main studies are as follows.（1）Compounding with highly conductive MXene to construct self-supporting composite films can solve the problems of conductivity and self-stacking of NiCo LDH.The highly conductive MXene and NiCo LDH were compounded by electrostatic self-assembly,which effectively improved the conductivity of the composite.In addition,the NiCo LDH nanosheets were distributed between the layers and on the surface of the MXene film,effectively suppressing the self-stacking phenomenon of MXene and NiCo LDH during the energy storage process.Benefiting from the synergistic effect of MXene and NiCo LDH,the3:1 MXene@NiCo LDH composite had a high specific capacitance of 629.7 C g^-1 at a current density of 1 A g^-1.Furthermore,it still possessed a capacitance retention of 87.6%after15,000 cycles at a current density of 10 A g^-1.The asymmetric supercapacitor assembled with 3:1 MXene@NiCo LDH as the positive electrode and activated carbon as the negative electrode has a high energy density of 22.9 Wh kg^-1 at a power density of 750 W kg^-1 and a high capacitance retention rate of 99.5%after 10,000 cycles.（2）From the perspective of resource utilization of solid waste,NiCo LDH was grown on an array of active carbon nanowires derived from cattail to solve the issues of electrical conductivity and self-stacking of NiCo LDH in one step.The NiCo LDH@BC was obtained via a facile hydrothermal method.The massive pore structure and large specific surface area of the BC provide more active sites and shorten the ion transport distance during the energy storage process.Thanks to the excellent electrical conductivity of BC and the synergistic effect between BC and NiCo LDH,20-NiCo LDH@BC-700 achieved excellent electrochemical performance.It achieved a high specific capacitance of 519.0 C g^-1 at a current density of 1 A g^-1 and still maintained a high rate capability of 63.1%when the current density was increased to 20 A g^-1.Furthermore,it still possessed a capacitance retention of52.7%after 3000 cycles at a current density of 10 A g^-1.The asymmetric supercapacitor assembled with 20-NiCo LDH@BC-700 as the positive electrode and activated carbon as the negative electrode delivered a high energy density of 28.7 Wh kg^-1 at a power density of800 W kg^-1 and a high capacitance retention rate of 90.0%after 10,000 cycles.