Application And Research Of Transition Metal Sulfide Quantum Dots Materials In Hybrid Supercapacitors

Relied on the advantages of high power density,fast charging and discharging,security and easy for maintenance,supercapacitors have become attractive energy storage equipment with great research value and application potential.The energy density of symmetric supercapacitors based on carbon-based electric double layer electrodes,which have been commercialized at present,still has large gap in comparison with secondary batteries such as lithium-ion batteries.In addition,although various pseudocapacitive materials break through the limitation of electrostatic capacitance storage mode,they still perform poorly in terms of structural design and charge and discharge stability.The capacity performance of the supercapacitor unit depends on the crystal phase,composition,structure and morphology of the electrode material to a large extent.Thereby,exploring and properly engineering suitable positive electrode materials are expected to essentially increase the energy storage density per unit mass for supercapacitors.Electrochemically active transition metal sulfides have emerged as novel supercapacitor cathode materials due to their large theoretical specific capacity,excellent electrical conductivity,abundant reserves and low toxicity.Although considerable work on various morphological designs and synthetic methodologies for different metal sulfides has been carried out and their electrochemical energy storage characteristics systematically investigated,the application of metal sulfides for electrode materials of supercapacitors still has some nonnegligible problems and deficiencies as far as the current research status,such as nomic bulk structure inhibiting exposure of reactive sites and hindering the kinetics of ion diffusion,the unstable lattice structure of metal sulfides material themselves during cyclic charging and discharging,methodological contriving for commercial scalable synthesis and phase control as well as possible underlying mechanism from coordinated modulation of electrochemical activity between bimetal ions.In order to solve these existing problems,this dissertation focuses on quantum dots(QDs)structure tailoring of transition metal sulfides.Based on the experimental synthesis,we conduct in-depth discussions combined with varieties of characterization and theoretical calculation results.The main research contents include the following aspects.Firstly,to tackle the problem that the lattice structure of traditional transition metal sulfide bulk materials collapses during the continuous electrochemical redox process and thus damages the cyclic stability,and also to maximize the exposure of the active sites on the surface of the materials,we design a ligands induced quantum dots structure strategy.Ultra-small NiS2 quantum dots are synthesized through high-efficiency hot injection method,and we assemble these NiS2 QDs with pyridine ligands containing highly electronegative heterocyclic nitrogen atoms.The induction of electron cloud of heterocyclic nitrogen shortens the Ni-S bond with stronger structural stability.Besides,the localized lone pair of electrons near nickel atoms reduces the repulsive force to the diffused OH-ions,thus increasing affinity of the redox reaction.Benefiting from the tailor-made of ligands and size effect of QDs,a great promotion in both specific capacity of 651.8 C·g-1 at 1 A·g-1 and stability of 94.7%retention at 5 A·g-1 over 8000 cycles is achieved as compared to ordinary bulk materials.These results demonstrate that ligands induced QDs is a promising strategy for precisely regulating metal sulfide electrode materials of advanced energy storage devices.Secondly,although the structural design of the transition metal sulfide QDs shows unique advantages in optimizing the electrochemical performance of the electrode,the current synthetic methodology for these compounds QDs limits its scalability.Therefore,we are committed to developing a simple,green and scalable one-pot method for the synthesis of ten-gram-scale monodisperse nickel sulfides QDs powders.Interestingly,the composition of the target QDs products can be precisely regulated through adjustment of the concentration of 6-mercapto-1-hexanol ligands in precursor solution and thus pure phase Ni3S4 QDs are obtained.Benefiting from the advantages of high specific area carrying more metallic active sites and rich pore structure in favor of the diffusion of electrolyte ions,the optimal Ni3S4 QDs manifest extraordinary electrochemical performance as advanced electrodes for hybrid supercapacitors.Impressively,the constructed hybrid supercapacitor achieves high energy density of 49.3 Wh·kg-1 and power density of 21718 W·kg-1 with a robust stability of maintaining 91.7%after 8000 cycles.Thirdly,compared to the binary nickel sulfide materials above studied,the ternary NiCo2S4 can produce more active redox reaction,but the advantages of this capacitive performance and their intrinsical mechanism of certain promotion in alkaline electrolyte are still not clear so far.In order to reveal the essential reason,we successfully synthesize high quality NiCo2S4 QDs with good monodispersity and size distribution.Combining DFT theoretical calculation with ex-situ XPS characterization results,we elucidate that the synergistic modulation between reactive Ni and Co donates more active electrons near the Fermi level,as well as advantageous reaction surface and decreased energy barrier.This synergistically enhanced reactivity boosts larger proportion of Ni and Co from the NiCo2S4 QDs involved in the redox reaction,contributing to more than three times augment in specific capacity over the NiS2 or CoS2 QDs.Meanwhile,electrochemical studies indicate that the structural superiorities of NiCo2S4 QDs accelerate ions diffusion and reaction kinetics.Benefiting from these aspects,a hybrid supercapacitor constructed by the NiCo2S4 QDs as cathode and nitrogen-doped reduced graphene oxide nanosheets as anode delivers an ultrahigh energy density of 67.5 Wh·kg-1 at power density of 850 W·kg-1,as well as excellent rate performance and robust cyclic stability.In this dissertation,an independent attempt is made to synthesize a variety of metal sulfides QDs electrode materials suitable for high-performance supercapacitors from the perspective of unique transition metal sulfide QDs structure design for the first time.On this basis,we strive to solve the current supercapacitor electrode materials problems encountered in the process of research performance,synthesis process and mechanism through various experimental strategies such as designing the microstructure of the material,optimizing the synthesis method and real-time characterization of changes in physical and chemical properties in the state of charge and discharge process combined with theoretical calculation results.We hope these work and research results could provide reference for the design of electrode materials for high-performance and applicable supercapacitors and other energy storage areas in the future.