A fuel cell consists of an anode—to which fuel, commonly hydrogen, ammonia, or hydrazine, is supplied—and a cathode—to which an oxidant, commonly air or oxygen, is supplied. The two electrodes of a fuel cell are separated by an ionic conductor electrolyte. In the case of a hydrogen-oxygen fuel cell with an alkali metal hydroxide electrolyte, the anode reaction is 2H2 + 4OH-
4H
2O + 4e and the cathode reaction is O2 + 2H2O
+ 4e4OH-.
The electrons generated at the anode move through an external circuit
containing the load and pass to the cathode. The OH- ions generated at the
cathode are conducted by the electrolyte to the anode, where they form
water by combining with hydrogen. The fuel cell voltage in this case is
about 1.2V but decreases as the load is increased. The water produced at
the anode has to be removed continuously in order to avoid flooding the
cell. Hydrogen-oxygen fuel cells using ion exchange membranes or
immobilized phosphoric acid electrolytes found early use in the Gemini and
Apollo space programs, respectively. Phosphoric acid fuel cells are in
limited use by electric utilities for power generation.Fuel cells with molten carbonate electrolytes are also under development. The electrolyte is solid at room temperature but is a liquid and a carbonate ion conductor at the fuel cell operating temperature (650° to 800° C/1200° to 1470° F). This system has the advantage of utilizing carbon monoxide as a fuel; therefore, mixtures of carbon monoxide and hydrogen, such as are produced in a coal gasifier, may be used as fuel.
Also being developed are fuel cells that employ solid zirconium dioxide as the electrolyte. These are called solid oxide fuel cells. Zirconium dioxide becomes an oxide ion conductor at about 1000° C (about 1800° F). Suitable fuels include hydrogen, carbon monoxide, and methane, with air or oxygen being supplied to the cathode. The high operating temperature of the solid oxide fuel cell permits the direct use of methane, a fuel that does not require expensive platinum catalysts on the anode. Solid oxide fuel cells have the advantage of being relatively insensitive to fuel contaminants, such as sulfur and nitrogen compounds, that impair the performance of the other fuel systems.
The relatively high operating temperature of both molten carbonate and solid oxide fuel cells facilitates the removal of water produced in the reaction in the form of steam. In low-temperature fuel cells, provision must be made for the removal of liquid water from the anode chamber.
Several companies are developing hydrogen fuel cells that they hope will replace conventional internal combustion automobile engines in the 21st century. Hydrogen-rich fuels such as methanol or gasoline enter this type of fuel cell, and the fuel cell strips electrons from the hydrogen atoms. While the electron-deficient hydrogen atoms (which are now just protons) continue passing through the fuel cell, the electrons are conducted through the automobile engine, providing electrical power. The returning electrons then recombine with the protons in the fuel cell to reform hydrogen. Oxygen (from the air) reacts with this reformed hydrogen, producing water, a non-polluting waste product that can be used to cool the engine.
HOW TO CITE THIS ARTICLE
"Fuel Cell," Microsoft® Encarta® Online Encyclopedia 2000
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