Author: admin
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Impedance Analysis of EDLCs
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…
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Porous Edlc Electrodes
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…
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Current–Voltage Relationship for Capacitors
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…
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Electrical Double-Layer Capacitance
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…
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Capacitor Introduction
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…
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Case Study
A fuel cell is being considered for a manned space flight. Because hydrogen and oxygen are needed for propulsion power, these are the preferred fuel and oxidant. The basic cell performance is shown in Figure 10.20. Our job is to recommend the current density and voltage at which the fuel cell operates. A high current…
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Structural–Mechanical Considerations
It might seem odd to include mechanical concerns in a text on electrochemical engineering. Experience has taught us that the mechanical design is critical in electrochemical systems. Many times performance difficulties can be traced to poor mechanical design or structural failure. Of course, mechanical engineering of materials is a large field in itself. Here our…
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Water and Thermal Management
For any fuel cell system at steady operation, water must be balanced; that is, the amount of water entering the fuel-cell system plus the water produced by the reactions must equal the amount of water in the exhaust. Similarly, water must be in balance around each component of the system. Just as a water balance…
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Flow-Field Design
As was noted in Section 10.4, the fuel and oxidant must be directed over the surface of anode and cathode in the fuel-cell stack. Most often this is done with rectangular channels. The set of channels make up the flow field. Drawing on what we have learned in the previous sections, we now have a…
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Utilization of Oxidant and Fuel
For both the fuel and oxidant streams, utilization is an important design factor. The utilization of fuel plays a key role in the fuel efficiency, and the utilization of the oxidant, typically air, affects the mechanical efficiency of the system and polarization losses in the cell. We’ll first consider the oxidant by examining the cathode of a…