Types of fuel cells
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Various types of fuel cells have been developed. They are generally classified on the basis of the electrolyte used, because the electrolyte determines the operating temperature of a system and in part the kind of fuel that can be employed.
Alkaline fuel cells
These are devices that, by definition, have an aqueous solution of sodium hydroxide or potassium hydroxide as the electrolyte. The fuel is almost always hydrogen gas, with oxygen (or oxygen in air) as the oxidizer. However, zinc or aluminum could be used as an anode if the by-product oxides were efficiently removed and the metal fed continuously as a strip or as a powder. Fuel cells generally operate at less than 100 °C (212 °F) and are constructed of metal and certain plastics. Electrodes are made of carbon and a metal such as nickel. Water, as a reaction product, must be removed from the system, usually by evaporation from the electrolyte either through the electrodes or in a separate evaporator. The operating support system presents a significant design problem. The strong, hot alkaline electrolyte attacks most plastics and tends to penetrate structural seams and joints. This problem has been overcome, however, and alkaline fuel cells are used on the U.S. space shuttle orbiters. Overall efficiencies range from 30 to 80 percent, depending on the fuel and oxidizer and on the basis for the calculation.
Phosphoric acid fuel cells
Such cells have an orthophosphoric acid electrolyte that allows operation up to about 200 °C (400 °F). They can use a hydrogen fuel contaminated with carbon dioxide and an oxidizer of air or oxygen. The electrodes consist of catalyzed carbon and are arranged in pairs set back-to-back to create a series generation circuit. The framing structure for this assembly of cells is made of graphite, which markedly raises the cost. The higher temperature and aggressive hot phosphate create structural design problems, particularly for joints, supporting pumps, and sensors. Phosphoric acid fuel cells have been proposed and tested on a limited scale for local municipal power stations and for remote-site generators.
Molten carbonate fuel cells
Fuel cells of this type operate quite differently from those so far discussed. The fuel consists of a mixture of hydrogen and carbon monoxide generated from water and a fossil fuel. The electrolyte is molten potassium lithium carbonate, which requires an operating temperature of about 650 °C (1,200 °F). Warming up to operational temperatures may take several hours, making these cells unsuitable for vehicles. In most cases, the electrodes are metallic-based, and the containment system is made of metals and special engineering plastics. Such combinations of materials are anticipated to be relatively inexpensive, perhaps only three times that of the alkaline fuel cell and less than that of the phosphoric acid variety. The cells combine the hydrogen and carbon monoxide first with the carbonate electrolyte and then with oxidizing oxygen to produce a reaction product of water vapour and carbon dioxide.
Molten carbonate fuel cells are expected to be useful in both local and larger power stations. Efficiencies of 45 percent may be attained where fossil fuels are already used. Operation at high temperatures creates a design problem for long-lived system parts and joints, especially if the cells must be heated and cooled frequently. The toxic fuel and high temperature together make power plant safety an area of special concern in engineering design and testing as well as in commercial operation.
Solid oxide fuel cells
In some ways solid oxide fuel cells are similar to molten carbonate devices. Most of the cell materials, however, are special ceramics with some nickel. The electrolyte is an ion-conducting oxide such as zirconia treated with yttria. The fuel for these experimental cells is expected to be hydrogen combined with carbon monoxide, just as for molten carbonate cells. While internal reactions would be different in terms of path, the cell products would be water vapour and carbon dioxide. Because of the high operating temperatures (900 to 1,000 °C, or 1,600 to 1,800 °F), the electrode reactions proceed very readily. As in the case of the molten carbonate fuel cell, there are many engineering challenges involved in creating a long-lived containment system for cells that operate at such a high-temperature range.
Solid oxide fuel cells would be designed for use in central power-generation stations where temperature variation could be controlled efficiently and where fossil fuels would be available. The system would in most cases be associated with the so-called bottoming steam (turbine) cycle—i.e., the hot gas product (at 1,000 °C) of the fuel cell could be used to generate steam to run a turbine and extract more power from heat energy. Overall efficiencies of 60 percent might be possible.
Solid polymer electrolyte fuel cells
A cell of this sort is built around an ion-conducting membrane such as Nafion (trademark for a perfluorosulfonic acid membrane). The electrodes are catalyzed carbon, and several construction alignments are feasible. Solid polymer electrolyte cells function well (as attested to by their performance in Gemini spacecraft), but cost estimates are high for the total system compared with the types described above. Engineering or electrode design improvements could change this disadvantage.