Graphene, CNT, manganese dioxide who will become the electrode material of the future "killer" supercapacitor?

LED semiconductor lighting network super capacitor is mainly composed of four parts: electrode, current collector, electrolyte and diaphragm. The electrode material is the most important factor affecting the performance and production cost of supercapacitor. Research and development of high-performance, low-cost electrode materials is an important part of the development of supercapacitors. At present, many supercapacitor electrode materials are mainly carbon materials, metal oxides (or hydroxides), conductive polymers, etc., and the commercialization of carbon materials and metal oxide electrode materials is relatively mature, which is a hot spot of current research. .

1. What is a supercapacitor?

Supercapacitors or ultracapacitors, also known as electrochemi calcapacitors, are new energy storage devices between secondary batteries and conventional capacitors. They combine high secondary battery energy density with conventional capacitor power density. In addition, supercapacitors have the characteristics of no pollution to the environment, high efficiency, long cycle life, wide temperature range, high safety, etc., and are widely used in electric vehicles, new energy power generation, information technology, aerospace and other fields. prospect.

The supercapacitor can also form a composite power system with the rechargeable battery, which can meet the high power requirements of the electric vehicle during starting, accelerating and climbing, and can prolong the cycle life of the rechargeable battery, thereby optimizing the performance of the electric vehicle power system. At present, commercial production of supercapacitors has been realized at home and abroad, but there are also problems of high price and low energy density, which greatly limits the large-scale application of supercapacitors.

Supercapacitors are mainly composed of four parts: electrode, current collector, electrolyte and diaphragm. The electrode material is the most important factor affecting the performance and production cost of supercapacitor. Research and development of high-performance, low-cost electrode materials is an important part of the development of supercapacitors.

At present, many supercapacitor electrode materials are mainly carbon materials, metal oxides (or hydroxides), conductive polymers, etc., and the commercialization of carbon materials and metal oxide electrode materials is relatively mature, which is a hot spot of current research. . Therefore, this paper will focus on the latest research progress and commercial application prospects of high-performance electrode materials such as carbon materials, metal oxides and their composite materials.

2. The latest research progress of carbon materials as electrode materials for supercapacitors

History of carbon materials

Carbon materials are currently the most widely studied and widely used supercapacitor electrode materials, including activated carbon, template carbon, carbon nanotubes, activated carbon fibers, carbon aerogels and graphene. The carbon material has the advantages of high electrical conductivity, large specific surface area, good electrolyte wettability, wide potential window, and the like, but its specific capacitance is low. The carbon material mainly uses the electric double layer formed by the electrode/solution interface to store energy, which is called the electric double layer capacitor. Increasing the specific surface area of ​​the electrode active material can increase the area of ​​the interface double layer, thereby increasing the electric double layer capacitance.

Carbon nanotube

Carbon nanotubes were discovered in the early 1990s as a nano-conductivity and chemical stability, a large specific surface area, pores suitable for electrolyte ion migration, and cross-winding to form a nano-scale network structure, which was once considered It is the ideal electrode material for high power supercapacitors.

Niu et al. first reported the use of carbon nanotubes as electrode materials for supercapacitors. They used catalytic pyrolysis to form hydrocarbons into multiwalled carbon nanotube film electrodes in a mass fraction of 38% H2SO4 electrolyte. At different frequencies from 0.001 to 100 Hz, the specific capacitance reaches 50-110 F/g, and its power density exceeds 8 kW/kg. However, the free-growth carbon nanotubes have different shapes, disordered orientations, and even inclusions with amorphous carbon, which is difficult to purify, which increases the difficulty of practical application.

In recent years, research on highly ordered carbon nanotube arrays has once again attracted attention. This array of carbon nanotubes grown directly on the current collector not only reduces the contact resistance between the active material and the current collector, but also simplifies the electrode. Preparation process.

Activated carbon

Due to its stable service life, low price and large-scale industrial production base, activated carbon has been widely used in the production of commercial supercapacitors. In the 1960s, Becker applied for the first patent on electrochemical capacitors for activated carbon materials. He applied activated carbon with a high specific surface area on a metal substrate and then immersed it in H2SO4 solution with the help of double electricity formed at the interface of the activated carbon. Layer structure to store charge.

The raw materials for preparing activated carbon are very rich, and coal, wood, nut shell, resin and the like can be used for preparing activated carbon powder. The raw materials are prepared and activated, and the activation methods are chemically activated and physically activated. Chemical activation is carried out at a temperature of 500 to 700 ° C using phosphoric acid, potassium hydroxide, sodium hydroxide and zinc chloride as activators; physical activation usually means in an oxidizing atmosphere such as water vapor, carbon dioxide and air. The carbon material raw material is treated at a high temperature of 800 to 1200 °C. The activated carbon pore structure prepared by the activation process generally has a pore size distribution with a wide size span, including micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm).

It should be pointed out that when the specific surface area of ​​activated carbon is as high as 3000m2/g, only a relatively small specific capacitance (<11μF/cm2) can be obtained, which is smaller than the theoretical value of the double-layer electrical capacitance of activated carbon (15~25μF/cm2). It is shown that not all pore structures have an effective charge accumulation.

Although the specific surface area is an important parameter for the performance of an electric double layer capacitor, the pore distribution, the shape and structure of the pores, the conductivity and the surface functionalization modification also affect the electrochemical performance of the activated carbon material. Excessive activation leads to large porosity, and also reduces the bulk density and conductivity of activated carbon, thereby reducing the volumetric energy density of the activated carbon material. In addition, some active groups and dangling bonds remaining on the surface of the activated carbon will make it the same electrolyte. The increased reactivity between the two also causes the degradation of the performance of the electrode material. Therefore, designing activated carbon materials with narrow pore distribution and cross-linked pore structure, short ion transport distance, and controlled surface chemistry (such as surface tension, surface free energy, etc.) will help to improve the performance of supercapacitors. Energy density without affecting its power density and cycle life.

At present, the preferred material for commercial supercapacitor electrode materials is still activated carbon, but with the continuous development of other new carbon materials (such as carbon nanotubes, graphene, etc.), it is possible to replace activated carbon materials in the future.

Graphene

British scientist Geim et al. discovered in 2004 a single-layer graphite sheet composed of carbon atoms, graphene (graphene. Graphene is not only the thinnest of the known materials, but also extremely strong and hard. As a simple substance The rate at which electrons are transported at room temperature is the fastest among all conductors. Carbon nanotubes and graphene are representative of one-dimensional nanomaterials and two-dimensional nanomaterials, respectively, which are complementary in structure and performance. .

From the current point of view, graphene has more excellent properties, such as high conductivity and thermal conductivity (5000 W/m·K), high carrier mobility (2×10 5 cm 2 /V·s), and free electron mobility. Space, high strength and stiffness (Young's modulus is about 1.0 TPa), high theoretical specific surface area (2600 m2/g), and the like. Therefore, graphene has broad application prospects in the fields of single-electron devices, ultra-sensitive sensors, electrode materials, and drug carriers. Utilizing the unique advantages of high specific surface area and high electrical conductivity of graphene materials, it is expected to obtain the next-generation high-performance supercapacitor electrode materials with low cost and superior performance.

3. Research progress of metal oxides as electrode materials for supercapacitors

Metal oxides mainly store energy by rapid redox reaction on the surface of the electrode and near the surface of the electrode active material. Its working principle is the same as the chemical power supply, but the charging and discharging behavior is similar to that of a conventional capacitor, so it is called a Faraday tantalum capacitor. The Faraday tantalum capacitor has a relatively high capacity and is 10 to 100 times that of the electric double layer capacitor. Accelerating the electrochemical reaction rate of the electrode active material and increasing the utilization rate of the electrode active material are effective ways to increase the specific capacitance of the metal oxide supercapacitor.

Cerium oxide material

SEM image of yttrium oxide used as electrode material for supercapacitors

The cerium oxide material has the advantages of high specific capacitance, good electrical conductivity, and stability in the electrolyte, and is currently the best performance supercapacitor electrode material. The US Army Research Laboratory reported in 1995 that the amorphous hydrated cerium oxide has a specific capacitance of up to 768 F/g and an energy density of 26.7 Wh/kg based on the electrode material. At present, the United States has used cerium oxide materials in important fields such as aerospace and military science. However, the resources are limited and the prices are very expensive and difficult to apply universally. In order to further improve performance and reduce costs, domestic and foreign companies are actively looking for other low-cost metal oxide electrode materials.

Studies have shown that manganese dioxide, cobalt oxide, nickel oxide, iron oxide and vanadium oxide have similar electrochemical properties to cerium oxide, of which manganese dioxide is one of the most studied electrode materials.

Manganese dioxide material

Manganese dioxide materials have significant advantages such as low cost, environmental friendliness, and wide electrochemical working window (up to 1000 mV in aqueous systems, comparable to yttria electrode materials). More importantly, the manganese dioxide-based supercapacitor can use a neutral electrolyte solution (such as Na2SO4 aqueous solution, KCl aqueous solution, etc.), unlike other metal oxide or carbon-based supercapacitors, it is necessary to use a strong acid or a strong alkali electrolyte, which makes The assembly and use of manganese dioxide based supercapacitors is safer and more convenient. In addition, nanotechnology is applied to the field of supercapacitor electrode materials, and the electrochemical activity of the nanometer manganese dioxide electrode material can be greatly improved by using a high specific surface area, a short ion diffusion distance and an electron transport distance.

In 1999, Goodenough et al. first studied the application of amorphous manganese dioxide electrode materials in supercapacitors. They prepared a high specific surface area amorphous manganese dioxide material (303 m2/g) by coprecipitation method, and a specific capacitance of 203 F/g in a 2 mol/L KCl electrolyte.

4. Research progress of composite materials as electrode materials for supercapacitors

The use of composite materials for supercapacitors is a research hotspot in recent years, and the comprehensive performance of supercapacitors is improved by utilizing the synergistic effect between the components of composite materials. Composite materials mainly include carbon/metal oxide composite materials, carbon/conductive polymer composite materials, and metal oxide/conductive polymer composite materials. For the disadvantage that the carbon material (such as graphene material) has lower specific capacitance, the surface of the metal oxide or conductive polymer having a large Faraday tantalum capacitor can be modified to significantly increase the specific capacitance; When the conductivity of the material such as manganese dioxide is recombined, its performance is also significantly improved, and the power characteristics are also improved accordingly.

Graphene composite

Wang et al. prepared a Ni(OH)2 nanosheet on graphene by hydrothermal crystallization. In a 1mol/L KOH electrolyte, when the constant current charge and discharge current density is 2.8A/g, based on the whole composite The specific capacitance of the material quality can reach 935F/g, while the specific capacitance based on the mass of Ni(OH)2 is as high as 1335F/g (potential window is -0.05-0.45V, reference electrode is Ag/AgCl). They also studied the effect of different preparation conditions and the difference in oxygen content of the graphene precursor on the specific capacitance of the composite. When the scanning speed was 40mV/s, the in-situ growth of Ni(OH)2 and graphene on the graphene surface was used. The mechanical capacitance of Ni(OH)2 and the growth of Ni(OH)2 on the surface of graphene oxide, the specific capacitance of the composite prepared is 877F / g? 339F/g and 297F/g. The above results indicate that the highly conductive graphene contributes to rapid and efficient charge transport between the macro-aggregated Ni(OH)2 and the current collector, accompanied by rapid energy storage and release.

Manganese dioxide composite

Since manganese dioxide is a semiconductor material, its conductivity is poor compared with noble metal oxides, which seriously affects the electrochemical performance of manganese dioxide materials. Therefore, researchers often use doping or compounding methods to improve the conductivity of manganese dioxide materials. Carbon nanotubes, mesoporous carbon, and recently discovered carbon materials such as graphene have been reported in the literature. In addition, the combination of conductive polymers and manganese dioxide has also attracted great attention. This organic-inorganic composite material can fully utilize the respective advantages of the two types of materials, greatly improving the overall performance of the electrode.

5. Outlook

Supercapacitors have an extremely broad market prospect as an emerging energy storage component, and high performance electrode materials are the focus of current supercapacitor research. In order to meet the high energy/high power density requirements of electric vehicles and renewable energy generation, supercapacitors must have high specific capacitance, large specific surface area, high electrical conductivity, long cycle life and low cost.

The pore size control of activated carbon is difficult, and the specific surface area utilization rate is low. The price of carbon nanotubes is relatively expensive and difficult to purify, which greatly affects the practical application of carbon nanotubes in supercapacitors; graphene is a new type of carbon material. It has excellent electrical conductivity and an open surface structure, and has excellent energy storage properties. If large-scale preparation can be achieved, and cost and performance can be controlled, graphene electrode materials will have attractive application prospects and are expected to be industrialized in the near future.

For the cheap metal oxide, manganese dioxide, if it can effectively solve the problem of poor conductivity and cycle stability, further improving the utilization of electrode materials will help achieve large-scale application of manganese dioxide supercapacitors.

On the other hand, the use of composite materials as electrode materials, the length of each material to avoid short, that is, through the "synergy effect" is conducive to improve the overall electrochemical performance of the material. At present, research hotspots for preparing new composite materials with high energy density, high power density and low cost (such as graphene-manganese dioxide composite materials) at home and abroad are the screening of composite systems and new nanocomposites. However, in general, the synthesis method, mechanism of action and electrochemical performance of composite materials are still in the development stage. To fully meet the requirements of practical use, further research and material performance improvement are still needed.