Today, most graphite — a form of carbon used in electronics and batteries — comes from environmentally damaging mining or a synthetic process that relies on crude oil. Both ways of obtaining graphite have a large carbon footprint and energy demand.
Now, researchers at the University of Chicago Pritzker School of Molecular Engineering have invented a new method to produce graphite from charred plant material. Their approach, developed in collaboration with scientists from Northwestern University and University of Illinois Urbana-Champaign, is described in the journal Small.
“Graphite plays a critical role in lots of things that we use every day, including batteries, so making sure that we can produce it sustainably is really important,” said Stuart Rowan, the Barry L. MacLean Professor for Molecular Engineering Innovation and Enterprise at UChicago Pritzker Molecular Engineering and a staff scientist Argonne National Laboratory. “If we can convert biomass into graphite, it gives us almost infinite access.”
The new method uses a plant-based “biochar” material that is otherwise thrown away by manufacturers to produces graphite with a significantly smaller impact on the environment than other methods.
Crystals of carbon
Graphite is composed of many layers of carbon arranged in stacked, hexagonal patterns. Compared to other forms of carbon, graphite is softer but can conduct electricity, giving it important applications in electronics, energy storage and materials science. For many of these applications, however, graphite must be formed in perfect crystals – something hard to do when starting with the disordered carbon found in plant material.
With the ultimate goals of both creating a more sustainable source of graphite, and capturing carbon that would otherwise be in the environment, Rowan and collaborators set out to improve methods of producing graphite from plants. Their program, known as MADE-PUBLIC (for Manufacturing ADvanced Electronics through Printing Using Bio-based and Locally Identifiable Compounds) was funded by the National Science Foundation’s Future Manufacturing Program.
About half of a plant’s dry mass is carbon and, in the past, researchers have tried to use plant material to create graphite. However, the resulting graphite has been of too low quality to be useful in high-tech applications.
Rather than start with whole plants, Rowan’s group turned to a material that is usually discarded by manufacturers of the eco-friendly fuel known as bio-oil. The waste, known as biochar, is a dense and carbon-rich substance left behind when plant oils are removed.
“Starting with this biochar gives us a much more concentrated source of carbon,” said UChicago PME graduate student Haoyang You, the first author of the new paper. “It also lets us generate a high-value product from what is essentially throw-away waste right now.”
Finding the critical temperature
To coax disordered carbon to organize into the strict hexagons of graphite, researchers often turn to iron. When a carbon-iron mixture is heated and then cooled, the carbon organizes itself into layers on the iron’s surface.
In their new work, Rowan, You and their colleagues discovered that cooling this mixture more slowly than usual led to larger, more organized crystals of graphite. Their bio-graphite, cooled over about 8 hours rather than the more standard 3 hours, contained crystals that were five times thicker and more than 15 times wider than the crystals in older methods.
In addition, calculations carried out by the group showed that their graphite production method, compared to other bio-graphite approaches, had a far lower fossil fuel demand and led to less greenhouse gas emissions.
“Not only is this method better than other bio-graphite preparation methods, but it actually beats out the current natural graphite preparation and synthetic graphite preparation when it comes to its environmental impact,” said You.
Inks and more
To test the utility of their bio-graphite, the group used it to produce graphene inks, which are used to print sensors and other small electronics. They showed that the resulting inks had a higher conductivity than inks made from other bio-graphite and could be successfully used in creating functioning electronics.
To use their graphite in other applications, such as batteries, the research team will need to continue refining their methods to yield even larger crystals of graphite – something they are already working on. They also plan to work on cutting costs to the bio-graphite production method as they scale it up.
The U.S. has designated graphite a critical mineral for national security due to its use in the defense, energy, aerospace, communications, and transportation industries. Despite an increasing demand for graphite, the US is currently reliant on other countries for nearly all its graphite.
“Our work is a big first step toward getting a more sustainable source of graphite and relieving the U.S. of some of its supply chain issues around batteries and other electronics,” said Rowan. “We are excited to keep working on this project.”
Citation: “Sustainable Production of Biomass-Derived Graphite and Graphene Conductive Inks from Biochar,” You et al, Small, October 22, 2024. DOI: 10.1002/smll.202406669
Funding: This work was primarily supported by the National Science Foundation MADE-PUBLIC Future Manufacturing Research Grant Program (NSF Award Number CMMI-2037026).