Preparation Of Nanostructured Mo-based Compounds As Electrode Materials For Lithium Storage And Electrocatalysis

Nanostructured materials have unique effects such as small size effect,surface effect,quantum size effect,quantum tunneling effect and dielectric confinement effect,which make them with excellent physical and chemical properties.Therefore,nanostructured materials have been widely studied in various fields.Due to the excellent electrochemical properties of nanostructured electrode materials,the design,synthesis and performance research of nanostructured electrode materials have received extensive attentions,especially in the field of electrochemical energy materials.However,nanostructured electrode materials still have some problems,such as poor stability and unclear electrochemical reaction mechanisms,etc.It is crucial to design and construct nanostructures ingeniously to achieve their applications.In this thesis,nanostructured molybdenum-based compounds are studied as electrode materials.C/MoO_2+?,oxygen-vacancy MoO₃,MoP@C with rich grain boundaries,and Mo₂C/C nanomaterials with different crystalline phases were designed and constructed.The relationship between experimental conditions and electrochemical properties as well as the related chemical reaction mechanisms has been explored.The main research contents and results are listed as follows:1.A novel free-standing electrode of C/MoO_2+?was prepared by combining hydrothermal process and,electrospinning,with the subsequent heat treatment process.The ultrafine MoO_2+?nanocrystals with a particle size of 10 nm were homogeneously embedded in the interconnected conductive carbon naonofibers with a diameter of 200 nm.The energy density of this kind of electrode is improved without the use of the inactive binder,the conductive agent and the current collector.Since the production process is simplified,the cost of the battery manufacturing is also reduced.When used as an anode for lithium ion batteries,the free-standing electrode of C/MoO_2+?exhibits excellentreversible specific capacity and superoir rate performance.After 500 charge/discharge cycles at a high current density of 2 A g^-?16?,the specific capacity of the free-standing C/MoO_2+?thin film can still maintain at 453.2 m Ah g^-?16?.The fabricated C/MoO_2+?thin film is expected to provide a new choice for next-generation advanced electrochemical energy-storage devices.2.MoO₃ nanobelts with controlled oxygen vacancy concentrations were synthesized rapidly by cominbing microwave solvothermal process with N₂ plasma etching for the first time.Owing to the relatively low conductivity of MoO₃,it is unsuitable for uses in electrode materials directly.Through N₂ plasma etching,oxygen vacancies with different concentrations can be introduced into MoO₃ nanobelts rapidly and controllably,which not only improves the electronic conductivity,but also increases the lithium-storage active sites.MoO₃ with different oxygen vacancies show different electrochemical performances.At a current density of 100 mA g^-?16?,the sample with N₂ plasma etching for 3 min exhibits a specific capacity of 1088 mAh g^-?16?after 120 cycles.This work provides an efficient,controllable and rapid method for the preparation of oxygen-vacancy MoO₃ nanobelts.In addition,MoO₃ nanobelts can be easily vacuum filtered into thin films and can be extended for flexible energy-storage devices.3.Novel MoP@C hierarchical nanofibers with rich grain boundaries were constructed by elecrospinning a single Mo/P source of MoO₂?PO₃OH?H₂O and subsequent heat treatment.The construction that a thin carbon layer of 2 nm coated MoP nanoparticles closely interconnected with each other makes the one-dimensional nanofibers featuring with rich grain boundaries and a porous structure.Compared with irregular MoP/C?I-MoP/C?,MoP nanoparticles and pure carbon nanofibers,the MoP@C with rich grain boundaries exhibites excellent electrocatalytic hydrogen evolution in both 1 M KOH and0.5 M H₂SO₄ solution.The excellent electrocatalytic capability of MoP@C may originate from the following essential aspects.The unique network of conformal carbon,which iscomposed of the interconnected nanocrystals,could provide effective electron transport routes.The enriched pores in the porous conformal MoP@C nanofibers are conducive to the full infiltration of the electrolyte with the catalyst,which could facilitate the molecular transport of the reactants and products.Besides,the hierarchical nanoarchitecture made of interconnected MoP nanocrystals induces rich crystal interfaces and even crystal defects.The grain boundaries may provide a large amount of catalytic sites with reduced atomic density and higher generalized coordination numbers,thus offering more electrocatalytic active sites.4.The materials with different crystalline phases show different electrocatalytic hydrogen evolution activities.Based on the consideration that there are few studies on the catalytic hydrogen evolution properties of different crystal phases of Mo₂C and how to further enhance the electrocatalytic activities of Mo₂C.The phase-selective ultrafine Mo₂C nanoparticles embedded in porous carbon nanofibers were prepared by electrospinning and heat treatment.It is worth noting that the crystalline phase,content and particle size of Mo₂C can be regulated by changing the sintering atmosphere,the content of Mo source in precursor,and the heating temperature.The obtained Mo₂C/C with different crystalline phases shows different catalytic activities in different electrolytes.The composite of hexagonal phase molybdenum carbide/carbon?H-Mo₂C/C?exhibites excellent electrocatalytic hydrogen evolution activities.The formed porous carbon nanofibers not only effectively prevent the agglomeration of Mo₂C nanoparticles but also improve the electronic conductivity as well as specific surface area,thereby providing more active reaction sites to enhance the activity of the electrocatalytic hydrogen evolution reaction.