What sparks will appear when supercapacitors meet graphene?

　　Supercapacitors are efficient and practical energy storage components, and graphene as an electrode material has superior performance in all aspects compared to traditional activated carbon. People are familiar with dry batteries and lithium-ion batteries, but may not know much about capacitors. In fact, these energy storage devices are composed of positive and negative electrodes (cathode and cathode), diaphragms, current collectors, electrolytes and shells. By replacing the electrode materials, the battery becomes a capacitor. Let's follow the mobile phone portable editor to learn about the relevant content.

　　Capacitors and Supercapacitors

　　Due to the different positive and negative electrode materials, the performance of lithium-ion batteries and capacitors is very different. For example, the energy density of lithium-ion batteries based on lithium iron phosphate as the positive electrode material is more than 20 times higher than that of the best supercapacitors currently on the market . The power density of supercapacitors can be 30 to 100 times that of lithium-ion batteries. If we use runners as an analogy, supercapacitors are 100-meter athletes with super explosive power, and lithium-ion batteries are marathon runners with outstanding endurance.

　　The differences between capacitors and supercapacitors are mainly in the following aspects:

　　First, different types of capacitors can store different amounts of electricity. The smallest capacitor can only store a few microvolts of electricity and is used exclusively in electronic controllers. For example, there are many capacitors in old radios to adjust circuit functions. A supercapacitor the size of a 560 ml beverage bottle can store 3,000 to 6,000 farads of electricity.

　　Secondly, supercapacitors can provide a large current instantly. The initial current of heavy machinery startup is 3 to 6 times that of normal operation, and the general power supply system does not have such a large setting margin. The use of supercapacitors can greatly simplify the configuration of the starting system and save costs. Therefore, supercapacitors form modules that can be used to start the blades in wind turbines; assist in the start-up of cranes, large trucks, light rail vehicles, etc.

　　In addition, supercapacitors can be reversibly charged and discharged 500,000 to 1 million times, while the most advanced lithium-ion batteries can hardly exceed 10,000 times (mostly 3,000 times), not to mention that the batteries (lead-acid batteries) in ordinary family cars can only be reversibly charged more than 300 times. Therefore, supercapacitors are often used as backup power sources for aircraft cabin doors. Once the aircraft encounters an accident and loses power, supercapacitors that have not been used for a long time but are on standby at any time can play a key role. It is precisely because of this ultra-long service life that the following two interesting situations have emerged.

　　(1) Although supercapacitors are far less expensive than lithium-ion batteries in terms of storage cost per watt-hour, they can store much more electricity than lithium-ion batteries over their entire life cycle.

　　(2) Since supercapacitors can be cycled 500,000 to 1 million times, and the lifespan of the motor vehicle equipped with supercapacitors is not as long as that of the motor vehicle itself (motor vehicles are generally scrapped after about 15 years), when the motor vehicle is scrapped, the supercapacitors with good performance can be removed and recycled elsewhere. The super-long life of supercapacitors may also explain why their market development is far less than that of lithium-ion batteries.

　　Opportunities for developing supercapacitors in China

Schematic diagram of capacitor structure and types of electrode materials

　　For a long time, supercapacitors have played a supporting role in energy storage in the European and American markets due to their low energy density. At the same time, due to the small size of European and American cities, low population density and saturated markets, the world is increasingly looking to China's huge market.

　　First, in the application of energy recovery systems, such as vehicle braking and crane deceleration, traditionally, mechanical energy is completely dissipated as heat energy through friction and wasted. Supercapacitors can convert mechanical energy into electrical energy storage through electromechanical conversion systems, and release it in pre-constructed backup circuits, thereby saving energy. This market is very huge and is also one of the important ways to improve energy efficiency in my country.

　　At present, my country has become the country with the longest mileage of highways in the world. The numerous buses shuttling on the highway will be ideal tools for recycling energy using supercapacitors. At the same time, my country's real estate industry is well developed, and elevators in high-rise offices and residences are in frequent operation. If capacitors that can respond quickly are used, they will be easy to start and can recycle energy.

　　Although the supercapacitor has a small charge capacity, it can be charged very quickly, generally within half a minute to one minute. Imagine that at any bus stop, the bus can be fully charged and run to the next stop during the time when passengers get on and off the bus, which can fully realize low energy operation, environmental protection, greenness and pollution-free. For my country's established large-city bus system, it has very practical significance.

　　For small and medium-sized cities with relatively small urban areas and relatively uncrowded traffic, the use of supercapacitors with fast charging but low charging capacity is also a favorable choice, as it will not consume all the power in the crowded waiting process. In comparison, electric vehicles powered by lithium-ion batteries, which take several hours to charge, occupy a large number of parking lots and road resources, and their development in large cities is restricted.

　　At the same time, supercapacitors have excellent stability and can also be used in municipal lighting systems such as street lamps, making these lighting systems free from repair and maintenance. This will be an effective option to reduce power storage costs and infrastructure costs throughout the entire life cycle.

　　Automotive systems have always been a strategic application area for lithium-ion batteries and supercapacitors. The current view in Europe and the United States is to use the two together. That is, electric vehicles use supercapacitors when starting, climbing and braking, and lithium-ion batteries when they are running at a steady speed. Although this energy utilization route is reasonable, it also limits the function of supercapacitors, that is, supercapacitors are in a subordinate position and cannot be used as a main power source. In my country, through a lot of practice, pure supercapacitor-driven urban light rail demonstration lines and urban bus demonstration lines have been produced, which effectively meet the immediate or transient rapid transportation of a large number of passengers and represent a development trend.

　　In addition, the road density in my country's big cities is insufficient, the number of vehicles is large, and the absolute speed is slow. The exhaust emissions at idle speed account for the majority of car emissions. Since the number of reliable charge and discharge times of the batteries (lead-acid batteries) currently used in cars is too small, if supercapacitors with reversible charge and discharge of 500,000 to 1 million times are used, the internal combustion engine can be extinguished at idle speed and quickly restarted when needed, effectively reducing exhaust emissions and realizing green transportation.

　　Graphene helps supercapacitors develop

Approximate performance range of different energy storage devices

　　Small cars have a huge market. Due to the limited space of the vehicle system and the increase in weight will increase energy consumption, supercapacitors must have the characteristics of high energy density and small size. Therefore, improving its energy density has become the key to application breakthroughs. This requires upgrading the current commercial products. Taking the double-layer capacitors currently on the market as an example, most of them have an operating voltage of 2.7 volts, use activated carbon as the electrode material and use organic electrolytes. The capacitance of the activated carbon electrode material is less than 200 farads/gram, and the energy density of the capacitor device is less than 6~7 watt-hours/kilogram (or watt-hours/liter).

　　Theoretically, energy density is proportional to the capacitance of the electrode material and proportional to the square of the operating voltage, which determines that increasing the operating voltage is the key to achieving high energy density. In fact, mobile phone batteries and lithium-ion power batteries are also working hard to increase the operating voltage.

　　In order to increase the working voltage, in addition to replacing the electrolyte with higher chemical stability, it is also necessary to use carbon electrode materials with higher purity. Generally speaking, activated carbon is obtained by carbonization of coconut shells, apricot shells, petroleum coke, etc., and may contain metal impurities and impurities such as oxygen, nitrogen, and phosphorus introduced during the activation process. Impurities can undergo redox reactions in aqueous electrolytes (1 volt) and contribute Faraday pseudocapacitance. However, under high voltage, these impurities will cause the electrolyte to continue to decompose, causing the device to swell, resulting in increased internal resistance and even damage to the device, so they must be removed.

　　At the same time, activated carbon is a microporous carbon with an "inwardly concave" structure, and the pore size is mostly less than 0.7 nanometers. For electrolytes such as organic liquids and ionic liquids, the transmission of ions inside the activated carbon is like a maze, which will lead to greater diffusion resistance and lower surface utilization.

Graphene is a kind of SP2 hybridized carbon, and its chemical stability is much higher than that of SP3 hybridized activated carbon. At the same time, 　　the surface of graphene is all "convex", which is very conducive to the approach and adsorption or desorption of electrolyte ions, and realizes a fast charge and discharge process. It is particularly important to point out that graphene can be prepared by chemical vapor deposition of high-purity hydrocarbons at high temperatures. In principle, it can ensure a large specific surface area and high purity without metal doping, thus possessing many excellent properties.

　　At a time when electrochemical energy storage has been included in the country's "13th Five-Year Plan for Renewable Energy Development" and supercapacitors are in urgent need of improving their quality, graphene materials have undergone more than a decade of development and recognition, and finally have a good opportunity at the right time.

　　Development and progress of graphene

Graphene 's characteristics and some application examples

　　Graphene was first discovered by scientists at the University of Manchester in 2004. It caused a sensation in the international physics community, not because of its well-known strength, electrical conductivity, thermal conductivity or energy storage properties, but because before that, physicists did not believe in the stable existence of two-dimensional atomic-level crystals.

　　When British scientists used tape to stick together a piece of high-quality graphite (a macroscopic body with more than a million layers of single-layer graphene), and persistently peeled off layer by layer until they obtained a single crystal of carbon atoms with a thickness of only 0.12 nanometers, graphene demonstrated a series of excellent properties such as sound, light, electricity, force, heat, and magnetism, and promoted the development of the preparation and self-assembly technology of other atomic-level two-dimensional materials.

　　Since the magic of nanotechnology was revealed in 1991, driven by the research boom of C60 and carbon nanotubes, graphene has encountered a golden age of abundant scientific research talents, abundant scientific funding, and active venture capital funds. In just a decade, graphene has completed a huge transformation from a "rising star" to a "Nobel Prize darling" and achieved great results.

　　(1) The normal direction of a single-layer graphene is the strongest material, which is more than 100 times stronger than steel. Therefore, graphene can be widely used in the field of composite reinforcement of various materials.

　　(2) Graphene is a non-polar material composed of a carbon-carbon six-membered ring structure, but it is hydrophilic on a macroscopic level, so it has the potential to adjust its surface hydrophilicity and hydrophobicity in a variety of ways.

　　(3) It has a perfect planar carbon structure and can be loaded with various metals. Its performance is also well studied and it can become a loading research platform.

　　(4) Single-atomic-layer films are transparent, conductive, and flexible, and can become the darling of flat-panel displays and flexible devices.

　　(5) By making a very small hole on the regular surface of graphene, seawater can be desalinated by forward osmosis, which is a great supplement to the current reverse osmosis desalination membrane.

　　(6) In the field of capacitors, American scientists have tried every possible means to stand up several sheets of graphene to make micro-capacitor devices, proving that this capacitor does have an ultra-fast response capability of millions of hertz.

　　The premise for achieving many excellent properties and application prospects is to obtain excellent materials, so its development directions mainly include:

　　(1) Prepare single crystals of increasingly larger sizes;

　　(2) Prepare powders with increasingly controllable number of layers, specific surface area, and purity;

　　(3) Directly prepare various composite materials with substrates.

　　For graphene, which is intended to replace activated carbon, it belongs to the category of powder material and looks like a pile of toner. For capacitor devices that are intended to replace activated carbon, more and more graphene materials must be loaded into a very small space, and measures include rolling and bonding. These engineering characteristics also put forward requirements for the preparation of graphene, because graphene is a two-dimensional material with a huge specific surface area. Once two single-layer graphene sheets are superimposed, the huge van der Waals force will make it impossible to separate them, and the specific surface area will immediately decrease by 50%. So people dispersed carbon nanotubes or grew them directly between graphene sheets.

　　Later, the template method was developed to grow graphene directly into a "honeycomb"-like nanostructure. Each graphene sheet is slightly curved, naturally connected, and does not overlap. It has a huge specific surface area and diffusion channels. Therefore, in general, the current level of graphene preparation is getting closer and closer to the various stringent requirements required for supercapacitor applications.

　　Phased development of graphene-based supercapacitors

　　Performance of electrode materials in supercapacitors and voltage window of suitable electrolytes

Graphene nanofibers suitable for capacitance properties

　　Due to the small output and the lack of scale of production, the current price of high-end graphene is equivalent to that of silver, which is 4,500 to 6,000 yuan per kilogram. This objectively hinders the various applications of graphene in fields including supercapacitors. Looking at the scale-up preparation and price rules of various materials, the maturity, expansion and quality improvement of the application, the increase in output and the reduction in price are complementary.

　　Therefore, looking at the role of graphene in supercapacitors from a development perspective is both in line with historical laws and not arbitrary speculation. The author attempts to divide the development of graphene-based capacitors into three stages.

　　Graphene helps activated carbon capacitors

　　The characteristic of this period is that activated carbon is still the dominant electrode material for capacitors, and the amount of graphene added is usually less than 3%~4%, which only acts as a conductive agent to help activated carbon capacitors reduce internal resistance, increase service life or appropriately increase power density. At present, the consumption of high-end activated carbon electrode materials in my country is about 1,000 tons/year, and the consumption of graphene is about 30~40 tons/year. For example, Cnano Technology was the first to apply carbon nanotubes to the conductive agent of lithium-ion batteries in 2007. At present, carbon nanotubes have become the preferred choice of conductive agent for power lithium-ion batteries, and a considerable industry is forming. By analogy, the time period for graphene materials to realize the function of conductive agent does not need to be too long.

　　Due to the small amount of graphene used, the current activated carbon slurry processing, electrode processing and assembly technology, voltage platform and testing system do not require revolutionary changes. Therefore, it is the breakthrough point with the highest possibility of industrial practice and the greatest opportunities.

　　Graphene partially replaces activated carbon electrode material stage

　　This period is characterized by the fact that graphene not only acts as a conductive agent, but also as a part of the main electrode material, coexisting with activated carbon, and its mass fraction can fluctuate between 20% and 40%. The annual demand for graphene will increase to about 200 to 400 tons, which will form a relatively considerable industry. However, since graphene coexists with activated carbon, it will be subject to the operating voltage platform of activated carbon. In addition, due to the large volume proportion of graphene, how to maintain a surface density similar to that of the original activated carbon on the pole piece will become the key to material processing. If the loss caused by the decrease in pole piece density is to be offset, it is required to improve the structural control technology of graphene and obtain materials with a larger accessible specific surface area.

　　Graphene completely replaces activated carbon electrode material stage

　　If graphene completely replaces activated carbon electrode materials, a market with an annual demand of 1,000 tons will be formed. The advantage is that a new electrolyte system can be used to increase the voltage of the capacitor, giving full play to the many advantages of graphene such as high chemical stability, high conductivity, and easy ion adsorption. However, it may cause the stacking density of the electrode material to be lower, and to increase the density of the electrode sheet, a new structure will be needed, which is a big challenge.

　　In short, supercapacitor energy storage is a complex high-tech field. There is an objective "barrel short board theory" for the requirements of electrode materials, that is, the water that a barrel can hold depends on the shortest board, not the longest board. In terms of specific surface area, purity, pore topology, electrochemical stability and conductivity, graphene is better than activated carbon. Once the shortcoming of graphene's "small stacking density and large liquid absorption" is overcome, graphene can replace activated carbon. This depends on the improvement of chemical vapor deposition preparation technology and the in-depth theoretical research on the complex interaction and control of the soft matter level between liquid and solid.

Graphene double-layer supercapacitor development roadmap

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