SOLID-STATE HIGH-TEMPERATURE FUEL CELLS

An anonymous wit once paraphrased the three laws of thermodynamics thus: First, you can never win, you can only break even. Second, you can never break even. Third, you can't get out of the game. The second law limits the efficiency of a fossil-fuel-fired power plant—only a fraction (typically less than 40%) of the chemical energy released by burning the fuel is converted to electricity. The balance is dissipated in various unavoidable ways—as friction between moving parts, as waste heat up smokestacks and cooling towers, and so forth. Fuel cells, however, translate chemical energy directly into electrical energy without any mechanical or thermal intermediaries. Fuel cells can have efficiencies as high as 90%, depending on their applications. With such high efficiencies, power plants based on fuel cell technology would consume much less fossil fuel and emit proportionally fewer pollutants than would conventional power plants.

All fuels burn by reacting with oxygen to release energy, and the key to a fuel cell's efficiency lies in using catalysts to control that reaction. There are a number of different types of fuel cells. One promising variety is the high-temperature, solid-state fuel cell (Figure 3.3), which is essentially a barrier made of ceramic, typically zirconium oxide doped with traces of yttrium oxide, whose structure conducts oxygen ions (negatively charged oxygen atoms). On one side of the barrier—the fuel cell's negative terminal—the fuel, typically a mixture of carbon monoxide and hydrogen gas created by gasifying coal or steam-refining natural gas, reacts catalytically with oxygen ions to liberate water, carbon dioxide, and electrons. The electrons go out over the wire as an electric current. On the other side of the barrier—the positive terminal—the electrons returning through the wire are catalytically added to molecules of oxygen from the atmosphere to create more oxygen ions that diffuse through the barrier and perpetuate the cycle. The yttria-doped zirconia fuel cell typically operates at around 1000° C.

So what are we waiting for? Why aren't power plants based on fuel cells? Unfortunately, several technological hurdles must be overcome before this catalytic technology can be a commercial success. A worldwide effort has been under way for more than two decades to clear these hurdles, which include devising ways to keep the catalyst from breaking down at such high temperatures, avoiding cracks and leaks in the ceramic structure, and designing a ceramic that conducts enough oxygen ions through a sufficiently small volume to make the size of the equipment economical. Some current efforts are focused on so-called cross-flow monolithic designs in which a solid ceramic block is laced with sets of fuel channels perpendicular to, and alternating with, sets of air channels. This design provides a high interior surface area upon which reactions can occur and thus occupies a very compact volume per unit of energy generated.



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