A fuel cell is an electrochemical device which combines a fuel and an oxidant, typically oxygen from air, to deliver power. Unlike a battery, which is closed, a fuel cell is open on at least one side, the air side being invariably open. Like a battery, individual cells can be combined together to form a stack and hence delivering whatever power is needed for the given application. The fuel at point of use is commonly hydrogen, but can be a hydrocarbon or other hydrogen containing fuel, decomposed by heat, catalytically, or simply stored at pressure.
The fuel cell stack combines fuel and air to form water and potentially CO2, cleanly and efficiently. While similar to a battery, a fuel cell system is far more complex, involving pumps, blowers, condensers, etc., (collectively denoted as the balance of plant) which impact on cost and reliability.