Synthesis And Electrochemical Properties Of Nitrogen-containing Carbon And Its Nano-metal Composites As Electrodes For Supercapacitors

Efficient electrochemical energy storage devices are an important part of renewable energy technologies,because they can effectively solve the problem of continuous stability in energy technology storage.As a new type of high efficiency electrochemical energy storage device,supercapacitor is considered to have a very large application prospect in the field of electrochemical energy storage due to its high power density,long cycle life and fast charge and discharge performance,and this also aroused widespread concern and research in the scientific community.Benifiting from these advantages,the supercapacitor has been widely used in communications,transportation,electronics,aviation and some other related fields.At present,most of the commercial supercapacitor electrodes are made of cheap activated carbon,which was the most commonly used as conductive materials,considering its rich resources,high surface area,conductive thermal conductivity,good chemical stability and broad operating temperature range.However,compared to conventional lithium-ion battery,the carbon-based supercapacitor with typically restricted surface area and pore size distribution has a relatively low specific capacity(?150 F g-1),and the energy density of the current commercial carbon-based supercapacitor is low(usually less than 10 Wh kg-1),which limits its use as the energy storage material.In order to solve this problem and improve the electrochemical performance,carbon materials can be combined with pseudo-capacitive materials such as transition metal oxides and conductive polymers in recent years,which integrate the double layer capacitance behaviour and pseudocapacitance behavior to exhibit excellent electrochemical performance.This is mainly due to the synergistic effect of the two materials,and the electrochemical performance of the final composite electrode material is remarkably improved.On the basis of literature research,this paper pays particular attention to another approach for heterogeneous structured materials with multi-components,and each component play a different role in enhancing electrochemical performance,exhibiting synergistic properties by integrating the individual components.The effects of different structural properties on the final products were elucidated by simple and effective methods,combined with microstructure characterization and electrochemical analysis,which provides the necessary physical and chemical parameters for further studying the relationship between the structure and electrical properties of nitrogen-containing carbon composites.The basic content of the study is summarized as follows:1.Nitrogen-doped carbon materials with multi-porous structure was prepared through carbonizing melamine-urea-formaldehyde(MUF)resins under N2 atmosphere,followed by KOH chemical activation.In the preparation process,the SBA-15 template can produce the mesoporous structure,and the final products exhibit high specific surface area and high nitrogen content.As expected,the obtained carbon(MUFCx)exhibits a high surface areas of up to 2563.4 m2 g-1 with nitrogen content changing from 14.25 to 11.68 at.%,respectively.Electrochemical evaluation of the sample carbonized at 750 °C displays notable capacitance properties,such as an enhanced specific capacitance(375 F g-1 at 0.5 A g-1),excellent rate capability(272 F g-1 at 10 A g-1),as well as excellent electrochemical stability(-95%capacitance retention over 2000 cycles).Furthermore,an as-fabricated ASC device shows an excellent energy density of 23.43 W h kg-1 at a power density of 450 W kg-1 operated in a wide operating voltage window of 1.8 V,suggesting their ideal candidates for future supercapacitors application.2.The graphene materials with high conductivity,large surface area and good cycle performance are considered as the most promising candidates for supercapacitors.In this work,the two-step process involving chemical activation and high temperature calcination is provided for the synthesis of nitrogen-containing hierarchical porous carbon/graphene composite(3DHCG),and subsequently investigate the effects of different contents of graphene on the pore structure,nitrogen content and electrochemical properties of the obtained materials.It was found that the introduction of graphene could significantly improve electrochemical performance of activated carbon materials,because the specific surface area of 3DHCG composites was enhanced with the increase of graphene content.However,with the high concentration of graphene,defective graphene hinders the transport of electrons,thereby reducing the electrochemical performance of the overall composite electrode.When the concentration of graphene oxide rises to 0.5 mg ml-1,the prepared 3DHCG composites have the best electrochemical performance,and the multi-porous network structure is favorable for the rapid diffusion and transport of ions during rapid charge and discharge.Compared with pure carbon,the resulted 3DHCG composites exhibit a high specific capacitance of 320 F g-1 at 1 A g-1 in the 6 M KOH solution,and when the current density increased form 1 A g-1 to 20 A g-1,the specific capacitance of 3DHCG still maintains 225 F g-1 with a retention rate of 70%.Moreover,the specific capacitance of the composites only decreases by 4%after the 2000 cycle.3.On the basis of the synthesis of carbon-based composites,homogeneous nano precipitation and multi-stage freeze-drying were used to prepare homogeneous inorganic nanoparticles-carbon aerogels composites,combining original relatively independent carbon aerogels and the subsequent formation of metal oxides.This in situ precipitation method facilitates the uniform incorporation of inorganic nanoparticles in organic frameworks,while improving multi-stage freeze-drying hole making technology.When compared to other currently existing technology,this novel one-step fabrication method greatly simplifies the preparation process steps.First,we used this method to prepare nitrogen-doped carbon aerogels/cobalt oxide(NCA/Co3O4)composites.The effects of different amounts of Co3O4 on the structural properties,specific surface area and electrochemical properties of the composite electrode material were investigated.The results show that the composite electrode with Co3O4 content of 75%exhibits excellent electrochemical performance,and the specific capacitance was calculated to 616 F g-1 at a current density of 1 A g-1,even the current density was increased to 20 A g-1,the specific capacitance still remains at 445 F g-1.Moreover,the NCA/Co3O4-3 composite was used as the positive electrode and NCA as the negative electrode,and then they was pressed individually and assembled into an asymmetric supercapacitor.Finally,the solid-state asymmetric supercapacitor with a maximum voltage of 1.5 V delivers a high energy density of 33.43 Wh kg-1 and and a maximum power density of 375 W kg-1.In order to further expand the operating voltage window of solid state devices,thereby improving the energy density and power density of device,we designed a manganese oxide/carbon aerogels(MnO/NCA)composite as a positive electrode,iron oxide/carbon aerogels(Fe2O3/NCA)composites as a negative electrode for high performance asymmetric supercapacitors(ASC).The prepared MnO/NCA composites have a highly interconnected network structure in which the MnO nanoparticles are uniformly embedded into the three-dimensional nitrogen-doped carbon skeleton.Compared to other MnOx-based materials,the MnO/NCA electrode exhibits a higher specific surface area due to the synergistic effect of the highly conductive carbon aerogels and the high pseudo-capacitive metal oxide,which greatly enhances the ion transport and obtains excellent electrochemical performance.By matching the Fe2O3/NCA with this electrode,the novel ASC device with high voltage window of 2.0 V was fabricated,and delivers an high energy density of 48.67 Wh kg-1 at a power density of 1000 W kg-1,showing a good potential for application in supercapacitor.4?With the natural bamboo-derived carbon fiber as the flexible substrate,a simple two-step hydrothermal method was used to synthesize the mixed ternary transition metal sulfide as the electrode material for high performance energy storage device.During the preparation process,the new Ni-Co-Zn-S ternary nanowires are grown on the bamboo fiber substrate and applied directly to the supercapacitor electrode materials,avoiding the use of polymer adhesives and other conductive additive materials.The new novel bamboo fiber@Ni-Co-Zn-S composite exhibited excellent electrochemical behavior,having a high specific capacitance of 1631 F g-1 at low current density of 2 A g-1,which was better than the previously reported binary metal sulfide.More importantly,the all solid state supercapacitor was fabricated using two bamboo fiber@Ni-Co-Zn-S as the positive and negative electrode,and the cells exhibits an energy density of 12.36 Wh kg-1 at a power density of 500 W kg-1.Electrode cycle life test displays a remarkable cycling performance with 92%retention after 5000 cycles.This outstanding capacitance performance is mainly due to the synergistic effect between the porous acicular nanostructures and the multiple elements of Ni,Co and Zn in the multiple metal oxides.