Study On Construction Of Micro-nano Structure Materials Based On Conductive Framework-transition Metal Compounds And Their Electrochemical Performance

As a new energy storage device,supercapacitors have effectively overcome the limitations of traditional capacitors and batteries.They have received extensive attention in scientific research and practical applications due to the highly power density,long cycle life and fast charge and discharge capability.However,its relatively low energy density compared to batteries is the key to limiting the wide application.According to the energy density equation E = 0.5(CV)2,it can be seen that the key for increasing the energy density is to increase the specific capacity C and broaden the operating voltage window V of the supercapacitor.As we all know,the electrode materials play the most important role among all of the components for the supercapaciotrs.The energy storage reactions in supercapacitors mainly occurs on the surface or in the bulk phase of the electrode materials Therefore,the micro-nano structure of the electrode material such as specific surface area,pore size distribution,interfacial effect,chemical and thermodynamic stability,etc.,have the important influence on the exposure of the reactive sites and the extension of cycle life for the electrodes as well as the electrolyte ion diffusion and transportation,which plays a crucial role in the specific capacity and energy density.Therefore,design and construct the ideal electrode materials with reasonable micro-nano structure and high electrochemical performance is the main concept of this paper.Firstly,based on the principle of "natural law",in this paper,we select the biomass-derived materials as the source to obtain hollow fiber-like hierarchical porous carbon materials with large specific surface area and high conductivity by regulating the micro-nano structure of the carbon materials,which in turn would enhances the energy density of the supercapacitors.Secondly,in order to further improve the energy density of the supercapacitors and overcome the limitations of the theoretical specific capacity of the carbon materials,the pesudocapacitor electrode materials with high theoretical specific capacity are used to obtain the high conductivity of three-dimensional self-supporting micro-nano structure hybrid electrode materials.The hybrid electrode materials are enriched with redox electrochemical active sites.At the same time,the assembled asymmetric supercapacitors with the carbon-based materials have the high energy density enhanced by the high specific capacity and wide working voltage window.Subsequently,metal organic frameworks(MOFs)riched with porous carbon-based specific surface area and pore structure and abundant pesudocapaciotr reactive metal centers were designed and controlled to obtain a three-dimensional self-supported conductive MOFs array structure to improve the energy density of the supercapacitors.Finally,we fabricated the three-dimensional self-supported hybrid micro-nano array electrode material with the special structural metal-organic frameworks,and the high theoretical specific capacity pesudocapacitor materials to improve the specific capacity and energy density of the capacitors.The details are as follows.In supercapacitors,carbon-based materials store charge mainly through electric double layer theory,and the electrochemical energy storage performance of electrode materials can be effectively improved by reasonably regulating the micro-nano structure of carbon materials,such as the specific surface area and pore size distribution.Firstly,we prepare the hierarchical porous carbon materials by utilizing the structural characteristics materials in nature,such as biomass raw materials,through high-temperature carbonization and activation.Therefore,the biomass materials with abundant specific surface area and pore size distribution occupy an important position in the preparation of carbon-based electrode materials due to their advantages of low price and wide sources.In this paper,the platanus fruit catkin fibers(PFs)and willow catkins fibers(WFs),which are more common in daily life and have obvious physical and chemical structural features,are used as precursors to prepare graded porous hollow tubular carbon materials through carbonization and subsequent KOH activation.In supercapacitors,the micropores are mainly used to store energy,mesopores are responsible for the transport of electrolyte ions,and macropores are used to store the electrolyte.Therefore,the co-existence of ordered pore structures at all levels is the key to improving the electrochemical performance of electrode materials.The carbon-based materials derived from biomass are mostly powdery and highly dependent on the activation process of the activator such as KOH.The PFs and WFs selected in this paper are rich in inorganic salt ions such as K+ and special hollow tubular structures,which endows the biomass self-activation ability in the initial carbonization process.Especially for the PFs,there would be some meso and macropores on the surface of the hollow fibers owning to the self-activation in the process of carbonization.Part of the mesopores and macropores ensure the subsequent mixture of the activator KOH with the carbon material abundantly,then promoting the generation of the pores and increasing its specific surface area.At the same time,influenced by the reactivity between KOH and carbon materials,the ratio of activator to carbon material and reaction temperature all affect the activation degree of carbon materials.There would be an ideal physicochemical structure and electrochemical performance for the PFs when the activated temperature was fixed at 900 ? and the mass ratio of KOH to carbonized PFs was fixed at 3:1.WFs can obtain the best porous carbon materials at 800 ? under the same KOH dosage,meanwhile,the structural properties and electrochemical properties of PFs are superior to that of WFs.The highest specific capacity of PFs in the three-electrode system was 304.65 F/g(WFs,276.13 F/g).In the two-electrode system,the capacity retention of PFs at 4 A/g was 97.03%(WFs 800,91.12%)after cycling 104 circles.The highest specific energy density is 39.44 Wh/kg for PFs,(WFs 800,36.19 Wh/kg).In the above work,it's an effect way to improve the electrochemical properties of electrode materials by the preparation of ordered porous carbon materials with ideal micro-nano structure.However,the strucrural design of the electrode were mainly based on the pristine materials and high temperature calcination.At the same time,limited by the reaction mechanism of carbon materials,the lower theoretical specific capacity of carbon materials limits the increase of the energy density of the electrode materials.Therefore,we use a chemical method with higher controllable means to adjust the micro-nano structure of the pesudocapacitor material whose reaction mechanism is Faraday redox reaction to improve the electrochemical properties such as the energy density of the capacitor more scientifically and effectively.In this paper,the author synthesized the three-dimensional self-supporting Ni/Co bimetallic porous oxide nanosheet arrays through electrochemical deposition method using the flexible and highly electrochemical conducted graphite fibers as the substrate.The surface nitridation leads to a 3D "pearled-veil" network structure,in which Ni-Co-N nanospheres are in situ mounted on NiCo2O4 nanosheets' electrode forming the Ni-Co-N/NiCo2O4 nanosphere/nanosheets hybrid electrode.It is demonstrated that the hybrid materials are more potent than the pure NiCo2O4 in energy storage applications due to a cooperative effect between the constituents.In the hybrid structure,the electrode electtochemical conductivity and the redox reaction intensity of the electrode materials are enhanced by the metal-level electrochemical conductivity,low valence states of Ni-Co-N,and the interfacial effects between Ni-Co-N and NiCo2O4,and the oxygen deficiency in NiCo2O4.Meanwhile,ownig to the cooperative effect between the constituents,the hybrid electrode promotes the transportation of electrolyte ions on the surface of the electrode material or its bulk phase,reduces the reaction barrier and accelerates the reaction kinetics process,thereby improving the electrochemical properties such as the specific capacity and energy density of the electrode materials.At the same time,the author assembled asymmetric flrxible supercapacitors using graphite fiber supported nitrogen-doped carbon nanotubes as the with the 3D self-supporting Ni-Co-N/NiCo2O4 electrode materials with solid state electrolyte,widening the voltage window and realizing the concept of flexible wearable electronic devices.It is well kown that the reduction activities of ammonia gas are dependent on the reaction temperatures,which has a great influence on the physicochemical properties and structure of the electrode materials during the anneling process.According to the result of this paper,there would an ideal hybrid structure when the temperature was fixed at 400 ?.The highest specific capacity of Ni-Co-N/NiCo2O4 in the three-electrode system is 384.75 mAh/g at 4 A/g,and the capacity retention is 86.5%as the current is increased to 20 A/g,which is better than that of the electrochemical performance of NiCo2O4(256.86 mAh/g,76%).As for the flexible supercapacitor,the Ni-Co-N/NiCo2O4/GFs?N-CNTs/GFs has higher specific capacity,energy density,rate performance and cycling stability in the asymmetric capacitors.The maximum specific energy density and power density are 125.89 Wh/kg and 36.23 kW/kg,respectively.In the above two schemes,it attempts to improve the physical and chemical structure characteristics and the electrochemical properties through regulating the micro-nano structure of the electrode materials.At the same time,we also fabricated the flexible wearable electronic devices with the three-dimensional self-supporting micro-nano structured electrodes.However,it is a high temperature depended process for the regulation of the micro-nano electrode materials.Therefore,how to prepare the electrode materials safely,controllably,easily and in batch by cryogenic chemical method and make it obtain the advantages of double layer and pseudocapacitor electrode materials in the same time is a new direction of electrode material preparation in the future.Metal organic framework materials(MOFs),as an emerging energy storage material,have attract widespread attention owning to the large specific surface area(1000-10000 m2/g),highly ordered and adjustable pore size distribution and the rich faradic reactive metal centers.In this paper,we select the proper organic ligands and metal coordination center using hydrothermal method to regulate the micro-nano structure of the electrode materials.which overcome the defects of the high temperature regulation in the above two solutions and are more secure,simpler and easy to operate for the structure design of electrode materials.At the same time,MOFs structure can effectively improve the electrochemical properties of electrode materials,such as energy density,by combining the characteristics of large specific surface area,abundant pore size distribution and pseudocapacitor-based metal reactive metal centers.In this paper,the author used the carbon cloth as the substrate,nickel/cobalt atoms as the metal reactive center,and 2,3,6,7,10,11-hexahydroxytriphenylene(HHTP)as the organic ligand to prepare the three-dimensional self-supporting Ni-MOFs and Co-MOFs nanopillar array.The MOFs array prepared in this paper overcoming the defects of the traditional MOFs which occupy relatively lower conductivity and can be directly used as the electrode materials owing to the enhanced electrochemical conductivity.At the same time,the MOFs array has a large specific surface area and a rich pore size distribution and can to promote the transmission of electrolyte ions on the surface or in the bulk of the MOFs,shorten the ion diffusion distance and promote the electrolyte reacts with the metal reactive center adequately.When used as an electrode material,the MOFs is mainly based on faradic redox reactions and accompanied by the double layer phenomenon,which reduces the polarization phenomenon of the electrode material during the reaction.At the same time,the MOFs have the better specific capacity than that of conventional electric double layer capacitor.Compared with the Ni-MOFs,the Co-MOFs demonstrate better electrochemical properties due to their physicochemical characteristics.In the three-electrode system,the specific capacity of Co-MOFs at 16 A/g was 161.8 mAh/g(Ni-MOFs 157.64 mAh/g).The assembled Co-MOFs/CC ? Co-MOFs/CC flexible supercapacitors have a maximum specific energy density of 75.6 Wh/kg and an energy density of up to 52.02 Wh/kg at a maximum specific power density of 1.65 kW/kg.In addition,considering the structural characteristics of MOFs,this paper combines the traditional high theoretical specific capacity layered double transition metal hydroxide Ni,Co-LDH and Co-MOFs to prepare a three-dimensional self-supporting nanopillar/nanosheet hybrid arrays.The regulation of the microstructure of layered bimetallic transition hydroxide is also mainly dependent on the low temperature chemical method and has been widely used in supercapacitors with its high theoretical specific capacity.But the limitation of the specific surface area and the spacing between layers hinders the full demonstration of the specific capacity and the widening of the voltage window.Based on the physical and chemical structure and the synergistic effects between the two materials,the regulation of the hybrid micro-nano structure formed MOFs/LDH can promote the transport of electrolyte ions between LDH layers and enhance the electrochemical properties.In this paper,the Ni,Co-LDH nanosheet array structure was prepared by traditional hydrothermal method using carbon x cloth as the substrate.Then,the Co-MOFs were synthesized on the Ni,Co-LDH nanosheets array by the second hydrothermal method according to the above MOFs preparation scheme.A Co-MOFs/Ni,Co-LDH hybrid array was obtained.The high specific surface area and abundant pore size distribution of the MOFs in the hybrid structure act as the high-speed ion channels and ion storage libraries.At the same time,the oxygen defects and interface effects between the Co-MOFs and Ni,Co-LDH accelerate the transmission process for the electrolyte ions and reduces the reaction barrier and polarization degree of the electrode material,then increasing the specific capacity and rate performance for it.In the three-electrode system,the specific capacity of Co-MOFs/Ni,Co-LDH at 2 A/g is 302.57 mAh/g,which is greatly improved compared with the Ni,Co-LDH(176.11 mAh/g).Based on the design of the micro-nano structure of electrode materials,this paper firstly uses the structural characteristics of biomass materials to control the micro-nano structure of the electrode materials through calcination and activation to abtain the high specific surface area and functional pore size distribution.The hollow tubular porous carbon materials improve the electrochemical performance of the electrode material and enhance the energy density of the supercapacitor.Subsequently,in order to break through the limitation of the carbon-based electrode materials owning to the structural characteristics and lower theoretical specific capacity,we prepared the three-dimensional self-supporting binary transition metal oxide nanosheet arrays by controllable chemical synthesis method,and through structural regulation,the introduction of a highly conductive metal nitride nanosphere to prepare a three-dimensional self-supported"pearled-vile" nanosheet connected nanosphere composite structure.The composite nanostructure effectively increases the specific surface area and exposes abundant active sites.These features in turn enhances the electrochemical performance of the electrode material.In the future,the struactural design of the electrodes with high electrochemical performance should based on the concepts of low temperature,controllable,safe and easy to operate.Therefore,in this experiment,the structural three-dimensional self-supported metal organic frame work arrays prepared by low-temperature hydrothermal method was synthesized,which have large specific surface area,highly ordered and controllable pore size distribution and rich metal active center.The structural design of the metalorganic frameworks and the layered double transition metal hydroxide hybrid structure through low temperature hydrothermal synthesis method providing a promising value for the electrode materials' structure design and construction in the future.