Hydrogen fuel cell, illustration. This is a clean and efficient power source. Hydrogen (red) is liberated from a natural source such as methanol or natural gases. It is then pumped into the cell, where the hydrogen nuclei are separated from their electrons (yellow). The electrons flow around a conducting loop as an electric current, which can be harnessed. The hydrogen nuclei pass through a membrane (centre) and then combine with the electrons and oxygen (blue) from the air to form steam, and water is the only waste product.