There are several types of fuel cells, each of which is generally defined by there operating temperature and the type of electrolyte they use. The choice of which type to use is often based on the targeted application of the fuel cell. For example, some types of fuel cells work well for use in stationary power generation plants while others may be useful for powering cars or small portable applications. The best-known types of fuel cells are alkaline (AFC), molten carbonate (MCFC), phosphoric acid (PAFC), proton exchange membrane (PEMFC) and solid oxide (SOFC). Each work on the same principle, striping electrons from hydrogen atoms to create electricity.

An example of the differences between fuel cell types and there ideal target output

Proton Exchange Membrane (PEMFC)
PEM fuel cells use a platinum coated solid polymer that require only hydrogen and oxygen to produce electricity. The advantages of a PEM fuel cell is that they have a high power density and are low in weight. They are also able to operate in low temperatures, typically around 80ºC which allows them to start start quickly (less warm uptime). The major disadvantage of PEM fuel cells is that they require an expensive catalyst, in this case platinum, which adds to the overall unit cost. Hydrogen delivery is also another significant barrier is it requires careful handling and storage. This makes PEM fuel cells more applicable for industrial uses where delivery and storage can be in a controlled environment.

Direct Methanol Fuel Cells (DMFC)
Direct Methanol Fuel Cells are PEM based fuel cell that use methanol instead of pure hydrogen. As such, they share many of the same advantages of a PEM fuel cell, with the added benefit that methanol is a much easier and safer fuel for transportation and storage. The disadvantage of methanol is that the reaction is often not as efficient as pure hydrogen because methanol can often move across the membrane without reacting with the catalyst. To address these concerns, some fuel cells reform the methanol (RMFC) before it reacts with the catalyst. While this approach improves the efficiency of methanol, it still requires an expensive reforming process that needs to operate at much higher temperatures (250ºC) to first burn-off the methanol in order to release hydrogen.

Alkaline Fuel Cells (AFC)
Alkaline Fuel Cell is one of the oldest types of fuel cell. The electrolyte they use is made of liquid potassium hydroxide. One of the major advantages for an alkaline fuel cell is that non-precious metals can be used as a catalyst. However, the catalysts can be easily poisoned by CO2 (carbon dioxide). As such, the hydrogen and oxygen used in an AFC needs to be purified, which is a more costly process. The operating temperature of AFC's is between 100ºC ~ 250ºC.

Solid Oxide Fuel Cells (SOFC)
Unlike other fuel cells, Solid Oxide Fuel Cell is composed of entirely solid materials. The electrolyte is a hard ceramic material made up mainly of zirconium oxide with small amounts of ytrria. The systems operate at about 600ºC~1000ºC. The high temperature means these are slow starting systems and most are designed for larger, stationary applications in the 25 kW to 100 kW range

Phosphoric Acid Fuel Cells (PAFC)
Phosphoric acid is the electrolyte used in an PAFC. As an acid, it requires a very high temperature(150ºC ~ 200ºC) to start the reaction. It also uses platinum as a catalyst, similar to PEM fuel cells, which is costly. However, PAFC's are more tolerant to impurities in fuels than PEM fuel cells, and they are very efficient at generating electricity and heat - which makes them ideal for power plants. However, they are typically large and heavy which makes them less than ideal for smaller or portable applications.