Graphene was meant to transform everything from the car tyre to the condom. There has never been so much hype around a new material. It’s easy to see why: its sheets of carbon atoms are incredibly strong, super-elastic and conduct heat better than most metals. The problem with graphene, however, is that it has yet to live up to its expectations commercially.
When it was discovered in 2004 by two professors at the University of Manchester, Andre Geim and Kostya Novoselov, graphene was hailed as a wonder material that could disrupt many industries.
The academics, who have since won the Nobel Prize for their work, produced the material during a Friday night experiment by playing around and using Scotch tape to create ultra-thin flakes from a chunk of graphite. And, although graphene proved to be the thinnest material known to scientists and just a single atom thick, it was also found to be 200 times stronger than steel.
The latest development is that graphene could revolutionise smartphone batteries. Researchers at the Samsung Advanced Institute of Technology found that, when a lithium-ion battery was coated with the material, its capacity was increased and charging was five times faster – the findings were published in the journal Nature Communications in November. However, the researchers admit that the challenge will be mass-production, and it could be some time before people are walking around with graphene-coated batteries in their pockets.
While on paper graphene has the qualities that, collectively, should make it an ideal material to work with, the challenge is that defect-free graphene is generally too expensive to make. Accurate data is difficult to get hold of, but the price of the material can vary depending on the production conditions, and the methods for producing it in bulk aren’t entirely cost effective.
“When graphene was first isolated by the team in Manchester, it came from high-quality graphite but was peeled off layer by layer using tape,” explains Krzysztof Koziol, professor of composites engineering at Cranfield University and head of its Enhanced Composites and Structures Centre. “This is a perfectly good method to get a few flakes in a lab environment, but not really suitable for significant volume generation.”
Researcher Antonios Oikonomou at the University of Manchester (Credit: University of Manchester/ NGI)
Although mechanically exfoliated graphene, like that produced by the
Scotch tape technique, has the best physical properties, individual flakes acquired through this method are too expensive for bulk production, adds Koziol.
One of the main techniques used instead is chemical vapour deposition (CVD), a relatively straightforward process that involves growing the graphene on a substrate, typically copper foil. Here, the price is dependent on the volume of the material being produced, and the cost of transferring it from the substrate. Although industrial technology, such as a patented graphene transfer method developed by leading producer Graphenea, is helping to reduce the cost of the process, CVD also has its flaws.
“The problem is that, when you exfoliate graphene mechanically through force or by taking a chemical-based approach, you can introduce defects into the structure of the material,” says Koziol. “With the CVD technique, harmful acids might be used to dissolve the substrate and separate it from the graphene. You can end up creating graphene oxide which is damaged to such an extent that it’s no longer electrically conductive.”
A greener graphene
In order for graphene to have an impact commercially and be used to develop better-quality products, such as transistors, it not only needs to be cost-effective, but also environmentally friendly. That’s the thinking behind new research from the University of Illinois at Urbana-Champaign’s department of mechanical science and engineering, where academics have discovered a cleaner and greener way to manufacture graphene and isolate it from its substrate. The magic ingredient? Carbonic acid, as found in fizzy drinks and champagne.
“Current strategies to produce graphene use and waste copious amounts of harsh chemicals and also waste high-purity deionised water. Not only that, they expend the copper substrate and contaminate the graphene with chemical residue that needs to be rinsed off,” says Michael Cai Wang, a PhD student who led the research project, the findings of which were published in the Journal of Materials Chemistry C earlier this year.
“By using carbonic acid as a conducting solution, this allowed us to delaminate the graphene from the copper substrate. The carbonic acid evaporated away as carbon dioxide and water vapour, meaning that the graphene didn’t have to be cleaned. We were also able to reuse the substrate, which of course could have a big impact on cost savings if the transfer process was ever scaled up,” adds Wang.
To prepare the graphene so that the process, which Wang likes to call “pantry chemistry-based manufacturing,” was environmentally friendly, he and his fellow researchers coated the material with a food-grade ethyl cellulose. This biologically-derived coating can be found in pill capsules and is used as an anti-clumping agent in Parmesan cheese.
They found that it could replace the conventional thin film that graphene is normally coated with during the CVD process, and which requires the use of solvents that can be carcinogenic and toxic. In doing so, they also discovered that it was compatible with other materials that aren’t able to tolerate harsh solvents.
Wang hopes that the research shines a spotlight on the need for cleaner transfer processes in graphene production. As demand for graphene increases in the future, so will the need for faster production methods. Yet sustainable manufacturing is also an important factor that should be taken into consideration, he says.
One company that has struck a balance between a production process that is cost-effective, fast and green is Cambridge Nanosystems, which Koziol co-founded in 2012. With the help of a £500,000 grant from the Technology Strategy Board, he and fellow University of Cambridge alumnus, Catharina Paukner, developed a method of creating graphene free of substrates and solvents, using methane gas.
“The process uses a unique plasma reactor to first break the feedstock [methane gas] into hydrogen and elemental carbon atoms, and then these atoms are recombined into graphene sheets by floating them in the hydrogen atmosphere,” explains Koziol. “The time it takes for methane gas to enter the plasma reactor to the point when graphene is formed is less than a second.”
According to Koziol, the chemical-free nature of the process results in very pure graphene. “Imagine a snowflake falling on the ground. In our reactors we have pure graphene flakes falling in the collection chamber,” he effuses.
Universal standards needed
As the work of Wang and Koziol demonstrates, graphene production is full of opportunities and potential. However, this in itself presents a stumbling block, because different methods of production mean there are varying degrees of quality in graphene being produced. This can be confusing for end users and, to some degree, is hindering uptake of the material.
To address the ambiguity, the National Graphene Institute (NGI) at the University of Manchester has partnered with the National Physical Laboratory to draw up a set of standardised measurements. In November, the two organisations published their first guide, entitled Characterisation of the Structure of Graphene, aimed at informing producers on how to reliably measure the structural properties of the material.
“Standardisation will help accelerate uptake. It will give users assurance over the quality and consistency of the properties of the graphene they’re buying,” says Antonios Oikonomou, who was a research associate at the NGI until September. His work was instrumental in setting the wheels in motion for establishing universally recognised standards.
Failing to address the issue could be critical and restrict the use of graphene to the R&D stage.
On the other hand, an awareness of the material’s structure, characteristics and potential defects could give researchers and innovators a much-needed shot in the arm and the confidence to scale-up and bring products to market, says Oikonomou, who has co-founded Eksagon, a company exploring graphene-based energy generation, storage and conversion technologies.
Kostya Novoselov shows former Chancellor George Osborne a graphene light bulb (Credit: University of Manchester/ NGI)
It wasn’t until 2015, more than a decade after graphene was discovered, that the NGI unveiled the first graphene-enhanced commercial product: an LED light bulb. Produced by the aptly-named Graphene Lighting, a company spun out of the institute, the bulb’s filament is coated with graphene, giving it greater conductivity and increased efficiency.
“The main advantage of using graphene lies in its ability to transfer heat more efficiently. LEDs produce a large amount of heat over a small area and, unless this heat is removed, the temperature rises and leads to significant degradation in the LEDs’ performance,” says Stephen Bignell, a director at UK lighting manufacturer Sera Technologies, which recently became one of the first adopters of the graphene technology anywhere in the world. “By transferring the heat away more efficiently, graphene improves the light output of LED lamps and extends their life expectancy.”
Bignell sees no reason why graphene-enhanced bulbs won’t be a commercial success, particularly as they will be sold at roughly the same price as non-graphene models.
As for other products, it could be five, 10 or 20 years before the graphene revolution arrives on shelves for consumers to buy. But they will come, it’s just a question of when. Graphene will not just remould manufacturing processes, says Koziol, it will also improve the performance of everyday objects.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.