Fuel cell

A fuel cell is an electrochemical “device” that continuously converts chemical energy into electric energy (and some heat) for as long as fuel and oxidant are supplied.

Different types of fuel cells

Alkaline Fuel Cell (AFC)

The AFC was one of the first modern fuel cells in the beginning of 1960’s. Liquid potassium hydroxide is used as electrolyte in AFC. The maximum operating temperature in this type of fuel cell is around 200°C. The AFC is mainly used as a power generator in spacecrafts. Pure hydrogen is the only fuel used in AFC which limits its application. The power density of the AFC is in the range of 0.1–0.3 Wcm^-2. Alkaline fuel cells are especially available in the kW range.

Proton Exchange Membrane Fuel Cell (PEMFC)

The electrolyte in the PEMFC is a cationic-exchange membrane of excellent proton conductor. The operating temperature is around 80°C and it can be started even below 0°C. PEMFC is used in stationary and transport applications. The present hydrogen storage technology could not meet the application requirements of PEMFC.  The power density of the PEMFC is in the range of 0.35 - 1 W cm^-2.  PEM fuel cells are available in the 1 W to 250 kW range.

Schematic diagram of PEM Fuel cell

Direct Methanol Fuel Cell (DMFC)

DMFC consists of cationic-exchange membrane as solid electrolyte. Aqueous methanol solution is used as fuel in DMFC. The operating temperature is around 90°C.  Low power density of DMFC compared to PEMFC limits its application within portable electronic devices (1 - 100 W range). Maximum peak power density of about 0.25 Wcm^-2 is obtained at a cell voltage as low as 0.2 - 0.3 V. High loading of noble metal catalyst in DMFC (1.2 mg cm^-2 and higher) accounts for the high cost. The high energy density of methanol provides a potential to replace batteries with micro fuel cell systems.

Phosphoric Acid Fuel Cell (PAFC)

PAFC uses liquid phosphoric acid as the electrolyte. It is operated at 200°C. The PAFC can use reformate with CO concentrations up to 1-2%. Commercially the most successful fuel cell at this moment, in 2003, 245 of the 200 kW systems has already been installed. The power density of the PAFC is in the range of 0.14 Wcm^-2.

Molten Carbonate Fuel Cell (MCFC)

The electrolyte used in MCFC is a molten mixture of lithium, sodium and potassium carbonate. The operating temperature is around 650 °C. Internal reforming of hydrocarbon fuels is possible due to the high operating temperature. The power density of the MCFC is around 0.12 Wcm^-2.  MCFC systems is available in the 50 kW to 5 MW range.

Solid Oxide Fuel Cell (SOFC)

SOFC uses yttrium stabilized zirconia as the solid electrolyte. Operating temperature of SOFC varies from 600 to 1000 °C depending on the electrolyte and the material composition of the electrodes. Hydrogen to natural gas and higher hydrocarbons can be used as fuel. From 1 kW to 5 MW stationary power is generated in SOFC. It is also considered as an important option for auxiliary power units on board of vehicles in the 5 kW range. The power density of the SOFC is in the range of 0.15 - 0.7 Wcm^-2.