Construction Of Polypyrrole/metal Compound Composite Electrodes And Their Energy Storage

Supercapacitors(SCs)have great application prospects in the fields of portable electronic devices,electrified vehicles,and energy storage devices due to their fast charging and discharging,high power density,long cyclic lifetime,and environmental friendliness.However,the relatively low energy density of SCs,compared to lithium-ion batteries,severely restricts their commercial application.Therefore,it is a hot and frontier topic in this field to optimize the design and regulate the structure and composition of electrode materials to improve their charge storage capabilities toward improving energy density of SCs.Cobalt-based electrode materials have the characteristics of high theoretical specific capacity,multiple valence states,good reversibility of electrochemical reactions,and abundant raw materials,making them an attractive class of SC electrode materials.However,their practical specific capacities,rate performance,and cycling stability still need to be further improved to push the commercial application of SCs.In this thesis,cobalt molybdate(CoMoO4),cobalt hydroxide(Co(OH)2),cobalt sulfide(CoS),and cobalt selenide(Co0.85Se)were respectively combined with conductive polypyrrole(PPy)to improve the conductivity,number of electroactive sites and structural stability of cobalt-based composite electrode materials(CEMs).By modulating the morphology,composition,composite structure,size or introducing surface modification of CEMs,the structure-activity relationship of composition-microstructure-energy storage performance of CEMs was systematically studied,and the electrochemical energy storage mechanism of CEMs was also clarified.The above research will provide the scientific basis and technical support for the structural design,controllable preparation and energy storage performance regulation of cobalt-based CEMs for high-performance SCs.The main research contents and results are as follows:1.A self-templated method combined with a gas-phase polymerization was adopted to prepare CoMoO4@PPy CEMs by wrapping CoMoO4 nanotubes with PPy.The results show that the flexible PPy conductive layer was uniformly wrapped on the surface of CoMoO4 nanotubes,which improves the electrical conductivity of CoMoO4@PPy CEMs and helps to maintain their structural stability during cycling.In addition,the charge storage process of CoMoO4@PPy CEMs was found to depend on the surface capacitance due to the introduction of PPy,thereby improving the rate capability.Based on the synergistic energy storage effect of CoMoO4 nanotubes and PPy conductive layer,the specific capacity of CoMoO4@PPy CEMs is as high as 541 C g-1 at a current density of 2 A g-1,and the specific capacity retention at 20 A g-1 is as high as 81%.The initial capacity retention reaches 96%after 5000 cycles at 10 A g-1.2.The PPy@Co(OH)2@Ag CEMs on nickel foam were prepared using a two-step electrochemical method combined with a liquid phase reduction reaction.The research results show that Co(OH)2 nanosheets were grown on the surface of PPy nano wires,generating a large number of voids between these nanosheets.This results in full contact of active materials with the electrolyte and thereby increases the utilization of active materials.When Ag nanoparticles were attached to the surface of Co(OH)2 nanosheets,density functional theory calculation results indicate that the introduction of Ag nanoparticles further improves the electrical conductivity of CEMs,while electron transfer occurs at the Co(OH)2/Ag interface.As a result,the surface of Co(OH)2 is positively charged,thereby reducing the surface adsorption energy of OH-on Co(OH)2.This leads to the rapid adsorption of OH-on Co(OH)2 and accelerates the electrode reactions,thereby increasing the electrochemical activity of electrodes.Therefore,the PPy@Co(OH)2@Ag CEMs exhibit a high specific capacity(998 C g-1 at 2 A g-1),good rate performance(a capacity retention of 72%at 20 A g-1)and cycling stability(an initial capacity retention of 92%after 5000 cycles).Furthermore,the assembled PPy@Co(OH)2@Ag‖N-CNTs(nitrogen-doped carbon nanotube)asymmetric supercapacitor(ASC)device exhibits an energy density of 54.4 Wh kg-1 at a power density of 800 W kg-1.3.The PPy@CoS CEMs were grown directly on nickel foam using a two-step electrochemical reaction.The research results indicate that compared with the CoS nanosheets grown directly on nickel foam,the PPy nanowires served as skeletons to grow CoS nanosheets on its surface,which is beneficial to reducing the size and thickness of the CoS nanosheets.As a result,the PPy@CoS CEMs possess more electroactive sites and shorter ion diffusion pathways,ensuring higher rate capacities.In addition,XPS analysis results show that the Co-N covalent bonds exist between PPy and CoS,which make the CoS nanosheets firmly anchored on the surface of PPy nanowires,increasing the mechanical strength of CEMs and thereby improving their cycling performance.The electrochemical evaluation results show that compared with the CoS electrode material grown directly on nickel foam,the PPy@CoS CEMs demonstrate higher charge storage performance,including specific capacities of 860 C g-1 and 595 C g-1 at 1 A g-1 and 20 A g-1,respectively,and an initial capacity retention of 92%after 5000 cycles at 10 A g-1.In addition,the assembled PPy@CoS‖N-CNTs ASC device has a specific energy of 51.1 Wh kg-1.4.A two-step electrochemical deposition method was used to fabricate binder-free PPy@Co0.85Se CEMs by decorating Co0.85Se nanoparticles onto PPy nanowires.Different from the obtained Co0.85Se microparticles with sizes of 1-2 μm on nickel foam by an electrodeposition reaction,the PPy nanowires as supports were found to grow Co0.85Se with smaller sizes of 20-150 nm on its surface using the same electrodeposition reaction.The small nanoparticle size endows Co0.85Se active materials with more electroactive sites and shorter ion diffusion distances,which are conducive to improving the rate capability.Meanwhile,the Co-N covalent bonds between PPy and Co0.85Se ensure the structural stability of CEMs during cycling,resulting in higher cycling performance.By optimizing the electrodeposition time of Co0.85Se on PPy nanowires,the obtained PPy@Co0.85Se CEMs deliver a specific capacity of as high as 827 C g-1 at 1 A g-1,and the capacity retention reaches 67%at 20 A g-1.The initial capacity retention of 93%is achieved after 5000 cycles.