A fuel cell is an electrochemical cell that uses a pair of redox processes to transform the chemical energy of a fuel (typically hydrogen) and an oxidising agent (commonly oxygen) into electricity. Fuel cells differ from most batteries in that they require a constant supply of fuel and oxygen (usually from air) to keep the chemical reaction going, whereas chemical energy in a battery is usually derived from metals and their ions or oxides that are already present in the battery, with the exception of flow batteries. As long as fuel and oxygen are available, fuel cells can create power indefinitely. The catalyst aids the reaction of oxygen and hydrogen in a fuel cell. It's often made of platinum powder that's been thinly coated on carbon paper or fabric. The catalyst is rough and porous, allowing the platinum to be exposed to as much hydrogen or oxygen as possible.