A fuel cell consists of a negatively charged electrode (cathode), a positively charged electrode (anode) and an electrolyte. The fuel is oxidized on the anode and free electrons are conducted through the electrode to a load or power application via the external circuit. From the load, electrons are conducted to the cathode, where the oxidant is reduced. Both the anode and the cathode contain a catalyst to speed up the chemical processes. The electrodes provide an interface between the reactants and the electrolyte. The electrolyte prevents fuel and oxidant molecules from mixing and, therefore, it prevents direct combustion while allowing ions to move through the electrolyte towards the oppositely charged electrode to maintain electrical balance — and thus complete the circuit.
Fuel cells are similar to batteries in that both systems have two electrodes separated by an electrolyte and electrical energy can be withdrawn from the cell reaction. However, unlike batteries, the reactants in a fuel cell are supplied from an external source and it operates as long as it is supplied with fuel and oxidant. The fuel is usually hydrogen and the oxidant is usually oxygen (typically from the air). In a hydrogen-oxygen fuel cell, hydrogen molecules are split at the anode by the catalyst into protons and electrons. The protons then travel through the electrolyte to the cathode while the electrons are conducted to the cathode through the external circuit and the load or power application. On the cathode, oxygen, protons, and electrons combine to form water. Depending on the input fuel and the electrolyte, different chemical reactions will occur.
Fuel cells have several highly attractive characteristics. Carnot’s law does not govern electrochemical processes in fuel cells, and therefore high operating temperature is not necessary for achieving high efficiency. The efficiency of a fuel cell can be higher than in conventional energy conversion processes. Low operating temperature guarantees that no nitrous oxide is produced. The only waste product is oxidized fuel. Normally, the fuel is hydrogen and consequently, the product is water. Carbon dioxide may be present as well, if a hydrocarbon fuel is used. Apart from being efficient and nonpolluting, fuel cells are silent, modular in design and respond rapidly to load changes. Fuel reformulation in fuel preprocessors adds the benefit of fuel flexibility.
From CIE