The relatively low power densities of most batteries mean that they do not have a long enough life time for many applications. One alternative to the battery is a fuel cell that runs on lightweight hydrocarbons such as methanol. However, these cells have previously been too big for portable applications.

Now Sossina Haile of the California Institute of Technology and co-workers at the University of Southern California and Northwestern University have overcome these problems by making a micro fuel cell that runs on propane. This material has a high energy density and can successfully be stored in miniature devices because it can be easily compressed into a liquid.

The new device belongs to family of fuel cells known as solid-oxide fuel cells that work using a mixture of fuel and oxygen. It is simple and compact because it contains just one inlet for the oxygen and fuel, and just one outlet for the exhaust gases. The fuel and oxygen mixture is partially oxidised in an exothermic reaction that heats the fuel cell up to between 500 and 600°C.

One of the most important breakthroughs in the new work is that the cell is able to keep itself hot, which is a prerequisite for producing power. The Caltech team achieved this by using novel catalysts that release enough heat to maintain the temperature of the cell, and by adding the fuel slowly so that only a small amount is consumed at any one time. This strategy also eliminates the need for bulky insulation to keep the cell warm, which can add to the size of the device. Finally, a heat exchanger ensures that the hot gases exiting from the fuel cell transfer their heat to the incoming cold gases.

The scientists obtained a power output of 350 mW from a surface area of 1.42 centimetres squared, which is among the highest values ever achieved in a micro fuel cell. The teams says that if such a cell could be commercialised it would be able to drive a small device like an MP3 player for much longer than the best lithium batteries available today.