Frequently Asked Questions
Various types of fuel cell have been developed, differing in the electrolyte used between the anode and the cathode. The different electrolytes operate efficiently under different conditions of temperature and pressure, with different fuels, resulting in a natural fit of types for particular applications.
Proton-exchange membrane or polymer-electrolyte membrane (PEM) fuel cells are currently receiving considerable attention as they are the most promising type for transport applications such as cars and buses. They make it possible to combine the anode, cathode and membrane electrolyte in a compact unit known as the 'membrane electrolyte assembly'. Systems up to 500kW are readily available on the market.
Alkaline fuel cells were the first type to be used, although only in specialised applications such as space travel (e.g. Apollo and Space Shuttle missions). This is because this type of fuel cell reacts easily with carbon dioxide in the air or in hydrogen produced from liquid fuels. However, they have a possible application in land vehicles and submarines.
Phosphoric acid fuel cells (PAFC) operate at relatively high temperatures (220°C) and achieve moderate current densities. This type is most suited to medium-scale, combined heat and power (CHP) plants. Units of up to 400kW are in commercial production by Doosan Fuel Cell in the USA.
Fuel cells based on solid oxide (SOFC) and molten carbonates operate at high temperatures (500-1000°C) and are most suitable for land-based power generation. Solid oxide fuel cells are still in the development phase but, potentially, have a wide range of industrial applications, including combined heat and power (CHP) systems of all sizes from 2kW to multi-MW. Molten carbonate fuel cells are mainly used in medium to large-scale CHP systems, and up to 1MW trial plants are currently under test.
Direct methanol fuel cells (DMFC) are a relatively new member of the fuel cell family. These cells are similar to the proton exchange membrane cells in that they both use a polymer membrane as the electrolyte. However, in the direct methanol fuel cell, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer*. They have the potential to replace batteries in portable devices such as laptop computers and mobile phones, subject to regulatory changes.
* A fuel reformer produces hydrogen from the base fuel.
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