Carbon-coated molybdenum carbide nanosheets derived from molybdenum disulfide for hydrogen evolution reaction

The design and synthesis of efficient non-noble metal catalyst is important for the practical application of hydrogen evolution reaction (HER) from water electrolysis. In this study, molybdenum carbides were prepared by a novel synthetic method, which involved the first exfoliation of 2D MoS₂ based on the principle of cold expansion of water below 4 °C and then carburization of exfoliated MoS₂. In this method, MoS₂ played roles of morphology template and molybdenum source simultaneously. Carbon-coated molybdenum carbide nanosheets were obtained and confirmed by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The stripping degree of MoS₂ was found to have an important influence on physical properties and catalytic performance of molybdenum carbides. Interestingly, Mo₂C nanosheets encapsulated in carbon nanotubes were observed when the MoS₂ with a high peeling degree was used in the preparation. It showed high activity and good durability towards HER in acid solution.     

Regulating surface wettability and electronic state of molybdenum carbide for improved hydrogen evolution reaction

Molybdenum carbide (Mo₂C) is a cost-effective transition metal carbides (TMCs) electrocatalyst for hydrogen evolution reaction (HER) due to its electronic structure similar to Pt-group noble metal. Herein, we report that an effective strategy of regulating the surface wettability and electronic state of Mo₂C by ammonia and hydrothermal co-treatment to enhance its HER activity. The electrochemical results demonstrate that Mo₂C undergoing ammonia and hydrothermal co-treatment (Mo₂C-wh) exhibits remarkably improved electrocatalytic HER activity as compared to the pristine Mo₂C (Mo₂C-p) catalyst. The activity-structure relationship studies manifest that ammonia and hydrothermal co-treatment increases the content of hydroxyl group and pyridinic-N, thus endowing Mo₂C-wh with lower charge transfer resistance, larger electrochemical active surface area and higher surface wettability. DFT calculations reveal that ammonia and hydrothermal co-treatment enhances the Mo 3d-band center and reduces the hydrogen adsorption free energy. These changes in electronic states of Mo sites and physical properties of Mo₂C positively contribute to the improvement of electrocatalytic HER activity.     

One-pot synthesis of pure phase molybdenum carbide (Mo2C and MoC) nanoparticles for hydrogen evolution reaction

Herein, we report the one-step synthesis of pure phase molybdenum carbide (Mo₂C and MoC) nanoparticles via the in-situ carburization reduction route without using any reducing agent. The X-ray diffraction (XRD) results confirm the formation of pure phase Mo₂C and MoC at 800 °C for 8 h and 15 h respectively. The as-synthesized powders have been investigated for hydrogen production and energy storage applications. The pure phase Mo₂C shows high performance towards the hydrogen evolution reaction (HER) with a Tafel slope of 129.7 mV dec⁻¹ however, MoC exhibits a low activity towards HER with a Tafel slope of 266 mV dec⁻¹. Both the phases show high stability up to 5000 cyclic voltammetry (CV) cycles in the potential range of 0–0.4 V. In the case of MoC, the specific capacitance increases during the initial 2000 CV cycles which may be attributed to the electrode activation during the CV test. The Mo₂C powder shows a double layer capacitance (C_dl) value of 2.47 mF cm⁻² and a specific capacitance of 2.24 mF g⁻¹. The MoC phase shows a higher C_dl value of 8.99 mF cm⁻² and a specific capacitance of 8.17 mF g⁻¹.     

Zeolite imidazolate framework-8 derived molybdenum carbide/nitrogen-doped carbon for highly-efficient hydrogen evolution reaction

A metal-organic framework-derived method was developed to synthesize highly efficient non-noble metal electrocatalyst for alkaline hydrogen evolution reaction (HER). Zn²⁺, phosphomolybdic acid were coordinated with 2-methylimidazole, and zinc (Zn) and phosphorus (P) species were removed by annealing at 850 °C in N₂ atmosphere, resulting in micro/mesoporous molybdenum carbide (Mo₂C) composited with nitrogen-doped carbon (denoted as ZIF8-xMo-850). The optimized sample ZIF8-12Mo-850 displayed a low overpotential of ~85.7 mV to deliver a current density of 10 mA cm⁻², with a corresponding Tafel slope of ~69.7 mV dec⁻¹ in 1 M KOH. This HER catalytic activity was competitive with the most recently developed Mo₂C-based HER electrocatalysts. From further investigation, the high HER catalytic activity of ZIF8-12Mo-850 is owing to three aspects: (i) The appropriate Mo feeding amount of ZIF8-12Mo-850 resulted in the highest surface content of Mo²⁺ active site; (ii) The evaporation of Zn and P in the ZIF8-12Mo precursor formed its largest average pore diameter of 32.3 nm, which leaded to the highest electrochemically active surface area (ECSA) of 64.29 cm² (iii) The 2-methylimidazole in the precursor resulted in the highest surface content of pyridinic N in ZIF8-12Mo-850 (14.63%), which efficiently improved its conductivity and charge transfer efficiency.     

Porous N-doped Mo2C@C nanoparticles for high-performance hydrogen evolution reaction

Exploration for an earth-rich and competent electrocatalyst for the hydrogen evolution reaction (HER) is a significant and challenging approach to confronting the resources shortage and environmental crisis. Porous N-doped Mo₂C@C (N-Mo₂C@C) nanoparticles self-encapsulated in nanospheres are presented as a high-performing HER electrocatalyst fabricated through a one-pot solvothermal method followed by hydrogen calcination. Structural analyses show that acetamide can regulate the size of the nanospheres, provide a N source for doping and form porous structures composed of Mo₂C, which suggests the exposure of extensive active sites as well as the contact and diffusion among the medium, electrodes, and gas. Theoretical calculations show that the N doping can enhance the activity of the Mo-C bond, reduce the energy of capturing hydrogen intermediates, and increase the catalytic conductivity. This work offers a simple and promising strategy to understand the catalytic mechanism required to optimize the activity of Mo-based electrocatalysts via N doping.     

Porous molybdenum carbide microspheres as efficient binder-free electrocatalysts for suspended hydrogen evolution reaction

Generally, the electrocatalysts are immobilized on conductive electrodes or in-situ grown on current-collecting substrates, which causes some disadvantages. For the first time, the obtained porous molybdenum carbide microspheres with diameters of 200–400 μm are employed as binder-free electrocatalysts in the novel model of suspended hydrogen evolution reaction (SHER), which possess the perfect catalytic stability and high practicability. Herein, porous molybdenum carbide microspheres synthesized by ion exchange reaction and subsequent calcining process are employed as electrocatalysts for HER, which possess a low onset potential of ?79 mV vs. RHE and a low overpotential of 174 mV achieving a current density of 10 mA/cm² in 0.5 M H₂SO₄. This work may provide a new methodology for rational design and fabrication of reaction pattern for the electrolysis of water.     

Monolayer palladium supported on molybdenum and tungsten carbide substrates as low-cost hydrogen evolution reaction (HER) electrocatalysts

Active and low-cost hydrogen evolution reaction (HER) electrocatalysts are needed to minimize capital costs associated with large-scale hydrogen production from water electrolysis. Catalysts based on monolayer (ML) amounts of precious metals supported on carbides are a promising concept for this purpose. In the current study Pd supported on tungsten carbide (WC) and molybdenum carbide (Mo₂C) were evaluated for HER activity. Carbide foils were synthesized using temperature programmed reaction of W or Mo in a CH₄/H₂ atmosphere. Physical vapor deposition was used to deposit Pd on WC or Mo₂C while X-ray Photoelectron Spectroscopy (XPS) was used to determine the Pd surface coverage. Linear sweep voltammetry and chronopotentiometry were used to evaluate the HER activity and electrochemical stability of the catalysts, demonstrating the possibility of using ML Pd on either WC or Mo₂C as active, stable and lower-cost HER catalysts.     

Coupling molybdenum carbide nanoparticles with N-doped carbon nanosheets as a high-efficiency electrocatalyst for hydrogen evolution reaction

Searching for earth-abundant and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) is of critical importance for future energy conversion devices. To facilitate the HER on a nonprecious metal-based catalyst, integration of catalytically active nanoparticles with highly conductive carbon supports represents a promising strategy since the formed nanohybrid can offer available active sites and improved electron transfer capability. Herein, we demonstrate a feasible and scalable approach to fabricate well-dispersed Mo₂C nanoparticles firmly anchored on 2D ultrathin N-doped carbon nanosheets (denoted as Mo₂C@NC nanosheets) using inexpensive NaCl as recyclable templates. The adoption of NaCl template provides a 2D space for the one-step concurrent growth of Mo₂C nanoparticles and N-doped carbon nanosheets. Benefiting from the synergy between fine Mo₂C nanoparticles with high dispersity and N-doped C nanosheets, the resultant Mo₂C@NC nanosheets exhibit an outstanding HER performance with a low overpotential, a small Tafel slope and excellent stability under acidic medium, making them a promising noble-metal-free HER catalyst.     

Surficial charge state tuning of tungsten carbide for catalyzing alkaline hydrogen evolution reaction

Rational tuning the surface charge state for catalysts is an attractive strategy to promote their catalytic activity toward catalyzing diverse chemical and electrochemical reactions. In this work, we modified the surface of a pure tungsten carbide (WC) material with an ultra-thin nitrogen doped carbon (NC) layer (WC@NC) for catalyzing alkaline hydrogen evolution reaction (HER). The XPS studies revealed an increased electron density on WC when the NC layer was introduced on its surface. Thus, a significant decrease of 282 mV in overpotential at 10 mA cm^?2 was observed on WC@NC electrode in comparison with the pristine WC electrode. The higher electrochemical active surface area, lower activation energy barrier, and excellent durability were identified by various electrochemical measurements. The three-dimensional charge density difference was also calculated and the results indicated that an electron-transfer process occurred from the NC layer (donor) to the WC layer (acceptor) through the interface of WC@NC composite material. The analysis explained the underlying mechanism for the promotion of the HER activity. Such a comprehensive study is expected to provide useful guidance for promoting the catalytic performance of diverse catalysts toward different reactions.     

Highly graphitic carbon shell on molybdenum carbide nanosheets by iron doping for stable hydrogen evolution

The graphitic degree of carbon layer on molybdenum carbide (MC) surface might affect its catalytic performance toward the hydrogen evolution reaction (HER). In this study, an iron (Fe)-doping method was investigated to adjust the graphitic degree of the carbon layer, and subsequently explore the catalytic activity of the generated MCs. A series of Fe-doped MCs was synthesized, and the optimization of their doping level led to distinctly higher HER activity compared to the undoped MC, in both acidic and alkaline solutions, due to the electronic effect. Transmission electron microscopy was applied to determine carbon-coated structure of the obtained Fe-doped MCs while Raman spectra demonstrated the successful adjustment of the carbon layer's graphitic degree with the change of the doping level. As a result, the well-preserved catalytic activity after a 10,000-circles stability test indicated the stable performance of HER on the Fe-doped MC, whereas the enhanced durability was supported by the high graphitic degree of carbon coating layer.     

Rational design of Ni-induced NC @Mo2C@MoS2 sphere electrocatalyst for efficient hydrogen evolution reaction in acidic and alkaline media

Exploiting efficient and low-cost electrocatalyst for Hydrogen Evolution Reaction (HER) of water electrolysis remains a challenge. Herein, we designed an efficient electrocatalyst of Ni-induced nitrogen-doped carbon @ molybdenum carbide @ molybdenum disulfide sphere (NC@Mo₂C@MoS₂-(Ni)) by two simple coating steps following pyrolysis process. Benefiting from the regular spherical morphology, unique structure, synergistic effect between Mo₂C and MoS₂, inducement effect of elemental Ni that initial added and removed in final synthesis procedure, heteroatom N and P doping. The catalyst NC@Mo₂C@MoS₂-(Ni) exhibits relatively good catalytic performance of overpotentials of 205 and 216 mV at 10 mA cm^?2 and Tafel slopes of 61.4 and 42.7 mV dec^?1 in acidic and basic media, respectively. This work not only fabricate the electrocatalyst of NC@Mo₂C@MoS₂-(Ni) towards HER, but also provides a way to rationally design more efficient other functional electrocatalysts.     

Effect of Mo content on hydrogen evolution reaction activity of Mo2C/C electrocatalysts

Recent research suggests that molybdenum carbide (β-Mo₂C) has the potential to be a cheap and active substitute for Pt-based electrocatalyst for hydrogen evolution reaction. In this article molybdenum carbide (Mo₂C) electrocatalysts immobilized on carbon support were synthesized and evaluated for hydrogen evolution reaction (HER). The quantity of Mo in the samples was varied to understand the effect of Mo content in Mo₂C/C electrocatalyst on the structure, morphology, electrochemical properties and HER. The Mo weight percentages determined by ICP-OES technique in four Mo₂C/C samples prepared were found as ~9.3, 15.8, 20.4 and 28.0. SAXS studies revealed that the pore size of the carbon increased with an increase in Mo content, most probably to accommodate the Mo₂C motifs. X-ray photoelectron spectra showed that the amount of low valent Mo increased as we increased the Mo content up to 20 wt % but decreased in the 28 wt % sample. All the samples were active for electrochemical HER with the sample having ~20 wt % Mo showing the highest activity and exhibited a Tafel slope of 69 mVdec⁻¹. Among all samples the 20 wt% Mo sample exhibited the highest electrochemical surface area (ECSA) of ~2.92 mFcm⁻² and minimum charge transfer resistance for the HER. Thus, it is concluded that 20 wt% Mo in Mo₂C/C electrocatalyst evolves with ideal pore size, highest ECSA, smooth charge transfer and thus exhibits the best electrochemical properties for HER.     

Molybdenum carbide MXene embedded with nickel sulfide clusters as an efficient electrocatalyst for hydrogen evolution reaction

Molybdenum-based MXene materials (Mo₂CT_x) have recently demonstrated great potential in electrocatalytic hydrogen production. Herein, we fabricated a novel NiS/Mo₂CT_x hybrid via chemical etching in an NH₄F/HCl solution followed by solvothermal reactions, where nickel sulfide (NiS) clusters were embedded between the interlayers of Mo₂CT_x. The intrinsic structure and electrochemical properties were experimentally investigated to explore the potential of an electrocatalyst for hydrogen evolution. As expected, the heterostructure by embedding NiS into the Mo₂CT_x MXene interlayers not only brings about large electrochemical surface areas with abundant active site exposure but also enhances the intrinsic kinetics to facilitate the electrolysis process. Electrochemical tests revealed that the NiS/Mo₂CT_x catalyst exhibited the HER performance with a small overpotential of 157 mV to drive the current density of 10 mA cm⁻² and long-term stable durability, which are superior to that of pristine Mo₂CT_x MXene and nickel sulfides. This study can provide a synthetic strategy for designing and developing Mo₂CT_x MXene-based electrocatalysts for hydrogen production.     

Reduced graphene oxide-supported Ni-MoxC electrocatalyst for hydrogen evolution reaction prepared by ultrasonication and lyophilization

A Ni and Mo_xC hybrid (Ni-Mo_xC) supported on N-doped reduced graphene oxide (N-rGO) electrocatalyst with high hydrogen evolution reaction (HER) activity was prepared by ultrasonication and lyophilization. Notably, benefiting from the synergistic effect between Ni and Mo_xC nanoparticles, the optimized electrocatalyst displayed excellent catalytic activity with low overpotentials of 183 mV and 216 mV for the HER at the current density 10 mA cm⁻² in 1.0 M KOH and 0.5 M H₂SO₄ solution. The stability of the electrocatalyst could be well maintained for 24 h. These results indicate that the method to prepare hybrid (Ni-Mo_xC) is a simple way to produce cost-effective and high-efficient molybdenum carbide for hydrogen evolution.     

Cobalt-molybdenum carbide@graphitic carbon nanocomposites: Metallic cobalt promotes the electrochemical hydrogen evolution reaction

In this work, a series of metallic cobalt-molybdenum carbide@graphitic carbon (CoMo(x:y)-T@GC) nanocomposites for the electrochemical hydrogen evolution reaction (HER) were synthesized by a sol-gel method. In the as-prepared nanocomposites, β-Mo₂C and metallic Co coexisted and were encapsulated by graphitic carbon. The presence of metallic Co effectively enhanced the crystallinity of β-Mo₂C, charge transfer efficiency and electrochemical active surface area (ECSA), thus resulting in the improved HER catalytic activities of the CoMo(x:y)-T@GC nanocomposites. The optimized electrocatalyst CoMo(0.5:0.5)-800@GC required the lowest overpotential of ～165 mV to deliver a current density of 10 mA cm^?2 in 0.1 M KOH, which was at the forefront compared with recently reported Mo₂C-based electrocatalysts.     

Mo2C regulated by cobalt components doping in N-doped hollow carbon nanofibers as an efficient electrocatalyst for hydrogen evolution reaction

Hydrogen is a viable substitute to fossil fuels and electrochemically catalyzed hydrogen evolution has attracted wide attention due to its stability and effectiveness. Nevertheless it is still a major challenge to design and prepare highly active noble metal-free electrocatalysts with controllable structure and composition for efficient hydrogen evolution reaction (HER). Herein, Mo₂C regulated by cobalt components (Co and CoO) doping in N-doped hollow carbon nanofibers (marked as Mo₂C/Co/CoO-NHCNFs) are firstly designed and prepared via a facile coaxial electrospinning followed by calcination process. The one-dimensional conductive carbon host, hollow structure and synergistic effect among CoO, Co and Mo₂C can jointly promote electron transfer, augment exposure of active sites and adjust the electronic structure of the active sites, resulting in the excellent of HER performances. The optimized catalyst has a high specific surface area of 101.27 m² g^?1. Meanwhile, it has a low overpotential of 143 mV at a current density of 10 mA cm^?2 and a small Tafel slope of 74 mV dec^?1 in 1.0 M KOH.Satisfactorily, the overpotential is reduced by 231 mV at the same current density compared with Mo₂C doped in N-doped carbon nanofibers (named as Mo₂C-NCNFs). Moreover, the Mo₂C/Co/CoO-NHCNFs also demonstrate superior long-term stability. The formative mechanism of Mo₂C/Co/CoO-NHCNFs is expounded, and the construction technique is established. The design philosophy and the simple and economical method are of significance for development of HER electrocatalysts.     

Ultrasmall Mo2C in N-doped carbon material from bimetallic ZnMo-MOF for efficient hydrogen evolution

The exploration and development of cost-effective and highly stable electrocatalysts with the highest possible energy efficiency remain a constant pursuit in the catalyst design and synthesis for electrocatalytic hydrogen evolution reaction (HER). In this work, a convenient approach is proposed to synthesize a type of ultrafine Mo₂C nanoparticles in average sizes of 3–4 nm embedded in hierarchically porous N-doped carbon material calcined from bimetallic ZnMo-MI (MI = 2-methylimidazole) is obtained at 1000 °C, denoted as ZnMo-MI-1000. First of all, the crystalline hybrid metal-organic framework of ZnMo-MI is fabricated from zeolitic imidazolate framework of Zn-MI precursors via solvothermal reaction, in which the conversion from Zn-MI to ZnMo-MI occurs gradually over time. After calcination, the as-obtained ZnMo-MI-1000 sample shows a satisfying HER performance with the small overpotential of 83.0 mV in 0.5 M H₂SO₄ and 100.1 mV in 1.0 M KOH to reach a current density of 10 mA cm^?2, which is attributed to ultrasmall Mo₂C, Mo and N-doped graphitic carbon matrix. The multiporous network of ZnMo-MI-1000 can provide continuous mass transportation with a minimal diffusion resistance that produce effective electrocatalytic kinetics in both acidic and alkaline media, which is utilized as a highly active and durable nonprecious metal electrocatalyst for HER.     

Insight into the influence of doped oxygen on active sites of molybdenum sulfide materials in hydrogen evolution reaction

Molybdenum disulfide has received great attention as a promising non-noble catalyst for electro-catalyzed hydrogen evolution reaction. The active sites originated from the limited edge of crystalline molybdenum disulfide is the key to restrict its HER performance. To increase the active sites of molybdenum disulfide through the heteroatom doping with effective synthetic strategy has become the focus of activity improvement. Herein, a facile and efficient strategy was adopted to synthesize oxygen-doped molybdenum sulfide catalyst by utilizing thiourea and sodium molybdate as precursors. It was found that the number of active sites could be regulated by controlling the dosage ratio of thiourea to sodium molybdate. The doped oxygen and abundant S endows molybdenum sulfide a great deal of lattice disorder or defects, thus providing adequate active sites. When the optimized ratio of thiourea to sodium molybdate (40:1), the double layer value of oxygen-doped molybdenum sulfide reached 34.14 mF/cm² (mass loading on glassy carbon electrode was 0.142 mg/cm²) which is considered to be proportional to the electrochemical active area. Raman spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of Mo–O bond and bridging S₂^2? bonds which endows molybdenum sulfide with a great deal of lattice disorder or defects, thus providing plentiful active sites.     

N,P-codoped molybdenum carbide nanoparticles loaded into N,P-codoped graphene for the enhanced electrocatalytic hydrogen evolution

Based on the ever-growing interest of heteroatoms (e.g., P, N, S, transition metals) doping into molybdenum carbide and graphene for electrochemical reactions, herein, a ternary phosphomolybdic acid-polyethyleneimine/graphene oxide nanocomposite as a suitable precursor was developed to not only uniformly hybridize molybdenum and carbon source to perform the controllable in situ growth of well-defined molybdenum carbide nanostructure on graphene by annealing, but also synchronously dope N and P atoms into molybdenum carbide crystal lattice and graphene. The as-prepared hybrid showed remarkable electrocatalytic activity and high stability for hydrogen evolution reaction in basic media, due to the following favorable features, i.e. a large accessible active sites afforded by the ultrafine molybdenum carbide, the heteroatoms doped, the regulated electronic structure, the balanced thermodynamics between hydrogen adsorption and desorption, the accelerated charge/mass transfer ability by the ultrathin and defective carbon layer, and the protection of molybdenum carbide by carbon layer. As a result, it only needed a small overpotential of 47 mV to drive 10 mA cm^?2 and a low onset potential of 10 mV, as well as a small Tafel slope of 56.8 mV·dec^?1, thus suggesting its promising potential for hydrogen evolution electrocatalyst.     

Study of molybdenum electrodes for hydrogen evolution reaction

The molybdenum electrode, Mo, has been investigated for hydrogen production via water electrolysis in 10 vol.% aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) using electrochemical impedance spectroscopy (EIS). The EIS measurements show that the Mo system has much higher interfacial capacitance, and correspondently the electrical double layer formed on this electrode is thicker than those formed on nickel or platinum. The positive displacement of potential of zero charge (PZC) values indicates the specific adsorption of the imidazolium cation on the Mo surface. This study provides an elegant explanation for the better performance of Mo electrodes in the hydrogen evolution reaction (HER): the BMI cation acts as an intermediate for the proton transfer from water to the electrode surface, thereby decreasing the overpotential of HER. This model explains the synergism between Mo and the BMI cation in the HER process.