Nowadays, energy storage systems are playing an essential role in various industries, including electric vehicles, power systems, and a variety of other fields. In this perspective, a supercapacitor is a boon as an energy storage device that are possessing great performance features like high power density, maintenance-free operation, and extended life, and have attracted academic and industrial attention.
What Are Supercapacitors?
Supercapacitors are technological devices that can store a massive amount of electrical power. They’re also known as ultracapacitors or double-layer capacitors. Supercapacitors store electric power comprising two mechanisms: double-layer capacitance and pseudocapacitance, rather than a traditional dielectric. Double layer capacitor is electrostatic, whereas pseudocapacitance is electrochemical, implying that supercapacitors combine the functions of conventional capacitors and those of ordinary batteries. Capacitances of up to 12000 F can be made by utilizing this technique.
On the other hand, the self-capacitance of the entire Earth is only about 710 F, which is less than 15 million times that of a supercapacitor. While the maximum working voltage of an ordinary electrostatic capacitor may be high, the maximum charging voltage of a supercapacitor is typically between 2.5 and 2.7 volts. Supercapacitors, like electrolytic capacitors, are polar devices, which means they must be linked to the circuit properly. The electrical features of these devices, particularly their quick charge and discharge periods, are intriguing for some applications in which supercapacitors could fully replace batteries.
Structure and properties of supercapacitor
Supercapacitors are equal to electrolytic capacitors in that they are made up of two foil electrodes, an electrolyte, and a foil separator. The separator is placed between the electrodes, and the foil is wrapped or folded into a cylindrical or rectangular shape. This folded structure is placed in a housing that has been electrolyte-impregnated and hermetically sealed. The electrolyte and electrodes used in the construction of supercapacitors differ from those used in ordinary electrolytic capacitors. A supercapacitor uses compounds as separators to store ions at the atomic level to store electrical charge. Activated charcoal is the most commonly used material in advanced supercapacitors. Because carbon is a poor insulator, the maximum operating voltage is limited to less than 3 V. Another reason that activated charcoal is not the ideal material is that the charge carriers are comparable in size to the pores in the material, and some of them cannot fit into the smaller pores, leading to a reduced storage capacity.
Graphene is one of the most fascinating materials used in supercapacitor research. Graphene is a rich carbon substance arranged in a one-atom-thick planar sheet. It has a high porosity and functions as an ion “sponge.” Energy densities achieved in supercapacitors using graphene are similar in batteries. Even though graphene supercapacitors have been prototyped as a proof of concept, graphene is difficult and expensive to produce in industrial quantities, delaying the use of this technology. Graphene supercapacitors, on the other hand, are the most promising candidate for future supercapacitor technology advances.
Attributes of Supercapacitor
Some of the attributes of supercapacitors are listed below-
Charging and discharge times of supercapacitors are equivalent to those of conventional capacitors. Considering their low internal resistance, they can obtain significant charge and discharge currents. Batteries can take many hours to fully charge – a cell phone battery is a good example – whereas supercapacitors can reach the same charge state in less than two minutes.
The specific energy of a battery or supercapacitor is a metric used to evaluate different technologies based on maximum power output divided by total device mass. Supercapacitors have a specific power that is 5 to 10 times that of batteries. For example, while Li-ion batteries have a specific power of 1 – 3 kW/kg, a typical supercapacitor has a specific power of around 10 kW/kg. This property is particularly important in applications that demand the storage device to release energy in short bursts.
Life Cycle and Safety of a Supercapacitor
When treated unfairly, supercapacitor batteries are safer than standard batteries. When batteries are short-circuited, they are known to spontaneously combust due to excessive heating; however, supercapacitors do not heat up as much due to their low internal resistance. Shorting a fully charged supercapacitor will result in a rapid release of stored energy, which may cause electrical arcing and damage to the device, but unlike batteries, the generated heat is not a concern.
Supercapacitors can be charged and discharged millions of times and have a nearly infinite cycle life, whereas batteries have a cycle life of 500 times or less. This makes supercapacitors extremely useful in applications requiring frequent energy storage and release.
Working Methodology of a Supercapacitor
Standard capacitors are composed of two metallic plates or electrodes separated by a dielectric substance. When a voltage is applied, electrons accumulate at one of the electrodes, storing the electrical charge.
In the meantime, the dielectric material tucked between the electrodes undergoes a process known as “dielectric polarization,” which contributes to increased capacitance.
Supercapacitors operate on the same principles, with the exception that the wedging material is an electrolytic solution rather than a dielectric content. When applied voltage, an “electrical double layer” is formed, which aligns both negative and positive charges along the boundaries of the electrodes and the electrolytic solution.
This location serves as a warehouse for electric charges. Activated carbon is frequently used to increase the size of boundary areas. This is because the capacitance of a supercapacitor is proportional to the area of the “electrical double layer.” This activated carbon is a well-known porous material with numerous surface holes that aid in covering a large surface area.
Supercapacitors have the same structure as a typical battery for electricity storage due to the electrolytic solution and electrodes. In a battery, chemical reactions occur between the electrolytic solution and the electrodes; however, supercapacitors only allow electrons to move between electrodes. Because of these distinctions, the properties of a battery and a supercapacitor differ.
Types of Supercapacitors
As previously stated, supercapacitors store energy using two basic principles: electrochemical pseudocapacitance and static double-layered capacitance. Supercapacitors are classified into three types based on this. They are as follows:
- Double Layered Capacitors
- Hybrid Capacitors
Double Layered Capacitors
Charge separation in the Helmholtz double layer is used to store electrical energy in double-layered supercapacitors. This serves as a separation between the conductor electrode and the electrolyte. The electrodes are made of activated carbon or derivatives with higher electrostatic double-layered capacitance than electrochemical pseudocapacitance.
Pseudocapacitors use polymer electrical conduction electrodes or transition metal oxides with high electrochemical pseudocapacitance. The storage of electrical energy is an electrochemical process that involves redox reactions and the intercalation of ions that have been specifically absorbed on the electrode surface.
The electrodes are ambiguous, with one exhibiting electrostatic property and the other exhibiting electrochemical capacitance. Because Pseudocapacitance and Double-Layered capacitance contribute inseparably to an electrochemical capacitor’s total capacitance, the concepts of Supercabattery and Supercapattery have been proposed. This clarifies hybrid devices that function as both a battery and a supercapacitor.
Drawbacks of Supercapacitors
Supercapacitors have some drawbacks as well. One disadvantage is that the specific energy is relatively low. The specific energy is calculated by dividing the total amount of energy stored in the device by its weight. Supercapacitors typically store 5 Wh/kg of energy, whereas Li-ion batteries commonly used in cell phones have a specific energy of 100-200 Wh/kg. It means that a supercapacitor with the same capacity (not capacitance) as a standard battery can weigh up to 40 times as much. The specific energy should not be confused with the specific power, which is a measure of a device’s maximum output power per weight.
A sequential discharge voltage is another disadvantage of supercapacitors. A battery rated at 2.7V, for example, would still output a voltage close to 2.7V at 50% charge, whereas a supercapacitor rated at 2.7V at 50% charge would output approximately half its maximum charge voltage of 1.35V. This means that the output voltage would fall below the minimum operating voltage of a device powered by a supercapacitor, such as a cell phone, and the device would have to shut down before using up all of the charges in the capacitor. The use of DC-DC converters is one solution to this problem. This method introduces new challenges, such as efficiency and power noise.
The third major disadvantage of currently available supercapacitors is their high cost. The cost of a supercapacitor per Wh is more than 20 times that of Li-ion batteries. However, through new technologies and mass production of supercapacitor batteries, the cost can be minimized. The main reasons for supercapacitors not replacing batteries in most applications are their low specific energy, linear discharge voltage, and high cost.
Implementations of Supercapacitor
Supercapacitors can be used in a wide range of applications because they bridge the gap between batteries and capacitors. In the automotive industry, one advanced form is energy storage in KERS, or dynamic braking systems (Kinetic Energy Recovery System). The main challenge in such systems is developing an energy storage device capable of storing large amounts of energy quickly. One method is to use an electrical generator to convert kinetic energy to electrical energy, which is then stored in a supercapacitor. This energy can then be repurposed to provide acceleration power.
One more example is low-power uses where a large capacity is not required but a long-life cycle or quick recharging is required. Photographic flash, MP3 players, static memories (SRAM) that require a low power constant voltage source to retain information, and so on are examples of such uses.
Future of Supercapacitor
The ultracapacitor’s (supercapacitor’s) future is very bright and promising. Among them is the plan to combine a double-layered interface with existing energy storage technologies. The addition of electrochemical capacitors to fuel-cell applications has resulted in significant improvements in charge and discharge life cycle performance.
This is especially noticeable in electric vehicle and hybrid applications. Several cities with hybrid public transportation systems have seen improvements in charge cycles and energy storage mechanisms.
Such fast charging and energy storage facilities are already on the market, causing a massive shift in our thinking about storage capabilities. Supercapacitors are also expected to be used in laptops, cell phones, electric cars, and other battery-powered devices.