Nanotube Membrane Could Revolutionize Osmotic Power

Two years ago, Stanford University researcher Yi Cui took a break from his work on solving the silicon-versus-graphite conundrum of anodes in Lithium-ion batteries to look into an alternative method for generating electricity that has become known as pressure-retarded osmosis.

Pressure-retarded osmosis exploits the difference in salinity between fresh water and salt water to generate electricity. Norway-based Statkraft has been a leading commercial proponent of the technology, building its first pilot plant in 2009.

Despite new plants being planned and this alternative energy source theoretically capable of generating 1 terawatt—the equivalent of 1000 nuclear reactors—a group of European researchers believed the technology was still not a truly viable energy source.

Now researchers at the Institut Lumière Matière in Lyon (CNRS/Université Claude Bernard Lyon 1), in collaboration with the Institut Néel (CNRS) claim to have developed a new membrane technology that will make new pressure-retarded systems 1000 times more efficient than today's systems.

The researchers didn’t set out to create a new membrane. The original aim of the research was just to measure dynamics of fluids confined in nanometric spaces. The research, which was published in the journal Nature ("Giant osmotic energy conversion measured in a single transmembrane boron-nitride nanotube"), did succeed in achieving the world’s first measurement of osmotic flow through a single nanotube. However, in achieving their initial aim they also managed to create a membrane design that could revolutionize the nascent industry.

The device the researchers developed employed a membrane material that was both impermeable and electrically insulating. The researchers poked a hole through the membrane with a scanning tunneling microscope probe and passed a boron nitride-based carbon nanotube through it. The researchers then placed the membrane with the boron nanotube into a reservoir where it separated fresh water and salt water. They then attached an electrode to either end of the boron nitride nanotube to measure the electric current that passed through the membrane.

The results were pretty spectacular. Because of the strong negative surface charge of the boron nitride nanotubes, the cations in the salt water were strongly attracted to it so that the current passing through the nanotubes were on the order of the nanoampere. When this is extrapolated to a larger scale device a 1 m2 boron nitride nanotube membrane should have a capacity of about 4 kW and be capable of generating up to 30 megawatt-hours per year.

While the new membrane design is three orders of magnitude greater than the prototype plants currently in operation, the researchers will focus their next research in investigating other materials besides boron nitride to test their capabilities in addition to looking at the production of boron nitride nanotubes.

Image: Laurent Joly

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