A fuel cell is a device akin to a continuously recharging battery and generates electricity by the low-temperature electrochemical reaction of hydrogen and oxygen from air. An important difference is that batteries store energy, while fuel cells can produce electricity continuously as long as fuel and air are supplied. Any hydrogen-rich fuel can be used in different types of fuel cells (employing an external or internal fuel-reforming process), but using a hydrocarbon-based fuel inevitably leads to CO2 emission. Hydrogen-powered fuel cells emit only water and have virtually no pollutant emissions, even nitrogen oxides, because they operate at temperatures that are much lower than internal combustion engines. However, even fuel cells fuelled by hydrocarbon fuels have the potential to provide efficient, clean and quiet energy conversion, which can contribute to a significant reduction in greenhouse gases and local pollution. Because fuel cells are not subject to the limitations of the Carnot cycle, they convert fuel into electricity at more than twice the efficiency of internal combustion engines. In transportation, hydrogen fuel cell engines operate at an efficiency of up to 65%, compared to 25% for present-day petrol-driven car engines. When heat generated in fuel cells is also utilised in combined heat and power (CHP) systems, an overall efficiency in excess of 85% can be achieved ( Dutton, 2002)