Introduction of 3D graphene and its composite materials

1 Introduction

Graphene (graphene) is a single-layer two-dimensional (2 d) carbonaceous material that is closely packed from carbon atoms. Graphene has attracted much attention due to its excellent electrical, optical, mechanical and other properties. At present, graphene (graphene G or chemically reduced graphene oxide rGO) and functionalized derivatives thereof can be prepared by mechanical stripping method, epitaxial growth method, chemical vapor deposition method, chemical reduction method, and the like. The integration of 2 d graphene to construct a graphene assembly with a specific three-dimensional (3 d) structure and the preparation of functional devices with excellent performance is of great significance for expanding the macroscopic application of graphene. The 3D structure can impart unique properties to the graphene assembly. Such as flexibility, porosity, high activity specific surface area, excellent mass transfer performance, etc. Therefore, in recent years, the preparation and application of graphene materials at home and abroad are very active. Researchers have successfully prepared a variety of 3 d graphene materials with micro/nanostructures using directional flow self-assembly methods, template synthesis methods, etc. 3D micro/nanostructures can be assembled by graphene sheets, porogens are introduced, and replication Methods such as template structure are obtained. In addition, graphene can be effectively compounded with other functional materials to form composite materials during the formation of 3 d micro/nanostructures. Studies have shown that 3 d graphene and its composite materials have the inherent properties of graphene, and have superior performance and wider performance than 2 d graphene materials in energy storage, catalytic reaction, environmental protection, and flexible/expandable conductive materials. Application prospects.

At present, researchers have reviewed the preparation methods and applications of 3 d graphenes from different classification perspectives. This paper combines the current research status, catalytic reaction, hydrogen storage, environmental restoration, sensor construction for 3 d graphene and its composite materials. , the application of five aspects of supercapacitors is reviewed. At the same time, a brief review of the current 3d graphene materials in the application of research challenges and development direction.

Preparation of 2, 3 d graphene

Graphene is a planar two-dimensional layered material with a honeycomb lattice structure. 3D graphene is composed of 2 d graphene sheets and has a specific three-dimensional micro/nanostructure. So far, researchers have established preparations 3 d various methods of graphene, for example: (1) directional flow assembly method: the graphene oxide (de) solution is filtered through a porous membrane, and then chemically reduced to obtain unsupported 3 drgo paper; (2) Solvent/hydrothermal method: For hydrothermal reduction of the film, the volume of rGO is expanded by the CO2 and H2O produced by the additive to obtain a 3 d porous material; (3) Template interface assembly method: for example, the surface of the solution is condensed The water droplets are induced by the template to self-assembly, and then subjected to subsequent drying and pyrolysis of the film to promote de-thermal reduction to form an elastic hydrophobic 3 drgo film; (4) chemical vapor deposition (CVD): such as using a three-dimensional porous nickel film as a template, high temperature Decomposition of methane to grow graphene, etching of template nickel with hydrochloric acid or FeCl3 to obtain a three-dimensional graphene foam (3 dgf) having a penetrating pore structure.

It can be seen that the 3 d micro/nanostructure can be generated by random or pore formation during the 2 d graphene integration process, or by replicating the template material morphology. In summary, the 3 d graphene material is formed by the integration of 2 d graphene sheets. In addition to the inherent properties of graphene, the specific 3 d micro/nanostructure gives it new properties.

3, 3 d application of graphene and its composite materials

3.1, application in catalysis

Graphene not only acts as a catalyst itself but also serves as a carrier for other catalysts. The porous through-network structure of 3D graphene material is not only beneficial for ion diffusion and reduces mass transfer dynamics, but also provides unique transfer of charge for rapid transfer and conduction. Conductive path. Therefore, the catalyst based on 3 d graphene and its composite materials has unique structure and properties and has been used to catalyze alcohol oxidation, hydrazine oxidation, oxygen reduction, peroxidation, organic coupling reactions, etc.

Mulchandani et al first modified the carbon nanotube pillared graphene (G-MWNTs) prepared by CVD method onto a glassy carbon electrode (GCE), and then electrochemically deposited Pt nanoparticles to finally obtain Pt / G-MWNTs GCE.G- The MWNTs complex has a large surface area and facilitates the diffusion of substances, and Pt and MWNT can promote the charge transfer. Therefore, Pt / rGO-MWNTs / GCE can efficiently catalyze the oxidation of methanol. Qu et al. Dpt / PdCu rGO complex. The complex has a strong oxidation effect on ethanol. Its catalytic performance is much higher than that of pure Pt and Pd-Cu electrodes, which is four times the catalytic ability of commercial Pt / C catalysts.

Chendeng prepared N-doped 3 d porous graphene (NHG) by one-step hydrothermal method. Since the pore area is as high as 25% of the surface area, there are more active catalytic sites at the edge of the NHG sheet, and the doping of N further increases the catalytic activity. The material can effectively catalyze the oxidation reaction of hydrazine and the reduction reaction of oxygen. At the same time, the 3 d porous structure not only effectively prevents the accumulation of graphene, but also facilitates the diffusion of reactants and electrolytes. Studies have shown that 3 dnhg is superior to the commercial mass fraction of 10%-20% Pt / C catalyst in terms of power generation, current limiting, and methanol permeation resistance. Fan et al. prepared N-doping by pyridine pyrolysis. Dmwnts / graphene composite, this material can be electrocatalyzed by oxygen reduction. Feng et al. successfully prepared N-doped, Fe3O4 composite graphene aerogel (Fe3O4 / smoke) by hydrothermal self-assembly, freeze-drying, and heat treatment. Gas composition). Due to its porous structure and high specific surface area, the material can be electrocatalyzed by oxygen reduction and has high current density, low ring current, low hydrogen peroxide production, high electron transfer number, and higher endurance than commercial Pt / C catalysts. Features and can be used in fuel cells.

The 3d porous rGO was synthesized by an indirect cold template method and then formed with 3 dag / rGO with silver nanoparticles. The material not only has a good catalytic effect on the reduction reaction of 4-nitrophenol and Suzukie Miyaura coupling reaction but also is easy to remove from the reaction system, thereby avoiding cumbersome post-treatment. Zhao et al. use CVD method with acetonitrile as a carbon source. Nickel nanoparticles were used as catalysts to prepare MWNTs in situ on 3 drgo. The unique porous structure and electron transfer properties of the composite are effective for photocatalytic degradation of the dye Rhodamine B.

3.2 Application in hydrogen storage and other gas adsorption

The increase in human demand for environmentally friendly fuels has led to the development of high-capacity hydrogen storage materials. Through theoretical calculations and experimental studies, the scientists investigated the gas adsorption properties of carbon nanotubes composited, elementally doped rGO 3d composites.

Fang Zhouzi et al. studied the effects of different environmental factors on the hydrogen adsorption capacity of MWNTs pillared 3 d graphene materials by molecular dynamics simulation. The results show that low temperature, high pressure, large gap and increasing the diameter of MWNTs are beneficial to the adsorption of hydrogen. Froudakis et al. demonstrated that the 3 d composites of MWNTs and graphene can increase the hydrogen adsorption capacity through multi-scale theoretical studies. Wang et al studied chemical reduction. The 3 drgo obtained by the method adsorbs the properties of nitrogen, hydrogen, carbon dioxide and water vapor. The results show that the material can adsorb 1.40% and 1.25% (mass ratio) of H2 (106.6 kpa, 77 k / 77 k), 2.98% of carbon dioxide (106.6 kpa, 273 k), and can absorb 18.7% of water vapor (97). Kpa, 293 k). Li et al. Chemically reduced Ni-B alloy doped 3 d graphene material. The results show that the H2 adsorption capacity of the materials doped with Ni (0.83 wt%) and B (1.09 wt%) can reach 4.4% (106 kpa, 77 k), which is three times that of undoped graphene material H2. In addition, the H 2 adsorption amount of the 3 d porous graphene material obtained by the Ca cluster modification can be increased to 5% to 6%.

3.3, application in sensor construction

The 3 d graphene material has a high specific surface area, excellent electron conductivity and special microstructure, which can effectively increase the immobilization capacity and electrical conductivity of active molecules, and has potential application value in the construction of ultrasensitive biosensors. At present, the 3 d graphene materials used for sensor construction include CVD growth of 3 dgf and its composites, composite graphene aerogels, graphene modified films on electrodes, unsupported graphene paper, and the like.

Using a porous nickel film as a template, the CVD-grown foamed 3 dgf has a penetrating pore structure, a high specific surface area, and good mass transfer performance. Since the advent of 3 dgf, great progress has been made in the construction of sensors using their and their composites as unsupported (independent) electrodes. Chen et al. used this material directly as an unsupported electrode, through hydrophobic interaction with dopamine (DA). The function achieves highly sensitive detection of DA (sensitivity is 619.6μa·mM -1·cm - 2 ), the detection limit is as low as 25 nm, and the detection of DA is highly selective in the presence of uric acid. Xi et al. Dgf is the basic carbon electrode, and the electron mediator thiopurine is covalently immobilized on the electrode surface by in-situ polymerization of polydopamine as a linking agent, realizing the real-time detection of the secretion of hydrogen peroxide by the cancer cells, the detection limit is 80 nm, and the sensor has Good stability. Zhang et al. Electrodeposited Pt nanoparticles, MWNTs, and nanoparticles on 3 dgf surface to prepare a composite modified electrode and used for hydrogen peroxide detection. The minimum detection limit is 8.6 nm.dong, etc. in 3 dgf The 3 dgf / Co3O4 complex was prepared by synthesizing Co3O4 nanowires. The complex has good stability and high selectivity for glucose detection and can achieve enzyme-free detection of glucose in serum. The detection limit (25 nautical miles) is much lower than that of single Co3O4 nanowire material (970 nautical miles). The team also synthesized 3 dg / MWNTs using a two-step CVD method. The composite is directly used as an electrode for DA detection with detection limits as low as 20 nm. The electrode obtained by modifying with horseradish peroxidase and electrolyte can also detect hydrogen peroxide with a detection limit of 1 μm. Subsequently, Dong et al. combined the zinc oxide nanorods with 3 dgf in situ, and the modified electrode was used to detect (Fe(CN)6)3 + and DA with detection limits of 1 μm and 10 nm, respectively.

In addition to 3 dgf grown by CVD, the researchers also prepared three-dimensional aerogels based on rGO and their composite materials and examined the application of these materials to GCE electrodes to prepare sensors. Zhang et al. will Prussian blue (PB) Porous PB@rGO aerogel was prepared by compounding with rGO. For the first time, 3 drgo materials were prepared by supercritical fluid drying hydrogel precursors. The aerogel precursors were prepared by reducing L-ascorbic acid as reducing agent and FeCl3 in the presence of ferricyanide. PB@ rGO aerogel is not only light in weight (40-60 mg/cm3), but also has a large specific surface area (316 - 601 m2 / g) and excellent electrical conductivity (38 s / m). It exhibits a low detection limit (5 nm) and a wide linear detection range (5 nm - 4 mm) in hydrogen peroxide detection. In addition to the unsupported aerogel, the researchers also prepared a 3 d modified film on the electrode surface. Yang et al. will be ultrasonically blended with silver nitrate and applied dropwise to GCE to obtain three-dimensional rGO-Ag by electrochemical reduction. The GCE electrode is used to detect hydrogen peroxide. Chang et al. assembled 3 daunps / rGO complexes by supercritical carbon dioxide fluids by AuNPs and rGO. Subsequently, the complex was dispersed in isopropyl alcohol and an electrolyte solvent, dropped onto GCE, and further coated with BMP-TFSI ionic liquid (IL) to obtain a 3 Diller/ Au / rGO electrode, which enabled sensitive detection of glucose. The detection limit was 62 nm. The research group prepared 3 drgo-aunps / GCE by GEM on one step by electrochemical co-deposition, and the thiol-modified DNA was immobilized on the electrode by forming Au-S bond. The DNA and biotin labeling form a sandwich electrode, which realizes the detection of osteosarcoma with a detection limit as low as 3.4, and the electrode has good selectivity, reproducibility, and stability.

The 3 d sandwich immunoelectrode prepared by the two-step method achieves ultra-sensitive detection of carcinoembryonic antigen (CEA) with a detection limit as low as 0.35 pg / mL, and has good stability and reproducibility. Hua et al. The acetic acid-treated N-butyl benzimidazole and rGO were assembled by π-π bond to obtain a 3 d complex, which was applied onto the AU electrode to obtain PBBIns-rGO/AU electrode, and further glucose was dispensed. An oxidase (God) solution is obtained from the enzyme electrode. The electrode can realize the rapid detection of glucose. The preparation process of HRP-Ab2 / TH / porous silver nanoparticle (NPS) nanomaterial containing hexyl pyridine hexafluorophosphate and the detection method of electrochemical immunosensor; (b) Preparation process of Pt-MnO2 / rGO paper. On the ionic liquid carbon paste electrode (CILE), a mixture of hemoglobin (Hb), rGO, and MWCNT was applied dropwise, and a perfluorosulfonic acid membrane was modified to obtain 3 dnafion / Hb-GR-MWCNT / CILE. The three-dimensional composite electrode can realize the detection of hydrogen peroxide, trichloroacetic acid and sodium nitrite. Chen et al. layered the AuNPs and bovine serum albumin-modified graphene (BSA-rGO) by electrostatic action, and then heat-treated. 3 d porous graphene composite doped with AuNPs and used for hydrogen peroxide detection. Cui et al found that under alkaline conditions, deoxidation occurs between acidified MWNTs due to van der Waals force or π-π stacking. The dmwnts / rGO complex, modified on GCE, enables direct electrochemistry of God.

Burckel et al. used carbon crystallization to prepare a 3 d porous graphene/Ni composite coated with solid nickel as a core and multilayer graphene. Since the presence of nickel in the composite increases its electrochemical activity, the complex can be prepared to detect glucose after preparing the modified electrode.

In addition to modifying the 3D graphene interface on the conventional GCE electrode to construct a three-dimensional sensing interface, the researchers also tried to prepare a three-dimensional flexible electrode. Duan et al. obtained the rGO paper by evaporation and reduction, and embedded the rGO paper by electrodeposition. A composite paper base having a 3 d network structure was constructed. Subsequently, Pt nanoparticles are deposited in the composite by ultrasonic-electrodeposition to obtain Pt-MnO2 / rGO paper, which can realize enzyme-free detection of in situ release of hydrogen peroxide by living cells. It is worth noting that the 3 d graphene paper has good flexibility and can be used as a flexible electrode.

In addition to detecting biological active molecules such as glucose, hydrogen peroxide, and tumor markers, 3 d graphene can also be constructed with chemical resistance sensors to achieve highly sensitive detection of environmentally contaminated gases in the ppm range. Taking 3 dgf as an example, the detection mechanism is The resistance of 3 dgf varies with the concentration of the analyzed gas. Therefore, the conductivity of GF can be measured to achieve gas detection. When the NH3 concentration was reduced from 1000 ppm to 20 ppm, the ΔR / R (resistance change) of the 3 dgf active layer was reduced from 30% to 5%. Compared to single-walled carbon nanotubes (carbon) and polymer-conducting systems, detection based on the 3 dgf system is more sensitive. In addition to NH3 detection, the device has the advantage of being as operative and low-power as metal oxide sensors at room temperature and atmospheric pressure. Of particular importance is that macroscopically sized 3 dgf detection can be achieved by direct bonding of conductive adhesives to conductive wires, while separately deposited or individual graphene sheets must be photolithographically used for electronic connections. In addition to 3 dgf, Lin et al. synthesized a 3 dsno2 / rGO complex with a network structure by hydrothermal reduction. The material can produce a highly sensitive response to NH3 at room temperature, with a detection range of 10 - 100 ppm.li. The 3D porous composite gel was prepared by mixing graphene with an ionic liquid (1-butyl 3-methylimidazolium hexafluorophosphate). The electrochemical sensor based on the composite gel can be used for high-sensitivity detection without The detection limit is as low as 16 nm.

3.4 Application in environmental restoration

At present, environmental pollution is becoming more and more serious. How to remove harmful substances in the water, such as organic matter or leaked petroleum products, has become a hot spot in scientific research. Modification of 3 d graphene or preparation of composite materials can effectively control the properties of pro-/hydrophobicity, achieve the removal of environmental pollutants, and exhibit the advantages of large adsorption capacity, stable performance, and reusability. In a recent study, the hydrophobic three-dimensional porous rGO film prepared by Chen et al. showed great potential as a selective adsorbent. This material can absorb more than 37 times its own weight of oil and 26 times its own weight of organic solvent. This is much higher than graphene foam and graphene sheets. In addition, the porous rGO film is very stable and can be recycled after removal of the adsorbed oil layer by hexane. The high adsorption capacity and long cycle life (at least 10 cycles) make the three-dimensional porous rGO film an ideal material for removing organic matter, especially suitable for cleaning crude oil leakage. Liu et al. prepared 3 d graphene/poly by the multi-step method. The pyrrole foam achieves rapid absorption of oils, up to 100 g / g, and has excellent recyclability. The Tiwari team used 3 sodium hyaluronate as a reducing agent to prepare 3 drgo hydrogels through π-π bonding. And electrostatic action can fully absorb methylene blue (MB) and rhodamine B in water, the removal rate is 100% and 97%, respectively, at the same time, the toxicity test proved that the quality of water treated with this material is consistent with distilled water.

Graphene composites are also a research hotspot. Dong et al. used a two-step CVD growth method to synthesize a 3d composite of graphene and carbon nanotube hybridization. This material exhibits both superhydrophobicity and superlipophilicity, and can effectively adsorb oil in water. Classes and organic solvents. Liu and the like will be mixed with resorcinol and formaldehyde, using Lewis acid Ni2 + ions as catalyst and cross-linking agent, heated, lyophilized, carbonized to obtain Ni-doped 3 dg / hydrogel ( NGCC). The material can adsorb oils greater than 20 times its own weight. In addition to oils, dyes are also a research hotspot. The NGCC absorbs MB in water by 151 mg/g. It is particularly noteworthy that the material can withstand an object that is more than 3,500 times its own weight, with a compressive strength of 0.038 mpa. Based on the π-π action, Shi et al. used gallic acid-assisted chemical reduction to prepare gallic acid-graphene aerogel (GaA-GA), which can effectively purify oils, organic solvents and dyes in sewage. The material realizes complete adsorption of water surface layer Sudan III dye-labeled kerosene. Titanium dioxide is also used in combination with graphene. Yan et al. Solvent heat treatment method, 3 dtio2- with mesoporous structure is prepared by direct sol-gel method. The rgo complex can effectively degrade the organic pollutants rhodamine B and norfloxacin. The TiO2-rGO hydrogel (TGH) prepared by Liu et al. has excellent adsorption to MB, and the maximum adsorption value is 120 mg / g. The adsorption capacity is higher than that of pure titanium dioxide, which is 3-4 times that of graphene hydrogel. In addition, TGH can be reused after being irradiated by ultraviolet light after adsorption. Wang and Li are co-reduced with L-ascorbic acid and hydrazine hydrate, and MWNTs or titanium dioxide nanoparticles (P25) are embedded at room temperature to synthesize 3 d of water. Glue (P25-MWNTs-rGO). This material can be used to purify MB in water with a removal rate of 2 times that of P25-MWNT and 10 times that of P25. This fully demonstrates the advantages of graphene composites.

In addition, Cheng Enhua et al used chitosan (CS) and de-heat treated chitosan-graphene complex (3 dcs-rgo) to remove active black (RB5) in aqueous solution, the removal rate was 97.5% (the initial concentration of RB5 was 1.0 mg / ml). ClO4 - has high solubility in water and strong chemical stability, can exist in water for decades, and the body absorbs ClO4 - it will hinder the secretion of thyroid hormone, thus affecting health. Zhang et al. The 3d graphene-polypyrrole (rGO-Ppy) nanocomposite was synthesized by electrochemical method, and the composite was used for the first time to remove ClO4 in water. The Duan research team synthesized unsupported, pDA functionalized 3 d graphene hydrogel (3 dpda-gh), can effectively adsorb a variety of water pollutants, such as heavy metals, synthetic fuels, aromatic pollutants. The adsorption effect of the material on the graphene hydrogel synthesized by hydrothermal method is more prominent. 3 dpda-gh can be regenerated after treatment with inexpensive chemicals.

3.5, application in supercapacitors

Supercapacitors are also called electrochemical capacitors (ECs). The ideal supercapacitor has a high energy density, fast charge, discharge rate, and long cycle life. Classification from charging and discharging mechanisms, ECs include electric double layer capacitor plates and virtual capacitors. The study found that EDLCs are superior in power density and cycle life. The performance of ECs depends largely on its construction materials, such as metal oxides, polymer materials, and carbon-based materials. However, the ECs based on the first two materials often have shortcomings such as charging, low discharge rate, short service life, and high cost, and the ECs constructed by carbon-based materials have high chemical stability, low cost and environmental friendliness. Therefore, the research on the preparation of ECs from carbon-based materials has attracted extensive attention. 3D graphene and its composites have high capacitance, and the 3 d through microstructure can provide high contact area and promote electron and electrolyte transport. Therefore, 3 d graphene and its composite materials are widely used in ECs construction research.

The Miller research team prepared a vertically oriented 3 d graphene sheet using a direct growth method on a metal current collector. The constructed EDLCs reduce the electron and ionic resistance and give an RC time constant of less than 200 seconds. In addition, the EDLCs can effectively achieve the current filtering of 120 hz. Shi group used a one-step electrochemical method to prepare 3 drgo electrodes. The method is similar to the electroplating process, which is fast, simple, inexpensive, easy to control, and can realize industrial scale production. The obtained electrode has excellent rate performance. It not only has the potential to replace the commercial aluminum electrolytic AC filter capacitor as the AC line filter but also greatly reduces the scale of the electronic circuit. Shi and other hydrothermal methods are used to reduce the 3 d hydrogel. (GH-Hs), which is further reduced by hydrazine or hydrazine to increase its conductivity. The resulting material has a capacitance of 220 f / g.

However, although the films prepared based on graphene have high-quality specific capacitance (80 - 200 f / g), due to the low thickness of such electrodes, the low load capacity causes their area-specific capacitance to be low (3 - 50 mf / Cm2). Therefore, in addition to the direct use of rGO materials, the researchers also tried to use element doping. Feng and Mullen and other 3 d aerogels (BN-Gas) based on doping N and B elements to prepare all solid supercapacitors (butt) . The supercapacitor not only has a thin thickness but also has high rate performance, excellent energy density (8.65 wh / kg) and power density (1600 w / kg).

Among the 3 d graphene composites, 3 d graphene and aggregated composites are more studied. MnO2 can effectively improve capacitor capacitance and has the characteristics of low cost, environmental friendliness, and high capacitance. Summary materials with outstanding performance after compounding include aggregated nanoparticles and nanowires prepared by hydrothermal or electrochemical deposition. For example, the shield research group synthesized the nanoparticles in situ by hydrothermal method on 3 dgf. The capacitance of the composite was increased to 560 f / g (current density was 0.2 a / g). Choi et al. further electrochemically deposited the aggregated nanoparticles on the 3 d porous graphene paper, and the composite capacitance was twice that before deposition. The supercapacitors (as shown in Figure 6) obtained by asymmetrically assembling the two papers showed excellent battery performance. The Cheng research team used the aggregate nanowire/3 drgo composite as the anode and graphene as the negative electrode. A high voltage asymmetric electrochemical capacitor (EC) is constructed. With Na2SO4 as the electrolyte, the reversible cycle is in the range of 0,2.0 v, and the energy density is 30.4 wh / kg, which is much larger than the symmetric EC with graphene as the anode and cathode. However, after 1000 cycles of charging and discharging, the capacitance retention rate of the electrode is 79%. Lu et al. are summarized/porous graphene gel/nickel foam composite (summary/G-gel/NF) as positive electrode and G- The gel / NF complex was used as the negative electrode to prepare an asymmetric supercapacitor and exhibited excellent electrochemical stability. After 10,000 charges and discharge, its specific capacitance decreased by only 1.35%, which is significantly better than the summary or graphene composite.

In addition to the summary, the researchers also investigated 3 d graphene and Co3O4, CoS2, NiO2, Ni (OH) 2, Li4Ti5O12, polyaniline, polymethyl methacrylate (PMMA), polypyrrole (Ppy) and other composite materials. Application in capacitor preparation. Some composite materials have excellent properties. A 3 d graphene/Co3O4 composite was synthesized by Rudong et al. The capacitance of the capacitor was as high as 1100 f / g. Wang et al. used graphene-nickel/cobalt acid composite as electrode anode and activated carbon as electrode cathode. An asymmetric electrochemical supercapacitor. This capacitor exhibits excellent energy and power density. After charging and discharging for 10,000 times, the capacitance retention rate is 102%. Duan et al. used a one-step hydrothermal method to prepare a 3 drgo / Ni(OH) 2 hydrogel with a maximum capacitance of 1247 f / g (sweep rate of 5). Mv / s). The capacitance value is twice the capacitance of the composite material obtained by physical mixing of rGO and Ni(OH)2. Zhang et al. prepared Ni3S2@Ni(OH)2/3DGN composite by one-step hydrothermal method, and its specific capacitance is higher than that of the previous one. The reported NiS hollow sphere, NiO / 3 dg. Its area specific capacitance is also higher than the reported Co3O4@MnO2, summary / MWNTs, and Co3O4 / NiO. At the same time, after 2000 charge and discharge, the capacitance retention rate is as high as 99.1%. Chen et al. use the G-LTO composite prepared by inserting graphene with Li4Ti5O12 (LTO) as the anode and 3d porous graphene-sucrose composite. As a cathode, a lithium ion-graphene-based hybrid supercapacitor was obtained, which achieved complete discharge in 36 seconds. This performance is excellent in hybrid capacitors. The capacitance of the 3 d graphene/polyaniline hydrogel prepared by Yan et al. is 1.5 times that of a simple graphene hydrogel capacitor. Chen et al. will be mixed with the PMMA sphere. The composite film was obtained by suction filtration, and then the film was calcined to remove the template to obtain an unsupported three-dimensional macroporous film (MGF). It has a high rate of electrochemical capacitance. Interestingly, the MGF response current measured by the CV method increases with the sweep speed (3 - 1000 mv / s), and the calculated capacitance value is 67.9 at a sweep rate of 1000 mv / s. %, while the retention value of graphene film is very small, only when the sweep speed drops to 50 mv / s, GCF shows a narrower resume curve, and MGF shows excellent rate performance in experiments, 1500 hz The peak frequency, while the graphene film is only 0.5 Hz, which indicates that the MGF open macroporous structure is beneficial to increase the electron transport speed. The team also found that MGF capacitance did not change much at high current densities, and the capacitance of GCF was barely detectable.

Carbon-based materials composed of carbon nanotubes and graphene composites are also important research directions for supercapacitor materials. Tour et al. constructed 3 d graphene/carbon nanotube-based micro-supercapacitors (G / MWNTs-MCs) on nickel electrodes in situ. . When water is used as an electrolyte, its maximum power density can reach 115 w / cm 3 . The volumetric energy density of the material in ionic liquids (2.42 kWh/cc) is two orders of magnitude higher than that of aluminum electrolytic AC filter capacitors. Therefore, G / MWNTs-MCs provide a way to solve the future demand for micro-sized energy storage devices. Xu et al. prepared cobalt phthalocyanine (CoPc) and acid functionalized MWNTs as precursors, which were prepared by microwave heating and subsequent carbonization. Sponge 3 drgo / MWNTs complex. Even when the power density is as high as 48,000 w / kg, the energy density of the composite can reach 7.1 wh / kg. At the same time, the composite in the ionic liquid and sulfuric acid, after 10,000 charges and discharge, the capacitance maintains 90% and 98% of the initial capacitance, respectively. The hydrothermal method, freeze-drying method and subsequent in the presence of pyrrole N-doped 3 drgo-mwnts composite was prepared by carbonization. After 3,000 cycles of charging, the capacitance retention rate was 96%, which was higher than that of pure N-doped graphene (76%).

Although carbon-based materials constructed by graphene and MWNTs have high electrical conductivity, they also have disadvantages. At high current densities, the presence of micropores makes their capacitance unsatisfactory. In order to further increase the energy density of carbon-based materials without sacrificing power density, the researchers further doped high-energy electrode materials such as transition metal oxides in 3 d graphene and carbon nanotube composite carbon-based materials. Conductive polymers, among which the better performance is the summary, Ni(OH)2, Al-Ni double hydroxide, to prepare high capacity supercapacitors. Ma et al. blended MWNTs with rGO and applied it to graphite. On the electrode of the substrate, the surface-deposited amorphous manganese oxide was used to construct the a-MnOx / G-CNT electrode by dynamic voltage deposition. The material has a very high capacitance value (1200 f / g), which is significantly higher than the pure a-MnOx electrode (233 f / g). In the fast charge, discharge process (5 s charge or discharge), the electrode exhibits higher power density and energy density (46.2 wh / kg and 33.2 kW / kg). Du and other vertically aligned carbon nanotubes Ni (OH)2-VACNTs-G composites were obtained by embedding in pyrolytic graphite to prepare a 3 d composite, followed by coating nickel (OH) 2 on the material. The composite has a capacitance of up to 1065 f / g (current density of 22.1 to / g). After 20,000 charges, only 4% of the capacitance is lost after discharge. It has excellent electrochemical stability. Wang et al. The 3 dni-al layered double hydroxide/carbon nanotube/rGO nanocomposite were synthesized by solvothermal method, and the composite showed a porous structure by nitrogen adsorption/desorption experiment. The results show that the capacitance is as high as 1562 f / g (current density is 5 ma / cm2). And its cycle stability and service life are much higher than the traditional Ni-Al layered double hydroxide composite.

It is worth noting that some 3 d graphenes and their composites not only have superior performance but also have excellent flexibility. The segment attempts to develop solid-state flexible 3 d graphene hydrogel (GH) supercapacitors and show excellent performance. Capacitor performance. The flexible supercapacitor not only has high-quality specific capacitance (186 f / g), extremely high area specific capacitance (372 mf / cm2), extremely low leakage current (10.6μa), but also excellent cycle stability. And mechanical flexibility. The specific preparation method is as follows: a solid flexible 3 dgh is pressed on a gold-plated polyimide substrate sheet, and a 3 dgh film (having an area mass of 2 mg/cm 2 with a thickness of 120 μm) is prepared, and further coated with H 2 SO 4 -PVA solution and dried. The solid-state flexible supercapacitor. 3rdgo /summary / /rGO / Ag) asymmetric supercapacitor prepared by Li et al., the curve of the resume curve when bending shows that the specific capacitance only decreases by 2.8%, showing excellent mechanical flexibility. Liu After the de/polymethacrylic acid composite was prepared and the amorphous batch was immobilized thereon, a de/polymethacrylic acid//aggregate complex (GOPM) was obtained. Studies have shown that GOPM has a specific capacitance of 372 f / g (charge and discharge rate of 0.5 a / g), which is much higher than the previously reported chemical synthesis of GO-MnO2 nanocomposites, rGO / summary / activated carbon Nanofibers, activated carbon / / summary. In addition, the composite has good mechanical properties. The 3 d graphene all-solid core-sheath microfiber (GF@3D-G) prepared by the Qu group has been bent at 500 times, and the capacitance is still maintained at 30-40 μf. Between, it shows excellent flexibility. And its surface area capacitance is 1.2 - 1.7 mf / cm2, which is significantly better than zinc oxide nanowire / graphene film composite (0.4 mf / cm2), graphene / AU nanowire composite (0.7 mf / cm2 And the common electrochemical microcapacitors. GF@3D-G has the same energy density and power density as zinc oxide nanowire-based fiber supercapacitors.

4. Conclusions and prospects

The 3 d graphene material developed on the basis of 2 d graphene material is of great significance for expanding the macroscopic application of graphene. In addition to the excellent properties of 2 d graphene, the 3 d graphene material also has a laminated or porous structure, exhibiting unique properties in energy storage, catalysis, environmental restoration, sensors and supercapacitors, and is expected to be flexible. , use of stretchable materials. However, there are still many challenges in the preparation and application of current 3 d graphene materials.

In the preparation of 3 d graphene materials, first of all, the framework and performance of the 3 d graphene structure largely depend on the building block and the preparation method. The ideal 3 d graphene should be composed of a highly conductive single-layer graphene structure. Despite the directional flow assembly method, solvent water/thermal method, template interface assembly method, cardiovascular disease, etc., 3 d graphite can be successfully prepared. Alkene material. However, in addition to the direct growth of graphene by chemical vapor deposition, most of the current 3 d graphene materials are prepared by mechanical stripping, epitaxial growth, chemical stripping, and reduced functionalized derivatives. The development of a graphene building module with excellent performance is critical to the improvement of 3 d graphene performance. Secondly, how to effectively prevent the re-deposition of graphene nanosheets during the formation of 3 d structure, it is still difficult to maintain the properties of graphene sheets intact. Thirdly, the microstructure control technology of 3 d graphene materials still needs to be further improved. Currently, the pores of the 3 d graphene material are usually between several hundred nanometers and several tens of micrometers. The porous structure increases the volume but weakens the mechanical properties of the material. At present, there are still few research results of 3 d graphene with nanometer pore structure. Finally, in addition to directly replicating the template structure, the microscopic pore structure of the 3 d graphene material is mostly generated by random formation or pore formation during the 2 d graphene integration process, and the pore structure is poor in controllability and repeatability, and therefore, in a wide pore size range. It is still difficult to control the 3D graphene pore size. In terms of application, the potential application of 3 d graphene in high-strength materials and high thermal conductivity materials needs to be further expanded. At present, most of the applications of 3 d graphene are still focused on the detection of small molecules, biosensor preparation, supercapacitors, environmental repair, hydrogen storage, and 3 d graphene in the preparation of high-strength materials, high thermal conductivity, flexibility The application of materials is progressing slowly. At the same time, 3 d graphene can be used in medical fields, such as sensitive detection of genetic material, micro-robot, etc. Therefore, there are still a lot of researches on the preparation and application of 3 d graphene. The application waits for analysis and resolution by scientists.

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