Scientists said Wednesday they had found a chink in the impermeable armour of graphene, the world's thinnest material, and one of the strongest.
The team said they were surprised to see the super material known to repel all gases and liquids, let through sub-atomic particles called protons.
And they hailed the unexpected finding as a potential breakthrough for fuel cells -- a non-polluting version of a traditional battery.
"The discovery could revolutionise fuel cells and other hydrogen-based technologies as they require a barrier that only allow protons -- hydrogen atoms stripped of their electrons -- to pass through," said a statement from Manchester University, whose researchers took part in the work.
Protons, along with neutrons, make up the nuclei of atoms, the building blocks of matter.
At just one atom thick -- many times thinner than a human hair, graphene is stronger than steel and acts as a barrier to even the smallest of atoms, hydrogen -- making it a perfect ingredient for impermeable coatings and packaging materials.
Led by physicist Andre Geim, awarded the Nobel in 2010 for his work on graphene, a research team set out to test whether protons, like atoms and molecules, would also be repelled by graphene.
They "fully expected that protons would be blocked, as existing theory predicted as little proton permeation as for hydrogen," said the statement of the findings published in the journal Nature.
"Despite the pessimistic prognosis, the researchers found that protons pass through the ultra-thin crystals surprisingly easily, especially at elevated temperatures..."
This made graphene an excellent candidate for proton-conducting membranes key to efficient fuel cell technology, said the statement.
Fuel cells, a kind of battery used in some electric cars and backup power generators, for example, use oxygen and hydrogen as a fuel and convert chemical energy into electricity.
They require membranes that allow protons to pass through, but not other particles.
"It looks extremely simple and equally promising. Because graphene can be produced these days in square metre sheets, we hope that it will find its way to commercial fuel cells sooner rather than later," said study co-author Sheng Hu.
Graphene was aired as a theoretical substance in 1947. But for decades, physicists thought it would be impossible to isolate, as such thin crystalline sheets were bound to be unstable.
The problem was resolved in 2004 by Geim and Konstantin Novoselov, who used ordinary sticky tape to lift a layer from a piece of graphite.
That layer was itself pulled apart using more tape, and the process repeated until just the thinnest of layers remained -- a graphene sheet.