CitationAll-MXene (2D titanium carbide) solid-state microsupercapacitors for on-chip energy storage 2016 Energy Environ. Sci. On-chip energy storage is a rapidly evolving research topic, opening doors for integration of batteries and supercapacitors at microscales on rigid and flexible platforms. Recently, a new class of two-dimensional (2D) transition metal carbides and nitrides (so-called MXenes) has shown great promise in electrochemical energy storage applications. Here, we report the farbication of all-MXene (Ti3C2Tx) solid-state interdigital microsupercapacitors by employing a solution spray-coating, followed by a photoresist-free direct laser cutting method. Our prototype devices consisted of two layes of Ti3C2Tx with two different flake sizes. The bottom layer was stacked large-size MXene flakes (lateral dimensions of 3-6 μm) serving mainly as current collectors. The top layer was made of small-size MXene flakes (~1 μm) with a large number of defects and edges as the electroactive layer responsible for energy storage. Compared to Ti3C2Tx micro-supercapacitors with platinum current collectors, the all-MXene devices exhibited much lower contact resistance, higher capacitances and better rate-capabilities. The areal and volumetric capacitances of ~27 mF cm -2 and ~357 F cm -3 , respectively, at a scan rate of 20 mV s -1 were achieved. The devices also demonstrated their excellent cyclic stability, with 100 % capacitance retention after 10,000 cycles at a scan rate of 50 mV s -1 . This study opens up a plethora of possible designs for high-performance on-chip devices employing different chemistries, flake sizes and morphologies of MXenes and their heterostructures. Eprint version Broader contextThe continuous development and further miniaturization of portable electronic devices and microelectromechanical systems has led to the increasing demands for micro or nanoscale power sources and energy storage units. Supercapacitors, also called electrochemical capacitors, are energy storage devices with long service life and high power densities that can be fully charged and discharged in seconds. Small-scale supercapacitors, or micro-supercapacitors (MSCs), can be integrated with self-powered microscale devices and provide the required power for a long duration of time without maintenance, serving as ideal stand-alone power sources. The intrinsic properties of electrode materials play a crucially important role in the performance of MSCs. Here, a novel MSC is fabricated by employing a new material, two-dimensional titanium carbide (MXene). The MXene MSCs offer long lifetime and higher areal and volumetric capacities compared to most of the previously reported devices. This work opens up a door for the design of on-chip devices with high energy storage capability by employing a large family (~ 20 members) of 2D MXenes and their heterostructures.

The discovery of liquid crystalline (LC) phases in dispersions of two-dimensional (2D) materials has enabled the development of macroscopically aligned three-dimensional (3D) macrostructures. Here, we report the first experimental observation of self-assembled LC phases in aqueous Ti 3 C 2 T x MXene inks without using LC additives, binders, or stabilizing agents. We show that the transition concentration from the isotropic to nematic phase is influenced by the aspect ratio of MXene flakes. The formation of the nematic LC phase makes it possible to produce fibers from MXenes using a wet-spinning method. By changing the Ti 3 C 2 T x flake size in the ink formulation, coagulation bath, and spinning parameters, we control the morphology of the MXene fibers. The wet-spun Ti 3 C 2 T x fibers show a high electrical conductivity of ∼7750 S cm −1 , surpassing existing nanomaterial-based fibers. A high volumetric capacitance of ∼1265 F cm −3 makes Ti 3 C 2 T x fibers promising for fiber-shaped supercapacitor devices. We also show that Ti 3 C 2 T x fibers can be used as heaters. Notably, the nematic LC phase can be achieved in other MXenes (Mo 2 Ti 2 C 3 T x and Ti 2 CT x ) and in various organic solvents, suggesting the widespread LC behavior of MXene inks.

Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response. In this study, rheological behavior of single- and multilayer TiCT in aqueous dispersions was investigated. Viscous and viscoelastic properties of MXene dispersions were studied over a variety of concentrations from colloidal dispersions to high loading slurries, showing that a multilayer MXene suspension with up to 70 wt % can exhibit flowability. Processing guidelines for the fabrication of MXene films, coatings, and fibers have been established based on the rheological properties. Surprisingly, high viscosity was observed at very low concentrations for solutions of single-layer MXene flakes. Single-layer colloidal solutions were found to exhibit partial elasticity even at the lowest tested concentrations (<0.20 mg/mL) due to the presence of strong surface charge and excellent hydrophilicity of MXene, making them amenable to fabrication at dilute concentrations. Overall, the findings of this study provide fundamental insights into the rheological response of this quickly growing 2D family of materials in aqueous environments as well as offer guidelines for processing of MXenes.