Exposure of active sites in Mn–SnS2 nanosheets to boost hydrogen evolution reaction

Designing and synthesizing high-activity, durable, and low-cost catalysts for the electrochemically transformation of water to hydrogen are vitally important to future energy systems. Herein, a simple but effective strategy for manganese-metal-heteroatom doping is adopted to intrinsically elevate the electrocatalytic activities of SnS₂ nanosheets by a facile two steps hydrothermal-sulfurization approach. Electrocatalytic hydrogen evolution (HER) performance of Mn–SnS₂ nanosheets grown on 3D nickel foam (Mn–SnS₂/NF) is efficiently optimized since the dopants and defects endow the Mn–SnS₂/NF vast active sites. An overpotential as low as 71 mV is required to drive a current density of 10 mA/cm² with a low Tafel slope of 72 mV dec^?1 in alkaline environment (1 M KOH). In addition, the Mn–SnS₂/NF exhibits prominent stability in 1 M KOH electrolyte, which is an indispensable index for the potential HER electrocatalysts. The present work demonstrates that the heteroatom manganese doping strategy renders a meaningful route for synthesizing cost-efficient HER electrocatalysts in alkaline condition.     

Iron oxide ores as carriers for the production of high purity hydrogen from biogas by steam–iron process

Production of high purity hydrogen (<50 ppm CO) by steam–iron process (SIP) from a synthetic sweetened biogas has been investigated making use of a natural iron ore containing up to 81 wt% of hematite (Fe₂O₃) as oxygen carrier. The presence of a lab-made catalyst (NiAl₂O₄ with NiO excess above its stoichiometric composition) was required to carry out the significant transformation of mixtures of methane and carbon dioxide in hydrogen and carbon monoxide by methane dry reforming reaction. Three consecutive sub-stages have been identified along the reduction stage that comprise A) the combustion of CH₄ by lattice oxygen of NiO and Fe₂O₃, B) catalyzed methane dry reforming and C) G–G equilibrium described by the Water-Gas-Shift reaction. Oxidation stages were carried out with steam completing the cycle. Oxidation temperature was always kept constant at 500 °C regardless of the temperature used in the previous reduction to minimize the gasification of eventual carbon deposits formed along the previous reduction stage. The presence of other oxides different from hematite in minor proportions (SiO₂, Al₂O₃, CaO and MgO to name the most significant) confers it an increased thermal resistance against sintering respecting pure hematite at the expense of slowing down the reduction and oxidation rates. A “tailor made” hematite with additives (Al₂O₃ and CeO₂) in minor proportions (2 wt%) has been used to stablish comparisons in performance between natural and synthetic iron oxides. It has been investigated the effect of the reduction temperature, the proportion of methane to carbon dioxide in the feed (CH₄:CO₂ ratio) and the number of repetitive redox cycles.     

The noble metal loading binary iron–zinc electrode for hydrogen production

The binary FeZn was electrodeposited on graphite surface for hydrogen evolution reactions. Then, zinc was leached from the surface by an etching method in alkaline solution. A trace amount of platinum and ruthenium electrochemically deposited on binary FeZn, respectively. All electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy and potentiodynamic polarization techniques in KOH solution. The surface structures were characterized by scanning electron microscopy and energy dispersive X-ray. The results show that etching process enhanced the surface area. The prepared electrodes exhibited much higher activity after noble metal loadings. It is seen from experimental research that the most active electrode is C@FeZn/Ru.     

Metal–organic framework-derived Co@NMPC as efficient electrocatalyst for hydrogen evolution reaction: Revealing the synergic effect of pyridinic N–Co

Developing hydrogen economy is one of the feasible routes to reduce carbon emission in response to the energy crisis and global warming. The hydrogen generation by electrochemical water splitting has received widespread attention, but it is still challenging to fabricate high-efficient electrocatalysts to decrease the kinetic energy barrier of hydrogen evolution reaction (HER). Loading transition metal (TM) nanoparticles (NPs) into heteroatom-doped carbon materials (HCM) has been reported as a capable scheme to increase the electrochemical activity and stability, but the synergic effect between TM surface and HCM is still worth exploring. Ascertaining that, we used metal-organic frameworks (MOFs) as the sacrificial precursor to synthesis a series of Co NPs encapsulated in N-doped microporous carbon (NMPC) nanocatalysts (denoted as Co@NMPC) with different N species (such as pyrrolic, pyridinic and graphitic N). The nanocatalyst prepared at an appropriate condition displayed an outstanding HER activity with an overpotential of 193 mV in 1 M KOH solution and 132 mV in 0.5 M H₂SO₄ solution to reach 10 mA cm^?2 current density. Furthermore, the results of in situ shielding tests indicate that the synergy of pyridinic N–Co site owing to the intimate contact between Co surface and NMPC play the pivotal role in boosting HER performance. Density functional theory (DFT) calculations were employed to obtain an in-depth mechanism of synergic effect between Co and NMPC.     

Partial phosphorization of porous Co–Ni–B for efficient hydrogen evolution electrocatalysis

Design of cost-effective and high-efficient electrocatalysts for hydrogen evolution reaction (HER) is of vital significance for the current renewable energy devices — fuel cells. Herein, we report a facile strategy to prepare partial phosphorization of Co–Ni–B material with porous structure via a water-bath boronizing and subsequent phosphorization process at moderate temperature. The optimal atomic proportion of Co to Ni is investigated via physical and electrochemical characterization. As a result, Co₉–Ni₁–B–P exhibits the best HER activity, which require an lower overpotential of ~192 mV to deliver a current density value of 10 mA cm⁻² and a smaller Tafel slope of 94 mV dec⁻¹ in alkaline media, relative to P-free Co–Ni–B catalysts, Co₉–Ni₁–B–P with other Co: Ni proportion and mono metallic borides The excellent electrocatalytic performance of Co₉–Ni₁–B–P is mainly ascribed to the three-dimensional (3D) porous structure and the coordinate functionalization between the borides and phosphides. This work provides a promising strategy for the exploration of quaternary composites as efficient and cost-effective electrocatalysts for HER.     

Study on Ni–Fe–C cathode for hydrogen evolution from seawater electrolysis

Hydrogen evolution reaction in 3.5 wt% NaCl (simulated seawater) was investigated using Ni–Fe–C cathode, prepared by cathode electrodeposits method on the matrix of A3 steel. The as-prepared Ni–Fe–C cathode coating materials has reached nanometer grade, what is more, the limit of average grain size was about 4.3 nm. As decreasing of the average coating grain sizes, hydrogen evolution overpotential was not decreasing linearly. There was a boundary average coating grain sizes of about 4.3–6.4 nm. The optimal preparation process of Ni–Fe–C cathode was listed as electroplating current density 200 A/m², temperature 30 °C, pH 1.5 and 60 min. The hydrogen overpotential was only about 65 mV, which was tested in the 3.5 wt% NaCl of 90 °C at pH 12.     

Platinum–rare earth electrodes for hydrogen evolution in alkaline water electrolysis

In the new “Hydrogen Economy” concept, water electrolysis is considered one of the most promising technologies for hydrogen production. Novel electrocatalytic materials for the hydrogen electrode are being actively investigated to improve the energy efficiency of current electrolysers. Platinum (Pt) alloys are known to possess good catalytic activities towards the hydrogen evolution reaction (HER). However, virtually nothing is known about the effects of rare earth (RE) elements on the electrocatalytic behaviour of Pt towards the HER. In this study, the hydrogen discharge is evaluated in three different Pt–RE intermetallic alloy electrodes, namely Pt–Ce, Pt–Sm and Pt–Ho, all having equiatomic composition. The electrodes are tested in 8 M KOH aqueous electrolytes at temperatures ranging from 25 °C to 85 °C. Measurements of the HER by linear scan voltammetry allow the determination of several kinetic parameters, namely the Tafel coefficients, charge-transfer coefficients, and exchange current densities. Activation energies of 46, 59, 39, and 60 kJ mol⁻¹ are calculated for Pt, Pt–Ce, Pt–Sm and Pt–Ho electrodes, respectively. Results show that the addition of REs improves the activity of the Pt electrocatalyst. Studies are in progress to correlate the microstructure of the studied alloys with their performance towards the HER.     

Noble metal-free NiCo nanoparticles supported on montmorillonite/MoS2 heterostructure as an efficient UV–visible light-driven photocatalyst for hydrogen evolution

In this study, a noble-metal-free photocatalyst, based on NiCo nanoparticles supported on montmorillonite/MoS₂ heterostructure (MMT/MoS₂/NiCo), was successfully synthesized and applied for photocatalytic water reduction to produce H₂. Under UV–visible light irradiation, the composite showed improved photocatalytic performance for H₂ evolution compared to MMT/MoS₂, MMT/MoS₂/Ni, MMT/NiCo, and MoS₂/NiCo. The as-synthesized MMT/0.79MoS₂/Ni_8.14Co_6.4 (0.79, 8.14 and 6.4 denote the weight ratios % of MoS₂, Ni and Co in the catalyst) photocatalyst exhibited a high H₂ production rate of 8.7 mmol g^?1 h^?1, 26.5 and 2.3 times higher than for MMT/0.79MoS₂ and MMT/Ni_8.14Co_6.4, respectively. The enhanced photocatalytic performance was attributed to the loaded MoS₂ and NiCo nanoparticles, introducing active sites, increasing the light-absorbing capacity and accelerating the charge transfer from the Eosin Y dye owing to their appropriate Fermi level energy alignment. This work presents a cost-effective method combining the 2D sheets of MMT and MoS₂, and NiCo nanoparticles to form a quaternary photocatalytic system showing highly efficient hydrogen evolution from water without using noble metals.     

Novel strategies to tailor the photocatalytic activity of metal–organic frameworks for hydrogen generation: a mini-review

This review provides a recompilation of the most important and recent strategies employed to increase the efficiency of metal–organic framework (MOF)-based systems toward the photocatalytic hydrogen evolution (PHE) reaction through specific strategies: tailoring the photocatalytic activity of bare MOFs and guest@MOF composites, formation of heterojunctions based on MOFs and various photocatalysts, and inorganic photocatalysts derived from MOFs. According to the data reported in this mini-review, the most effective strategy to improve the PHE of MOFs relies on modifying the linkers with new secondary building units (SBUs). Although several reviews have investigated the photocatalytic activity of MOFs from a general point of view, many of these studies relate this activity to the physicochemical and catalytic properties of MOFs. However, they did not consider the interactions between the components of the photocatalytic material. This study highlights the effects of strength of the supramolecular interactions on the photocatalytic performance of bare and MOF-based materials during PHE. A thorough review and comparison of the results established that metal–nanoparticle@MOF composites have weak van der Waals forces between components, whereas heterostructures only interact with MOFs at the surface of bare materials. Regarding material derivatives from MOFs, we found that pyrolysis destroyed some beneficial properties of MOFs for PHE. Thus, we conclude that adding SBUs to organic linkers is the most efficient strategy to perform the PHE because the SBUs added to the MOFs promote synergy between the two materials through strong coordination bonds.     

Electrodeposition of nickel–copper alloys to use as a cathode for hydrogen evolution in an alkaline media

The electrocatalytic activity of nickel–copper (Ni–Cu) alloy coated electrodes for the hydrogen evolution reaction (HER) in an alkaline media was studied. The Ni–Cu alloys were electrodeposited on a Cu substrate by direct current (DC) and pulse current (PC) electrodeposition in a fixed plating bath. A wide alloy composition range (6–81 mol% Ni) was achieved by controlling the applied current density between 5 and 300 mA cm⁻². It was found that the electrocatalytic activity for the HER depended on the composition of the Ni–Cu alloys, where electrodes having low Ni content gave high electrocatalytic activities. DC electrodeposition resulted in better electrocatalytic performances than PC. Pulse plating parameters other than the magnitude of the applied current density did not substantially influence the electrocatalytic performance of the Ni–Cu alloy electrodes. Ni content was found to have a stronger effect on the electrocatalytic activity for the HER than the deposit morphology.     

Tuning of electrocatalytic activity of Mn–O–Co composite for alkaline hydrogen evolution reaction

We report the enhancement in electrocatalytic activity of Mn–O–Co composite electrode developed through chemical reduction method. The Mn–O–Co composite electrode exhibits high catalytic activity with a low Tafel slope of 123 mV dec⁻¹ and a low overpotential of 117 mV at a current density of 10 mA cm⁻². The enhancement in electrocatalytic activity of Mn–O–Co composite electrode is due to the synergistic activity of MnO and CoO with the NiP matrix. The intermetallic interaction among the half-filled orbitals of manganese with the fully occupied orbitals of cobalt and nickel leads to an effective electron delocalization in the catalytic system which enhances the HER performance of the coating. The C_dl value of the composite electrode is in the order of 254 μF, which is approximately ten fold higher than the bare NiP coating, due to the enhancement in interaction between the Mn–O–Co composite electrode and the reactive species in the HER medium. The Mn–O–Co composite electrode shows promising characteristics as an electrocatalyst with long term stability and remarkable competency with the commercially available electrodes.     

Direct-current electrodeposition of Ni–S–Fe alloy for hydrogen evolution reaction in alkaline solution

Ni–S–Fe alloy has been successfully fabricated on a copper foil substrate through direct-current electrodeposition as an electrocatalyst for hydrogen evolution reaction (HER) in alkaline solution. The Ni–S–Fe alloy is characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The electrocatalytic performance of Ni–S–Fe alloy for HER is studied in 30 wt% KOH solution. The results show that the Ni–S–Fe alloy exhibits much higher catalytic activity for HER relative to Ni–S alloy, as manifested by smaller overpotential of 222 mV at 10 mA cm^?2 and higher exchange current density of 1.60 × 10^?2 mA cm^?2. The Tafel slope of 84.5 mV·dec^?1 implies an underlying Volmer-Heyrovsky mechanism. The outstanding catalytic performance of the Ni–S–Fe alloy may originate from the synergistic effects of Ni and Fe, refined grain, and enlarged surface area of Ni–S–Fe alloy upon Fe doping. In addition, the Ni–S–Fe alloy has better anti-corrosion property than Ni–S alloy as a result of the poorer crystallinity of Ni–S–Fe alloy.     

Co-catalyst free direct Z–scheme photocatalytic system with simultaneous hydrogen evolution and degradation of organic pollutants

Photocatalytic hydrogen evolution coupled with simultaneous pollutant degradation is a promising but challenging strategy for both wastewater treatment and renewable energy production. In this work, a series of direct Z-scheme heterostructural catalysts, denoted as CeO₂/Cu–I-bpy, were designed and constructed by in-situ synthesizing CeO₂ nanoparticles on the surface of a coordinate polymer, named Cu–I-bpy. The simultaneous photocatalytic evolution of hydrogen and photo-degradation of organic pollutants was successfully achieved with this hybrid catalyst, even without any co-catalyst. The hybrid catalyst exhibited a 6.9 folds higher hydrogen evolution efficiency than the corresponding individual photo-reduction catalyst (Cu–I-bpy here), and 2.5 times degradation rate of the corresponding individual photo-oxidation catalyst (CeO₂ here), after optimizing the CeO₂ loading. After extensively characterized via XRD, XPS, Raman spectroscopy, UV–Vis absorption spectroscopy, SEM, TEM, BET, photo-electrochemical method and photoluminescence, it was found that the hybrid catalysts show higher charge separation and transfer efficiency than corresponding individual CeO₂ and Cu–I-bpy. Further mechanism investigation via active species trapping, EPR and electron flow experiments indicates a direct Z-scheme charge transfer mechanism in the CeO₂/Cu–I-bpy, which not only boosts the photogenerated electron-hole separation efficiency, but also retains the high reduction and oxidation ability the corresponding individual catalyst for hydrogen evolution and pollutant degradation, respectively. This work demonstrates the possibilities of turning waste water into energy resources using carefully designed photocatalytic systems.     

Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm^?2 OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm^?2 galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.     

Preparation of Ni–P–La alloy as a novel electrocatalysts for hydrogen evolution reaction

Ternary Ni–P–La alloy was synthesized by the co-electrodeposition method on the copper substrate. The energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used for characterization of the synthesized alloy. The electrochemical performance of the novel alloy was investigated based on electrochemical data obtained from steady-state polarization, Tafel curves, linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in alkaline solution and at ambient temperature. The results showed that the microstructural properties play a vital purpose in determining the electrocatalytic activity of the novel alloys. Also, the HER on investigated alloys was performed via the Volmer-Heyrovsky mechanism and Volmer step as RDS in this work. Ni–P–La catalyst was specified by ƞ₂₅₀ = −139.0 mV, b = −93.0 mV dec⁻¹, and j_o = −181.0 μA cm⁻². The results revealed that the Ni–P–La catalysts have a high potential for HER electrocatalysts in 1M NaOH solution.     

A porous 3d-4f heterometallic metal–organic framework for hydrogen storage

A heterometallic metal–organic framework, {[Ce(oda)₃Zn_1.5(H₂O)₃]·0.75H₂O}_n (1, H₂oda = oxydiacetic acid), has been synthesized under hydrothermal condition. The single-crystal X-ray diffraction analysis reveals that compound 1 belongs to hexagonal crystal system with space group P6/mcc and exhibits 3D porous framework. The hydrogen adsorption experiments suggest that 1 possesses reversible hydrogen storage capacity, up to 1.34 wt.% at 77 K and 0.86 wt.% at 298 K, respectively.     

Fluff spherical Co–Ni3S2/NF for enhanced hydrogen evolution

Ni₃S₂ is a kind of HER catalyst electrode with high efficiency and easy preparation. However, due to the weak electrochemical adsorption capacity of water molecules at the Ni site, it is not conducive to the dissociation of water molecules. At the same time, strong sulfur-hydrogen bond is easily formed at the S site, which greatly hinders the desorption and bonding of hydrogen atom to produce hydrogen. Hydrogen evolution performance of Ni₃S₂ in alkaline media needs to be improved. In this paper, fluff spherical Co–Ni₃S₂ was grown in situ on nickel foam by two-step hydrothermal method successfully. By doping cobalt ions, the strong interaction of S–H bond on Ni₃S₂ surface was weakened, the adsorption and dissociation of water molecules were promoted, and the catalyst was exposed to more reactive centers, so as to improve the hydrogen evolution performance of cathodic reduction reaction. Electrochemical test and Transient Photovoltage (TPV) tests show that Co–Ni₃S₂ has fast reaction kinetics and high electron transfer rate, especially it only needs 148 mV low overpotential to reach 10 mA cm^?2 in 1.0 M KOH alkaline electrolyte, which is better than Ni₃S₂/NF (250 mV). In addition, Co–Ni₃S₂ also has excellent electrochemical stability. Density functional theory (DFT) calculations confirm that the optimized adsorption energy enables the catalyst to exhibit excellent HER activity. This work provides useful guidance to construction of effective nickel related HER catalysts.     

Self-supported cobalt–molybdenum oxide nanosheet clusters as efficient electrocatalysts for hydrogen evolution reaction

In this paper, we report the three-dimensional self-supported CoMoO₄ nanosheet clusters on the nickel foam (denoted as CoMoO₄/NF) by a facile hydrothermal-calcination method for efficient hydrogen generation. As a result, the freestanding CoMoO₄ electrode exhibits an efficient electrochemical activity towards hydrogen evolution reaction, showing overpotentials as low as 68 and 178 mV at current densities of 10 and 100 mA cm⁻² in the alkaline condition (1 M KOH), respectively, a Tafel slope value of 82 mV per decade. Moreover, the electrode exhibits remarkable electrochemical durability for 1000 cycles. Significantly, the water splitting electrolyzer assembled with CoMoO₄/NF || NiFe LDH/NF (the nickel iron layered double hydroxide supported on the nickel foam) system achieved 20 mA cm⁻² at 1.63 V, showing the CoMoO₄/NF is promising for practical water splitting applications.     

Heterofullerene-linked metal–organic framework with lithium decoration for storing hydrogen and methane gases

Hydrogen and methane are considered as the promising fuels in the future, however, one of the obstacles to their utilizations is the lack of efficient storage and safe transportation materials. In this theoretical work, a novel kind of metal–organic framework (MOF) is designed using heterofullerene as linker from density functional theory calculations and first-principles molecular dynamics simulations. Based on grand canonical Monte Carlo simulations, we explore the adsorption performances of H₂ and CH₄ in the proposed porous MOF materials, which exhibit spectacular capacities for hydrogen storage as well as for methane storage after Li doping, both achieving the targets set by U.S. Department of Energy at workable conditions.     

Cu–MoS2/rGO hybrid material for enhanced hydrogen evolution reaction performance

Cu doped MoS₂ (Cu–MoS₂)/reduced graphene oxide (rGO) (Cu–MoS₂/rGO) hybrid material is fabricated by a facile one-step solvothermal method. The X-ray diffraction (XRD) results suggest that the doping of Cu does not alter the crystal structure of MoS₂. X-ray photoelectron spectroscopy (XPS) analysis reveal that the doping of Cu atoms influences the electronic structure of MoS₂, which is favorable to increase active sites of edges. Electrochemical impedance spectroscopy (EIS) results indicate that Cu–MoS₂/rGO performed a faster charge-transfer in comparison to MoS₂/rGO hybrid. In addition, the resultant Cu–MoS₂/rGO catalyst with Cu/Mo mole ratio of 9% exhibits a lower overpotential of 199 mV at 10 mA cm⁻², small Tafel slop of 44 mV dec⁻¹ and cycling stability, indicative of enhanced electrocatalytic activity towards HER. The improved performance is attributed to the increased active sites and a synergistic effect between copper and molybdenum, leading to electronic structure change and charge redistribution of MoS₂.