Graphene is a nanomaterial consisting of carbon atoms which are made up of a single layer of atoms arranged in a two-dimensional honeycomb lattice. The name is derived from graphite and the suffix -ene and describes the nature of the graphite allotrope of carbon which contains numerous double bonds. In a graphene sheet, each atom is connected to its three nearest neighbours by a strong σ-bond and contributes one electron to a valence band which spreads over the whole sheet. The valence band is connected to a conduction band. This makes graphene a semimetal with unusual electronic properties that are best described by theories for massless relativistic particles.
Graphene is a valuable and useful nanomaterial which is not least due to its exceptionally high tensile strength, electrical conductivity and transparency. It is in high demand in industrial areas concerning, for example, semiconductors, electronics, electric batteries etc.
To further improve the production process of graphene, from 2019 to 2021 scientists launched a project whose aim it was to find a method to use flash Joule heating (FJH) on anthracite coal and produce high value graphene in minimal amounts in less than 1 second per conversion step. The method included various approaches, such as electrothermally converting various carbon sources, e.g. carbon black, coal, and food waste into micron-scale flakes of graphene. In more recent studies the use of mixed plastic waste, waste rubber tires, and pyrolysis ash as carbon feedstocks was tested. The graphenisation process was kinetically controlled, and the energy dose was chosen in a manner in order to preserve the carbon in its graphenic state, as excessive energy input would cause subsequent graphitization through annealing.
The process the scientists devised was remarkable in itself because no furnaces or solvents or reactive gases were used. The anthracite coal was produced at a quantity of 85-95% and had a purity of about 99%. Flash graphene is the lowest defect graphene and also turbostratic by powder X-ray diffraction (XRD) analysis which means that little order can be witnesses between the graphene layers. This circumstance facilitates its dispersion in composites. The turbostratic FG (tFG) had a rotational mismatch between neighbouring layers. These tFG sheets exhibited moiré patterns, large-scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar pattern, which were analysed through TEM imaging. TEM imaging also revealed that what remained of the FG was made up of wrinkled graphene sheets that resemble non-graphitising carbon. To generate high quality tFG sheets, a FJH duration of 30 – 100 ms was employed. Beyond 100 ms, the turbostratic sheets had enough time to AB-stack and form bulk graphite. Theoretical simulations showed that wrinkled graphene was mainly formed because of thermal annealing and exhibited almost no alignment of graphitic planes. The situation was different with high-quality tFG which was formed under the direct influence of current conducted through the material. The tFG was easily exfoliated via shear, hence the FJH process has the potential for bulk production of tFG without the need for chemical exfoliation or high energy mechanical shear.
Improving materials through flash Joule heating has been afforded a lot of scientific interest by many research institutions around the globe in the past few years. In May 2022, scientists demonstrated the usefulness of graphene as a reinforcing agent in automotive polyurethane foam composites. It was found that graphene was responsible for improved tensile strength and low frequency noise absorption properties. The continuity of the process was demonstrated by upcycling the resulting foam composite back into equal-quality flash graphene. Through a prospective cradle-to-gate life cycle assessment it was shown that the method might use lower cumulative energy and water and decrease the global warming compared to traditional graphene synthesis methods. Also, a life cycle assessment suggested environmental benefits compared to other graphene synthetic routes.
Image: Process schematic, workflow, and current discharge for FJH conversion of ELV-WP into FG. a: Block diagram of the custom designed dual capability FJH station for low current (LC) and high current (HC) discharge, with a working procedure displayed below. b: A typical current discharge profile of the FJH procedure to convert ELV-WP into ELV-WP-FG
Source: Kevin Wyss, Robert De Kleine, Rachel Couvreur, Alper Kiziltas/ Upcycling end-of-life vehicle waste plastic into flash graphene/ Communications Engineering 1, Article number: 3 (2022), 26 May 2022/ DOI:10.1038/s44172-022-00006-7/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)
Also in 2022, scientists performed a detailed analysis of graphite material before and after recovery by multiple means of characterisation and found that the regenerated graphite displays electrochemical properties nearly the same as new graphite. Flash Joule heating provided a large current for defect repair and crystal structure reconstruction in graphite. It also allowed for the solid electrolyte interphase coating to be removed during ultra‐fast annealing. The electric field controlled the conductive agent and binder pyrolysis products and made them form conductive sheet graphene and curly graphene covering the graphite surface. Thus, the recycled graphite became even better than new commercial graphite in terms of electrical conductivity. Regenerated graphite was shown to have good multiplier performance and cycle performance. This FJH method was not only universal for the regeneration of spent graphite generated by various devices but also enabled multiple use‐failure‐regeneration steps of graphite, showing great potential for commercial applications.
Image: Schematic illustration of FJH discharge equipment and regeneration of spent graphite electrode materials
Source: Shu Dong, Yali Song, Ke Ye, Jun Yan/ Ultra‐fast, low‐cost, and green regeneration of graphite anode using flash joule heating method/ EcoMat 4(5), April 2022/ DOI: 10.1002/eom2.12212/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)
Using flash Joule heating to produce graphene has many advantages: Graphene is a versatile material that could open up new markets and even replace existing technologies or materials. Graphene is extremely thin and also very strong. It consists of a single layer of carbon atoms and is transparent. It is a very good conductor of both heat and electricity. Graphene is used in the production of high-speed electronic devices as well as membranes for more efficient separation of gases. It can be employed in transistors that operate at higher frequency. Graphene has enabled the production of low-cost displays in mobile devices by replacing the indium-based electrodes in organic light emitting diodes (OLED). Moreover, it can be used in the production of lithium-ion batteries and make them recharge faster. These batteries use graphene on the anode surface. Finally, graphene can be used to store hydrogen for fuel cell powered cars.
The outcome of the project showed that flash graphene might be a solution to the problem of producing graphene in bulk at reasonable economical parameters. Their approach used inexpensive coal, furnace-free, solvent-free and chemical-free processing, and low-energy input to render it into a product suitable for bulk plastic, metal and even concrete composites. There is hope that this process will soon be ready for large-scale application.