Peculiarities of the absorption and desorption of hydrogen by opal matrices

The absorption and desorption of hydrogen at high pressure and temperature by opal matrices formed by amorphous silica spheres with a diameter of 0.235 and 1.6 μm have been studied. The Raman spectra of hydrogen-saturated opal matrices measured at a temperature of 80 K and ambient pressure show that hydrogen molecules are adsorbed in two different ways, directly into silica spheres and mesopores between them. The kinetics of hydrogen desorption was studied in-situ from a change in the relative intensity of rotational modes in the Raman spectra under annealing at 163–213 K. The hydrogen content decreases exponentially under isothermal heating, while the exponential decay time constant τ increases with a decreasing temperature showing the activation nature of desorption. The data for various temperatures are well described by the Arrhenius dependence τ(Т) = A × exp(E_A/k_BT) with the activation energy E_A=(162 ± 13) meV and time constant А= (1.8 ± 0.5) × 10⁻² s.     

Hydrogenation of Pd/Mg films: A quantitative assessment of transport coefficients

Due to increasing energy demands, interest in hydrogen storage is substantial. Magnesium is one of the most attractive systems, yet, development for practical application remains challenging. By combination of X-ray diffraction, electron microscopy and in-situ measurements of resistivity we determine the diffusion coefficient of hydrogen in MgH₂ at technically relevant pressure (20 bar). Pd coated thin films of well-defined thickness enable a quantitative evaluation of the hydrogenation rate. From this, we detect linear to parabolic kinetic transition and obtain the diffusion coefficient of hydrogen in MgH₂. Measurements at different temperatures (RT-300 °C) demonstrate an Arrhenius behaviour with an activation energy E_a = 28.1 kJ mol⁻¹. This low value and the transformation into a nanocrystalline microstructure upon hydrogenation indicate grain boundary diffusion as the essential mechanism. In completion, the interface Pd/Mg is studied. Mg₅Pd₂ and Mg₆Pd form at elevated temperatures required for dehydrogenation. These phases affect, but do not prevent, further hydrogen loading.     

Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage: (I) absorption

The Lithium–Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB₂/2 LiBH₄ + MgH₂) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy ΔH and entropy ΔS are determined to amount to −34 ± 2 kJ∙mol H₂⁻¹ and −70 ± 3 J∙K⁻¹∙mol H₂⁻¹, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 °C and 412 °C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 ± 3 kJ∙mol H₂⁻¹. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations.     

Reversible dehydrogenation of Mg(BH4)2–LiH composite under moderate conditions

The Mg(BH₄)₂-xLiH (0.1 ≤ x ≤ 0.8) composites which exhibit favorable dehydrogenation and encouraging reversibility are experimentally investigated. LiH additive reduces the onset temperature for dehydrogenation to 150 °C. And hydrogen release exceeds 10 wt.% from the new binary material below 250 °C. Furthermore, rehydrogenation results show that 3.6 wt.% hydrogen can still be recharged after twenty cycles at 180 °C. It should be emphasized that the long-term reversibility of borohydride under 200 °C is long overdue. TPD, PCT, and high-pressure DSC measurements are used to characterize the improvements in thermodynamic and kinetic ways. In addition, FT-IR and NMR studies indicate that the composite has a significant synergistic effect during (de)hydrogenation processes. This work suggests that controlled cation stoichiometry combined with doping by metal Li⁺ subvalent to Mg²⁺ facilitate the formation of polyborane intermediates [B₃H₈]⁻ and [B₂H₆]²⁻. They improve the dehydrogenation properties and make the material reversible under mild conditions.     

Fundamentals of hydrogen storage processes over Ru/SiO2 and Ru/Vulcan

Hydrogen adsorption and desorption over Ru/SiO₂ and Ru/Vulcan are investigated in terms of hydrogen storage and release characteristics by both dynamic and static experiments. Ru particle dispersions as a function of metal loading were determined by HR-TEM and volumetric chemisorption experiments. Vulcan was more accommodating for spillover hydrogen than SiO₂. High Ru dispersions, i.e., small particle sizes, favored the amount of hydrogen spillover to Vulcan, as revealed by temperature programmed desorption (TPD) of hydrogen. TPD of hydrogen under He flow experiments over Ru/SiO₂ and Ru/Vulcan materials revealed a low temperature process (up to 200 °C) attributed to desorption of weakly bound hydrogen from Ru metal surface. A high temperature process (above 450 °C) was attributed to diffusion of hydrogen from the support to the Ru particle and desorption at the Ru sites. Hydrogen adsorbs strongly on Ru metal, as indicated by the initial heats of H₂ adsorption measured as 100 kJ/mol over 1 wt% Ru/Vulcan by adsorption calorimetry. At higher coverages, heat of adsorption of hydrogen was measured as 10 kJ/mol. Low heat of adsorption of hydrogen at high coverages indicate multilayer weak adsorption of hydrogen over the storage material, which can desorb at lower temperatures.     

Influence of microstructure on hydrogen trapping and diffusion in a pre-deformed TRIP steel

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (D_{app, φ = 0} = 3.6 × 10^?12 cm² s^?1, D_{app, φ = 0.32} = 1.6 × 10^?11 cm² s^?1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (D_{app, φ = 0.39} = 5.3 × 10^?11 cm² s^?1, D_{app, φ = 0.49} = 1.1 × 10^?10 cm² s^?1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts

In this study, magnetic Fe₃O₄ modified Ru/Carbon nanotubes (CNTs) catalysts were used to achieve the catalytic transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA), with alcohols as the solvent and hydrogen donors. According to the result of the catalyst characterization, Fe₃O₄ promoted the formation of Ru⁰ species. The effects of Fe₃O₄ loading and different hydrogen donors on the catalytic transfer hydrogenation of FF were tested, and the reaction parameters and catalyst stability were also analyzed. It is found that Fe₃O₄ effectively enhanced the activity of Ru/CNTs in catalytic transfer hydrogenation of FF, the catalytic activity was optimized at the Fe₃O₄ loading of 5 wt%, and the optimal hydrogen donor was i-propanol. Moreover, the Ru–Fe₃O₄/CNTs could be easily collected for further use and possessed excellent stability. The mechanism of the catalytic transfer hydrogenation of FF using Ru–Fe₃O₄/CNTs was discussed, and the corresponding catalyst activity groups included metal Ru sites and RuO_x-Fe₃O₄ Lewis acid sites, which account for the excellent catalytic activity of transfer hydrogenation.     

Poly(N-vinyl-2-pyrrolidone)-stabilized ruthenium supported on bamboo leaf-derived porous carbon for NH3BH3 hydrolysis

In this work, poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized ruthenium nanoparticles (NPs) supported on bamboo leaf-derived porous carbon (Ru/BC) has been synthesized via a one-step procedure. The structure and morphology of the as-synthesized samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM) and transmission electron microscope (TEM). As a catalyst for hydrogen generation from the hydrolysis of ammonia-borane (AB, NH₃BH₃) at room temperature, Ru/BC stabilized with 1 mg of PVP exhibited high activity (TOF = 718 mol_H2·mol_Ru⁻¹·min⁻¹) and low activation energy (E_a = 22.8 kJ mol⁻¹). In addition, the catalyst could be easily recovered and showed fairly good recyclability with 55.6% of the initial catalytic activity retained after ten experimental cycles, which confirmed that PVP could stabilize the Ru NPs and prevent their agglomeration on BC surface. Our results suggest that PVP-stabilized Ru/BC is a highly efficient catalyst for the hydrolysis of AB.     

Synthesis and characterization of bifunctional Ru doped La-based perovskites for magnetic refrigeration and energy storage systems

In this study, the effect of Ru additive on the crystallographic, magnetic, and electrocatalytic properties of LaBiBaMnO₃ manganite was investigated for magnetic refrigeration and energy storage systems. XRD analysis of La_0·62Bi_0·05Ba_0·33MnO₃ (LBBM) and La_0·62Bi_0·05Ba_0·33Mn_0·90Ru_0·10O₃ (LBBMR) manganites prepared by solid state method reveals that the samples have a rhombohedral structure and the average grain size of perovskites are found to be below 3 μm. From the temperature-dependent magnetization measurements, it was observed that the value of Curie temperature (T_C) decreased from 334 K for LBBM to 327 K for LBBMR with the substitution of Mn with 10% Ru. In parallel with the decrease observed in T_C with Ru contribution, a decrease in magnetic entropy value was also observed. The substitution of 10% Ru reduced the onset potential 51 mV for oxygen evolution reaction (OER) and 74 mV for oxygen reduction reaction (ORR). The electrode activity of OER at 10 mA cm^?2 was obtained at around 0.976 V. Ru substitution and increasing scan rate to 100 mV s^?1 increased the current density and lowered onset potential for OER. The ORR efficiency of LBBM catalysts boosted up to 60% with the durability test for 6 h. The increased cathodic Tafel slope from ?255.1 mV dec^?1 to ?120.4 mV dec^?1 with 10% Ru substitution suggests that a fast charge transfer for ORR activity due to Ru-occupied Mn-sites in the structure. LBBMR sample achieved bifunctional properties as ORR/OER performance and magnetic cooling applications by substituting Mn with 10% Ru.     

Superior hydrogen generation from sodium borohydride hydrolysis catalyzed by the bimetallic Co–Ru/C nanocomposite

Sodium borohydride has been widely regarded as a promising hydrogen carrier owing to its greatly hydrogen storing capability (10.8 wt%), high weight density and excellent stability in alkaline solutions. Herein, we first design and synthesize a series of bimetallic M-Ru/C nanocomposites (including Fe–Ru/C, Co–Ru/C, Ni–Ru/C and Cu–Ru/C), via simply alloying of commercial Ru/C with nonprecious metal, for superior H₂ evolution from the NaBH₄ hydrolysis. The result exhibits that H₂ generation is synergetically improved by alloying Ru/C with Co or Ni, while it is hindered by alloying Ru/C with Fe or Cu. Indeed, Co–Ru/C presents the highest efficient catalytic activity for H₂ generation, with the TOF of 117.69 mol(H₂)·mol_Ru^?1·min^?1, whereas Ru/C is only 57.08 mol(H₂)·mol_Ru^?1·min^?1. In addition, the TOF of Co–Ru/C reaches to 436.51 mol(H₂)·mol_Ru^?1·min^?1 (96.7 L(H₂)·g_Ru^?1·min^?1) in the presence of NaOH.     

MgH2–TiF4-MWCNTs based hydrogen storage tank with central tube heat exchanger

Improvement of hydrogen sorption kinetics of MgH₂–TiF₄-MWCNTs based tank by addition of central tube heat exchanger and enhancement of hydrogen diffusion is proposed. After doping with TiF₄ and MWCNTs, dehydrogenation temperature of MgH₂ decreases significantly (ΔT = up to 90 °C). Superior hydrogen permeability, favoring hydrogen sorption kinetics is detected at hydrogen supply side to the middle of the tank, while effective heat transfer during exothermic hydrogenation is assured by the temperature increment of heat exchanger fluid (compressed air at room temperature). Hydrogen desorption and absorption can be completed within 120–150 and 25 min, respectively, up to twice as fast as the tank without heat exchanger from the previous studies. Due to fast hydrogenation rate resulting in short reaction time at high equilibrium temperature (up to 390 °C), particle agglomeration and/or sintering of MgH₂ upon cycling are prevented. Enhanced de/rehydrogenation rates and suppression of MgH₂ particle growth during cycling yield to considerable reversibility upon 20 de/rehydrogenation cycles with storage capacity up to 5.60 wt % H₂ (82% theoretical value). By increasing operating temperature to 330–335 °C, hydrogen released with constant flow rate of 0.30 standard L/min is prolonged up to three times, favoring electrical power production of PEMFC stack. Electrical performances obtained from PEMFC stack (13 single cells) supplied with hydrogen gas from MgH₂-based tank are also investigated.     

The influence of LiH and TiH2 on hydrogen storage in MgB2 I: Promotion of bulk hydrogenation at reduced temperature

Mg(BH₄)₂ is an attractive hydrogen storage material, owing to its high gravimetric capacity of 14.9 wt %. However, the dehydrogenated material MgB₂ is very difficult to rehydrogenate, requiring excessive pressures and temperatures. Here we report the influence of LiH and TiH₂ on hydrogen storage reactions involving Bulk MgB₂ using XRD, XAS, FTIR and NMR. In ball-milled mixtures of LiH/MgB₂, the LiH loses crystallinity but remains undissociated, forming a weakly bound complex with MgB₂. The weak interactions produce minor variations in the local electronic structure at B and Mg, but do not markedly affect the underlying MgB₂ hexagonal crystal structure. No evidence is found for a mixed-metal boride Mg_1-xLi_xB₂ in the as-prepared LiH/MgB₂ materials. The presence of LiH dramatically improves the hydrogenation of MgB₂ at 700 bar, forming borohydride 100 °C below the minimum hydrogenation temperature of pure MgB₂ and without the formation of undesirable intermediates such as [B₃H₈]^-, [B₁₀H₁₀]^2- or [B₁₂H₁₂]^2-. Evidence is reported for a mixed-metal borohydride of the type Mg_(3-x)/2Li_x(BH₄)₃ produced by the hydrogenation. Subsequent desorption is also improved compared to pure Mg(BH₄)₂ and LiBH₄, showing single-step hydrogen release up to ～8 wt% by 380 °C, whereas Mg(BH₄)₂ and LiBH₄ still retain significant amounts of hydrogen at this temperature. The material produced by desorption contains both MgB₂ and Mg metal, revealing the original LiH/MgB₂ system is not fully reversible. In contrast to LiH, TiH₂ is essentially inert when ball-milled with MgB₂, and high-pressure hydrogenation leaves only unreacted TiH₂ and MgB₂. Thus, added TiH₂ provides no benefit to MgB₂ hydrogenation.     

Influence of Ni interlayer on hydrogen absorption and cyclicity in Gd thin films

The Pd/Gd bilayers and Pd/Ni/Gd trilayers were prepared onto glass substrates by magnetron sputtering. Detailed XPS in-situ studies of the Pd/Gd bilayers allowed to estimate the thickness of the mixed layer at about 4 nm. Further studies showed that the additional Ni interlayer can significantly reduce the interface mixing during the deposition process, leading to the formation of a rather narrow Ni–Gd interface. The structure of the obtained layers before and after the hydrogenation was examined by the standard X-ray diffraction method. The hydrogenation kinetics was studied in-situ at room temperature and pressure up to 1 bar using simultaneous measurements of resistivity and optical transmittance. Furthermore, hydrogen absorption by electrochemical method was monitored using optical transmittance. It was found that the use of an additional 4 nm Ni layer significantly improved the cyclicity of the hydrogen absorption/desorption from the electrolyte, while maintaining low switching time (10 s).     

Thermodynamics, kinetics and microstructural evolution during hydrogenation of iron-doped magnesium thin films

New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg-4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH₂ is the mechanism controlling the hydride layer growth.Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg-Fe powders of 1-2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.     

A small pilot reactor containing active MgTi–based hydrides at low operational temperatures

This study was investigated to utilize innovatively oil-free diaphragm pump to forcibly desorb the hydrogen from the small pilot MgH₂–TiH₂ based hydride reactor below the theoretical temperature of 278 °C. Active MgH₂-0.1TiH₂ composites were prepared using ball milling. Their hydrogenation performances at 25–300 °C were measured under a constant H₂ flow mode using a modified Sieverts apparatus. The dehydrogenation rates at 250–350 °C with or without diaphragm pump were investigated to examine whether the pilot reactors could be integrated with a proton exchange membrane fuel cell (PEMFC) for power generation. At a H₂ flow rate of 25 ml min⁻¹ g⁻¹, the reactors exhibited excellent hydrogenation, achieving gravimetric hydrogen storage capacities of 2.9–5.2 wt% (excluding the weight of the reactors) at 25–300 °C after 22 min. All hydrided MgTi–based reactors could be dehydrogenated at 250 °C at an average rate of 5 ml min⁻¹ g⁻¹ under vacuum. This is the first demonstration of Mg-based reactors that were hydrogenated at 100 °C and dehydrogenated at 250 °C to power a small PEMFC, yielding a measured conversion efficiency of 18%.     

Ruthenium supported on nitrogen-doped porous carbon for catalytic hydrogen generation from NH3BH3 hydrolysis

In this paper, ruthenium supported on nitrogen-doped porous carbon (Ru/NPC) catalyst is synthesized by a simple method of in situ reduction using ammonia borane (AB) as reducing agent. The composition and structure of Ru/NPC catalyst are systematically characterized. This catalyst can efficiently catalyze the hydrolysis of AB. The hydrogen production reaction is completed within about 90 s at a temperature of 298 K and the maximum rate of hydrogen production is 3276 ml·s⁻¹·g⁻¹ with a reduced activation energy of 24.95 kJ·mol⁻¹. The turnover frequency (TOF) for hydrogen production is about 813 mol_H2·mol_Ru⁻¹·min⁻¹. Moreover, this catalyst can be recycled with a well-maintained performance. After five cycles, the maximum rate of hydrogen generation is maintained at 2206 ml·s⁻¹·g⁻¹, corresponding to 67.3% of the initial catalytic activity. Our results suggest that Ru/NPC prepared by in situ reduction is a highly efficient catalyst for hydrolytic dehydrogenation of AB.     

The retardation kinetics of magnetite reduction using H2 and H2–H2O mixtures

Cylindrical compacts of magnetite were isothermally reduced at 773–1273 K with pure H₂ or H₂–H₂O mixtures. The initial reduction rates increased with temperature and partial pressures of H₂ in the H₂–H₂O mixtures. However, with progressing reduction, a dense iron layer formed around the wüstite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51–89%, depending on the temperature and H₂ partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k₁^mixed/k₂^mixed, (k₁^mixed, k₂^mixed: contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H₂, H₂–10%H₂O, 773 K//H₂–10, 20%H₂O, 873 K//H₂–20%H₂O, 973 K) the retardation was absent because of the formation of a microporous iron layer product.     

Kinetics of NaBH4 hydrolysis on carbon-supported ruthenium catalysts

In this work, different shapes (powder and spherical) of ruthenium-active carbon catalysts (Ru/C) were prepared by impregnation reduction method for hydrogen generation (HG) from the hydrolysis reaction of the alkaline NaBH₄ solution. The effects of temperature, amount of catalysts, and concentration of NaOH and NaBH₄ on the hydrolysis of NaBH₄ solution were investigated with different shapes of Ru/C catalysts. The results show that the HG kinetics of NaBH₄ solution with the powder Ru/C catalysts is completely different from that with the spherical Ru/C catalysts. The main reason is that both mass and heat transfer play important roles during the reaction with Ru/C catalysts. The HG overall kinetic rate equations for NaBH₄ hydrolysis using the powder Ru/C catalysts and the spherical catalysts are described as r = A exp (−50740/RT) [catalyst]^1.05 [NaOH]^−0.13 [NaBH₄]^−0.25 and r = A exp (−52,120/RT) [catalyst]^1.00 [NaOH]^−0.21 [NaBH₄]^0.27 respectively.     

Temperature imaging of sub-millimeter-thick water using a near-infrared camera

This paper presents a method of imaging temperature distributions of sub-millimeter-thick water using a near-infrared camera and optical narrow-bandpass filter. The principle is based on the temperature dependence of the ν₁ + ν₃ absorption band of water. Temperature images are constructed by measuring the absorbance of water at the wavelength of 1412 nm through the filter for all pixels of the camera. From calibration measurements on 0.5-mm thick water at temperatures from 26.0 °C to 40.0 °C, the temperature coefficient was 6.3 × 10^?4 K^?1 and the standard deviation of absorbance was 1.9 × 10^?4. Thermal diffusion in 0.5-mm thick water caused by a thin heating wire was visualized with this method. The obtained images were verified against temperature distributions calculated by solving a two-dimensional thermal conduction model. This method would be useful for temperature measurement applications and control of aqueous solutions in microchips.