Modification of Ti3C2Tx MXene with hyperbranched polyethylene ionomers: stable dispersions in nonpolar/low-polarity organic solvents, oxidation protection, and potential application in supercapacitors

Abstract

MXenes have attracted enormous research interest due to their 2D structure and advantageous properties. However, they show low/poor dispersibility in a wide variety of organic solvents, particularly nonpolar or low-polarity solvents, and tend to get oxidized in the presence of oxygen and water during storage, handling, delamination, and applications under ambient conditions. In the present study, we report a simple yet convenient noncovalent modification strategy with the use of hyperbranched polyethylene ionomers containing quaternary ammonium ions to prepare stable, highly concentrated Ti₃C₂T_x MXene dispersions in various nonpolar and low-polarity organic solvents. Constructed with nonpolar hydrophobic hyperbranched polyethylene backbones, the ionomers contain covalently tethered quaternary ammonium cations, which can bind tightly onto the negatively charged Ti₃C₂T_x MXene surface by ionic interactions. Simple mixing provides the ionomer-modified MXenes (I-MXenes) with the hyperbranched polyethylene ionomers efficiently intercalated within the MXene sheets. Accordingly, the interlayer spacing of Ti₃C₂T_x MXene can be expanded to more than 5 nm with an increase of at least 400% compared to the original spacing of 1 nm. The surface modification effectively adjusts the surface properties of MXene sheets and facilitates their compatibility with various nonpolar or low-polarity organic media. The resulting modified MXenes are readily stably dispersible in a broad range of nonpolar or low-polarity organic solvents, including tetrahydrofuran (THF), chloroform, xylene, and toluene, at high concentrations (as high as 30 mg mL⁻¹ in THF) after simple sonication. Meanwhile, the modification also markedly improves the oxidation stability of MXene sheets due to the presence of the tightly surface-bound hydrophobic hyperbranched polyethylene protecting layer. In contrast to the easy oxidation of unmodified MXenes, the ionomer-modified MXenes are stable in air-saturated water even for weeks with no/minimum oxidation. Subsequently, a symmetrical two-electrode supercapacitor made from an I-MXene suspension in THF exhibits a capacitance of 220 F g⁻¹ at a scan rate of 2 mV s⁻¹. This strategy thus facilitates the processing of MXenes in the form of organic dispersions and greatly expands their opportunities for different applications in various areas, particularly in electrochemical energy storage devices.