An Overview of Supercapacitors

Catalog

I Introduction

In brief, a supercapacitor is a very large polarized electrolytic capacitor. 'Large' here refers to capacity, not their physical size.

For ordinary electrolytic capacitors, the larger the capacitance and voltage value, the larger the overall package. Electrolytic capacitors usually provide capacitance values on the microfarad order of magnitude, from about 0.1uF to about 1F, with nominal voltage up to 1kVdc. In general, the higher the rated voltage, the smaller the capacitance value; the larger the capacitance value, the larger the package, and the operating voltage may decrease.

These rules also basically apply to supercapacitors. The capacitance of the supercapacitor is above 1F, and the operating voltage ranges from 1.5V to 160V or even higher. As capacitance and voltage increase, their volume increases.

Early supercapacitors with capacitance values around ten Farads were very big, mainly used in large power supplies. Small-volume supercapacitors can work under low voltage are often used as short-term backup power sources in consumer electronic devices.

Although there is a great similarity between supercapacitors and electrolytic capacitors, there are also large differences in electrical performance and physical size. For example, the size of a common 10uF, 25Vdc rated-voltage electrolytic capacitor may be slightly smaller or even equivalent to a 1F to 10F, 2.7Vdc supercapacitor. With the recent technological progress, when the working voltage of the supercapacitor is increased to 25Vdc, the size is increased by less than 1 time. This volume change may not be very significant depending on the specific applications.

Inside Look Of A Supercapacitor vs Electrolytic Capacitor

II Structure of Supercapacitors

In principle, a supercapacitor can be regarded as a rechargeable battery. It can store the charge that is proportional to its capacity and release the charge when a discharge is required. The supercapacitors have electronic double-layer structure, which is the biggest difference from electrolytic capacitors and achieve higher capacity.

Figure 1. Structure of a Standard Capacitor

The structure of a standard capacitor is a dielectric layer sandwiched between two electrodes attached to a metal plate (Fig1). Depending on the type of capacitor, the dielectric can be alumina, tantalum tetroxide, barium titanium oxide, or polypropylene polyester. Different materials will have different capacity and voltage characteristics. The amount of dielectric and the distance between the polar plates will also affect the capacitance. However, the maximum allowable distance between the plates limits the number of dielectrics.

Figure 2. Capacity and Voltage Characteristics of Different Materials

In this single-layer structure, it's usually feasible to increase capacity by increasing the number of dielectrics. Specifically speaking, there are mainly three methods, increasing the package width and plate size, increasing the package length and the plate distance or a combination of these two methods. All three methods will increase the volume of the capacitor, which is a sacrifice that must be made to increase the capacitance.

An electric double-layer capacitor (EDLC) can solve the above problem. It has a second dielectric layer in the package, which works in parallel with the first layer on both sides of the intermediate isolator( Fig3). EDLC also uses non-porous dielectrics, such as activated carbon, carbon nanotubes, and carbon black gels, and it also uses conductive polymers. Its storage capacity is much higher than standard electrolytic materials. This combination of extra layers and more efficient dielectric materials can increase the capacitance by almost four orders of magnitude.

Figure 3. Structure of Super Capacitors

However, voltage capability is a weakness of supercapacitors as a result of dielectric materials. The dielectric in EDLC is extremely thin, only on the order of nanometers, so it has a large surface area and thus forms a larger capacity. However, these thin layers do not have the ideal insulating properties of traditional dielectrics and therefore can only be used under lower operating voltages.

III Working Principle of Super Capacitors

The supercapacitor is a new type of component that stores energy through a double-layer interface formed between electrodes and electrolytes. When the electrode is in contact with the electrolyte, due to the Coulomb force(Fig4.) and intermolecular force, the solid-liquid interface has a stable double-layer charge with the opposite signs.

Figure 4. Coulomb Force

The electric double-layer supercapacitor can be seen as two inactive porous plates suspended in the electrolyte, and the voltage is applied to the two plates. The potential applied to the positive plate attracts negative ions in the electrolyte, and the negative plate attracts positive ions, thereby forming an electric double-layer capacitor on the surfaces of the two electrodes.

The electrical energy stored in traditional physical capacitors comes from the separation of charges on two plates, and the two plates are separated by the vacuum (relative permittivity is 1) or a layer of dielectric substance(relative permittivity is ε).

C(Capacitance) =ε·A / 3.6πd·10-6 (μF)

A is the area of the plate and d is the thickness of the dielectric.

E(The stored energy) = C (ΔV) 2/2

where C is the capacitance value and ΔV is the voltage drop between the plates. It can be seen that if you want to obtain a larger capacitance and store more energy, you must increase the area of the plate or reduce the thickness of the dielectric. However, the flex space is limited, resulting in a small amount of stored electricity and energy.

The supercapacitor is made of activated carbon material into a porous electrode, and an electrolyte solution is filled between the opposite carbon porous electrodes. When a voltage is applied at both ends, positive and negative electrons are collected on the opposite porous electrode, and the positive and negative ions in the electrolyte solution will be collected on the interfaces opposite to the positive and negative plates due to the effect of the electric field, thereby forming two current collecting layers, which is equivalent to 2 capacitors connected in series.

The activated carbon material has an ultra-high specific surface area of over 1200m²/g (a very large electrode area A is obtained), and the interface distance between the electrolyte and the porous electrode is less than 1nm (a very small value of dielectric thickness d is obtained).

According to the previous calculation formula, it can be seen that the capacitance of this electric double-layer capacitor is much larger than that of traditional physical capacitors, and the specific capacity can be increased by more than 100 times, so that the capacitance per unit weight can reach 100F/g, and the internal resistance of the capacitor can also be kept at a very low level.

IV Classification of Supercapacitors

As the supercapacitor is a type of new product, different updates and adjustments have been made in terms of structure, material, and performance.

According to different contents, the method of classifying supercapacitors is different. At present,  supercapacitors are classified in the light of the two major elements: the working principle and electrolyte type.

1. Classification by Principle

(1) Electric Double-layer Capacitors

It is produced by the confrontation of charges caused by the alignment of electrons or ions at the electrode or solution interface. The double electric layer is formed at the interface between the electrode where electrons are conductive and the electrolyte solution where ions are conductive.

When an electric field is applied to the two electrodes, the anions and cations in the solution respectively migrate to the positive and negative electrodes, and an electric double layer is formed on the electrode surface. After the electric field is removed, the positive and negative charges on the electrodes are attracted by the ions with opposite charges in the solution, which stabilize the electric double layer, and a relatively stable potential difference(PD) is generated between the positive and negative electrodes.

Figure 5. Operating Principle in Electric Double-layer Capacitors

At this time, a certain amount of ionic charge charges of different polarity equal to the charge on the electrode is generated within a certain distance (dispersion layer) to keep electrically neutral. When the two electrodes are connected to an external circuit, the charges on the electrodes migrate, generating a current in the external circuit. The ions in the solution migrate to the solution and become electrically neutral. This is the charging and the discharging principle of the electric double-layer capacitor.

(2) Faraday Quasi-capacitors

The theoretical model of the Faraday Quasi-capacitor was first proposed by Conway. Its capacitance is generated on the electrode surface and near-surface or in a two-dimensional or quasi-two-dimensional space of the bulk phase. The electroactive material is performing the underpotential deposition(UPD), resulting in highly reversible chemical adsorption-desorption redox reactions, which generate a capacitance related to the electrode charging potential.

Figure 6. The Mechanism of Underpotential Deposition(UPD)

The charges of Faraday quasi-capacitor are not only stored on the electric double layer, but also generated by the redox reactions between electrolyte ions and electrode active materials. Ions in the electrolyte (such as H +, OH-, K + or Li +) first diffuse from the solution to the electrode/ solution interface under the action of an electric field, and then enter the bulk phase of activating oxide on the electrode surface through the redox reaction at the interface, thus a large amount of charges is stored in the electrode.

During discharge, these ions entering the oxide will be returned to the electrolyte again through the reverse reaction of the above redox reaction, and the stored charge is released through the external circuit. This is the charging and discharging mechanism of the Faraday quasi-capacitor.

2. Classified by Electrolyte

Electrolytes are compounds that have conductivity when dissolved in an aqueous solution. Electrolytes in supercapacitors are mainly aqueous electrolytes and organic electrolytes.

Aqueous electrolytes are subdivided into acidic, alkaline, and neutral types, and the composition of electrolytes with different characteristics is also different. For example, the acidic electrolyte is composed of 36% H2SO4 aqueous solution, and the alkaline electrolyte is constituted by strong bases such as KOH and NaOH.

Organic electrolytes generally use lithium salts based on LiClO4, quaternary amine salts based on teABF4, etc. as electrolytes, and sometimes corresponding solvents, such as PC, ACN, GBL, THL, etc., can be added according to the needs, which will greatly improve the properties of supercapacitors.

Figure 7.(a) CVs showing comparison of aqueous and organic electrolyte at 2 mVs −  1 .

(b) Ragone plots of five cells treated in aqueous electrolyte (2 M KOH) and their comparison with a cell assembled in organic electrolyte (TEABF 4 /PC).

V Main Parameters and Performance of Supercapacitors

1. Main Parameters

(1) Service Life

If the internal resistance of the supercapacitor increases, the capacity will decrease within the specified parameter range. Its effective service time can be extended, which is generally related to its characteristics. If the lost liveness is lost and the internal resistance increases, the working time will also be shortened. When the ability to store electrical energy drops to 63.2%, the supercapacitor can not be used anymore.

(2) Voltage

The supercapacitor has a recommended voltage and an optimal working voltage. If the used voltage is higher than the recommended voltage, the life of the capacitor will be shortened. However, the capacitor can work continuously for a long time in an over-voltage state, and the activated carbon inside the capacitor will decompose and become gas, which is beneficial for storing electrical energy. It cannot exceed 1.3 times the recommended voltage, otherwise, the supercapacitor will be damaged due to the excessive voltage.

(3) Temperature

The normal operating temperature of the supercapacitor is from -40 to 70 °C. Temperature and voltage are important factors affecting the life of supercapacitors. Every 5 °C increase in temperature will reduce the life of the capacitor by 10%. At low temperatures, increasing the working voltage of the capacitor will not increase the internal resistance of the capacitor, which can improve the efficiency of the capacitor.

(4) Discharge

In pulse charging technology, the internal resistance of the capacitor is an important factor; in the small current discharge, capacity is also an important factor.

(5) Charging

There are many ways to charge capacitors, such as constant current charging, constant voltage charging, and pulse current charging. During the charging process, connecting a resistor in series with the capacitance loop will reduce the charging current and increase the battery life.

Figure 8. Constant Current & Constant Voltage Charging

2. Performance Characteristics

(1) Advantages

As a new energy device, the supercapacitor has the following main advantages:

1) High Power Density

The internal resistance of the supercapacitor is very small, and the rapid storage and release of charges can be realized at the electrode/solution interface and the electrode material body. Therefore, its output power density is as high as several kilowatts/kg, which is unmatched by any chemical power source, and is dozens of times that of ordinary storage batteries.

2) Long Charge and Discharge Cycle Life

In the process of charging and discharging, the supercapacitor only transfers ions and charges. There is no phase change caused by the electrochemical reaction. Therefore, its capacity has almost no attenuation. The cycle life can reach more than 10,000 times, which is much longer than the charge and discharge cycle life of the battery.

3) Short Charging time

From the results of supercapacitor charging tests that have been made so far, when the current density is 7mA/cm² (equivalent to the charging current density of a general battery), the full charging time is only 10-12 minutes, and the battery cannot be fully charged in such a short time.

4) Special Power Density and Moderate Energy Density

For ordinary storage batteries, if the energy density is high, its power density will not be too high; and if the power density is high, its energy density will not be too high. However, while the supercapacitor provides a high power density output of 1 to 5 kW/kg, its energy density can reach 5 to 20 Wh/kg. If it is combined with a battery, it will form an energy storage system with both high energy density and high power density output.

Figure 9. Power vs. Energy Density

5) Long Storage Life

During the storage of the supercapacitor after charging, although there is a slight leakage current, the ion or proton migration movement that occurs inside the supercapacitor is generated under the action of the electric field rather than chemical or electrochemical reaction. The electrode material is also relatively stable in the electrolyte, so the storage life of the supercapacitor is almost unlimited.

6) Wide Operating Temperature Range

The supercapacitor can work under the temperature of -50 to + 75℃, and its performance is better than traditional capacitors and batteries.

(2) Disadvantages

Through the performance test of supercapacitors, this new type of capacitor also has disadvantages. Such as:

1) Leak

Unreasonable installation position of the supercapacitor will easily cause problems such as electrolyte leakage and damaging the structural performance of the capacitor.

2) Circuit

Supercapacitors can only be used in DC circuits. Compared with aluminum electrolytic capacitors, supercapacitors have greater internal resistance and are not suitable for AC circuit operating requirements.

Figure 10. An Aluminum Electrolytic Capacitor

3) Price

Since the supercapacitor is a new generation of high-tech products, its price was relatively high when it was first introduced to the market, increasing the cost of equipment operation.

VI The Identification of Super Capacitors

In the market, the concept of supercapacitors is very confusing, so it is necessary to know how to identify various types of supercapacitors.

1. Identification of Supercapacitors and Batteries

(1) There are no electrochemical reactions during the charge and discharge process of the supercapacitor based on the electric double layer theory. Therefore, the voltage of the supercapacitor can be released to zero, and the two ends of the supercapacitor are shorted during storage, which means that the supercapacitor is not expected to have a charge or voltage when not in use.

The battery voltage is not allowed to be released to zero, and the positive and negative electrodes are not allowed to be shorted. This will cause a short circuit and damage the battery.

Therefore, we can easily distinguish between the two only by judging whether the two electrodes are shorted. Of course, it is also effective to differentiate them by testing whether it's able to be discharged to zero.

Batteries and Supercapacitors

(2) In theory, the two electrodes of the supercapacitor are symmetrical, so the reverse voltage is allowed to work. Reverse voltage operation is not allowed or impossible for storage batteries.

(3) The relationship between the voltage and charge of the supercapacitor during the charging process is linear, and the relationship between the battery voltage and charge is not linear.

2. Identification of Electric Double-Layer Supercapacitor and Electrochemical Supercapacitor

Because the characteristics of electrochemical supercapacitors are very similar to those of batteries. Electric double-layer supercapacitors can release voltage to zero, and electrochemical supercapacitors are not allowed to release the terminal voltage to zero. And the ESR of double-layer supercapacitors is much lower than the electrochemical supercapacitor.

In addition, due to the electrochemical reaction of the electrochemical supercapacitor during the charge and discharge process, the corresponding energy storage and release is higher than that of the electric double layer supercapacitor. Therefore, the equivalent farad number of an electrochemical supercapacitor is larger than that of an electric double layer supercapacitor.

3. Identification of Supercapacitors in Aqueous and Organic Solutions

The rated voltage of organic system supercapacitors is between 2.3 and 2.7V, while aqueous supercapacitors are below 1.6V. For single supercapacitors, it is possible to distinguish between them based only on their rated voltage. And the energy density of organic supercapacitors is higher than that of water-based supercapacitors. Besides, aqueous supercapacitors are usually relatively large and heavy.

In terms of packaging, the packaging of small-capacity organic supercapacitors is similar to that of electrode capacitors or button batteries, while water-based supercapacitors do not have similar packaging forms.

Figure 11. Button Batteries

In the aspect of ESR, the ESR of the aqueous supercapacitor based on the electric double layer principle is lower than that of the organic system, so the discharge current of the supercapacitor is higher than that of the organic system.

VII Precautions for Use

Not all aspects of supercapacitors are superior in the using process. Due to the limitations of manufacturing technology, there are still deficiencies in installation and debugging. Many circuit failures are caused due to the blind use of supercapacitors, which affects the performance of the entire device. Precautions for using supercapacitors include:

1. Supercapacitors have fixed polarities. Confirm the polarity before use.

2. Supercapacitors should be used at nominal voltage. When the capacitor voltage exceeds the nominal voltage, the electrolyte will decompose, at the same time the capacitor heats up, the capacity decreases, and the internal resistance will increase, which will shorten the service life.

3. Supercapacitors should not be used in high-frequency charging and discharging circuits. High-frequency fast charging and discharging will heat up the capacitor, which will decrease the capacity and increase the internal resistance. 

4. The ambient temperature has an important effect on the working life of the supercapacitor. Therefore, supercapacitors should be kept as far away from heat sources as possible.

5. When a supercapacitor is used as a backup power supply, there is a voltage drop at the moment of discharge because of its large internal resistance.

Figure 12. A Backup Supply Circuit Diagram

6. Supercapacitors should not be placed in an environment with a relative humidity greater than 85% or containing toxic gases. Under these circumstances, the leads and the capacitor case will be corroded, causing disconnection.

7. Supercapacitors should not be placed in high temperature and high humidity environments. It should be stored in an environment with a temperature of -30 to 50 ° C and relative humidity of less than 60%.

8. When a supercapacitor is used on a double-sided circuit board, it should be noted that the connection should stay away from places where the capacitor can reach, or it will cause a short circuit.

Figure 13. A Double-sided PCB Board

9. When the capacitor is soldered on the circuit board, the capacitor case must not be contacted with the circuit board, or the solder will penetrate into the capacitor threaded hole and affect the performance of the capacitor.

10. After installing a supercapacitor, do not forcibly tilt or twist the capacitor. This will lose the capacitor leads and degrade its performance.

11. Avoid overheating capacitors during soldering. If the capacitor is overheated during welding, it will reduce the service life of the capacitor.

12. After the capacitor is soldered, the circuit board and the capacitor need to be cleaned, because some impurities may result in the short circuit of the capacitors.

13. When supercapacitors are used in series, there is a problem with voltage balance between the monomers. A simple series connection will cause overvoltage in one or more single capacitors, which will damage these capacitors and affect the overall performance. Therefore, when the capacitors are used in series, technical supports from the manufacturers is needed.

14. When other application problems occur during the use of supercapacitors, we should consult the manufacturer or refer to the relevant technical data of the instructions.

VIII Main Application Fields of Super Capacitors

1. Application in Solar Energy System

The use of solar energy ultimately comes down to two aspects: solar energy utilization and sunlight utilization. Solar power generation is divided into photovoltaic power generation and solar thermal power generation, of which photovoltaic power generation uses photovoltaic(PV) cells to directly convert solar energy into electricity. Photovoltaic power generation is far better than solar thermal power generation in terms of conversion efficiency, equipment cost, and development prospects. Since the advent of practical polycrystalline silicon photovoltaic cells, the application of solar photovoltaic power generation has emerged in the world.

Figure 14. The Basic Operation of a PV cell

At present, solar photovoltaic power generation systems have three development directions: independent operation, grid-connected, and hybrid photovoltaic power generation systems. In an independent operating system, an energy storage unit is generally required, and it can store the remaining electrical energy emitted by the sun for use when there is insufficient or no sunlight. At present, the demand for the international photovoltaic energy industry has begun to develop from remote rural and special applications to the power supplied by the combination of grid-connected power generation and building, and photovoltaic power generation has transitioned from supplementary energy to alternative energy.

The energy storage system constituted by the battery set can iron out the fluctuation of electric energy caused by fluctuations in solar light intensity, and can also compensate for sudden voltage drops or sudden rises in the power grid system. However, due to its limited number of charge and discharge times and slow charging and discharging times at high currents, its service life is short and its cost is high. Therefore, the use of supercapacitor banks in solar photovoltaic power generation systems will make their grid-connected power generation more feasible.

2. Application in Wind Power System

As the fastest growing renewable energy power generation technology, wind power generation has broad application prospects. However, wind energy is a randomly changing energy source, and changes in wind speed will cause fluctuations in the output power of wind turbines, which will affect the power quality of the power grid.

Figure 15. Traditional Wind Power Generation

At present, the active power fluctuation of wind power mostly directly adjusting the operating state of the wind turbine to smooth its output power, but the power adjustment capacity of this method is limited. The reactive power fluctuation usually uses parallel static reactive power compensation devices for reactive power adjustment, but this device cannot suppress active power fluctuations.

By adding additional energy storage equipment, the reactive power can be adjusted, the wind farm bus voltage can be stabilized, and active power can be adjusted over a wide range.

The wind power research shows that the fluctuating power located at 0.01Hz-1Hz has the greatest impact on the power quality of the power grid. The smoothing of wind power fluctuations in this frequency band has the greatest impact on the power quality of the power grid, which can be stabilized by using short-term energy storage. Therefore, small-capacity energy storage equipment capable of realizing short-term energy storage has high application value for wind power generation.

Because of its tens of thousands of times of charge and discharge cycle life and high-current charge and discharge characteristics, supercapacitors can adapt to large current fluctuations in wind energy. It can absorb energy when there is sufficient sunlight or strong wind during the day and can be discharged at night or when the wind is weak so that the fluctuation of wind power can be smoothed, which makes the grid connection more efficient.

3. Super Capacitors in the Development of New Energy Vehicles

In the field of new energy vehicles, supercapacitors can be used in conjunction with secondary batteries to store energy and protect the battery. Usually, supercapacitors are used with lithium-ion batteries. The perfect combination of the two forms a stable, energy-saving, and environmentally-friendly power source for power vehicles, which can be used in hybrid and pure electric vehicles.

Figure 16. New Energy Vehicles

Lithium-ion batteries solve the problems of car charging and energy storage and providing durable power for cars, while supercapacitors are applied to provide high-power auxiliary power when the car starts and accelerates, and to collect and store energy when the car is braking or idling.

Supercapacitors can quickly recover and store energy when the car is decelerating, downhill, and braking. It can safely convert the excess irregular power generated by the car to the battery's charging energy to achieve safe and stable operation. When the car starts or accelerates, the battery transfers energy into the supercapacitor, and the supercapacitor can provide the required peak energy in a short time.

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