By applying voltage to electrically control a new "transistor" membrane, researchers at Lawrence Livermore National Laboratory (LLNL) achieved real-time tuning of ion separations - a capability previously thought impossible. The recent work, which could make precision separation processes like water treatment, drug delivery and rare earth element extraction more efficient, was published in Science Advances.
The membranes are made of stacks of MXenes - 2D sheets that are only a few atoms thick. Ions squeeze through nanoscale channels formed in the gaps between the stacked MXene layers.
Until now, scientists thought MXene membrane properties were intrinsic and unchangeable once created. The rate of ion transport was thought to be baked in from the beginning.
But in an advancement for the field, the team discovered that the MXene membranes instead can be configured to act like transistors. Because the MXene material is electrically conductive, applying an electric field can change the efficiency of the molecular transport through the membrane.
"This work was inspired by the transistor's ability to regulate current through a device by applying a gate voltage," said lead co-author and LLNL scientist Aleksandr Noy. "It's just like how you can regulate the flow through a garden hose with a valve, or by using your foot to step on it."
Like transistors that control electrical current with voltage, these membranes control molecular flow with an applied electric field. That surface electrical charge determines how many ions can fit in between the MXene layers and how easily they move. With its transistor-like behavior, the MXene membrane's transport properties can be turned on and off in real time throughout a separation process.
"We also demonstrated that by applying an alternating positive and negative voltage, we were able to enhance the ion transport through the membrane and make it essentially self-pumping," said co-author Aaditya Pendse, a former LLNL postdoctoral researcher. "That increases the efficiency of the ion travel through the membrane."
"This oscillating voltage approach represents a particularly significant discovery, as it enables the membrane to actively drive molecular transport rather than relying solely on passive diffusion," said co-author Arjun Yennemadi, a graduate student at the Massachusetts Institute of Technology.
Going forward, the team aims to test how these membranes might work for transporting and separating rare earth element ions - critical materials that are required for a robust U.S. supply chain.
