An engineering team at the Massachusetts Institute of Technology has developed a continuous manufacturing process that produces long strips of high-quality graphene, and the process is scalable. Graphene with holes, or nanopores, that can be tailored for specific applications could be used to filter a variety of molecules, including salts, larger ions, proteins, and nanoparticles. Such filters would be useful for desalination, biological separation, and other applications.
“For several years, researchers have thought of graphene as a potential route to ultrathin membranes,” says John Hart, associate professor of mechanical engineering at MIT. “We believe this is the first study that has tailored the manufacturing of graphene toward membrane applications, which require the graphene to be seamless, cover the substrate fully, and be of high quality.”
For many researchers, graphene is ideal for use in filtration membranes. A single sheet of it resembles atomically thin chicken wire composed of carbon atoms joined in a pattern that makes the material extremely tough and impervious–even to the smallest atom, helium.
A new manufacturing process produces strips of graphene, at large scale, for use in membrane technologies and other applications. (Image courtesy of Christine Daniloff, MIT)
Researchers have fabricated graphene membranes with tailored nanopores, but they generally used chemical vapor deposition, in which they first heat a sample of copper foil and then deposit onto it a combination of carbon and other gases. But that limits graphene production to small batches made in labs. The MIT team is confident that if graphene membranes are to commercialized, they will have to be made in large quantities, at high rates, and with reliable performance.
“We know that for industrialization, it would need to be a continuous process,” Hart says. “You would never be able to make enough by making just pieces. And membranes that are used commercially need to be so big you would have to send a poster-wide sheet of foil into a furnace to make a membrane.”
The team’s innovative setup combines a roll-to-roll approach, a common industrial approach for continuous processing of thin foils. Two spools are connected by a conveyor belt that runs through a small furnace. The first spool unfurls a long strip of copper foil, less than a centimeter wide. When it enters the furnace, the foil is fed through first one tube and then another, in a “split-zone” design.
While the foil rolls through the first tube, it is heated to a certain temperature and then rolls through the second tube. There, it is exposed to a specified ratio of methane and hydrogen gas, which are deposited onto the heated foil to produce graphene.
“Graphene starts forming in little islands, and those islands grow together to form a continuous sheet,” explains Hart. “By the time it’s out of the oven, the graphene should be fully covering the foil in one layer.”
As the graphene exits the furnace, it’s rolled onto the second spool. Researchers could feed the foil continuously through the machine, creating graphene at 5 centimeters per minute. Their longest run lasted almost four hours, during which they turned out about 10 meters of continuous graphene.
“If this were in a factory, it would be running 24/7,” Hart says. “You would have big spools of foil feeding through, like a printing press.”
After using their roll-to-roll method to make graphene, the researchers unwound the foil from the second spool and cut small samples. They cast the samples with a polymer mesh, or support, and etched away the underlying copper.
The process consists of a “roll-to-roll” system that spools out a ribbon of copper foil from one end, which is fed through a furnace. Methane and hydrogen gas deposited on the foil form graphene, and the foil then exits the furnace and is rolled up for further development. (Image courtesy of the researchers)
“If you don’t support graphene adequately, it will just curl up on itself,” Hart says. “So you etch copper out from underneath and have the graphene supported by a porous polymer.”
The polymer covering contains holes larger than graphene’s pores, which act as microscopic “drumheads,” keeping the graphene sturdy and its nanopores open.
The researchers performed diffusion tests with the graphene membranes, flowing a solution of water, salts, and other molecules across each one. The membranes withstood the flow while filtering out molecules. Their performance was comparable to graphene membranes made using conventional, small-batch approaches.
The team also ran the process at different speeds, with different ratios of methane and hydrogen gas, and characterized the quality of the resulting graphene after each run. The researchers drew up plots to show the relationship between graphene’s quality and the speed and gas ratios of the manufacturing process. If other designers build similar setups, they can use the team’s plots to identify the settings needed to produce a certain quality of graphene.
The new process gives engineers a great degree of flexibility in terms of what they want to tune the graphene for, all the way from electronic to membrane applications. Looking forward, Hart says he would like to find ways to include polymer casting and other steps currently performed by hand in the roll-to-roll approach.
“In the end-to-end process, we would need to combine more operations into the manufacturing line,” he notes. “For now, we’ve demonstrated that this process can be scaled up, and we hope this increases confidence and interest in graphene-based membrane technologies, and provides a pathway to commercialization.”