Super capacitors and the wonders of the double layer

The double layer phenomenon and the development of supercapacitors are at the forefront of research, both academically and industrially all over the world. In this field too, Israel leads global research, presenting advanced and practical developments in academic-industrial cooperation. A lot of research is being done within the National Center for Electrochemical Propulsion (INEP) led by Professor Doron Orbach from Bar Ilan University.

As far as chemistry is concerned, which deals with bonds between atoms, it is precisely the smaller parts of the atom that have the most decisive effect. The electrons, frenetic and negative particles in themselves, are responsible for the "foreign policy" of the atoms. With the help of these charged particles, the atoms form bonds with each other. Atoms can bond by sharing electrons (covalent bonds) or transfer electrons from one to another (ionic bond). In forming chemical bonds, the atoms change their oxidation state, i.e. the electric charge they carry. A chemical reaction in general, and an electrochemical one in particular, deals with changing the oxidation states of the atoms participating in it (redox reaction), when adding an electron makes the atom's specific charge negative and giving up an electron makes its charge positive. The energy contained in such a bond is of the order of several hundred kilojoules (kJ).

But the electrons can form intermolecular bonds even without moving from one atom to another. These bonds are sometimes called 'electrostatic', because the electrons stay around their atom, so it is more correct to call them interactions than bonds. These interactions are weaker by several orders of magnitude, and the energy stored in them is relatively small, on the order of a few kilojoules.

Electrostatic bonding is the force behind a phenomenon called "Electric Double Layer Capacity" (EDLC). When a conductive surface is immersed in a solution containing ions, and an electric charge is applied to it from an external voltage source, the ions in the solution, which have the opposite charge, are attracted and stick to the surface by attracting opposite charges. This weak attachment is called electrostatic adsorption, and the energy contained in it is relatively low. But sometimes the trick is knowing how to turn a disadvantage into an advantage. With the help of a little creativity and a lot of optimization, science managed not only to recognize this phenomenon, but also to harness it for the benefit of humanity.

But scientists have to put every idea into formulas so that they can effectively deal with a phenomenon they discovered. And to put it correctly, two conductive surfaces must be placed in the solution, and in this way they are called electrodes. Now, instead of applying an electrical voltage to one surface, we will apply a voltage between the two electrodes. In order to intuitively understand what electric voltage is, we call it by its more accurate name 'potential difference', that is, charge difference - one electrode is loaded with a positive charge and the other with a negative charge. Such a system is a well-known system in physics and is called an electronic capacitor. It turns out that there is a limit to the amount of charge such a capacitor can carry and it depends on the size of the electrodes and the distance between them and insists on behaving according to the equation:
C=Aε/d

That is, the capacitance (C) increases as the area of ​​the electrodes (A) increases and decreases as the distance between them (d) increases. (ε is a constant that depends on the system, and for the sake of simplicity we will ignore it in this article). Such capacitors are widely used in many electronic systems and their role is to provide the necessary current for these systems, such as: electronic memories, detection and control systems, microphones, energy conversion facilities and more. Our industrialized and skilled world was very thirsty for mobile systems capable of storing and delivering energy. And why then are the capacitors not advertised to the average person who does not deal with electronics? The answer is that the energy capacity of these systems is extremely limited. In fact, to provide electrical energy equal to that of an AA finger battery, two electrodes had to be placed at the same time in an area of ​​about ten football fields each. It's definitely uncomfortable.

This is where the double layer phenomenon comes into play. It turns out that a double layer capacitor of the same size is capable of reaching a capacitance a million times greater than a classical capacitor (up to 200 farads per gram). No wonder it is called a super capacitor.

How does he do it? The answer involves the design of the electrodes of the supercapacitor: they are not smooth surfaces but are made of a highly porous material with a consequently high surface area. Activated carbon is usually used, which is inexpensive, a good conductor and mainly due to the fact that it can be pierced easily like a sponge so that it reaches a surface area very close to the theoretical limit - over 2,000 square meters per gram. If we go back and look at the capacitance equation, we will immediately understand the decisive implication of the huge surface area (A) on the increase in the capacitance of supercapacitors.

But that's not all. There is another parameter in the beacon and it is the distance between the electrodes (d). Classic capacitor manufacturers run into a limitation when trying to reduce the distance. Below the minimum distance, the electrodes simply "spit" electricity at each other (the minimum distance depends on the voltage between them). Supercapacitors make use of the double layer phenomenon: the electrodes can be located at a great distance from each other (several hundred micrometers) because the distance for the energy storage is measured between the electrode and the ions adhering to it from the solution. Let's draw an abstract model: the ions with the opposite charge to the one with which we charged the electrode, stick to the electrode and cover the entire surface and essentially create a sort of second charged layer corresponding to the electrode layer (hence the name - double layer). What is the distance between the two surfaces? This is the key point: inside the solution, each ion is surrounded like an onion by layers of solvent molecules known as a solvation layer, since this way, when the ion comes to stick to the electrode, the solvent layer stands between it and the object of its attraction and does not allow contact between them. If so, the answer to the question of what is the distance between the two charged surfaces is the diameter of the solvent molecule, which is usually estimated in a few angstroms (10-10 meters) only. If we recall the position of the letter d in the denominator of the capacitance equation, we will understand the importance of reducing the distance between the electrodes. The double layer phenomenon overcomes the technical difficulties of placing two electrodes so close together in a sophisticated and beautiful way: "establishing" a new electrode of ions which by definition is adjacent to the old electrode. In this way such a dramatic improvement in the capacitance of supercapacitors is achieved.

Kabali Super are taking their place in the energy storage market. The field is dominated by batteries, whose energy storage mechanism involves high-energy chemical reactions. Although there are also notable advantages to the weak interactions of supercapacitors.

Electrostatic reactions are much faster than chemical reactions involving the creation of new materials, so while batteries discharge the energy stored in them slowly and at low currents, supercapacitors can deliver the energy stored in them in a few seconds. In systems where short, high-energy pulses are needed, there is a preference for using supercapacitors. An important example that requires a lot of energy in a short time is the emergency system that was installed in Airbus planes. Another field of use that is in accelerated development in recent years is the electric vehicle. Its main power source is a high-energy battery, but to accelerate or jump on it, a fast and nervous pulse of electronic current is required, and this can be provided by a supercapacitor attached to the battery.

The double layer phenomenon and the development of supercapacitors are at the forefront of research, both academically and industrially all over the world. In this field too, Israel leads global research, presenting advanced and practical developments in academic-industrial cooperation. A lot of research is being done within the National Center for Electrochemical Propulsion (INEP) led by Professor Doron Orbach from Bar Ilan University.