Molybdenum oxide/carbon composites derived from the CO2 oxidation of Mo2CTx (MXene) for lithium ion battery anodes

Oxidation of Molybdenum-Based Single-Metallic/bimetallic Carbide MXenes in Aqueous Suspensions: Mechanistic Insights

Molybdenum carbide MXenes have garnered considerable attention in electronics, energy storage, and catalysis. However, they are prone to oxidative degradation, but the associated mechanisms have not been systematically explored. Therefore, the oxidation mechanisms of Mo-based single-metallic/bimetallic carbide MXenes including Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx in aqueous suspensions were investigated for the first time in this study. Similar to Ti3C2Tx MXene, Mo-based MXenes were found to undergo oxidative degradation in their aqueous dispersions, leading to the disruption of their crystal structure and subsequent loss of optical and electronic properties. Notably, the Mo2CTx MXene deviated from this typical oxidation behavior as it produced an amorphous product with Mo ions instead of highly crystalline Mo-oxides during oxidation. Similarly, the Mo2TiC2Tx and Mo2Ti2C3Tx MXenes did not yield crystalline Mo-oxides; instead, they produced highly crystalline anatase TiO2 and a Mo-ion-containing amorphous product simultaneously. Furthermore, high-temperature annealing of the oxidized Mo2CTx MXene powder at 800 °C transformed the amorphous Mo-containing product into highly crystalline MoO2 crystals. These findings highlight the unconventional oxidation behavior of Mo-based MXenes, which suggests that the formation of crystalline Mo-based oxides requires a higher activation energy during oxidation than that of TiO2. The unique oxidative pathway reported herein can help elucidate the oxidation mechanisms of Mo-based MXene dispersions and their products. The insights from this study can pave the way for fundamental studies in academia as well as broaden the applications of Mo-based MXenes in various industries.

Exfoliated MXene-AuNPs hybrid in sensing and multiple catalytic hydrogenation reactions

The increasing use of nanomaterials in consumer products is expected to lead to environmental contamination sometime soon. As water pollution is a pressing issue that threatens human survival and impedes the promotion of human health, the search for adsorbents for removing newly identified contaminants from water has become a topic of intensive research. The challenges in the recyclability of contaminated water continue to campaign the development of highly reusable catalysts. Although exfoliated 2D MXene sheets have demonstrated the capability towards water purification, a significant challenge for removing some toxic organic molecules remains a challenge due to a need for metal-based catalytic properties owing to their rapid response. In the present study, we demonstrate the formation of hybrid structure AuNPs@MXene (Mo2CTx) during the sensitive detection of Au nanoparticle through MXene sheets without any surface modification, and subsequently its applications as an efficient catalyst for the degradation of 4-nitrophenol (4-NP), methyl orange (MO), and methylene blue (MB). The hybrid structure (AuNPs@MXene) reveals remarkable reusability for up to eight consecutive cycles, with minimal reduction in catalytic efficiency and comparable apparent reaction rate constant (Kapp) values for 4-NP, MB, and MO, compared to other catalysts reported in the literature.

Mo-based MXenes: Synthesis, properties, and applications

Ti-MXene allows a range of possibilities to tune their compositional stoichiometry due to their electronic and electrochemical properties. Other than conventionally explored Ti-MXene, there have been ample opportunities for the non-Ti-based MXenes, especially the emerging Mo-based MXenes. Mo-MXenes are established to be remarkable with optoelectronic and electrochemical properties, tuned energy, catalysis, and sensing applications. In this timely review, we systematically discuss the various organized synthesis procedures, associated experimental tunning parameters, physiochemical properties, structural evaluation, stability challenges, key findings, and a wide range of applications of emerging Mo-MXene over Ti-MXenes. We also critically examined the precise control of Mo-MXenes to cater to advanced applications by comprehensively evaluating the summary of recent studies using artificial intelligence and machine learning tools. The critical future perspectives, significant challenges, and possible outlooks for successfully developing and using Mo-MXenes for various practical applications are highlighted.

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 Brief Review of MoO3 and MoO3-Based Materials and Recent Technological Applications in Gas Sensors, Lithium-Ion Batteries, Adsorption, and Photocatalysis

Molybdenum trioxide is an abundant natural, low-cost, and environmentally friendly material that has gained considerable attention from many researchers in a variety of high-impact applications. It is an attractive inorganic oxide that has been widely studied because of its layered structure, which results in intercalation ability through tetrahedral/octahedral holes and extension channels and leads to superior charge transfer. Shape-related properties such as high specific capacities, the presence of exposed active sites on the oxygen-rich structure, and its natural tendency to oxygen vacancy that leads to a high ionic conductivity are also attractive to technological applications. Due to its chemistry with multiple valence states, high thermal and chemical stability, high reduction potential, and electrochemical activity, many studies have focused on the development of molybdenum oxide-based systems in the last few years. Thus, this article aims to briefly review the latest advances in technological applications of MoO3 and MoO3-based materials in gas sensors, lithium-ion batteries, and water pollution treatment using adsorption and photocatalysis techniques, presenting the most relevant and new information on heterostructures, metal doping, and non-stoichiometric MoO3-x.

Synthesis of Fluoride-Substituted Layered Perovskites ZnMoO4 with an Enhanced Photocatalytic Activity

Oxyfluorides possess considerable attention for their multiple excellent properties, but the conventional high-temperature solid-state syntheses have seen bottlenecks in the synthesis of new compounds. Herein, we report a novel layered oxyfluoride ZnMoO4:F, which is prepared by a facile hydrothermal method using ZnF2 as the fluoride source. The fluoride anions are successfully introduced into the oxygen sublattice, which is confirmed by a combined analysis using XRD, STEM, and TGA techniques. The as-synthesized ZnMoO4:F has an absorption edge at around 550 nm, indicating a red shift of Eg to the visible region compared to the oxide counterpart. The layered oxyfluoride exhibits an enhanced photocatalytic active for hydrogen evolution under simulated sunlight (λ > 350 nm), and the activity of ZnMoO4:F (651.9 μmol g-1) was 2 times higher than that of ZnMoO4 (309.7 μmol g-1). Further electrochemical analysis has shown that the conduction band position plays a critical role in the high performances of ZnMoO4:F. This work sheds new light on the future design and synthesis of novel fluoride-doped materials for photocatalysis applications.

A Laser-Induced Mo2 CTx MXene Hybrid Anode for High-Performance Li-Ion Batteries

MXenes, a fast-growing family of two-dimensional (2D) transition metal carbides/nitrides, are promising for electronics and energy storage applications. Mo2 CTx MXene, in particular, has demonstrated a higher capacity than other MXenes as an anode for Li-ion batteries. Yet, such enhanced capacity is accompanied by slow kinetics and poor cycling stability. Herein, it is revealed that the unstable cycling performance of Mo2 CTx is attributed to the partial oxidation into MoOx with structural degradation. A laser-induced Mo2 CTx /Mo2 C (LS-Mo2 CTx ) hybrid anode has been developed, of which the Mo2 C nanodots boost redox kinetics, and the laser-reduced oxygen content prevents the structural degradation caused by oxidation. Meanwhile, the strong connections between the laser-induced Mo2 C nanodots and Mo2 CTx nanosheets enhance conductivity and stabilize the structure during charge-discharge cycling. The as-prepared LS-Mo2 CTx anode exhibits an enhanced capacity of 340 mAh g-1 vs 83 mAh g-1 (for pristine) and an improved cycling stability (capacity retention of 106.2% vs 80.6% for pristine) over 1000 cycles. The laser-induced synthesis approach underlines the potential of MXene-based hybrid materials for high-performance energy storage applications.

MXene Derivatives for Energy Storage and Conversions

Associated with the rapid development of 2D transition metal carbides, nitrides, and carbonitrides (MXenes), MXene derivatives have been recently exploited and exhibited unique physical/chemical properties, holding promising applications in the areas of energy storage and conversions. This review provides a comprehensive summarization of the latest research and progress on MXene derivatives, including termination-tailored MXenes, single-atom implanted MXenes, intercalated MXenes, van der Waals atomic layers, and non-van der Waals heterostructures. The intrinsic relationship between structure, properties, and corresponding applications for MXene derivatives are then emphasized. Finally, the essential challenges are addressed and perspectives for the MXene derivatives are also discussed.

MXene: fundamentals to applications in electrochemical energy storage

A new, sizable family of 2D transition metal carbonitrides, carbides, and nitrides known as MXenes has attracted a lot of attention in recent years. This is because MXenes exhibit a variety of intriguing physical, chemical, mechanical, and electrochemical characteristics that are closely linked to the wide variety of their surface terminations and elemental compositions. Particularly, MXenes are readily converted into composites with materials including oxides, polymers, and CNTs, which makes it possible to modify their characteristics for a variety of uses. MXenes and MXene-based composites have demonstrated tremendous promise in environmental applications due to their excellent reducibility, conductivity, and biocompatibility, in addition to their well-known rise to prominence as electrode materials in the energy storage sector. The remarkable characteristics of 2D MXene, including high conductivity, high specific surface area, and enhanced hydrophilicity, account for the increasing prominence of its use in storage devices. In this review, we highlight the most recent developments in the use of MXenes and MXene-based composites for electrochemical energy storage while summarizing their synthesis and characteristics. Key attention is paid to applications in supercapacitors, batteries, and their flexible components. Future research challenges and perspectives are also described.

MXene chemistry, electrochemistry and energy storage applications

The diverse and tunable surface and bulk chemistry of MXenes affords valuable and distinctive properties, which can be useful across many components of energy storage devices. MXenes offer diverse functions in batteries and supercapacitors, including double-layer and redox-type ion storage, ion transfer regulation, steric hindrance, ion redistribution, electrocatalysts, electrodeposition substrates and so on. They have been utilized to enhance the stability and performance of electrodes, electrolytes and separators. In this Review, we present a discussion on the roles of MXene bulk and surface chemistries across various energy storage devices and clarify the correlations between their chemical properties and the required functions. We also provide guidelines for the utilization of MXene surface terminations to control the properties and improve the performance of batteries and supercapacitors. Finally, we conclude with a perspective on the challenges and opportunities of MXene-based energy storage components towards future practical applications.

In Situ Architecting Endogenous Heterojunction of MoS2 Coupling with Mo2 CTx MXenes for Optimized Li+ Storage

Endogenous heterojunction of 2D MXenes with unique structure shows inspiring potential in energy applications, which is impeded by complex synthesis method and finite MAX materials. Herein, an in situ hydrothermal strategy is implemented to successfully synthesize unique endogenous hetero-MXenes of amorphous MoS₂ coupling with fluoride-free Mo₂ CT_x (hetero-Mo₂ C) directly from Mo₂ Ga₂ C MAX. The distinctive morphology and heterojunction structure caused by the introduction of MoS₂ endow the hetero-MXenes with extraordinary structural stability and optimized Li⁺ storage mechanism with improved charge transport and lithium ion adsorption capabilities. As a result, hetero-Mo₂ C exhibits excellent electrochemical performance with a high discharge specific capacity of 1242 mAh g^-1 at 0.1 A g^-1 and long cycle stability of 683.9 mAh g^-1 after 1200 cycling. This work provides new insights into rational design of novel MXenes heterojunctions, practically important for the development of MXenes and their applications in high-performance energy storage systems.

Synergistic catalytic effects of MoO2 and Vulcan carbon on the oxygen reduction reaction

The MoO₂/C composite exhibits superior electrocatalytic activity towards the ORR via a near four-electron reaction path.

2H-MoS2 on Mo2CTx MXene Nanohybrid for Efficient and Durable Electrocatalytic Hydrogen Evolution

The development of highly efficient and durable earth-abundant hydrogen evolution reaction (HER) catalysts is crucial for the extensive implementation of the hydrogen economy. Members of the 2D MXenes family, particularly Mo₂CT_x, have recently been identified as promising HER catalysts. However, their inherent oxidative instability in air and aqueous electrolyte solutions is hindering their widespread use. Herein, we present a simple and scalable method to circumvent adventitious oxidation in Mo₂CT_x MXenes via in situ sulfidation to form a Mo₂CT_x/2H-MoS₂ nanohybrid. The intimate epitaxial coupling at the Mo₂CT_x/2H-MoS₂ nanohybrid interface afforded superior HER activities, requiring only 119 or 182 mV overpotential to yield -10 or -100 mA cm^-2_geom current densities, respectively. Density functional theory calculations reveal strongest interfacial adhesion was found within the nanohybrid structure as compared to the physisorbed nanohybrid, and the possibility to tune the HER overpotential through manipulating the extent of MXene sulfidation. Critically, the presence of 2H-MoS₂ suppresses further oxidation of the MXene layer, enabling the nanohybrid to sustain industrially relevant current densities of over -450 mA cm^-2_geom with exceptional durability. Less than 30 mV overpotential degradation was observed after 10 continuous days of electrolysis at a fixed -10 mA cm^-2_geom current density or 100,000 successive cyclic voltammetry cycles. The exceptional HER durability of the Mo₂CT_x/2H-MoS₂ nanohybrid presents a major step forward to realize practical implementation of MXenes as noble metal free catalysts for broad-based applications in water splitting and energy conversion.

Strongly Coupled NiCo2O4 Nanocrystal/MXene Hybrid through In Situ Ni/Co-F Bonds for Efficient Wearable Zn-Air Batteries

Recently, owing to the high energy density and excellent security, wearable Zn-air batteries (ZABs) have been known as one of the most prominent wearable energy storage devices. However, sluggish oxygen reaction kinetics of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the air-breathe cathode seriously has limited further practical applications. In this work, we synthesize a NiCo₂O₄ nanocrystal/MXene hybrid with strong Ni/Co-F bonds. The prepared MXene-based hybrid composites show remarkable ORR and OER electrocatalytic activity, which results in the fabricated solid-state ZAB device to achieve an open-circuit voltage of 1.40 V, peak power density of 55.1 mW cm^-2, and energy efficiency of 66.1% at 1.0 mA cm^-2; to the best of our knowledge, this is the record performance among all reported flexible ZABs with MXene-based air cathodes and comparable with some noble metal catalysts. Moreover, even after cutting and suturing, our flexible solid-state ZAB devices are tailorable with high rate of performance.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

CO2-Oxidized Ti3C2Tx-MXenes Components for Lithium-Sulfur Batteries: Suppressing the Shuttle Phenomenon through Physical and Chemical Adsorption

Lithium-sulfur (Li-S) batteries are one of the main challenges facing Li-ion technology because the insulating nature of sulfur and the shuttle phenomenon of dissolved lithium polysulfides (LPSs) in liquid electrolytes result in critical problems, including low Coulombic efficiency, loss of active material, and rapid capacity decay. Here, we oxidized delaminated transition metal carbides (MXenes) using CO₂ (Oxi-d-MXenes) and used them as both cathode electrode with sulfur and modified separator coated onto the glass fiber without a conductive material and binder to suppress the diffusion of LPSs. Oxi-d-MXenes annealed at 900 °C using CO₂ gas formed perfectly converted rutile-TiO₂ nanocrystalline particles on their two-dimensional sheets. Li-S batteries fabricated with the Oxi-d-MXenes cathode and the Oxi-d-MXenes-modified separator exhibited high Coulombic efficiency (nearly 99%) and retained a capacity of about 900 mAh g^-1 after 300 cycles at a current density of 1C. These results were attributed to the chemical and physical adsorption between the Oxi-d-MXenes and the LPSs. Our results imply that Oxi-d-MXenes prepared by the CO₂ treatment exhibit physical and electrochemical properties that enhance the performance of Li-S batteries.

Transformation of carbon dioxide into carbon nanotubes for enhanced ion transport and energy storage

The synthesis of carbon nanotubes (CNTs) from CO₂ is an attractive strategy to reduce CO₂ emission, but involves extreme reaction conditions and has low scalability. This work introduces continuous chemical vapor deposition for the conversion of CO₂ to CNTs using the NaBH₄ reductant and NiCl₂ catalyst. Multi-walled CNT fibers were synthesized from gaseous CO₂ under mild conditions (500-700 °C and 1 atm). Based on in situ analyses, the proposed mechanism behind the formation of CO₂-derived CNTs (CCNTs) is CO₂ activation and subsequent hydroboration for the generation of methane, which can induce the growth of CCNTs on the catalyst. Their intrinsic properties give rise to an enhanced capacitive performance. The boron and oxygen of CCNTs provide a pseudo-capacitance of 302 F g^-1 at a low charging rate of 0.1 A g^-1 in 1 M TEABF₄/acetonitrile. The mesoporous networks between CCNT fibers enhance ion transport at a high current density of 205 A g^-1, leading to an outstanding energy density of 13 W h kg^-1 at a high power density of 115 kW kg^-1. A well-developed graphitized structure of CCNTs contributes to the reduction of the electrochemical resistance and leads to their superior stability at 65 °C during 10 000 cycles.

2D organ-like molybdenum carbide (MXene) coupled with MoS2 nanoflowers enhances the catalytic activity in the hydrogen evolution reaction

Our work introduces an emerging route for the synthesis of MoS₂ nanoflowers decorating organ-like Mo₂CT_x MXene. This effective synthesis strategy of MoS₂@Mo₂CT_x nanohybrid structure can shed some light on energy-related applications.

Formation of toroidal Li2O2 in non-aqueous Li-O2 batteries with Mo2CT x MXene/CNT composite

Due to the growing demand for high energy density devices, Li-O2 batteries are considered as a next generation energy storage system. The battery performance is highly dependent on the Li2O2 morphology, which arises from formation pathways such as the surface growth and the solution growth models. Thus, controlling the formation pathway is important in designing cathode materials. Herein for the first time, we controlled the Li2O2 formation pathway by using Mo2CT x MXene on a catalyst support. The cathode was fabricated by mixing the positively charged CNT/CTAB solution with the negatively charged Mo2CT x solution. After introducing Mo2CT x , important battery performance metrics were considerably enhanced. More importantly, the discharge product analysis showed that the functional groups on the surface of Mo2CT x inhibit the adsorption of O2 on the cathode surface, resulting in the formation of toroidal Li2O2 via the solution growth model. It was supported by density functional theory (DFT) calculations that adsorption of O2 on the Mo2CT x surface is implausible due to the large energy penalty for the O2 adsorption. Therefore, the introduction of MXene with abundant functional groups to the cathode surface can provide a cathode design strategy and can be considered as a universal method in generating toroidal Li2O2 morphology.

Quantum-Dot-Mediated Controlled Synthesis of Dual Oxides of Molybdenum from MoS2 : Quantification of Supercapacitor Efficacy

The versatile technological applications of molybdenum oxides requires the efficient synthesis of various stoichiometric molybdenum oxides. Thus, herein, a controlled method to synthesize both MoO₃ and MoO₂ from MoS₂ via quantum dot intermediates is reported. Microscopic, spectroscopic, and X-ray studies corroborate the formation of orthorhombic α-MoO₃ with a microbelt structure and monoclinic MoO₂ nanoparticles that self-assemble into hollow tubes. Quantitative investigations into charge-storage kinetics reveal that MoO₂ exhibits an excellent pseudocapacitive response up to a mass loading of 5 mg cm^-2 with an areal capacity of 327.2 mC cm^-2 at 5 mV s^-1 , with 41.9 % retention at 100 mV s^-1 . In contrast, above a mass loading of 0.5 mg cm^-2 , the charge-storage nature of MoO₃ electrodes switches from that of a supercapacitor to battery type. At a sweep rate of 50 mV s^-1 , 87.2 % of the total charge is contributed by a capacitive response in a 1 mg cm^-2 MoO₂ electrode. The charge-storage kinetics of MoO₃ and MoO₂ reflect on the respective asymmetric supercapacitors. A MoO₂ //graphite asymmetric supercapacitor holds an outstanding energy density of 341 mW h m^-2 at a power density of 4949 mW m^-2 and delivers an ultrahigh power density of 28140 mW m^-2 with an energy density 142 mW h m^-2 and energy efficiency of 87 %.