Two-dimensional (2D) materials such as graphene and molybdenum disulfide (MoS2) have been investigated widely for applications in energy storage, including supercapacitors, due to their high specific surface area, potential redox activity, and mechanical flexibility. However, electrodes comprising either pure graphene and MoS2 have failed to reach their potential due to restacking of the layered structure and poor electrical conductivity. It has been shown previously that composite electrodes made from a mixture of graphene and MoS2 partially counteract these issues; however, performance is still limited by poor mixing at the nanoscale. Herein, we form a true composite electrode by chemically functionalizing the graphene so that the negatively charged surface can self-assemble with the positively charged 1T-MoS2 to give an alternating layer structure. These alternately restacked 2D materials were then used to produce supercapacitor electrodes, and their energy storage properties were characterized. This stacked structure has increased the interlayer spacing of 1T-MoS2 which was indicated by the increase in the intensity of the (001) peak in the X-ray diffraction spectra. Furthermore, the typically metastable 1T-MoS2 was stabilized by the interaction with the functionalized graphene, preventing it reverting back to the 2H phase, which was observed when pristine graphene was used. The graphene was functionalized using either 4-bromobenzenediazonium or 4-nitrobenzenediazonium, with the latter giving optimal capacitance when mixed with the MoS2. The alternative layer graphene–MoS2 structure was confirmed by Raman spectroscopy and electron microscopy, leading to a high specific capacitance (290 F cm–3 at 0.5 A g–1) and 90% retention of capacitance after 10 000 cycles.

Transitional metal carbides and nitrides (MXenes) have promise for incorporation into multifunctional composites due to their high electrical conductivity and excellent mechanical and tribological properties. It is unclear, however, to what extent MXenes are also able to improve the mechanical properties of the composites and, if so, what would be the optimal flake size and morphology. Herein, Ti 3 C 2 T x MXene is demonstrated to be indeed a good candidate for mechanical reinforcement in polymer matrices. In the present work, the strain-induced Raman band shifts of mono-/few-/multilayer MXenes flakes have been used to study the mechanical properties of MXene and the interlayer/interfacial stress transfer on a polymer substrate. The mechanical performance of MXene was found to be less dependent upon flake thickness compared to that of graphene. This enables Ti 3 C 2 T x MXene to offer an efficient mechanical reinforcement to a polymer matrix with a flake length of >10 μm and a thickness of 10s of nanometers. Therefore, the degree of exfoliation of MXenes is not as demanding as other two-dimensional (2D) materials for the purpose of mechanical enhancement in polymers. In addition, the active surface chemistry of MXene facilitates possible functionalization to enable a stronger interface with polymers for applications, such as strain engineering and mechanical enhancement, and in materials including membranes, coatings, and textiles.

The demand for efficient electrochemical energy storage technology, such as supercapacitors, continues to increase as both the energy and power demands of devices grow. Graphene has attracted wide interest in addressing this energy challenge due to its high conductivity and specific surface area. However, in reality the hydrophobic properties and the restacking of the graphene sheets during device manufacture leads to significantly lower storage performance than that theoretically predicted for isolated sheets. Herein, functionalized graphene was prepared by a convenient one-pot process, where graphene was functionalized with aryl diazonium salts (4nitrobenzenediazonium tetrafluoroborate (NBD) and 4-bromobenzenediazonium tetrafluoroborate (BBD)) simultaneously during oxidative electrochemical exfoliation of graphite. It was found that the specific capacitance for functionalized graphene was significantly improved compared to pristine graphene due to the introduction of pseudocapacitance by the aryl diazonium salts. The dispersibility of functionalized graphene in water was also found to be improved, implying a better hydrophilicity. NBD functionalized graphene which had been exfoliated/functionalized for a total of 30 minutes exhibited the best energy storage properties with a 5 times increase in specific capacitance (17 mF cm-2) compared to pristine graphene (3 mF cm-2).