It has long been known that it is possible to generate electricity from the flow of ions through a suitable membrane separating salt water and fresh water. Pilot projects to generate energy at estuaries, where rivers flow into the sea, have been conducted in places including Norway, the Netherlands, Japan and the United States. Until now, the membranes used have been organic and fragile, and have delivered low yields.

Now, researchers at the Ecole Polytechnique Fédérale in Lausanne have reported in Nature that they have developed an osmotic power generation system that delivers high yields of electricity. Their innovation lies in a three atoms thick membrane used to separate the two fluids.

To generate the electricity, a semipermeable membrane separates two fluids with different salt concentrations. Salt ions travel through the membrane until the salt concentrations in the two fluids reach equilibrium. Since an ion is simply an atom with an electrical charge, the movement of the salt ions can be harnessed to generate electricity. The thinner the membrane, the more electricity can be generated.

The researchers at the Ecole Polytechnique Fédérale have developed a membrane made of molybdenum disulfide just a few atoms thick. Until now, the researchers have worked on a membrane with just a single nanopore but, according to their calculations, a one square metre membrane with 30% of its surface covered by nanopores should be able to produce 1 megawatt of electricity 

Since molybdenum disulfide is easily found in nature or can be grown by chemical vapour deposition, the system could feasibly be ramped up for large-scale power generation at low cost. The challenge now is to scale up the process by developing larger membranes with relatively uniform pores.