Long-term stable solid concentrated graphene dispersion assisted by a highly aromatic ionic liquid

The sonochemical exfoliation of graphite in solution has been demonstrated as a promising and easy technique for producing graphene dispersions. This is usually done in organic solvents and leads to unstable dispersions with very low graphene concentration. Ionic liquids (ILs) represent a versatile and safe alternative to traditional organic solvents. A few recent studies reported the use of commercial ILs with bulky anions, such as bis(trifluoromethylsulfonyl)imide, and aromatic cations, such as imidazolium, which favour the exfoliation of graphite through π-π and cation-π interactions. Although recently investigated, the role of aromatic groups on imidazolium cations is still controversial and systematic studies are still necessary. Besides, these studies were limited to liquid dispersions at room temperature.

Herein, we prepared four highly aromatic imidazolium-based ILs, including the newly reported 1-(naphthylmethyl)-3-benzylimidazolium bis(trifluoromethanesulfonyl)imide, [(Np)(Bn)im][NTf₂]. These ILs were used for the sonochemical exfoliation of graphite and compared with a commercial benchmark, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][NTf₂].

Interestingly, [(Np)(Bn)im][NTf₂] allowed reaching solid dispersions at room temperature containing thin few layer graphene sheets with long-term stability (up to 2 years) and high concentration (3.6 mg/mL). Such graphene dispersion combines long-term stability in the solid-state and high processability in the liquid state, by a simple heating above 60 °C.