Graphene membrane technology update

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Since announcing last March that they were developing a graphene membrane under the trademark ‘Perforene’, Lockheed Martin engineers have received at least four patents, which cover the selective perforation of graphene membranes, functionalization of membrane perforations and the use of various perforated graphene membranes in various configurations to desalinate water. Although WDR understands that Lockheed continues development work on Perforene products for desal applications, a spokesperson said that no additional information is available for release at this time.

Lockheed may have gone silent (except for its public relations department, which ridiculously and very prematurely claims that it has developed a membrane that will desalt water “at a fraction of the cost of industry-standard RO systems”), but university research on graphene continues to grow and graphene has become one of the membrane industry’s hottest topics. In the past four weeks, at least three new journal papers have been published, which review development work on graphene as a potential desal membrane material.

Even with all of the activity, one thing is very clear: readers who are impatiently waiting to test new graphene membranes should relax. It will likely be years before commercial elements are ready for desal applications.

This is not meant to sound discouraging, it simply means that developing the ability to mass produce a one-atom thick, defect-free material with functionalized pores or channels that are measured in angstroms will not happen overnight.

WDR has reviewed the most recent papers:

Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes, O’Hern, et al., published in Nano Letters, 3 February 2014

Massachusetts Institute of Technology (MIT) researchers have been evaluating graphene as a membrane material since 2009. Working in conjunction with Oak Ridge National Laboratory and King Fahd University of Petroleum and Minerals, MIT’s Mechanical Engineering and Electrical Engineering and Computer Science departments have been fabricating single-layer graphene sheets and evaluating various methods of creating controlled, subnanometer-sized pores in ranges suitable for use as nanofiltration and desalination membranes.

MIT graduate student Sean O’Hern told WDR that the manufacturing process’ first step involves bombarding 1-cm2 graphene sheets with high-energy gallium ions to weaken or strain some of the carbon bonds.

“The process is similar to the way an electron gun produces pictures on old cathode ray TV screens: the gun shoots a beam of gallium ions at the graphene, scanning the sheet from left to right and top to bottom. Unlike the electron beam used to create the frames on TV screens, the gallium ion beam is diffuse instead of focused. As the gallium ions travel towards the graphene after being shot out of the gallium gun, they spread apart and hit the graphene with a distribution not unlike what happens when you shoot a target with a shotgun. This process is repeated until the entire sheet is uniformly covered,” said O’Hern.

Once some of the carbon bonds have been weakened, the sheets are etched in an oxidant bath, where the bonds break to form pores. By increasing or decreasing bath times, pore sizes are able to be controlled.

The ability to tune graphene’s selectivity through the controlled generation of subnanometer pores addresses one of its most significant development challenges. He said that the group’s two-step process is repeatable and reproducible, but it is merely the first step towards reaching the membrane’s full potential, adding, “Opportunities also exist to tailor the membrane porosity, which is currently less than one percent, the chemistry of pores and the pore size distribution using our process as a template for optimization.”

Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes, RK Joshi, et al., published in Science, 14 February 2014

This paper’s authors include Professor Andre Geim, who was the co-recipient of the 2010 Nobel Prize in physics for isolating graphene. Unlike much of the other membrane work being done with graphene, this research group from the University of Manchester is focusing on a new class of membranes made of laminated sheets of graphene oxide.

According to Rahul Nair, the paper’s corresponding author, if the sheets are stacked so that the spaces between them are less than 0.7nm, sodium and chloride ions are effectively excluded. “A capillary-like pressure acts on the smaller water molecules, facilitating permeation through the interconnected nanochannels between the sheets.

“We found graphene oxide laminates to be particularly attractive because they are easy to fabricate, mechanically robust and should be more amenable to industrial-scale production,” he told WDR.

Baoxia Mi, an assistant professor at the University of Maryland who wrote a perspective that accompanied the paper, told WDR that she agrees that it should be much easier to fabricate and scale-up laminated membranes for commercial use than some of the alternatives being considered by others.

“Graphene oxide membranes represent a next generation of ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solutions,” she said.

Quantifying the potential of ultra-permeable membranes for water desalination, D Cohen-Tanugi, et al., published in Energy & Environmental Science, 4 February 2014

Working in conjunction with the Mechanical Engineering department, MIT’s Materials Science & Engineering depart-ment has done extensive work using molecular dynamics simulations to demonstrate that perforated graphene membranes, assuming a porosity of 10 percent, would have a permeability that is two to three orders of magnitude higher than conventional RO membranes.
The group’s latest paper considers how ultra-permeable membranes (UPMs) could improve the performance and cost of RO by realistically explaining and quantifying the potential benefits for BWRO and SWRO systems.

In one example presented, a threefold increase in the flux of a 100,000 m3/d (26.4 MGD) SWRO plant is shown to result in a 15 percent reduction in specific energy consumption, or a 44 percent reduction in the number of pressure vessels.

MIT’s David Cohen-Tanugi told WDR, “UPMs provide an opportunity for lowering both energy consumption and capital costs. However, beyond certain levels, they exhibit a law of diminishing returns due to thermodynamics and concentration polarization. For the example given in the paper, we show that any further improvements in membrane permeability, beyond 3 Lmh/bar, would have essentially no effect on energy consumption, since the lower feed pressure is already within one percent of the osmotic limit for SWRO at the chosen recovery.”

Volume 50
Issue 9

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