Hydride System Research Articles

Modelling and simulation of a wind-hydrogen CHP system with metal hydride storage

Abstract This paper describes the modelling and simulation of a wind-hydrogen system aimed at supplying electrical and thermal residential loads, where the thermal load is in part supplied by a catalytic hydrogen combustion device with hydrogen stored in a metal hydride system composed of a cluster of five metal hydride tanks equipped with a metal foam heat exchanger. The complete mathematical model has been developed from models available in literature and describing the different sub-systems that constitute the overall wind-hydrogen system. It has been later implemented in a multi-domain software environment to simulate system operations. Results over a year-long simulation show complete stand-alone capabilities, with an electrical efficiency and a combined heat and power efficiency of 8.2% and 12.5% respectively. At the end of the simulation period, a hydrogen annual surplus of 110.5 kg is left over which can, for instance, be used to feed a hydrogen powered car for about 9500 km.

Stable Hydrogen Storage Cycling in Magnesium Hydride, in the Range of Room Temperature to 300 °C, Achieved Using a New Bimetallic Cr-V Nanoscale Catalyst

We created a bimetallic chromium vanadium hydrogen sorption catalyst for magnesium hydride (MgH2). The catalyst allows for significant room-temperature hydrogen uptake, over 10 cycles, at absorption pressures as low as 2 bar. This is something that has never been previously achieved. The catalyst also allowed for ultrarapid and kinetically stable hydrogenation cycling (over 225 cycles) at 200 and at 300 °C. Transmission electron microscopy analysis of the postcycled samples revealed a nanoscale dispersion of Cr-V nanocrystallites within the Mg or MgH2 matrix. TEM analysis of the partially absorbed specimens revealed that even at a high absorption pressure, that is, a high driving force, relatively few hydride nuclei are formed at the surface of the pre-existing magnesium, ruling out the presence of any contracting volume (also termed contracting envelope or core shell) type growth. HRTEM of the cycled and desorbed powder sample demonstrated that the bcc Cr-V phase is crystalline and nanoscale. We experimentally demonstrated that the activation energy for hydrogen absorption is not constant but rather evolves with the driving force. This finding sheds new insight regarding the origins of the wide discrepancy in the literature – reported values of the hydrogenation activation energy in magnesium hydride and in related metal hydride systems.

Influence of Nanoconfinement on Reaction Pathways of Complex Metal Hydrides

Nanoconfinement is receiving increasing interest in the field of energy storage and has been applied to different metal hydride systems in order to improve their properties for hydrogen storage. Modified hydrogen sorption kinetics and thermodynamics of metal hydrides have been achieved by nano size effects. In case of metal complex hydrides, in addition to kinetic limitations, competing side reactions and stable intermediates during de/rehydrogenation processes are obstacles for a practical utilization. The impacts of nanoconfinement on dehydrogenation reaction pathways of the eutectic borohydrides LiBH4–Mg(BH4)2 have been studied in the presented work.

Desorption kinetics of lithium amide/magnesium hydride systems at constant pressure thermodynamic driving forces

Lithium amide and magnesium hydride are lightweight materials with high hydrogen-holding capacities and thus they are of interest for hydrogen storage. In the present work mixtures with initial molar compositions of (LiNH2 + MgH2) and (2LiNH2 + MgH2) were ball milled with and without the presence of 3.3 mol% potassium hydride dopant. Temperature programmed desorption, TPD, analyses of the mixtures showed that the potassium hydride doped samples had lower onset temperatures than their corresponding pristine samples. The dehydrogenation kinetics of the doped and pristine mixtures was compared at 210 °C. In each case a constant pressure thermodynamic driving force was applied in which the ratio of the plateau pressure to the applied hydrogen pressure was set at 10. Under equivalent conditions, the (LiNH2 + MgH2) mixture desorbed hydrogen about 4 times faster than the (2LiNH2 + MgH2) mixture. The addition of potassium hydride dopant was found to have a 25-fold increase on the desorption rates of the (2LiNH2 + MgH2) mixture, however it had almost no effect on the desorption rates of the (LiNH2 + MgH2) mixture. Activation energies were determined by the Kissinger method. Results showed the potassium hydride doped mixtures to have lower activation energies than the pristine mixtures.

Economic potential of complex hydrides compared to conventional hydrogen storage systems

Novel developments of materials for solid hydrogen storage show promising prospects. Complex hydrides exhibit great technical potential to store hydrogen in an efficient and safe way. Nevertheless, so far an evaluation of economic competitiveness is still lacking. In this work, an assessment about the economic feasibility of implementing complex hydrides as hydrogen storage materials is presented. The cost structure of hydrogen storage systems based on NaAlH4 and LiBH4/MgH2 is discussed and compared with the conventional high pressure (700bar) and liquid storage systems. The vessel construction for the complex hydride systems is much simpler than for the alternative conventional methods because of the milder pressure and temperature conditions during the storage process. According to the economical analysis, this represents the main cost advantage of the complex hydride systems.

Resonant ultrasound spectroscopy at high temperatures and pressures: Palladium hydride near the tri-critical point

Much fundamental quantum and critical phenomenon physics has been learned from precision elastic constant and attenuation measurements at low temperatures. Less attention has been paid to phenomenon in the high temperature regime outside of specialized areas such as geophysics. However, interesting fundamental and applied physics does not stop at room temperature. In this talk, I will review methods developed for applying resonant ultrasound spectroscopy at high temperature (<1300 K) and high hydrostatic pressure (<140 atm). One of the phenomenon we have studied using this apparatus is palladium hydride near the tri-critical point in the temperature, pressure phase diagram. The results from these measurements have validated a recent theory by Schwarz and Khachaturyan (2006) regarding the hysteresis in metal hydride systems which predicts a strong softening in the elastic moduli.

Effects of Different Ti-compounds on the Reversibility of NaAlH4

SUMMARY TiCl3 has been considered as the best catalyst for the hydrogen desorption/re-absorption of NaAlH4 in terms of kinetic enhancement. However, the formation of NaCl as a by-product leads to the decrease in the reversible hydrogen capacity of NaAlH4. In this work, TiO2 and metallic Ti were selected as catalysts for the reaction to avoid the formation of the by-product. The comparison of the catalytic activity of Ti, TiCl3, TiO2 and Ti(OBu)4 on the hydrogen desorption/absorption NaAlH4 were carried out. It was found that TiO2 doped NaAlH4 exhibits similar behavior as TiCl3 doped NaAlH4 with the reversible hydrogen capacity about 3.8 wt% (H/M). In addition, TiO2 doped NaAlH4 exhibits the superior hydrogen re-absorption rate to the one doped with TiCl3. That may be due to the Ti3+ defect sites on the surface of TiO2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO2 prevents the segregation and the morphological change of the desorbed substances (NaH and Al). This benefits to the mass transfer into the hydride system. However, doping with TiO2 also produces sodium oxide and hydroxide as by-products. Unexpectedly, metallic Ti doped NaAlH4 shows the lowest hydrogen desorption/re-absorption among the tested samples. Its hydrogen reversible capacity is around 1 wt% (H/M). The formation of TiHx (1 < x < 2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright © 2011 John Wiley & Sons, Ltd.

System modeling methodology and analyses for materials-based hydrogen storage

In the global efforts to develop advanced materials-based hydrogen storage, the various on-board reversible hydrides, adsorbents and chemical storage candidate materials and systems each have their individual strengths and weaknesses. An overarching challenge in associated research and development is to devise material/system architectures which satisfy all requirements for viability in a particular application area, such as light-duty vehicular transportation. System modeling at the level which encompasses not only the storage material and vessel/reactor, but also integration with a fuel cell and balance-of-plant components, provides a more complete assessment of viability and guides options for improvement. The current work covers the methodology developed for conducting such system modeling consistently across multiple organizations and will present performance results from studies focused on reversible hydride systems. Connecting this high level modeling to more detailed finite element design simulations will be one aspect of our framework approach. The complex hydride NaAlH 4 is representative of novel materials under development and will be used as the basis for properties, such as temperature dependent kinetics, which influence the integrated system configurations and component sizing. While system charging is included through the sizing of certain components, emphasis is placed on hydrogen discharge by the storage system, interrogated through drive cycle transients. Comparisons of performance relative to requirements, including effective gravimetric capacity, effective volumetric density and energy utilization, are given for the baseline material and for a sensitivity study on material density.

The Effect of H2 Partial Pressure on the Reaction Progression and Reversibility of Lithium-Containing Multicomponent Destabilized Hydrogen Storage Systems

It is known that the reaction path for the decomposition of LiBH(4):MgH(2) systems is dependent on whether decomposition is performed under vacuum or under a hydrogen pressure (typically 1-5 bar). However, the sensitivity of this multicomponent hydride system to partial pressures of H(2) has not been investigated previously. A combination of in situ powder neutron and X-ray diffraction (deuterides were used for the neutron experiments) have shed light on the effect of low partial pressures of hydrogen on the decomposition of these materials. Different partial pressures have been achieved through the use of different vacuum systems. It was found that all the samples decomposed to form Li-Mg alloys regardless of the vacuum system used or sample stoichiometry of the multicomponent system. However, upon cooling the reaction products, the alloys showed phase instability in all but the highest efficiency pumps (i.e., lowest base pressures), with the alloys reacting to form LiH and Mg. This work has significant impact on the investigation of Li-containing multicomponent systems and the reproducibility of results if different dynamic vacuum conditions are used, as this affects the apparent amount of hydrogen evolved (as determined by ex situ experiments). These results have also helped to explain differences in the reported reversibility of the systems, with Li-rich samples forming a passivating hydride layer, hindering further hydrogenation.

ChemInform Abstract: Reversible Dehydrogenation—Hydrogenation of Tetrahydroquinoline‐Quinoline Using a Supported Copper Nanoparticle Catalyst.

AbstractTitania‐supported copper nanoparticles act as a heterogeneous catalyst for the reversible dehydrogenation and hydrogenation of tetrahydroquinoline/quinoline under an atmospheric pressure of hydrogen.This reversible organic hydride system can be applied under mild conditions and repeated with retention of high efficiency.

System simulation models for on-board hydrogen storage systems

System simulation models for automotive on-board hydrogen storage systems provide a measure of the ability of an engineered system and storage media to meet system performance targets. Thoughtful engineering design for a particular storage media can help the system achieve desired performance goals. This paper presents system simulation models for two different advanced hydrogen storage technologies – a cryo-adsorption system and a metal hydride system. AX-21 superactivated carbon and sodium alanate are employed as representative storage media for the cryo-adsorbent system and the metal hydride system respectively. Lumped parameter models incorporating guidance from detailed transport models are employed in building the system simulation models. Simulation results to test the storage systems’ ability to meet fuel cell demand for different drive cycles and varying operating conditions are presented. Systems are engineered to provide the ability to refuel a vehicle in a short time guided by DOE targets. Gravimetric and volumetric hydrogen densities are computed for the engineered systems and compared to the DOE system goals.

Affects of Mechanical Milling and Metal Oxide Additives on Sorption Kinetics of 1:1 LiNH2/MgH2 Mixture

The destabilized complex hydride system composed of LiNH2:MgH2 (1:1 molar ratio) is one of the leading candidates of hydrogen storage with a reversible hydrogen storage capacity of 8.1 wt%. A low sorption enthalpy of ~32 kJ/mole H2 was first predicted by Alapati et al. utilizing first principle density function theory (DFT) calculations and has been subsequently confirmed empirically by Lu et al. through differential thermal analysis (DTA). This enthalpy suggests that favorable sorption kinetics should be obtainable at temperatures in the range of 160 °C to 200 °C. Preliminary experiments reported in the literature indicate that sorption kinetics are substantially lower than expected in this temperature range despite favorable thermodynamics. Systematic isothermal and isobaric sorption experiments were performed using a Sievert’s apparatus to form a baseline data set by which to compare kinetic results over the pressure and temperature range anticipated for use of this material as a hydrogen storage media. Various material preparation methods and compositional modifications were performed in attempts to increase the kinetics while lowering the sorption temperatures. This paper outlines the results of these systematic tests and describes a number of beneficial additions which influence kinetics as well as NH3 formation.

Hydrogen release from sodium borohydrides at low temperature by the addition of zinc fluoride

The major challenge of complex hydride for hydrogen storage is to release hydrogen at a moderate temperature with rapid kinetics. This paper reports a new composite system (NaBH 4/ZnF 2) to generate hydrogen at a low temperature below 100 °C. The NaBH 4 and ZnF 2 mixture was prepared by a mechanical milling in a 2:1 M ratio. Dehydrogenation of the mixture during a heating process shows that the initial dehydrogenation temperature was below 100 °C with favorable kinetics. The structural analysis confirmed the formation of NaBF 4 and Zn in the reaction products. The study concerning the reaction mechanism reveals that ZnF 2 plays a role of reagent rather than catalyst. The produced intermediate of NaZnF 3, however, affects the decomposition of the emitted B 2H 6. This work suggests that the extension of the decomposition mechanism of NaBH 4/ZnF 2 composite system would favor the development of new catalysts for complex hydride systems.

The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH 2 + 1.2LiH) system

The addition of 5 wt.% of graphite was incorporated into the (LiNH 2 + 1.2LiH) hydride system in order to study its effect on the prevention of LiH from hydrolysis/oxidation which leads to the escape of NH 3. The composite hydride system was processed by ball milling for 25 h. Thermal behavior in DSC up to 500 °C and isothermal desorption in a Sieverts-type apparatus were carried out. XRD was used to obtain information about phase changes. It is found that after ball milling graphite becomes amorphous. DSC analysis shows that for the mixture ((LiNH 2 + 1.2LiH) + 5 wt.% graphite) graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH since no melting peak of retained LiNH 2 is observed. Both the DSC and Sieverts-type tests show that the addition of graphite increases the apparent activation energy of desorption from the ∼57–58 to ∼85–90 kJ/mol range. On the other hand, the graphite additive increases measurably the desorbed/absorbed capacity of hydrogen at 275, 300 and 325 °C. The ((LiNH 2 + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing ∼5 wt.% H 2 at 325 °C in the following reaction: (LiNH 2 + LiH ↔ Li 2NH + H 2). Step-wise pressure–composition–temperature (PCT) tests show that the enthalpy and entropy change of this reversible reaction is −62.4 and −61.0 kJ/mol H 2 and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH 2 + 1.2LiH) system, respectively. It shows that within an experimental error there is no measurable effect of graphite additive on the thermodynamic properties. The Van’t Hoff analysis of the obtained thermodynamic data shows that the equilibrium temperature at atmospheric pressure of hydrogen (1 bar H 2) is 256.8 and 253.9 °C for the undoped and 5 wt.% G doped (LiNH 2 + 1.2 LiH) system ball milled for 25 h, respectively. Such high equilibrium temperatures render it rather obvious that both of these hydride systems cannot be employed for hydrogen desorption/absorption below 100 °C as required by the DOE targets for the automotive hydrogen storage materials.

Structure and Thermodynamic Properties of the NaMgH3 Perovskite: A Comprehensive Study

One of the bottlenecks in the implementation of a hydrogen economy is the development of storage materials that can uptake high content of H2 and release it within a suitable temperature and pressure range. Among the proposed hydride systems, the perovskite NaMgH3 is receiving increasing attention, not only as the Mg ternary based hydride with the highest hydrogen gravimetric (6 wt %) and volumetric density (88 g L−1) but also as a stable hydride likely to be formed in the transformation reactions of mixed hydrides. However, there is a large scatter in the literature for both the structure of the NaMgH3 compound and the thermodynamics of the hydrogenation/dehydrogenation processes. In this paper a critical review of the literature data, supported by a new set of experimental (in situ synchrotron X-ray diffraction, infrared spectroscopy, high-pressure differential scanning calorimetry, pressure composition isotherms) and theoretical data is presented. The influence of ball milling on the microstructure is studied in the NaMgH3 in comparison to NaH and MgH2. The infrared spectrum of NaMgH3 compound, assigned by calculated and experimental results, is characterized by vibrational regions around 1100 and 600 cm−1. In situ synchrotron X-ray diffraction measurements show the desorption reaction of NaMgH3 into NaH and Mg at about 673 K under 0.2 MPa H2, and the successive reabsorption of NaH and Mg back to NaMgH3 at 623 K under 0.5 MPa H2. From high-pressure differential calorimetry, it was measured a formation enthalpy of 141 kJ/mol f.u for NaMgH3 compound. It was confirmed the possible reaction of NaH with Mg with observation of NaMgH3 formation in 1.0 MPa H2. Finally, this work provides a thermodynamic description of the NaMgH3 phase by a critical assessment of the available information using the CALPHAD approach and the equilibrium pressure−temperature phase diagram is presented.

High level theoretical study of binding and of the potential energy surface in benzene–hydride system

High level ab initio calculations were performed on the interaction of the hydride anion with benzene, a system of potential interest for modelling the interactions occurring in hydrogen rich planetary atmospheres. We investigated both non-covalent and covalent binding, exploring the complete basis set limit using highly correlated MP2 and CCSD(T) levels of theory. Two non-covalent minima on the potential energy surface have been characterized, and found to correspond to moderately strong hydrogen bonding interactions. To gain further insight on the nature of binding, the total interaction energy was decomposed into its physically meaningful components and selected sections of the potential energy surface were calculated. Moreover, we found that H− can easily covalently bind to one of the carbon atoms of benzene to form a stable C6H7− anion, a global minimum on the potential energy surface, characterized by a puckered geometry, with a carbon atom bending out of the benzene plane. A slightly less stable planar C6H7− structure was also identified, corresponding to the transition state for the flipping motion of the puckered species.

Non-harmonic electro-chemical and pneumato-chemical impedance spectroscopies for analyzing the hydriding kinetics of palladium

Analysis of phase transformation processes observed in most hydrogen-absorbing materials (pure metals, alloys or compounds) is still a matter of active research, in view of electrochemical or chemical hydrogen storage applications. Most metal hydride systems present a large and irreducible hysteresis, indicating that non-linear phenomena are taking place during hydrogen sorption/desorption reactions. Investigation of hydriding kinetics therefore requires non-harmonic spectroscopies to fulfill the requirements of linearity and time invariance imposed by the theory of systems. The purpose of this paper is to report on the insertion of atomic hydrogen into a palladium foil. The process is analyzed using electrochemical (the electrolyte is the source of hydrogen) and pneumato-chemical (the gas phase is the source of hydrogen) experimental spectroscopies. Pneumato-chemical impedance spectroscopy (PIS) provides additional information to electro-chemical impedance spectroscopy (EIS) since (i) there is no electrolyte and no electrified interface; (ii) experiments can be performed over long periods of time, without any pollution (closed systems); (iii) there is no parasite reaction such as hydrogen evolution encountered in electrochemical experiments; (iv) the temperature range of investigation is larger; (v) there is no kinetic limitations on the electrolyte side of the interface. Results obtained in single and two-phase domains of the model PdH system, using both EIS and PIS spectroscopies, are reported and discussed. Surface and bulk microscopic rate parameters are compared.

Intrinsic defects and dopants in LiNH2: a first-principles study

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ∼0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.

The impact of carbon materials on the hydrogen storage properties of light metal hydrides

The safe and efficient storage of hydrogen is still one of the remaining challenges towards fuel cell powered cars. Metal hydrides are a promising class of materials as they allow the storage of large amounts of hydrogen in a small volume at room temperature and low pressures. However, usually the kinetics of hydrogen release and uptake and the thermodynamic properties do not satisfy the requirements for practical applications. Therefore current research focuses on catalysis and the thermodynamic tailoring of metal hydride systems. Surprisingly, carbon materials used as additive or support are very effective to improve the hydrogen storage properties of metal hydrides allowing fast kinetics and even a change in the thermodynamic properties. Even though the underlying mechanisms are not always well understood, the beneficial effect is probably related to the peculiar structure of the carbon materials. This feature article gives an introduction to the different carbon materials, an overview of the preparation strategies to synthesize carbon/hydride nanocomposites, and highlights the beneficial effect of carbon by discussing two important hydrides: MgH2 and NaAlH4.

Effective hydrogen storage: a strategic chemistry challenge

This paper gives an overview of the current status and future potential of hydrogen storage from a chemistry perspective and is based on the concluding presentation of the Faraday Discussion 151--Hydrogen Storage Materials. The safe, effective and economical storage of hydrogen is one of the main scientific and technological challenges in the move towards a low-carbon economy. One key sector is transportation where future vehicles will most likely be developed around a balance of battery-electric and hydrogen fuel-cell electric technologies. Although there has been a very significant research effort in solid-state hydrogen storage, high-pressure gas storage combined with conventional metal hydrides is still seen as the current intermediate-term candidate for car manufacturers. Significant issues have arisen in the search for improved solid-state hydrogen storage materials; for example, facile reversibility has been a major challenge for many recently studied complex hydrides while physisorption in porous structures is still restricted to cryogenic temperatures. However, many systems fulfil the majority of necessary criteria for improved hydrogen storage--indeed, the discovery of reversibility in multicomponent hydride systems along with recent chemistry breakthroughs in off-board and solvent-assisted regeneration suggest that the goal of both improved on-board reversible and off-board regenerated hydrogen storage systems can be achieved.