Fuel cell

A fuel cell is an electrochemical cell (much like a battery) in which fuels are consumed as energy is released.

A fuel cell can be considered as a battery that operates on flowing reactants. Typical reactants are hydrogen on the anode side and oxygen on the cathode side. Batteries consume their reactants and, once these reactants are depleted, must be discarded or recharged with electricity by running the chemical reaction backwards. Fuel cells do not retain the reaction products. They continue to operate as long as they are supplied with a continuous flow of reactants.

Fuel cells are attractive for their high efficiency and low pollution. They have been suggested for a number of applications, including baseload utility power plants, emergency backup generators, off-grid power storage, portable electronics, and vehicles.

Their use is controversial in some applications. The hydrogen typically used as a fuel isn't a primary source of energy. It is only a source of stored energy that must be manufactured using energy from other sources. Some critics of the current stages of this technology argue that the energy needed to create the fuel in the first place may reduce the ultimate energy efficiency of the system to below that of highly efficient gasoline internal-combustion engines; this is especially true if the hydrogen is generated from electrolysis of water by electricity. On the other hand, hydrogen can be generated from methane (the primary component of natural gas) with approximately 80% efficiency. The methane conversion method releases greenhouse gases, however, and the ideal environmental system would be to use renewable energy sources to generate hydrogen through electrolysis.

Science

Fuel cells are electrochemical devices, so they are not constrained by the maximum thermal (Carnot) efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy.

In the archetypal example of a hydrogen/oxygen polymer electrolyte membrane (PEM) fuel cell, a proton-conducting polymer membrane separates the anode ("fuel") and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.

On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.

On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) protons to form water.

In this example, the only waste product is water.

History

Although the first fuel cell was developed in the late 19th century by British scientist William Grove, fuel cells did not see practical application until the 1950's, where they were used in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications.

Further technological advances in the 1980's and 1990's, like the use of Nafion as the electrolyte, and reductions in the quantity of expensive platinum catalyst required, have made the prospect of fuel cells in consumer applications such as automobiles more realistic.