Water-based and tannin-assisted liquid-phase exfoliation for a sustainable production of graphene

Graphene is a material with high potential in the development of advanced applications, such as 3D printing. In the current scenario, it is obvious that technological progress should align with sustainability, therefore, it is necessary to search for efficient and sustainable alternatives to the liquid phase exfoliation of graphene. In this work, chestnut tannins were employed as surfactants for graphene exfoliation in aqueous media, via ultrasound exfoliation and high-shear exfoliation. The effect of the initial concentration of graphite, the initial concentration of tannin and the exfoliation time over the concentration of the obtained graphene suspensions and mass yield was assessed, showing that comparable values to NMP based exfoliation were obtained. Obtained graphene flakes were morphologically characterized by Raman spectroscopy and Transmission Electron Microscopy, and results showed that high-shear exfoliation resulted in graphene flakes with the highest ID/IG ratio and more homogenous thickness. Life Cycle Assessment of the exfoliation processes revealed the environmental benefits of water-based exfoliation processes compared to traditional process based on NMP solvent. A sensitivity analysis showed that environmental impacts could be reduced if renewable energy sources are considered. High-shear exfoliation with initial concentration of graphite of 5 mg·mL−1, tannin concentration of 1 mg·mL−1 and shearing time of 15 min emerged as the most balanced choice between quality and sustainability.