Fuel cells

Fuel cells are electrochemical devices that directly convert chemical energy to electrical energy. They consist of an electrolyte medium sandwiched between two electrodes (Fig. 1). One electrode (called the anode) facilitates electrochemical oxidation of fuel, while the other (called the cathode) promotes electrochemical reduction of oxidant. Ions generated during oxidation or reduction are transported from one electrode to the other through the ionically conductive but electronically insulating electrolyte. The electrolyte also serves as a barrier between the fuel and oxidant. Electrons generated at the anode during oxidation pass through the external circuit (hence generating electricity) on their way to the cathode, where they complete the reduction reaction. The fuel and oxidant do not mix at any point, and no actual combustion occurs. The fuel cell therefore is not limited by the Carnot efficiency and, theoretically (although not practically), can yield 100% efficiency. Fuel cells are primarily classified according to the electrolyte material. The choice of electrolyte material also governs the operating temperature of the fuel cell. Table I lists the various types of fuel cells along with electrolyte used, operating temperature, and electrode reactions.