The first membraneless alkaline fuel cell has been built by exploiting the way liquids do not mix in ultra-narrow channels. It could offer cheaper and more efficient fuel cells.

Doing away with membranes not only simplifies a fuel cell's design, it has also enabled the first alkaline fuel cells to be built. These could potentially be 40% more efficient than the acidic units used today, says Paul Kenis of the University of Illinois at Urbana Champaign, Illinois, US, who has developed the system.

His system exploits a phenomenon known as "laminar flow", where tiny streams of liquid become so viscous they do not mix when squeezed past one another.

"The concept of a membraneless fuel cell is a great idea, because between 20% and 40% of the cost of a fuel cell is the membrane," says chemist Shelley Minteer at the University of St Louis in Missouri, US.

She has also built a membraneless fuel cell, but her system uses two different, selective enzymes to stop the two liquids in the cell mixing, and has not yet been harnessed for an alkaline fuel cell. "This is the only other way to do it," she says, but admits that her method introduces new lifetime and power density problems.

Ten times more

Fuel cells work by breaking down a fuel such as methanol into protons and electrons, combining them with oxygen from the air and using the energy liberated to create an electric current.

Scientists hope they could be an environmentally friendly, longer-lasting replacement for the lithium-ion batteries that currently power cell phones and laptops. Fuel cells produce water as their main by-product and they could potentially be made to produce at least ten times more power for the same volume of fuel.

In the past, fuel cells have been made up of two chambers, one housing the liquid fuel that produces protons and supplies electrons to the anode, the other containing an oxygen-water mixture, which absorbs electrons.

The polymer membrane separating the chambers is punctured by tiny pores that allow protons through but are small enough to stop the larger methanol and oxygen molecules from diffusing across.

Limiting chemistry

The problem is that using these membranes means that all fuel cells must be based on the exchange of acidic protons. Because alkaline hydroxide ions are much bigger than protons, there are no membranes that can allow the hydroxide ions through without also causing mixing between the two chambers, explains Kenis. "The membrane limits the chemistry of the fuel cell," he says. But he insists that alkaline fuel cells are nevertheless more efficient.

So he decided to do away with the membrane altogether. Kenis realised that if he shrank the chambers down to about 0.25 millimetres and ensured that the liquids were always moving, the two could flow past each other and would not mix, even with no separating a membrane. And they would still allow the diffusion of protons or hydroxide ions from one side of the cell to the other.

Thin streams

The reason that two liquids normally mix - as with cream poured into coffee, for example - is a physical effect known as turbulence. But turbulence disappears when streams of liquid become thinner than about 1 centimetre, explains Kenis.

Kenis has patented the concept of his membraneless fuel cell system and presented a demonstration of the first membraneless alkaline fuel cell at the American Physical Society meeting in Los Angeles on Tuesday.

The cell is a cuboid of 3cm by 1mm by 1mm and produces 0.25 watts of power. He has already used it to power a miniature fan in the lab, but to power laptops or battery chargers in future, hundreds of tiny cells would have to be arranged in parallel.

However, this prototype model has not yet shown the high efficiency that might be expected from an alkaline fuel cell because the oxygen does not dissolve well in water. Kenis claims to have a solution, but will not divulge it until he has patented the idea.

NewScientist.com news service

Celeste Biever