High Performance Asymmetric Supercapacitors Based On Nickel Phosphides And Its Nanocomposite

At present,our planet is facing huge energy challenges.The increasing depletion of fossil fuels and non-renewable energy sources.How to reduce carbon dioxide emissions and how to integrate sustainable energy into the hybrid energy system are urgent issues in the current energy field.The development of new energy,energy storage and efficient utilization are considered to be the key to solving the above problems.Among many new energy storage devices,supercapacitors have attracted wide attention from researchers due to their higher power density and longer cycle life than lithium batteries and fuel cells,and higher energy storage capacity than traditional capacitors.Based on the analysis of the constraints on the performance of supercapacitors,the nickel phosphides and its nanocomposite materials are designed and prepared as the supercapacitor positive electrode materials.Then,those materials are matched with carbon-based negative electrode materials and appropriate electrolytes to construct a novel asymmetric supercapacitor with large working voltage to improve the energy density and cycle life of supercapacitors.The main research contents and results are as follows:1.A one-step hydrothermal method was used to prepare the intertwined stacked Ni?OH?_2·0.75H₂O nanosheets,and then 3D self-supporting Ni₅P₄ nanoballs were obtained by low-temperature phosphating.The interlaced nanosheets of Ni₅P₄ nano flower balls can promote the rapid transfer of electrolytes and electrons.The 3D self-supporting nano flower balls can ensure their structural stability and help capacity retention.Due to its unique structure,Ni₅P₄ material exhibits high specific capacity,excellent capacity rate performance,and good cycle stability.In addition,the Ni₅P₄//AC asymmetric supercapacitor assembled based on the Ni₅P₄ nano flower positive electrode and activated carbon?AC?negative electrode shows a high energy density of 25 Wh kg^-11 at a power density of 434 W kg^-1,even at 1685 W kg^-11 the energy density up to 16.8Wh kg^-1.2.A nickel nitrate-assisted polyvinylpyrrolidone?PVP?in situ"blow-molded"high-temperature pyrolysis method is used to synthesize nickel nanoparticle-modified honeycomb porous carbon?Ni-HCPC?.Then,porous carbon strutted nickel phosphide nanoparticles?Ni₂P-HCPC?is prepared by low-temperature phosphation process.The carbon nanosheet skeleton not only provides fast conductive channels but also prevents a large amount of agglomeration of Ni₂P nanoparticles.Therefore,the Ni₂P-HCPC as the positive electrode of a supercapacitor can reach high specific capacity of 116 mAh g^-11 at a current density of 1 A g^-1.In addition,the highly cross-linked honeycomb porous carbon nanosheet framework?A-HCPC?is prepared by KOH activation and acid etching of Ni-HCPC material,which further matched the Ni₂P-HCPC anode to assemble a new Ni₂P-HCPC//A-HCPC asymmetric supercapacitor?ASC?.The Ni₂P-HCPC//A-HCPC ASC has a large working voltage?1.7 V?and high energy density(32 Wh kg^-1)in 2 M KOH electrolyte,and excellent electrochemical stability?98%capacitor retention after 8,000 charge cycles?.The high-performance positive and negative electrode materials were prepared from the same precursor?Ni-HCPC?and through low-temperature phosphating and acid etching methods,respectively.This novel synthesis strategy can further develop high-performance metal compounds with self-decorated porous carbon nanomaterials for energy conversion and storage devices.3.A novel core-shell porous nanocomposite?T-Nb₂O₅@Ni₂P?based on orthorhombic niobium pentoxide?T-Nb₂O₅?nanowires and metallic phosphides nanoflakes is reasonably designed and synthesized by a highly efficient solvothermal and subsequently moderate phosphorization procedure.The T-Nb₂O₅ nanowires act as structure-directing agent to directionally grow hierarchical ultrathin porous nickel phosphide?Ni₂P?nanoflakes,thus providing high electrical conductivity,large surface area and sufficient redox active sites.Owing to the distinctive nanostructure and synergistic effects of the T-Nb₂O₅@Ni₂P core-shell heterostructure,its electrochemical properties have vastly improved.When the current density is 1 A g^-1,a high specific capacity of 105 mAh g^-11 can be obtained.In addition,an asymmetric supercapacitor assembled based on the T-Nb₂O₅@Ni₂P composite positive electrode and activated carbon negative electrode exhibits a high energy density of 30.2 Wh kg^-11 at a power density of 453 W kg^-11 and impressive cycling stability?90%?after 5000 cycles.The well-defined oxide/phosphide core-shell materials can be expected to a new type of electrode materials for the high-performance supercapacitors.