Fuel cells convert chemical energy to electrical energy by combining hydrogen from fuel with oxygen from the air. Hydrogen fuel can be supplied in two ways—either directly as pure hydrogen gas or through a "fuel reformer" that converts hydrocarbon fuels such as methanol, natural gas, or gasoline into hydrogen-rich gas. A fuel cell's only emission is water.

All fuel cells have the same basic configuration; an electrolyte and two electrodes. But there are different types of fuel cells, based mainly on what kind of electrolyte they use.