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As a recently emerging group of 2D materials, MXene has attracted extensive attention in the energy storage field in recent years owing to their outstanding features. However, the notorious issues of inevitable oxidation stability and surface‐to‐surface self‐restacking for MXene significantly prevent its further wide‐ranging application. Herein, the 3D macroporous oxidation‐resistant Ti3C2Tx MXene/graphene/carbon nanotube (MRC) hybrid hydrogels are prepared by a simple gelation method assisted by l‐cysteine as crosslinker and l‐ascorbic acid (VC) as reductant. Benefitting from the effectively alleviated restacking, excellent electrical conductivity, and the 3D inter‐crosslinked macroporous architecture, as a supercapacitor electrode, the obtained MRC aerogel exhibits a superior specific capacitance of 349 F g−1, unparalleled rate capability (52.0% at 3000 mV s−1) and amazing cyclic stability (retention of 97.1% after 100 000 cycles). Moreover, the 3D MRC‐30 aerogel exhibits an impressive oxidation‐resistant performance with just a 9.3% increase in electrical resistance after storing in ambient condition for 60 days, effectively alleviating the oxidation problem of MXene. This work demonstrates a new method for construction of 3D oxidation‐resistant MXene hydrogel, shedding new light on the promising applications of MXene materials, especially in high humidity and oxygen environment. [ABSTRACT FROM AUTHOR]

2D MXene materials have attracted intensive attention in energy storage application. However, MXene usually undergoes serious face‐to‐face restacking and inferior stability, significantly preventing its further commercial application. Herein, to suppress the oxidation and self‐restacking of MXene, an efficient and fast self‐assembly route to prepare a 3D porous oxidation‐resistant MXene/graphene (PMG) composite with the assistance of an in situ sacrificial metallic zinc template is demonstrated. The self‐assembled 3D porous architecture can effectively prevent the oxidation of MXene layers with no evident variation in electrical conductivity in air at room temperature after two months, guaranteeing outstanding electrical conductivity and abundant electrochemical active sites accessible to electrolyte ions. Consequently, the PMG‐5 electrode possesses a striking specific capacitance of 393 F g−1, superb rate performance (32.7% at 10 V s−1), and outstanding cycling stability. Furthermore, the as‐assembled asymmetric supercapacitor possesses a pronounced energy density of 50.8 Wh kg−1 and remarkable cycling stability with a 4.3% deterioration of specific capacitance after 10 000 cycles. This work paves a new avenue to solve the two long‐standing significant challenges of MXene in the future. [ABSTRACT FROM AUTHOR]

Potassium‐ion hybrid capacitors have attracted increasing attention due to good energy density, high power density, and low cost. Ti3C2Tx‐MXene is considered as a promising anode material for K ion storage. However, undesirable stacking issues decrease its exposed area and breeds sluggish K ion transport. Herein, a facile spray‐lyophilization strategy is proposed to construct stacking‐resistant Ti3C2Tx with 3D structures. As‐prepared Ti3C2Tx hollow spheres/tubes present stack resistance, a large specific surface area, and a short ion diffusion pathway. When serving as an anode material, it shows enhanced capacity and thickness‐independent rate performance compared to 2D Ti3C2Tx. After 10 000 cycles, a specific capacity of 122 mAh g−1 is obtained at 1 A g−1. Systematic kinetics analyses demonstrate the significance of concentration polarization on the electrode's rate ability. Furthermore, a 3D Ti3C2Tx‖hierarchical porous activated carbon (HPAC) K‐ion hybrid capacitor is assembled and displays remarkable energy and power densities with energy retention of 100% after 10 000 cycles at 1 A g−1. Following this strategy, other 3D structures from nanosheets can also be obtained, such as 3D Ti3C2Tx microtubes and graphene oxide nanoscrolls. This study provides a viable approach to solve the stacking issues of 2D nanosheets to promote the application of 2D materials. [ABSTRACT FROM AUTHOR]

Abstract: Hollow structures are considered as promising electrode materials. Their enhanced surface‐to‐volume ratio results in an increased contact area between the electrolyte and electrode and, thereby, a higher accessibility of active material. Here, a facile hydrothermal method is applied to synthesize hollow Cu2O spheres, assembled by well‐aligned nanofibers. Based on the investigation of the samples at different reaction times, a growing mechanism is proposed. Nano building‐blocks create porous channels within the shell, which could facilitate the transfer and shorten the transportation pathway for both ions and charge. The Cu2O with its unique structure displays a high specific capacitance of 1075 F g−1 at a current density of 5 mA cm−2, a remarkable rate capability of 82 %, and an excellent cycling property of nearly 100 % of the initial value remaining after 5000 charge‐discharge cycles. Moreover, the hollow Cu2O spheres are first used for the preparation of a positive electrode in an asymmetric supercapacitor, using carbon as a negative electrode. The device exhibits a high energy density of 44.7 Wh kg−1 at a power density of 420.2 W kg−1, and when the power density reaches values as high as 4201.7 W kg−1, an energy density of 16 Wh kg−1 is still attained. [ABSTRACT FROM AUTHOR]

For the first time, hierarchically porous carbon materials with a sandwich-like structure are synthesized through a facile and efficient tri-template approach. The hierarchically porous microstructures consist of abundant macropores and numerous micropores embedded into the crosslinked mesoporous walls. As a result, the obtained carbon material with a unique sandwich-like structure has a relatively high specific surface (1235 m2 g−1), large pore volume (1.30 cm3 g−1), and appropriate pore size distribution. These merits lead to a comparably high specific capacitance of 274.8 F g−1 at 0.2 A g−1 and satisfying rate performance (87.7 % retention from 1 to 20 A g−1). More importantly, the symmetric supercapacitor with two identical as-prepared carbon samples shows a superior energy density of 18.47 Wh kg−1 at a power density of 179.9 W kg−1. The asymmetric supercapacitor based on as-obtained carbon sample and its composite with manganese dioxide (MnO2) can reach up to an energy density of 25.93 Wh kg−1 at a power density of 199.9 W kg−1. Therefore, these unique carbon material open a promising prospect for future development and utilization in the field of energy storage. [ABSTRACT FROM AUTHOR]

Developing advanced supercapacitors with both high areal and volumetric energy densities remains challenging. In this work, self‐supported, compact carbon composite electrodes are designed with tunable thickness using 3D printing technology for high‐energy‐density supercapacitors. The 3D carbon composite electrodes are composed of the closely stacked and aligned active carbon/carbon nanotube/reduced graphene oxide (AC/CNT/rGO) composite filaments. The AC microparticles are uniformly embedded in the wrinkled CNT/rGO conductive networks without using polymer binders, which contributes to the formation of abundant open and hierarchical pores. The 3D‐printed ultrathick AC/CNT/rGO composite electrode (ten layers) features high areal and volumetric mass loadings of 56.9 mg cm−2 and 256.3 mg cm−3, respectively. The symmetric cell assembled with the 3D‐printed thin GO separator and ultrathick AC/CNT/rGO electrodes can possess both high areal and volumetric capacitances of 4.56 F cm−2 and 10.28 F cm−3, respectively. Correspondingly, the assembled ultrathick and compact symmetric cell achieves high areal and volumetric energy densities of 0.63 mWh cm−2 and 1.43 mWh cm−3, respectively. The all‐component extrusion‐based 3D printing offers a promising strategy for the fabrication of multiscale and multidimensional structures of various high‐energy‐density electrochemical energy storage devices. The all‐graphene‐oxide (GO)‐based symmetric supercapacitor is assembled with a thin GO separator and compact active carbon/carbon nanotube/reduced graphene oxide composite electrodes with tunable thickness, which are fabricated using the advanced extrusion‐based 3D printing technology. The 3D‐printed ultrathick and compact symmetric supercapacitor with a high mass loading can achieve both high areal and volumetric energy densities. [ABSTRACT FROM AUTHOR]