Mo2C Nanocrystals Research Articles

Liquid metal-assisted solid-flame combustion synthesis of transition metal carbide nanocrystals for enhanced hydrogen evaluation reaction

Efficient hydrogen production relies on cost-effective and durable non-noble metal electrocatalysts for the hydrogen evolution reaction (HER). In this work, a liquid metal-assisted solid-flame combustion synthesis (SF-CS) method was used to create transition metal carbide TMC) nanocrystals with exposed facets. The TMCs include TiC, VC, NbC, TaC, Ta2C, WC, and Mo2C. In acidic solutions, the TMC nanocrystals show enhanced conductivity and superior HER performance, as evidenced by their lower Tafel slope, higher current density, and decreased overpotential. In particular, Mo2C nanocrystals exhibit an overpotential of 95 mV and a Tafel slope of 26 mV dec−1. These findings highlight a noteworthy improvement over previously reported nanostructures in recent years in terms of the HER properties of Mo2C with particular facets. This improvement is attributed to the unique surface characteristics, such as energy and active sites, which are impacted by various factors and require more investigation to meet the intended goals.

Highly dispersed conductive and electrocatalytic mediators enabling rapid polysulfides conversion for lithium sulfur batteries

The practical application of Li-S batteries encounters hurdles in maintaining cycling durability and rate capability under realistic conditions. Electrostatic self-assembly stands as a captivating avenue for exploring new frontiers in nanoscale catalysis. Leveraging renewable bio-oil both as a carbon source and self-polymerization precursor, ultrafine Mo2C nanocrystals anchored on bio-oil-derived 3D hierarchical carbon matrix (Mo2C@BOC) are fabricated by a facile NaCl template-assisted electrostatic self-assembly and followed annealing procedure. Densely anchored ultrafine Mo2C nanocrystals, intertwined with highly conductive carbon nanosheets, unveil an enhanced exposure of catalytically active sites for lithium polysulfide immobilization, conversion, and supply amply nucleation sites to mediate the fast precipitation of Li2S2/Li2S. Attributed to these favorable features, Li-S cells based on the as-designed catalytic host attain satisfactory cyclability with a minimum capacity fading rate of 0.0144 % over 1000 cycles and excellent rate capability. Decent performance can be achieved even at a high sulfur loading of 8.0 mg cm−2 and a low electrolyte-to-sulfur ratio of 3.5 µL mg−1. This strategy for solving the shuttle effect under high sulfur loading provides a promising solution for the further development of high-performance Li-S batteries. This work provides a rational strategy for solving the shuttle effect under high sulfur loading and sheds light on the great potential of Mo2C@BOC for developing more practical Li-S batteries.

Mo2C nanocrystalline coupling with Ni encapsulated in N-doped fibrous carbon network as bifunctional catalysts towards advanced anion exchange membrane electrolyzers

Anion exchange membrane water electrolysis (AEMWE) has been considered to be the promising approach for cost-effective hydrogen production. However, the noble metal catalysts of AEM enable it still under the early stage of development. Herein, Mo2C nanocrystalline coupling with Ni have been successfully encapsulated into robust carbonaceous network (Mo2C/NC@0.5Ni) as inexpensive transition metal catalysts for AEM electrolyzers. The Mo2C nanocrystals are uniformly confined in N doped porous carbon network which is derived from the decomposition of PVP and urea during carbonation process. Importantly, rational designed Ni layers are also coated on the surface of the network structure. When utilized as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the Mo2C/NC@ 0.5Ni can achieve desirable overpotentials of 103 mV and 307 mV at a current density of 10 mA cm−2 in 1 M KOH, respectively. Furthermore, the Mo2C/NC@ 0.5Ni‖Mo2C/NC@ 0.5Ni AEM electrolyzers requires only a voltage of 1.9 V to reach a current density of 110.5 mA cm−2 with remarkable durability over 50 h. Numerous interconnected porous structure and defects caused by N doping can provide abundant active sites, facilitate electrons transportations and suppress the aggregation of inner Mo2C. Particularly, the conductive Ni layers could effectively improve the structural strength and induce the synergistic effect with Mo2C component, thereby, improving the catalytic activity and stability. In addition, the DFT calculation results indicate that the Mo2C/NC@ 0.5Ni catalyst has a lower Gibbs free energy of hydrogen adsorption (ΔGH*=−0.21 eV), which is beneficial to its HER catalytic activity. This work provides an insight in developing advanced AEMWE for high efficient and green hydrogen production.

Synergistic Effect of Dual-Doped Carbon on Mo2C Nanocrystals Facilitates Alkaline Hydrogen Evolution

HighlightsThe B and N dual-doped carbon layer that encapsulated on Mo2C nanocrystals (Mo2C@BNC) were fabricated for accelerating HER under alkaline condition.Theoretical calculations reveal that the H2O could be decomposed spontaneously over the introduced B sites, and the defective C atoms in the dual-doped carbon layer provide the best H binding sites.The optimized dual doped Mo2C catalyst with synergistic effect of non-metal sites delivers superior HER performances.

Tailoring the phase evolution of molybdenum-based nanocrystals in carbon nanofibers for enhanced performance of lithium-ion batteries

The design and development of flexible and free-standing molybdenum (Mo)-based composite nanomaterials have aroused intensive attention in the fields of electrocatalysis, energy storage, and wearable electronics. In this study, novel nanocomposite membranes constructed with Mo-based compound nanocrystals loaded carbon nanofibers (CNFs) were successfully generated by a combination of electrospinning molybdenum acetylacetone-contained polyacrylonitrile (PAN) solution and subsequent annealing process from 580 to 900 ºC. The relationship and mechanisms between the generation of different types of Mo-based nanocrystals and the annealing temperature were systematically explored. It was found that MoO2 nanocrystals were formed when the annealing temperature was 580 °C. With the temperature increasing, Mo2N nanocrystals began to form due to the reaction between the formed MoO2 and the N atoms in the PAN macromolecules. At higher temperature like 669 °C, Mo2C nanocrystals started to form due to the reaction between Mo2N and C atoms in the carbonized PAN macromolecules, and single-phase Mo2C nanocrystals were produced in CNFs at 900 ºC. Then, a series of flexible CNF-based nanocomposite membranes embedded with various different types of Mo-based compound nanocrystals were generated by effectively controlling the annealing temperature, which were all utilized as anodes for the construction of lithium-ion batteries. Electrochemical measurements showed that the anode materials annealed at 600 oC possessed the highest specific capacity, which were due to the high theoretical specific capacity of MoO2 and significantly improved conductivity of Mo2N. Moreover, all the as-developed electrodes exhibited excellent cycling stability during the charge-discharge process. The present study provides a facile and broadly-applicable routine to tailor the component, structure, and electrochemical properties of Mo-based nanocrystals in CNFs, which finds huge application potential in the fields of various flexible electrochemical devices.

Rational design of ultrathin Mo2C/C nanosheets decorated on mesoporous hollow carbon spheres as a multifunctional sulfur host for advanced Li-S batteries

Developing sulfur hosts with reasonable structure and effective catalyst is essential for lithium-sulfur (Li-S) batteries. Herein, a novel multifunctional sulfur host composed of hollow carbon spheres with mesoporous channels (MHCS) coupled with few ultrathin Mo2C/C nanosheets embedded with Mo2C nanocrystals ([email protected]2C/C) has been prepared via one-pot self-assembly and in-situ grown strategies. The Mo2C/C nanosheets with heterostructure can shorten the electron/ion transport path, and ultra-small Mo2C nanocrystals with abundant active sites contribute to enhancing chemical adsorption towards polysulfides and propel their conversion. Meanwhile, the MHCS facilitates physical adsorption towards polysulfides, effective sulfur/electrolyte permeation, fast electron/ion transportation, and sufficient buffering for volume change. Given these, the multifunctional [email protected]2C/C-S cathode demonstrates an initial specific capacity of 1316.8 mAh g−1 at 0.1 C and outstanding cycling stability with an average capacity decay rate of 0.045 % for 500 cycles at 0.5 C. Moreover, [email protected]2C/C-S cathode with high areal sulfur loading of 4.5 mg cm−2 can deliver a high initial discharge specific capacity of 797.4 mAh g−1, and a reversible capacity of 542.7 mAh g−1 after 500 cycles at 0.5 C. It is expected this work could provide a new perspective to the rational design of sulfur hosts for Li-S batteries with enhanced performance.

Anchoring ultra-small Mo2C nanocrystals on honeycomb-structured N-doped carbon spheres for efficient hydrogen evolution.

Molybdenum carbide (Mo2C) has attracted considerable research interest as one of the most efficient non-noble electrocatalysts for the hydrogen evolution reaction (HER). Supporting nanosized Mo2C on a conductive carbon matrix with high porosity and large surface area represents an efficient strategy to enhance its HER performance. Herein, we constructed a Mo2C based HER catalyst consisting of ultra-small Mo2C nanocrystals anchored on honeycomb-structured N-doped carbon spheres (Mo2C-HNCS). The as-prepared Mo2C-HNCS manifests a high HER catalytic activity in alkaline media with an overpotential of 128 mV at 10 mA cm-2, a Tafel slope of 60 mV decade-1, and good stability.

NaCl-Assisted Ultrasmall Mo2C Nanocrystals Confined in N, S Double-Doped Hierarchical Porous Carbon for Efficient Hydrogen Generation

Refining the size of nonprecious metal-based catalysts and restricting them in multiheteroatom-doped hierarchical porous carbon can achieve efficient electrical energy conversion. In this study, ultrasmall Mo2C nanocrystals with an average size of 4.9 nm are fitted to N and S double-doped hierarchical porous carbon (us-Mo2C/N,S-HPC) through a template strategy and used for efficient hydrogen production. First, the metal molybdenum salt is captured and fixed by natural bamboo leaf fibers which are pulverized by a ball mill. Meanwhile, the soluble sodium salt is largely filled in the bamboo leaf tissue. Subsequently, Mo2C nanocrystals formed by high-temperature reduction are uniformly anchored on the bamboo-derived N, S double-doped hierarchical porous carbon. As a result, the prepared us-Mo2C/N,S-HPC requires 150, 197, and 148 mV overpotential to drive a current density of 10 mA cm–1 at pH = 0, 7, and 14, respectively. This improvement is attributed to the synergistic effects of size controllable Mo2C, multidoped heteroatoms, and multiscale assembly structures. Significantly, this study is conducive to the construction of ultrafine nanocrystals and the high-value conversion of waste biomass.

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.

Enhanced Electrochemical Kinetics with Highly Dispersed Conductive and Electrocatalytic Mediators for Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are promising energy-storage devices because of their high theoretical energy densities. However, the practical application of Li-S batteries is still impeded by the poor cycling performance and rate capability at practical conditions. In order to improve the performance of practical Li-S batteries, a hierarchical Mo2 C nanocluster/carbon nanosheets hybrid based hollow spherical material (Mo2 C/CHS) is designed and prepared. The hollow spheres composed of stacked carbon nanosheets can facilitate the infiltration of electrolyte. The ultrasmall and highly conductive Mo2 C nanocrystals are confined in the carbon nanosheets and expose more active sites for anchoring and conversion of lithium polysulfides and increase the number of the nuclei for Li2 S2 /Li2 S precipitation. Benefitting from the synergistic effects, Mo2 C/CHS greatly promotes electrochemical kinetics in Li-S batteries with high sulfur loading (5 mg cm-2 ). Even under lean electrolyte conditions (E/S = 7 μL mgsulfur -1 ), the Li-S batteries with Mo2 C/CHS added exhibit a discharge capacity of 904 mAh g-1 at the high current rate of 0.5 C, and with 894 mAh g-1 maintained after 200 cycles. This work provides a fundamental understanding of the electrochemical processes and guides the rational design of host and additive materials for practical Li-S batteries.

Ultrasmall Mo2C nanocrystals embedded in N-doped porous carbons as a surface-dominated capacitive anode for lithium-ion capacitors.

In situ uniform confinement of ultrasmall Mo2C nanocrystals into micropore-enriched N-doped carbons was achieved by carbonizing phosphomolybdic acid/polyimide precursors to craft a surface-dominated capacitive battery-type anode. Upon coupling with a capacitor-type cathode, the as-fabricated lithium-ion capacitors exhibit superior power and energy outputs by improving the kinetics and capacity imbalance between two electrodes.

Thermodynamics guided ultrafast and continuous preparation of Mo2C nanocrystals for hydrogen evolution electrocatalysis

Existing nano-Mo2C preparation methods suffer from expensive raw materials, subtle operation demands and complex synthesis steps. Herein, both theoretical and experimental studies were performed to develop a more efficient and economical method for nano-Mo2C preparation. Reaction thermodynamics of gas-vapor system of CH4-MoO3-N2 indicates that high-proportion inert gas significantly increases the required carbonization potential (CO/CO2) for Mo2C formation, but surplus carbon produced from methane decomposition can trigger water gas producing reaction and Boudouard reaction to help meet the requirement. Nano-Mo2C was then synthesized by mixing N2-carried MoO3 vapor with CH4 under varying conditions including temperature, gas flowrate and vapor concentration. It is found that MoO3 vapor experienced conversions to monoclinic MoO2 to cubic Mo to hexagonal Mo2C, and all these stages could be finished in as short as about 6 s at 1100 °C with initial molar ratio of CH4 to MoO3 over 3.5. The final products were present as porous plate-like single crystals or chain-like polycrystals, which are determined by initial vapor concentration and gas flowrate. Furtherly, electrocatalytic properties of Mo2C nanocrystals for hydrogen evolution reaction were measured, and the sample with the largest specific surface area and cleaner surface showed competitive performance over reported nano-Mo2C.

Binary electrocatalyst composed of Mo2C nanocrystals with ultra-low Pt loadings anchored in TiO2 nanotube arrays for hydrogen evolution reaction

Pt is the most effective electrocatalyst for hydrogen evolution reaction (HER). But its practical applications are severely hindered by the high cost and low abundance. Herein, we proposed a facile strategy to significantly reduce the Pt loadings for HER application. By introducing a Pt counter electrode in electrochemical cycling, trace Pt species were dissolved and redeposited on the working electrode composed of Mo2C nanoparticles anchored on the inner wall of highly ordered TiO2 nanotube arrays (TiO2 NTAs). Highly-dispersed Pt species were embedded in the crystal defects of Mo2C nanostructures with high dispersion and ultra-low Pt loading of ~13 μg/cm2. The as-prepared Pt-Mo2C/TiO2 NTAs exhibit highly electrocatalytic activities towards HER with an overpotential of 67 mV at the current density of 10 mA cm−2. Excellent stability has also been demonstrated. The mass activity of Pt-Mo2C/TiO2 NTAs is about 8-folds greater than that of commercial Pt/C electrode.

Design of house centipede-like MoC–Mo2C nanorods grafted with N-doped carbon nanotubes as bifunctional catalysts for high-performance Li–O2 batteries

Developing effective bifunctional catalysts to promote Li2O2 oxidation and to realize high-performance Li–O2 batteries with long cycle life is a key challenge. Herein, a house centipede-like bifunctional catalyst consisting of a molybdenum carbide nanorod grafted with N-doped carbon nanotubes (denoted as MoC–Mo2C/NCNTs) is developed for enhancing the performance of Li–O2 batteries. This nanocatalyst is successfully fabricated by the post-treatment of MoO3 nanorods coated with a Co-based zeolitic imidazolate framework (ZIF-67). The obtained MoC–Mo2C/NCNT nanorods have a unique structure with a body comprising small MoC–Mo2C nanocrystals and Co@NCNTs extending like legs from it. This structural architecture not only provides numerous catalytic active sites and sufficient space for accommodating Li2O2 but also facilitates electron transport through the catalytically active NCNT conductive pathways. Consequently, the MoC–Mo2C/NCNTs show substantially improved electrocatalytic performance in Li–O2 batteries with a significantly reduced discharge–charge overpotential (0.50 V), high specific discharge capacity (34,862 mA h g−1 at 200 mA g−1), and long cycle stability (up to 162 cycles at current density of 200 mA g−1) and cut-off capacity of 500 mA h g−1. Preliminary results suggest that the MoC–Mo2C/NCNT nanorod is an efficient, stable bifunctional catalyst for Li–O2 batteries.

Porous Mo2C nanorods as an efficient catalyst for the hydrogen evolution reaction

Generate hydrogen fuel from electrochemical water splitting has been considered as a promising approach. However, to obtain the low-cost and high performance catalysts towards hydrogen evolution reaction (HER) which can be applied in both alkaline and acid solution remains a challenge. Herein, we synthesized an active and stable HER catalyst composed of Mo2C nanocrystals embedded in the nanocarbon layers (Mo2C@C) by using MoO3 nanorods as precursor. Benefiting from the porous one dimensional structure and ultrafine Mo2C nanocrystals, Mo2C@C exhibits high HER catalytic activity for 10 mA cm−2with the overpotential of 119 mV in 1 M KOH solution and 170 mV in 0.5 M H2SO4solution, respectively. Moreover, Mo2C@C displays long durability during the HER process with almost no decay and maintains the porous one dimensional architecture after the HER stability test. This study offers the guideline for the further design and fabrication of the nanostructured HER electrocatalysts in wide pH range.

Surfactant-assisted hydrothermal synthesis of nitrogen doped Mo2C@C composites as highly efficient electrocatalysts for hydrogen evolution reaction

We describe a facile surfactant-assisted hydrothermal route to synthesize nitrogen doped Mo2C@C composites in the presence of cetyltrimethylammonium bromide (CTAB) as carbon source and structure guiding agent. The resulting Mo2C@C composites consist of Mo2C nanocrystals with sheet-like morphology and well-dispersed nitrogen element doping. Controllable experiments indicate that the additive amount of CTAB can efficiently tune porous structure and electrochemical activity of the as-prepared Mo2C@C materials. This unique nitrogen doped Mo2C@C composite provides several advantages for electrocatalytic applications: (1) nitrogen doped carbons can prevent the aggregation of Mo2C nanocrystals and render it high conductivity; (2) the homogeneous dispersion of Mo2C nanocrystals provides abundant active sites; (3) 2D morphology, the hierarchical porosity, and high surface areas allow large exposed field of active sites and facilitate mass transfer. As a result, the nitrogen doped Mo2C@C composites deliver superior HER electrocatalytic activities with a low overpotential of only 100 mV and also a low Tafel slope of 53 mV/dec in alkaline condition. Such CTAB-assisted strategy may open up an opportunity towards synthesis of low cost and high performance Mo-based electrocatalysts for various applications, such as water splitting.

Fine Tuning Electronic Structure of Catalysts through Atomic Engineering for Enhanced Hydrogen Evolution

AbstractAn efficient, durable, and low‐cost hydrogen evolution reaction (HER) catalyst is an essential requirement for practical hydrogen production. Herein, an effective approach to facilitate the HER kinetics of molybdenum carbide (Mo2C) electrocatalysts is presented by tuning its electronic structure through atomic engineering of nitrogen implantation. Starting from the organoimido‐derivatized polyoxometalate nanoclusters with inherent MoN bonds, the formation of N‐implanted Mo2C (N@Mo2C) nanocrystals with perfectly adjustable amounts of N atoms is demonstrated. The optimized N@Mo2C electrocatalyst exhibits remarkable HER performance and good stability over 20 h in both acid and basic electrolytes. Further density functional theory calculations show that engineering suitable nitrogen atoms into Mo2C can regulate its electronic structure well and decrease MoH strength, leading to a great enhancement of the HER activity. It could be believed that this ligand‐controlled atomic engineering strategy might influence the overall catalyst design strategy for engineering the activation sites of nonprecious metal catalysts for energy conversions.

Paragenesis of Mo2C nanocrystals in mesoporous carbon nanofibers for electrocatalytic hydrogen evolution

Cost-effective Mo2C nanocrystals homogeneously distributed in interconnected mesoporous carbon nanofibers are prepared through an electrospinning route combined with a subsequent pyrolysis process. The crystal phases of the Mo2C nanocrystals are controllable and their phase-dependent electrocatalytic activities for the hydrogen evolution reaction (HER) are explored. The nanocomposite catalyst made of the hexagonal-phase Mo2C nanocrystals in carbon nanofibers exhibits outstanding electrocatalytic kinetics and high activities for the HER in both acidic and basic media. It is found that the overpotentials are 160 mV in 0.5 M H2SO4 and 92 mV in 1 M KOH at a current density of 10 mA cm−2. The Tafel slopes of 66 mV dec−1 in 0.5 M H2SO4 and 63 mV dec−1 in 1 M KOH are achieved. Also, the as-obtained Mo2C/C electrocatalyst shows long-term durability.

Ultrafine Molybdenum Carbide Nanocrystals Confined in Carbon Foams via a Colloid‐Confinement Route for Efficient Hydrogen Production

AbstractPrecise size‐controlled synthesis and multiscale assembly of ultrafine non‐noble‐metal‐based catalysts play a key role in electrochemical energy conversion. Herein, a novel colloid‐confinement strategy for facile synthesis of ultrafine molybdenum carbide nanocrystals with sub‐2 nm average size assembled in carbon foams (uf‐Mo2C/CF) is reported. First, uniformly sized colloidal SiO2 nanospheres are chosen to confine the metal source (NH4)6Mo7O24 molecules and the glucose is as carbon source. Subsequently, by a high temperature reduction–carbonization, Mo2C nanocrystals are achieved and uniformly assembled on the in situ formed amorphous carbon foams. The as formed uf‐Mo2C/CF demonstrates superior hydrogen evolution activity and outstanding stability in the whole pH range. These results indicate the validity of size control and multiscale structural assembly of metal carbide nanocrystals by consideration of the overall mass transport, accessibility, and quantity, as well as the capability of the active sites toward efficient electrocatalytic hydrogen evolution reaction. This strategy can also be expanded to other energy‐related application.

A novel two-dimensional hierarchical Mo2C/C-N hybrid fabricated via ionothermal route as a robust electrocatalyst for hydrogen evolution reaction

Design of highly efficient noble-metal-free hydrogen evolution electrocatalysts is of great significance to the practical application of water-separation devices. Herein, we design a highly active and durable two-dimensional (2D) hierarchical Mo2C/C-N hybrid with well-dispersed small Mo2C nanocrystals decorated on nitrogen-doped carbon (C-N) frameworks via one-step ionothermal route. Encouragingly, the optimal 2D hierarchical Mo2C/C-N hybrid shows a low onset potential of 6 mV, an overpotential of 98 mV at a cathodic current density of 10 mA cm−2, a Tafel slope of 60 mV dec−1, and a large exchange current density of 0.311 mA cm−2 in acidic medium. Moreover, it also displays outstanding long-term electrochemical stability, exhibiting almost 100% activity retention after 3000 cycles and 20 h of continuous testing at fixed overpotentials. The unique 2D structure of electrocatalyst possesses highly dispersed ultrasmall Mo2C nanocrystals embedded in 2D layered C–N frameworks, excellent electron transportation, abundant active sites (Mo-C), and strong interaction between Mo2C nanocrystals and C-N frameworks, leading to the remarkable catalytic activity and endurance for hydrogen evolution reaction over a wide pH range. The synthetic strategy is facile and inexpensive for large-scale production and also can be extended to fabricate various 2D hierarchical nanomaterials for electrocatalytic applications.