Hottest graphene materials and graphene based supe

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Graphene materials and graphene based supercapacitors

graphene, a two-dimensional material with a single atomic layer thickness of honeycomb structure, is formed by the hybridization of carbon atoms with SP2. It is the basic structural unit for building zero dimensional fullerenes, one-dimensional carbon nanotubes and three-dimensional graphite. Graphene was first obtained by scientists at the University of Manchester in the UK in 2004 by mechanical stripping, and it was proved to be the thinnest material that really exists at room temperature

countries all over the world attach great importance to the basic research of graphene and actively develop related industrial applications. The investment in graphene research in the United States was earlier, with a total annual investment of more than US $50million; Singapore has invested more than US $150million in graphene research; South Korea plans to invest US $350million in graphene research and has formulated a detailed business development roadmap; The EU launched the "flagship graphene research project" as early as 2013, and plans to invest 1 billion euros in the R & D, industrialization and application exploration of graphene within 10 years to maximize the promotion of scientific and technological innovation. In addition, the EU has listed graphene based energy storage and conversion as one of the four important research directions of graphene in the future

China is also very active in graphene research, and has formed an industry university research cooperation and docking mechanism for collaborative innovation by the government, scientific research institutions and enterprises, which has greatly promoted graphene technology research. The "made in China 2025" key field technology roadmap issued in 2015 has included graphene in one of the key development new materials of the 13th five year plan, clearly pointed out the direction and path of graphene industry development in the next 10 years, set the goal of reaching 10000 tons of industrial scale, and proposed a "package" of breakthrough actions for new materials and a "one-stop" application plan for graphene

in 2016, graphene was approved in the key project of the national key R & D plan "nanotechnology", and two key projects related to graphene were approved: macro controllable preparation of graphene macro bulk materials and its application research in optoelectronics, key technologies and major Sciences for the industrialization of nano carbon materials. All this has enabled mankind to continuously obtain new results on the road to uncover the secrets of metal fatigue. Some measures have listed graphene as an advanced basic material, key strategic material and cutting-edge new material

the project approval of graphene will continue to strengthen China's original exploration and forward-looking technology research and development in the field of graphene research, strengthen the accumulation of foundation and technology, strive to implement the key industrialization transformation of important and revolutionary application achievements in time, and gradually expand the application field of graphene materials

it is worth noting that the scientific and industrial circles agree that graphene will first be industrialized in energy memory devices. It is believed that in the next 10 years, more and more graphene products will be circulated in the market by building a number of graphene industrial chains and establishing a number of industrial clusters

supercapacitor and graphene

supercapacitor is composed of two electrode plates, diaphragm, fluid collector and electrolyte. Compared with batteries, supercapacitors have the characteristics of high power density and fast charge and discharge, and have the advantages of long service life, maintenance free, wide temperature range, no memory effect and more safety. Unlike traditional capacitors, which store energy by electrostatic adsorption of electrons, supercapacitors can store electric energy by adsorption of ions in electrolyte, and have a specific capacity at least 3 orders of magnitude higher than traditional capacitors

according to the energy storage mechanism, supercapacitors are mainly divided into two categories, one is electric double-layer capacitor, the other is pseudo capacitor supercapacitor. The former mechanism is that the ionic charge accumulates at the interface between the electrode material and the electrolyte solution, and the illegal lardy reaction occurs; The latter is a reversible redox reaction on the surface of the electrode material, or a Faraday reaction occurs when electrolyte ions enter the electrode material

the common electric double-layer materials are mainly activated carbon, carbon fiber, carbon nanotubes, graphene and other carbon materials, while oxides and conductive polymers are common pseudocapacitor materials

the main disadvantage of supercapacitors is their low energy density. The energy density of supercapacitors is watt hour/kg, which is lower than that of lead-acid batteries (watt hour/kg), nickel hydrogen batteries (watt hour/kg) and commercial lithium-ion batteries (watt hour/kg). Graphene materials with unique ultra-thin two-dimensional structure, excellent conductivity (5000 w/cm), high specific surface area (2620 m2/g), high theoretical specific capacity (550 f/g), high area specific capacity (21 μ f/cm 2) and good mechanical properties have been proved to be a very ideal material for supercapacitor electrodes. Graphene electrode materials have been applied to supercapacitors, It can significantly increase its energy density by more than ten times, and greatly improve the power density at the same time

graphene can be directly used as the electrode material of electric double-layer capacitors because of its unique physical and chemical properties. The main preparation methods include mechanical stripping, vapor deposition, reduced graphite oxide, liquid phase stripping and organic synthesis. Among them, the reduced graphite oxide method is considered to be a relatively low-cost method that can produce graphene on a large scale. At present, it is the most common commercial method

the modified Hummer method is usually used, that is, graphite is oxidized with concentrated sulfuric acid and potassium permanganate to obtain graphene oxide, and then reduced graphene is obtained through various reduction methods, such as chemical reduction of hydrazine hydrate, urea, ascorbic acid, potassium hydroxide, high temperature treatment, electrochemical reduction, laser treatment, active metals, etc

the morphology and structure of graphene obtained by different reduction methods are different, mainly in the aspects of surface oxygen-containing groups, structural defects, specific surface area and conductivity, which will lead to great differences in the electrochemical properties of different graphene materials. The specific capacity of graphene is about law/g, which is far from the theoretical value (550 law/g). The main reason is that there is a strong T-T interaction between graphene sheets, which makes the phenomenon of re stacking and agglomeration between graphene sheets serious. In this case, the electrolyte ions cannot fully infiltrate and reach the inner surface of agglomeration or stacking graphene, which greatly reduces the available specific surface area, Finally, the specific capacity is relatively low

in addition, the surface of graphene and electrolyte also show the characteristics of "similar miscibility". For example, graphene with less (or no) oxygen-containing functional groups on the surface shows hydrophobicity, so the aqueous electrolyte cannot be soaked naturally, and the effective specific surface area cannot be fully utilized, resulting in low specific capacity. However, it shows good wettability and large specific capacity in organic electrolyte. On the contrary, graphene with relatively more surface functional groups can show higher electrochemical performance in aqueous electrolyte

in order to avoid agglomeration and stacking between graphene sheets and improve electrolyte ion transport, scientists have developed a variety of effective methods. For example, introducing functional groups with redox function (such as benzoquinone) on the surface of graphene oxide; Through structural design and assembly regulation, new wrinkled graphene, graphene ball, graphene roll, graphene nanobelt, graphene fiber, graphene film, graphene three-dimensional complex, etc. are obtained; Pre embedded nano space fillers, such as electrolyte; Porous graphene nanosheets were prepared by soft and hard template methods; Strong alkali and oxide are used to make holes on the surface of graphene

these methods can effectively improve the specific surface area of graphene, prevent graphene from stacking each other, and obtain graphene electrode materials with high specific capacity. In addition, the formation of developed ion electron complex channels between graphene can significantly accelerate the rapid transmission and migration of electrolyte ions and electrons, thus effectively enhancing the magnification performance of these graphene materials

doped graphene

doping heteroatoms in graphene lattice can significantly improve its electrochemical performance. The introduction of heteroatoms can change the intrinsic physicochemical properties of graphene, including basic electronic properties, mechanical properties, hydrophilicity and lipophilicity. Common doped heteroatoms include nitrogen atom, boron atom, sulfur atom and phosphorus atom, among which N atom is the most widely studied. According to the different doping positions of N atoms, graphitized nitrogen, pyrrole nitrogen and pyridine nitrogen can be obtained. The latter two can significantly improve the electrochemical properties of graphene. The specific capacity of graphene electrode materials doped with nitrogen atoms is generally in F/g, which is nearly 4 times higher than that of undoped graphene

in addition to single element doping, two or more elements can also be doped together to enhance the electrochemical performance of graphene. However, doping graphene can not avoid the stacking and agglomeration between graphenes, and other structural design and assembly methods need to be combined to avoid the stacking and agglomeration of graphene

graphene composites

graphene/metal oxide and graphene/conductive polymer are two kinds of graphene composite electrode materials that have been studied most deeply at present. Common metal oxides include manganese oxide and oxide nails, and conductive polymers include polyaniline and polypyrrole. Metal oxides and conductive polymers can be used as pseudocapacitor electrode materials to undergo rapid and reversible oxidation-reduction reactions on their surfaces, thereby delivering high specific capacity. However, due to the shortcomings of low conductivity and poor cycle performance of these materials, their practical application in supercapacitors is greatly limited

in order to improve this situation, graphene with high specific surface area, high conductivity and inert at room temperature is usually used to compound with metal oxide and conductive polymer to form a new pseudo capacitor electrode material. This kind of graphene composite combines the advantages of graphene and metal oxide or conductive polymer, and the two can produce significant synergistic effect

first, the pseudocapacitive material loaded on the surface of graphene can prevent the re stacking between graphene layers, which is not only conducive to ion transmission, but also increases the active specific surface area of graphene that can be used, thereby improving charge storage. Secondly, pseudocapacitive materials can be uniformly bonded on the surface of conductive graphene in the form of special nanostructures or particles, which not only greatly promotes the reversible redox reaction on the surface of pseudocapacitive materials, but also accelerates the transmission of electrons, increasing the specific capacity of pseudocapacitive materials

moreover, the pseudocapacitive nano material is fixed on the surface of graphene, which can effectively prevent the particles from gradually agglomerating and growing, electrode pulverization or destruction in the repeated Faraday reaction process, so as to improve the cycle stability of the material. Therefore, the synergistic effect of composite materials can not only increase the conductivity of oxide or polymer materials, the specific capacity of pseudocapacitance and graphene, but also greatly improve the cycle stability of pseudocapacitance electrode materials

it should be pointed out that the mass specific capacity of pseudocapacitor materials obtained by different preparation methods is very different from that of graphene composites. For example, poly (phenylamine) and graphene composites are fa/g. The mass specific capacity of composites tends to increase with the increase of pseudo capacitance material content, but its conductivity decreases. Compared with pure pseudocapacitor materials, the mass specific capacity of graphene composites may be slightly reduced, but its cycle performance and power density will be significantly improved

graphene based flexible supercapacitors

in recent years, more and more civil electronic devices are developing in the direction of lightweight, flexible and wearable

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