Nickel hydroxide armour promoted CoP nanowires for alkaline hydrogen evolution at large current density

The development of hydrogen evolution activity (HER) electrocatalyst that can run durably and efficiently under the large current density is of special significance but still challengeable for the massive production of hydrogen. Herein, a CoP/Ni(OH)₂ nanowire catalysts grown on Co foam (CF) with a three-dimensional heterojunction structure has been successfully prepared by electrodepositing nickel hydroxide on the surface of cobalt phosphide. The prepared CoP/Ni(OH)₂–15 min sample reveals a superior HER activity and stability. It merely requires ultralow overpotentials of 108 and 175 mV to 100 and 500 mA cm^?2, respectively. In addition, the long-term stability test shows that the catalyst (CoP/Ni(OH)₂–15 min) can operate stably for at least 70 h at 400 mA cm^?2. Utilizing NiFe-LDH/IF with high OER activity, the NiFe-LDH/IF || CoP/Ni(OH)₂–15 min catalyst system possesses the same outstanding performance for overall water splitting (OWS), which can accomplish ≈ 500 mA cm^?2 at 1.74 V in 1 M KOH electrolyte. Moreover, the NiFe-LDH/IF || CoP/Ni(OH)₂–15 min couple can work for more than 80 h at 500 mA cm^?2, indicating its a great prospect in the area of electrolysis water. Such excellent catalytic performance is mainly attributed to the armor effect of Ni(OH)₂, which can not only promote the rapid decomposition of water molecules, but also prevent the loss of phosphorus and enhance the synergistic effect of CoP and Ni(OH)₂. This work can offer a significant reference for the design with high-performance and durable transition metal phosphide electrocatalysts.     

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.     

NiCo2O4 nanowire arrays rich in oxygen deficiencies for hydrogen evolution reaction

The rational design of highly efficient electrocatalysts to generate hydrogen by catalyzing hydrogen evolution reaction still remains a challenge. Herein, we report a simple strategy to significantly enhance the catalytic activities of NiCo₂O₄ nanowire arrays by simply tuning the amount of oxygen vacancies. Remarkably, the oxygen-deficient NiCo₂O₄ catalysts obtained in Ar environment show significantly improved catalytic activities toward hydrogen evolution reaction with the requirement of 104 mV overpotential to afford 10 mA cm⁻², 122 mV less than that for air-sintered NiCo₂O₄ (226 mV). Moreover, such catalysts also exhibit superior long-term durability for 24 h at 100 mA cm⁻². The present study further promotes the application of NiCo₂O₄ in other energy storage and conversion system.     

New insights into high-valence state Mo in molybdenum carbide nanobelts for hydrogen evolution reaction

Hydrogen evolution reaction (HER) is considered to be one of the most promising strategies to create hydrogen. Recently, searching high-efficient, stable, and earth-abundant electrocatalysts to replace precious metals for practical utilizations of HER is attracting more and more attentions. Herein, novel molybdenum carbide nanobelts containing Mo of high-valence state derived from MoO₃-ethylenediamine inorganic/organic hybrid precursors are successfully synthesized via a facile one-pot pyrolysis method. The molybdenum carbide nanobelts are characterized using XRD, SEM, TEM and XPS. Moreover, the high-valence state Mo and their relative content in the molybdenum carbide nanobelts can be identified by XPS. The high-resolution XPS spectra of Mo 3d indicates in the molybdenum carbide nanobelts the proportion of high-valence state Mo in active Mo components is 51.3%. More importantly, the as-synthesized products exhibit excellent electrocatalytic activity for HER with a low onset overpotential of 50 mV and a small Tafel slope of 49.6 mV dec^?1 in acidic medium (0.5 M H₂SO₄). Besides, the catalysts require only overpotentials of 143 and 234 mV to achieve current densities of 10 and 220 mA cm^?2, respectively. Furthermore, they also exhibit good durability after 2000 cycles and constant current density test. Such excellent electrocatalytic HER performance can be ascribed to the high intrinsic activity of high-valence state Mo in Molybdenum Carbide. Synthesizing molybdenum carbide with high-valence state Mo electrocatalysts for HER will open up an exciting alternative avenue to acquire outstanding HER electrocatalytic activity.     

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.     

Facile in-situ formation of high efficiency nanocarbon supported tungsten carbide nanocatalysts for hydrogen evolution reaction

Electrochemical hydrogen evolution reaction (HER) is one of the key techniques for hydrogen production. Much great effort has been made so far to develop highly efficient HER catalysts to replace expensive precious metals (e.g. Pt). Unfortunately, the synthesis processes were generally not cost-effective and/or scalable. So it is highly desirable to develop a facile technique to enhance HER activity of conventional inexpensive but less active materials. In this work, monodispersed tungsten carbide (WC) nanoparticles (<5 nm) were in-situ formed/anchored on nanosized carbon black (CB) and carbon nanotube (CNT) via a simple low temperature molten salt synthesis technique. Owing to this special hybrid structure, both the exposed surface area of active species and the electrical conductivity of the catalysts were increased effectively, making the catalysts perform considerably better in HER than pure WC and WC based catalysts prepared via other conventional routes. WC nanocrystals in-situ formed/anchored on CNTs showed small onset overpotential (90 mV), low Tafel slope (69 mV dec^?1), high current density (93.4 and 28 mA cm^?2 at 200 and 300 mV, respectively) and excellent stability (remaining stable even after 3000 cycles). Such a performance is one of the best among those of WC based electrocatalysts developed to date. We demonstrate here significantly improved HER performances of inexpensive tailored WC materials, along with a facile synthesis strategy which could be also readily extended to prepare a range of other types of mono-dispersed nanocatalysts for more potential applications.     

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.     

Exfoliated MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution

Porous graphene (P-rGO) was synthesized from graphene oxide (GO) via a one-pot calcination method with CO₂ as an activation agent at 800 °C. Due to the special porous structure, the surface area of P-rGO can be increased to ～759 m²/g. The P-rGO was then used as a support to incorporate with chemical exfoliated molybdenum disulfide (MoS₂) for the fabrication of MoS₂/P-rGO composite. Compared to bulk MoS₂, the exfoliated MoS₂ is in the 1T phase with a metallic property and smaller charge transfer resistance, thus has a better activity in electrochemical hydrogen evolution reaction (HER). The HER activity of 1T MoS₂ could be further increased after the combination with P-rGO. The overpotential of 1T MoS₂/P-rGO was only ～130 mV vs. RHE, and the corresponding Tafel slope was ～75 mV Dec^?1. The special porous structure and good electric conductivity of P-rGO decrease the charge transfer resistance of the composite without sheltering too many active sites of MoS₂, thus leading to the enhanced HER activity. As an efficient noble metal free HER catalyst, the 1T MoS₂/P-rGO has great potential for large-scale hydrogen production.     

Facile synthesis of porous dendritic Pt68Ag32 nanodandelions for greatly boosting electrocatalytic activity towards oxygen reduction and hydrogen evolution

Contrary to single counterparts, Pt-based bimetallic nanocrystals are attractive and feasible to decrease the cost, improve the catalytic activity and enhance the stability via morphology-, structure- and composition-engineering. Herein, porous dendritic alloyed Pt₆₈Ag₃₂ nanodandelions (NDs) were synthesized by a one-pot successive co-reduction method, using amrinone as the new stabilizer and structure director, without any template, seed or complicated instrument. The correlative shapes, composition, and crystalline structure were examined by a range of characterization techniques. It is found that the molar ratio of the precursors (AgNO₃ and H₂PtCl₆), the dosage of amrinone, and different reductants play the essential roles during the synthesis process. The Pt₆₈Ag₃₂ NDs exhibited dramatically enlarged electrochemically active surface area, enhanced catalytic activity and stability for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) as compared to commercial Pt/C, Pt black, home-made Pt₇₅Ag₂₅ nanocrystals (NCs) and Pt₅₆Ag₄₄ NCs catalysts.     

Facile and large-scale preparation of Co/Ni-MoO2 composite as high-performance electrocatalyst for hydrogen evolution reaction

Facile fabrication of high-performance catalyst based on low-cost metals for sustainable hydrogen evolution is still a matter of cardinal significance. However, synthetic approaches for electrocatalyst are usually complicated and the yields are often low. Herein, we report a one-step simple method for the large-scale synthesis of Co/Ni-MoO₂ composite as efficient and stable hydrogen evolution reaction (HER) electrocatalyst to drive 10 mA cm^?2 current density with a low overpotential of 103 mV in basic media. Co-MoO₂ and Ni-MoO₂ were also prepared using this method with overpotential of 137 and 130 mV, respectively, to gain the same current density. These results indicate that this facile synthesis approach is of great practical importance as it can be easily used for large-scale preparation of electrocatalysts in industry.     

In-situ synthesis of porous Ni2P nanosheets for efficient and stable hydrogen evolution reaction

The design and development of highly efficient and stable non-noble metal electrocatalysts for hydrogen evolution reaction (HER) have attracted increasing attention. However, some key issues related to large overpotential, high cost and poor stability at high current density still remains challenging. In this work, we report a facile in-situ integration strategy of porous Ni₂P nanosheet catalysts on 3D Ni foam framework (PNi₂P/NF) for efficient and stable HER in alkaline medium. The two-step method can creates high density of ultra-thin porous Ni₂P nanosheets firmly rooted into Ni foam substrate which can guarantee excellent electrical contacts, strong substrate adherence and large amount of active sites. Such a binder-free flexible HER cathode exhibits superior electrocatalytic performance with an overpotential of 134 mV at current density of 10 mA cm⁻². It also shows superior stability at higher current densities of 100 and 500 mA cm⁻² for at least 48 h and negligible performance degradation is observed.     

Facile synthesis of porous 3D CoNiCu nano-network structure and their activity towards hydrogen evolution reaction

Nanostructured alloys have recently attracted great attention for hydrogen evolution due to their unique electron structure and large surface area. Herein, we reported a facile electrochemical method to prepare three-dimensional (3D) nano-network structures and demonstrated their feasibility as efficient electrocatalyst for hydrogen evolution. These 3D CoNiCu nano-network structure exhibited good performance in hydrogen evolution reaction (HER).     

Facile synthesis of NiS2 nanowires and its efficient electrocatalytic performance for hydrogen evolution reaction

Recently, the first-row transition metal dichalcogenides MX₂ (M = Fe, Co, Ni; X = S, Se) have been widely reported as promising catalysts for hydrogen evolution reaction (HER) because of its excellent catalytic activity and earth-abundance. The rational nanostructure designs have been proved as an effective way to improve their catalytic performance. However, the reported one dimension (1D) NiS₂ nanowires for HER suffer from a large Tafel slope. Here, we report a facile synthesis of 1D NiS₂ nanowires and its high efficient catalytic activity in HER. This nanowire structure with large surface area and active sites enables highly efficient electrocatalytic performance in HER with a much smaller Tafel slope (83.5 mV/dec) compared to that of bulk NiS₂ (136 mV/dec) as well as long-term stability. Our work builds up a structure–performance relationship and enriches the synthetic strategy to other efficient catalysts such as first-row transition metal dichalcogenides or transition metal phosphide.     

Facile synthesis of MoS2/CuS nanoflakes as high performance electrocatalysts for hydrogen evolution reaction

The fabrication of metal sulfides heterostructure is a promising strategy for enhancing catalytic activity. Herein, the MoS₂/CuS heterostructure was successfully grown on carbon cloth (MoS₂/CuS/CC) through an efficient method. The SEM results confirmed that the fabricated MoS₂/CuS/CC composites have a flake morphology, which can not only improves the surface area but also offers ample surface catalytic active sites. Particularly, the optimized MoS₂/CuS/CC-2 electrocatalyst showed a small overpotential of 85 mV@10 mA cm^?2 and exceptional long-term cycling durability for hydrogen evolution in 1 M KOH. The outstanding catalytic activity is attributed to the fact that the combination of MoS₂ with CuS can greatly enhance the charge transport rate and improve the structural stability. These results suggest that the MoS₂/CuS/CC heterostructure is a potential electrocatalyst for hydrogen production.     

Synergistic coupling of CoFe-layered double hydroxide nanosheet arrays with reduced graphene oxide modified Ni foam for highly efficient oxygen evolution reaction and hydrogen evolution reaction

Novel CoFe-LDH (layered double hydroxide) nanosheet arrays in situ grown on rGO (reduced graphene oxide) uniformly modified Ni foam were synthesized by a citric acid-assisted aqueous phase coprecipitation strategy. Systematic characterizations indicates that the series of Co_xFe₁-LDH/rGO/NF (x = 4, 3, 2) all show Co_xFe₁-LDH nanosheets (150–180 × 15 nm) grown vertically on the surface of rGO/NF. Especially, the Co₃Fe₁-LDH/rGO/NF exhibits the best performance with overpotentials of 250 and 110 mV at 10 mA cm^?2 in 1 M KOH for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. When it is used as cathode and anode simultaneously for overall water splitting, they require 1.65 and 1.84 V at 10 and 100 mA cm^?2, respectively. Excellent performance of Co₃Fe₁-LDH/rGO/NF is due to the nanosheet arrays structure with open channels, synergistic coupling between Co₃Fe₁-LDH and rGO enhancing electrical conductivity, and in-situ growth of Co₃Fe₁-LDH on rGO/NF enhancing stability.     

Facile one-step synthesis of nitrogen-doped carbon sheets supported tungsten carbide nanoparticles electrocatalyst for hydrogen evolution reaction

Exploring non-precious metal catalysts with high activity and stability to replace Pt-based materials is vital for electrochemical water splitting. In this work, a facile one-step method was put forward to synthesize WC/NC composite. Due to the couple effect of KCl/NaCl salt and dicyandiamide, pure WC phase was obtained at 900 °C. Meanwhile, KCl/NaCl salt eliminated the runaway pyrolysis expansion of glucose. Besides contributing to the special surface area, dicyandiamide as N source significantly alleviated W mass loss trigged by KCl/NaCl salt and ensured the appropriate WC content in WC/NC composite. As a HER electrocatalyst in acid media, WC/NC composite exhibits the small overpotential of 156 mV at a current density of 10 mA cm⁻², the low Tafel slope of 64 mV dec⁻¹, as well as the robust stability. This work offers a feasible option to fabricate low-cost and effective transition metal carbide electrocatalysts on a large-scale for hydrogen evolution reaction.     

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.     

Defect-rich MoSx/carbon nanofiber arrays on carbon cloth for highly efficient electrocatalytic hydrogen evolution

Engineering MoS₂ catalysts with more active sites and higher conductivity is an effective way to improve its electrochemical activity. Herein, defect-rich amorphous MoS_x/carbon nanofiber (CF) arrays on carbon cloth (CC) support (denoted as MoS_x/CF/CC) was designed and fabricated, which served as an efficient free-standing electrocatalyst for hydrogen evolution reaction (HER) in acid media. This architecture was beneficial to expose more active catalytic sites and improve the electron/ion transport. In addition, abundant defects altered preferred growth direction of MoS_x, resulting in the formation of irregular MoS_x particles at the surface of CF arrays. The as-synthesized MoS_x/CF/CC-2 exhibited excellent stability and superior HER activity, with a small onset overpotential (107 mV) and low Tafel slope (51 mV dec^?1). Such excellent electrochemical performance was attributed to the enriched active sites and shortened charge diffusion distance. This work would pave a new way for rational design and fabrication of defect-rich MoS_x-based composite electrode for renewable energy applications.     

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.     

Facile hydrothermal synthesis of rare earth phosphate for boosting hydrogen evolution reaction

The water electrolysis process has attracted great attention due to the production of high energy density pure hydrogen. However, the involved cell reactions in this process such as hydrogen and oxygen evolution reactions are kinetically sluggish and demands high input energy to accelerate the rate of these reactions. Therefore, the development and application of efficient electrocatalyst is essential for hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). In the present work, we have successfully synthesized two rare earth phosphates through the hydrothermal route and used as a catalysts towards HER in an acidic medium. The rare earth phosphate PrPO₄ exhibits better catalytic activity than YPO₄ catalyst. The overpotential of PrPO₄, YPO₄ and standard Pt/C were found as 147, 484.3 and 58 mV vs. reversible hydrogen electrode, respectively, to reach current density 10 mA·cm^?2 and corresponding Tafel slopes were found as 107.58, 118.73 and 80.89 mV decade^?1, respectively in 0.5 M H₂SO₄. The catalytic activity of PrPO₄ (472.83 mA·cm^?2) overcome standard Pt/C (179.60 mA·cm^?2) at high overpotential 450 mV vs. reversible hydrogen electrode. The prepared PrPO₄ shows efficient electrocatalytic activity towards HER in acidic medium because it possess high BET surface area, large ECSA value and small charge transfer resistance than YPO₄.