 - A fuel cell is an electrochemical cell that derives its energy from combustible substances such as hydrogen, methane, propane, methanol, diesel fuel or even gasoline. The most widely discussed type is the hydrogen fuel cell, in which energy is obtained from the oxidation of hydrogen. The only byproducts are water and a small amount of nitrous oxide if air is used as the oxidizer.
The fuel cell was originally conceived by a German scientist, Christian Schoenbein, in the middle of the 19th century. A Welsh physicist, William Grove, developed a working device in 1843. More than a century later in the 1950s, the American scientists Thomas Grubb and Leonard Niedrach improved on the design by incorporating an ion-exchange membrane and making use of hydrogen as the fuel source. Fuel cells of this type were used in some of the American space missions conducted by the National Aeronautics and Space Administration (NASA) during the 1960s. Modern hydrogen fuel cells can provide up to 200 kilowatts (kW) of power as alternative energy sources for cars, light trucks and space vehicles.
A fuel cell does not actually burn its fuel. The oxidation process occurs at a far lower temperature than that produced by active combustion. A fuel cell can be recharged by filling a tank or from a continuously available external supply of fuel. In a common form of hydrogen fuel cell, known as the proton exchange membrane (PEM), hydrogen is delivered to a positive electrode called the anode. At the anode, hydrogen atoms are broken down or ionized into their constituent protons and electrons. The protons permeate through an electrolyte membrane to a negative electrode called the cathode. Electrons travel from the cathode to the anode through an external load, which converts the resulting current to useful power. Within the cell, oxygen molecules react with the protons permeating through electrolyte membrane and the electrons arriving through the external load. The result is water, the principal byproduct of all hydrogen-based energy sources.
A single PEM hydrogen fuel cell generates approximately 0.7 direct-current (DC) volts of electricity. The voltage decreases as the current demand increases. Combinations of cells in series and parallel can produce higher voltage and deliver more current than a single cell. A set of fuel cells connected together to obtain useful power is called a stack. A typical stack, capable of powering a small electric vehicle or large home appliance, is roughly the size and mass of a small tank full of gasoline. Fuel cells can be used to provide power for most electrical or electronic devices designed for operation from batteries or from conventional utility power sources.
Fuel cells, especially the hydrogen type, offer several advantages over conventional power sources. These include:
- Reduced dependence on fossil fuels
- Long useful life
- High efficiency
- Relative safety
- Essentially zero toxicity
- Minimal maintenance costs
- Reduced pollution, particularly carbon emissions
- Tax breaks for users and producers
Significant limitations of hydrogen fuel cells include:
- High cost of manufacture
- Relatively high cost of operation
- Difficulty in transporting and storing hydrogen
- Low fuel energy density
These problems can be largely overcome by the use of conventional fossil fuels rather than hydrogen as the primary energy source. American purists object to the use of petroleum diesel or gasoline as a fuel source because reliance on these substances perpetuates dependence on foreign oil. Biofuels such as ethanol, as well as lesser fossil fuels such as methane or propane, provide higher energy density than hydrogen but they produce more pollution. Some fossil fuels, notably petroleum diesel and biodiesel, become viscous in extremely cold weather, rendering them useless. Some alternative fuels and all fossil fuels can be toxic, while hydrogen is benign in that respect.
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28 Nov 2007
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