Understanding the etching mechanisms of MXene and obtaining direct insights into the influence of etchants on structural features and defects are of critical importance for improving MXene properties, optimization of etching protocols, and exploring new MXene compositions. Despite their importance, such studies have been challenging because of the monoatomic thickness of the A-element layers being etched and aggressive etchants that hinder in situ studies. Here, we visualize the etching behavior of the Ti3AlC2 MAX phase in different etchants at the atomic scale using focused ion beam and electron microscopy. We also report on the structural changes in the Ti3AlC2 phase as a function of etching time and etchant type (LiF/HCl, HF, or NH4HF2) to reveal the etching mechanism for the first time. Interestingly, direct visualization reveals an unexpected stepwise etching where the edge Al atoms at the middle layers of the MAX particle are not etched despite contact with the acidic etchant counterintuitively. Also, while the propagation of the etching front occurs in the direction normal to the inner basal plane for all etchants, we reveal that HF and NH4HF2 etch the grain boundaries of polycrystal MAX particles to expose more edge sites to the etchant, which is not observed for LiF/HCl.
Lithium-ion batteries (LIBs) are rechargeable batteries that have attracted great interest as next-generation energy storage devices that will lead future energy technologies because of their various excellent advantages. Two-dimensional (2D) MXene-based LIBs have been actively investigated because of their high energy/power density and good performance at high charge/discharge rates. However, three major limitations of 2D MXene electrodes -self-stacking, low specific surface area, and disturbance of Li + diffusion by surface terminations -have hindered the commercialization of MXene-based LIBs. Herein, we fabricate 1D hollow Ti 3 C 2 T x MXene/carbon (MX/C) nanofibers via an electrospinning process and use them as anode materials in LIBs. Compared with the pristine 2D MXene (MX) paste electrode and MXene/carbon (MX/C) paste electrode, the hollow MX/C nanofibers electrode exhibits a greater specific surface area, less self-stacking of flakes, and surface functional groups tailored for LIBs. The LIBs based on the hollow electrode exhibit a higher energy density (306.5 mA h g À 1 at 40 mA g À 1 ) than those with the MX paste electrode (81.08 mA h g À 1 at 40 mA g À 1 ) and MX/C paste electrode (196.9 mA h g À 1 at 40 mA g À 1 ). In addition, the hollow MX/C nanofiber electrode shows a high reversible capacity, proving that it is a promising anode material for LIBs.
Electro-ionic soft actuators, capable of continuous deformations replacing noncompliant rigid mechanical components, attract increasing interest in the field of next-generation metaverse interfaces and soft robotics. Here, a novel MXene (Ti 3 C 2 T x ) electrode anchoring manganese-based 1,3,5-benzenetricarboxylate metal-organic framework (MnBTC) for ultrastable electro-ionic artificial muscles is reported. By a facile supramolecular self-assembly, the Ti 3 C 2 T x -MnBTC hybrid nanoarchitecture forms coordinate bond, hydrogen bond, and hydrophilic interaction with the conducting polymer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), resulting in a mechanically flexible and electro-ionically active electrode. The superior electrical and electrochemical performances of the electrode stem from the synergistic effects between intrinsically hierarchical nanoarchitecture of MnBTC and rapid electron transport behavior of Mxene, leading to fast diffusion and accommodation of ions in the ion-exchangeable membrane. The developed artificial muscle based on Ti 3 C 2 T x -MnBTC is found to exhibit high bending displacement (12.5 mm) and ultrafast response time (0.77 s) under a low driving voltage (0.5 V), along with wide frequency response (0.1-10 Hz) and exceptional stability (98% retention at 43,200 s) without any distortion of actuation performance. Furthermore, the designed electro-active artificial muscle is successfully used to demonstrate mimicry of eye motions including eyelid blinking and eyeball movement in a doll.
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