A fuel cell is a primary voltaic cell that directly converts the chemical energy of a fuel into electricity. Not being a heat engine its efficiency is not subject to the Carnot limitation and in suitable circumstances very high efficiencies can be attained, although progress has been limited by a number of constraints.

In this article low temperature fuel cells operating with acid and alkaline electrolytes are described as well as high temperature cells with both fused carbonate and solid oxide electrolytes. In all cases catalysis of the reactions at both fuel and oxygen (air) electrodes is crucial to satisfactory performance.