Fuel cells are electrochemical devices sometimes compared to conventional automobile batteries. However, there is a fundamental difference in that fuel and oxidizer are supplied to fuel cells continually so that they can generate continuous electric power. Therefore, a fuel cell is important as an energy vector, combining with the stored fuel as a potential energy source. Whereas fuel cells are based purely on the electrochemical reaction of hydrogen or other fuel gases, in ordinary batteries the reactants are either consumed or must be regenerated through electric recharge, as in automotive lead-acid batteries [1,2]. The hydrogen combines with the oxygen inside a combustion-free process, liberating electric power in a chemical reaction that is very sensitive to the operating temperature. Despite initial use in aerospace applications, new experimental
materials have been contributing significantly to improved use, efficiency, and cost of fuels for terrestrial applications. A very significant figure that shows a lot of improvement is the amount of platinum catalyst needed in the electrodes. It has decreased in the last few years from 28 mg/cm2 of electrode area to about 0.1 mg/cm2 [3].
There are different types of fuel cells classified according to their operation at low or high temperatures. Low-temperature PEM fuel cells (up to about 100°C) are available and ready for mass production. They are used in applications where high – temperature cells would not be suitable, such as in commercial and residential power sources as well as in electric vehicles. The most compact systems are appropriate for powered electric vehicles and are available from 5 to 100 kW. High-temperature (about 1000°C) SOFCs are typically used for industrial and large commercial applications and operate as decentralized stationary units of electric power generation. Because of such high operating temperatures, great heat dissipation is expected to be recovered, and integration with gas-powered microturbines is very successful. A 70% overall efficiency is reported in such applications, representing a great contrast with respect to the energy generation in coal power plants, whose efficiency is between 35 and 40%.
Figure 7.1 shows the principles of an operating fuel cell. They are powered by the flow of a fuel gas such as hydrogen, which could be pure or derived form (e. g., methanol, coal gas, natural gas, gasoline, naphtha) and that of an oxidizer (e. g., oxygen, air). After the hydrogen passed through a porous anode separated by an electrolyte of a porous cathode, the resulting ion meets the oxygen to form water. As a result of this reaction, the accumulated electrons in the anode form an electrical field and find their way through an external conductive connection
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Figure 7.1 Simplified diagram of a fuel cell. |
between the anode and the cathode. An electrical current is established through an external load. As in any other chemical reaction, heat is produced which has to be dissipated in some way. As a result of the gas reaction, there is by-product generation of heat and water, in contrast with the conventional mixture and burning of fossil fuel.
From a physiochemical point of view, fuel cells are the electrolysis antipode process in the sense that the first consumes H2 and O2 and produces electricity, heat, and water, whereas the second consumes electricity and water and produces heat, H2, and O2. This duality widely motivates practical possibilities for fully clean and sustainable systems, as discussed in Chapter 2.
