Preparation Of Ni-based Phosphate And Microporous Carbon Electrodes And Investigation Of Supercapacitor Properties

With the continuous advancement of science and technology,the rapid growth of population and economy around the world has led to an increasing demand for energy.Fossil fuels are non-renewable energy sources and will one day be exhausted.It is very necessary to develop clean and renewable energy such as wind energy and solar energy.However,due to the relatively large dependence of wind energy and solar energy on geographical and natural conditions,and the unstable energy output,we urgently need energy storage devices with good performance.It is well known that energy storage can be achieved by batteries or supercapacitors.In recent years,supercapacitors have attracted widespread attention due to their excellent performance,such as high power density,fast charge and discharge,and long cycle life.Of course,the performance of a supercapacitor depends critically on its electrode materials,therefore people have long been committed to finding high-performance supercapacitor electrode materials.Since there are few studies on transition metal phosphates in pseudocapacitive electrode materials,and in situ growth of electrochemically active nanostructured metal hydroxides from transition metal phosphates can improve the charge transfer efficiency between the active material and the current collector in the electrode,thereby improving the electrochemical performance of the electrode.Therefore,low-cost and environmentally friendly transition metal phosphate Na₄Ni₃P₄O₁₅（SNP）was synthesized by solid-phase reaction method as a pseudocapacitive electrode material in this work,the phosphate reacted with the electrolyte in an alkaline electrolyte to in situ deposit the electrode electrochemically active material nanosheets of Ni（OH）₂,SNP-C/CC electrode as the positive electrode of the hybrid supercapacitor.For supercapacitors,its negative pole is also very important.Since the carbon materials synthesized by carbides in the laboratory are microporous carbons,the specific capacity can be improved through sub-nano pores,compared with other carbon materials,it has a good capacity performance（its specific capacity is about 50%higher than that of ordinary activated carbon）.Therefore,in this paper,microporous carbon（MPC）with large specific surface area（1337 m~2/g）was prepared by carbonization and chemical activation using plant dried apricot kernel shell as raw material as the negative electrode material of hybrid supercapacitor（MPC-C/CC electrode is EDLCs type electrode）.The electrode materials（SNP and MPC）were characterized by a variety of characterization methods,and the electrochemical performance of the electrodes（SNP-C/CC and MPC-C/CC）was tested with an electrochemical workstation.Finally,the SNP-C/CC electrode and the MPC-C/CC electrode,which are the positive and negative electrodes of the supercapacitor,respectively,were assembled into a complete SNP-C/CC//MPC-C/CC hybrid supercapacitor in 6 M KOH electrolyte,and the electrochemical performance of this supercapacitor was tested with an electrochemical workstation.The main work is as follows:1.Transition metal phosphate Na₄Ni₃P₄O₁₅（SNP）was synthesized by solid-state reaction method as the cathode material of supercapacitor,SNP will in situ deposit high electrochemically active material nanosheet Ni（OH）₂ in alkaline electrolyte.Mix SNP, conductive agent graphitized carbon black Super-P,and binder polyvinylidene fluoride PVDF in a ratio of 85:10:5,after adding an appropriate amount of organic solvent N-methyl pyrrolidone to obtain a uniform suspension slurry.The SNP-C/CC electrode was fabricated by coating it on the current collector carbon cloth and drying it in a drying oven at 60°for 12 h.In order to gain a deeper understanding of SNP、the in-situ transformation characteristics of SNPs in alkaline electrolytes and the chemical and electrochemical processes of SNP Na₄Ni₃P₄O₁₅ to Ni（OH）₂ and Ni OOH conversion,various physical and chemical characterization methods（XRD,SEM EDS,FTIR,TEM,XPS）were used to characterize the SNP,the SNP after 30 days of exposure to 6 M KOH electrolyte,and the electrochemical cyclic voltammetry test,that is,the SNP after CV.A series of electrochemical tests were carried out on the SNP-C/CC electrode using a typical three-electrode system（SNP-C/CC electrode as the working electrode,platinum sheet as the counter electrode,and Hg/Hg O as the reference electrode）.The results show that the SNP-C/CC electrode not only has a high specific capacity of 687.93 F·g^-~1 in 6 M KOH electrolyte,but also has good rate capability（when the current density is increased to six times the original,the electrode specific capacity retention rate is as high as about 80%）and electrical conductivity（the equivalent series resistance Rs is 1.7～2Ω,and the resistance Rct during charge transfer/migration after the electrochemical reaction is 45～55Ω）,it is a good candidate for pseudocapacitor electrodes.2.In this paper,a special method was used to prepare a microporous carbon material MPC with a large specific surface area by using the dried apricot kernel and shell of plants as the raw material,through two stages of carbonization（calcination）and chemical activation（acid treatment）.Afterwards,the MPC was characterized by various characterization methods（XRD,SEM EDS,nitrogen adsorption and desorption test, FTIR,XPS）.And by using the above method of preparing electrodes,MPC-C/CC electrodes（electric double layer supercapacitor electrodes）were prepared as negative electrodes of hybrid supercapacitors.This electrode was then combined with a platinum sheet（counter electrode）and Hg/Hg O（reference electrode）to form a three-electrode system,and a series of electrochemical tests were performed in 6 M KOH electrolyte.The results show that the MPC-C/CC electrode not only has high specific capacity(138 F·g^-1),good electrical conductivity（equivalent series resistance Rs=1.224Ω,charge transfer resistance Rct=1.208Ω）and good rate capability（when the current density is increased to 6 times the original,the specific capacity retention rate of the MPC-C/CC electrode is as high as 83%）,and the production cost is low and environmentally friendly,it is a potential electric double layer supercapacitor electrode.3.The SNP-C/CC electrode and the MPC-C/CC electrode are the positive and negative electrodes of the hybrid supercapacitor,respectively,and a complete SNP-C/CC//MPC-C/CC hybrid super capacitor was simply assembled in 6 M KOH electrolyte.The electrochemical test results of this hybrid supercapacitor show that when the current density is 0.25 A·g^-1,the maximum specific capacity of this capacitor is 92.26 F·g^-1; when the power density is 200 W·Kg^-1,the highest energy density is 32.8 Wh·Kg^-~1,when the power density is as high as 2400 W·Kg^-1,the energy density still remains at 21.7 Wh·Kg^-1;after 5000 constant current charge-discharge GCD cycles,the specific capacity retention rate of this capacitor is as high as 82.4%,only a decrease of 17.6%.That is,this SNP-C/CC//MPC-C/CC hybrid supercapacitor not only achieved good power density and energy density values,but also has good cycling stability.In conclusion,this study provides an inexpensive,green,and environmentally friendly method for the production of high-energy energy storage devices.