The description of the operation of a fuel cell above is a simplification because it omits one key feature of the reaction between hydrogen and oxygen. Although hydrogen atoms and oxygen atoms will react spontaneously to form water, both hydrogen and oxygen are found (at room temperature) in the molecular forms H2 and O2. These will not react spontaneously and the hydrogen and oxygen molecules must be split before the reaction will proceed.
One method of splitting the molecules is to raise their temperature. Thus a flame will split sufficient of the molecules to start the reaction which then generates enough heat spontaneously to keep the reaction going. Some fuel cell designs use high temperatures too.
The alternative is to use a catalyst. A metal such as platinum will promote the splitting of both hydrogen and oxygen molecules at low temperatures and the resulting atoms will then react in a fuel cell. However platinum is very expensive. This has a significant effect on the cost of low-temperature fuel cells.
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.