Constructing Hierarchical Porous MoO2 @Mo2 N@C Composite via a Confined Pyrolysis Synthetic Strategy Towards Lithium-Ion Battery Anodes.

Molybdenum dioxide (MoO2 ) demonstrates a big potential toward lithium-ion storage due to its high theoretical capacity. The sluggish reaction kinetics and large volume change during cycling process, however, unavoidably lead to inferior electrochemical performance, thus failing to satisfy the requirements of practical applications. Herein, we developed a molybdenum-based oxyacid salt confined pyrolysis strategy to achieve a novel hierarchical porous MoO2 @Mo2 N@C composite. A two-step successive annealing process was proposed to obtain a hybrid phase of MoO2 and Mo2 N, which was used to further improve the electrochemical performance of MoO2 -based anode. We demonstrate that the well-dispersed MoO2 nanoparticles can ensure ample active sites exposure to the electrolyte, while conductive Mo2 N quantum dots afford pseudo-capacitive response, which conduces to the migration of ions and electrons. Additionally, the interior voids could provide buffer spaces to surmount the effect of volume change, thereby avoiding the fracture of MoO2 nanoparticles. Benefiting from the aforesaid synergies, the as-obtained MoO2 @Mo2 N@C electrode demonstrates a striking initial discharge capacity (1760.0 mAh g-1 at 0.1 A g-1 ) and decent long-term cycling stability (652.5 mAh g-1 at 1.0 A g-1 ). This work provides a new way for the construction of advanced anode materials for lithium-ion batteries.

Gradient Heating Epitaxial Growth Gives Well Lattice-Matched Mo2 C-Mo2 N Heterointerfaces that Boost Both Electrocatalytic Hydrogen Evolution and Water Vapor Splitting.

An optimized approach to producing lattice-matched heterointerfaces for electrocatalytic hydrogen evolution has not yet been reported. Herein, we present the synthesis of lattice-matched Mo2 C-Mo2 N heterostructures using a gradient heating epitaxial growth method. The well lattice-matched heterointerface of Mo2 C-Mo2 N generates near-zero hydrogen-adsorption free energy and facilitates water dissociation in acid and alkaline media. The lattice-matched Mo2 C-Mo2 N heterostructures have low overpotentials of 73 mV and 80 mV at 10 mA cm-2 in acid and alkaline solutions, respectively, comparable to commercial Pt/C. A novel photothermal-electrocatalytic water vapor splitting device using the lattice-matched Mo2 C-Mo2 N heterostructure as a hydrogen evolution electrocatalyst displays a competitive cell voltage for electrocatalytic water splitting.

In-situsputtered 2D-MoS2nanoworms reinforced with molybdenum nitride towards enhanced Na-ion based supercapacitive electrodes.

We report the fabrication of binder-free, low-cost and efficient hybrid supercapacitive electrode based on the hexagonal phase of two-dimensional MoS2nanoworms reinforced with molybdenum nitride nanoflakes deposited on stainless steel (SS) substrate using reactive magnetron sputtering technique. The hybrid nanostructured MoS2-Mo2N/SS thin film working electrode delivers a high gravimetric capacitance (351.62 F g-1at 0.25 mA cm-2) investigated in 1 M Na2SO4aqueous solution. The physisorption/intercalation of sodium (Na+) ions in electroactive sites of MoS2-Mo2N composite ensures remarkable electrochemical performance. The deposited porous nanostructure with good electrical conductivity and better adhesion with the current collector demonstrates a high-energy density of 82.53 Wh kg-1in addition to a high-power density of 24.98 kW kg-1. Further, excellent capacitance retention of 93.62% after 4000 galvanostatic charge-discharge cycles elucidated it as a promising candidate for realizing high-performance supercapacitor applications.

A Highly Active Porous Mo2C-Mo2N Heterostructure on Carbon Nanowalls/Diamond for a High-Current Hydrogen Evolution Reaction.

Developing non-precious metal-based electrocatalysts operating in high-current densities is highly demanded for the industry-level electrochemical hydrogen evolution reaction (HER). Here, we report the facile preparation of binder-free Mo2C-Mo2N heterostructures on carbon nanowalls/diamond (CNWs/D) via ultrasonic soaking followed by an annealing treatment. The experimental investigations and density functional theory calculations reveal the downshift of the d-band center caused by the heterojunction between Mo2C/Mo2N triggering highly active interfacial sites with a nearly zero ∆GH* value. Furthermore, the 3D-networked CNWs/D, as the current collector, features high electrical conductivity and large surface area, greatly boosting the electron transfer rate of HER occurring on the interfacial sites of Mo2C-Mo2N. Consequently, the self-supporting Mo2C-Mo2N@CNWs/D exhibits significantly low overpotentials of 137.8 and 194.4 mV at high current densities of 500 and 1000 mA/cm2, respectively, in an alkaline solution, which far surpass the benchmark Pt/C (228.5 and 359.3 mV) and are superior to most transition-metal-based materials. This work presents a cost-effective and high-efficiency non-precious metal-based electrocatalyst candidate for the electrochemical hydrogen production industry.

Pseudocapacitive Kinetics in Synergistically Coupled MoS2-Mo2N Nanowires with Enhanced Interfaces toward All-Solid-State Flexible Supercapacitors.

Pseudocapacitive kinetics in rationally engineered nanostructures can deliver higher energy and power densities simultaneously. The present report reveals a high-performance all-solid-state flexible symmetric supercapacitor (FSSC) based on MoS2-Mo2N nanowires deposited directly on stainless steel mesh (MoS2-Mo2N/SSM) employing DC reactive magnetron co-sputtering technology. The abundance of synergistically coupled interfaces and junctions between MoS2 nanosheets and Mo2N nanostructures across the nanocomposite results in greater porosity, increased ionic conductivity, and superior electrical conductivity. Consequently, the FSSC device utilizing poly(vinyl alcohol)-sodium sulfate (PVA-Na2SO4) hydrogel electrolyte renders an outstanding cell capacitance of 252.09 F·g-1 (44.12 mF·cm-2) at 0.25 mA·cm-2 and high rate performance within a wide 1.3 V window. Dunn's and b-value analysis reveals significant energy storage by surface-controlled capacitive and pseudocapacitive mechanisms. Remarkably, the symmetric device boosts tremendous energy density ∼10.36 µWh·cm-2 (59.17 Wh·kg-1), superb power density ∼6.5 mW·cm-2 (37.14 kW·kg-1), ultrastable long cyclability (∼93.7% after 10,000 galvanostatic charge-discharge cycles), and impressive mechanical flexibility at 60°, 90°, and 120° bending angles.

Promoting electrochemical ammonia synthesis by synergized performances of Mo2C-Mo2N heterostructure.

Hydrogen has become an indispensable aspect of sustainable energy resources due to depleting fossil fuels and increasing pollution. Since hydrogen storage and transport is a major hindrance to expanding its applicability, green ammonia produced by electrochemical method is sourced as an efficient hydrogen carrier. Several heterostructured electrocatalysts are designed to achieve significantly higher electrocatalytic nitrogen reduction (NRR) activity for electrochemical ammonia production. In this study, we controlled the nitrogen reduction performances of Mo2C-Mo2N heterostructure electrocatalyst prepared by a simple one pot synthesis method. The prepared Mo2C-Mo2N0.92 heterostructure nanocomposites show clear phase formation for Mo2C and Mo2N0.92, respectively. The prepared Mo2C-Mo2N0.92 electrocatalysts deliver a maximum ammonia yield of about 9.6 µg h-1 cm-2 and a Faradaic efficiency (FE) of about 10.15%. The study reveals the improved nitrogen reduction performances of Mo2C-Mo2N0.92 electrocatalysts due to the combined activity of the Mo2C and Mo2N0.92 phases. In addition, the ammonia production from Mo2C-Mo2N0.92 electrocatalysts is intended by the associative nitrogen reduction mechanism on Mo2C phase and by Mars-van-Krevelen mechanism on Mo2N0.92 phase, respectively. This study suggests the importance of precisely tuning the electrocatalyst by heterostructure strategy to substantially achieve higher nitrogen reduction electrocatalytic activity.

Characteristics of the Structure, Mechanical, and Tribological Properties of a Mo-Mo2N Nanocomposite Coating Deposited on the Ti6Al4V Alloy by Magnetron Sputtering.

Mo-Mo2N nanocomposite coating was produced by reactive magnetron sputtering of a molybdenum target, in the atmosphere, of Ar and N2 gases. Coating was deposited on Ti6Al4V titanium alloy. Presented are the results of analysis of the XRD crystal structure, microscopic SEM, TEM and AFM analysis, measurements of hardness, Young's modulus, and adhesion. Coating consisted of α-Mo phase, constituting the matrix, and γ-Mo2N reinforcing phase, which had columnar structure. The size of crystallite phases averaged 20.4 nm for the Mo phase and 14.1 nm for the Mo2N phase. Increasing nitrogen flow rate leads to the fragmentation of the columnar grains and increased hardness from 22.3 GPa to 27.5 GPa. The resulting coating has a low Young's modulus of 230 GPa to 240 GPa. Measurements of hardness and Young's modulus were carried out using the nanoindentation method. Friction coefficient and tribological wear of the coatings were determined with a tribometer, using the multi-cycle oscillation method. Among tested coatings, the lowest friction coefficient was 0.3 and wear coefficient was 10 × 10-16 m3/N∙m. In addition, this coating has an average surface roughness of RMS < 2.4 nm, determined using AFM tests, as well as a good adhesion to the substrate. The dominant wear mechanism of the Mo-Mo2N coatings was abrasive wear and wear by oxidation. The Mo-Mo2N coating produced in this work is a prospective material for the elements of machines and devices operating in dry friction conditions.

Mo2 N-W2 N Heterostructures Embedded in Spherical Carbon Superstructure as Highly Efficient Polysulfide Electrocatalysts for Stable Room-Temperature Na-S Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries are highly desirable for a sustainable large-scale energy-storage system due to their high energy density and low cost. Nevertheless, practical applications of RT Na-S batteries are still prevented by the shuttle effect of sodium polysulfides (NaPS), slow reaction kinetics of S, and incomplete conversion process of NaPS. Here, Mo2 N-W2 N heterostructures embedded in a spherical carbon superstructure (Mo2 N-W2 N@PC) are designed to efficiently suppress the "polysulfide shuttle" and promote NaPS redox reactions. The designed Mo2 N-W2 N@PC heterostructure with abundant heterointerfaces, high conductivity, and porosity can facilitate electron/ion diffusion and provide high catalytic activity for efficient NaPS conversion. The obtained Na-S battery delivers high reversible capacity with superior long-term cyclability (517 mAh g-1 at 1 A g-1 after 400 cycles) and unprecedented rate capability (417 mAh g-1 at 2 A g-1 ). Furthermore, the electrocatalysis mechanism is revealed by combining in situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), UV-vis spectra, and precipitation experiments. This work demonstrates a novel heterostructure design strategy that enables high-performance Na-S batteries.

Interfacial Engineering of 3D Hollow Mo-Based Carbide/Nitride Nanostructures.

Molybdenum carbide and nitride nanocrystals have been widely recognized as ideal electrocatalyst materials for water splitting. Furthermore, the interfacial engineering strategy can effectively tune their physical and chemical properties to improve performance. Herein, we produced N-doped molybdenum carbide nanosheets on carbonized melamine (N-doped Mo2C@CN) and 3D hollow Mo2C-Mo2N nanostructures (3D H-Mo2C-Mo2N) with tuneable interfacial properties via high-temperature treatment. X-ray photoelectron spectroscopy reveals that Mo2C and Mo2N nanocrystals in 3D hollow nanostructures are chemically bonded with each other and produce stable heterostructures. The 3D H-Mo2C-Mo2N nanostructures demonstrate lower onset potential and overpotential at a current density of 10 mV cm-2 than the N-doped Mo2C@CN nanostructure due to its higher active sites and improved interfacial charge transfer. The current work presents a strategy to tune metal carbide/nitride nanostructures and interfacial properties for the production of high-performance energy materials.

Molybdenum Nitride Electrocatalysts for Hydrogen Evolution More Efficient than Platinum/Carbon: Mo2N/CeO2@Nickel Foam.

To produce hydrogen economically by electrolysis of water, one needs to develop a non-precious-metal catalyst that is as efficient as platinum metal. Here, we prepare such a catalyst by growing a layer of Mo2N over a layer of CeO2 deposited on nickel foam (NF) [hereafter, Mo2N /CeO2@NF] and show that the activity of this self-supported catalyst for hydrogen evolution in 1.0 M KOH is more efficient than that of the Pt/C electrode, achieving a current density of 10 mA/cm2 at a fairly low overpotential of 26 mV. Furthermore, after a long-time electrochemical stability test for 24 h at a fixed current density, the overpotential needed to attain a current density of 10 mA/cm2 is increased only by 6 mV, implying the huge potential of this method to prepare a super HER activity electrode for water splitting.

Mo2N nanobelt cathodes for efficient hydrogen production in microbial electrolysis cells with shaped biofilm microbiome.

High cost platinum (Pt) catalysts limit the application of microbial electrolysis cells (MECs) for hydrogen (H2) production. Here, inexpensive and efficient Mo2N nanobelt cathodes were prepared using an ethanol method with minimized catalyst and binder loadings. The chronopotentiometry tests demonstrated that the Mo2N nanobelt cathodes had similar catalytic activities for H2 evolution compared to that of Pt/C (10 wt%). The H2 production rates (0.39 vs. 0.37 m3-H2/m3/d), coulombic efficiencies (90% vs. 77%), and overall hydrogen recovery (74% vs. 70%) of MECs with the Mo2N nanobelt cathodes were also comparable to those with Pt/C cathodes. However, the cost of Mo2N nanobelt catalyst ($ 31/m2) was much less than that of Pt/C catalysts ($ 1930/m2). Furthermore, the biofilm microbiomes at electrodes were studied using the PacBio sequencing of full-length 16S rRNA gene. It indicated Stenotrophomonas nitritireducens as a putative electroactive bacterium dominating the anode biofilm microbiomes. The majority of dominant species in the Mo2N and Pt/C cathode communities belonged to Stenotrophomonas nitritireducens, Stenotrophomonas maltophilia, and Comamonas testosterone. The dominant populations in the cathode biofilms were shaped by the cathode materials. This study demonstrated Mo2N nanobelt catalyst as an alternative to Pt catalyst for H2 production in MECs.

Monocyclic aromatic hydrocarbons production from catalytic cracking of pine wood-derived pyrolytic vapors over Ce-Mo2N/HZSM-5 catalyst.

A series of Mo2N/HZSM-5 and transition metal modified Mo2N/HZSM-5 catalysts were prepared for the catalytic upgrading of pine wood-derived pyrolytic vapors for the selective production of monocyclic aromatic hydrocarbons (MAHs), while restraining the formation of polycyclic aromatic hydrocarbons (PAHs). Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed to determine the effects of several factors on selective MAHs production, including Mo2N loading on HZSM-5, transition metal (Fe, Ce, La, Cu, Cr) modification of Mo2N/HZSM-5, pyrolysis temperature, and catalyst-to-biomass ratio. In addition, quantitative experiments were conducted to determine the actual yields of major aromatic hydrocarbons and the source of aromatic hydrocarbons from basic biomass components. Results indicated that among the various catalysts, the Ce-10%Mo2N/HZSM-5 exhibited the best performance on promoting the formation of MAHs and restraining the generation of PAHs. Under the optimal conditions, the actual yields of MAHs and PAHs from Ce-10%Mo2N/HZSM-5 catalytic process were 99.8mg/g and 7.5mg/g, while those from HZSM catalyst were only 77.2mg/g and 23.7mg/g respectively. Furthermore, the possible catalytic mechanism of the Ce-Mo2N/HZSM-5 catalyst was proposed based on the catalyst characterization.

Efficient Electrocatalytic Hydrogen Evolution from MoS2-Functionalized Mo2N Nanostructures.

Molybdenum-based compounds and their composites were investigated as an alternative to Pt for hydrogen evolution reactions. The presence of interfaces and junctions between Mo2N and MoS2 grains in the composites were investigated to understand their role in electrochemical processes. Here we found that the electrocatalytic activity of Mo2N nanostructures was enhanced remarkably by conjugation with few-layer MoS2 sheets. The electrocatalytic performance of Mo2N-MoS2 composites in the hydrogen evolution reaction (HER) was revealed from the high catalytic current density of ∼175 mA cm-2 (at 400 mV) and good electrochemical stability (more than 18 h) in acidic media. Increasing the amount of MoS2 in the composite, decreases the HER activity. The mechanism and kinetics of the HER process on the Mo2N-MoS2 surface were analyzed using Tafel slopes and charge transfer resistance.