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

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Nature loves balance. Ever sprinkle salt on a piece of eggplant? Over time, the eggplant starts to sweat out water. This happens because there is a different ratio of salt to water on the surface versus inside the eggplant, so water seeps out onto the surface to try and equalize how salty the water is inside and out. This balancing act is called osmosis. It happens throughout nature – even in massive bodies of water – like where rivers meet the sea. Scientists have been trying to harness the power of this balancing act –called blue energy by sticking thin membranes in between sea water and fresh water. The holes in the membranes are too small to allow water to move through, like in the eggplant, so instead, the charged particles from the salt move. This flow of charged particles can be harnessed to create electricity. In order for the charges to continue flowing, the membrane needs to be selective— meaning it should only let one type of ion through. Since manufacturing the membranes relies on nanotechnology, it is often expensive to build and maintain them, and the system can lose efficiency over time. It’s a challenge to scale up these systems to produce enough power to make them worth the cost. Now, scientists have built a 3-d Janus membrane named after the mythical god with two faces. The Janus membrane has different properties on each side, which makes it a good gatekeeper for encouraging a steady flow of charged particles from one side to the other. The membrane can be tuned to have different sized pores and accept different kinds of charged particles. The researchers measured how efficient this process was at converting chemical energy to electrical energy. At about 35.7 percent it’s better than a solar cell but not as good as a wind turbine. The team hopes that Janus membranes can be used to create more efficient and less expensive blue energy generators. These systems can be deployed to generate power in coastal communities that don’t have large enough waterfalls or strong enough currents for other forms of water power – like underwater turbines. In order to find new forms of renewable energy and limit carbon emissions, researchers are looking in unexpected places – even tapping into salty solutions.

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