A fuel cell is an electrochemical energy conversion device. Fuel cells differ from batteries insofar as they are designed for continuous replenishment of the reactants consumed; they produce electricity from an external supply of fuel and oxygen as opposed to the limited internal energy storage capacity of a battery. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable. Typical reactants used in a fuel cell are hydrogen on the anode side and oxygen on the cathode side (a hydrogen cell). Usually, reactants flow in and reaction products flow out. Virtually continuous long-term operation is feasible as long as these flows are maintained.