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A standard plate capacitor is made by separating two plates by a dielectric. The capacitance is the area times the permittivity of the dielectric, divided by the distance between them.
C = εA/d
So, the thinner the dielectric, the larger the capacitance for the same area device. I wonder how many of you cut long strips of aluminum foil and wax paper and rolled your own capacitors when you were teenagers. I know I did.
A Farad you say?
I remember an introductory class at the University where the professor was asked about capacitor values. He said, “A Farad is a big unit, really big. We deal with millionths or maybe thousandths of a Farad. A 1 Farad capacitor would be the size of a small room.” Today Supercaps come in sizes from 1-100 Farads. Times have changed.
The supercap is a different animal than a parallel plate capacitor; and it did not just spring into existence overnight. It was in the early 1950’s at General Electric that the exploration of different materials for the construction of caps really got going. They borrowed technologies used in fuel cells and rechargeable batteries. This research was not carried on exclusively by electrical engineers, as you might think, or even physicists, but by chemists.
Let’s look under the hood
For those of us who grew up with carburetors, the innerworkings of supercaps will leave you with the same feeling you had when you pulled up the hood on one of those newfangled fuel injection motors. You’re likely to look around you and ask, “Does anyone know what we are looking at?”
A supercap is an electrochemical device that looks, at first glance, more like a battery than a capacitor, more on this later. The two carbon electrodes are placed in an ionic solution and are separated by a porous, ion-permeable membrane. When the cap is charged, a voltage is placed across the electrodes. At each of the two electrodes there is a boundary layer consisting of two phases of matter, a non-soluble solid and an ion rich liquid.
At the positive electrode boundary, positive charges will accumulate on the electrode and negative charged ions will accumulate next to the electrode in the solution. They will be separated by a monolayer of solvent material. The solvent must be a polar molecule to form the boundary layer. In an aqueous solution, water is the solvent. This places the charges between 0.3 and 0.8 angstroms apart. Remembering the equation for capacitance separating the charges by a very small ‘d’ dramatically increases the capacitance.
The boundary layer above is called the Helmholtz plane. There is an inner and an outer plane. The inner plane defines the boundary between the electrode and the solvent. The outer plane defines the boundary between the ion and the solvent. There is a Helmholtz plane on each electrode so it is commonly referred to as the Helmholtz double layer. Each side forms a capacitor. The two capacitors are in series so the capacitance of each one is twice the value that the user sees, Ctotal = ((C1*D2)/C1+C2)). A one farad supercap actually has two two-farad capacitors in series.
But wait there is more
There is something called a Redox reaction which is sort for Reduction-Oxidation reaction. Some of the ions undergo a reversible transfer of charge with the electrode. This leaves them charged (cations and anions). Some of the ions can be adsorbed into the matrix of the electrode. Only a single electron participates in the transfer. There is no chemical reaction between the electrode and the ion, only a charge transfer takes place.
The first energy storage device described above is called double-layer capacitance. It is where electrostatic storage is achieved by the Helmholtz double layer. The second device is called pseudocapacitance. It is where energy is stored by a faradaic redox reaction with charge-transfer.
What is it?
So, a supercap has a lot of very sophisticated chemistry going on, does that make it a battery? A battery provides electrical energy like a supercap but the storage device is different. In a battery, the potential energy is stored in chemical bonds. As the chemical reaction takes place electrons are liberated and pushed out one end while positive charges are pushed the other direction. There are no chemical reactions required to make a supercaps store and liberate charge.
Confusing? In the 1990’s David Evans developed a hybrid Electro-chemical capacitor that combined features of the electrolytic and electrochemical capacitors and called it the Capattery. One of the most exciting things about engineering is that it is an ever-evolving field. In the 1950 only Dick Tracy had a wrist phone. Today you can purchase them at BestBuy.
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