Hydrogen Fuel Cells
The "fuel cell" is a device that produces electricity by combining hydrogen and oxygen, with water and heat as its only
byproducts. The fuel cell was invented by a Welshman by the name of William Grove in 1839. Grove was experimenting with electrolysis
and discovered that when combined, hydrogen and oxygen produced an electric current. The basic chemical reaction occuring in a fuel
2H2 + O2 ® 2H2O + 2e-
Fuel cell technology has been refined over the years and was used in the Apollo space missions in the 1960s. Today fuel cells continue to evolve and fall under six major categories:
Direct methanol fuel cells are similar to PEMFCs and feed methonol fuel directly to the fuel cell. DMFCs are a very recent invention, having been developed in the early 1990s. Their advantage lies in the fact that methanol is a stable liquid when comapred to pressurized hydrogen and has a much higher energy density. On the downside, methanol is poisonous and a DMFC is less efficient due to the high permeation of methanol through the membrane. Research continues to increase the efficiency of direct methanol fuel cells with the possibility that they may eventually reach the 40% efficiency level of current proton exchange membrane fuel cells. DMFCs are also being developed for use in portable electronic equipment like laptops and PDAs.
Solid oxide fuel cells are high temperature devices that use a hard ceramic where oxygen ions are transferred through this solid oxide electrolyte material to react with hydrogen on the anode side. Research into solid oxide electrolytes began with Baur and Preis in the late 1930s and culminated with important research conducted in the United States and Europe during the 1950s. SOFCs are large and generate a lot of heat, making them best suited for stationary applications like powerplants with an output between 1 kW and 100 kW. In these applications, the excess gases generated can be used to run a secondary gas turbine to improve electrical efficiency up to 70%.
Alkaline fuel cells have been used over the years with their most high profile use being in NASA's Apollo mission to the Moon, providing the mission with both electricity and drinking water. AFCs use compressed hydrogen and oxygen along with a solution of potassium hydroxide in water as their electrolyte. They can produce between 300 W to 5 kW of electricity at a 70% efficiency level and operate at temperatures in the 150 to 200 degree celsius range. Alkaline fuel cells produces power through a redox reaction between hydrogen and oxygen where hydrogen is oxidized at the anode. The electrons flow through a circuit and return to the cathode. One oxygen molecule and two hydrogen atoms are used to produce two water molecules with heat and electricity generated as by-products of this reaction.
Phosphoric acid fuel cells use liquid phosphoric acid as their electrolyte. PAFCs can be expensive as they use carbon paper coated with a finely-dispersed platinum catalyst for their electrodes. Phosphoric acid fuel cells operate at temperatures between 150 200 degrees celsius. They can achieve efficiencies of upto 80% if the the expelled water can be converted to steam and used for heating. PAFCs are often used for stationary fuel cell applications and can output up to 200 kW of electricity. They can use a wide variety of fuels, including non-sulfphur gasoline.
Molten Carbonate fuel cells operate at very high temperatures (in the 600 degrees celsius range) and are among the most efficient type of fuel cells developed to date. When the waste heat is captured and used, the overall fuel efficiencies of a molten carbonate fuel cell can be as high as 85%. MCFCs are being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications with output ranging up to 2 megawatts (MW). Designs supposedly exist for units generating up to 100 MW of electricity.
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