There are six principal types of fuel cell currently under development of which four are useful for power generation applications. Their typical characteristics are shown in Table 7.1. The alkaline fuel cell, developed by Francis Bacon in the 1930s has been used in to provide power for US space ships including the space shuttle. It is extremely efficient but uses pure platinum electrodes making it too expensive for earth-bound power generation. A second type of cell which burns methanol rather than hydrogen is at an early experimental stage. Neither of these will be discussed further.
The remaining four fuel cells are all being developed for a variety of uses including power generation. Of these the phosphoric acid fuel cell (PAFC) was the first, with commercial units appearing in 1992. However these have proved expensive and this type of cell may be superseded by newer designs.
The proton-exchange membrane (PEM) fuel cell has attracted considerable attention from automotive manufacturers as an electric power source to replace the internal combustion engine. This has provided the primary stimulus for its development but there are a number of stationary power applications being developed too as well as portable applications for computers.
Both the phosphoric acid and the PEM fuel cells operate at relatively low temperatures, so they require expensive catalysts to assist the fuel cell reaction to proceed at a usable rate. The two other types under development are both high-temperature devices that do not need special catalysts. The molten carbonate fuel cell (MCFC) uses a molten carbonate electrolyte which must be heated to around 650°C while the solid oxide fuel cell (SOFC) uses a solid electrolyte that will only operate effectively at around 1000°C. The latter may eventually prove the most competitive of all fuel cell systems.
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