Fuel cells generate electricity by an electrochemical reaction in which oxygen and a hydrogen-rich fuel combine to form water. Unlike internal combustion engines, the fuel is not combusted, the energy instead being released electrocatalytically. This allows fuel cells to be highly energy efficient, especially if the heat produced by the reaction is also harnessed for space heating, hot water or to drive refrigeration cycles.
A fuel cell is like a battery in that it generates electricity from an electrochemical reaction. Both batteries and fuel cells convert chemical potential energy into electrical energy and also, as a by-product of this process, into heat energy. However, a battery holds a closed store of energy within it and once this is depleted the battery must be discarded, or recharged by using an external supply of electricity to drive the electrochemical reaction in the reverse direction. A fuel cell, on the other hand, uses an external supply of chemical energy and can run indefinitely, as long as it is supplied with a source of hydrogen and a source of oxygen (usually air).
There are several different types of fuel cell but they are all based around a central design. A fuel cell unit consists of a stack, which is composed of a number of individual cells. Each cell within the stack has two electrodes, one positive and one negative, called the cathode and the anode. The reactions that produce electricity take place at the electrodes. Every fuel cell also has either a solid or a liquid electrolyte, which carries ions from one electrode to the other, and a catalyst, which accelerates the reactions at the electrodes. The electrolyte plays a key role - it must permit only the appropriate ions to pass between the electrodes. If free electrons or other substances travel through the electrolyte, they disrupt the chemical reaction and lower the efficiency of the cell.
Fuel cells are generally classified according to the nature of the electrolyte (except for direct methanol fuel cells which are named for their ability to use methanol as a fuel), each type requiring particular materials and fuel. Each fuel cell type also has its own operational characteristics, offering advantages to particular applications. This makes fuel cells a very versatile technology.
As a result, fuel cells have a broader range of application than any other currently available power source - from toys to large power plants, from vehicles to mobile chargers, and from household power to battlefield power.
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