What is a Super Capacitor?
A super capacitor is an energy storage device with a higher power density than batteries. It uses a material with a high specific surface area to store charges on its electrodes.
When the two electrodes of a supercapacitor make contact with the electrolyte solution, an electric double layer builds up at their common boundary. This causes oppositely charged ions to build up on the surfaces of each electrode.
Capacitance
In a capacitor, the capacitance is the ability to store energy in an electrostatic field. This field is a result of unequal charges on the plates of the capacitor. The difference in these charges imposes an electric force that cannot be overcome by current flow.
The charges are separated by some form of non-conductive insulator known as the dielectric. The material determines the dielectric constant of the capacitor, which affects many of its other properties. The higher the dielectric constant, the more capacitance it has.
A capacitor is polarized, meaning that the positive (+) and negative (-) leads must be connected to DC ground. If you reverse the battery monitoring system polarity, you will burn out your super capacitor and possibly cause an explosion.
The capacitance is a function of the surface area, A, and the distance between the plates. The greater the plate size and the closer the plates are, the higher the capacitance. The capacitance is also affected by the material that separates the plates. The lower the dielectric constant, the less the capacitance. The factors that determine the capacitance of a capacitor are a complex combination of physical and chemical characteristics. Our interactive Java tutorial demonstrates how these factors affect capacitance and shows changes in capacitance with changes in plate size, distance, and dielectric material.
Voltage
Supercapacitors do not produce gas like batteries, and they can also be charged and discharged much faster. This allows them to be used in applications that require low power, high life cycles and quick recharging, such as photographic flash, MP3 players and static memory (SRAM) for computers.
When a super capacitor is charged, the voltage increases linearly and the current drops by default when it’s full without needing a separate “full-charge detection” circuit. On discharge, the voltage decreases quickly and can even reverse polarity, which could damage the supercapacitor. Therefore, it is important to use a constant current supply and a voltage limit that is lower than the maximum rated voltage of the supercapacitor.
Supercapacitors are typically polarized, with a positive (+) and negative (-) lead. The positive lead should be connected to DC ground, and the negative lead should be connected to a power source that can provide enough current to pull the electrons out of the electrode material. This can be done by using a DC motor, a generator or a battery. It’s important to note that supercapacitors must be handled with care, since they can generate a lot of current and may burn you if you touch them while they are discharging. Also, they are not as durable as batteries and can be damaged by shock, vibration and exposure to extreme temperatures.
Current
The electrical current produced during charging and discharging of a super capacitor is proportional to its capacitance. The ions moving within the electrolyte generate Joule heat, which is dissipated through the conductor. This energy loss offsets the kinetic energy that the electrons use to move through the electrode, which results in a lower discharge voltage than that of a traditional battery.
The difference between a supercapacitor and a battery is that a supercapacitor uses porous materials as separators, which store electrolyte ions at an atomic level. Activated carbon is the primary material used in contemporary supercapacitors, but it is not a good insulator and limits its maximum operating voltage to under 3 V. Porous material with very small pores, such as graphene, can increase the capacity of a supercapacitor to over 10 farads.
Another difference is that supercapacitors have a low internal Programmable Logic Device resistance, while batteries have a high external resistance. Supercapacitors are also much quicker to charge than batteries.
Supercapacitors are used in medical devices like defibrillators that control irregular heart beats by delivering an electric shock. They are also being used in mobile phones, laptops, electric cars and other devices that use batteries. They are a practical alternative to batteries for applications that need a large storage capacity, long life cycle and fast recharging. For example, supercapacitors are ideal for photographic flash and static memory (SRAM) that need a low power constant voltage source to retain data.
Energy
Unlike batteries, which use chemical reactions to hold and discharge electricity, supercapacitors store electrical charge by creating a difference between two conductive plates. This difference can be maintained for a long time and discharged quickly when needed.
A typical supercapacitor has two metal foils (electrodes) sandwiched with an ion-permeable separator such as graphene. During charging, the electrodes and electrolyte exchange charged ions to generate electricity. During discharge, the separated ions release electrons to neutralize the negative charges on the electrode surfaces. As the ions release, they create a voltage across the capacitor that is equal to the applied charging voltage.
As the capacitor discharges, the applied voltage decreases to zero. The current decreases by default to stop when the rated voltage is reached. This means that a simple circuit can be used to monitor the state of charge, with no need for a full-charge detection system.
To achieve high capacitance, the separator in a supercapacitor can be made from porous materials, such as activated carbon. These structures have a fractal-like structure that creates large surface areas. When soaked in an electrolyte solution, the carbon stores charged particles on its surfaces and at the point where the liquid electrolyte meets the solid electrode. This builds up opposite electric charges and doubles the capacity of the supercapacitor. The MIT researchers used a patented carbon-based material with a nanoscopic, pore-rich metal organic framework (MOF) that makes the structure more conductive. The new supercapacitor can be used for low-power applications, such as photographic flash and MP3 players, as well as static memories that require a constant voltage source to retain information.