Scalable production of graphene slurry

4 Apr 2018. NUS chemists have developed an economical and industrially scalable method to produce high concentration graphene slurry for 3D printing.

Graphene is a two-dimensional material made of honeycomb sheets of carbon atoms which are just one atom thick. It exhibits unique electronic properties which can find potential uses in a wide variety of applications (such as touch screens, transparent electrodes and batteries). However, the large-scale application of graphene is in part hampered by its considerable production cost. Conventional dispersion methods of producing graphene uses sonication (applying sound energy to agitate the sample) or the application of shearing forces to peel off graphene flakes from graphite and disperse them in an organic solvent. As graphene flakes have low solubility (1 mg mL^-1), they remain suspended in the solvent. This production method is environmentally unfriendly and costly as a large amount of solvent is needed. Typically, the production of one kg of graphene by such dispersion methods requires the use of at least one ton of solvent. If there is insufficient solvent, the exfoliated graphene may reattach itself back to the graphite due to van der Waals interactions, therefore affecting the efficiency of the exfoliation process.

A collaborative team led by Prof LOH Kian Ping from the Department of Chemistry, NUS and Prof LU Hongbin from Fudan University has developed a more efficient method for exfoliating graphite which uses up to 50 times less solvent compared to conventional dispersion methods. This method is suitable for mass production. The research team achieved this by exfoliating pre-treated graphite under a highly alkaline condition which causes the exfoliated graphene to undergo a flocculation process. The flocculation process is similar to wastewater treatment, in which coagulants are added to precipitate out impurities. This means that the exfoliated graphene, which is usually suspended in the solvent, forms graphene flakes that then settle at the base of the reaction tank. The exfoliation process can then continue without the need to increase the solvent usage. The presence of absorbed ions on the flocculated graphene flakes introduces electrostatic repulsive forces to prevent re-stacking and this enables it to be separated easily into monolayers when required.

Although this method operates under high alkaline conditions, the higher yield and significant reduction in the amount of solvent required makes it attractive for industrial purposes.

Prof Loh said, “This water-phase, non-dispersion method is able to produce highly crystalline graphene flakes in the form of a concentrated slurry (50 mg mL^-1) or filter cake, which can be stored for months and directly used as printing inks to make functional materials such as conductive aerogels and graphene-polymer composites.”

Figure shows the dispersion of graphene in aqueous solution with pH ranging from 1 to 14. At pH level 1 to 8, the graphene concentration is low and the vial appears transparent. At pH level 9 to 12, the vials are filled with graphene which has a dark colour. This is the conventional way of graphene exfoliation. Due to the low solubility of graphene, overall exfoliation efficiency is low (more solvent has to be added to exfoliate more flakes from the graphite in the vial). At pH level 13 to 14, flocculation occurs and exfoliated graphene in “solid” form settles at the bottom of the vials which can be removed by filtration and centrifugation without the need to add more solvent. These flocculated flakes can be re-dispersed into monolayers easily by adding water.

Reference

Lei D; Chen Z; Zhao X; Ma J; Lin S; Li M; Bao Y; Chu L; Leng K; Lu HB*; Loh KP*, “A non-dispersion strategy for large-scale production of ultra-high concentration graphene slurry in water” NATURE COMMUNICATIONS Volume: 9 Pages: 14548 (1-9) DOI: 10.1038/ ncomms14548 Published: 2018.