Design and Synthesis of Two-Dimensional Nickel-Cobalt Phosphide (Sulfide) and the Properties of Asymmetric Supercapacitors

Author:Zhang Xiao Meng

Supervisor:fu hong gang tian chun gui


Degree Year:2019





Energy shortages and environmental pollution accompanying economic development are the focus of current social concerns.Using of renewable energy,such as solar energy,is a necessary way to alleviate environmental energy problems.Solar power generation is the most direct way for humans to use solar energy,but it has the characteristics of large intermittent and strong regional characteristics,and it is difficult to use it in parallel.Therefore,the development of efficient and safe energy storage devices to store solar power is necessary to make full use of solar energy.Supercapacitors(SCs)have the advantages of high power density,good stability,environmental friendliness and fast charging speed,which provide an opportunity for efficient storage of solar power.It is well known that the performance of SCs is closely related to electrode materials.Transition metal phosphides(TMPs)have significant advantages over conventional carbon-based,transition metal oxide/hydroxide and other electrode materials due to their high conductivity,high stability,high reserves and low cost.However,the current application of phosphide in supercapacitor is mainly focuses on the morphology of single-metal TMPs nanoparticles and nanowires.In order to improve the application performance of TMPs in terms of supercapacitor,it is necessary to properly control the morphology and composition of the TMPs to meet the needs of supercapacitor’s requirements.Taking the advantage of two-dimensional porous materials in energy storage,this thesis is devoted to design and synthesis of two-dimensional porous TMPs electrode materials,and optimization and enhancement of energy storage performance through structural regulation.The main research contents of the thesis are as follows:1.Maintaining effective contact between the active materials and efficient transfer of the electrolyte is essential for the performance of the electrode material.The lamellar structure can be layered to achieve effective contact,while the porous structure facilitates electrolyte transport.Based on the above characteristics,we designed and synthesized a sheet of porous cobalt phosphide nickel for use as a efficiency supercapacitor electrode material.First,a high yield NiCo hydroxide sheet precursor was prepared with sodium acetate as an additive in polyethylene glycol solvent.The process has the characteristics of simple operation and good reproducibility.During the phosphating process,the PH3 released by the phosphorus source(NaH2PO2)can react with NiCo in the precursor to form NiCoP,and the gas released by the hydroxide in situ can lead to the formation of abundant pores,and finally obtain a high specific surface area two-dimensional porous NiCoP(216.39 m2 g-1).Due to the special structure,S-NiCoP-300 has a high specific capacitance(1206 F g-1 at 1 A g-1)and exhibits good rate performance(612 F g-1 at 20 A g-1)).The performance is superior to the corresponding oxides and phosphides with other morphologies reported in the literature.In particular,the electrode still exhibits a high(1095 F g-1)mass capacitance at high active species loaded(13.5 mg cm-2).This is due to the porous structure which allows efficient transport of the electrolyte along the channels while the thick active material is coated.2.Nano-assemblies will have some improvement in activity while maintaining the advantages of the assembly unit.Based on this,we envisage assembling the sheets to further improve the energy storage performance of nickel cobalt phosphide.Through optimized experiment conditions,PVP was used as the surfactant and CH3COONa as the regulator,NiCo-OH nanoflower precursors with uniform morphology and two-dimensional ultra-thin nanosheets were obtained by hydrothermal method.Controlling the phosphating conditions to obtain the hydrated cobalt nickel phosphide that maintains the topography,with improved supercapacitor properties.In order to further improve the activity,the NiCo-OH nanoflower precursor was coated with glucose as a carbon source,and then the carbon-coated sheet-like NiCoP(NiCoP/C)assembly structure was obtained by phosphating.Testing it as a supercapacitor electrode material,the prepared NiCoP/C-2 nanostructures have a high mass capacitance value(1258 F g-1 at a current density of 1 A g-1).The asymmetric supercapacitor assembled with N-doped porous carbon has excellent stability,after 6,000 cycles,the discharge capacitance remains at 90.8%.The obtained carbon-coated sheet assembly nanoflower-like NiCoP exhibits a promising supercapacitor electrode material.3.When the powder sample is coated on the current collector to prepare the electrode,there is some problems that the contact with the current collector does not closely increase the impedance and the use of the binder increases the quality of the electrode.We prepared a self-supporting NiCoOH nanowall precursor by controlling the deposition rate of Ni2+and Co2+on NF by a NaAc-assisted(fluorine-free)process.After phosphating,a high loading of NiCo-P nanowalls having a high loading of about 8.6 mg cm-2 was formed on the nickel foam(NF).The electrode combines several advantages that are advantageous for supercapacitor’s performance:a rich pore structure that facilitates ion transport,a space surrounded by nanowalls that easily accommodate electrolytes,good conductivity of NiCo-P and close contact with current collectors facilitates electronics transmission.As expected,porous NiCoP nanowalls supported on NF(NiCo-P/NF),used as supercapacitor electrodes with a win-win high-area capacitance(17.31 F cm-22 at 5 mA cm-2)and mass capacitance(1861 F g-1 at 1 A g-1 and1070 F g-1 at 10 A g-1).The NiCo-P-6/NF//AC asymmetric supercapacitor has an energy density of 44.9 Wh kg-11 at a power density of 0.75 kW kg-1.At a higher power density of4.5 kW kg-1,the energy density can still reach 20.37 Wh kg-1.4.Due to the interaction between components,the construction of the heterojunction further promotes the performance of the material.Based on the characteristics that the periodic table of sulfur and phosphorus is prone to substitution,we constructed a Nickel cobalt sulfide-nickel cobalt phosphide heterostructure by stepwise vulcanization-phosphating of the cobalt-nickel oxide precursor.The heterojunction structure that inheriting the precursor sheet structure can be obtained by controlling the experimental conditions.The test results indicate that the energy storage performance of the dianion heterojunction structure obtained by the partial substitution method is further improved.In the three-electrode test,the current density at 1 A g-1,the specific capacitance is as high as 1400 F g-1,and the colleagues have excellent rate performance(820 F g-1,10 A g-1).The water-based asymmetric capacitor assembled with AC exhibited a higher energy density of 37.69 Wh kg-1(power density 800 W kg-1),and the all-solid capacitor exhibited a higher energy density of 33.78 Wh kg-1(power density800 W kg-1).