Coal is often seen as a negative due to its role in contributing to climate change, but researchers at Ohio University have found ways to transform it into useful, environmentally-friendly materials like graphite and carbon nanotubes. The team used the Bridges-2 system at the Pittsburgh Supercomputing Center to run simulations exploring this possibility.
Coal has received criticism due to its negative impact on the environment and its role in climate change. Many scientists believe that burning fossil fuels like coal will lead to an increase in average global temperatures, potentially causing significant changes in weather patterns, agricultural production, and sea levels.
However, researchers at Ohio University are working on ways to utilize coal in a more environmentally-friendly way. David Drabold, a physics professor at the university, explained that they are exploring the potential to convert coal into valuable construction materials like graphite rather than burning it. He and a graduate student are specifically interested in finding out if it is possible to produce graphite from coal.
Switching to electric vehicles and using carbon-neutral energy sources to power them can help to reduce carbon emissions. However, the lithium-ion batteries used in electric vehicles, like Tesla's Model S, require a large amount of graphite. It has long been known that it can transform coal into graphite by applying high pressure and temperature.
Using Pittsburgh Supercomputer
David Drabold and his team at Ohio University are using computer simulations to study the potential for converting coal into valuable materials like graphite. To do this, they are using the Bridges-2 supercomputer at the Pittsburgh Supercomputing Center to recreate the chemical conversion process virtually.
Pure graphite comprises sheets of six-carbon rings held together by aromatic bonds, a specific type of chemical bond. The pi electrons in these bonds create a "slippery" effect that allows the sheets to slide easily past one another. This is why low-grade graphite forms, like pencil "lead," can leave a mark on paper when the sheets rub off onto the surface.
One of the key benefits of aromatic bonds is that they allow for the easy movement of pi electrons from ring to ring and sheet to sheet, making graphite a good conductor of electricity despite not being a metal. This characteristic makes graphite ideal for a battery's use as the anode or positive pole. In contrast, coal is chemically complex and contains various atoms, such as hydrogen, oxygen, nitrogen, and sulfur, that can disrupt the formation of graphite. It also has connections in three dimensions, whereas graphite is strictly two-dimensional.
Kickstarting the Analysis
To start their research, Drabold's team created a simplified version of coal made only of carbon atoms in random positions. They then subjected this simplified coal to high pressure and temperature (around 3,000 Kelvin or nearly 5,000 Fahrenheit) to study its potential conversion into graphite. To analyze the process and create an "amorphous-graphite paper," they relied on the fast and accurate capabilities of the Bridges-2 supercomputer. According to Chinonso Ugwumadu, a physics doctoral student at Ohio University working with Drabold, their home systems would have taken around two weeks to simulate 160 atoms, while the Bridges system could run 400 atoms in just six to seven days using density functional theory.
Initially, the Ohio researchers used density functional theory, based on basic physical and chemical principles, to run their simulations on the Bridges-2 supercomputer, which has many computing cores and is well-suited to carrying out parallel computations. Later, they switched to using GAP (Gaussian approximation potential), a software tool developed by researchers at the University of Cambridge and the University of Oxford in the UK that utilizes machine learning to perform similar calculations more efficiently. Graduate students Rajendra Thapa and Ugwumadu took turns leading the initial computational work.
The results of the simulations were somewhat unexpected, as the carbon atoms did not form simple, six-carbon rings as expected. Instead, a fraction of the rings had five or seven carbons, which caused the rings to "pucker" in opposite ways (i.e., one with positive curvature and the other with negative curvature). The researchers initially thought these puckers would prevent the formation of graphite sheets, but they could form anyway, possibly due to the balance of pentagons and heptagons in the simulations. The resulting sheets were technically considered "amorphous graphite," as they were not made up entirely of six-ringed carbon atoms. However, they still form in layers.
Simulation Series and Outcome
In a separate series of simulations, Ugwumadu explored how molecules rather than solids would behave under the same conditions. The simulations resulted in the sheets curving in on themselves to form nested, amorphous carbon nanotubes (CNTs) made up of single-atomic-layer tubes nested inside one another. CNTs have been of interest in materials science due to their potential use as tiny wires for conducting electricity at small scales and for various other applications such as fuel cell catalysis, production of supercapacitors and lithium-ion batteries, electromagnetic interference shielding, biomedical sciences, and nano-neuroscience. Ugwumadu also studied how the amorphous wrinkles on the walls of the CNTs affected the movement of electricity through the structures.
These irregularities could potentially be utilized by engineers to fine-tune the behavior of CNTs to meet the specific requirements of electronic devices. The scientists posted their developments in two reports, one on the appearance of amorphous graphite sheets in Physical Review Letters in June 2022 and the other on CNTs in Physica Status Solidi B published last December 2022. Another paper discussing the role of five- and seven-membered rings in forming the sheets is in press in the European Journal of Glass Science and Technology.
The Ohio team continues to work on simulating the conversion of carbon atoms into graphite and related materials, including nested, amorphous fullerenes, which are soccer-ball-shaped structures of interest in nano-neuroscience. They also published a paper on fullerenes in November 2022 in Carbon Trends. The team is also exploring using graphics processing units on the Bridges-2 system to potentially speed up their VAST computations using machine learning, intending to make more complex materials like real-world coal accessible to their simulations.
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