A battery stores a lot of energy in its chemicals, but chemical reactions are slow and cause a battery to charge and discharge slowly. A capacitor, on the other hand, can charge and discharge extremely fast. It stores energy with two electrodes and a separator the way a battery does, but it does not involve chemical reactions. It also can be recycled limitlessly and does not decay with time. Capacitors are used in almost all electronic circuits and come in many sizes. Millions of small ones can be made on a computer chip, and large ones the size of a waste basket (trash bin to Anglophiles) are used by power companies. Supercapacitors are capacitors that still use no chemicals but can hold much more energy than previously possible. Used in combination with batteries, they help overcome some of the drawbacks of batteries. Pseudocapacitors are supercapacitors with reacting chemicals, thus combining the virtues of capacitors and batteries. A few diagrams will show how interesting these new developments in transportable energy storage are.

Figure 3.56a shows a normal capacitor. The positive and negative electrodes are metal sheets separated by an insulator called a dielectric. When the capacitor is charged by applying voltage between the electrodes, the charges move to the inner surfaces of the dielectric, and they attract opposite charges onto the surfaces of the dielectric. There are then sheets of opposite charges on each interface, and they stay

there when the switch is opened. These charges cannot move together to annihilate one another because the dielectric is an insulator. The energy is stored in the dielec­tric. When the switch is closed to hook up a load, the opposite charges on the electrodes move through the load to combine with one another, thus applying the energy that was stored. The dielectric, which had zero total charge all along, then redistributes its charges to match the charges left on the metal sheets, if any. The energy storage capacity of a capacitor (hence its name) depends on three factors: the area of the sheets, the thinness of the dielectric, and “dielectric constant.” The latter is a number varying from 1 (for air or vacuum) to 3 (for plastic), to 5 (for glass), and as high as 80 for water. The higher the number, the more energy the dielectric can hold for a given voltage between the electrodes.

To get more energy into a capacitor, one can work with these three factors. Capacitors are already made as thin as possible and rolled up to get the largest area for their size. Supercapacitors, however, can have much thinner dielectrics and much larger areas by virtue of nanotechnology. This can be explained step-by-step. In Fig. 3.56b, we show two simple capacitors connected in series. The inner elec­trodes are not metal but conducting liquids (electrolytes). The gaps are filled not with a dielectric but with air. This lowers the dielectric constant to 1, but thickness of the gap is much, much smaller. Now if we connect the two capacitors not by a wire but by simply extending the electrolyte as in Fig. 3.56c, we have a capacitor whose capacitance depends on the thicknesses of the two gaps, and not by the thickness of the electrolyte layer. Next, we can increase the area by roughening up the inner surfaces, as shown in Fig. 3.57a. This is done by coating the electrodes with a layer of “activated” carbon, which consists of fine particles. Special process­ing techniques make the surfaces of these particles break up into channels nanometers in size, as shown in Fig. 3.57b. The electrolyte goes into these channels but does not actually touch the carbon because of a nanoscopic surface tension effect.

This forms an air gap of nanometers thick. The capacitance is increased to tens of thousands of times.

Capacitance is measured in farads (named after Michael Faraday). The energy a capacitor can hold is proportional to its capacitance and the square of the voltage it can take before arcing over. While usual capacitors have capacitances of picofarads to microfarads and a rare one may have a farad, supercapacitors (also called ultra­capacitors) can have 5,000 farads. They can hold 5% as much energy as a automo­tive Li-I cell in the same size package.61 They can supplement Li-I batteries in electric cars by storing and releasing braking energy more quickly than the batteries can. They can store enough energy to be used on short trips by buses, garbage trucks, and the like.

Pseudocapacitors add porous electrode structures like those of Fig. 3.57 to a Li-I battery using molybdenum trioxide (MoO3). The trick is to find a material that can make a chemical battery and yet can be processed in such a way as to have a large area, rough surface. This has been accomplished in the laboratory by Brezesinski et al [35]. Still in their infancy, pseudocapacitors have the potential to store enough energy fast enough to be useful in smoothing the output of intermittent energy sources such as wind and solar.62 The development of such electrochemical capacitors will fill the gap in Fig. 3.58 between batteries and capacitors in their abilities to store large amounts of energy and to cycle the storage fast. There are still other types of batteries which lurk in the future, such as metal-air batteries, especially zinc-air and lithium-air batteries. Since the cathodes are air, these could have very large storage per unit weight. They are the only batteries that could approach the energy density of gasoline. However, there are several performance defects, most seriously inability to be recharged completely. The physics of the reversible reaction is still unknown;62 but, with intensified research, there is hope for a paradigm­changing advance with these new types of batteries.