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The fuel cell is basically an energy-conversion device in which chemical energy is isothermally converted into dc electricity. Several types of fuel cell systems have been developed, including the hydrogen-oxygen system used in certain space missions, but many of them employ fuels that are expensive and not readily available. Considerable research and development work is being done on hydrocarbon-air systems, which use air and "real-world" fuels. In the direct-oxidation cells, the hydrocarbon fuel is oxidized directly at the fuel electrode. Because of reaction problems at the electrode, these cells are still in their early stages of development. In the indirect-oxidation cells, now in the systems engineering development stage, the hydrocarbon fuel is converted into an impure hydrogen, which may then be purified to a certain degree and injected into the fuel cell modules. Initial applications will probably be in the military, because of the fuel flexibility and ease of maintenance inherent in these fuel cell systems.