General Principles of Fuel Cells

The efficiency e of a fuel cell operating at standard conditions, in which the hydrogen is oxidized to liquid water. 

If the product is water vapor, e becomes 98.1%. Such theoretical efficiencies would require intractably high temperatures in a heat engine. 

In practice, there are several factors that reduce e, but even in the worst case e is higher than the 25 to 30% efficiency realized by conventional or nuclear power stations. These factors include the following  

To minimize overpotential effects, cathodes are usually made of finely divided platinum on a porous support, for aqueous electrolytes. The catalytic surfaces of the anodes are particularly susceptible to poisoning by CO, olefins, sulfur compounds, and other impurities in the fuel. These lie above H2 in the chemisorption series (Eq. 6.3). 

Consequently, the direct use of hydrocarbon gases as fuel is usually considered to be impractical, although Whitesides and co-workers describe an aqueous fuel cell in which methane reacts with aqueous iron(III) ions over a platinum black catalyst to form CO2 and iron(II) the Fe solution 

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