Researchers from Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) have demonstrated an efficient desalination process using a porous graphene-based desalination membrane. While the thinness of the freestanding graphene (roughly 0.3 nm) allows for significantly higher flux than traditional reverse-osmosis processes, the major breakthrough in the membrane’s efficiency came with targeting the optimal pore configuration. The size and spacing of the pores is key, and the team applied atom-resolution imaging to optimize these parameters for desalination purposes. The resulting pore size, 0.5–1.0 nm, was found to be large enough for water molecules to pass through, while remaining small enough to prevent salt ions from penetrating.
To create the pores, the graphene layer, which resides on a silicon nitride skeleton, was exposed to a highly reactive oxygen plasma that etches away at the graphene’s carbon atoms, until holes are formed in the layer. The pores themselves are punctuated with silicon atoms — not oxygen or carbon — a noteworthy phenomenon that the researchers attribute to the silicon’s potential stabilizing effect on the pores. The size of the pores depends on the amount of time that the membrane is exposed to the oxygen plasma.
Controlling pore size is among the most challenging tasks in scaling up this technology beyond the currently demonstrated milliliter scale. As the membrane surface area gets larger, there will be added difficulty in maintaining the optimal pore density of one pore per 100 nm2. Ensuring mechanical stability (while remaining at the desired pore density) as the membranes get larger will also be key to moving to pilot and commercial levels. In addition to the oxygen-plasma approach, the team is also researching alternative, more controllable methods of pore production to help alleviate some of these concerns.