Researchers from École Polytechnique Fédérale de Lausanne (EPFL) have developed a system that they say efficiently generates electricity using seawater, fresh water and a membrane atoms thick.

The concept uses a semipermeable membrane that separates two fluids with different salt concentrations. Salt ions travel through the membrane until the salt concentrations in the two fluids reach equilibrium. Because an ion is simply an atom with an electrical charge, the researchers say that movement of the salt ions can be harnessed to generate electricity.

EPFL’s osmotic power generation system consists of two liquid-filled compartments separated by a membrane made of molybdenum disulfide. The membrane has a hole, or nanopore, through which seawater ions pass into the fresh water until the two fluids’ salt concentrations are equal. As the ions pass through the nanopore, their electrons are transferred to an electrode, which generates an electric current.

The membrane allows positively charged ions to pass through, while they push away most of the negatively charged ones. That action creates voltage between the two liquids as one builds up a positive charge and the other a negative charge. This voltage causes the current generated by the transfer of ions to flow.

By playing with the diameter of the nanopore, the researchers were able to find the ideal compromise between current and voltage to obtain the highest possible power. Image credit: EPFL.By playing with the diameter of the nanopore, the researchers were able to find the ideal compromise between current and voltage to obtain the highest possible power. Image credit: EPFL.What may set EPFL’s system apart from others is its membrane. In similar types of systems, the current increases with a thinner membrane. And EPFL’s membrane is just a few atoms thick. The material it is made of—molybdenum disulfide—appears to be well suited for generating an osmotic current.

“This is the first time a two-dimensional material has been used for this type of application,” says Aleksandra Radenovic, head of EPFL's Laboratory of Nanoscale Biology.

According to the researchers, their calculations suggest that a 1m2 membrane with 30% of its surface covered by nanopores could produce 1 Megawatt of electricity. And since molybdenum disulfide (MoS2) is readily found in nature or can be grown by chemical vapor deposition, the system could be scaled up for power generation.

One challenge in scaling this process is finding out how to make relatively uniform pores. Until now, researchers have worked on a membrane with a single nanopore.

EPFL’s research is part of a growing trend. For the past several years, scientists around the world have been developing systems that leverage osmotic power to create electricity. Pilot projects have sprung up in Norway, the Netherlands, Japan and the United States to generate energy at estuaries, where rivers flow into the sea.

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