http://www.sciencemag.org/news/2018/10/two-faced-membrane-can-create-electricity-nothing-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.