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.

However, these membranes are often expensive to manufacture and they tend to get leaky over time. Researchers have now developed a new kind of cheaper 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.