US chemical engineers have created the world's smallest working fuel cell, a tiny power unit that is just 3 millimetres across, the New Scientist has reported.

Fuel cells have being touted for years as the future for powering electronic devices. Unlike conventional batteries, which are made up of highly toxic ingredients to hold a charge, fuel cells promise a much greener way of powering devices, as they instead generate an electrical current from a chemical reaction between, say, a hydrogen-containing fuel and oxygen.

The downside is that unlike conventional batteries that can be recharged repeatedly, fuel cells consume their reactant (or fuel), which must be replenished (or the cell replaced). However fuel cells are able to store more energy in the same space than conventional batteries. Even the most advanced batteries have an energy density an order of magnitude smaller than that of a hydrogen fuel tank.

That said, conventional batteries are still easier to manufacture when considering a very small form factor. This is because fuel cells typically require pumps and control electronics, and when considering a tiny fuel cell, the small pump can actually use more energy than the cell generates.

"It's not practical to make a pump, a pressure sensor, and the electronics to control the system in such a small volume," says Saeed Moghaddam at the University of Illinois at Urbana-Champaign, speaking to the New Scientist of the tiny fuel cell. "Even if they are magically made at that scale, their power consumption would probably exceed the power generated."

So apparently Moghaddam, along with colleague Mark Shannon, has come up with a more efficient design for a tiny fuel cell, measuring just 3 mm by 3 mm by 1 mm.

Their tiny fuel cell has just four components. In the bottom of the cell is a metal hydride chamber that contains an assembly of electrodes. A water reservoir above the chamber is separated by a thin membrane.

Tiny holes in the membrane allow the water molecules to reach the adjacent chamber as vapour. Once there, the vapour reacts with the metal hydride to form hydrogen, which fills the chamber, pushing the membrane upwards and blocking the flow of water.

As is typical with fuel cells though, the hydrogen is gradually depleted, as it reacts at the electrodes beneath the chamber to create a flow of electricity. And when the hydrogen pressure drops, more water can enter to keep the reaction going, prolonging its life.

The inventors say that because the device is so small, surface tension and not gravity, controls the flow of water through the system. This means that the cell operates even if moved and rotated, and therefore broadens its appeal for use inside portable electronic gadgets.