The world's smallest working fuel cell has been created by US chemical engineers, at just 3 millimetres across. Future versions of the tiny hydrogen-fuelled power pack could replace batteries in portable gadgets.
While batteries are used to do that today, fuel cells are able to store more energy in the same space. Even the most advanced batteries have an energy density an order of magnitude smaller than that of a hydrogen fuel tank.
Yet batteries are much easier to make at the small scale than the pumps and control electronics of a fuel cell. And small pumps can use more energy than they generate.
Give and take
"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. "Even if they are magically made at that scale, their power consumption would probably exceed the power generated."
So Moghaddam, along with a team of Illinois colleagues including Mark Shannon and Richard Masel, has come up with a design for a tiny fuel cell that generates power without consuming it.
The new device has just four components. A thin membrane separates a water reservoir above from a chamber containing metal hydride below. Beneath the metal hydride chamber there is an assembly of electrodes.
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 (see image, top right).
The hydrogen is gradually depleted, though, 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.
Zero gravity
Because the device is so small - just 3 mm by 3 mm by 1 mm - surface tension, not gravity, controls the flow of water through the system. This means that the cell operates even if moved and rotated - perfect for life inside a pocket gadget.
The first designs generated 0.7 volts and a current of 0.1 milliamps for 30 hours before the fuel ran out, but Moghaddam says the latest designs give currents of around 1 milliamp at a similar voltage. That's not yet enough to drive cellphones, which use batteries typically rated at a few volts, but it's enough for simpler electronic systems or microrobots, he says.
At just 9 mm3 the fuel cell certainly is small, says Steve Arscott at the University of Lille in France, an expert on micro fuel cells.
However, it may not be powerful enough to be truly useful, Arscott adds. His own latest micro fuel cell uses methanol, not a metal hydride, as a hydrogen source, as do many micro fuel cells. It is three times the volume of Moghaddam's device and has a power density of 12.5 milliwatts per cm2 - more than 10 times as much as the smaller fuel cell (Journal of Micromechanics and Microengineering, DOI: 10.1088/0960-1317/18/12/125019).
But Shannon points out that the two devices cannot be directly compared. Most fuel cells are fed by an external fuel supply, whereas the new device carries its fuel internally. That bulky fuel takes up a lot of room and so the power density falls. But measured as a volume, the power density of the new fuel cell is a relatively high 100 watts per litre, he says.
Journal reference: Journal of Microelectromechanical Systems (DOI: 10.1109/JMEMS.2008.2007250)
Blog
