Microstructure-controlled Synthesis Of Transition Metal Compound Nanomaterials For Energy Storage And Conversion

Energy is one of the biggest challenges that humanity faces over the next decades.Ever-growing energy demands lead to excessive usage of fossil fuels coupled with increasing global environmental issues,which are stimulating a broad,intensive search for clean and sustainable alternative energy sources.As a result,various energy storage and conversion systems are being developed,which aim for utilization of different clean energy sources.Among them,lithium?sodium?ion batteries?LIBs/SIBs?,electrocatalytic water splitting and fuel cells have recently emerged as several kinds of promising candidates to realize efficient energy storage and conversion.Hovever,in order to improve their high performance,it is most important to design highly efficient electrode materials.As the anode materials of LIBs?SIBs?,transition metal compounds have the advantage of higher theoretical capacities.However,their electrochemical performances are greatly hindered by their lower electrical conductivity and poor stability that is caused by the drastic volume change.Meantime,hydrogen evolution reaction?HER?in electrocatalytic water splitting or oxygen reduction reaction?ORR?in fuel cells are sufferring from the drawbacks of high cost and poor stability of precious metal catalysts.However,transition metal compounds can be alternative catalysts because of their relatively low cost and environmental benignity.The goal of this dissertation is to design and controllably fabricate novel,multi-structured,multi-property transition metal compound electrode materials based on the challenges of the electrochemical processes on the LIBs?SIBs?,ORR and HER,thus improving their performances of the energy storage and conversion.The details of this dissertation are summarized briefly as follows:?1?Porous ZnFe₂O₄/?-Fe₂O₃ micro-octahedrons were prepared by a facile solvothermal method and subsequent thermal treatment.Compared with bulk material,the porous ZnFe₂O₄/?-Fe₂O₃ micro-octahedrons have high surface area,which can greatly enhance the contact area of the electrode-electrolyte and shorten the diffusion length of Li⁺and electron,thus improving the Li⁺storage.Moreover,the heterojunctions between ZnFe₂O₄ and?-Fe₂O₃ nanoparticles can ensure a high electrical conductivity of the overall electrode,hence realizing much improved rate capability.?2?Two types of amorphous and mixed-valence vanadium oxides or/and molybdenum oxide have been successfully constructed via solvothermal or freeze-drying method followed by subsequent calcination treatment.First,hydrangea-like-structured amorphous mixed-valence VO_x microspheres??-VO_x MSs?were easily prepared by solvothermal and annealing process.Its high surface area can increase the contact area with the electrolyte and shorten the Li⁺and electron diffusion paths.Moreover,the amorphous nature can avoid lattice stress and provide open vacancies for faster Li⁺diffusion,greatly improving the reversible capacity.Second,a three-dimensional?3D?ordered hierarchically porous amorphous hybrid by hybridizing amorphous VO_x with amorphous MoO_y?donated as3D-OHP-?-VO_x/MoO_y?via freeze-drying followed by subsequent calcination treatment.The hierarchically porous structure has high surface area,allowing the diffusion of electrolyte into the electrode easily.In addition,the stable interconnected 3D nanostructure based on the ultrafine nanocrystals uniformly loaded on carbon network significantly improves the electronic conductivity.?3?With freeze-drying method followed by subsequent calcination treatment,we successfully constructed hierarchically porous Li₃VO₄/C hybrid?HP-Li₃VO₄/C?and novel multivoids-assembledhierarchicallymeso-macroporousVPO₄nanospheres?MVHP-VPO₄@C NSs?.First,because of its unique microstructure with Li₃VO₄nanoparticles embedded in a continuous,interconnected hierarchically porous,highly graphitized carbon network,the resultant Li₃VO₄@HPC exhibits excellent lithium storage performance in terms of specific capacity,cycling stability,and rate capability when used as an anode material in LIBs.Second,MVHP-VPO₄@C NSs based on VPO₄ encapsulated in porous carbon not only inherit the advantage of the hollow spheres,but slso evidently decrease their excessive interior space to gain high tap density.As expected,MVHP-VPO₄@C NSs show good lithium storage behavior with gravimetric discharge capacity of 628 mAh g^-1 after 100 cycles at a current density of 100 mA g^-1.Furthermore,the full cell?LiFePO₄ cathode//MVHP-VPO₄@C NSs anode?also exhibits outstanding lithium storage performance.?4?Two types of novel VN nanomaterials have been successfully constructed via template-assisted solvothermal strategy followed by ammoniation.First,VN hollow spheres?VN HSs?assembled from porous nanosheets?NSs?were prepared by a facile template-assisted strategy.The VN HSs exhibit good cycling performance and high specific capacity as an anode for LIBs and excellent electrocatalytic activity as an electrocatalyst for the ORR,both mainly due to their unique nanostructure,which can provide shortened ion diffusion pathways and more active sites for Li⁺storage and O₂ adsorption and reduction.Second,integrated,binder-free,and lightweight porous VN network in situ grown on nitrogen-rich?N-rich?carbon textile network?N-C@VN?was constructed using the cheap and widely available cotton cloth as the template.Porous feature of VN network would provide more active sites and enable quick electron transfer.The N-C substrate not only serves as cuttrent collector,but also can store Li⁺,thus improving the electrode capacity.?6?The novel MoP/MoS₂ heterostructure nanocrystallines encapsulated into N,S,P-codoped,spherical carbon framework with a unique pomegranate-like architecture were constructed through freeze-drying method followed by an annealing process?MoP/MoS₂@NSPC NSs?.The resultant hybrid exhibits excellent catalytic activity for HER in both acid and alkaline media and also demonstrates high specific capacity and a long life span as the anodes materials in LIBs and SIBs.The good energy conversion and storage performance can be attributed to its unique pomegranate-like architecture,abundant interfaces,porous structure,good structural robustness and N,S,P-codoped carbon framework.