Synthesis of Nickle-cobalt Based Electrodes for All-solid-state Supercapacitor

Author:Qin Qing Qing

Supervisor:wu yu cheng


Degree Year:2019





With the development of hybrid electric vehicles,portable and wearable electronic equipment,aerospace electronics,power grids and energy-storage technology,new efficient energy-storage devices are highly desirable.Supercapacitors are very attractive owing to their high power density,long cycling life and good safety.The low energy density hinders their practical applications.A promising approach to overcome this drawback is to develop alkaline based all-solid-state hybrid supercapacitors using battery-type electrode materials with high energy density.Nickel-cobalt based electrode materials have attracted large attention in terms of good electrochemical activity and high theoretical specific capacity.In this paper,the morphology of nickel-cobalt based electrode material was controlled,compounded and doped.In addition,the all-solid-state supercapacitor based on polymer electrolyte(polybenzimidazole,PBI)with high mechanical strength were assembled.The main research work and results are as follows:(1)PBI with a high tensile strength of100 MPa was fabricated.Activated carbon(AC)used as electrode material and PBI doped with KOH or H3PO4 were assembled an integrated all-solid-state device.These flexible supercapacitors exhibit low equivalent series resistance and good cycling stability of capacitance retention above90%after 10000 cycles.Under mechanical deformations of bending,twisting and rolling-up as well as repeated bending test,electrochemical performance of the flexible supercapacitors is well maintained,demonstrating the good flexibility.(2)An insightful study on hybrid supercapacitors has been carried out using devices based on flower-like NiCo layered double hydroxide(NiCo-LDH,777 C g-1)and AC in KOH.It has been found that the upper potential limit of NiCo-LDH is about 0.5 V(vs Ag/AgCl)while the lower potential limit of AC is about-1.1 V to-1.3 V.The stable potential window of hybrid supercapacitor is 1.6 V.A model to disclose the effect of materials performance,cell potential window and mass ratio on the cell performance have been developed.Following the optimized cell parameters,a demo of all-solid-state hybrid supercapacitor has been assembled using solid polymer electrolyte.It exhibits a high specific energy of 69.5 Wh kg-11 at 450 W kg-1 within 1.8 V and good cycling stability with 95.7%capacitance retention after 5,000 cycles.(3)Here,the unique three-dimensional hierarchical porous Ni(OH)2/CNTs composite electrode material were designed.The CNTs can improve the electronic conductivity of the electrode material.Ni(OH)2 flakes directly grown on the surface of CNTs can provide high electrochemical utilization.With good structural stability,the composite electrode material shows a high specific capacity of 854 C g-1.The PBI electrolyte was assembled into an all-solid-state integrated device,which shows a high specific energy of 50.6 Wh kg-1 within 1.5 V.The electrochemical performance of all-solid-state flexible supercapacitors under folding and bending tests were explored.This chapter opens up new opportunities for the development of alkaline based flexible supercapacitors with high mechanical strength and high energy density.(4)The Mn-doped NiCo-LDH(Mn-NiCo/LDH)electrode material was synthesized at 60°C by simple chemical precipitation method.The synthesis yield can be more than1.5 g in a 1 L container,which enables controlled macro synthesis of nanomaterials.The synthesized nickel-cobalt based hydroxide shows ultra-thin structure,which can expose the active site sufficiently and adjust the band gap energy,improve the electrochemical reaction kinetics of the electrode material.The electrode activity is further improved by Mn doping,and the Mn-NiCo/LDH nanosheet shows high specific capacitance of 925 C g-1.The assembled all-solid-state supercapacitor shows high energy density of 75.6 Wh kg-1 at a power density of 450 W kg-11 within 1.8 V.This work suggests that PBI based ion conducting polymer electrolytes are very promising for developing highly robust flexible supercapacitors in future practical applications.This work sheds light on comprehensive understanding towards design and fabrication high performance hybrid supercapacitor.