Supercapacitors Battery Chemistry FAQ

Supercapacitors

Unlike batteries, which store energy chemically, capacitors store energy as an electrostatic field. Typically, a battery is known for storing a lot of energy and little power; a capacitor can provide large amounts of power, but low amounts of energy. A capacitor is made of two conducting plates and an insulator called the dielectric, which conducts ionically, but not electrically. In a capacitor,

E_cap = qV = ½CV²

where the capacitance, C, is directly proportional to the surface area of the plates and inversely proportional to the distance between them.

So in other words, as the plate surface area increases and the distance between the plates decreases, the energy you can store in a capacitor increases. Normal every-day capacitors have capacity on the orders of millifarads per cubic foot. Aluminum electrolytics are about a farad per cubic foot. But for useful energy storage we need farads per cubic inch. That is where supercapacitors come in.

First let's see how clever we can get to obtain a big surface area in a small volume. Imagine a polymer foam cleaning sponge. It has a tremendous amount of surface area in a small area because of all the crenulations (OK, nooks and crannies). Now, put it in a furnace, excluding the oxygen and bake it until only the carbon is left. You now have a conductive carbon surface with an incredible surface area in a small volume.

But to get a high capacitance there has to be two plates. You can't just go in there and create complimentary surface as the other electrode—or can you? Yes, just fill it with a conductive liquid (e.g., an aqueous acid or salt solution). The last thing you need is an ultra-thin insulator on the carbon. Ultra thin to get high capacitance, and insulator so the carbon and the liquid don't short out. This is also easy, you can electrochemically deposit an insulator on the carbon surface (or electrochemically deposit something that could be turned into an insulator upon baking).

Now attach one electrode to the carbon, one to the liquid, and you can have a capacitor that can have Farads of capacitance per cubic inch. Very nice.

Most practical supercapacitors have low voltage (2 to 5 V—remember that insulator is ultra-thin and so can break down at low voltages), which is a problem for energy storage, since the stored energy is proportional to the square of the voltage. Also, conduction through an ionic liquid is slow, so these capacitors cannot be discharged quickly compared with standard capacitors, but can be discharged very quickly compared to batteries!

Typical numbers for capacitors and batteries are given below:

device	volumetric energy density Wh/L	power density W/L	number of charge/discharge cycles	discharge time s
batteries	50-250	150	1 - 103	> 1000
capacitors	0.05 - 5	105 - 108	105 - 106	<1

Supercapacitors have several advantages over batteries: they can experience virtually indefinite number of cycles (charging and discharging), they are maintenance free, they work well in high-rate discharge, they recharge quickly, and they have no negative environmental impact. Their disadvantages are low energy density, linear voltage drop during discharge, and high cost.