There are five main types of fuel cells currently undergoing research. They are all at differing levels of commercialisation and are being considered for different applications. Each of the fuel cells operates at different temperatures and utilises different electrolytes. All of the fuel cells considered operate essentially on hydrogen though those operating at higher temperatures offer the possibility of internal conversion (reforming) of hydrocarbon fuels to yield hydrogen. The higher temperature fuel cells also offer advantages in terms of efficiency. It is these fuel cells which rely on ceramic electrolytes for their operation.

The five main types of fuel cell are:

· Phosphoric Acid Fuel Cell (PAFC)

· Alkaline Fuel Cells (AFC)

· Molten Carbonate Fuel Cells (MCFC)

· Solid Polymer Fuel Cells (SPFC)

· Solid Oxide Fuel Cells (SOFC)
Phosphoric Acid Fuel Cell (PAFC)

In this design the phosphoric acid electrolyte is contained between two porous graphite electrodes coated with a platinum catalyst. Hydrogen is used as the fuel and oxygen as the oxidant. The operating temperature is between 180°C and 210°C. This fuel cell is the closest to commercialisation for large-scale power generation. This is because its development started earlier and a great deal of effort and finance was put into the PAFC during the initial stages [1]. The electrical efficiency of the PAFC units are ~40% with combined heat and power units achieving ~70% [2].

As of August 1999, a total of 180 plants with a total rated power of 46.26MW had been installed in Japan. Among these plants, 106 were installed by gas utilities with the other 74 being sited in breweries, water and sewage works, electrolyser plant and at Epson’s Nagano Works [3].
Disadvantages with PAFC’s

There are several disadvantages associated with the PAFC design. These include the need to use the expensive noble metal, platinum, as electrodes. Furthermore the electrodes are susceptible to CO poisoning and the electrolyte in the fuel cells is a corrosive liquid which, is consumed during operation [4].
Alkaline Fuel Cells (AFC)

In the AFC the aqueous potassium hydroxide acts not only as an electrolyte but also as a coolant. It conducts hydroxyl ions from cathode to anode and depending upon the weight percentage of the hydroxide in the electrolyte can operate at temperatures between 60 and 120°C or as high as 250°C [2]. The cells require very pure hydrogen and oxygen as the fuel and oxidant as they cannot tolerate even the atmospheric levels of carbon dioxide. Due to the requirement for pure gases, the AFC would not be economically viable for terrestrial power applications. However, they do demonstrate high power densities and have an established space application. Today there are three AFC power plants used in the space shuttle Orbiter, supplying 12kW maximum power [16].
Molten Carbonate Fuel Cells (MCFC)

The MCFC has a molten mixture of alkali carbonates, Li2CO3 and K2CO3, as the electrolyte. Both the electrodes are nickel based, the anode consisting of a nickel chromium mix. The 10% chromium is required to maintain the porosity of the anode structure. These cells can be run on a variety of fuels and one of their interesting features is their operating temperature (~ 650°C), which is high enough for the direct conversion of natural gas to be performed at the anode [5].

The potential market for MCFC systems is thought to be in co-generation applications. The electrical efficiency of the MCFC unit is ~ 50% and in combined heat and power applications efficiencies could reach up to 90% [7]. Currently, the MCFC technology is entering the 0.1-2MW demonstration phase in order to confirm the initial indications of performance and efficiency [7].