Capacitors have a wide range of applications in both analog and digital circuits. A typical capacitor features two conductive plates that are separated by a layer of a dielectric material. For a given capacitor, the capacitance value depends on the characteristics of the dielectric material as well as the geometry of its structure.
Most of today’s electronic systems require energy storage solutions with higher energy and power densities. The demand for such storage solutions is growing rapidly as packing densities of ICs increase. This demand has pushed manufacturers to explore alternative energy storage solutions. Since it is quicker and more efficient to extract energy that is stored statically, capacitors have been the primary focus in the effort to develop better energy storage solutions. Compared to batteries, capacitors store energy in electric fields. Supercapacitors, also known as ultracapacitors, have high power capability and long cycle life.
Although a conventional capacitor has a high power density, its energy density is usually relatively low. Electrochemical cells have higher energy density compared to these passive components. This means that electrochemical cells are capable of storing more energy compared to conventional capacitors. However, in terms of delivering the energy, capacitors are quicker. Supercapacitors have very high energy and power densities, and this makes them suitable energy storing solutions for today’s high density integrated circuits.
Supercapacitors are electrochemical capacitors with very high energy and power densities. These capacitors are constructed with thinner and higher surface area electrodes. This design enables supercapacitors to achieve greater energy densities and capacitance compared to conventional capacitors. Despite these performance differences, supercapacitors and conventional capacitors are governed by the same fundamental principles.
Compared to electrochemical cells, ultracapacitors have shorter charging times, higher power density, and longer cycle life and shelf life. Moreover, supercapacitors can deliver high amounts of energy in short bursts. Despite their impressive power density, these capacitors have a lower energy density compared to high-end electrochemical batteries. There are two types of supercapacitors: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The two ultracapacitors use different energy storage mechanisms.
Electrochemical Double-Layer Capacitors (EDLCs)
A typical electrochemical double-layer capacitor (EDLC) consists of an electrolyte, two electrodes, and a separator. An EDLC stores charge electrostatically by utilizing an electrochemical double-layer of charge. Applying voltage to this capacitor causes charge to accumulate on its electrode surfaces. Furthermore, there is no movement of charge between the electrolyte and the electrodes of an EDLC.
Unlike conventional capacitors, the electrodes of EDLCs are constructed with materials that provide a large surface area. A double-layer of charge is created by preventing recombination of ions in these electrodes. The large surface area of these electrodes and the small separation distance between them enables electrochemical double-layer capacitors to achieve very high energy densities.
In an electrochemical double-layer capacitor, there is no charge transfer between the electrodes and the electrolyte. The non-Faradaic process, therefore, does not cause the materials to undergo chemical changes. This property enables these capacitors to achieve high cycling stabilities. Compared to supercapacitors, electrochemical batteries have lower cycling stabilities and fewer charge-discharge cycles.
The performance of a supercapacitor is greatly determined by the characteristics of the electrolyte and electrode material used. Carbon offers a large surface area, and is used for manufacturing electrodes for electrochemical double-layer capacitors. The key properties of an electrode material that determine the characteristics of an EDLC include surface area, porosity, and electrical conductivity.
For high capacitance to be achieved, the electrolyte must be able to access the carbon pores of the electrodes. This is made possible by ensuring that the size of the pores is large enough to allow electrolytic ions to penetrate. If the pores are too large, then it may be impossible to fully exploit the available large surface area. To achieve high capacitance, it is also necessary to use a highly conducting electrolyte.
The electrodes of EDLCs are made from high effective surface materials. Some of the commonly used materials include carbon aerogels, active carbon, and CNTs. Active carbon is highly porous and has very high surface area. The properties of this material are enhanced through a process known as activation. Carbon aerogels have large surface area and high porosity, and this makes them a suitable choice for manufacturing electrodes for supercapacitors.
CNT, an allotrope of carbon, has a hollow tube structure, mesoporous structure, very high surface area, and high chemical stability, electrical conductivity, and mechanical strength. These properties, as well as good pore distribution, make carbon nanotubes popular materials for manufacturing electrodes for EDLCs.
Pseudocapacitors, unlike electrochemical double-layer capacitors, store charge Faradaically. During this process, charge transfers occur between the electrodes and the electrolyte. The Faradaic process involves reduction-oxidation (redox) reactions, electrosorption, and intercalation processes. Pseudocapacitors can achieve higher energy densities and greater capacitances than electrochemical double-layer capacitors.
The performance of pseudocapacitors is greatly dependent on the properties of the material used to construct them. Some of the materials used to construct electrodes for pseudocapacitors include transition metal oxides and conducting polymers. Hydrous ruthenium oxide has excellent properties, and it is commonly used for constructing electrodes for pseudocapacitors. However, this oxide is expensive and can only be used with aqueous electrolytes.
Some polymers are capable of storing energy like batteries. Such conducting polymers include polyaniline, polypyrrole, and polythiophene. These materials are cheap and have properties that allow redox reactions. Their characteristics make them suitable options for making electrodes for high capacitance pseudocapacitors. Although pseudocapacitors operate like batteries, they are faster. As compared to electrochemical double-layer capacitors, pseudocapacitors are slower and have shorter cycle life. Their shorter cycle life is due to the inability of conducting polymers to allow repeated redox reactions.
Benefits and applications of supercapacitors
The performance characteristics of supercapacitors make them suitable energy storage solutions for a wide spectrum of applications. They are becoming increasingly popular for many applications including automotive, aerospace, transportation, maritime, and heavy industry applications.
To start with, electrochemical capacitors have very high coulombic efficiency and high current capability. Compared to electrochemical batteries, EDLCs allow more efficient use of energy. This efficiency makes them a suitable energy storage option for motorsports and automotive industry. In automobiles, these capacitors are becoming popular for regenerative braking and start-stop systems. Moreover, supercapacitors can be charged at a faster rate.
The energy storage process of electrochemical double-layer capacitors is not dependent on chemical reactions. This means that these capacitors can operate over a broad range of temperatures, -40°C to 65°C for most ultracapacitors. Unlike batteries, these capacitors have excellent cold performance, making them suitable for many applications including engine-starting applications for trains and trucks. The high tolerance to temperature variations, high power-to-weight ratio, and impressive performance of ultracapacitor-based energy storage solutions make them an excellent choice for aerospace, automotive, and heavy industry applications.
Electrochemical capacitors have long cycle life and operational life. The energy storage mechanism of EDLCs does not affect chemical bonds, and this means that these capacitors can be charged thousands of times with minimal effects on their performance. Unlike batteries, EDLCs can be cycled frequently or infrequently without negligible effects on their performance. Moreover, long-term storage of electrochemical capacitors does not have significant effects on their performance.
Batteries require regular maintenance, and this increases the cost of maintaining battery-operated systems. In comparison, EDLCs can operate for many years without requiring replacement. Moreover, supercapacitors allow easy system integration and require simple voltage management systems as compared to battery systems.
The increasing demand for energy has pushed authorities and governments to promote renewable energy resources, energy saving, and efficient energy storage solutions. Electrochemical double-layer capacitors and pseudocapacitors are designed to deliver very high energy and power densities with high efficiency. An electrochemical double-layer capacitor stores energy electrostatically while a pseudocapacitor stores it Faradaically. Ultracapacitor-based energy storage solutions offer high energy and power densities, efficiency, reliability, and power-to-weight ratio. In addition, they have longer cycle life and operational life than electrochemical batteries. These characteristics make supercapacitor-based energy storage solutions suitable for many applications including transportation, aerospace, automotive, and heavy industry applications.
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