Controllable Synthesis And Supercapacitor Performance Of Three-dimensional Nanoporous Graphene-based Materials

Graphene possesses unique two-dimensional?2D?monolayer or few-layers carbon atoms nanostructure.Due to the numerous excellent performances of graphene,intense attention has been drawn in the scientific and industrial communities,such as electronic,optics and mechanical performances.However,2D graphene tends to stack which influences its properties.The integration of 2D graphene to three-dimentional?3D?graphene with designated architectures is an effective way to solve this issue.Moreover,the applications of the 3D graphene are usually limited by some drawbacks,such as poor conductivity,too large and uncontrollable porous structure.Therefore,it is urgent to explore a novel method to fabricate the free-standing 3D porous graphene films with high electrical conductivity and controllable pore size and structure.In the dissertation,using nanoporous copper as the substrate and catalyst,free-standing 3D nanoporous graphene and its hybrids films with 2D coherent electronic properties of graphene and controllable pore size are successfully fabricated by chemical vapor deposition?CVD?approach.The electrochemical performances and interaction mechanism are studied systematically.The detailed research content is as follows:1)Continuously hierarchical nanoporous graphene?hnp-G?films with interconnected spherical-like structure are synthesized by a combination of low-temperature CVD and rapid catalytic pyrolysis process.The influences of calcination temperature and time on morphology evolution of NPC,and the structure,specific surface area and electrochemical performance of hnp-G film are investigated.It is revealed that low-temperature growth of HG coating on NPC can obviously delay the coarsening evolution of NPC at high temperature,providing the precondition to obtain hnp-G with small pore size?<150 nm?and large specific surface area?1160m²/g?with external surface area of 954.7m²/g.More importantly,the continuously 3D hierarchical nanoporous structure and fully wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic squeezing but also guarantee the continuous and short electron/ion diffusion pathway in the whole electrodes.The symmetric SC offers high specific capacitance?38.2 F/cm³?,good rate performance,high energy density?2.65 mW h/cm³?and power density?20.8 W/cm³?,as well as good bending stability and excellent lifetime?94%retention after 10000 cycles?,suggesting its wide application potential in powering wearable/miniaturized electronics.2)Nitrogen and oxygen co-doped 3D multi-level nanoporous graphene?NO-3DnpG?films are synthesized through a rapid catalytic pyrolysis of hydrogenated graphite in NH₃ atmosphere using nanoporous copper as catalyst and further concentrated nitric acid treatment.The influence of calcination temperature,ammonia and acidification on the structure,dopant,specific surface area and electrochemical performances of 3D graphene films are investigated.Results show that the coarsen process of NPC will be accelerated under the ammonia atmosphere,resulting in the fabrication of 3D graphene film with macro-meso-microporous architectures,which preserves a coherent electrons/ions transmission channel and provides larger specific surface area and more active sites to improve the supercapacitor performances of electrode materials.The N and O atoms are introduced by annealing under ammonia atmosphere and further HNO₃ modification.The working voltage window,electrical conductivity,specific surface area and electrochemical performance of NO-3DnpG films are dramatically improved by the synergistic effects of N and O dopants.The as-prepared NO-3DnpG based flexible solid-state symmetric supercapacitors?SSCs?device displays high energy and power densities,good cycling durability and can be reversibly charged/discharged at a voltage of 1.6 V.3)Flexible high-quality nitrogen and oxygen co-doped 3D nanoporuos duct-like graphene@carbon nano-cages films are obtained by CVD approach under high temperature,low pressure and short-term condition using nanoporous copper?NPC?as the substrate and further modified by HNO₃.The influences of the CVD process parameters,gas species,ammonia and HNO3 modification on the morphology,dopants and electrochemical performances of the prepared 3D graphene films are investigated.It is revealed that the 3D nanoporous duct-like graphene coated with carbon nano cages with thinner layers due to the etching of NH₃ are obtained by short-term CVD approach.The 3D graphene retains the 2D coherent electronic properties of graphene,and the existence of carbon nano-cages combining with the microporous structure caused by heteroatom-doping greatly increase the effective specific surface area.Moreover,heteroatom-doping and oxygen-containing groups on the surface of graphene can obtain extra redox capacitance and good wettability between the electrode and the electrolyte.Due to the high electronic conductivity and large specific surface area,3D graphene film with abundant controllable micro-meso-macro porous structure is an ideal electrode material for supercapacitors,such as aqueous-system,ionic-system,and lithium-ion capacitors,which all show high energy density,power density and exlcellent cycle stability,4)With the similar above process,free-standing 3D nanoporous duct-like graphene?3D-DG?films with high flexibility are obtained under low carbon sources and robustness as the backbones to deposit MnO2 nanosheets?3D-DG@MnO2?.The growth mechanism of 3D-DG and the influence of electrochemical deposition time on the morphology and electrochemical performances of MnO₂ are studies.Furthermore,the flexible asymmetric supercapacitors?ASCs?device is fabricated using3D-DG@MnO₂ hybrid film as the positive electrode and 3D nanoporous graphene?3D-npG?film as the negative electrode.It is revealed that the 3D-DG is the ideal support for the deposition of large amount of active materials because of its large surface area,high conductivity,appropriate pore structure,and negligible volume,in which charge carries move rapidly with a small resistance through the high-quality and continuous graphene building blocks,which results in a high rate performance.Marvelously,ultrathin?⁵0?m?ASCs device displays ultrahigh volumetric energy density(28.2 mW h cm^-3),power density(55.7 W cm^-3)and cycle stability at 2.0 V.5)Nitrogen and oxygen co-doped 3D rivet graphene?H-3DRG?film with high flexibility,conductivity,superhydrophilic and microscale porous structure is obtained using the combination of short-time pre-calcination and CVD approachs,where the coarsing process of NPC at high temperature is controlled.Furthermore,arranged NiCo double hydroxide?NiCo-LDH?nanoneedles are deposited onto the surface of H-3DRG film via hydrothermal approach,and hierarchical NiCo₂S₄ nanostructures by further in-situ anion exchange process.The growth mechanism of the H-3DRG films and the influence of dopants on the hydrophilicity of H-3DRG are investigated.Moreover,the influence of hydrothermal time and anion exchange on the morphology and electrochemical performances of products are also studied.Results show that the existence of carbon nanorivets improve the mechanical strength and structural stability of 3D graphene.Vertical orientated NiCo-LDH and NiCo₂S₄ are strongly anchored onto the inner and outer side of hydrophilic H-3DRG film due to the abundant heteroatoms dopants,which significantly reduces the volume and weight percentage of the carbon-based substrate in the entire hybridized electrodes.Such outstanding structural advantages accelerate the reaction kinetics and enhance the electrochemic alactivities which make the H-3DRG@NiCo-LDH and H-3DRG@NiCo₂S₄ based asymmetric supercapacitors devices both show high energy densities of 78.8 Wh/kg and 75.1 Wh/kg based on the entire devices,respectively.