Boosting the volumetric capacitance of MoO3-x free-standing films with Ti3C2 MXene

Facile preparation of MoO3@Mo2CTxnanocomposite with high lithium storage performance byin situoxidation

In this work, a new MoO3@Mo2CTxnanocomposite was prepared from two-dimensional (2D) Mo2CTxMXene byin situoxidization in air, which exhibited wonderful lithium-storage performance as anodes of lithium-ion batteries (LIBs). The precursor Mo2CTxwas synthesized from Mo2Ga2C by selective etching of NH4F at 180 °C for 24 h. Thereafter, the Mo2CTxwas oxidized in air at 450 °C for 30 min to obtain MoO3@Mo2CTxnanocomposite. In the composite,in situgenerated MoO3nanocrystals pillar the layer structure of Mo2CTxMXene, which increases the interlayer space of Mo2CTxfor Li storage and enhances the structure stability of the composite. Mo2CTx2D sheets provide a conductive substrate for MoO3nanocrystals to enhance the Li+accessibility. As anodes of LIBs, the final discharge specific capacity of the MoO3@Mo2CTxcomposite was 511.1 mAh g-1at a current density of 500 mA g-1after 100 cycles, which is about 36.7 times that of pure Mo2CTxMXene (13.9 mAh g-1) and 3.2 times that of pure MoO3(159.9 mAh g-1). In the composites, both Mo2CTxand MoO3provide high lithium storage capacity and can enhance the performance of each other. Moreover, this composite can be made by a facile method ofin situoxidation. Therefore, the MoO3@Mo2CTxMXene nanocomposite is a promising anode of LIB with high performance.

A Transient Pseudo-Capacitor Using a Bioderived Ionic Liquid with Na Ions

A pseudo-capacitor with transient behavior is applied in implantable, disposable, and bioresorbable devices, incorporating an Na ion-doped bioderived ionic liquid, molybdenum trioxide (MoO₃ )-covered molybdenum foil, and silk sheet as the electrolyte, electrode, and separator, respectively. Sodium lactate is dissolved in choline lactate as a source of Na ions. The Experimental results reveal that the Na ions are intercalated into the van der Waals gaps in MoO₃ , and the pseudo-capacitor shows an areal capacitance (1.5 mF cm^-2 ) that is three times larger than that without the Na ion. The fast ion diffusion of the electrolyte and the low resistance of the MoO₃ and Mo interface result in an equivalent series resistance of 96 Ω. A cycle test indicates that the pseudo-capacitor exhibited a high capacitance retention of 82.8% after 10 000 cycles. The transient behavior is confirmed by the dissolution of the pseudo-capacitor into phosphate-buffered saline solution after 101 days. Potential applications of transient pseudo-capacitors include electronics without the need for device retrieval after use, including smart agriculture, implantable, and wearable devices.

Tightly intercalated Ti3C2Tx/MoO3-x/PEDOT:PSS free-standing films with high volumetric/gravimetric performance for flexible solid-state supercapacitors

Ti₃C₂T_x-MXenes have extremely promising applications in electrochemistry, but the development of Ti₃C₂T_x is limited due to severe self-stacking problem. Here, we introduced oxygen vacancy-enriched molybdenum trioxide (MoO_3-x) with pseudocapacitive properties as the intercalator of Ti₃C₂T_x and PEDOT with high electronic conductivity as the co-intercalator and conductive binder of Ti₃C₂T_x to synthesize Ti₃C₂T_x/MoO_3-x/PEDOT:PSS (TMP) free-standing films by vacuum-assisted filtration and H₂SO₄ soaking. The tightly intercalated free-standing film structure can effectively improve the self-stacking phenomenon of Ti₃C₂T_x, expose more active sites and facilitate electron/ion transport, thus making TMP show excellent electrochemical performance. The volumetric and gravimetric capacitance of optimized TMP-2 can reach 1898.5 F cm^-3 and 523.0 F g^-1 at 1 A g^-1 with a rate performance of 90.5% at the current density from 1 A g^-1 to 20 A g^-1, which is significantly better than those of MXene-based composites reported in the literature. The directly-assembled TMP-2//TMP-2 flexible solid-state supercapacitor displays high energy/power output performances (25.1 W h L^-1 at 6383.1 W L^-1, 6.9 W h kg^-1 at 1758.4 W kg^-1) and there is no obvious change after 100 cycles at a bending angle of 180°. As a result, the tightly intercalated TMP-2 free-standing film with high volumetric/gravimetric capacitances is a promising material for flexible energy storage devices.

Flexible MXene-Based Composite Films: Synthesis, Modification, and Applications as Electrodes of Supercapacitors

MXenes, as a 2D planar structure nanomaterial, were first reported in 2011. Due to their large specific surface area, high ductility, high electrical conductivity, strong hydrophilic surface, and high mechanical flexibility, MXenes have been extensively explored in the development of various functional materials with desired performances. This review is aimed to summarize the current progress in synthesis, modification, and applications of MXene-based composite films as electrode materials of flexible energy storage devices. In the synthesis of MXenes, the evolution and exploration of etchants are emphasized. Furthermore, in order to develop MXene-based composite films, the components used to modify the MXene nanoflakes, including 0D, 1D, and 2D nanomaterials, are summarized, and the perspectives and research direction of such materials are also discussed.

A Free-Standing α-MoO3/MXene Composite Anode for High-Performance Lithium Storage

Replacing the commercial graphite anode in Li-ion batteries with pseudocapacitor materials is an effective way to obtain high-performance energy storage devices. α-MoO3 is an attractive pseudocapacitor electrode material due to its theoretical capacity of 1117 mAh g−1. Nevertheless, its low conductivity greatly limits its electrochemical performance. MXene is often used as a 2D conductive substrate and flexible framework for the development of a non-binder electrode because of its unparalleled electronic conductivity and excellent mechanical flexibility. Herein, a free-standing α-MoO3/MXene composite anode with a high specific capacity and an outstanding rate capability was prepared using a green and simple method. The resultant α-MoO3/MXene composite electrode combines the advantages of each of the two components and possesses improved Li+ diffusion kinetics. In particular, this α-MoO3/MXene free-standing electrode exhibited a high Li+ storage capacity (1008 mAh g−1 at 0.1 A g−1) and an outstanding rate capability (172 mAh g−1 at 10 A g−1), as well as a much extended cycling stability (500 cycles at 0.5 A g−1). Furthermore, a full cell was fabricated using commercial LiFePO4 as the cathode, which displayed a high Li+ storage capacity of 160 mAh g−1 with an outstanding rate performance (48 mAh g−1 at 1 A g−1). We believe that our research reveals new possibilities for the development of an advanced free-standing electrode from pseudocapacitive materials for high-performance Li-ion storage.

Three-Dimensional Self-Supporting Ti3C2 with MoS2 and Cu2O Nanocrystals for High-Performance Flexible Supercapacitors

The three-dimensional (3D) architecture of electrode materials with excellent stability and electrochemical activity is extremely desirable for high-performance supercapacitors. In this study, we develop a facile method for fabricating 3D self-supporting Ti₃C₂ with MoS₂ and Cu₂O nanocrystal composites for supercapacitor applications. MoS₂ was incorporated in Ti₃C₂ using a hydrothermal method, and Cu₂O was embedded in two-dimensional nanosheets by in situ chemical reduction. The resulting composite electrode showed a synergistic effect between the components. Ti₃C₂ served as a conductive additive to connect MoS₂ nanosheets and facilitate charge transfer. MoS₂ acted as an active spacer to increase the interlayer space of Ti₃C₂ and protect Ti₃C₂ from oxidation. Cu₂O effectively prevented the collapse of the lamellar structure of Ti₃C₂-MoS₂. Consequently, the optimized composite exhibited an excellent specific capacitance of 1459 F g^-1 at a current density of 1 A g^-1. Further, by assembling an all-solid-state flexible supercapacitor with activated carbon, a high energy density of 60.5 W h kg^-1 was achieved at a power density of 10³ W kg^-1. Additionally, the supercapacitor exhibited a capacitance retention of 90% during 3000 charging-discharging cycles. Moreover, high mechanical robustness was retained after bending at different angles, thereby suggesting significant potential applications for future flexible and wearable devices.