Atomic Modulation Triggering Improved Performance of MoO3 Nanobelts for Fiber-Shaped Supercapacitors

MoO3 with the Synergistic Effect of Sulfur Doping and Oxygen Vacancies: The Influence of S Doping on the Structure, Morphology, and Optoelectronic Properties

Molybdenum trioxide (MoO3) is an attractive semiconductor. Thus, bandgap engineering toward photoelectronic applications is appealing yet not well studied. Here, we report the incorporation of sulfur atoms into MoO3, using sulfur powder as a source of sulfur, via a self-developed hydrothermal synthesis approach. The formation of Mo-S bonds in the MoO3 material with the synergistic effect of sulfur doping and oxygen vacancies (designated as S-MoO3-x) is confirmed using Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). The bandgap is tuned from 2.68 eV to 2.57 eV upon sulfur doping, as confirmed by UV-VIS DRS spectra. Some MoS2 phase is identified with sulfur doping by referring to the photoluminescence (PL) spectra and electrochemical impedance spectroscopy (EIS), allowing significantly improved charge carrier separation and electron transfer efficiency. Therefore, the as-prepared S-MoO3-x delivers a sensitive photocurrent response and splendid cycling stability. This study on the synergistic effect of sulfur doping and oxygen vacancies provides key insights into the impact of doping strategies on MoO3 performance, paving new pathways for its optimization and development in relevant fields.

Designing transition metal-based porous architectures for supercapacitor electrodes: a review

Over the past decade, transition metal (TM)-based electrodes have shown intriguing physicochemical properties and widespread applications, especially in the field of supercapacitor energy storage owing to their diverse configurations, composition, porosity, and redox reactions. As one of the most intriguing research interests, the design of porous architectures in TM-based electrode materials has been demonstrated to facilitate ion/electron transport, modulate their electronic structure, diminish strain relaxation, and realize synergistic effects of multi-metals. Herein, the recent advances in porous TM-based electrodes are summarized, focusing on their typical synthesis strategies, including template-mediated assembly, thermal decomposition strategy, chemical deposition strategy, and host-guest hybridization strategy. Simultaneously, the corresponding conversion mechanism of each synthesis strategy are reviewed, and the merits and demerits of each strategy in building porous architectures are also discussed. Subsequently, TM-based electrode materials are categorized into TM oxides, TM hydroxides, TM sulfides, TM phosphides, TM carbides, and other TM species with a detailed review of their crystalline phase, electronic structure, and microstructure evolution to tune their electrochemical energy storage capacity. Finally, the challenges and prospects of porous TM-based electrode materials are presented to guide the future development in this field.

TiO2-modified two-dimensional composite of nitrogen-doped molybdenum trioxide nanosheets as a high-performance anode for lithium-ion batteries

Nitrogen-doped molybdenum trioxide (MoO3/NC) has drawbacks such as volume expansion during long-term charging and discharging cycles, which severely limit its further application. This work proposes the addition of titanium dioxide nanoparticles (TiO2 NPs) for performance improvement of MoO3/NC. TiO2 NPs embedded on the surface of a MoO3/NC nanosheet can alleviate its volume expansion and the accumulation of lithiated products and improve the conductivity of the electrode material. The results show that the MoO3/NC nanosheet decorated with TiO2 NPs (TiO2@MoO3/NC), when used as an electrode material, exhibited a discharge specific capacity of 419 mA h g-1 at a current density of 0.05 A g-1 and retained a discharge specific capacity of 517 mA h g-1 after 600 cycles at a current density of 1 A g-1.

Heterostructured MoO3@CoWO4 nanobelts towards high electrochemical performances via oxygen vacancies generation

Heterostructured nanomaterials tend to have a high proportion of oxygen vacancies (VO) due to the presence of heterogeneous interfaces. Herein, a new kind of heterostructured MoO3@CoWO4 nanobelts was successfully evaluated as fascinating cathode material. SEM and TEM analysis indicated that the MoO3 nanobelts were fully blanketed with CoWO4 nanodots. The generation of Vo species was confirmed by XPS and EPR data. By profiting both synergistic and Vo generation effects, MoO3@CoWO4 electrode displayed an excellent capacitance of 246 mAh·g-1 (1966 F·g-1at 0.5 A·g-1) with high-rate capability of 174 mAh·g-1 (1394 F·g-1 at 30 A·g-1) as well as superb stability of 94 % (over 15,000 cycles). Notably, all-solid-state device delivered a good energy value of 63.1 Wh·kg-1 at 375 W·kg-1. Interestingly, the supercapacitor device showed super-low self-discharge comportment of only 12.1 % during 24 h. Importantly, the generation of the VO defects could control the ions diffusion process and lead a sharp decrease in the self-discharge process.

Designed formation of C/Fe3O4@Ni(OH)2 cathode with enhanced pseudocapacitance for asymmetric supercapacitors

The rational design and synthesis of advanced electrode materials are significant for the applications of supercapacitors. Ferroferric oxide (Fe₃O₄), with its high theoretical capacitance is a renowned cathode material. Nevertheless, its low electronic conductivity and poor cycling stability during a long-term charge/discharge process limit its large-scale applications. In this work, the precise modulation of multiple components was reported to enhance electrochemical performance. The ternary heterostructures were fabricated by wrapping ultrathin nickel hydroxide (Ni(OH)₂) nanosheets on the surfaces of Fe₃O₄ nanoparticles-loaded on sodium carboxymethyl cellulose (CMC)-derived porous carbon, named as C/Fe₃O₄@Ni(OH)₂. Due to the large specific surface area and excellent conductivity of CMC-derived porous carbon and the abundant reaction sites of Ni(OH)₂ nanosheets, the optimized C/Fe₃O₄@Ni(OH)₂-1.0 sample exhibited the highest specific capacitance of 3072F g^-1 at a current density of 0.5 A g^-1. Furthermore, the assembled asymmetric supercapacitor (ASC) with activated carbon and C/Fe₃O₄@Ni(OH)₂-1.0 as the negative and positive electrodes, respectively, showed an energy density of 123 W h kg^-1 at 381 W kg^-1, and a long-life stability with an excellent capacitance retention of 90.04 % after 10,000 cycles. The route for preparing composite electrode materials proposed in this work provides a reference for realizing high-performance energy storage devices.

Storage of Lithium-Ion by Phase Engineered MoO3 Homojunctions

With high theoretical specific capacity, the low-cost MoO3 is known to be a promising anode for lithium-ion batteries. However, low electronic conductivity and sluggish reaction kinetics have limited its ability for lithium ion storage. To improve this, the phase engineering approach is used to fabricate orthorhombic/monoclinic MoO3 (α/h-MoO3) homojunctions. The α/h-MoO3 is found to have excessive hetero-phase interface. This not only creates more active sites in the MoO3 for Li+ storage, it regulates local coordination environment and electronic structure, thus inducing a built-in electric field for boosting electron/ion transport. In using α/h-MoO3, higher capacity (1094 mAh g-1 at 0.1 A g-1) and rate performance (406 mAh g-1 at 5.0 A g-1) are obtained than when using only the single phase h-MoO3 or α-MoO3. This work provides an option to use α/h-MoO3 hetero-phase homojunction in LIBs.

Ultrathin Polypyrrole Layers Boosting MoO3 as Both Cathode and Anode Materials for a 2.0 V High-Voltage Aqueous Supercapacitor

An aqueous supercapacitor is an emerging energy storage unit on account of its low cost, fast energy delivery rate, and long service life. The energy density of an aqueous supercapacitor can be enlarged via extending the voltage window of electrode materials, while the aqueous electrolyte remains thermodynamically constant at 1.23 V. Herein, an aqueous supercapacitor with a 2.0 V high-voltage window is realized by core-shell MoO_3-x/polypyrrole (MP) nanocomposites as both cathode and anode materials. The ultrathin PPy layer on the MoO₃ core not only improves the conductivity and cycle stability of the nanocomposites but also acts as a reductant, leading to the formation of oxygen vacancies in the MoO₃ core. When used as a cathode material, the potential range of the as-obtained MP nanocomposite is up to 1.0 V. As an anode material, the stable potential range could reach -1.0 V. Due to the large potential range of the cathode and anode, the as-obtained 2.0 V aqueous supercapacitor shows a remarkably high delivery energy of 58.5 Wh kg^-1. The synthesis of MP nanocomposites is simple and the electrode performance is significantly enhanced; thus, it is a suitable candidate for high-energy-density aqueous supercapacitors.

Enhancing Li-Ion Affinity of Molybdenum Dioxide/Carbon Fabric to Achieve High Pseudocapacitance

High-energy electrodes at high mass loadings (usually >8.0 mg cm^-2 ) are desired for aqueous pseudocapacitors. Yet, how to overcome the thickness-dependent resistance increase of ion/electron transport in pseudocapacitive materials is still challenging. Herein, a high-performance electrode (denoted as AMC) adapted to high mass loading is achieved by promoting the Li-ion affinity of 3D MoO₂ /carbon fabric. The experimental results and corresponding computational results reveal that the oxygen-activated surface of AMC, combined with the wettability and conductivity superiority of 3D graphite network, significantly facilitates the Li-ion adsorption and diffusion at the electrode/electrolyte interface, even at large thicknesses. Consequently, even at a high mass loading up to 8.1 mg cm^-2 , the AMC electrode also displays an impressive specific capacity (567.5 C g^-1 at 2.5 A g^-1 ), substantially superior to most advanced pseudocapacitive electrodes. The strategy of boosting energy characteristic by enhancing the affinity of charge carriers is applicable to other pseudocapacitive electrodes.

An Amorphous-Crystalline Nanosheet Arrays Structure for Ultrahigh Electrochemical Performance Supercapattery

Hybrid supercapacitors (HSCs), also called supercapattery, which can substitute for low power density batteries have attracted extensive interest. However, when HSCs comes to commercial applications, there is still space for improvement in energy density. It seems that designing of electrode with high capacity is an effective measure. Herein, amorphous-crystalline MoO₃ -Ni₃ S₂ /NF-0.5 nanosheet arrays are developed as battery-type electrodes. Specifically, the sheet-like structure of crystalline Ni₃ S₂ can achieve rich structural nanocrystallization, improving the redox reaction efficiency. Meanwhile, the disordered structure and irregular surface of the amorphous MoO₃ are conducive to maximize the contact between the electrode and electrolyte, slowing down the volume change caused by the continuous charge-discharge process. As a result, it displays an ultrahigh areal specific capacity of 8.52 C cm^-2 at 5 mA cm^-2 , and superior lifespan up to 7500 cycles with 90.0% retention. Further, when assembled into HSCs, the specific capacity reaches 1.47 C cm^-2 , corresponding to an energy density of 4.18 mWh cm^-2 at a power density of 0.34 mW cm^-2 . Totally, the design of the unique structure displays a valuable measure for rational development of high energy density hybrid energy storage devices that are not limited to supercapacitors.

Fast synthesis of MoO3-x and its catalytic effect on the thermal decomposition of ammonium perchlorate based molecular perovskite (DAP-4)

In this work, it is shown that MoO3-x has a positive effect on the thermal decomposition of ammonium perchlorate based molecular perovskite (H2DABCO)[NH4(ClO4)3] (DAP-4). MoO3-x was prepared by heat treatment, and the morphology, structure, and thermal decomposition performance were characterized. The morphology and structure characterization results showed that MoO3-x was an irregular layered structure material and the Mo element was mainly in the +6 chemical valence state, with a small amount of Mo5+. Thermal analysis results showed that the thermal decomposition peak temperature of DAP-4 was effectively reduced from 394.4 °C to 353.7 °C, 321.4 °C, and 312.5 °C in the presence of 1%, 5%, and 10% MoO3-x, respectively. It is particularly worth noting that the maximum heat release rate of the DAP-4/10% MoO3-x mixture was increased by 4.9 times compared with pure DAP-4. Through the two classic thermal decomposition kinetic methods, Kissinger and Starink, the reliable kinetic parameters of DAP-4/MoO3-x were obtained. The increase of the reaction rate constant k indicated that the maximum thermal decomposition reaction rate of DAP-4 was effectively improved. This work provided a feasible technology for using MoO3-x as an effective catalyst to improve the thermal performance of DAP-4.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

Fiber‐Shaped Electronic Devices

Textile electronics embedded in clothing represent an exciting new frontier for modern healthcare and communication systems. Fundamental to the development of these textile electronics is the development of the fibers forming the cloths into electronic devices. An electronic fiber must undergo diverse scrutiny for its selection for a multifunctional textile, viz., from the material selection to the device architecture, from the wearability to mechanical stresses, and from the environmental compatibility to the end‐use management. Herein, the performance requirements of fiber‐shaped electronics are reviewed considering the characteristics of single electronic fibers and their assemblies in smart clothing. Broadly, this article includes i) processing strategies of electronic fibers with required properties from precursor to material, ii) the state‐of‐art of current fiber‐shaped electronics emphasizing light‐emitting devices, solar cells, sensors, nanogenerators, supercapacitors storage, and chromatic devices, iii) mechanisms involved in the operation of the above devices, iv) limitations of the current materials and device manufacturing techniques to achieve the target performance, and v) the knowledge gap that must be minimized prior to their deployment. Lessons learned from this review with regard to the challenges and prospects for developing fiber‐shaped electronic components are presented as directions for future research on wearable electronics.

Laser-Induced MoOx/Sulfur-Doped Graphene Hybrid Frameworks as Efficient Antibacterial Agents

Rational design and scalable construction of antibacterial mediators based on unique graphene architectures with highly efficient antibacterial ability and significant biocompatibility are challenging. Herein, sulfur-doped graphene skeletons uniformly decorated with metal oxide nanoparticles were designed and constructed via one-step laser-induced microexplosive techniques and demonstrated for the first time as highly efficient antibacterial agents. The optical density and flat colony counting methods demonstrated that the as-designed laser-induced MoO_x/sulfur-doped graphene hybrids exhibited exceptional activity inhibition of Escherichia coli and Staphylococcus aureus. Moreover, the bacteria were treated with an impressive laser-induced MoO_x/sulfur-doped graphene colloidal solution of concentration as low as 1 mg/mL for 4 h, leading to an excellent viability loss of 85% for the two bacteria. Cell toxicity experiments proved that the biological toxicity of laser-induced MoO_x/sulfur-doped graphene to pig sperm cells was negligible. The molecular dynamics calculations proposed that the intrinsic interaction with N-acetylglucosamine at the cell wall and the high-efficiency synergistic effect of sulfur-doped graphene and MoO_x played the key role in inhibiting the viability of bacteria. This work provides new insights for a novel structure design and opens up a potential route to construct antibacterial agents with high efficiency for clinical application.