Synthesis and hydrogen adsorption properties of a new phthalocyanine-based metal–organic framework

The first member of a new class of phthalocyanine-based metal–organic frameworks, MOF-IP, has been synthesized in order to gauge the potential of this type of materials for hydrogen storage. Preliminary studies indicate that MOF-IP is a robust material with a hydrogen uptake capacity of 1.15 wt% at 77 K under atmospheric pressure.     

Synthesis and hydrogen-storage behavior of metal–organic framework MOF-5

Metal–organic framework MOF-5 (Zn₄O(BDC)₃), a microporous material with a high surface area and large pore volume, was synthesized by three approaches: direct mixing of triethylamine (TEA), slow diffusion of TEA, and solvothermal synthesis. The obtained materials were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption, and their hydrogen-storage capacities were measured. The different synthesis methods influenced the pore-structure parameters, morphologies and hydrogen-storage behavior of the obtained MOF-5. MOF-5 synthesized by the solvothermal approach showed a higher surface area and larger pore volume than the samples prepared by the other two approaches. Measurements of the hydrogen-storage behavior showed that the hydrogen-storage capacity was correlated with the specific surface area and pore volume of MOF-5.     

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.     

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.     

Cobalt metal–organic framework derived cobalt–nitrogen–carbon material for overall water splitting and supercapacitor

Metal-organic frameworks (MOFs) have been the subject of intensive structural tuning via methods like pyrolysis for superior performance in electrocatalytic oxygen and hydrogen evolution processes (OER and HER) and supercapacitors. Here, a Co-MOF based on 2-methylimidazole was synthesized using a precipitation approach, and its electrochemical characteristics were tuned via pyrolysis at different temperatures, including 600, 700, and 800 °C. Characterization findings corroborated the formation of Co–N–C moieties from Co-MOF, and XPS analyses indicated that 700 °C was the optimal temperature for achieving a high density of Co–N–C moieties. The optimized Co-MOF-700 sample displayed remarkable HER and OER performance in terms of lower overpotentials of 75 mV and 370 mV as well as small Tafel slopes of 118 mV/dec and 79 mV/dec, respectively. Furthermore, at a current density of 1 A/g, the Co-MOF-700 sample had a specific capacitance of 210 F/g. The enhanced electrochemical properties of Co-MOF-700C as compared to other samples can be attributed to the availability of a high density of Co–N–C sites for catalytic reaction and its porous architecture. This study will expand the knowledge of how compositional and morphological changes in MOFs affect their utility in energy conversion and storage applications.     

Synthesis and characterization of g/Ni–SiO2 composite for enhanced hydrogen storage applications

Hydrogen is considered as one of the most important clean energy carriers for the future. Many experimental and theoretical investigations have focused on the adsorption and activation of H₂ on the metal surfaces. Metal oxides and semiconductors are suitable materials for this purpose. Gelatin assisted Ni loaded SiO₂ (g/Ni–SiO₂) was prepared and its structural properties, morphology, composition and surface properties were analyzed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM), Elemental mapping and energy dispersive spectrum (EDS), High resolution transmission electron microscopy (HR-TEM) and Brunauer-Emmett-Teller (BET) surface area measurements. The prepared material was effectively utilized for H₂ storage applications at room temperature. The H₂ storage capacity of g/Ni–SiO₂ was twice that of pristine SiO₂. This may be due to large change in pore volume and pore diameter of g/Ni–SiO₂, which may enhances the H₂ storage capacity of the sample. The H₂ storage capacity of other materials such as ZnO, anatase TiO₂, g/TiO₂, g/ZnO, g/TiO₂–SO₄^2-, Sb doped TiO₂, Ag₂S/TiO₂, Sb₂O₃, CdS and SiO₂/CdS also studied and compared with g-Ni/SiO₂.     

Synthesis and characterization of macroporous Ni,Co and Ni–Co electrocatalytic deposits for hydrogen evolution reaction in alkaline media

In this work, macroporous Ni, Co and Ni–Co electrodes have been developed by co-deposition at high current density on stainless steel (AISI 304) substrates. The obtained materials were characterized both morphologically and chemically by confocal laser scanning microscopy, and SEM coupled with EDX analysis. The activity for hydrogen evolution reaction (HER) on the obtained layers was assessed by using pseudo-steady-state polarization curves and electrochemical impedance spectroscopy (EIS) in alkaline solution (30 wt.% KOH). The electrochemical results show that HER on these electrodes takes place by the Volmer–Heyrovsky mechanism. The synthesized coatings present higher catalytic activity for HER than commercial smooth Ni electrode. As the Co content increases in the electrodeposition bath the obtained structures show lower surface roughness factors. Ni–Co deposit with a Co content of 43 at.% manifests the highest intrinsic activity for HER as a consequence of the synergetic combination of Ni and Co.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.     

Investigation on hydrogen storage properties of as-cast,extruded and swaged Mg–Y–Zn alloys

In this work, three different states of Mg-9.1Y-1.8Zn alloys including as-cast, extruded and swaged were prepared by semi-continuous casting, extrusion and swaging processes, respectively. Their compositions, microstructures and hydrogen storage properties were investigated. The results show that Mg-9.1Y-1.8Zn alloys in three different states are all composed of Mg and long-period stacking ordered (LPSO) phases. The LPSO phases occurs to break and decompose after hydrogenation and in-situ forms the YH_{χ(χ = 2,3)} nano-hydrides. The nano-hydrides can be used as in-situ catalysts to improve the hydrogen storage properties of alloys. Meanwhile, many nanocrystalline grains appear in the core of alloy after swaging, and the average grain size ranges from 80 to 200 nm. The presence of nanocrystals may increase the specific surface area of alloy, facilitating the diffusion and absorption of hydrogen. Comparatively, the swaged alloy exhibits the largest hydrogen storage capacity and excellent hydrogen sorption kinetics relative to other states of alloys.     

Synthesis and electrochemical properties of layered Li–Ni–Mn–O compounds

An attempt to prepare solid solutions in the system of LiNiO₂, LiMnO₂ and Li₂MnO₃ was performed by heating metal acetates. The solid solutions between end members LiNiO₂ and Li₂MnO₃ can be successfully prepared in the overall compositional ranges. Both the structure and capacity were compared based on Rietveld analysis and electrochemical investigation on solid solutions between LiNiO₂ and Li₂MnO₃. The result showed that the cationic disorder as well as capacity was closely related to the ratio of Li, Mn and Ni in formula. The investigation of chronopotentiogram and ex situ XRD on the solid solutions indicated that the complex phase transitions in LiNiO₂ during delithiation were strongly suppressed with low Mn content (Mn/(Mn+Ni) ratio was 0.1 or 0.2) and completely suppressed with the ratio more than 0.5.     

Study of two-dimensional and dynamic heat and mass transfer in a metal–hydrogen reactor

To analyse heat and mass transfer in a metal–hydrogen reactor, the hypothesis that disregards the radiative heat transfer in the reactor, is typically used. In this paper, we take into account the radiative heat transfer and we test the validity of this hypothesis in the case of the LaNi₅ and in the case of the magnesium. A theoretical model is conducted for the two-dimensional system where conduction, convection radiation and chemical reaction take place simultaneously. This model is solved by the finite volume method. The numerical simulation is used to present the time–space evolutions of the temperature and the hydride density in the reactor and to determinate the sensitivity to some parameters (absorption coefficient, scattering coefficient, reactor wall emissivity).     

Stacked nano rods of cobalt and nickel based metal organic frame work of 2–amino benzene dicarboxylic acid for photocatalytic hydrogen generation

Stacked nanorods of cobalt and nickel based hetero bimetallic organic frameworks (MOFs) of 2–amino benzene dicarboxylic acid are developed as photocatalyst for hydrogen evolution reaction. The ratio of metals in the catalyst is tuned to achieve a narrow band gap, and the MOF with optimized Ni to Co ratio of 1: 0.5 (1Ni0.5Co@NH₂BDC) exhibited the lowest band gap (2.2 eV) and electron–hole recombination rate. The catalyst exhibits enhanced photocatalytic activity for hydrogen evolution reaction due to the absorption of photons by 2–amino benzene dicarboxylic acid and excitation of electrons from HOMO to LUMO of the organic linker. The excited electrons relay to the cluster of the framework and reduce the protons gather around the cluster to hydrogen. The holes in the HOMO state are occupied by the electrons from the sacrificial agents and it supports the photocatalytic hydrogen evolution by avoiding electron–hole recombination. The stability of MOF catalyst in water splitting medium even with sacrificial agents confirms its competency with the state–of–the–art photocatalytic materials.     

Dynamic behavior of metal–hydrogen reactor during hydriding process

The present study involves numerical simulation of transient transport of hydrogen and heat within a metal–hydrogen reactor connected to a hydrogen tank during the hydriding process. This problem is of particular interest in the design of many installations in the field of energy technology (compressor, heat pumps, thermal or hydrogen storage systems). The reactor presents an expansion volume for hydrogen. The hydrogen flow is described by general momentum conservation equations instead of Darcy's law. The evolutions of the temperature, of the hydrogen concentration and of the hydrogen flow velocity are presented. The effects of the reactor dimensions, the inlet diameter, the volume of the expansion part, the tank volume, the initial pressure and the amount of hydrogen in the tank, on the heat and hydrogen transfer are determined.     

Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys

The non-stoichiometric C15 Laves phase alloys namely Zr_0.9Ti_0.1V_x (x = 1.7, 1.8, 1.9, 2.1, 2.2, 2.3) are designed and expected to investigate the role of defect and microstructure on hydrogenation kinetics of AB₂ type Zr-based alloys. The alloys are prepared by non-consumable arc melting in argon atmosphere and annealed at 1273 K for 168 h to ensure the homogeneity. The microstructure and phase constitute of these alloys are examined by SEM, TEM and XRD. The results indicate the homogenizing can reduce the minor phases α-Zr and abundant V solid solution originating from the non-equilibrium solidification of as-cast alloys. Twin defects with {111}<011 > orientation relationship are observed, and the role of defects on hydrogenation kinetics is discussed. Hydrogen absorption PCT characteristics and hydrogenation kinetics of Zr_0.9Ti_0.1V_x at 673–823 K are investigated by the pressure reduction method using a Sievert apparatus. The results show the hypo-stoichiometric alloys preserve faster hydrogenation kinetics than the hyper-stoichiometric ones due to the decrease of dendritic V. The excess content of Zr₃V₃O phase decreases the hydrogenation kinetics and the stability of hydrides. In addition, the different rate controlled mechanisms during hydrogen absorption are analyzed. The effects of non-stoichiometry on the crystal structure and hydrogen storage properties of Zr_0.9Ti_0.1V_x Laves alloys are discussed.     

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.     

Prediction of transient heat and mass transfer in a closed metal–hydrogen reactor

The metal–hydrogen reactor is usually composed of a porous medium (hydride bed) and an expansion volume (gaseous phase). During the sorption process, the hydrogen flow and the heat transfer in the expansion part are badly known and can have some effects on the sorption phenomena in the hydride medium. At our knowledge, the hypothesis that neglects those effects is typically used. In this paper, a 2D study of heat and mass transfer has been carried out to investigate the transient transport processes of hydrogen in the two domains of a closed cylindrical reactor. A theoretical model is conducted and solved numerically by the control-volume-based finite element method (CVFEM). The result on temperature and hydride density distribution are presented and discussed. Moreover, this paper discusses in detail the effects of some governing operating conditions, such as dimensions of the expansion volume, height to the radius reactor ratio, and the initial hydrogen to metal atomic ratio, on the evolution of the pressure, fluid flow, temperature and the hydrogen mass desorbed.     

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.     

Synthesis and electrochemical properties of binary MgTi and ternary MgTiX (X = Ni,Si) hydrogen storage alloys

Mg-based hydrogen storage alloys are promising candidates for many hydrogen storage applications because of the high gravimetric hydrogen storage capacity and favourable (de)hydrogenation kinetics. In the present study we have investigated the synthesis and electrochemical hydrogen storage properties of metastable binary Mg_yTi_1?y (y = 0.80–0.60) and ternary Mg_0.63Ti_0.27X_0.10 (X = Ni and Si) alloys. The preparation of crystalline, single-phase, materials has been accomplished by means of mechanical alloying under controlled atmospheric conditions. Electrodes made of ball-milled Mg_0.80Ti_0.20 powders show a reduced hydrogen storage capacity in comparison to thin films with the same composition. Interestingly, for a Ti content lower than 30 at.% the reversible storage capacity increases with increasing Ti content to reach a maximum at Mg_0.70Ti_0.30. The charge transfer coefficients (α) and the rate constants (K₁ and K₂) of the electrochemical (de)hydrogenation reaction have been obtained, using a theoretical model relating the equilibrium hydrogen pressure, electrochemically determined by Galvanostatic Intermittent Titration Technique (GITT), and the exchange current. The simulation results reveal improved values for Mg_0.65Ti_0.35 compared to those of Mg_0.80Ti_0.20. The addition of Ni even more positively affects the hydrogenation kinetics as is evident from the increase in exchange current and, consequently, the significant overpotential decrease.