Category: 11. Electrochemical Double-Layer Capacitors

Other than self-discharge, our discussion of EDLCs has not involved faradaic reactions; that is, we have assumed that no charge is transferred across the double layer, and that there is no change in oxidation state due to reaction. The resulting current–voltage behavior is purely capacitive and would approximate the ideal box shown earlier in Figure 11.9. The…

Construction of a typical cell sandwich for an EDLC is shown in Figure 11.18. The differences in how energy is physically stored aside, these EDLCs have many similarities to batteries, two porous electrodes coated onto current collectors and separated by an electrolyte. The cell designs are similar too. Typical configurations for EDLCs include cylindrical, prismatic, button,…

Both the energy and power density are important characteristics of capacitors, and are considered in this section. The change in energy associated with a change in capacitor voltage is (11.36)The total energy stored in the capacitor can be obtained by integration (11.37)where we have assumed that C is constant. Recall that capacitance has units of…

In this section, we apply impedance spectroscopy to electrochemical double-layer capacitors in order to gain insight into their transient behavior. Additionally, we use the impedance results as a basis for a simplified EDLC model that will facilitate our analysis of these devices. Analysis for Highly Conductive Solid PhaseIn order to use impedance to examine the…

As we noted in Chapter 5, a porous electrode is an effective means of increasing the surface area of an electrode. As with any porous electrode, the local resistance is a function of position in the electrode. An equivalent circuit diagram for an EDLC using a porous electrode is shown in Figure 11.10. The resistances along the…

We are interested in how a capacitor behaves under a variety of circumstances. In particular, we seek out the current–voltage behavior of a capacitor under different conditions. This section explores these aspects. We begin with the expression for current from a capacitor (see Chapter 6): (6.6) This equation describes an ideal capacitor, where the capacitance, C, is…

Recall from Chapter 3 that the interface between an electrode and the electrolyte is generally charged. There can be excess positive or negative charge in the metal that is balanced with an equal and opposite charge in the electrolyte adjacent to the surface. The counterbalancing charge may consist of adsorbed ions in the inner Helmholtz plane (IHP), solvated…

A conventional electrostatic capacitor consists of two conductors separated by a dielectric (electronic insulator). Energy storage is accomplished by charge separation, with positive charge accumulated on one conductor and negative on the other (see Figure 11.1a). The charge, Q, is the amount of charge on either conductor (not the sum of the two). Capacitance is defined as…