A type of galvanic cell which promises to become increasingly important in the future is the fuel cellAn electrochemical cell in which the reactants are supplied on a continuing basis.. By contrast to a conventional cell, where only limited quantities of oxidizing agentA chemical species that accepts electrons in order to oxidize another species. In the process the oxidizing agent is itself reduced. and reducing agentA chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized. are available, a continuous supply of both is provided to a fuel cell, and the reaction productA substance produced by a chemical reaction. is continually removed. A somewhat oversimplified diagram of a fuel cell in which the cell reaction is the production of water from hydrogen and oxygen is shown in Fig. 1. Hydrogen enters the cell through a porous
carbon electrodeIn an electrochemical cell, a surface on which oxidation or reduction occurs; an electrode conducts electric current into or out of a cell. which also contains a platinum catalyst. Oxygen is supplied to a similar electrode except that the catalyst is silver. The electrolyte is usually a warm solution of potassium hydroxide, and the two electrode reactions can be written as
H2(g) + 2OH–(aq) → 2H2O(l) + 2e– (1a)
and ½O2(g) + H2O + 2e– → 2OH–(aq) (1b)
giving the overall result
H2(g) + ½O2(g) → H2O
Unless it is removed, water produced by the reaction will gradually dilute the potassium hydroxide, rendering the cell inoperative. Hence the electrolyte is kept warm enough that water evaporates just as fast as it is produced by the cell reaction. A fuel cell like this will continue to operate and produce electrical energyA system's capacity to do work. as long as a supply of hydrogen and oxygen are available.
Fuel cells have an important advantage over all other devices which burn fuel in order to obtain useful energy: their efficiency. While an internal-combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. engine is only about 25 percent efficient and a steam engine about 35 percent efficient, the H2–O2 cell just described can already operate at an efficiency of 45 percent. The theoretically highest possible efficiency of such a cell, set by the second law of thermodynamicsA formal statement that any spontaneous (product-favored) process is accompanied by an increase in the entropy (dispersal of energy) of the universe., is 83 percent. Because of this high efficiency many possible uses and developments for fuel cells have been proposed. One of these scenarios for the future envisions large nuclear power plants floating on the sea producing hydrogen gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. by the decomposition of water rather than producing electrical power. This hydrogen gas could then be piped to individual homes where it could either be burned for heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. or converted to electricity with the aid of a fuel cell. A second scenario involves automobiles powered by cells fueled by conventional gasoline or perhaps hydrogen. These automobiles would run virtually noiselessly without any pollutionThe contamination of the air, water, and earth by personal, industrial, and farm waste. problems and deliver twice as many kilometers per literA unit of volume equal to a cubic decimeter. of fuel as a conventional vehicle. Alas for such scenarios, many technological problems still intervene, but further development of fuel cells is certainly one approach to our current energy problems that should be thoroughly investigated.