Electrochemical And Photocatalytic Properties Of Fe-based Composite System

With the increasingly prominent energy and environmental issues,efficient conversion and utilization of solar energy have received increasing attention.However,the solar energy is intermittent and unstable due to the seasonal variations and day-night alternation.Thus,it is imperative to investigate and design a more appropriate battery energy storage system with high energy density,superior cycling stability,and excellent rate capability to make the solar resource more stable and continuous.Lithium ion batteries(LIBs)have been widely used in portable electronics,because of their high energy density and environmental friendliness.However,the poor rate and cycling performances restrict their further application in photovoltaic energy storage system.Consequently,in this thesis,the cycling stability and rate capability of electrode materials were mainly improved through the rational design of microstructure,leading to the guidance for the development of advanced LIBs,which are suitable for the photovoltaic energy storage system.Besides,a preliminary exploration on the photocatalytic degradation of organic wastewater was also given.Iron-based materials are widely used in energy storage and photocatalytic fields due to their earth-abundance,environment friendliness,and excellent electrochemical/photocatalytic performance.In this thesis,a scale of iron-based/carbon composites were constructed by optimizing the microstructure to mainly enhance their rate performance,cyclability,and photocatalytic performance.The main researches are as follows:(1)Here,three-dimensional(3D)graphene networks by hybridizing ZnFe2O4 nanoparticles with the ultra-small(USGN)synthesized by Photo-Fenton reaction and large graphene sheets(GN)were constructed.Such 3D interconnected double graphene networks can provide fast electron/ion transfer paths,accommodate the volume change,and prevent the pulverization of the electroactive materials during cycling.As a result,the ZnFe2O4/USGN/GN electrode exhibits a large reversible capacity of 1257 mAh g-1 at 0.1 A g-1,excellent rate capability(575 mAh g-1 at 0.1 Ag-1),and superior cycling stability(475 mAh g-1 at 1 Ag-1 even after 2000 cycles)simultaneously.(2)Fe2O3 nanowires with a diameter of?2 nm uniformly anchored on conductive substrate of 3D cross-linked graphene ribbons were successfully fabricated by rapid solvothermal method.Owing to the unique 3D cross-linked conductive networks(electron transport pathways)and interconnected hierarchical porous structures(Li+ diffusion channels),the Fe2O3/GR hybrid can effectively improve the electrical conductivity and ion diffusion,especially at high rate and mass loading.Besides,the well-developed elastic networks can protect the bulk electrode from further pulverization and self-aggregation during repetitive cycling.As a result,the Fe2O3/GR hybrid exhibits high-rate performance(632 mAh g-1 at 5 A g-1)and long cycle life Li-storage performance(92%capacity retention after 3000 cycles).Moreover,the Fe2O3/GR with high mass loading(?4 mg cm-2)can still exhibit excellent rate capability(223 mAh g-1 even at 2 A g-1).(3)Here,flexible CoFe2O4/N-doped carbon nanofiber film(CoFe2O4@N-CNF)was successfully synthesized by electrospinning and consequent post-treatment,which possesses 3D cross-linked conductive networks and interconnected hierarchical porous structure with fast ion/electron diffusion pathways.Meanwhile,the CoFe2O4 nanosheets with stable carbon shell uniformly anchored in carbon fibers can effectively mitigate the volumetric expansion/constriction during cycling.Moreover,the defects in N-doping(12.7 at%)disordered carbon are also beneficial for enhancing specific capacity.As a consequence,the optimized CoFe2O4@N-CNF exhibits a high reversible capacity(858 mAh g-1 at 0.1 A g-1),excellent rate capability(306 mAh g-1 at 30 A g-1),and superior cycling stability(10000 cycles with almost no capacity loss at 10 A g-1).(4)Here,multi-layer stacked LiFePO4/N-GQD composite with N-GQD intercalated in-between LiFePO4 sheets was successfully synthesized though solvothermal method,which owns a relatively narrow band gap,leading to high utilization of visible-light.Moreover,the incorporation of N-GQD into LiFePO4 sheets can also effectively prevent the self-aggregation and increase the surface areas of photocatalyst,resulting in high photo-fenton degradation efficiency of RhB.