The electrode is the key component of the supercapacitor which will fully determine its capacitance and partially determine its series resistance and selfdischarge. Different strategic options are available for the choice of the electrode technology to use:
- carbon [i,ii,iii]
- metal oxide
- solid polymer
In the case of carbon electrodes, there are two contributions to the supercapacitor capacitance: the double layer capacitance and the pseudocapacitance [iv]. The latter is attributed to functional groups mainly located on carbon edges on the surface. The double layer capacitance value is built proportionally to the accessible carbon surface. Today the stability over time under electrical and thermal stresses of the different capacitance contributions is not well established. It seems that the CO generating functional groups during Thermo Programmed Desorption (TPD) have no capacitance contribution contrarily to CO2 generating ones [v,vi,vii,viii]. To maximize the double layer capacitance the engineers develop high surface carbon with a morphology which provides a good accessibility for the ions [ix,x,xi].
From a practical point of view, the supercapacitor size and the electrode thickness being determined, the important carbon parameter to optimize is the volumetric capacitance density. This is achieved by choosing the correct pore size which must be tuned on the ions size. Typical commercial carbons are in the range of 50 F/cm3 while their mass capacitance density is in the range of 100 F/g. The most performing carbons available, mostly derivates from metal carbide (CDC) [xii,xiii] may have a capacitance of 130 to 140 F/g.
The basic problem with the activated carbon is that intrinsically it's a very poor electrical conductor. Moreover the use of small particles instead of a bulk adds a contact resistance contribution. A binder must be mixed with the powder to stick the carbon particles together. The binder material type and amount are conditioned by the carbon surface properties.
In general this low conductivity weakness is circumvented by coating the high resistive active carbon layer on a thin aluminum current collector which provides a much better conductivity by minimizing the charge path length in the carbon. The electrode performance is very sensitive to the adhesion of the carbon layer on the current collector surface [xiv]. It is important to insure a good quality of this contact over the time, even in presence of a solvent at elevated temperature, in order to maintain a low series resistance of the device.
The activated carbon conductivity may also be improved by adding some small amount of higher conductive carbon [xv]. There are carbon categories which are more conductive than others: for example carbon black, graphite or glassy carbon are better conductors than activated carbon (AC). The proportion between the high surface high resistive carbon, the conductive carbon and the binder are manufacturer's recipes.
A lot of investigations have been performed on Carbon Nanotube (CNT) utilization in the supercapacitor electrodes [xvi,xvii,xviii]. Today it is well established that the CNTs offer a poor surface accessibility for the ions with the resulting low capacitance density. The studies are going in the direction of using CNT in small proportion has an additional material to enhance the electronic conductivity and the mechanical properties of the electrode. The CNT are also used as a support for polymeric redox material again in order to increase the conductivity [xix].
The electrode properties are not only determined by the carbon properties, but also by some geometric factors. They are the result of a compromise between the required energy and power densities. To get high power, the ionic and the electronic paths must be minimized in the system. The electrode thickness is made as small as possible to reduce the path length for the ions. The electrode width is made as short as possible to reduce the path length for the electrons. The direct consequence of these two measures is that in a given volume the number of parallel layers which are stacked or rolled is increased. It follows that the cross section for the current is increased while the distance to be crossed is decreased. These geometric consideration leads to low series resistance and to high power capabilities.
The carbon purity is one of the key properties which must be surveyed during the production process. This will condition the potential window in which the supercapacitor is stable during its life. The impurity oxido-reduction reactions will be the root of the pressure build up in the supercapacitor cell. A too big amount may lead to the opening of the cell with the consequence of electrolyte leakage. They will also accelerate the selfdischarge and the aging of the component. The capacitance drop and the series resistance increase are accelerated.
There are activated carbons of high purity available on the market. The general rule is that synthetic based activated carbons have much lower impurity content than natural based activated carbons. They are nevertheless not widely used because their price is 4 to 10 times much more expensive.
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