Fuels for the future | Archive Titles

Table 1: Fuel cell types
Cell type	Electrolyte	Operating temperature	Efficiency	Output	Applications	Disadvantages
Alkali (AFC)	Generally potassium hydroxide in water	150 – 200°C	70%	300 W – 5 kW	Used on Apollo spacecraft to provide electricity and drinking water and in prototype taxi by ZEVCO in London.	Require pure hydrogen fuel; platinum electrodes are expensive, although companies are examining ways of reducing costs; can leak.
Molten carbonate (MCFC)	High temperature compounds of salt carbonates	650°C	60 – 80%	Up to 2 MW with 100 MW units designed	Creation of waste heat to make steam for heating, industrial processing or in steam turbines to generate more electricity.	High temperature limits materials and safe use of MCFCs – too hot for home use. Also, carbon dioxide needs to be injected to compensate for carbonate ions used in reactions.
Phosphoric acid (PAFC)	Phosphoric acid	150 – 200°C	40 – 80%	Existing cells up to 200 kW; 11 MW units have been tested	Used in public transport, but limited applications in private cars as warm-up time needed. Have been used in stationary power generation.	Platinum electrode catalysts are needed and internal parts must be able to withstand acid.
Proton exchange membrane (PEM)	Polymer electrolyte in thin permeable sheet	80°C	40 – 50%	50 – 250 kW	Operate at low enough temperatures to be suitable for homes and cars.	Fuels must be purified; platinum catalyst used raises costs.
Solid oxide (SOFC)	Hard ceramic compound of metal oxides	1000°C	60%	Up to 100 kW	Most common use is in large stationary power plants.	High temperature limits applications and electrolytes can crack.