Construction And Synergy Mechanism Of High Performance Electrode For Lithium Storage And Catalysis

In the case of people excessive dependence on non-renewable fossil energy resources,and demand grow rapidly,the energy crisis and environmental problems have become the greatest threat to the sustainable development of human society.In order to protect the environment and rationally utilize energy,new clean renewable energy such as solar energy,wind energy,geothermal energy and hydrogenic energy have been widely used,Lithium ion battery and hydrogenic energy are among the select best in a number of new clean energies.Lithium ion battery has been applied to all aspects of people's lives because of its excellent performance,safe and nontoxic.While hydrogenic energy is also regarded as the best carrier to replace fossil fuels due to its advantages such as high combustion value,high efficiency,combustion product is water.However,the current commercial lithium ion batteries have been gradually unable to meet people's requirements for higher energy density and longer life.The popular hydrogen production by water splitting also can not meet the urgent demand for energy due to its low efficiency.Therefore,we must vigorously develop new anode materials of lithium ion battery with high energy density,long life and high safety performance to replace graphite electrode,and explore catalyst materials with high catalytic efficiency,cheap,easy to synthesized and environmentally friendly to replace noble metals such as platinum catalyst for catalyzing water splitting.Researches show that when material used alone usually with various disadvantages more or less such as poor conductivity,low stability and so on,combine two or more kinds of materials can compensate effectively for the deficiency of any kind of material when used solely,thus improve the performance of materials.Our paper is utilized simple preparation methods to combine different kinds of materials in consideration of improving the performance of them,and studied their performance when as anode for lithium ion batteries or catalyst for water splitting.The main contents and innovative points are as follows:(1)In chapter 3,from the point of structure designation,we firstly synthesized echinus-like SnO2 nanospheres by a facile hydrothermal method,then added the sulfur source for further reaction to achieve shell-shell-structured echinus-like SnO2@SnS2 nanospheres composite based on the Kirkendall effect,and researched its property of lithium storage when used as anode for lithium ion batteries.We can find that the structure of the composite developed into a hollow ball from a solid ball,which provides space for huge volume expansion of the material during charging/discharging processes.Compared with pure SnO2 nanospheres,shell-shell-structured SnO2@SnS2 showed better electrochemical performance of lithium storage.When the current density was 100 mA g-1,SnO2@SnS2 composites displayed a high capacity of 548 mAh g-1 after 100 cycles,while the capacity of the pure SnO2 after 60 cycles is only 403 mAh g-1.When the current density increased to 5 C,the reversible capacity of SnO2@SnS2 composites could still achieve 443.4 mAh g-1,compared to SnO2 displayed a reversible capacity of only 186.7 mAh g-1.Hierarchical and hollow structure could improve the problem of structure collapsed caused by volume expansion effectively,which could be proved by scanning electron microscopy(SEM)image characterized after cycle test.Electrochemical impedance spectra provides evidence for that the conductivity of SnO2 anode have improved after composed with SnS2.In addition,the synergistic effect between SnO2 and SnS2 also make contributions to improve electrochemical performance of electrode materials.(2)In chapter 4,from the angle of improving the shortcoming of the structure of material is easy to collapse,we prepared porous ZnCo2O4@TiO2 nanowall array composite based on ultilizing the excellent chemical stability of TiO2,and studied its performance of lithium storage when used as anode for lithium ion batteries.Compared to the pure ZnCo2O4,the ZnCo2O4@TiO2 nanowall array composite shows significantly improved electrochemical lithium storage performance.The ZnCo2O4@TiO2 electrode maintained a high discharge capacity of 827 mAh g-1 after 90 cycles at a current density of 100 mA g-1.While pure ZnCo2O4 electrode was displayed a discharge capacity of only 312 mAh g-1.The SEM image after the cyclic test proved that the excellent chemical stability of TiO2 played a crucial role for maintaining morphology of materials.The design of combining ZnCo2O4 with TiO2 is not only make full use of the high capacity of ZnCo2O4,but also take full advantage of TiO2 to maintain a stable structure,thereby improving the poor cycle stability of pure ZnCo2O4 nanowall array.(3)In chapter 5,to improve conductivity of materials and solve the problem of catalysts are easy to peel from the electrode,we prepared NiFe LDH@CNT nanosheet array composites with different quantity CNTs directly on nickel foam by hydrothermal method,and studied their catalytic performance when used as water electrolysis catalyst.The NiFe LDH nanosheets were coated on CNTs,and the CNTs are fixed on the surface of electrode.The NiFe LDH@CNT electrode combined with 0,3 and 6 mg CNTs requires over potentials of approximately 1.75,1.59 and 1.62 V versus reversible hydrogen electrode(RHE),respectively,to obtain current densities of 100 mA cm-2.The Tafel slope of NiFe LDH@CNT is 54 mV dec-1,much lower than the NiFe LDH@CNT electrode containing 0 mg and 6 mg CNTs.Cause CNTs can greatly improve the conductivity of the hydroxides,and their combination could fully utilize the high catalytic activity of NiFe LDH,which can effectively increase the catalytic performance of the composite for water splitting.(4)According to the results of chapter 4,the excellent chemical stability of TiO2 could prolong the life of composite materials in a large part.In chapter 6,from the perspective of the synergistic effect between different components and improving the stability of materials,we use atomic layer deposition method which with simplicity of operator and can control the thickness of the film accurately preparing dragon fruit-structured CuInS2/CdS quantum dots@ TiO2 composite nanofilms,and study its catalytic performance when used as photoelectrocatalyst.The electrochemical test results show that the photoelectrocatalyst with 4 nm TiO2 film has higher catalytic activity and better durability than that of the photoelectrocatalyst without TiO2 film or coated TiO2 films with diffirent thickness.The CuInS2/CdS quantum dots@TiO2 nanofilms with 4 nm TiO2 shell requires over potentials of approximately-0.219 V versus RHE to obtain current densities of 100 mA cm-2 under light illumination,compared with-0.382 V without light illumination.The CuInS2/CdS quantum dots@TiO2 nanofilms with 2 nm TiO2 shell,6 nm TiO2 shell,and CuInS2/CdS quantum dots require-0.306,-0.338,and-0.297 V,respectively,versus RHE to obtain current densities of 100 mA cm-2 under light illumination,compared with-0.364,-0.414,and-0.491 V,respectively,without light illumination.In addition,the catalyst that coated TiO2 nanofilm all showed a high durability for several days,while the current of pure CuInS2/CdS quantum dots arised a sharp decline after the test have been carried out two days.