A fuel cell is a device similar to a battery. Like a battery, it consists of an anode and a cathode, separated by an electrolyte. Both devices convert a fuel to useable energy (electricity and heat) through an electrochemical process, without combustion of the fuel. The main difference between a fuel cell and a battery is that a battery stores a finite quantity of energy, while the fuel cell can continuously produce energy, as long as its reactants (fuels) are replenished.Fuel cells directly convert the chemical energy in methanol or hydrogen fuel into electricity. They hold great promise as power sources because they deliver more energy than batteries of the same size and weight and their only byproduct is water vapor.

Fuel cells have been used for years in space and specialty applications; today, there is strong interest in commercializing fuel cell technology, especially from governmental organizations and automakers seeking a zeroemission power source. Fuel cells are now predicted to replace traditional power sources in the coming years—from tiny fuel cells in cell phones to high powered ones in race cars. However, to make fuel cells commercially viable, it is necessary to perfect the manufacturing of their components.

Fuel Cell Types (PEM, MCFC, SOFC, AFC)

PEM

Proton exchange membrane (PEM) fuel cells work with a polymer electrolyte in the form of a thin, permeable sheet. This membrane is small and light, and it works at low temperatures (about 80 degrees C, or about 175 degrees F). Other electrolytes require temperatures as high as 1,000 degrees C.

To speed the reaction a platinum catalyst is used on both sides of the membrane. Hydrogen atoms are stripped of their electrons, or "ionized," at the anode, and the positively charged protons diffuse through one side of the porous membrane and migrate toward the cathode. The electrons pass from the anode to the cathode through an exterior circuit and provide electric power along the way. At the cathode, the electrons, hydrogen protons and oxygen from the air combine to form water. For this fuel cell to work, the proton exchange membrane electrolyte must allow hydrogen protons to pass through but prohibit the passage of electrons and heavier gases.

Efficiency for a PEM cell reaches about 40 to 50 percent. An external reformer is required to convert fuels such as methanol or gasoline to hydrogen. Currently, demonstration units of 50 kilowatt (kw) capacity are operating and units producing up to 250 kw are under development.

MCFC

In a molten carbonate fuel cell (MCFC), carbonate salts are the electrolyte. Heated to 650 degrees C (about 1,200 degrees F), the salts melt and conduct carbonate ions (CO3) from the cathode to the anode. At the anode, hydrogen reacts with the ions to produce water, carbon dioxide, and electrons. The electrons travel through an external circuit, providing electrical power along the way, and return to the cathode. There, oxygen from air and carbon dioxide recycled from the anode react with the electrons to form CO3 ions that replenish the electrolyte and transfer current through the fuel cell.  

A component module from a 1966 molten carbonate fuel cell made for the U.S. Army High-temperature MCFCs can extract hydrogen from a variety of fuels using either an internal or external reformer. They are also less prone to carbon monoxide "poisoning" than lower temperature fuel cells, which makes coal-based fuels more attractive for this type of fuel cell. MCFCs work well with catalysts made of nickel, which is much less expensive than platinum.

MCFCs exhibit up to 60 percent efficiency, and this can rise to 80 percent if the waste heat is utilized for cogeneration. Currently, demonstration units have produced up to 2 megawatts (MW), but designs exist for units of 50 to 100 MW capacity.

SOFC

A solid oxide fuel cell (SOFC) uses a hard ceramic electrolyte instead of a liquid and operates at temperatures up to 1,000 degrees C (about 1,800 degrees F). A mixture of zirconium oxide and calcium oxide form a crystal lattice, though other oxide combinations have also been used as electrolytes. The solid electrolyte is coated on both sides with specialized porous electrode materials.

At these high operating temperature, oxygen ions (with a negative charge) migrate through the crystal lattice. When a fuel gas containing hydrogen is passed over the anode, a flow of negatively charged oxygen ions moves across the electrolyte to oxidize the fuel. The oxygen is supplied, usually from air, at the cathode. Electrons generated at the anode travel through an external load to the cathode, completing the circuit and supplying electric power along the way.

Generating efficiencies can range up to about 60 percent. In one configuration, the SOFC consists of an array of tubes (see image below). Another variation includes a more conventional stack of disks. Since SOFCs operate at such high temperatures, a reformer is not required to extract hydrogen from the fuel. Some demonstration units have capacities up to 100 kilowatts.

AFC

Alkali fuel cells operate on compressed hydrogen and oxygen and generally use a solution of potassium hydroxide in water as their electrolyte. Operating temperatures inside alkali cells are around 150 to 200 degrees C (about 300 to 400 degrees F).

In these cells, hydroxyl ions (OH-) migrate from the cathode to the anode. At the anode, hydrogen gas reacts with the OH- ions to produce water and release electrons. Electrons generated at the anode supply electrical power to an external circuit then return to the cathode. There the electrons react with oxygen and water to produce more hydroxyl ions that diffuse into the electrolyte.

Alkali fuel cells operate at efficiencies up to 70 percent and, like other fuel cells, create little pollution. Because they produce potable water in addition to electricity, they have been a logical choice for spacecraft. A major drawback, however, is that alkali cells need very pure hydrogen or an unwanted chemical reaction forms a solid carbonate that interferes with chemical reactions inside the cell. Since most methods of generating hydrogen from other fuels produce some carbon dioxide, this need for pure hydrogen has slowed work on alkali fuel cells in recent years.