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Nanoporous graphene for desalination

3 July 2019

Carbon nanotube reinforcement and template-based etching help scale up membranes.

The United Nations predicts that two-thirds of the world’s population will face freshwater shortages by 2025. Even today, some Middle Eastern and North African cities get most of their potable water through desalination of seawater. Commercial plants remove the salt by pumping water through semipermeable polymer membranes, a process that consumes vast amounts of energy. The easily clogged membranes require frequent cleaning that releases chemicals into the environment.

Membrane schematic.

One potentially stronger, thinner, and more water-permeable membrane alternative is a single layer of graphene etched with nanopores. But nanoporous graphene tends to lose mechanical strength when scaled up above the microscale, and etching it with a dense grid of pores reduces the membrane’s structural integrity. Now Xiangfeng Duan (UCLA), Quan Yuan (Wuhan University in China), and colleagues have designed a centimeter-sized graphene membrane that effectively filters ions from seawater.

The researchers started with an exceptionally strong, defect-free, single-layer graphene sheet, which they made by chemical vapor deposition. Then they reinforced it with a layer of carbon nanotubes. Finally, with a porous template as a guide, the researchers drilled a grid of uniform 0.3- to 1.2-nm-diameter holes. Removing the template left a precise network of pores in a strong, freestanding membrane.

As part of a benchtop filtration system, the membrane blocked 85% of NaCl and up to 98% of larger-molecule solutes. It also withstood pressures—up to 10 MPa—typical of commercial filtration and achieved permeability two orders of magnitude higher than that of commercial membranes. Although commercial membranes routinely filter out more than 90% of NaCl, the performance of the new membrane more than doubles that of other multilayer graphene-based systems, which filter 42% at best. Future work toward meter-scale membranes will focus on improving the filtering efficiency by modifying the pore size distribution and minimizing crack formation during etching. (Y. Yang et al., Science 364, 1057, 2019.)

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