A new breakthrough at MIT enables researchers to continuously mass-produce high-quality graphene sheets for membrane applications. This breakthrough will help bridge the gap between R&D and commercialization, enabling new products and technologies in the coming years.

What is graphene?

Graphene is a carbon allotrope consisting of a two-dimensional layer of carbon atoms arranged in a hexagonal lattice structure. It has semimetal characteristics, which arise due to a small overlap of conduction and valence bands. As such, graphene possess many unique properties, including exceptional mechanical strength, electrical conduction, and a customizable nanometer-scale pore in the hexagonal lattice. Applications of graphene span a wide-range of industries, including, but not limited to, fuel cells, semiconductors and microelectronics, dialysis, desalination, and more.

Current manufacturing process:

Graphene is presently manufactured using a technology called chemical vapor deposition (CVD). CVD utilizes a carbon source, such as methane gas, to produce graphene. Petroleum asphalt is another source that is used. Other materials (“catalysts”) are added during the process to alter the physical properties of the graphene produced. Notable catalysts include iron nanoparticles, nickel foam, and gallium vapor.

In the semiconductor industry, CVD of graphene on polycrystalline copper foil is the preferred approach for producing high-quality monolayer graphene. However, outside of this industry, optimizing the graphene CVD process (for example, in membrane applications) is only feasible under strict  laboratory conditions, and it remains commercially unviable. This is due to quality requirements of graphene used in the semiconductor industry that is significantly different from the quality used in other applications. For example, sub-nanometer defects in graphene material can be overlooked in micrometer-sized electronic devices. However, the presence of such defects could adversely affect permeability/barrier properties (by allowing for leakage or much higher flows) in membrane applications, such as fuel cells or desalination processes.

A novel continuous graphene membrane manufacturing process:

To industrialize graphene manufacturing for membrane applications, the MIT engineers setup a system encompassing two spools, linked by a conveyor belt that runs through a small furnace (see figure & video). The first spool unwinds a long piece of copper foil, less than 1 cm wide. When it enters the furnace, the foil is rolled first through one tube and then another, in a ‘split-zone’ design.

When the foil feeds through the first tube, it heats up to an ideal temperature, at which point it is ready to fed through the second tube. At this point, the scientists pump in a specified ratio of methane and hydrogen vapors, which are deposited onto the heated copper to produce graphene.

As the graphene leaves the furnace, it is rolled onto the second spool. The researchers found that they were able to feed the foil continuously through the system, producing high-quality graphene membranes at a speed of 5 cm per minute. Their longest run was almost 4 hours, during which they produced about 10 m of continuous graphene. They also tested the process at different speeds, pumping different ratios of methane and hydrogen gases to investigate the difference in the quality of the synthesized graphene after each run.

What’s next?

Researchers are carrying out diffusion tests with the produced graphene membranes, such as running a solution of water, salts, and other molecules across each membrane. Preliminary results indicate that the membranes resist the flow while filtering out molecules and the performance was comparable to the graphene membranes made using conventional, small-batch laboratory methods. The MIT team is making their results available to other designers in the industry looking to identify the parameters they would require to produce the quality of graphene sheets for industrial membrane applications. In addition, they are finding ways to include polymer casting and automate other steps that are currently performed by hand. According to the team, upgrading these functionalities in the process would pave a pathway for commercialization in the future.

Featured image courtesy of Christine Daniloff, MIT.

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