SUPERCAPACITOR
- Deeksha Sori
- Jan 23, 2021
- 9 min read
Updated: Jan 24, 2021
§ NAME: DEEKSHA SORI
§ Post Graduated, Integrated M.Sc. in Chemistry,
§ Center for Basic Sciences,
§ Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, 492010,
§ E-mail : chemdeeksha98@gmail.com
CONTENT
*INTRODUCTION
*HISTORICAL BACKGROUND
*WHAT IS SUPERCAPACITOR?
*STRUCTURE
*MECHANISM
*TYPES
*BATTERY VS SUPERCAPACITOR
*IMPORTANCE
*APPLICATIONS
*REFERENCES
* Introduction
In recent years, we have been observing that there are significant developments in the field of energy storage devices such as batteries and supercapacitors. Among them, supercapacitors are considered to be advantageous which have excellent properties like fast recharge capability, high power density, long cycle life, efficient fabrication process and low risk of device to explode as compared to batteries and they are clean and environmental friendly.
Supercapacitors are the most effective and simple way to store electrical energy and since the earliest days of electro-technology has been the primary building block of electronic circuits. Developing more effective electrical storage is now the key requirement for future, social and environmental needs.
This demand is important for more efficient, sustainable energy has given rise to renewed commercial and scientific interest in supercapacitor designs in which nanotechnology-including ideas and experimental techniques play a critical role. Supercapacitors are known as the primary building block of many forms of electrical circuits from microprocessors to large sales power supplies and can be charged and discharged rapidly.
*Historical Background
In 1957, while experimenting with porous carbon electrode-based devices, a group of General Electric Engineers invented the EDLC (electrochemical double layer capacitor) effect.
Their finding at the time was that the energy was stored in the pores of carbon and had an exceptionally high capacity.
Previously, the EDLC effect was rediscovered by a group of researchers at Standard Oil of Ohio (SOHIO) working on fuel cells in 1961.
Their cell design consisted of two layers of activated carbon as an electrode medium in which a thin porous insulator divided the electrodes. It serves as the basis for the design to date of the supercapacitors.
After that SOHIO did not commercialize its invention, the technology was sold to the Nippon Electric Company (NEC), which introduced the term Supercapacitor in 1978 and used its applications to provide backup power to maintain computer memory.
Afterwards number of companies began manufacturing supercapacitors to compete on the market in the late twentieth century.
For high-powered portable energy storage, the Pinnacle Research Institute (PRI) engineered supercapacitors with low internal resistance.
After that Maxwell Technologies took over the development of PRI in 1992 and produced its own supercapacitors called' Boost Caps. This ongoing research has led to the high-performance supercapacitors available today.
What is Supercapacitor?
Supercapacitors, also known as double-layer electrical capacitors (EDLCs) or ultra capacitors, are energy storage devices with very high capacity and low internal resistance capable of storing and transmitting energy at relatively higher rates than batteries due to the mechanism of energy storage, which involves a simple separation of charge at the interface between the electrode and electrolyte. A supercapacitor specific capacitance can be defined as
C = Q/V
where Q is the charge stored on the electrode per unit mass and V is the operating voltage window.
The high capacitance of SCs can be achieved by the combination of (i) an extremely small distance that separates the opposite charges, as defined by the electric double layer. (ii) Highly porous electrodes that provides very high surface area, relatively high electronic conductivity and acceptable cost
A supercapacitor consists of two electrodes, an electrolyte, and a separator that electrically isolates the two electrodes.
This capacitor can hold a large amount of charge that can be supplied at a higher power level than rechargeable batteries. It can be used in a range of energy storage applications, either separately or in combination with batteries or fuel cells.
Supercapacitors have several benefits over traditional devices, including a large number of charging cycles, higher power density, shorter charging time and longer life and shelf life.
Structure of Supercapacitor
Supercapacitors consist of (i) porous electrodes (ii) electrolyte (iii) separator (iv) current collectors.
1. Electrodes:
Supercapacitor electrodes are generally thin coatings applied and electrically connected to a conductive, metallic current collector. It must have good conductivity, high temperature stability, long-term chemical stability (inertness), high corrosion resistance and high surface areas per unit volume and mass.
The capacitance value of the supercapacitor is proportional to the surface of the electrode. Usually, highly porous powder-coated activated carbon compounds are used as an electrode material.
The porous nature of material allows many more charge carriers (ion or radical electrolyte) to be stored in a given volume. This increases the supercapacitors power value. The electrodes are placed on a collector and soaked in an electrolyte.
Figure 1. Structure of Supercapacitor[6]
2. Electrolyte:
Electrodes are attached electronically to the electrolytes called ionic liquids. This is made up of charge carriers such as positive cations and negative anions. This is the key factor in internal resistance (ESR) determination. The electrolyte solution in nature must be either aqueous or non-aqueous. As they provide high terminal voltage (V), non-aqueous electrolytes are preferred. Non-aqueous solution consists of dissolved conductive salts in solvents.
3. Separator:
A membrane that separates a positive and a negative plate is called a separator. It is used to provide insulation or to separate the electrodes in order to prevent short-circuit. The separator is mainly composed of material that is invisible to the ions. The separator is very thin like paper. The separator provides insulation between the electrodes, but allows the carrier to flow through it.
4. Current Collector:
The current collectors are used to connect the electrodes and the capacitor terminals. Generally, the new collections are made of foil metals. Aluminum is mostly used.
In the supercapacitor there are two current collectors, one for the positive electrode and the other for the negative electrode.
The design of the supercapacitor is unique and thus different from conventional batteries and capacitors. Using activated carbon as an electrode material increases the surface area of the electrode and thus increases efficiency. The low internal resistance of the electrolyte increases the power density. Such two incorporate the supercapacitor ability to quickly store and release energy. The supercapacitors power [W] is given by,
W = V**2/ 4R
where V [Volts] is the operating voltage and R [Ω] is internal resistance.
Charge Storage Mechanism of Supercapacitor
Charging of Supercapacitor :
When the voltage is applied, the charging begins and the electrical field starts to develop. The process of charging referring in figure 2
Figure 2. Charging of Supercapacitor [7]
A mixture of positive and negative ion electrodes is the electrolyte solution. It starts charging when the voltage is applied to the electrodes of the supercapacitor. The electrodes start to attract ions of opposite polarity. This means the positive electrode absorbs negative ions or charges and attracts positive ions or charges from the negative electrode.
As a result, the positive ions or charges create a layer near the negative electrode and the negative ions produce a layer near the positive electrode. The electrical charge carrier is called the Electrical Double Layer Capacitor (EDLC) because there are two layers.
Discharging of Supercapacitor:
When a load is attached to a supercapacitor, the electrodes cannot absorb the ions, and the ions begin to disperse through the solutions of the electrolytes and move to a mixed state.
The discharge mechanism is as follows:
Ø Ions are no longer strongly attracted to existing collectors.
Ø Ions are transmitted through the electrolyte.
Ø The fee on both existing holders reduces.
The discharge of the supercapacitor is shown in fig.3 below.
Figure 3. Discharging of Supercapacitor[8]
Types of Supercapacitor
Based on the electrode materials, the supercapacitors can be classified into three categories:
1. Electrochemical double layered capacitor (EDLC)
2. Pseudo capacitors
3. Hybrid capacitors.
The classification of the supercapacitors on the basis of electrode materials is illustrated in Fig 4
Figure 4. Classification of supercapacitors based on the electrode material and the charge storage Mechanism[9]
Electrochemical Double Layer Capacitor:
It consists of two carbon-based electrodes, an electrolyte, and a separator between the electrodes. Figure 5 shows a schematic diagram of a typical EDLC. Like conventional capacitors, charge is stored electrostatic or non-faradically (especially for carbon materials) and there is no transfer of charge between the electrode and the electrolyte. It uses a double-layer electrochemical charge to store energy.
When the voltage is applied, the charge is distributed around the surface of the electrode. The ions in the electrolyte solution are distributed across the separator in the pores of the electrodes with opposite charges reflecting the natural attraction of different charges.
However, the working electrode (cathode) and counter electrode (anode) are well designed to prevent the recombination of the ions. Double-layer charging on the surface of each electrode (working and counter electrodes) can therefore be produced.
These double-layers, combined with an increase in the surface area and a decrease in the distance between the electrodes, require a higher energy density than conventional capacitors.
Figure 5. Electric Double Layer Capacitor [10]
2. Pseudocapacitors
Pseudocapacitors store charge through a faradic process involving charge transfer between the electrode and the electrolyte. When a potential is applied to a pseudo-capacitor, the electrode material reduces and oxidizes, which means moving the charge through the double layer, resulting in a faradic current flowing through the electrode.
The faradic process of pseudo-capacitors allows them to achieve more accurate capacitance and energy density compared to EDLCs.
Figure 6.Pseudocapacitor[11]
There are two general types of pseudocapacitive materials: transition metal oxides and electronically conducting polymers. The most commonly known pseudocapacitive metal oxides include ruthenium oxide (RuO2), manganese oxide (MnO2) iron oxide (Fe3O4), nickel oxide (NiO), and others. Which leads to interest in these materials but due the faradic nature, it involves reduction-oxidation reaction just like in the case of batteries; hence they also suffer lack of stability during cycling and low power density.
3. Hybrid Supercapacitor
Hybrid capacitors comprising EDLCs and pseudocapacitors combine their advantages, namely high energy and power densities. The charge storage mechanisms in such devices are a combination of purely electrostatic adsorption–desorption phenomenon at the nonfaradaic electrode and a reversible faradaic reaction at the electrode. To achieve high energy density, hybrid capacitor systems comprising redox materials have been actively researched and developed in recent years. Both EDLCs and pseudocapacitors are essential for fabricating high-performance hybrid capacitors.
Battery vs Supercapacitor
Supercapacitor has a range of benefits over the battery, the most prominent being the charging time, which is much shorter for the supercapacitors than for the batteries.
Supercapacitors are risk-free, unlike batteries that are prone to explosions. Supercapacitors also have a much higher life cycle compared to batteries.
It has a very high power density, so it can pump a large amount of power in a short period of time.
Figure 7 shows the power drawn by supercapacitors versus the power drawn by the battery applications as a function of current drawn by the devices.
In Figure 7, the green curve is the power consumed by the battery due to it's own equivalent series resistance (ESR), blue curve is the power consumed by the supercapacitors.
The black dashed line shows that if the power is too high then one may require fin and fan to cool the battery. If the cooling is not done for a give battery operated system, approximately around 5-10 W, the battery may explode which was observed in various occasion in CE products that exploded due to overheating.
Figure 7. Battery vs. supercapacitor [5]
Importance
Developing relevant energy storage systems (eg. Batteries and supercapacitors) is essential to utilizing sustainable and renewable energy resources.
A supercapacitor is highly beneficial in storing renewable energy. For example , when light is not shining or wind is not blowing, the energy needs to be stored in devices like batteries and supercapacitor.
Among the efforts of building efficient supercapacitors, electrode materials with rational nanostructured designs have offered major improvements in performance over the past several years.
Supercapacitos have several advantages such as fast charging, long charge – discharge cycle and broad operating temperature ranges.
Low internal resistance with very high efficiency, no maintenance cost, higher lifetimes are the main reason for its high demand in the modern power source related market.
Applications
Figure 8. Some modern day diverse applications of supercapacitor[5]
By evolving technology in the modern age, the Common Era systems such as laptops and cell phones have become part of our everyday lives.
For energy storage, CE electronic systems and portable devices, including camcorders, laptops, and cell phones, use batteries. However, for the specification of these devices, high-capacity batteries (e.g. li-ion) are required. These high-capacity batteries need longer time to charge and discharge as compared to supercapacitors in order to meet the high-specification power demand for CE devices.
The idea of using supercapacitor technology in place of the battery came into being to deal with these problems. In the field of technology, supercapacitor technology has provided new possibilities by providing an alternative source of power storage and supply. This is because the time required to charge the supercapacitor is much less compared to the battery, as the supercapacitors equivalent series resistance is considerably less compared to the battery's internal resistance.
REFERENCES
[1] Halper, M., & Ellenbogen, J. (2021). Retrieved 16 January 2021,from https://www.mitre.org/sites/default/files/pdf/06_0667.pdf
[2] Kumar, L., Boruah, P., Das, M., & Deka, S. (2019). Superbending (0–180°) and High-Voltage Operating Metal-Oxide-Based Flexible Supercapacitor. ACS Applied Materials & Interfaces, 11(41), 37665-37674. doi: 10.1021/acsami.9b11963
[3] Zhao, X., Sánchez, B., Dobson, P., & Grant, P. (2011). The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices. Nanoscale, 3(3), 839.doi:10.1039/c0nr00594k
[4] Vangari, M., Pryor, T., & Jiang, L. (2013). Supercapacitors: Review of Materials and Fabrication Methods. Journal Of Energy Engineering, 139(2), 72-79. doi: 10.1061/(asce)ey.1943-7897.0000102
[5] Sengupta, A., Satpathy, S., Mohanty, S., Baral, D., & Bhattacharyya, B. (2021). Supercapacitors Outperform Conventional Batteries [Energy and Security] - IEEE Journal and Magazine. Retrieved 16 January 2021, from https://ieeexplore.ieee.org/document/8429941/
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