Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. Delamination of MXene flakes in water produces colloidal solutions that are used to manufacture all kinds of products (thin films, coatings, and electrodes, etc.). However, the stability of MXene colloidal solutions, which is of critical importance to their application, remains largely unexplored. Here we report on the degradation of delaminated-Ti3C2T x colloidal solutions (T represents the surface functionalities) and outline protocols to improve their stability. Ti3C2T x MXene solutions in open vials degraded by 42%, 85%, and 100% after 5, 10, and 15 days, respectively, leading to the formation of cloudy-white colloidal solutionss containing primarily anatase (TiO2). On the other hand, the solution could be well-preserved when Ti3C2T x MXene colloidal solutionss were stored in hermetic Ar-filled bottles at 5 °C, because dissolved oxygen, the main oxidant of the MXene flakes, was eliminated. Under such a recipe, the time constant of the solution was dramatically increased. We have found that the degradation starts at the edges and its kinetics follows the single-exponential decay quite well. Moreover, we performed size selection of the MXene solution via a cascade technique and showed that the degradation process is also size-dependent, with the small flakes being the least stable. Furthermore, a dependence between the degradation time constants and the flake size allows us to determine the size of the nanosheets in situ from UV–vis spectra and vice versa. Finally, the proposed method of storing the MXene colloidal solution in Ar-filled vials was applied to Ti2CT x to improve its stability and time constant, demonstrating the validity of this protocol in improving the lifetime of different MXene solutions.

2D transition metal carbides and/or nitrides (MXenes), by virtue of high electrical conductivity, abundant surface functional groups and excellent dispersion in various solvents, are attracting increasing attention and showing competitive performance in energy storage and conversion applications. However, like other 2D materials, MXene nanosheets incline to stack together via van der Waals interactions, which lead to limited number of active sites, sluggish ionic kinetics, and finally ordinary performance of MXene materials/devices. Constructing 2D MXene nanosheets into 3D architectures has been proven to be an effective strategy to reduce restacking, thus providing larger specific surface area, higher porosity, and shorter ion and mass transport distance over normal 1D and 2D structures. In this review, the commonly used strategies for manufacturing 3D MXene architectures (3D MXenes and 3D MXene-based composites) are summarized, such as template, assembly, 3D printing, and other methods. Special attention is also given to the structure-property relationships of 3D MXene architectures and their applications in electrochemical energy storage and conversion, including supercapacitors, rechargeable batteries, and electrocatalysis. Finally, the authors propose a brief perspective on future opportunities and challenges for 3D MXene architectures/devices.

The ongoing miniaturization of devices and development of wireless and implantable technologies demand electromagnetic interference (EMI)‐shielding materials with customizability. Additive manufacturing of conductive polymer hydrogels with favorable conductivity and biocompatibility can offer new opportunities for EMI‐shielding applications. However, simultaneously achieving high conductivity, design freedom, and shape fidelity in 3D printing of conductive polymer hydrogels is still very challenging. Here, an aqueous Ti3C2‐MXene‐functionalized poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate ink is developed for extrusion printing to create 3D objects with arbitrary geometries, and a freeze–thawing protocol is proposed to transform the printed objects directly into highly conductive and robust hydrogels with high shape fidelity on both the macro‐ and microscale. The as‐obtained hydrogel exhibits a high conductivity of 1525.8 S m–1 at water content up to 96.6 wt% and also satisfactory mechanical properties with flexibility, stretchability, and fatigue resistance. Furthermore, the use of the printed hydrogel for customizable EMI‐shielding applications is demonstrated. The proposed easy‐to‐manufacture approach, along with the highlighted superior properties, expands the potential of conductive polymer hydrogels in future customizable applications and represents a real breakthrough from the current state of the art.