The fuel cell, device that directly converts the energy potential of fuels into electrical power. (Electrical power is equivalent to work output.) Directly means without first burning the fuel to cause a temperature rise followed by a second-step, which is the conversion of heat into work. It is the heat-to-work step in conventional heat engines that results in the severe efficiency constraints imposed by the second law of thermodynamics. (Just about every technology we now use to convert fuel to power is a heat engine - automobile engines, aeroplane engines, outboard motors and even nuclear-generating stations, but not electric motors, which convert electricity to power.) By avoiding the heat-to-work step, fuel cells avoid the efficiency constraint imposed on today's technology for converting fuel to work (power).

The fuel cell concept has been known for more than a century. But modern fuel cell development dates from the late 1930s, when F.T. Bacon developed an alkaline electrolyte fuel cell that used hydrogen and oxygen. This development formed the basis of the fuel cells that were used to generate onboard electricity for today's spacecraft. The water waste product from these fuel cells is used as astronaut drinking water.

Terrestrial applications, for uses such as locomotives, mining vehicles, submarines and even electric utility stations, are now in stages of planning, design or development. A fuel cell is an electrochemical energy conversion technology - a classification shared with electricity storage batteries (which convert energy, stored within their own material, into electricity) and electrolysis plants (which convert electrical energy into chemical energy stored in hydrogen and oxygen, by splitting water molecules). Looking into the next century, the importance of electrochemical energy conversion technologies will grow perhaps to eclipse today's ubiquitous heat engines.

All living things convert their fuel (food) to work (power) by electrochemical processes - for growing, running, swimming or flying. But about 2 centuries ago, when mankind first invented machines to convert fuel to work, the electrochemical way of nature was not followed. Instead, heat engines were invented, of which the first was the steam engine. Electrochemical energy conversion is inherently more efficient, although today it suffers from low power densities when compared to heat engines. But with continuing advances in materials and catalysts, power densities could be increased dramatically. This, together with the parallel evolution towards the hydrogen age, will almost certainly make the fuel cell a common feature of everyday life in the 21st century.