Controllable Fabrication Of Nanostructured Transition Metal Sulfides/Selenides And Their Corresponding Hybrids For Electrochemical Energy Conversion And Storage

Because of the unique electronic structures and polymorphism characteristics,transition metal sulfides/selenides and their corresponding hybrids are rich in various functions and excellent prospects,and have attracted much attention in recent years.Especially in research and application fields such as energy conversion and storage,like electrocatalytic hydrolysis hydrogen and oxygen production,supercapacitors,lithium ion and sodium ion and other ion batteries,the transition metal sulfides/selenides and their corresponding hybrids have great potential for developments.Among them,the compositions and structures of the binary and ternary transition metal sulfides/selenides are relatively uncomplicated,and their electrochemical and other related properties can be effectively regulated and optimized by changing the components,morphology,microstructures,and even electronic structures of the materials,which could greatly enriched the development space of binary and ternary transition metal sulfides/selenides and their corresponding hybrids in the area of energy conversion and storage.This work aims to control the preparation of a variety of novel binary and ternary transition metal sulfides/selenides and their corresponding hybrids by wet chemical synthesis methods such as colloidal hydrothermal injection,and(1)study their compositions,structures,and structure effect relationship,(2)optimize their electrochemical activities,(3)improve their theoretical and application performance in energy conversion and storage,and(4)expand their actual development value.The main research progress achieved in this paper is as follows:1.A hot injection method of preparation composite nanomaterials with heterogeneous structural characteristics was developed to achieve the controlled synthesis of MoSe2/Bi2Se3 hybrid.By introducing topological insulators into electrochemically active materials,it is used to improve the overall performance of the active materials and expand the application of the prepared hybrid in electrocatalytic hydrogen production and supercapacitors.Among the hybrid,Bi2Se3 is a kind of topological insulator which has a special metallic surface,while the MoSe2 nanosheets have indeed grown on the whole surfaces of hexagonal Bi2Se3 nanoplates substrate evenly.In particular,XPS and UPS data confirmed that the construction of the heterojunctions in MoSe2/Bi2Se3 is beneficial to the regulation of the internal electronic structures,that is,the electron can transfer from Bi2Se3 to MoSe2 to improve the conductivity of the active material.At the same time,the formation of the composites also could help increase the reaction sites and enlarge the effective specific surface area of the active materials.Based on this,MoSe2/Bi2Se3 exhibits excellent electrochemical performance.Typically,in 0.5 M H2SO4 electrolyte,there is a higher current density of 85 mA cm-2 at 300 mV(η),far better than pure MoSe2,Bi2Se3+MoSe2 mixture and pure Bi2Se3 electrocatalysts under the same HER conditions.Meanwhile,build a supercapacitor test device in 3 M KOH electrolyte,the hybrid also demonstrate enhanced capacitive performances of 1451.8 F g-1 at 1 A g-1 and 750 F g-1 at 20 A g-1,respectively,which are more than two times of pure MoSe2 and three times of pure Bi2Se3.In addition,the experimental data also indicate excellent durability of the hybrid during charge and discharge examinations.2.A well-defined heteronanostructure which integrated by ZnSe quantum dots anchoring on single-layer porous MoSe2 nanosheets(hybrid-S)was successfully obtained through a simple one-step colloidal synthetic route,and the electrocatalytic hydrogen evolution reaction(HER)and supercapacitor performance of this hybrid were also studied in this work.Particularly,with the assistance of ZnSe spacers,the naturally stacked MoSe2 matrix could be thoroughly isolated into ultra-thin nanosheets which spread out extremely to form a flexible single-layer configuration with porous architecture so as to expose as many reaction edges as possible.Meanwhile,as for the hybrid,in-depth studies show that the electrons could transfer from ZnSe to MoSe2 continuously due to the well-defined heterojunctions,and subsequently,the injected electrons to MoSe2 would play an important role in activating the interface of electrode/electrolyte efficiently.Benefiting from the especial structure engineering and electronic modulation which performed synchronously,the dual-modified MoSe2-based hybrid-S was upgraded with increased active centers,enlarged specific surface areas,reduced volume changes,freed mechanical stress of structures and decreased diffusion pathways of electrons/ions as well as the adequate interfacial reactions between electrolyte and electrode.That is to say,the hybrid-S with special double modifications has improved dynamic and thermodynamic properties during electrochemical reaction.Based on this,for HER activity,the ZnSe/MoSe2 hybrid-S has a low onset overpotential of 60 mV(RHE),a small Tafel slope of 49 mV dec-1,and a large current density of 50 mA cm-2 at 200 mV(RHE)while in 0.5 M H2SO4 electrolyte,which is far superior to the performance of MoSe2 based electrode materials with only electronic structure regulation and no modification.For supercapacitors in 3 M KOH electrolyte,the ZnSe/MoSe2 hybrid-S possesses high specific capacities up to 1100 F g-1,1000 F g-1,875 F g-1,820 F g-1,802 F g-1,741 F g-1,699 F g-1,607 F g-1 and 520 F g-1 under the conditions of 1 A g-1,2 A g-1,4 A g-1,5 A g-1,6 A g-1,8 A at g-1,10 A g-1,15 A g-1 and 20 A g-1,respectively,and also has excellent cycle stability performance,ranking among the top in the transition metal sulfides/selenides system.3.A simple one-step colloidal reaction method for synthesizing nickel sulfide with metastable phase structure was developed to obtain the monodisperse uniform Ni3S4 nanorods.Based on this,the heterojunction-like architecture of m-Ni3S4 NRs/g-C3N4 NS/N-G(monodisperse Ni3S4 nanorods/graphitic C3N4 nanosheets/nitrogen-doped graphene)ternary hybrid was skillfully constructed via van der Waals interaction by using a solution-phase self-assembly strategy for the first time.Subsequently,the electrocatalytic hydrolysis and sodium ion battery performance of all products were also studied.Our research found that m-Ni3S4 NRs itself is an excellent multifunctional energy material,while its application potential is severely affected by poor conductivity,easy agglomeration and large volume expansion of m-Ni3S4 NRs.In this regard,when N-G,which improves conductivity,and g-C3N4 NS,which relieves volume expansion,were introduced,the formed m-Ni3S4 NRs/g-C3N4 NS/N-G ternary hybrid with robust and intertwined nanostructures possesses shorter diffusion pathways,smaller diffusion resistance and faster transfer speed of electrons and ions that could expedite transport of electrolyte to flood and interact with reaction sites,dynamically and thermodynamically.Notably,the active Ni3S4 nanorods within the unique integrated ternary hybrid as embedded in graphitic C3N4 nanosheets over a conducting substrate of nitrogen-doped graphene could avoid agglomeration,obscission,anisotropic volume expansion and irreversible mechanical failure intensively even during extreme electrochemical reactions.Owe to the integrated effect,the ternary hybrid delivers excellent electrochemical performances with long-term stability in water splitting and cyclic durability in sodium-ion battery.Especially,there is a low onset potential of 40 mV(RHE)to be observed in HER process,and the applied potentials are also conducted as small as 1.47 V(vs RHE)and 1.51 V for OER and overall water splitting,respectively,at a 10 mA cm-2 under the same alkaline conditions.As for sodium-ion battery,the ternary hybrid possesses an advanced specific capacity of～670 mA h g-1 at 100 mA g-1 with particularly excellent rate capability and could even work at a high current density.4.By screening out suitable organometallic precursor sources,a simple one-step colloidal method was designed to successfully obtain the Fe2NiSe4 and Fe2CoSe4 nanosheets,which could solve the problem of difficult preparation of bimetallic selenide ternary nanomaterials.Additionally,the lithium ion battery(LIB)and sodium ion battery(SIB)performance of these two products were also evaluated in this work.In detail,the phase analysis results of XRD,IPC and XPS in the experiment show that the prepared Fe2NiSe4 and Fe2CoSe4 are pure phases without any impurities.In order to further understand the electronic structures and other characteristics of the as-synthesized samples,the UPS of Fe2NiSe4 and Fe2CoSe4 was tested by using a synchrotron radiation light source.Through this way,the respective valence band position and work function of these two materials were obtained easily.From electrochemical results,the Fe2NiSe4 could deliver a specific capacity of 1050 mA h g-1 at 200 mA g-1 for LIB,and a specific capacity of 670 mA h g-1 at 100 mA g-1 for SIB,while the Fe2CoSe4 possesses a specific capacity of 950 mA h g-1 at 200 mA g-1 for LIB,and a specific capacity of 650 mA h g-1 at 100 mA g-1 for SIB.Moreover,both samples show excellent rate performance and durability,for instance,after thousands of cycles,their capacity has not been significantly attenuated,indicating great potential of Fe2NiSe4 and Fe2CoSe4 nanocrystallines to be the lithium/sodium ion hosts.