2.1.1 Types of fuel cell

A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity, heat and water. The hydrogen may be produced as a by-product of a chemical process, extracted from any hydrocarbon fuel such as natural gas, gasoline, diesel, or methanol via a fuel reformer, or by electrolysis of water. The oxygen is usually obtained from the ambient air around the fuel cell. In some cases where hydrogen is produced by electrolysis, the oxygen co-produced may be used in the fuel cell.

Fuel cells can be loosely grouped into those with acidic electrolytes, those where the electrolyte is alkaline, and cells that operate at very high temperatures.

Successful examples of acidic electrolyte fuel cells are the proton exchange membrane or polymer electrolyte membrane fuel cells (PEMFCs), that use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst, and the phosphoric acid fuel cells (PAFCs) that use liquid phosphoric acid as an electrolyte (the acid is contained in a Teflon-bonded silicon carbide matrix) and porous carbon electrodes containing a platinum catalyst. PEMFCs are generally designed to be operated at lower temperatures, although some may operate at around 80°C, while PAFCs typically operate at temperatures between 150°C to 200°C.

Alkaline electrolyte fuel cells (AFCs) use an aqueous solution of potassium hydroxide as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode.  Most AFCs operate at temperatures of between 100°C and 250°C, but new designs operate at lower temperatures of between 20°C to 70°C.

High temperature fuel cells include molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). MCFCs use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide and operate at 650°C and above. SOFCs use a hard, non-porous ceramic as the electrolyte and operate usually at around 1,000°C. Ongoing research is aimed at reducing this operating temperature down to 550-700°C.