Fuel Cells

There are five basic types of fuel cells, differentiated by the type of electrolyte separating the hydrogen from the oxygen. The cells types now in use or under development are alkaline, phosphoric acid, proton exchange membrane, molten carbonate, and solid oxide.

Long used by NASA on space missions, alkaline cells can achieve power-generating efficiencies of up to 70%. NASA's fuel cells use alkaline potassium hydroxide as the electrolyte and the electrodes of porous carbon. At the anode, hydrogen gas combines with hydroxide ions to produce water vapor. This reaction results in extra electrons that are forced out of the anode to produce the electric current. At the cathode, oxygen and water plus returning electrons from the circuit form hydroxide ions that are again recycled back to the anode. The basic core of the fuel cell, consisting of the manifolds, anode, cathode, and electrolyte, is generally called the stack. Until recently, such cells were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility.

The fuel-cell type most commercially developed today is the phosphoric acid, now being used in such diverse settings as hospitals, nursing homes, hotels, office buildings, schools, utility power plants, and airport terminals. They can also be used in large vehicles such as buses and locomotives. Phosphoric-acid fuel cells generate electricity at more than 40% efficiency. If the steam produced is captured and used for heating, the efficiency jumps to nearly 85%. This compares to only 30% efficiency for the most advanced internal combustion engines. Phosphoric-acid cells operate at around 400°F (205°C).

Proton exchange membrane cells operate at relatively low temperatures (about 200°F [93°C]) and have high power density. They can vary their output quickly to meet shifts in power demand, and are suited for small-device applications. Experts say they are perhaps the most promising fuel cell for light-duty vehicles where quick startup is required.

Molten carbonate fuel cells promise high fuel-toelectricity efficiencies and the ability to consume coal-based fuels such as carbon monoxide. These cells, however operate at very high temperatures (1,200°F [650°C]) and therefore cannot be used in small-scale applications.

The solid oxide fuel cell could be used in big, high-power applications including industrial and large-scale central electricity generating stations. Some developers also see a potential for solid oxide use in motor vehicles. A solid oxide system usually uses a hard ceramic electrolyte instead of a liquid electrolyte, allowing operating temperatures to reach 1,800°F (980°C). Power generating efficiencies could reach 60%.

Direct methanol fuel cells (DMFC), relatively new members of the fuel cell family, are similar to the proton exchange membrane cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120–190°F (50–90°C). Higher efficiencies are achieved at higher temperatures.

Regenerative fuel cells use sunlight as their energy source and water as a working medium. These cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen are fed into the fuel cell, which generates electricity, heat, and water. The water is then recycled back into the system to be reused.