Fuel cells convert chemical energy into electricity. They are similar to batteries in function, except that the chemical reactants in a fuel cell can be replenished when they run out. A fuel cell is typically characterized by the type of electrolyte used, its operating temperature and its possible applications. Most research into fuel cells has been centered around automobile applications, though they are also being researched for space exploration.
An important characteristic of a fuel cell is the type of electrolyte used. An electrolyte in a fuel cell connects the fuel and oxidizer electrically while keeping them physically separate. Electrolytes used in fuel cell technology can be either liquid or solid, leading to different design advantages and challenges.
A helpful way of classifying fuel cells is by their working temperature. Many designs, like the solid oxide fuel cell, require high operating temperatures to achieve high energy efficiency. Reactions at these high temperatures can often convert more internal chemical energy into electricity, rather than losing much energy through waste heat. Fuel cells with lower operating temperatures, on the other hand, are typically more portable. Polymer exchange member fuel cells (PEMFCs) are being studied for their promising application in transportation.
PEMFCs can achieve a relatively high efficiency while operating at a temperature less than 212 degrees Fahrenheit (100 degrees Celsius). Such a low operating temperature allows the fuel cell to start up rapidly. This type of fuel cell also uses a solid plastic film as an electrolyte, which makes sealing the fuel cell simpler than for other types of electrolytes. This combination of features has made the PEMFC an ideal candidate for replacement of automobile combustion engines.
Some have speculated that fuel cells will eventually replace gasoline-based transportation systems altogether. An economy based on hydrogen, rather than oil, could enjoy several key advantages. First, vehicle emissions would be limited to water vapor, which poses no obvious environmental threat. Second, vehicles powered by hydrogen may eventually prove more powerful per unit mass of fuel. Lastly, the supply of hydrogen could potentially be a renewable resource, unlike fossil fuels, which are not renewable.
Alkaline fuel cells are the type used in space exploration, including the Apollo spaceflights to the moon. They combine hydrogen and oxygen to generate electricity, releasing heat and water in the process. An aqueous alkaline solution is used as the electrolyte in this type of fuel cell. Alkaline fuel cells enjoy a high level of technology development and can have an electrical efficiency of up to 60%. The cost of these fuel cells, however, has prevented their widespread adoption in terrestrial applications.