Review of supercapacitor technology and applications

1. Introduction

Electric vehicles are an essential environmental step today. All-electric vehicles are called “zero-emission vehicles” because no harmful gases are released [1]. Gas emissions are a critical environmental problem. Due to the greenhouse effect created by the increasing CO₂ emissions in the atmosphere, serious changes have occurred in the earth’s surface temperature and climate. Moreover, according to 2002 data, it is estimated that 40,000 deaths per year are due to air pollution in Austria, France, and Switzerland combined (approximately 74.5 million population). These estimates are based on the assumption of linearity of exposure-response relationships [2]. CO₂ emissions from the transportation industry can be reduced by electric vehicles. In 2007, it was proposed by the European Union to reduce greenhouse gas emissions by 20 % by 2020 compared to 1990 levels. Goals like these are gaining importance every day. Environmental problems in the world are now one of the main issues that need to be dealt with. Considering that the transport industry accounts for approximately 31 % of European energy use and about 25 % of European CO₂ emissions (according to 2010 data), the importance of switching from conventional vehicles to electric vehicles can be better understood [3-4].

In all-electric vehicles, the main power source is usually a battery. A secondary battery or supercapacitor can be used as an additional source supporting the main battery. This paper discusses supercapacitors, a new version of the conventional capacitor, in detail. This paper explains the supercapacitor structure, types, classifications, and application areas. Although all application areas of supercapacitors are briefly mentioned, their use in electric vehicles is emphasized.

2. Capacitor

A capacitor is a passive electronic component consisting of a pair of conductive plates separated by a dielectric material. Figure 1 shows the schematic sketch of a capacitor. Capacitors store electrical energy as an electrostatic charge. Equal amounts of positive and negative charges are collected on opposite surfaces of the conductive plates, thus creating a voltage difference between these surfaces. When an external load connects these two surfaces, current flows through them until full charge balance is achieved, and then the stored energy is released. This is the charge-discharge cycle. If desired, it can be restored to the charged state by applying a voltage to the capacitor. No chemical reaction takes place during this process. The load is physically stored [5].

Fig. 1Schematic sketch of a capacitor [5]

The capacitance value of a capacitor in farads ($C$); is directly proportional to the surface area ($A$) of the electrodes and the dielectric constant ($\epsilon$) of the insulating separator. It is inversely proportional to the distance ($d$) between the electrodes. Capacitance is the ability of an object to store an electric charge. Equation 1 represents the capacitance value of a capacitor. The value of $Q$ (coulomb) is the amount of charge stored in the capacitor, and $V$ (volt) is the potential difference between the plates:

When finding the capacitor's power, the capacitor’s internal resistances must be considered. The internal resistances of the capacitor are called ESR (Equivalent Series Resistance). Eq. (2) shows the calculation of the maximum power value ($P_{MAX}$) to be obtained from the capacitor, depending on the voltage ($V$) applied to the capacitor and the ESR value of the capacitor [6]:

2.1. Types of capacitors

Conventional capacitors have been available since 1745 [7]. Later, they were developed and diversified and turned into technologies such as supercapacitors, which have an essential role in many applications, including electric vehicles today. Capacitors are basically of 3 types. These are electrostatic capacitors, electrolytic capacitors, and electrochemical capacitors. Fig. 2 shows the constructions of these three types of capacitors.

2.1.1. Electrostatic capacitor

The electrostatic capacitor is the most basic capacitor structure formed by placing an insulator between two conductors. Electrons are separated into opposite poles on opposite plates and stored in the electric field. The charge occurs when a potential difference occurs between the plates. It is the simplest type of capacitor.

Fig. 2Electrostatic, electrolytic, and electrochemical capacitors respectively [6]

2.1.2. Electrolytic capacitor

The structure of electrolytic capacitors is also similar to electrostatic capacitors. The only difference is that the electrodes are in direct contact with the electrolyte salt in electrostatic capacitors. Electrolytic capacitors are formed using a thin, structured dielectric material with a high dielectric constant on a rough metal surface. Although the energy densities of electrolytic capacitors are much higher than electrostatic capacitors, they have a shorter lifetime. They are much more likely to generate leakage current than an electrostatic capacitor.

2.1.3. Electrochemical capacitor

Electrochemical capacitors, also known as supercapacitors, have a much higher electrode surface area than other capacitor types. This way, it has much higher capacitance values than other capacitors. Although the working principles of conventional capacitors and supercapacitors are similar, energy storage systems differ [6]. In this study, supercapacitors are discussed in detail.

3. Supercapacitors

Supercapacitors are improved versions of conventional capacitors. Supercapacitors store electrical charge in a double layer at the interface between high surface area electrodes and electrode material. For this reason, it is also called a “double-layer capacitor”. Otherwise, they are also known by names such as ultracapacitor, electrical double-layer capacitor (EDLC), electrochemical capacitor (EC), power capacitor, gold capacitor, pseudo capacitor, and power caches. Different manufacturers have given these names, and there are slight differences between them. The evolution of supercapacitors is as follows; German physicist Hermann von Helmholtz first described the concept of a double-layer capacitor in 1853. The General Electric Company created the electrochemical capacitor operating on the basis of double-layer capacitance in 1957, but the device did not have any commercial use. Pinnacle Research Institute (PRI) named the device it produced an ultracapacitor. A double-layer capacitor was produced by the Standard Oil Company of Sohio (SOHIO) in 1966 in the format still used today. In this case, a group of Standard Oil engineers working on fuel cells accidentally discovered and patented the double-layer capacitor structure. However, supercapacitors were not used commercially for a long time. The first commercial device developed by the Japanese company Nippon Electric Company (NEC) in 1979 was called the supercapacitor, and it was the first commercially successful double-layer capacitor to come to market. After the 1980s, a number of companies were now producing supercapacitors [8-13].

3.1. Types of supercapacitors

According to the energy storage mechanism, supercapacitors are basically divided into three classes. These are electrical double-layer capacitors (EDLC), pseudo-capacitors, and hybrid capacitors. The classification of supercapacitors is shown in Fig. 3. Each class has a different charge storage mechanism. These mechanisms are non-faradaic, faradaic, and a combination thereof, respectively. Faradaic processes involve chemical reactions such as oxidation and reduction, and in these processes, charge transfer occurs between the electrode and the electrolyte. A non-faradaic process does not involve a chemical reaction. In this process, the charges are physically distributed on the surfaces [5, 14-16].

Fig. 3Types of supercapacitors

3.1.1. Electrical double layer capacitors (EDLC)

The charge accumulation mechanism of this structure is based on the principle of electrostatic separation of opposite charges in the double layer. In an ideal EDLC, only charge separation occurs at the electrode. No oxidation or reduction reactions take place. The process is not faradaic. Capacitance is independent of voltage. EDLCs store electrical charge directly across the bilayer of the electrode. No load transfer occurs across the interface. For this reason, a real capacitance effect is created. This structure is the basic supercapacitor structure [5, 10, 17].

3.1.2. Pseudo capacitor

Pseudo-capacitive supercapacitors involve the charge storage process by charging the double-layer layer at the electrode-electrolyte interface by faradaic charge transfer reactions. In addition to the capacitance formed by the separation of charges in the double layer, the additional capacitance value formed by the oxidation and reduction of the ions on the electrode surface is called pseudo-false capacitance. In pseudo capacitors, charge is stored through electrosorption, oxidation-reduction reactions and intercollation mechanism. In this structure, redox reactions occur, and the charge transferred is dependent on the voltage. For this reason, it is called a pseudo-capacitor [5, 18]. In short, pseudo capacitors, unlike other supercapacitors, store energy by chemical mechanism, just like in batteries [19]. Faradaic processes in pseudo capacitors enable these capacitors to achieve higher specific capacitance and higher energy density than EDLCs [20].

3.1.3. Hybrid supercapacitors

This construction is a combination of faradaic and non-faradaic components. In this way, a supercapacitor structure is obtained, providing continuous energy and exhibiting high capacitance for pulse power [5]. Hybrid supercapacitors use a carbon and a li-ion electrode. In this way, the faradaic electrode with high specific capacitance provides higher energy density. The non-faradaic electrode provides higher power density. The advantages of both processes (faradaic and non-faradaic) combine [20].

There are 3 types of hybrid supercapacitors based on the electrode configuration used. The first of these is the composite structure. In the composite structure, carbon-based materials are combined with polymer conductive or metal oxide materials. It is called binary composite or triple composite depending on the number of materials used in the electrode. Another type is the asymmetric structure. In this structure, EDLC is combined with the so-called capacitor electrode. Faradaic and non-faradaic processes coexist. In this structure, carbon material is used as the negative electrode, and polymer or metal oxide material is used as the positive electrode. Finally, there is the battery type hybrid supercapacitor structure. This structure is a combination of a battery electrode and a supercapacitor electrode [21].

EDLC is the most cost-effective and widely used supercapacitor type today. When talking about the supercapacitor concept, the structure usually discussed is the EDLC structure, which creates a real capacitance effect.

4. An overview of the supercapacitor structure

A supercapacitor consists of two electrodes, an electrolyte material and a separator. Although similar in structure to batteries, supercapacitors differ from batteries by containing a separator membrane that electrically separates the two electrodes [11, 17, 22]. Fig. 4 shows the supercapacitor structure. There are electrodes with increased surface area in the structure of the supercapacitor. In this way, the capacity value has increased. The energy density of a supercapacitor is much higher than that of a conventional capacitor [23], and a supercapacitor fills the gap between conventional capacitors and batteries in terms of energy density [22]. Electrode material, electrolyte type and separating membrane properties are the factors that determine performance criteria such as capacitance, cycle life, energy, and power densities in a supercapacitor [7, 17].

Fig. 4Schematic sketch of a supercapacitor [7]

When a voltage is applied between the two electrodes in the supercapacitor, the positive and negative ionic charges in the electrolyte accumulate as opposite charges on the surface of the two electrodes. This process is physical, and no chemical reaction occurs inside the supercapacitor (the fake capacitor is an exception). The main difference between the supercapacitor from other capacitors is the expanded surface area of the electrodes. In this way, supercapacitors have a very high capacitance value compared to other capacitors. While the capacitance values for electrostatic capacitors are generally in the micro and millifarad range, the capacitance values of supercapacitors rise to the kilofarad level [6].

Supercapacitors can be classified according to the electrode material used or the type of electrolyte.

5. Supercapacitor applications

A low ESR (Equivalent Series Resistance) value is required to store energy quickly. Supercapacitors are the devices with the lowest ESR value compared to other energy storage devices. Supercapacitors can be charged-discharged very quickly due to their exceptionally low internal resistance and the absence of any chemical reaction in their internal structures. So from a speed standpoint, the supercapacitor can be an ideal store. A lower ESR value also means less power loss. Power losses occur in devices with large ESR values when the output load situation changes. This deteriorates the quality of the power supply and the performance of the electronic circuit. This sometimes causes electronic components to 'stuck' out of the blue. This can be observed on laptops or mobile phones. A low ESR value reduces the likelihood of device “stuck” [36, 38]. This is one of the reasons why super capacitors are preferred. Fig. 5 shows the power loss in a supercapacitor and a battery when the current drawn (output load state) changes.

When looking at energy storage technologies, fuel cells are considered systems with high energy density, and super capacitors are considered systems with high power density. On the other hand, batteries have a higher power density than fuel cells and a higher energy density than supercapacitors. From this point of view, they have an average power and energy value [34]. Different systems have different performances between energy and power. Therefore, a single system cannot meet the demand of different applications. The demand for each application is different. By using these three technologies together, suitable systems can be obtained. In a study by Paladini et al., these three systems were used together. While the fuel cell is the main power source, the battery and supercapacitor are secondary storage devices. In this system, the fuel cell both powers the electric motor and charges the battery and supercapacitor with the stored energy. In this system, the battery is only activated when the fuel cell is insufficient in high-power situations. If the required power is at a level the fuel cell can supply, the system is operated only with the fuel cell. If the power demand increases, the battery, and supercapacitor come into play and close the gap. In this system, the supercapacitor captures most of the braking energy, significantly improving fuel economy [35]. The storage of some of the energy that occurs during braking is called regenerative braking [39]. Because supercapacitors work fast, they are much more effective than batteries in efficiently recovering the energy that occurs in the case of rapid braking in vehicles [37].

Fig. 5The power loss in a supercapacitor and a battery when the current drawn changes [38]

For this reason, supercapacitors can be used in buses providing urban transportation. Thanks to the supercapacitor in these vehicles, which make many stop-starts during the day, most of the energy lost can be recovered. Another application where supercapacitors are used is rubber-stained gantry cranes used to load and unload container ships in major ports. The primary power source in this system is a diesel engine. The supercapacitor, on the other hand, is used together with the diesel engine, easing the diesel engine's workload. In this way, the same work can be done using a smaller-sized diesel engine. This improves air quality while reducing emissions from the diesel engine [8, 40-41].

Supercapacitors are already available on many devices. For example, in cameras, supercapacitors provide power for high power-consuming functions such as zoom mode for close-up shooting and prolonging its life by lightening the load of the battery, which is the main power source [7]. For small portable devices, a supercapacitor can be preferred as an energy storage device with a small volume, lightweight, high reliability, and high mechanical strength [24].

6. Overview of supercapacitor technology

Environmentally friendly power supplies are gaining more and more importance. Energy obtained from sources such as solar energy and wind energy is also clean energy, but it depends on climatic conditions and varies. Therefore, these systems may not always meet the needs. This is one of the reasons why the supercapacitor, which is an environmentally friendly energy storage device, is preferred [38]. Supercapacitors are becoming increasingly popular due to their high specific power and ability to integrate these properties into batteries with high energy density. Hybrid energy storage systems in which battery and supercapacitor are used together are promising. The battery’s high energy density and the supercapacitor's high power density can be exploited simultaneously. This type of system cannot match the performance of conventional internal combustion engines [35]. However, if it is considered that the population in the world is increasing and environmental problems cannot be ignored, the importance of such environmental systems will be understood.

Supercapacitors are profitable systems in the long run in terms of cost. The cost of supercapacitors dropped from 80 cents per farad in 1996 to 10 cents per farad in 2010. It continues to decline [38].

A supercapacitor can operate from temperatures as low as –40 to as high as +85 without loss of performance [7, 42]. For batteries, weather conditions are essential, and they lose performance at shallow temperatures, while at very high temperatures, there is a risk of explosion. This reduces the reliability of the batteries. In order to give an electric vehicle its initial speed and lighten the load on the battery, using an additional supercapacitor on the battery is a useful solution.

The disadvantage of supercapacitors is that they have a low energy density. A supercapacitor can store about 5 % of the energy of a lithium-ion battery. The most important reason is that although batteries store energy in most of their materials, supercapacitors can only store energy on the electrode surface [7]. The larger the electrode surface, the greater the stored energy, but it is not currently possible to store as much energy as a battery. Therefore, hybrid systems are in the foreground instead of a single supercapacitor energizing a device.

As a result, besides the advantages of supercapacitors, such as high power density, fast charge-discharge time, long cycle life, cost-effectiveness, reliability, and durability. It also has disadvantages, such as low energy density. The use of supercapacitors in devices is mostly due to these reasons; a supercapacitor can operate at 85 % to 98 % efficiency, and it can run very fast thanks to its high power density. In addition, the materials from which they are produced are environmentally friendly [15, 36, 40, 43-44].

7. Conclusions

In this article, supercapacitors are examined from various perspectives. In order to better understand supercapacitors, first of all, capacitor technology from the past to the present has been discussed. Afterwards, the structure of the supercapacitor, which is an advanced type of capacitor, its working mechanism and properties such as specific power, specific energy, cycle life, cost, etc. are examined. Electrode materials and electrolyte types, which are of great importance in the structure of the supercapacitor, have been studied in detail. Types of supercapacitors and applications in which supercapacitors are used are other topics in the article.

Supercapacitor technology is a relatively new and emerging technology. A supercapacitor alone cannot currently provide sufficient power to an electrical device. Because supercapacitors have low energy density, it is more advantageous to use them with batteries in electric vehicles. Literature searches have shown that the advantages of supercapacitors and the integration of these advantages into batteries are promising for many applications.

The most important property to be developed in supercapacitors is energy density. The energy density of a supercapacitor is highly dependent on the electrode surface. Many studies have been carried out to increase the electrode surface area. While the electrode surface area of supercapacitors is increasing day by day, their costs are decreasing day by day. In addition to all these, the important point is that supercapacitors are produced entirely from environmentally friendly materials and do not have a negative impact on the environment when they become waste. One of the most important focal points of the 21st century is the concept of “sustainability”. Supercapacitors are one step ahead of other storage devices in this respect. As a result of all this, supercapacitors are promising for applications that require energy storage, both today and in the future.