This post, the third in our series on energy storage is all about supercapacitors. Also known as ultracapacitors or the slightly more cumbersome “electric double layer capacitors,” supercapacitors look like batteries at first glance, but have much greater power capabilities.
The trade-off, unfortunately, is that their energy density is much lower than batteries, which means they cannot store as much charge as a battery. But taken together, the two devices provide good energy and power performance. Batteries and supercapacitors are both electrochemical cells, but while batteries use chemical reactions to store energy, supercapacitors store energy in an “electrochemical double layer.” We’ll explain what this means in a minute, but first let’s briefly review basic capacitors.
Capacitors are very common elements in electrical circuits. They store a very small amount of energy but also perform important functions like acting as filters for different signal frequencies. A capacitor consists of two flat, conductive plates separated by an insulator. Charge builds up on these plates, with positive charge accumulating on one plate and negative charge on the opposing plate; energy is stored in the resulting electrostatic field between the plates. The amount of energy stored is proportional to the area on the plate and inversely related to the distance between the plates. The measure of capacitance is given by:
1) C = Q/V
- Q is the total charge stored (think of this as the number of electrons built up on the charged plate) and
- V is the potential difference between the plates.
The total energy stored is:
2) E = ½ CV².
However, the capacitance can also be written as
- εr and ε0 are constants called the dialectric constant (referring to properties of the insulator between the plates) and the permittivity of free space.
- A is the area of the plate, and
- d is the distance between them (and therefore the distance between the positive and negative charges).
Combining equations 2 and 3, we can rewrite the energy stored as:
4) E = ½ ε0εrA V²/d.
What this equation means is the larger the plates and the closer they are together, the more energy is stored.
Supercapacitors can store much more energy than regular capacitors because they incorporate a few tricks of the trade to increase the area of their plates (“A”) and decrease the separation (“d”) between positive and negative charge. To do so, they mimic the construction of a battery. Supercapacitors are made up of two electrodes sandwiching a separator and filled with electrolyte. However, unlike batteries, the materials used in supercapacitors do not undergo redox reactions at each electrode. Instead, ions collect on the surface of each electrode, attracted to the build-up of the opposite charge on the surface of the electrode. This forms something called an electrochemical double layer. This double layer of charge creates an electric field just like in a regular capacitor, but the distance “d” between charges has been shrunk by a few order of magnitudes (100x-1000x) and is now on the same length scale as the size of the ion. As a result, the energy storage skyrockets. Furthermore, the electrodes are highly porous, which increases the area and therefore the energy stored by a few more orders of magnitude. By adding electrolyte and porous electrodes, the energy density increases to about 10% of that of a battery.
Ten percent of the energy of a battery doesn’t sound too exciting; however, the supercapacitor earns the “super” in its name because of its power capabilities. In a battery, redox reactions can take time. However, building up layers of charge in a capacitor is really, really fast. Supercapacitors can deliver energy about 10 times faster than most batteries. This means that their “power density” is 10 times greater.
So, if you need to accelerate rapidly in a car, for example, an supercapacitor can give you that extra kick much more easily than a battery. Additionally, The electrochemical double layer doesn’t degrade a supercapacitor the same way that redox reactions slowly lead to capacity fade in a battery. Supercapacitors can be cycled on the order of one million times.
If supercapacitors could reach the same energy density as batteries, they would revolutionize the energy storage world. Imagine a battery that you could cycle a million times and charge and discharge 10 times faster. Research is underway to use new materials, like carbon nanotubes and novel electrolytes, to increase the energy density of supercapacitors.
For now, supercapacitors are most useful in conjunction with battery systems. A lot of work is currently focused on supercapacitor-battery combinations for electric vehicles. Batteries degrade quickly if charged or discharged quickly; supercapacitors can smooth out these charge curves and increase battery life in addition to providing better power capabilities. As devices and control systems develop in the coming years, supercapacitors will increasingly find their way into cars, renewable energy systems and electronics.