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.

In this study, graphene aerogel (GA)-supported Fe2O3 particles with three-dimensional (3D) architecture was prepared by a one-pot hydrothermal process. Fe2O3 particles were dispersed uniformly on the graphene sheets, and the resulting composites self-assembled into a 3D network via hydrothermal treatment. This strategy provides a facile and environmentally friendly method for the large-scale synthesis of Fe2O3/GAs without any additional reductant. As the anode material for lithium ion batteries, the Fe2O3/GAs in this study manifested an excellent reversible capacity of 995 mA h g(-1) after 50 cycles at a charge-discharge rate of 100 mA g(-1) and even delivered reversible capacity as high as 372 mA h g(-1) at a high rate of 5000 mA g(-1). The outstanding electrochemical performance of Fe2O3/GAs can be attributed to the synergistic interaction between uniformly dispersed Fe2O3 particles and graphene aerogel, in which a robust 3D framework of graphene provided highly conductive networks with a large surface area and short diffusion path length for the transport of lithium ions.

A 3D graphene/polyimide composite is fabricated by a one-step solvothermal strategy as a high-performance cathode for both rechargeable lithium and sodium batteries.

3D hierarchical tin oxide/graphene frameworks (SnO2 /GFs) were built up by the in situ synthesis of 2D SnO2 /graphene nanosheets followed by hydrothermal assembly. These SnO2 /GFs exhibited a 3D hierarchical porous architecture with mesopores (≈3 nm), macropores (3-6 μm), and a large surface area (244 m(2) g(-1) ), which not only effectively prevented the agglomeration of SnO2 nanoparticles, but also facilitated fast ion and electron transport in 3D pathways. As a consequence, the SnO2 /GFs exhibited a high capacity of 830 mAh g(-1) for up to 70 charge-discharge cycles at 100 mA g(-1) . Even at a high current density of 500 mA g(-1) , a reversible capacity of 621 mAh g(-1) could be maintained for SnO2 /GFs with excellent cycling stability. Such performance is superior to that of previously reported SnO2 /graphene and other SnO2 /carbon composites with similar weight contents of SnO2 .

Superior energy density and exceptional cycling stability of conducting polymer-based flexible all-solid-state supercapacitors have been successfully achieved.