Supercapacitors have been around since 1966 but were never considered as a serious alternative for batteries. Ironically, we could have the said the same for AM radio before Armstrong developed the superheterodyne receiver. It may not be long before a major discovery will change the fate of supercapacitors forever.
Supercapacitors have a lot of promise in them. But what exactly are the reasons why alternative energy industries are not so fond of this device (yet)?
Let us take a closer look at Supercapacitors, its inherent advantages, its glaring weaknesses and the current studies in improving it.
A supercapacitor or ultracapacitor is essentially similar to the basic capacitor in the sense that it stores energy in an electric field. As such, it can quickly deliver and store energy because there is no chemical reaction involved. It can also survive thousands of charge and discharge cycles – obviously more than a battery can.
Like the basic capacitor, a “super” also contains two conducting plates. But these are “virtual plates” and are actually substrates of the same material. The substrate is a sponge-like, porous material made from activated carbon. They are immersed in an electrolyte consisting of positive and negative ions dissolved in a solvent. Just like an ordinary capacitor, the substrates are separated by a dielectric separator.
Energy is stored in the electric field created by the two oppositely charged substrates or electrodes. When voltage is applied across the two electrodes, a charge builds on them—one positive, one negative. This makes the ions from the electrolyte go to the electrode with a charge opposite with them. As a result, the electrodes of a supercapacitor end up having two layers due to the addition of the ions. The supercapacitor is essentially two capacitors in series.
During discharging, the charge on the plates decreases as electrons flow through a circuit. This decrease in charge results in ions releasing their hold on the plates and returns to the electrolyte.
The Advantages Of Supercaps
We can recall that a battery stores energy in a chemical reaction. That is, ions are actually introduced inside the electrode’s atomic structure. Because ions in supercapacitors only “cling” on the plates, this results in much faster charge and discharge cycles. Also, capacitors can last much longer than batteries because they do not suffer the wear and tear caused by chemical reactions.
Both capacitors and supercapacitors store energy through the separation of charges. There is a difference however in the measurement of this separation. In a common capacitor, the plates are usually in the order of tens of microns. Recall that capacitance depends on how small the distance is between the plates. An ordinary capacitor cannot have larger capacitance because they are limited by the thickness of the dielectric. In supercapacitors, the distance is not between the plates but between the ions and the electrode. This distance is so tiny that is only measured in nanometers! Consequently the supercapacitor contains capacitance in the order of thousands of farads.
This is only one reason though, as the activated carbon in the substrate also has something to do why supercapacitors can have large capacitances. Recall again that aside from plated distance, capacitance is directly proportional to the surface area of the plates. The truth is it’s not how large the area of the plates is but how many electrons can cling on the plate. Since the plate of a supercapacitor is made from activated carbon, electrons have more space to cling on than with an ordinary metal plate.
Why Not Supercaps?
A supercapacitor might be a huge leap over a common capacitor but when it comes to energy storage and price, it still way behind batteries. Although the price of supercapacitors have gone way down since 2001($5000 to $50), it is still more expensive than a lithium-ion battery. Also, it can store only about five percent of the energy that a lithium-ion can.
It would be possible that supercaps will replace lithium-ion batteries in cell phones. Since it can last for 10-15 years, you will not need to change a supercap. The cellphone may go down but the battery would still be in top shape. Environmentalists would be happy with supercapacitors as their long life means less waste disposal. A flaw however is that the phone wouldn’t stay charged for long using, at least, today’s supercapacitors.
The greatest challenge with supercapacitors could be its ability to handle voltage. A 20 uF capacitor could handle as much as 300 volts, while a supercap would handle only 2.7 volts. The electrolyte inside the supercap would break down at a higher voltage.
A number of electric vehicle manufacturing companies are now using supercapacitors for acceleration. Aside from this, the devices can also be found in cell phone base stations, backup power systems and audio systems.
Supercapacitors have a future on the electric grid. To leave a buffer for power surges, most transmission lines today operate around 90% of their capacity. With the help of supercapacitors to absorb power surges, these transmission lines could run closer to its full capacity.
Supercapacitors could also become an important piece for the growing market of microhybrid cars. In this, a supercapacitor could provide the power during the stop and also provide power for the restart. The supercap will then be recharged while the car is traveling.
The Mission For a Better Supercap
Scientists have two goals for a better supercapacitor: increasing the plate coating surface area and increasing the maximum amount of voltage that it can handle.
These experts are battling the surface area challenge using carbon nanotubes. Other researchers are working with an improved activated carbon and graphene. In addition to boosting the plate surface area, the above mentioned materials can also withstand a higher voltage than activated carbon.
To increase the maximum amount of voltage researchers are toying with ionic liquid electrolytes. This all-ion electrolyte can operate up to three folds the voltage of conventional electrolytes under the right conditions.