Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Characterization of LaNi4.70Al0.30 handled in air and application to a scheme of thermal compression of hydrogen

LaNi_4.70Al_0.30 was characterized by SEM, EDS and XRD. The structure was refined by the Rietveld method. The intermetallic stability temperature range in air was analyzed by DSC. The intermetallic is destabilized at T > 160° C. The intermetallic was annealed at this temperature for 24 h in air. After that, the pressure-composition-isotherms were measured. The thermodynamic properties were calculated from the Van’t Hoff diagrams. Values obtained were ΔH_f = 30 ± 2 kJ/mol and ΔS_f = 0.13 ± 0.01 kJ/mol for absorption process and ΔH_d = 31 ± 2 kJ/mol and ΔS_d = 0.14 ± 0.01 ± kJ/mol for desorption process. From these results, a scheme of thermal compression of hydrogen (TCH) was proposed. The scheme has a practical compression ratio (R_c) of 7.2 in the 25–100 °C temperature range and 1–1000 kPa pressure range.     

New fan blade-like core-shell Sb2TixSy photocatalytic nanorod for hydrogen production from methanol/water photolysis

New photocatalysts of Sb₂Ti_xS_y (x = 0, 0.5, 1.0, 1.5 mol and y = 3, 4, 5, 6 mol) fan blade-like core-shell nanorods have been designed ultimately to enhance hydrogen production. The nanorods of 500 nm long and 60–100 nm wide are Sb₂S₃ nanorod surrounded by an amorphous TiS₂ membrane, showing absorption band edges of above 600 nm. The evolution of H₂ from methanol/water (1:1) photo-splitting over Sb₂Ti_xS_y nanorods in the liquid system is doubled, compared to that over pure Sb₂S₃. Particularly, 52 μmol of H₂ gas is produced after 10 h when 0.5 g of Sb₂Ti_1.0S₅ nanorods is used at pH = 7, and the performance is increased by more than 50% at higher pH. Based on cyclic voltammetry (CV) and UV-Visible absorption spectra, the high photocatalytic activity can be attributed to the existence of an appropriate band-gap state, which includes the scope of the redox potential of water in Sb₂Ti_1.0S₅ nanorods, resulting in the promotion of the redox reaction of water.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Effect of Al content on structure and electrochemical properties of LaNi4.4 − xCo0.3Mn0.3Alx hydrogen storage alloys

The structure and electrochemical properties of LaNi_4.4 − xCo_0.3Mn_0.3Al_x hydrogen storage alloys have been investigated by XRD and simulated battery test, including maximum capacity, cyclic stability, self-discharge, high-rate dischargeability (HRD). Samples A, B, C and D were used to represent alloys LaNi_4.4Co_0.3Mn_0.3Al, LaNi_4.3Co_0.3Mn_0.3Al_0.1, LaNi_4.2Co_0.3Mn_0.3Al_0.2 and LaNi_4.1Co_0.3Mn_0.3Al_0.3 respectively. The results indicated that as-prepared LaNi_4.4 − xCo_0.3Mn_0.3Al_x alloys are all single-phase alloys with hexagonal CaCu₅ type structure. The maximum discharge capacity is 330.4 mAh g⁻¹ (Alloy C). With the increase of Al content from A to D, cycle life of alloy electrode has been improved. Higher capacity retention of 89.29% (after 50 charge/discharge cycles) has been observed for electrode D, while with a smaller capacity loss of 12.5% in its self-discharge test. Better high-rate charge/discharge behaviors have been observed in electrode B, and the maximum data is 54.7% at charge current of 900 mA/g) and 68.54% at discharge current of 1800 mA/g). Furthermore, the electrochemical impedance spectroscopy (EIS) analysis shown that the reaction of alloy electrode is controlled by charge-transfer step. The addition of Al results in the formation of protective layer of aluminum oxides on the surface of the alloy electrode, which is good for the improvement of electrode properties in alkaline solution and is detrimental for the charge-transfer process. Therefore, a suitable addition of Al is needed to improve its electrode properties.     

Photocatalytic hydrogen production over CuGa2−xFexO4 spinel

Recovery of hydrogen from industrial H₂S waste using spinel photocatalyst was studied. Spinel metal oxide photocatalysts (CuGa_2−xFe_xO₄ for x = 0.8, 0.6 and 0.4) were synthesized by ceramic route. They were loaded with 0.5 and 1 wt% noble metal oxide, RuO_2. Their XRD pattern revealed a single phase cubic spinel crystalline structure for all the catalysts. SEM displayed small size cubic particles with the particle size decreasing with the decrease in iron content. 1 wt% RuO₂ loaded CuGa_1.6Fe_0.4O₄ decomposed H₂S in aqueous 0.5 M KOH solution under visible light (λ ≥ 420 nm) irradiation and generated H₂ to the tune of 10,045 μmol/h, giving rise to a high quantum efficiency of 21% at 510 nm.     

Investigations on the effect of Cu/Cu redox couples and oxygen vacancies on photocatalytic activity of treated LaNi1−xCuxO3 (x=0.1, 0.4, 0.5)

Perovskite-like oxides LaNi_1−xCu_xO₃ (x = 0.1, 0.4, 0.5) were prepared by means of the citric acid complexing method. TPR revealed the incorporation of Cu into the perovskite lattice increased the reducibility of the catalyst. After LaNi_1−xCu_xO₃ were pretreated in H₂ for 2 h at certain low temperature, the material still retained its perovskite structure and oxygen vacancies were generated in the lattice. DRS showed that narrowing of band-gap of reduced LaNi_1−xCu_xO₃ was governed by the crystalline structure and the defect in the catalyst. In the photocatalytic water splitting experiment, 200 and 250°C-reduced LaNi_0.6Cu_0.4O₃, 200°C-reduced LaNi_0.5Cu_0.5O₃ possessed the high and colse catalytic activity. XPS showed that the molar ratio of Cu²⁺/Cu¹≈1 and lattice oxygen/adsorb oxygen ≈ 0.2 in the catalysts had high catalytic activity. According to the outcome of our experiments, we conclude that there is a balance relation either between oxygen vacancies and catalytic activity or between Cu²⁺/Cu¹⁺ redox couples and catalytic performance of these materials for hydrogen production from photocatalytic water splitting. Enhancement of hydrogen yield can be attributed to the small band-gap and the lowering the recombination probability for electron-hole pairs.     

Novel photocatalysts based on Cd1−xZnxS/Zn(OH)2 for the hydrogen evolution from water solutions of ethanol

Multiphase photocatalysts Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂, Pt/Cd_1−xZn_xS/ZnO, and Pt/Cd_1−xZn_xS/Zn(OH)₂ were synthesized by a new two-step technique. The photocatalysts were characterized by a wide range of experimental techniques: X-ray diffraction, high-resolution transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, low-temperature N₂ adsorption/desorption, and UV/VIS spectroscopy. The photocatalytic activity was tested in a batch reactor in the reaction of H₂ evolution from aqueous solutions of ethanol under visible light irradiation (λ > 420 nm). The highest achieved photocatalytic activity was 2256 μmol H₂ per gram of photocatalyst per hour; the highest quantum efficiency was 10.4%. The activity of Pt/Cd_1−xZn_xS/Zn(OH)₂ was higher than that of Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂ and Pt/Cd_1−xZn_xS/ZnO. The explanation of enhanced activity of zinc–cadmium sulfide/ε-zinc hydroxide based on quantum calculations was suggested.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Hydrogen desorption performance of high-energy ball milled Mg2NiH4 catalyzed by multi-walled carbon nanotubes coupling with TiF3

The Mg₂NiH₄ complex hydrides were synthesized by high-energy ball milling (HEBM) MgH₂ + Ni mixtures. Multi-walled carbon nanotubes (MWCNTs) or TiF₃ as catalysts were added and the catalytic-dehydrogenation behaviors were investigated. All prepared samples are characterized by X-ray diffraction (XRD) spectroscopy, scanning electron microscope (SEM) and differential scanning calorimetry (DSC) to acquire information of microstructure, phase compositions, surface and dehydrogenation properties. The results indicate that the method of adding catalysts by HEBM is reasonable and the hydrogen desorption property of Mg₂NiH₄ is improved by catalysts. It is worth noting that the dehydrogenation temperature (T_D) and the activation energy (E_a) of Mg₂NiH₄ catalyzed by MWCNTs coupling with TiF₃ are reduced to 230 °C (243.6 °C of Mg₂NiH₄) and 53.24 kJ/mol (90.13 kJ/mol of Mg₂NiH₄), respectively. The addition of proper catalysts is proved to be an effective strategy to decrease T_D and E_a of Mg₂NiH₄ hydrides.     

Photocatalytic H2 evolution from NaCl saltwater over ZnS1−x−0.5yOx(OH)y-ZnO under visible light irradiation

Photocatalytic hydrogen production was investigated over ZnS_1−x−0.5yO_x(OH)_y-ZnO using sulfide ion (Na₂S-Na₂SO₃) as an electron donor from NaCl saltwater. NaCl can affect markedly the activity for photocatalytic hydrogen production, depending on NaCl concentration. When NaCl concentration is lower, the activity is lower than that in pure water, whereas when NaCl concentration is higher, the activity is higher than that in pure water. NaCl decreases not only the surface charge of ZnS_1−x−0.5yO_x(OH)_y-ZnO but also the surface hydration. When ZnS_1−x−0.5yO_x(OH)_y-ZnO was impregnated with the electron donor (Na₂S-Na₂SO₃), ZnO was transformed partly into ZnS. The impregnated ZnS_1−x−0.5yO_x(OH)_y-ZnO exhibits higher activity than the non-impregnated one. The possible mechanisms were discussed.     

Effect of Ce on the structural features and catalytic properties of La(0.9−x)CexFeO3 perovskite-like catalysts for the high temperature water-gas shift reaction

La_(0.9−x)Ce_xFeO₃ perovskite-like catalysts were investigated for the production of hydrogen from simulated coal-derived syngas via the water-gas shift reaction in the temperature range 450-600 °C and at 1 atm. These catalysts exhibited higher activity at high temperatures (T ≥ 550 °C), compared to that of a commercial high temperature iron-chromium catalyst at 450 °C. Addition of a low Ce content (x = 0.2), has little influence on the formation of the LaFeO₃ perovskite structure, but enhances catalytic activity especially at high temperatures with 19.17% CO conversion at 550 °C and 40.37% CO conversion at 600 °C. The LaFeO₃ perovskite structure and CeO₂ redox properties play an important role in enhancing the water-gas shift activity. Addition of a high Ce content (x = 0.6) inhibits the formation of the LaFeO₃ perovskite structure and decreases catalyst activity.     

Substitution of nickel in Mg2Ni and its hydride with elements from groups XIII and XIV: An ab initio study

We have performed ab initio calculations with equilibrium supercells of the Mg₂Ni compound and its hydride Mg₂NiH₄ doped with elements X = Al, Ga, In, Si, Ge and Sn. Two concentrations of X in both structures have been set: (1) every 16th, and (2) every fourth Ni atom has been substituted by X. Total energy calculations yielded the Mg₂NiH₄ hydrogen absorption enthalpy ΔH_abs according to the chemical reaction Mg₂Ni + 2H₂ → Mg₂NiH₄. Reduction of the hydrogen absorption enthalpy was reported for both concentrations of X. When doping the Mg₂NiH₄ hydride with X = In in a low concentration (1), the value of hydrogen desorption enthalpy decreases from 68.22 to 55.96 kJ(mol H₂)^?1. Doping with X = In in a high concentration (2) further decreases the hydrogen desorption enthalpy to 5.50 kJ(mol H₂)^?1. Further, the electronic structure of Mg₂(Ni–In)H₄ hydride with a low In concentration indicates weaker Ni–H bonds in comparison with the pristine Mg₂NiH₄. Attraction between H and In atoms induced enhanced bonding between Mg and H atoms compared to the pristine Mg₂NiH₄.     

Mg2NiH4 synthesis and decomposition reactions

A ternary Mg₂NiH₄ hydride was synthesized using method that relies on a relatively short mechanical milling time (one hour) of a 2:1 MgH₂–Ni powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and temperature (>400 °C). The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% as a mixture of two polymorphic forms. The mechanisms of formation and decomposition of ternary Mg₂NiH₄ under different hydrogen pressures were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high pressure DSC. The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high resolution STEM. Additionally, effects occurring during the desorption reaction were studied using DSC coupled with mass spectrometry.     

Microstructure and electrochemical hydrogen storage characteristics of La0.67Mg0.33−xCaxNi2.75Co0.25 (x = 0-0.15) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.67Mg_0.33−xCa_xNi_2.75Co_0.25 (x = 0, 0.05, 0.10 and 0.15) alloys are investigated. The results show that all alloys mainly consist of (La, Mg)Ni₃ and LaNi₅ phases, besides a small amount of (La, Mg)₂Ni₇ phase. The cycle stability (S₈₀) after 80 charge/discharge cycles of all alloy electrodes first increases from 60.1% (x = 0) to 64.2% (x = 0.05), then decreases to 45.9% (x = 0.15). The high rate dischargeability of all alloy electrodes first increases from 52.6% (x = 0) to 61.4% (x = 0.10), then decreases to 57.2% (x = 0.15). Moreover, the charge-transfer resistance (R_ct) first decreases from 168.2 mΩ (x = 0) to 125.7 mΩ (x = 0.10), then increases to 136.6 mΩ (x = 0.15). All the results indicate that the substitution of Mg with a certain amount of Ca can improve the overall electrochemical characteristics.     

Cd1−xZnxS supported on SBA-16 as photocatalysts for water splitting under visible light: Influence of Zn concentration

Cd_1−xZn_xS solid solutions (x = 0.05–0.3) supported on mesoporous silica SBA-16 substrate with 3D cubic structure were investigated for hydrogen production from water splitting under visible light. The influence of Zn concentration (x) in the Cd_1−xZn_xS solid solution and support morphology were investigated. The bare SBA-16 substrate was synthetized by the hydrothermal method whereas the Cd_1−xZn_xS photocatalysts were prepared by coprecipitation of metal sulfides from aqueous solutions of Cd²⁺ and Zn²⁺ using Na₂S as precipitating agent. An attempt has been made to determine the photocatalyst structures using several techniques including elemental analysis, N₂ adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS) and Raman spectroscopy. Surface characterization of the samples by XPS indicates that Cd_1−xZn_xS nanoparticles are unevenly distributed on both external surface and within the pore network. An increase of the band gap energy with increasing Zn loading up to x = 0.2 in the Cd_1−xZn_xS solid solution was observed. As a consequence, H₂ evolution increases gradually with an increase of the Zn loading in the photocatalysts from 0.05 to 0.2 wt% being the Cd_0.8Zn_0.2S/SBA-16 system the most active among the catalysts studied. The highest activity of this photocatalyst was explained in terms not only of its large band gap energy but also by the enhancement of the interaction between the particles of solid solution and the SBA-16 substrate.