Energy_Storage_Technologies.pdf

How it works as electrochemical capacitors are a type

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How it Works As electrochemical capacitors are a type of capacitor, first an overview of capacitors is called for. Conven- tional Electrostatic Capacitors This is the simplest form of capacitor, and works by storing energy in an electric field. Two plates (electrodes) are placed very close together, but not touching, with either air other non-conductive material (known as a dielectric) in between the plates. A power source is then connected across the plates which places a voltage across the plates, with one side being positively charged and the other negatively charged. Electrons migrate away from the positively charged plate to the negatively charged plate due to the applied voltage. So, when the power source is removed, there are more electrons on one plate than the other (this is a buildup of charge). But now that the power source is gone, which had caused the electrons to move, now the electrons on the negative plate want to make their way to the positive plate to equalize the charge. This desire of the electron to equalize the charge is their potential to do work, or the stored energy (stored in the electric field). The amount of energy stored in a capacitor is determined by the voltage applied to the capacitor, V , and the capacitance of the device, C , through the relationship E = 1 2 CV 2 . The capacitance is dependent on how the device is designed and is higher when the plates are larger (more surface area for the electrons to collect on), when the plates are closer together, and when the material in between the plates have a higher dielectric constant (which means the material is better for supporting an electric field). So the capacitance is defined as C = ϵ A d (where ϵ is the dielectric constant, A is the area of the plates, and d is the distance between the plates). Electrolytic Capacitors These devices operate essentially the same way as the electrostatic capacitor, except they use an electrolyte as one of the two plates, which means a larger capacitance per unit volume. Electrochemical Capacitors The design of “supercapacitors” is essentially a hybrid between batteries and capacitors. They have two electrode plates and an electrolyte in between (like batteries) and when a power source is connected, ions make their way to the electrodes with opposite charges due to the electric field (since oppositely charged objects attract). The difference is that a chemical reaction does not occur, merely the ions migrate; so the storage mechanism is still the electric field. Figure 3.4 shows the this idea through the stages of charge. Therefore, unlike batteries that would wear out after being cycled due to numerous chemical reactions, the lifetime of these devices is not significantly impacted by cycling. Also, the electrodes are often made of carbon nanotubes, which, under a microscope, appear as masses of tangled string. This significantly increases the surface area of the electrodes, increasing the storage capacity of these devices significantly. In
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