Application note from KEMET Electronics explains its supercapacitors structure, how it works and reliability / temperature load performance.
An electrical double-layer capacitor (EDLC) is different from a conventional capacitor that uses a dielectric substance. Instead of a dielectric material physically constructed between electrodes, a chemical process is used to deposit an “electrical double layer” on one electrode. An electrical double layer is a common barrier formed between two phases of matter – liquid and solid. When a suitable liquid and solid are used, and a voltage is applied, two layers of opposite polarity are formed, hence the “double layer.”
KEMET’s electrical double layer capacitor, also known as a “supercapacitor,” uses activated carbon as its solid part and an aqueous solution of dilute sulfuric acid as its liquid part. Figure 1 models the state in which the activated carbon and dilute sulfuric acid are brought into contact and then shows the modeled state in which two pairs of the solid and liquid parts are connected in series. In this scenario, both pairs share the same liquid part when an electrical field is applied externally.
Figure 1 – The Principle of Electrical Double Layer Capacitors
Figure 2 shows a conceptual drawing of the basic structure of a supercapacitor using the electrical double layer principles.
A supercapacitor typically has a much higher capacitance than a conventional capacitor. However, the voltage ratings are very low because of how small the separation of charge is (typically less than 1nm, on the order of a single molecule diameter). Supercapacitors can be considered as the gap between electrolytic capacitors and batteries in terms of their characteristics and performance.
How EDLC Supercapacitors Work
Suppose η is the amount of unitary charge of the solid part, d is the dielectric constant of the medium (liquid part), δ is the distance from the solid surface to the center of ions, and ψ is the potential of the double layer, then η is represented by expression (1).
According to Helmholtz’s theory, there is a potential gradient only in the electrical double layer, and their respective potential curves are shown in Figure 2. In Figure 2, if ψ and η are φ0 and η0, respectively, when no load is applied, then η0 is represented by expression (2).
Then, if an external electrical field is applied, a charge is accumulated on the boundary surface shown in Figure 2. At this time, suppose ψ0 becomes ψ , and η0 becomes η1 , then η1 is represented by expression (3).
From expressions (2) and (3) above, expression (4) is found.
The external electrical field allows charge corresponding to η1 in expression (4) to accumulate in the electrical double layer. Here, ψ0 is on the order of several mV. According to an experiment using mercury for the electrode, an accumulated capacitance of 20 to 40 μF/cm2 per unit area is obtained. If the activated carbon electrode shows the same action as that of mercury, then activated carbon with a surface area of 1000 m2/g will produce a capacitance of 200 to 400 F/g. Such a high capacitance cannot be obtained in real life, however. KEMET’s proprietary technology has made it possible to obtain a value very close to the above value by improving the quality of the activated carbon surface and increasing specific surface area.
It is not possible to apply a voltage higher than an electrolyte’s decomposition voltage based on the substance that makes up an electrical double layer capacitor without damaging it. Therefore, it is necessary to structure the capacitor base cells in series to obtain the desired breakdown voltage.
The Structure of a Supercapacitor
Figure 3 shows the basic structure (capacitor base cell) of a supercapacitor.
The electrical double layer phenomenon appears on the boundary surface between activated porous carbon powder (solid) and the electrolyte, dilute sulfuric acid (liquid). The separator (porous organic film) has a structure that prevents short-circuit between the positive and negative electrodes (activated carbon powder). It lets ions pass in the electrolyte (dilute sulfuric acid).
It also places a conductive current collecting electrode behind both electrodes (activated carbon powder), allowing a voltage to be applied to this capacitor base cell. In addition, it provides sealing rubber (mainly butyl rubber) at the electrode flank for sealing the electrolyte and isolating the conductive material. The amount of the electrolyte to be sealed into the capacitor base cell is equivalent to that needed for the impregnation of the pores inside activated carbon and the porous organic film, which is a minimal amount.
The breakdown voltage of the capacitor base cell depends on the electrolysis voltage of the electrolyte. The electrolysis voltage depends on the water content in the dilute sulfuric acid, which is approximately 1.2 V. To design the breakdown voltage for the maximum operating voltage of 5.5 V we must connect five or more sheets of capacitor base cells in series. (See Figure 4).
Pressure is applied inside the package to stabilize the electrical connection between the capacitor base cells, activated carbon powder particles, and conductive current collecting electrodes.