Osmotic power receives boost from atom-thick membrane

Researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL)’s Laboratory of Nanoscale Biology in Switzerland have developed an osmotic power generation system that delivers never-before-seen yields. Their innovation lies in the development of a three-atom-thick membrane for separating the two fluids. The results of their research are published in Nature.

The concept of osmotic power is fairly simple. A semipermeable membrane separates two fluids with different salt concentrations, such as seawater and fresh water. The salt ions naturally travel through the membrane until the salt concentrations in the two fluids reach equilibrium, and since an ion is simply an atom with an electrical charge, the movement of the salt ions can be harnessed to generate electricity.

EPFL's system consists of two compartments filled with seawater and fresh water, separated by a thin membrane made of the two-dimensional material molybdenum disulphide (MoS₂). The membrane has a tiny hole, or nanopore, through which positively-charged ions from the seawater can pass into the fresh water, but it pushes away most of the negatively-charged ions. That creates a voltage between the two liquids as one builds up a positive charge and the other a negative charge, and this voltage allows current generated by the transfer of ions to flow.

"We had to first fabricate and then investigate the optimal size of the nanopore. If it's too big, negative ions can pass through and the resulting voltage would be too low. If it's too small, not enough ions can pass through and the current would be too weak," said Jiandong Feng, lead author of the research.

What sets EPFL's system apart is its membrane. In osmotic power systems, the current increases with thinner membranes, and EPFL's membrane is just a few atoms thick. In addition, MoS₂ is an ideal material for generating an osmotic current. "This is the first time a two-dimensional material has been used for this type of application," said Aleksandra Radenovic, head of the Laboratory of Nanoscale Biology.

The potential of the new system is huge. According to the researchers’ calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity – or enough to power 50,000 standard energy-saving light bulbs. And since MoS₂ is easily found in nature or can be grown by chemical vapor deposition, the system could feasibly be ramped up for large-scale power generation. The major challenge in scaling-up this process is finding out how to make relatively uniform pores.

"This is the first time a two-dimensional material has been used for this type of application."Aleksandra Radenovic, EPFL.

Up to now, researchers have been working on membranes with a single nanopore, in order to understand precisely what was going on. ''From an engineering perspective, a single nanopore system is ideal to further our fundamental understanding of membrane-based processes and provide useful information for industry-level commercialization," said Jiandong Feng.

The researchers were able to run a nanotransistor from the current generated by a single nanopore and thus demonstrate that their system worked. These low-power single-layer MoS₂ transistors were fabricated in collaboration with Andreas Kis' team at EPFL, while molecular dynamics simulations were performed by collaborators at the University of Illinois at Urbana-Champaign, US.

EPFL's research is part of a growing trend. For the past few years, scientists around the world have been developing systems that leverage osmotic power to generate electricity. Pilot projects have sprung up in places such as Norway, the Netherlands, Japan and the US to generate energy at estuaries, where rivers flow into the sea. For now, the membranes used in most osmotic power systems are organic and fragile, and deliver low yields. Some systems use the movement of water, rather than ions, to power turbines that in turn produce electricity.

Once the systems become more robust, however, osmotic power could play a major role in the generation of renewable energy. While solar panels require adequate sunlight and wind turbines adequate wind, osmotic energy can be produced just about any time of day or night – provided there's an estuary nearby.

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.