CHEM-E4175 - Fundamental Electrochemistry, 09.01.2019-18.02.2019
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Kirja
9. Electrochemical energy conversion
9.1. Supercapacitors
Supercapacitors, also known as ultracapacitors, are electrochemical devices that utilize the double layer region of the potential window for short-term energy storage. In a sense, they are reminiscent of conventional capacitors, but the charge separation and electric field is created across the electrical double layers (Chapter 5) formed at the both electrodes (Figure 9.2), instead of dielectric material used in the conventional capacitors. These devices have two similar electrodes separated by an aqueous or organic electrolyte.
With supercapacitors, high energy density is achieved compared to conventional capacitors, which is based on two factors: First, combining porous 3D electrodes (in commercial devices made of activated carbon) and a liquid electrolyte (often TEABF4 in acetonitrile) results in a large surface area (see Equation (9.5)). Second, the distance between the electrode surface and the ions in the double layer region is very small, of an atomistic scale.
| \( \displaystyle C=\frac{Q}{U}=\varepsilon\frac{A}{d} \) | (9.5) |
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Because the energy storage mechanism in supercapacitors is based on the formation of the double layer on the electrode-electrolyte interface, no electrochemical reactions take place. The energy can therefore be stored and released quickly, leading to high power density and minimal degradation of the materials. As a result, these devices can withstand hundreds of thousands of charge-discharge cycles and are suitable for high-power applications, e.g. storing mechanical energy when vehicles brake[1]. The downside is that the energy density is relatively low (compared to batteries) and the self-discharge rate is relatively high.
In order to increase the energy density, redox supercapacitors that utilize fast and reversible electrochemical surface reactions for the charge storage have been developed. These include MnOx and RuOx redox processes. Hybrid supercapacitors that combine electrodes with dissimilar storage principles are also being investigated. A combination of an electrode type used in the lithium-ion battery with an EDLC or redox electrode can be mentioned as an example of hybrid devices.