Preparation Of Nickel-based Compounds And Research On The Performance Of Supercapacitors

Trucks,buses,and electric vehicles driven by high power density supercapacitors have gradually become widely used.However,the energy density of supercapacitors is still lower than that of batteries and fuel cells,which limits their practical applications.Therefore,the construction of a hybrid supercapacitor（the positive electrode stores charge through the Faraday reaction and the negative electrode stores charge through electrostatic action）is a means to solve the relatively low energy density.In the field of supercapacitors,nickel-based compounds with good electrochemical performance and high theoretical specific capacity are an ideal cathode material.However,nickel-based compounds are typical battery materials,and cycle stability is an important factor restricting their development.In addition,the characteristics of poor conductivity will also affect their actual specific capacity.Among various carbon materials,biomass-derived carbon has become a new direction for preparing hybrid supercapacitor anode materials due to its abundant,sustainable and non-toxic characteristics.At the same time,biomass activated carbon has good specific surface area,porosity and conductivity,which is beneficial to improve the electrochemical activity and stability of nickel-based materials.Synthesizing the characteristics of biomass carbon and nickel-based compound materials,this topic improves the conductivity,specific capacity and cycle stability of nickel-based compound materials by adjusting the morphology of the material and combining with high-conductivity biomass carbon materials.Finally,the modified nickel-based compound materials are used in water-based hybrid supercapacitors to broaden the working voltage and further increase the energy density.The main research contents are as follows:（1）Layered anion intercalation materials composed of different metal hydroxides are called layered double metal hydroxides（LDHs）.Among variousLDHs,the rich reversible Faraday reaction of NiCoLDH has attracted widespread attention.However,the specific capacity of pure NiCoLDH is still limited by the reduction of active centers and low conductivity during continuous charging and discharging.As a typical biological template,Poplar catkins have great potential in designing interconnected porous carbon（IPC）for hybrid supercapacitors.The collected natural poplar flocs have been degreasing,salt treatment,alkali treatment and high-temperature calcination to obtain a 3D porous network structure that is different from the original poplar flocs.Then,NiCoLDH was grown on the surface of the porous network structure biochar by hydrothermal,and it was used as a competitive material for supercapacitor electrodes.NiCoLDH/IPC electrode has better electrochemical performance(1 A g^-1 is 148.6 m Ah g^-1)than pure NiCoLDH electrode(1 A g^-1 is 209.7 m Ah g^-1).In addition,a hybrid supercapacitor was assembled with the prepared NiCoLDH/IPC positive electrode and IPC negative electrode.Compared with NiCoLDH||IPC,NiCoLDH/IPC||IPC provides a higher energy density of 29.6 Wh kg-1 and good cycle stability（the capacity retention rate after 4000 cycles is 88%）,which is due to the introduction of IPC with advanced porous structure synergistically improves the electrochemical performance of NiCoLDH.（2）Transition metal sulfides have been extensively studied due to their good electrical conductivity,theoretical specific capacity and thermal stability.Using biological yeast as the carbon source,rational design and synthesis of new NiS/NCS binary hybrid microspheres.Studies have found that biological yeast needs to use GA solution as a cross-linking agent in the hydrothermal process to inhibit the hydrolysis of cell membranes under high temperature and pressure conditions and promote the carbonization process to protect the morphology and structure of NCS.In addition,the introduction of biochar and a good NiS/NCS interface effect are key factors for the application of electrochemical capacitors.These NiS/NCS binary hybrid microspheres exhibit a high specific capacity of 221.4 m Ah g^-1 when the current density is 1 A g^-1.The energy density of NiS/NCS||AC reached 44.3 Wh kg^-1,the power density was810.9 W kg^-1,and the specific capacity retention rate was 82.5%after 4000 cycles.Density functional theory（DFT）calculations show that the NiS/NCS composite material enhances the electronic state of the Ni d orbital around the Fermi level,and the unique electronic arrangement of the d orbital synergistically promotes the entire composite redox reaction.The concept of electrode hybrid structure can be used as a promising choice for the development of high energy density supercapacitors.（3）Multiple transition metal sulfides have two or more metal elements,which can provide abundant redox reactions during the electrochemical reaction process.Among them,NiCo₂S₄ has received extensive research interest due to its superior theoretical specific capacity and good electrical conductivity.As a common street tree,Platanus vulgaris produces a lot of fluff every fall.Platanus fluff,as a common biomass template,has rich nitrogen elements and a large specific surface area.The porous carbon obtained by platanus fluff after degreasing,salt treatment,alkali treatment and high-temperature calcination is used as a conductive matrix to support the growth of NiCo₂S₄.Among them,biomass porous carbon can not only improve the electrical conductivity,but also inhibit the aggregation of active particles during the long cycle.NiCo₂S₄/IPC electrode has better electrochemical performance than pure NiCo₂S₄ electrode.In addition,a hybrid supercapacitor was assembled with the prepared NiCo₂S₄/IPC positive electrode and IPC negative electrode.Compared with NiCo₂S₄||IPC,NiCo₂S₄/IPC||IPC has enhanced energy density and stability.