Controllable Synthesis Of Novel Two Dimensional Nanocomposites For Electrochemical Energy Storage

Human's increasing demand of energy is sounding the alarm to establish effective,economic and sustainable systems for energy storage and conversion.Development of electrode materials with high energy/power densities,good cyclability,low cost and environmental friendliness is of great importance.Thanks to the unique features of two dimensional?2D?materials,including surface effect,small size effect and tunable electronic structures,they are promising to serve as efficient electrode materials.In this thesis,novel 2D nanohybrids with high performance have been fabricated via inspired hybridization,doping,pore creating or phase transformation,which are realized by cost-effecitve multicomponent assembly and inducing growth methods.The relationship between the morphology,structure,composition and electrochemical performance of these 2D nanohybrids has been explored.The following is the main contents and results.?1?Empolying graphene oxide?GO?as 2D substrate,facile step-wise approach was established to construct a ternary 2D nanohybrids with TiO₂and SnO₂ nanocrystals distributed on graphene in a staggered manner?TiO₂/SnO₂@G?,among which there were strong interfacial interactions.Graphene based SnO₂?SnO₂@G?was firstly synthesized by the redox reaction between Sn²⁺and GO and then used as template for controlled nucleation and crystallization of rutile TiO₂ at low temperature.Especially benefiting from the high conductive graphene substrate,TiO₂ nanocrystals with small volumetric expansion,nanosheet structure associated with mesopores and interfacial interactions among the nanoparticles,TiO₂/SnO₂@G exhibited superior cycling performance stabilized at 600 mAh g^-1 for more than 300 cycles at 160 mA g^-1 and excellent rate performance with 260 mAh g^-1 at 4000 mA g^-11 as anode in lithium ion battery?LIB?.The charging/discharging can be completed in as fast as 3 min.?2?Driven by the electrostatic attraction,GO substrate and protonated graphitic carbon nitride?g-C₃N₄-H⁺?were flocculated together via sonication to obtain g-C₃N₄-H⁺@GO,which was then employed as template to assemble MoS₃ nanoparticles?MoS₃@g-C₃N₄-H⁺@GO?.Owing to the mutual inducing effects among the components,the resulting 2D nanocomposite with MoS₂nanocrystals strongly coupled on nitrogen-enriched graphene(MoS₂@NG_g-C3N4)was obtained via mild temperature pyrolysis of MoS₃@g-C₃N₄-H⁺@GO.During the thermal treament,g-C₃N₄-H⁺effectively induced the formation of highly crystallized MoS₂ nanoplatelets with?002?d-spacing of 0.66 nm vertically grown on graphene.Simultaneously,molybdenum sulfide promoted the complete decomposition of g-C₃N₄-H⁺at mild temperature?550??and thereby imparted graphene with high N doping content?ca.13 at.%,most are pyridinic N and graphitic N?,rendering greatly enhanced coupling of MoS₂ on the N enriched graphene and boosting the conductivity of the hybrid.The achieved MoS₂@NG_g-C3N4 manifested outstanding capacity with 1450 mAh g^-1 at 0.1 A g^-1 for 200 cycles and exceptional rate capability with 830 mA h g^-1 retained at 10 A g^-1 as anode in LIBs.?3?2D nanocomposite with Co₉S₈ nanoplates anchored on highly crumpled nitrogen and sulfur co-doped graphene(Co₉S₈@NSG_g-C3N4)was produced via mild temperature pyrolysis of the self-assembled composite of GO,g-C₃N₄-H⁺and CoS.There are mutual inducing effects among the components during the thermal treatment:CoS promoted g-C₃N₄-H⁺decomposition at mild temperature?550??,which in return induced the growth of Co₉S₈ nanoplates exposed along c axis on the graphene substrate.The decompositions of g-C₃N₄-H⁺and CoS gave rise to the Co₉S₈ nanoplates embedded in the highly crumpled N and S co-doped graphene sheet with high surface area and hierarchical pore structure.The resulting Co₉S₈@NSG_g-C3N4showed excellent ORR catalytic activity(onset potential:-0.02 V,half wave potential:-0.10 V,limited current density at-1 V:6.1 mA cm^-2),long time stability and methanol tolerance,outperforming commercial Pt/C catalyst.In addition,Co₉S₈@NSG_g-C3N4 showed high cyclability as oxygen electrode in Zn-air battery.?4?Employing the inorganic crystal of MgAl-LDH obtained from the coprecipitation of Mg²⁺and Al³⁺as the novel 2D template,MoS₃nanoparticles were assembled to obtain hybridized precursor?MoS₃@MgAl-LDH?.Benefiting from the inducing effect of the substrate,mesoporous nanosheet hybridized with metallic and semiconducting MoS₂?meso-1T/2H-MoS₂NS?was achieved via thermal annealing and acid leaching strategy.The synthesis process was facile,efficient and reliable.During the thermal treatment,MgAl-LDH transformed into porous single crystal of MgAl-LDO plate,which induced the formation of 2H phase MoS₂nanocrystals with exposed?001?facet and same crystallization orientation on its surface?2H-MoS₂@MgAl-LDO?.After acid leaching of the template,the2D arrangement of MoS₂ nanocrystals was preserved.Moreover,the process of template etching induced the partial conversion of semiconducting 2H phase MoS₂ nanocrystals into metallic 1T phase MoS₂.Compared to exfoliated MoS₂ or MoS₂ nanoparticles,meso-1T/2H-MoS₂NS showed superior cycling and rate performances in LIB,in virtue of its 2D morphology,mesoporous structure and the hybridized highly conductive 1T phase MoS₂.