This ‘two-faced’ membrane can create electricity—from nothing but salty water

Imagine being stuffed into a crowded train car and noticing a less crowded one just down the platform. You’d probably want to move over as soon as possible. Particles that follow this balancing act—known as osmosis—spontaneously move from an area of high concentration to one of low concentration. Now, scientists have used this tendency to create a power-producing membrane that can harvest electric current from nothing but salty water.

When ionic salts, made of bundles positively and negatively charged particles, dissolve in water, the bundles break apart, leaving positively and negatively charged particles free to participate in osmosis. By placing charged, thin membranes in between salty water and freshwater, scientists can create an expressway for the flowing particles, generating electric current. But these membranes are often expensive to manufacture and they tend to get leaky over time. That lets particles pass back through in the wrong direction, cutting into how much electricity they can produce.

Now, researchers have developed a new kind of gatekeeper—a “two-faced” membrane that has different properties on either side, from the size of the pores to the charge of the membrane itself. This encourages a steady flow of charged particles from one side to the other while preventing them from drifting back in the wrong direction. These so-called Janus membranes, named after the ancient Roman god of gates and passages, can also be manufactured to have different-size pores and hold different charges, allowing them to accept different kinds of particles.

The researchers tested their Janus membranes with salty sea water on one side and fresh river water on the other. They found the devices were able to convert 35.7% of the chemical energy stored in the salty water into useable electricity. That’s as efficient as most wind turbines and higher than most solar cells, they report today in Science Advances.

Next, the researchers plan to build larger membranes and see whether they can withstand the conditions of real sea and river water. If the membrane performs as well in “the wild,” the new membranes could be used to power remote communities with no other sources of renewable energy in just a few years, the researchers say. That suggests that when it comes to harvesting power from moving particles, being a little two-faced is a good thing.