Types of Fuel Cells

The fuel cell type is distinguished by the electrolyte that is utilized and can be classified in two main categories according to their operating temperatures: the low temperature fuel cells (operating temperatures 60-250 °C) and high temperature fuel cells (operating temperatures 600-1000 °C). The formers' quick start times, compact volume and lower weight attributes have led to an almost exclusive use in transportation applications. The latters' features include a higher efficiency in energy conversion than the low temperature fuel cells. They also provide high-temperature waste heat, which is beneficial for stationary cogeneration applications but present a problem for transportation applications.
Common types of high temperature fuel cells are molten carbonate fuel cells and solid oxide electrolyte fuel cells.

· Molten carbonate fuel cells use high-temperature compounds of sodium or magnesium carbonates (chemically, CO2) as the electrolyte. Their operating temperature is about 650 °C and their efficiency ranges from 60-80%. Units with output up to 2 MW have been constructed and further plans for units of 100 MW are under development. The nickel electrode catalyst used is inexpensive but the high operating temperature limits the materials and safe uses of molten carbonate fuel cells. They would simply be too hot for transportation uses. Furthermore, carbonate ions from the electrolyte are used up in reactions making it necessary to inject carbon dioxide to compensate.

Molten carbonate fuel cell

· Solid oxide electrolyte fuel cells use a hard, ceramic compound of metal (preferably calcium or zirconium) oxides (chemically, O2) as the electrolyte. Their operating temperature reaches the 1000 °C barrier and they possess 60% efficiency. Cells output is about 100 kW. At such high temperature, a reformer is not needed to extract hydrogen from the fuel and waste heat can be recycled to generate additional electricity. The high operating temperature limits the solid oxide electrolyte fuel cells applications. Additionally, while solid electrolytes cannot leak, they can crack.

Solid oxide electrolyte fuel cell

Common types of low temperature fuel cells are alkaline fuel cells and proton exchange membrane fuel cells.

· Alkaline fuel cells operate on compressed hydrogen and oxygen. The electrolyte used is a solution of potassium hydroxide (chemically, KOH) in water. Their operating temperature ranges between 150 and 200 °C and their efficiency is about 70 %. Cell output ranges from 300 W to 5 kW. Alkaline fuel cells demand pure hydrogen fuel and their platinum electrode catalyst is expensive limiting their eventual applications.


Alkaline Fuel Cell

· Proton exchange membrane fuel cells work with a polymer electrolyte in the form of a thin permeable sheet (proton exchange membrane). The membrane is coated on both sides with highly dispersed metal alloys (platinum) that are active catalysts. Operating temperature is about 80 °C and efficiency ranges between 40 and 50 %. Cell outputs range from 50 to 250 kW. The electrolyte use is solid, yet flexible enough, so leaking and cracking are avoided. Operation is taking place at low temperature; fuels must be purified but the platinum catalyst used initially was raising the cost considerably. Fortunately, new techniques for coating very thin layers of catalyst on the polymer electrolyte have reduced costs considerably to around $150 per automobile, distinguishing proton exchange membrane fuel cells as ideal for stationary as well as transportation applications.

Proton exchange membrane fuel cell