Equilibrium Hydrogen Research Articles

Hydrogen Storage Properties of La1-xYxNi5-yAly Alloys

Abstract The feasibilities of La 1- x Y x Ni 5-y Al y alloys ( x =0.6, 0.7; y =0.1, 0.2) applied in metal hydride high-pressure compressor were investigated systematically in terms of the crystal structure, hydrogen absorption/desorption properties and pulverization resistance. XRD analyses show that all the investigated alloys present CaCu 5 type hexagonal structure and the unit cell volume decreases with the increase of Y, but increases with the increase of Al content. The pressure-composition isotherms and absorption kinetics were measured at 20, 30 and 40 °C by the volumetric method. Hydrogen absorption/desorption measurement reveals that Y and Al could effectively increase and decrease the equilibrium hydrogen pressure, respectively. They are ideal elements to adjust the equilibrium hydrogen pressure of LaNi 5 -based alloys without other remarkable negative influences. We think that the alloys could be matched as ‘alloy pairs’ and applied in a double-stage metal hydride compressor; the compressor could boost the feed hydrogen gas at room temperature from 2 MPa to 35∼40 MPa at 135∼155 °C.

Thermodynamic model for prediction of phase equilibria of clathrate hydrates of hydrogen with different alkanes, alkenes, alkynes, cycloalkanes or cycloalkene

In this work, structure H hydrate phase equilibrium of hydrogen in the presence of organic promoter is predicted using a proposed thermodynamic model. The investigated promoters are various n-alkanes, n-alkenes/alkynes, and cycloalkanes/cycloalkene. The van der Waals–Platteeuw solid solution theory is used for determination of the fugacity of water in hydrate phase. Phase behavior of the hydrogen+water system is modeled using the Valerama–Patel–Teja equation of state (VPT-EoS) with non-density dependent mixing rules. Due to the lack of experimental solubility data of hydrogen in the investigated promoters, the phase equilibria of the hydrogen+promoter system is treated using the VPT-EoS-GE method consisting the UNIFAC activity model and the modified Huron–Vidal (MHV1) mixing rules. The obtained results show reasonable agreement of the predictions with the existing experimental data from the literature. Finally, the hydrogen storage capacity of the corresponding clathrate hydrates is predicted as well as the occupancies of the clathrate hydrate cavities.

Hydrogen diffusion coefficient and mobility in palladium as a function of equilibrium pressure evaluated by permeation measurement

Hydrogen permeation flux is generally described by the square-root law, but deviation from the law has been widely reported. For more precise description, pressure-dependent permeability has been proposed. Based on this approach, the hydrogen diffusion coefficient in palladium was evaluated as a continuous function of equilibrium hydrogen pressure. Results showed a decreasing diffusion coefficient with pressure, in contrast to increasing permeability and solution coefficient. The pre-exponential factor and activation energy for the diffusion coefficient in the dilute limit, i.e., intrinsic diffusion coefficient, were, respectively, 2.40×10−7m2/s and 21.1kJ/molH. This study did not assume constant hydrogen mobility, differently from most studies, but evaluated the mobility as a function of pressure. This precise analysis revealed slightly increasing mobility with pressure, because of less deep hydrogen potential at higher hydrogen concentrations. Finally, a guideline to develop membrane materials was provided. Materials with large deviation from Sieverts' law possibly have high permeability at practical pressures for permeation, i.e., atmospheric pressure or higher, even if the mobility in the dilute limit is low.

Studies of Co, Al, and Mn substitutions in NdNi5 metal hydride alloys

The structure and gaseous phase hydrogen storage characteristics of a series of as-cast Nd(NiCoAlMn)5 alloys were studied. Without an annealing treatment, the main phase of these alloys was NdNi5-structured with Nd2Ni7 (may include Nd5Ni19 and NdNi4) as the main secondary phase. In most of the alloys, single-plateau pressure–concentration–temperature isotherms were observed due to the expansion of the unit cells brought on by additions of larger B-elements into the alloys. As the amount of replacement element increased, both the equilibrium plateau pressure and hydrogen storage capability reduced. A proper guideline for the amounts of Al- and Mn-additions was established for battery application: Co=8at.%, 8at.%⩽Mn+Al⩽14at.%. In the half-cell electrochemical measurements, the capacities of these un-annealed Nd-based AB5 alloys were lower than those of the commercially available misch metal (Mm)-based AB5 alloy but showed an advantage in high-rate dischargeability. In the sealed cell experiments, some of the Nd-based AB5 alloys showed superior high-rate and charge-retention performance compared to the Mm-based AB5 alloy. The abundance of secondary phase may have played an important role in influencing the high-rate performance. The alloy with the composition of Nd17Co8Al6Mn8Ni61 showed the best overall electrochemical performance and was recommended for further study.

Nanosize-Induced Drastic Drop in Equilibrium Hydrogen Pressure for Hydride Formation and Structural Stabilization in Pd–Rh Solid-Solution Alloys

We have synthesized and characterized homogeneous solid-solution alloy nanoparticles of Pd and Rh, which are immiscible with each other in the equilibrium bulk state at around room temperature. The Pd-Rh alloy nanoparticles can absorb hydrogen at ambient pressure and the hydrogen pressure of Pd-Rh alloys for hydrogen storage is dramatically decreased by more than 4 orders of magnitude from the corresponding pressure in the metastable bulk state. The solid-solution state is still maintained in the nanoparticles even after hydrogen absorption/desorption, in contrast to the metastable bulks which are separated into Pd and Rh during the process.

In situ measurement of alternating current magnetic susceptibility of Pd–hydrogen system for determination of hydrogen concentration in bulk

An alternating current magnetic susceptometer for use as a hydrogen gauge for hydrogen-storage materials was designed and developed. The experimental system can simultaneously measure the hydrogen equilibrium pressure and the magnetic susceptibility of metal hydrides. The background voltage of the susceptometer was stabilized for a long period of time, without any adjustments, by attaching an efficient compensation circuit. The performance of the susceptometer at a static hydrogen concentration was demonstrated by measuring the magnetic susceptibility of a Pd-hydrogen system under equilibrium conditions. The in situ measurement of the magnetic susceptibility of Pd during hydrogen absorption was carried out using the susceptometer. Since the in situ magnetic susceptibility obtained at a lower initial hydrogen pressure agreed with the magnetic susceptibility measured at a static hydrogen concentration, the susceptometer could be used to determine the hydrogen concentration in Pd in situ. At a higher initial hydrogen pressure, enhancement of the magnetic susceptibility was observed at the beginning of hydrogen absorption because the magnetic moments induced by the large temporary strain generated in the Pd affected the magnetic susceptibility.

High-yield hydrogen production from glucose by supercritical water gasification without added catalyst

Continuous supercritical water gasification of glucose is investigated with a recently developed updraft gasification apparatus under various conditions: temperatures of 600–767 °C, residence times of 15–60 s, glucose concentrations of 1.8–15 wt% and without added a catalyst. The experimental gas yields are compared with predicted values at equilibrium that are estimated via Gibbs free energy minimization. Total gas yields and hydrogen gas yield increase with temperature. At 740 °C and 1.8 wt%, hydrogen gas yields become very high (10.5–11.2 mol/mol glucose). The hydrogen gas yields do not vary significantly with different residence times. The hydrogen gas yield decreases to 5.7 mol/mol glucose at 15 wt%, a value very close to the predicted value (6.3 mol/mol glucose). Only acetic acid is detected in the liquid effluents at temperatures above 740 °C, while 42 products are detected at 600 °C. The highest hydrogen gas yield obtained in this study is 11.5 mol/mol glucose at 25 MPa, 767 °C, and 1.8 wt%, for 60 s; this value is very close to the theoretical equilibrium hydrogen yield of 11.9 mol/mol glucose. Under these conditions, the carbon efficiency is very high (91%) and total organic carbon (TOC) in the liquid product is very low (23 ppm), indicating that glucose is almost completely converted to gaseous products. Comparison with other work under similar operating conditions shows that the current reactor is capable of attaining higher hydrogen gas yields at temperatures above 650 °C. Possible explanations for the higher hydrogen gas yields are presented.

Zeolite-Templated Carbon Materials for High-Pressure Hydrogen Storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 m(2) g(-1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g(-1) (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g(-1) (0.46 wt %) uptake per 1000 m(2) g(-1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5-6.6 kJ mol(-1) in the Henry's law limit.

Statistical thermodynamic description of H2 molecules in normal ortho/para mixture

In this paper, an expression for the internal partition function of normal hydrogen has been obtained starting from the principles of classical statistical thermodynamics. The calculated contribution applies to ideal gas properties. Particular attention has been devoted to the entropy, which depends on the logarithm of the partition function. Comparison between normal and equilibrium hydrogen together with theoretical and experimental data available in literature has been discussed. Tables of the thermodynamic properties of hydrogen and hydrogen ion are also reported.

Effect of argon on the structure of hydrogenated nanocrystalline silicon deposited from tetrachlorosilane/hydrogen/argon plasma

AbstractHydrogenated nanocrystalline silicon films (nc‐Si:H) have been deposited by the decomposition of tetrachlorosilane (SiCl4) diluted with hydrogen (H2) and argon (Ar) by a plasma‐enhanced chemical vapor deposition (PECVD) method. Basing on the structural characterization of nc‐Si:H, we have found that Ar affects the growth of films deposited at two substrate temperatures (Ts = 250 and 120 °C) in different ways. At Ts = 250 °C, the Ar gas deteriorates the crystallinity of nc‐Si:H, whereas, at Ts = 120 °C, the crystallization of nc‐Si:H films is promoted with increasing argon flow rate (Hf) from 10 to 30 sccm; however, the extremely high Ar dilution adversely affects the ordered structure of films. Meanwhile, the hydrogen concentration in the films is also controlled by Hf. Finally, the effects of Ar in the SiCl4/H2/Ar system have been discussed in related to the equilibrium of atomic hydrogen and ionized argon.

Interfacial Alloy Hydride Destabilization inMg/PdThin Films

Recently, a large increase in the equilibrium hydrogen pressure has been reported for MG thin films capped with a Pd layer. We show that this increase is due to intermixing of Mg and Pd, as opposed to a strain effect as previously claimed. Transmission electron microscopy and depth profiling x-ray photoemission spectroscopy are used to directly measure interfacial intermixing between Mg and Pd, and we find that intermixing and equilibrium hydrogen pressure both increase with annealing. We present a thermodynamic model of the effect of alloying on equilibrium pressure, and find that the observed equilibrium pressure increase is consistent with the observed thickness of the intermixed region, which is of the order of a few nm. We also show that stress measured during hydrogenation corresponds to a negligible increase in equilibrium pressure.

Structure and hydrogenation study of nickel substituted NdCo3NiB

In an attempt to observe hydrogen (deuterium) induced atomic ordering of transition (T) metals in AB5-type derivative structures neutron diffraction and hydrogen cycling experiments were performed on a nickel substituted boride sample of composition NdCo3NiB. While uncycled NdCo3NiB crystallizes with the centrosymmetric CeCo4B-type structure (space group P6/mmm), hydrogenation (deuteration) and cycling of NdCo3NiB at 373K between 0 and 60bar hydrogen (deuterium) atmosphere induces a symmetry decrease to non-centrosymmetric space group P6mm. Evidence for partial cobalt/nickel ordering is found in pristine NdCo3NiB but not in its deuteride NdCo3NiBDx. Compared to boron free AB5-type compounds deuterium occupies only those interstices that have no boron atom in their coordination sphere. These findings are consistent with repulsive B–D interactions. At 303K the hydrogen equilibrium plateau pressure of the NdCo3NiB–H system is about 2bar, and the hydrogen content at 10bar is about 3.1H atoms per formula unit.

Strong Metal Support Interaction of Pt and Ru Nanoparticles Deposited on HOPG Probed by the H-D Exchange Reaction

The interaction between metals and support is investigated in the case of 50 A Pt and 50 A Ru films deposited on a HOPG substrate. The films are prepared by electron beam physical vapor deposition and annealed in UHV to temperatures up to 700 °C. The equilibrium hydrogen exchange rate between adsorbed and gas phase at 1 bar is measured before and after annealing. The rate is measured in the temperature range of 40–200 °C at 1 bar, by utilization of the H-D exchange reaction. Experiments are performed on fresh cleaved and sputtered HOPG, which give similar results. We find that annealing the films from 150 up to 700 °C increases the amount of carbon present in the films up to 95%, as derived by surface analysis, indicating the formation of a carbon layer on top of the metal films. The exchange rate decreases dramatically with increasing carbon content on the films for both metals, pointing to a decrease in the hydrogen adsorption on the films, due to the carbon poisoning. We show how to reverse this effect...

New insights into the mechanism of activation and hydrogen absorption of (2LiNH2–MgH2)

2LiNH2–MgH2 is considered an attractive material for reversible hydrogen storage. In an attempt to improve the hydrogen storage characteristics of the 2LiNH2–MgH2 system, the activation mechanism of the material, as well as the improvement in hydrogen absorption rates through doping were explored. Differential Scanning Calorimetry (DSC) investigations reveal that the initial and irreversible conversion process 2LiNH2+MgH2⇒2LiH+Mg(NH2)2 is exothermic, indicating it is energetically favorable for this initial conversion of the starting material. The exothermicity of this first step explains why the original starting material (2LiNH2 + MgH2) is never regenerated during re-hydrogenation of the desorbed product. Adding catalytic amounts (<4 mol %) of potassium hydride (KH) significantly increases the hydrogen absorption rate of the desorbed material, and has a less dramatic effect on the kinetics of hydrogen desorption. Pressure-Composition-Temperature (PCT) studies for the KH-catalyzed material indicate a substantial hydrogen equilibrium pressure of 20 atm at 180 °C. The fast absorption rate obtained via using KH catalysis allows a more accurate equilibrium measurement. The changes of enthalpy and entropy for the conversion of catalyzed (2LiH + Mg(NH2)2) to Li2Mg(NH)2 with hydrogen release were determined from the van’t Hoff plot. These values for the enthalpy and entropy of hydrogen desorption are ∆H = 40 kJ/mol H2, and ∆S = 99 J/K-mole H2, respectively. This ∆H value is similar to that reported previously by Wang et al. for the K-catalyzed material and previous measurements for the un-catalyzed material. The similarity of the ∆H values for both K-doped and un-doped material confirms KH is acting catalytically and not thermodynamically.

Interface reactions and stability of a hydride composite (NaBH4 + MgH2)

The use of the interaction of two hydrides is a well-known concept used to increase the hydrogen equilibrium pressure of composite mixtures in comparison to that of pure systems. The thermodynamics and reaction kinetics of such hydride composites are reviewed and experimentally verified using the example NaBH(4) + MgH(2). Particular emphasis is placed on the measurement of the kinetics and stability using thermodesorption experiments and measurements of pressure-composition isotherms, respectively. The interface reactions in the composite reaction were analysed by in situ X-ray photoelectron spectroscopy and by simultaneously probing D(2) desorption from NaBD(4) and H(2) desorption from MgH(2). The observed destabilisation is in quantitative agreement with the calculated thermodynamic properties, including enthalpy and entropy. The results are discussed with respect to kinetic limitations of the hydrogen desorption mechanism at interfaces. General aspects of modifying hydrogen sorption properties via hydride composites are given.

H2Splitting on Pt/Ru Alloys Supported on Sputtered HOPG

The equilibrium hydrogen exchange rate between adsorbed and gas phase hydrogen at 1 bar is measured on three Pt/Ru systems: Pt overlayers on Ru films, Ru overlayers on Pt films, and Pt/Ru bulk alloys, with different Pt/Ru compositions. The catalysts are prepared by electron beam physical vapor deposition and supported on a sputtered HOPG substrate. The hydrogen exchange rate is measured in the temperature range 40–200 °C at 1 bar, by utilization of the H-D exchange reaction. We find that the exchange rate, r, and the dissociative sticking probability, S, as a function of surface concentration exceed the values measured on the individual metals and have a maximum for a composition of 2/7 Pt in the bulk alloys. The Pt overlayers on Ru films give similar results for r and S, and maximum activity is found when 4 Å of Pt are deposited on 50 Å Ru films. ISS spectra show that both Ru and Pt are on the surface, which indicates the formation of a surface alloy already at 200 °C. We also find that the maximum activity is given by the alloys with a surface composition of Pt/Ru equal to a 1:1 ratio. Experiments are also carried out in the presence of 10 ppm CO for the Pt–Ru bulk alloys. We find that alloying Pt with Ru improves significantly the resistance toward CO poisoning with respect to pure Pt, and the resistance increases with an increasing amount of Ru in the bulk alloys. The faster hydrogen exchange rate with respect to the pure metals and the higher CO tolerance of the alloys are attributed to strain and ligand effects, caused by the compression of the surface due to the presence of the larger Pt atoms in the neighboring Ru atoms. The apparent energy of desorption at equilibrium, Eapp, for the three Pt–Ru systems is found to decrease with an increasing amount of Ru in the alloys, and it is attributed to geometrical ensemble effects.

A theoretical study of the molecular structures and vibrational spectra of the N2O⋯(HF)2

Abstract Theoretical calculations using both the MP2 and B3LYP levels of calculation with a 6-311++G(3df,3pd) basis set have been performed to determine stable structures and molecular properties for the H-bonded complexes involving nitrous oxide (N2O) and two HF molecules. Five complex have been characterized as minima since no imaginary frequency was found. Three complex are predicted to be relatively more stable with binding energies varying from 14 kJ mol−1 to 23 kJ mol−1 after BSSE and ZPE corrections. Our calculations have revealed that the second complexation with HF preferably occurs with the first complexed HF molecule, i.e., forming the X⋯H F⋯H F skeleton with X = O or N instead the F H⋯N N O⋯H F one. As expected, the H F chemical bonds are increased after complexation due to intermolecular charge transfer from “n” isolated pair of the X atom (X = N, O or F) to the σ∗ anti-bonding orbital of HF. For the strongly bounded complex, the doubly complexed HF molecule acts as a bridge between the two end molecules while transferring electrons from N2O to HF. Both possess the same amount of residual charge but with opposite signs. The H F stretching frequency of the monoprotic acid is shifted downward after complexation whereas its IR intensity is much enhanced. This increase has been adequately interpreted in terms of equilibrium hydrogen charge and charge-flux associated to the H F stretching using the CCFOM model for infrared intensities. This procedure has also allowed to analyze the new vibrational modes arising upon H-bond formation, especially those associated with the out-of-plane and in-plane HF bending modes, which are pure rotations in the HF isolated molecule.

Infrared spectroscopic and thermodynamic study on hydrogen adsorption on the metal organic framework MIL-100(Sc)

The thermodynamics of hydrogen interaction with coordinatively unsaturated ScIII cationic sites in the metal organic framework MIL-100(Sc) was studied by variable temperature infrared spectroscopy. Simultaneous measurement of the hydrogen equilibrium pressure (inside a closed IR cell) and the integrated IR absorbance over the temperature range 80–125K led to the determination of the corresponding values of standard adsorption enthalpy and entropy, which resulted to be ΔH0=−11.2 (±1)kJmol−1 and ΔS0=−120 (±10)Jmol−1K−1. The enthalpy value is among the largest reported so far for hydrogen adsorption on MOFs.

H2 splitting on Pt, Ru and Rh nanoparticles supported on sputtered HOPG

The equilibrium hydrogen exchange rate between adsorbed and gas phase hydrogen at 1bar is measured for Pt, Ru and Rh nanoparticles supported on a sputtered HOPG substrate. The particles are prepared by Electron Beam Physical Vapor Deposition and the diameter of the particles varies between 2 and 5nm. The rate of hydrogen exchange is measured in the temperature range 40–200°C at 1bar, by utilization of the H–D exchange reaction. We find that the rate of hydrogen exchange increases with the particle diameter for all the metals, and that the rate for Ru and Rh is higher than for Pt. In the case of Pt, the equilibrium dissociative sticking probability, S, is found to be nearly independent of particle diameter. For Ru and Rh, S is found to depend strongly on particle diameter, with the larger particles being more active. The apparent energy of desorption at equilibrium, Eapp, shows a dramatic increase with decreasing particle diameter for diameters below 5nm for Ru and Rh, whereas Eapp is only weakly dependent on particle diameter for Pt. We suggest that the strong variation in the apparent desorption energy with particle diameter for Ru and Rh is due to the formation of compressed hydrogen adlayers on the terraces of the larger particles. Experiments are also carried out in the presence of 10ppm CO. Pt is found to be very sensitive to CO poisoning and the H–D exchange rate drops below the detection limit when CO is added to the gas mixture. In the case of Ru and Rh nanoparticles, CO decreases the splitting rate significantly, also at 200°C. The variation of the sensitivity to CO poisoning with particle diameter for Ru and Rh is found to be weak.

Potassium Silanide (KSiH3): A Reversible Hydrogen Storage Material

KSi silicide can absorb hydrogen to directly form the ternary KSiH(3) hydride. The full structure of α-KSiD(3), which has been solved by using neutron powder diffraction (NPD), shows an unusually short Si-D lengths of 1.47 Å. Through a combination of density functional theory (DFT) calculations and experimental methods, the thermodynamic and structural properties of the KSi/α-KSiH(3) system are determined. This system is able to store 4.3 wt% of hydrogen reversibly within a good P-T window; a 0.1 MPa hydrogen equilibrium pressure can be obtained at around 414 K. The DFT calculations and the measurements of hydrogen equilibrium pressures at different temperatures give similar values for the dehydrogenation enthalpy (≈23 kJ mol(-1) H(2)) and entropy (≈54 J K(-1) mol(-1) H(2)). Owing to its relatively high hydrogen storage capacity and its good thermodynamic values, this KSi/α-KSiH(3) system is a promising candidate for reversible hydrogen storage.