N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen Production

Study of efficient catalytic electrode for hydrogen evolution reaction from seawater based on low tortuosity corn straw cellulose biochar/Mo2C with porous channels

Porosity and channel structure has important effects on the performance of hydrogen evolution reaction (HER) of nanostructured electrocatalysts in acid solution and seawater. Mesopore usually helps to enhance the reaction kinetics and mass transfer, while the macroporous channel structure also affects the electrocatalyst. Traditional graphene materials do not have such structure. Therefore, this paper designs a method to synthesize Mo2C composite nanomaterial in situ on corn straw biochar, inspires by the natural channel structure of conducting water, salt and organic matter in plants. Characteristic characterization shows that the material also has a large number of mesoporous and vertical distribution of large porous channel structure, through the decrease of tortuosity and porosity, ensure the catalyst surface electrolyte transport and hydrogen timely escape, alleviate the process of metal ion precipitation blocking pore channel, so as to improve the rate of hydrogen evolution reaction. The results shows that the overpotential of the catalyst was 48 mV and 251 mV under 10 mA cm-2 acidic electrolyte and simulated seawater electrolyte, respectively. This method provides new ideas for the design of efficient electrocatalysts for seawater decomposition, then the HER performance in alkaline and neutral environments needs to be further explored.

Synergistically Tuning Surface States of Hierarchical MoC by Pt-N Dual-Doping Engineering for Optimizing Hydrogen Evolution Activity

Catalytic performance can be greatly enhanced by rational modulation of the surface state. In this study, reasonable adjustment of the surface states around the Fermi level (EF ) of molybdenum carbide (MoC) (α phase) via a Pt-N dual-doping process to fabricate an electrocatalyst named as Pt-N-MoC is performed to promote hydrogen evolution reaction (HER) performance over the MoC surface. Systematically experimental and theoretical analyses demonstrate that the synergistic tuning of Pt and N can cause the delocalization of surface states, with an increase in the density of surface states near the EF . This is beneficial for accumulating and transferring electrons between the catalyst surface and adsorbent, resulting in a positively linear correlation between the density of surface states near the EF and the HER activity. Moreover, the catalytic performance is further enhanced by artificially fabricating a Pt-N-MoC catalyst that has a unique hierarchical structure composed of MoC nanoparticles (0D), nanosheets (2D), and microrods (3D). As expected, the obtained Pt-N-MoC electrocatalyst exhibits superb HER activity with an extremely low overpotential of 39 mV@10 mA cm-2 as well as superb stability (over 24 d) in an alkaline solution. This work highlights a novel strategy to develop efficient electrocatalysts via adjusting their surface states.

Developing Superior Hydrophobic 3D Hierarchical Electrocatalysts Embedding Abundant Catalytic Species for High Power Density Zn-Air Battery

It is essential but still challenging to design and construct inexpensive, highly active bifunctional oxygen electrocatalysts for the development of high power density zinc-air batteries (ZABs). Herein, a CoFe-S@3D-S-NCNT electrocatalyst with a 3D hierarchical structure of carbon nanotubes growing on leaf-like carbon microplates is designed and prepared through chemical vapour deposition pyrolysis of CoFe-MOF and subsequent hydrothermal sulfurization. Its 3D hierarchical structure shows excellent hydrophobicity, which facilitates the diffusion of oxygen and thus accelerates the oxygen reduction reaction (ORR) kinetic process. Alloying and sulfurization strategies obviously enrich the catalytic species in the catalyst, including cobalt or cobalt ferroalloy sulfides, their heterojunction, core-shell structure, and S, N-doped carbon, which simultaneously improve the ORR/OER catalytic activity with a small potential gap (ΔE = 0.71 V). Benefiting from these characteristics, the corresponding liquid ZABs show high peak power density (223 mW cm^-2 ), superior specific capacity (815 mA h g_Zn ^-1 ), and excellent stability at 5 mA cm^-2 for ≈900 h. The quasi-solid-state ZABs also exhibit a very high peak power density of 490 mW cm^-2 and an excellent voltage round-trip efficiency of more than 64%. This work highlights that simultaneous composition optimization and microstructure design of catalysts can effectively improve the performance of ZABs.

MoO3/S@g-C3N4 Nanocomposite Structures: Synthesis, Characterization, and Hydrogen Catalytic Performance

Hydrogen production as a source of clean energy is high in demand nowadays to avoid environmental issues originating from the use of conventional energy sources i.e., fossil fuels. In this work and for the first time, MoO₃/S@g-C₃N₄ nanocomposite is functionalized for hydrogen production. Sulfur@graphitic carbon nitride (S@g-C₃N₄)-based catalysis is prepared via thermal condensation of thiourea. The MoO₃, S@g-C₃N₄, and MoO₃/S@g-C₃N₄ nanocomposites were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field Emission Scanning Electron Microscope (FESEM), STEM, and spectrophotometer. The lattice constant (a = 3.96, b = 13.92 Å) and the volume (203.4 ų) of MoO₃/10%S@g-C₃N₄ were found to be the highest compared with MoO₃, MoO₃/20-%S@g-C₃N₄, and MoO₃/30%S@g-C₃N₄, and that led to highest band gap energy of 4.14 eV. The nanocomposite sample MoO₃/10%S@g-C₃N₄ showed a higher surface area (22 m²/g) and large pore volume (0.11 cm³/g). The average nanocrystal size and microstrain for MoO₃/10%S@g-C₃N₄ were found to be 23 nm and -0.042, respectively. The highest hydrogen production from NaBH₄ hydrolysis ~22,340 mL/g·min was obtained from MoO₃/10%S@g-C₃N₄ nanocomposites, while 18,421 mL/g·min was obtained from pure MoO₃. Hydrogen production was increased when increasing the masses of MoO₃/10%S@g-C₃N₄.

Sustainable Production of Molybdenum Carbide (MXene) from Fruit Wastes for Improved Solar Evaporation

Freshwater production using solar-driven interfacial evaporation is regarded as a green and sustainable strategy. The biggest barrier to practical deployment of solar desalination, however, continues to be the lack of options for renewable materials. Herein, we present a facile two-step carbonization approach that is sustainable for developing innovative two-dimensional (2D) molybdenum carbide (Mo₂ C) materials derived from carbonized fruit wastes. The resultant 2D Mo₂ C photothermal layer has an efficient water evaporation rate of 1.52 kg m^-2  h^-1 with a photothermal conversion efficiency of 94 % under one sun irradiation, which is among the best reported values so far. The broad solar absorption band, high specific surface area (555.1 m²  g^-1 ) with large micro- and meso porosity, of the Mo₂ C photothermal layer are responsible for these outstanding results. The conversion of food wastes into valuable products, in this case MXene, can potentially inspire greener developments of advanced materials for solar water evaporator.

Carbon Nanotube Supported Molybdenum Carbide as Robust Electrocatalyst for Efficient Hydrogen Evolution Reaction

Molybdenum carbide is considered to be one of the most competitive catalysts for hydrogen evolution reaction (HER) regarding its high catalytic activity and superior corrosion resistance. But the low electrical conductivity and poor interfacial contact with the current collector greatly inhibit its practical application capability. Herein, carbon nanotube (CNT) supported molybdenum carbide was assembled via electrostatic adsorption combined with complex bonding. The N-doped molybdenum carbide nanocrystals were uniformly anchored on the surfaces of amino CNTs, which depressed the agglomeration of nanoparticles while strengthening the migration of electrons. The optimized catalyst (250-800-2h) showed exceptional electrocatalytic performance towards HER under both acidic and alkaline conditions. Especially in 0.5 M H₂SO₄ solution, the 250-800-2h catalyst exhibited a low overpotential of 136 mV at a current density of 10 mA/cm² (η₁₀) with the Tafel slope of 49.9 mV dec^-1, and the overpotential only increased 8 mV after 20,000 cycles of stability test. The active corrosive experiment revealed that more exposure to high-activity γ-Mo₂N promoted the specific mass activity of Mo, thus, maintaining the catalytic durability of the catalyst.

3D Hierarchical Porous Fe/Ni-P-B as Practical Bifunctional Electrode for Alkaline Water Electrolysis

Bifunctional electrodes for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are extremely attractive as they can simplify the water electrolysis system. Here, a general and scalable strategy to prepare stable and efficient bifunctional electrode was reported, based on a novel hierarchical porous structure constructed by conductive electrocatalyst. The method involved the construction of 3D monolithic structure and its surface reconstruction by chemical etching process. This strategy produced an advanced 3D hierarchical porous Fe/Ni-P-B@MS electrode containing well-defined macropores (>100 μm) at the inter-wire space and mesopores (<100 nm) distributed uniformly over the entire catalyst surface. This highly efficient bifunctional electrode required only 79 and 279 mV to reach 100 mA cm^-2 toward HER and OER in 1.0 m KOH. An alkaline electrolyzer consisting of this electrode provided 100 mA cm^-2 at a low cell voltage of 1.61 V and survived at large current density of 800 mA cm^-2 for over 140 h without apparent degradation. This work provides a new perspective for the rational design of transition metal-based bifunctional electrodes with outstanding performance.

Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production

The development of highly active non-precious metal electrocatalysts is crucial for advancing the practical application of hydrogen evolution reaction (HER). Doping engineering is one of the important strategies to optimize the electrocatalytic activity of electrocatalysts. Herein, we put forward a simple strategy to optimize the catalytic activity of MoO3 material by incorporating the Cu atoms into the interlayer (denoted as Cu-MoO3). The prepared Cu-MoO3 nanosheet has a larger surface area, higher conductivity, and strong electron interactions, which contributes to optimal reaction kinetics of the HER process. As a result, the Cu-MoO3 nanosheet only needs a small overpotential of 106 mV to reach the geometric current density of 10 mA cm−2. In addition, it also delivers a low Tafel slope of 83 mV dec−1, as well as high stability and Faraday efficiency. Notably, when using the Cu-MoO3 as a cathode to construct the water electrolyzer, it only needs 1.55 V to reach the 10 mA cm−2, indicating its promising application in hydrogen generation. This work provides a novel type of design strategy for a highly active electrocatalyst for an energy conversion system.

Encapsulating N‑doped graphite carbon in MoO2 as a novel cocatalyst for boosting photocatalytic hydrogen evolution

Generally, it is important to ameliorate the co-catalyst used in photocatalytic hydrogen evolution reactions (PHERs) to achieve efficient transfer and separation of photogenerated carriers, decrease the surface reaction energy barrier, and hence improve the photocatalytic activity. In this study, N-doped graphite carbon (GC) was introduced in situ to MoO₂ to ensure the presence of well-dispersed active sites, lower the overpotential of hydrogen evolution, and further achieve high conductivity. Then, the MoO₂/GC composite obtained was used as a co-catalyst of ZnIn₂S₄ (ZIS) in a PHER, resulting in a great improvement in the photocatalytic activity. Given the metallicity and large work function of MoO₂/GC, a Schottky interface can form between MoO₂/GC and ZIS, which accelerates the transmission of photogenerated electrons. As a result, the separation efficiency of photogenerated carriers improves, whereas the surface overpotential of PHERs clearly decreases for ZIS. This study proposes a new idea for exploiting efficient co-catalysts and promotes the wide and heavy use of carbon materials in the field of solar energy conversion.

Priority Occupation of C-Sites by N-Confining P-Implantation in Pyrrodic N-Sites in NCNT@P,N-Mo2C for Highly Efficient Electrocatalytic Hydrogen Evolution

Optimizing the electronic configuration of Mo₂C by activating heteroatom(s)-neighboring carbon atoms to enhance the activity of hydrogen evolution reaction (HER) has been demonstrated. However, the development of heteroatom-doped Mo₂C to fabricate a water electrolyzer is still a challenge because of the limitation of a well-defined electronic structure of hybridization of Mo with heteroatom(s). Here, nitrogen (N) and phosphor (P) codoped Mo₂C embedded carbon nanotubes (NCNT@P,N-Mo₂C) with the priority occupation of C-sites by N, which well confines the P-implantation at the pyrrodic N-sites and brings out N-O bonding on the surface, which favorably modifies the electronic configuration of adjacent Mo, resulting in highly efficient pH-tolerant HER activity. This study not only presents a potential HER electrocatalyst candidate but also provides a strategy for the construction of a well-defined electronic structure of heteroatom(s)-neighboring carbon-based materials.

Probing the Tribochemical Impact on Wear Rate Dynamics of Hydrogenated Amorphous Carbon via Raman-Based Profilometry

Solid-liquid lubricating systems have received significant attention as a promising way for energy saving and emission control. For deeply understanding their tribological behaviors, it is necessary to study interaction mechanisms between solid and liquid lubricants from the tribochemical viewpoint, as tribofilms formed by tribochemical products on contact surfaces critically affect the whole tribological process. Continually or periodically monitoring tribofilm formation and evolution can contribute significantly to clarifying its dominating role in tribological behavior under boundary lubrication. However, detecting tribofilms in situ remains a big challenge for conventional surface analytical approaches, mainly due to their limitations in accessing tribofilms or low signal intensities of thin tribofilms. In this study, highly sensitive Raman-based profilometry with in situ potential has been developed for detecting molybdenum dialkyldithiocarbamate (MoDTC)-derived tribofilms and exploring their effect on a-C:H wear over time. The optical properties of tribochemical products formed on the coating surface in different wear stages could result in extra attenuation of Raman signal intensities in the form of measurement deviations in wear depth. By monitoring the deviations, key information of tribofilm compositions was obtained and a two-stage wear progression mechanism was proposed for the first time to clarify the detrimental effect of MoDTC-derived tribofilms on a-C:H wear by combining detailed structure and composition analyses.

An acid-base molecular assembly strategy toward N-doped Mo2C@C nanowires with mesoporous Mo2C cores and ultrathin carbon shells for efficient hydrogen evolution

Molybdenum carbides are promising electrocatalysts for the hydrogen evolution reaction (HER). Rational design of morphology, composition and interfacial structure in Mo₂C materials is essential to enhance their HER performance. Herein, an acid-base molecular assembly strategy is demonstrated for the synthesis of novel N-doped Mo₂C@C core-shell nanowires (NWs) composed of mesoporous Mo₂C cores with interconnected crystalline walls and ultrathin carbon shells. The strong interactions between the two precursors, adenine (Ade) and phosphomolybdic acid (PMA), lead to the formation of inter-molecular hybrid NWs during a hydrothermal process. The subsequent pyrolysis leads to confined growth of crystalline Mo₂C NWs with inter-crystal mesopores (5 ~ 10 nm), formation of ultrathin carbon shells (~1.5 nm in thickness), and effective N doping. Such a structure architecture can provide abundant active sites, fast and diverse mass and electron transport paths, as well as stable reaction interfaces. The typical N-doped Mo₂C@C NWs exhibit high HER performance with a low overpotential of 136 mV at 10 mA cm^-2, a small Tafel slop of 58 mV dec^-1, excellent durability and outstanding anti-poisoning performance against CO and H₂S gases. Furthermore, the influences of several important factors, including the pyrolysis temperature, hydrothermal temperature and precursor mass ratio, on the morphology, composition and structural configuration of the resulted materials are elucidated and correlated with their HER performance. This work may provide a general strategy for the synthesis of other nanoscale metal carbides for various catalytic applications.

High-Performance MoC Electrocatalyst for Hydrogen Evolution Reaction Enabled by Surface Sulfur Substitution

Molybdenum carbides have been expected to be one of the promising catalysts for the hydrogen evolution reaction (HER) due to their similar d-band electronic structures to the Pt-group metals. However, the weaker hydrogen-adsorption ability of MoC severely hinders its applications. Guided by density functional theory calculations, we put forward a strategy to design the novel MoC-based electrocatalyst with surface reconstruction through sulfur doping. The incorporation of minor sulfur not only greatly increases the number of active sites and intrinsic activity but also optimizes the electronic structure to improve the electron transfer efficiency. As a result, the as-prepared sulfur-substituted MoC tackles the limitation of the Volmer step and exhibits superior HER performance with a small Tafel slope of 48 mV dec^-1. Theoretical investigations demonstrate that the terminal sulfur plays a critical role in facilitating a close to zero hydrogen adsorption energy (ΔG_H*) and a lower hydrogen release barrier.

Engineering Two-Phase Bifunctional Oxygen Electrocatalysts with Tunable and Synergetic Components for Flexible Zn-Air Batteries

Metal-air batteries, like Zn-air batteries (ZABs) are usually suffered from low energy conversion efficiency and poor cyclability caused by the sluggish OER and ORR at the air cathode. Herein, a novel bimetallic Co/CoFe nanomaterial supported on nanoflower-like N-doped graphitic carbon (NC) was prepared through a strategy of coordination construction-cation exchange-pyrolysis and used as a highly efficient bifunctional oxygen electrocatalyst. Experimental characterizations and density functional theory calculations reveal the formation of Co/CoFe heterostructure and synergistic effect between metal layer and NC support, leading to improved electric conductivity, accelerated reaction kinetics, and optimized adsorption energy for intermediates of ORR and OER. The Co/CoFe@NC exhibits high bifunctional activities with a remarkably small potential gap of 0.70 V between the half-wave potential (E_1/2) of ORR and the potential at 10 mA cm^‒2 (E_j=10) of OER. The aqueous ZAB constructed using this air electrode exhibits a slight voltage loss of only 60 mV after 550-cycle test (360 h, 15 days). A sodium polyacrylate (PANa)-based hydrogel electrolyte was synthesized with strong water-retention capability and high ionic conductivity. The quasi-solid-state ZAB by integrating the Co/CoFe@NC air electrode and PANa hydrogel electrolyte demonstrates excellent mechanical stability and cyclability under different bending states.

Heteroatom-Doping of Non-Noble Metal-Based Catalysts for Electrocatalytic Hydrogen Evolution: An Electronic Structure Tuning Strategy

Electrocatalytic water splitting for hydrogen production is an appealing way to reduce carbon emissions and generate renewable fuels. This promising process, however, is limited by its sluggish reaction kinetics and high-cost catalysts. Construction of low-cost and high-performance non-noble metal-based catalysts have been one of the most effective approaches to address these grand challenges. Notably, the electronic structure tuning strategy, which could subtly tailor the electronic states, band structures, and adsorption ability of the catalysts, has become a pivotal way to further enhance the electrochemical water splitting reactions based on non-noble metal-based catalysts. Particularly, heteroatom-doping plays an effective role in regulating the electronic structure and optimizing the intrinsic activity of the catalysts. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the hetero-dopants in catalysts yet remains ambiguous. Herein, the recent progress is comprehensively reviewed in heteroatom doped non-noble metal-based electrocatalysts for hydrogen evolution reaction, particularly focus on the electronic tuning effect of hetero-dopants in the catalysts and the corresponding synthetic pathway, catalytic performance, and activity origin. This review also attempts to establish an intrinsic correlation between the localized electronic structures and the catalytic properties, so as to provide a good reference for developing advanced low-cost catalysts.

Surface and Interface Engineering: Molybdenum Carbide-Based Nanomaterials for Electrochemical Energy Conversion

Molybdenum carbide (Mo_x C)-based nanomaterials have shown competitive performances for energy conversion applications based on their unique physicochemical properties. A large surface area and proper surface atomic configuration are essential to explore potentiality of Mo_x C in electrochemical applications. Although considerable efforts are made on the development of advanced Mo_x C-based catalysts for energy conversion with high efficiency and stability, some urgent issues, such as low electronic conductivity, low catalytic efficiency, and structural instability, have to be resolved in accordance with their application environments. Surface and interface engineering have shown bright prospects to construct highly efficient Mo_x C-based electrocatalysts for energy conversion including the hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. In this Review, the recent progresses in terms of surface and interface engineering of Mo_x C-based electrocatalytic materials are summarized, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and others electronic conductive species. Finally, the challenges and prospects for energy conversion on Mo_x C-based nanomaterials are discussed in terms of key performance parameters for the catalytic performance.

In situ anodic dissolution–cathodic deposition route for preparation of the Pt–SiW11Co/SiW11Co–CNP/GC electrode: application as an efficient electrode for the hydrogen evolution reaction

A novel nanohybrid based on carbon nanoparticles, platinum nanoparticles, and SiW₁₁Co polyoxometalate is introduced as an efficient electrocatalyst for the hydrogen evolution reaction (HER).

α-MoC1−x nanorods as an efficient hydrogen evolution reaction electrocatalyst

Benefiting from their more exposed active sites, α-MoC_1−x nanorods exhibited significantly enhanced HER performance under both acidic and alkaline conditions.

Porous Plate-like MoP Assembly as an Efficient pH-Universal Hydrogen Evolution Electrocatalyst

Molybdenum phosphide is one of the most potential electrocatalysts for the hydrogen evolution reaction (HER), whereas it is still challenging to achieve an efficient molybdenum phosphide-based catalyst that performs well over a wide pH range. Herein, a porous nanoplate composed of small MoP flakes confined in thin N, P, S-triple-doped carbon (MoP@NPSC) was prepared by the assembly of phosphomolybdic acid (H₃PMo₁₂O₄₀·nH₂O, {PMo₁₂}) and egg white, followed by phosphorization. Given its small size (ca. 1 nm) in favor of deriving small particles and the oxygen-rich surface with strong coordination ability, the {PMo₁₂} cluster was selected to combine with egg white to obtain a lamellar hybrid precursor via a hydrogen bond. Through controllable phosphating, a nanoplate organized by interconnected MoP particles was generated, accompanied by the in situ formation of the N, P, S-doped carbon thin layer and pores from the pyrolysis of egg white. The plentiful pores, thin carbon coating, and multielement doping bring about promoted electrolyte/bubble diffusion, enhanced conductivity and stability, and lowered adsorption energy of hydrogen/hydroxyl, respectively. All of the above merits endow MoP@NPSC with prominent activity with low overpotentials of 50, 76, and 71 mV at 10 mA cm^-2 toward the HER in alkaline, neutral, and acid media, respectively, and nearly no attenuation after 40 h of testing. Especially, compared with commercial Pt/C, MoP@NPSC exhibits similar low onset potential and even better at large current density in 1 M KOH. The electrolyzer equipped with the MoP@NPSC cathode and the NiFe-LDH anode requires only 1.52 V to deliver 10 mA cm^-2 and can be powered by a solar cell (1.524 V) charged by sunlight.

Efficient H2O2 generation and spontaneous OH conversion for in-situ phenol degradation on nitrogen-doped graphene: Pyrolysis temperature regulation and catalyst regeneration mechanism

H₂O₂ is a green and valuable chemical that can be electrochemically synthesis from oxygen reduction, offering in-situ application for organic pollutants removal in environmental remediation. However, how to improve activity and further convert into powerful radicals is a still challenge. Herein, we show a facile and general approach to fabricate nitrogen-doped graphene (N-GE) catalyst via pyrolysis temperature regulation. The optimal N-GE at 400 °C exhibited the highest active N content (12.2 wt.%) and H₂O₂ selectivity (85.45 %) and spontaneous OH production (19.42 μM), achieving a high phenol degradation (93.58 %) at 180 min in neutral pH condition. Importantly, a simple catalyst regeneration method and mechanism was disclosed. It is proposed that the conversion of graphite N and pyridinic N in N-GE plays an important role in oxygen reduction reaction (ORR) and OH conversion, while the conversion of pyridinic N-oxide to pyridinic N is critical to catalyst stability and sustainability. This study provides a new insight into structure design of electro-catalyst about stability of nitrogen-doped carbon materials for efficient H₂O₂ generation and cost-effective pollutants removal.

Preparation of Robust Hydrogen Evolution Reaction Electrocatalyst WC/C by Molten Salt

Tungsten carbide (WC) is an alternative to the costly and resource-constrained Pt-based catalysts for hydrogen evolution reaction (HER). In this work, a one-step facile and easily scalable approach is reported, to synthesize ultrafine WC by molten salt. Benefiting from the ideal synergistic catalytic effect between the highly active WC nanoparticles and the conductive graphitic carbon, and strong charge transfer ability, the unique WC/C hybrids demonstrated excellent HER performance in both acid and alkaline medias with overpotentials of 112 and 122 mV, at a current density of 10 mA cm⁻² and Tafel slopes of 54.4 and 68.8 mV dec⁻¹, in acid and alkaline media, and remarkable stability. With the simplicity and low-cost of the synthetic approach, the strategy presented here can be extendable to the preparation of other transition metal-based/carbon hybrids for versatile applications.

Insights into N, P, S multi-doped Mo2C/C composites as highly efficient hydrogen evolution reaction catalysts

Heteroatom doping has been proved to be an effective strategy to optimize the activity of hydrogen evolution reaction (HER) catalysts. Herein, we report N, P, S multi-doped Mo₂C/C composites exhibiting highly efficient HER performance in acidic solution, which are facilely fabricated via annealing of N, P, S-containing MoO _x -polyaniline (MoO _x -PANI) hybrid precursors. The optimized N, P, S multi-doped Mo₂C/C catalyst with a moderate P dopant level (NPS-Mo₂C/C-0.5) exhibits excellent performance with an overpotential of 53 mV to achieve a current density of 20 mA cm^-2, a Tafel slope of 72 mV dec^-1 and good stability in acidic electrolytes. Based on the study of XPS, EPR and ³¹P MAS NMR, the excellent electrocatalytic performance could be attributed to the effective electronic configuration modulation of both Mo₂C nanorods and the carbon matrix, derived from stronger synergistic N, P, S multi-doping coupling effects. This work provides a promising methodology for the design and fabrication of multi-doped transition metal based electrocatalysts via electronic structure engineering.

Phosphorus-doped CoTe2/C nanoparticles create new Co-P active sites to promote the hydrogen evolution reaction

Doping has been widely recognized as an effective method for adjusting the performance of electrocatalysts. It can cause changes in the electronic structure of substances. Thereby, it can affect the intrinsic catalytic performance. Herein, we report a facile doping method in which phosphorus can be simultaneously doped into both CoTe2 and C. In the acidic solution, the hydrogen evolution reaction (HER) performance of the obtained P-CoTe2/C nanoparticles was significantly improved compared with that of undoped nanoparticles. At a current density of 10 mA cm-2, the overpotential decreased from 430 mV to 159 mV. Density functional theory (DFT) calculations show that phosphorus doping can produce new high activity Co-P catalytic sites. In addition, phosphorus can be doped into the carbon in the composite at the same time, which enhances the electrical conductivity of the composite. Moreover, in the process of calcination and doping, the electric double layer capacitance (Cdl) of the composite is significantly increased, which helps in exposing more active sites. This work has developed a multi-effect doping method that simultaneously increases the intrinsic activity, conductivity and active sites of the material. This method provides a new strategy for the performance regulation of other electrocatalysts.

Dual-Functional Template-Directed Synthesis of MoSe2/Carbon Hybrid Nanotubes with Highly Disordered Layer Structures as Efficient Alkali-Ion Storage Anodes beyond Lithium

Sodium/Potassium-ion batteries (SIBs/PIBs) have recently received tremendous attention because of their particular features of cost-effectiveness and promising energy density, which hold great potential for large-scale applications. Nevertheless, it still has a common bottleneck issue that is the sluggish kinetics of Na⁺/K⁺ intercalation, which raises more rigorous requirement on the electrode candidates regarding the morphology, dimension, and architecture. Herein, we have constructed unique MoSe₂-based hybrid nanotubes with wall structures composed of highly disordered MoSe₂ layers embedded in phosphorus and nitrogen co-doped carbon matrix (denoted MoSe₂⊂PNC-HNTs), by a facile two-step strategy using Se nanorods as the dual-functional template, i.e., shape-directed agent and in situ selenization resources. Benefitting from the combined features of the one-dimensional (1D) hollow interior, hybrid wall structure with high disorder, and the phosphorus and nitrogen co-doping-induced abundant defect sites in the carbon matrix, the MoSe₂⊂PNC-HNT anode exhibits high specific capacities of 280 and 262 mA h g^-1 over 200 cycles at the current density of 0.1 A g^-1 for Na⁺ and K⁺ storage, respectively, and achieves remarkable capacity retention rates of 87.0% at 2 A g^-1 over 3500 cycles for Na-ion storage and 80.1% at 1 A g^-1 after 500 cycles for K-ion storage.

Tungsten Carbide Encapsulated in Grape-Like N-Doped Carbon Nanospheres: One-Step Facile Synthesis for Low-Cost and Highly Active Electrocatalysts in Proton Exchange Membrane Water Electrolyzers

Tungsten carbide (WC) is an alternative to the costly and resource-constrained Pt-based catalysts. Herein, a one-step facile and easily scalable approach is reported to synthesize ultrafine WC nanocrystals encapsulated in porous N-doped carbon nanospheres (NC) by simple self-polymerization, drying, and annealing. It is worth mentioning that this developed method has four novel features: (1) the synthesis process, without any hard template or hydrocarbon gas feeding, is, notably, very facile and efficient with low cost; (2) the carbon coating on WC nanocrystals not only restrains coarsening of particles but also creates strong coupling interactions between the nanocrystallines and the conductive carbonaceous matrix; (3) uniform grape-like WC@NC nanospheres with high specific surface area can be obtained in a large scale; and (4) single-phase WC can be achieved. As a result, WC@NC demonstrates remarkable hydrogen evolution reaction (HER) electrocatalytic performance with overpotentials of 127 and 141 mV at a current density of 10 mA cm^-2 and Tafel slopes of 56.3 and 78.7 mV dec^-1 in acid and alkaline media, respectively. Our density functional theory calculations manifest that the strong synergistic electronic effect between WC and its intimately bonded carbon shell vastly boosts the HER electrocatalytic activity. WC@NC catalysts as a cathode are further tested in a home-made electrolyzer with 0.78 A cm^-2 achieved at a cell voltage of 2 V at 80 °C and operated stably at 200 mA cm^-2 for more than 20 h.

Ultrafine α-Phase Molybdenum Carbide Decorated with Platinum Nanoparticles for Efficient Hydrogen Production in Acidic and Alkaline Media

The development of efficient electrocatalysts is important to produce clean and sustainable hydrogen fuel on a large scale. With respect to cathodic reactions, Pt exhibits an overwhelming electrocatalytic capability in the hydrogen evolution reaction (HER) in comparison with other earth-abundant electrocatalysts, despite its rarity and high cost. So, a hybrid catalyst that combines a low-cost electrocatalyst with Pt would balance cost-effectiveness with catalytic activity. Herein, α-phase molybdenum carbide (MoC1- x ) nanoparticles (NPs) decorated with a small amount of Pt (MoC1- x /Pt-NPs) are designed to achieve high-performance hydrogen production in acidic and alkaline media. MoC1- x -NPs exhibit good electrocatalytic HER activity as well as stability and durability. They show favorable catalytic kinetics in an alkaline medium, suggesting an active water dissociation process. After Pt decoration, Pt-NPs that are 2-3 nm in diameter are well incorporated with MoC1- x -NPs. MoC1- x /Pt-NPs with a small amount of Pt (2.7-3 wt%) and are able to perform superior electrocatalytic HER activity, and possess stability and durability that is comparable to that of commercial Pt/C. Notably, they exhibit a higher intrinsic catalytic activity compared to that of Pt/C in an alkaline medium, indicating that they promote the sluggish catalytic kinetics of Pt in alkaline medium.

Structural Design and Electronic Modulation of Transition-Metal-Carbide Electrocatalysts toward Efficient Hydrogen Evolution

As the key of hydrogen economy, electrocatalytic hydrogen evolution reactions (HERs) depend on the availability of cost-efficient electrocatalysts. Over the past years, there is a rapid rise in noble-metal-free electrocatalysts. Among them, transition metal carbides (TMCs) are highlighted due to their structural and electronic merits, e.g., high conductivity, metallic band states, tunable surface/bulk architectures, etc. Herein, representative efforts and progress made on TMCs are comprehensively reviewed, focusing on the noble-metal-like electronic configuration and the relevant structural/electronic modulation. Briefly, specific nanostructures and carbon-based hybrids are introduced to increase active-site abundance and to promote mass transportation, and heteroatom doping and heterointerface engineering are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted HER kinetics. Finally, a perspective on the future development of TMC electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials in energy chemistry.

Air-stable phosphorus-doped molybdenum nitride for enhanced electrocatalytic hydrogen evolution

Molybdenum-based electrocatalysts for hydrogen evolution have been investigated extensively in recent years. However, unlike other non-oxides, molybdenum nitride generally shows a weak preference for hydrogen evolution and low performance owing to surface oxidation and the strong Mo–H bond. Here, we prepare an air-stable molybdenum nitride through a multi-step solid-state reaction. We find that a uniformly dispersed mixture of the precursors is optimal for preparation of the electrocatalyst. To further enhance hydrogen evolution performance towards practical device applications, phosphorus doping is carried out, using a few layered black phosphorus source. The phosphorus-doped molybdenum nitride (P–Mo–N) sample catalyzes hydrogen evolution with potentials of 105, 145, and 157 mV at the current densities of 10, 50, and 100 mA/cm2, respectively, in 0.5 M H2SO4 solution with a small Tafel slope of 43 mV/dec. Thus it outperforms many of the state-of-art molybdenum-based hydrogen evolution catalysts reported to date.

Walnut-like Transition Metal Carbides with Three-Dimensional Networks by a Versatile Electropolymerization-Assisted Method for Efficient Hydrogen Evolution

Mo₂C@NPC (N,P-doped carbon) electrocatalysts are developed on carbon cloth (CC) as binder-free cathodes for efficient hydrogen evolution through a facile route of electropolymerization followed by pyrolysis. Electropolymerization of pyrrole to form polypyrrole occurs with the homogeneous incorporation of PMo₁₂, driven by Coulombic force between the positively charged polymer backbone and PMo₁₂ anions. This electrochemical synthesis is easily scaled up, requiring neither complex instrumentation nor an intentionally added electrolyte (PMo₁₂ also acts as an electrolyte). After pyrolysis, the resultant Mo₂C@NPC/CC electrode exhibits a unique interconnected walnut-like porous structure, which ensures strong adhesion between the active material and the substrate and favors electrolyte penetration into the electrocatalyst. This method is effective with other monomers such as aniline and is readily extended to fabricate other metal carbide electrodes such as WC@NPC/CC. These carbide electrodes exhibit high catalytic performance for hydrogen production, for example, WC@NPC/CC can deliver an unprecedented current density of 600 mA cm^-2 at an overpotential of only 200 mV either in an acidic or an alkaline solution. Considering the simplicity, scalability, and versatility of the synthetic method, the unique electrode structure, and the excellent catalysis performance, this study opens up new avenues for the design of various novel binder-free metal carbide cathodes based on electropolymerization.

Scalable Synthesis of a Ruthenium-Based Electrocatalyst as a Promising Alternative to Pt for Hydrogen Evolution Reaction

Designing highly active, stable, and cost-efficient electrocatalysts as alternatives to replace Pt is extremely desirable for hydrogen evolution reaction (HER). Despite much progress that has been made based on complete nonprecious metals (NPMs), very few NPM catalysts have shown comparable performance to Pt-based catalysts. Herein, a cost-efficient, environmentally friendly, and scalable method to synthesize a novel ruthenium(Ru)-doped transition-metal carbide (Mo₂C) hybrid catalyst was proposed. The hybrid nanoparticles were uniformly distributed and strongly embedded in a biomass-derived highly porous N-doped carbon framework. In particular, Mo₂C@Ru exhibited a Pt-like remarkable electrocatalytic performance for HER, and it only required an extremely low overpotential of 24.6 mV to reach the current density of 10 mA cm^-2. Furthermore, our density functional theory calculations indicated that the nanocomposite exhibits improved metal-hydrogen binding and favorable hydrogen adsorption energy, which is comparable to that of Pt. The facile and scalable synthesis methodology, the relatively low cost, and the excellent electrochemical HER performance comparable to that of commercial Pt/C suggest that the Mo₂C@Ru electrocatalyst is a promising alternative to Pt for large-scale hydrogen production.

N,P-Codoped Carbon Layer Coupled with MoP Nanoparticles as an Efficient Electrocatalyst for Hydrogen Evolution Reaction

Efficient electrocatalyst plays a significant role on the development of hydrogen energy. In this work, an N,P-codoped carbon layer coupled with MoP nanoparticles (MoP/NPCs) was prepared through a facile high-temperature pyrolysis treatment. The obtained MoP/NPCs presented efficient activity for hydrogen evolution reaction (HER), with low onset potential of 90 mV, and a small Tafel slope (71 mV dec-1), as well as extraordinary stability in acidic electrolyte. This work provides a new facile strategy for the design and synthesis of sustainable and effective molybdenum-based electrocatalysts as alternatives to non-Pt catalysts for HER.