Synthesis Of Nanostructured Metal Sulfides And Their Applications In Energy Conversion And Storage

One of the great challenges in the 21st century is undoubtedly energy conversion and storage.The present energy-conscious society calls for light-weight,low-cost,high-efficiency and environmentally friendly renewable energies due to the increasing demand for oil and environmental issues.Much important progress has been made in the development of advanced energy conversion and storage technologies such as solar cells,electrochemical water splitting,supercapacitor,fuel cells and lithium-ion batteries.Nanostructured metal sulfides have been extensively studied due to their importance in interpreting quantum size effects and applications in a variety of devices.They are abundant and cheap since they usually exist in nature as minerals such as heazlewoodite?Ni₃S₂?,chalcocite?Cu₂S?,pyrite?FeS₂?and so on.So many efforts have been made on the synthesis,properties and application of metal sulfide.However,the fabrication and performance of nanostructured metal sulfides need additional improvement.Herein,we report the successful design of metal sulfides composites and their corresponding applications for energy conversion and storage devices.1.Ammonia intercalated flower-like MoS₂ nanosheet films and their properties.Noble metal catalysts,such as Pt,Ir and Au,are the most effective catalysts,but the cost and scarcity limit their wide application.MoS₂ has received tremendous attention due to the earth-abundant composition,high activity and high chemical stability.Earth-abundant MoS₂ has demonstrated its promise as an attractive non-noble metal HER catalyst with high catalytic efficiency.Both theoretical and experimental studies concluded that the HER activity arises from the sites located along the edges of the 2D MoS₂ layer.Hence,much effort has been devoted to the preparation of MoS₂nanostructures with a high fraction of exposed edges and lower intrinsic resistance using various engineering strategies to boost HER catalytic performance.Here,flower-like ammoniated MoS₂ nanosheet film grown on graphite sheet has been successfully developed via a simple hydrothermal method.Through XRD measurement,ammonia can effectively insert into the parallel plane of the MoS₂ nanosheets,leading to the expansion of?002?lattice with 3.4?.The XPS and TEM clearly shows that the existence of ammonium between the planes induce the formation of 1T-MoS₂.Compared to the 2H phase and large size MoS₂ nanosheets,the flower-like 1T MoS₂ electrodes display excellent activity for hydrogen evolution with a small Tafel slope and more positive overpotential.The ammoniated dense flower-like vertical orientated ultrathin nanosheets not only increase the density of active sites but also could decrease the charge transfer resistance during HER process.The high active area and high electrical contact conductivity lead to better HER performance.2.Ammonium intercalated MoS₂ nanosheet decorated Ni₃S₂ nanofiber core-shell nanostructure and their properties.To achieve sustainable production of H₂ through water splitting,non-noble electrocatalysts for the hydrogen-evolution reaction?HER?are required to replace Pt catalyst.Herein,we successfully design and fabricate ammonium intercalated MoS₂nanosheet decorated Ni₃S₂ nanofiber core-shell nanostructure on 3D graphene/Ni foam by a convenient one-step method.SEM and TEM demonstrate that vertical orientated metallic phase MoS₂ nanosheet array can be grown uniformly on Ni₃S₂ nanofibers.The evolution process of nanostructures was observed by adjusting the sulfur source content.On one hand,high active metallic phase MoS₂ nanosheet increase electrodes effective active area and be in favor of the escape of the hydrogen bubbles produced during the HER.On the other hand,the high conductive 1D core nanofiber grown on the 3D graphene-Ni foam endows the quick transport of electrons along Ni₃S₂ nanofibers to Ni foam.Benefiting from the quite large surface area,high conductivity,as well as more effective sites,compared to the bare Ni₃S₂ nanofibers,this advanced electrocatalyst exhibits extremely low overpotential of ca.80 mV at 10 mA cm^-2 and long-term durability in alkali medium for HER catalysis.Furthermore,according to the DFT calculations,the existence of ammonium between the MoS₂?002?planes facilitate the formation of 1T-MoS₂ and accelerate the chemisorption of hydrogen.These advantages make such Ni₃S₂@A-MoS₂ heterostructures exhibit a highly efficient HER performance in alkaline electrolyte.The approach can be readily extended to synthesize other self-supported transition metal sulfide HER cathodes.3.Hierachical carbon coating TiO₂@C@MoS₂ asymmetric sanwish hollow nanospheres and their properties.Rechargeable lithium ion batteries?LIBs?are considered as potential power sources for various applications.In recent years,layered transition-metal dichalcogenides are of great interest as the active materials for lithium storage.In particular,MoS₂ has been found to be a promising anode material for LIBs due to its layered structure and the capability to allow easy lithium ion insertion/extraction,enabling its assembly with diverse substrates and delivering a high theoretical specific capacity of 670 mA h g^-1.However,most MoS₂ anodes still suffer from capacity fading and poor rate capability.Here,we fabricate a carbon coating TiO₂@C@MoS₂ asymmetric sandwich hollow nanosphere.First,the TiO₂ core with high stability can ensure the hollow structure stable during the lithium ion insertion/extraction;second,the graphite carbon interlayer can facilities electron transport throughout the electrode;third,the MoS₂ nanosheets shell significantly increase the specific surface area of the materials and improve the effective reaction rate between the electrolyte and materials;finally,carbon coating layer can enhance electronic conductivity and prevent MoS₂ aggregation and pulverization over repeated discharge/charge processes.Attributed to the rational design and engineering of the unique nanostructure and composition,the obtained hierarchical hollow spheres exhibit enhanced electrochemical performances and to fulfill the advantage of both high reversible capacity and superior cycling stability.When such nanostructures are evaluated as an anode material for LIBs,they show remarkably improved electrochemical performance as high as 850 mAh g^-1(100 mA g^-1,50th cycle),compared with pure MoS₂and TiO₂.These results clearly demonstrate the advantage of hybrid sandwich hierarchical structures in electrochemical energy storage.4.Nickel framework nitrogen doped three-dimensional superstructure graphene aerogel and their properties.Graphene,as an ideal supercapacitor electrode material,is receiving growing attention due to their excellent electronic conductivity,good electrochemical stability,high surface area and flexibility.Assembling graphene sheets into three-dimensional interconnected porous micro-structures,namely,graphene aerogels has been considered the most effective approach to achieve high specific capacitance.The performances of rGO-based supercapacitor electrode materials strongly depend on the micro-structures and morphologies of their electrodes.Here,we have developed the nickel framework nitrogen doped three-dimensional superstructure graphene aerogel via two stage process through chemical reduction of the GO dispersion with EDA.The micropores and morphology can be facile engineered through the hierarchical simultaneous self-assembly and self-bending of EDA-mediated GO sheets into interconnect nanotubes with a typical GO and EDA concentration dependent structural evolution process.The controllable micropores of N-GR are exposed to the electrolyte for the access of ions to form electrochemical double-layers,and the nickel framework shortened the distances of charge transfer and ensures that the N-GR can be directly used in capacitors electrode.Due to the synergistic effect of aforementioned features,the exhibits high specific capacity of 262 F g^-1 at 1.5 A g^-1,excellent rate performance and cycle stability in 1 M KOH electrolyte.More importantly,microporous and morphology engineering of graphene gel can be further exploited,hybriding with other high active metal sulfides for supercapacitor applications.5.Synthesis of ZnS/GR composites and their properties.ZnS is a well-known photocatalyst.However,ZnS can solely absorb the UV light,which accounts for only 4%of the total sunlight,due to its large band gap of 3.75 eV.Numerous attempts such as doping metal ions?Au,Cu,Ni,Pb and Sr?and preparing of solid solutions have been applied to make ZnS generate visible light activity.Unfortunately,heavy metal cadmium and lead are toxic and harmful to human beings and environment,which limit their wide application.Here,a series of ZGx composites with different GR content were synthesized by a two-step hydrothermal method.For the blank ZnS,almost no photocatalytic activity is observed under visible light irradiation because the wide band gap ZnS is cannot be photoexcited by visible light irradiation.However,the as-prepared sample ZnS-GR with 0.1%GR reaches a high photocatalytic H₂-production rate of 7.42?mol h^-1g^-1 under the same conditions.These results suggest that under visible-light irradiation,the GR in the ZnS-GR nanocomposite can be excited from HOMO to LUMO,and the photoinduced electrons can be transferred to the CB of ZnS,leading to a charge injection and separation.Thus,ZnS-GR isable to show visible-light photoactivity toward hydrogen generation.The results indicate that the unique features of GR make it to behave like a photosensitizer rather than an electron reservoir to capture or transfer photogenerated electrons.