Hydrogen Sorption Reaction Research Articles

Hydrogen storage and handling with hydrides

Abstract After production and before the use in different applications, hydrogen may need to be purified, transported, compressed and stored. Hydrogen is conventionally stored in high pressure gas cylinders and, as a liquid phase at low temperatures, in opened tanks. These methods present several economic and security problems. So, hydrogen storage in liquid or solid carriers is a suitable method for future applications. Hydrogen absorption and desorption in metal and complex hydrides will be discussed. Examples are provided, including the role of additives in promoting hydrogen sorption reactions. Some case studies using metal hydrides as hydrogen carrier are presented. The HyCARE project, focussed on the development of an efficient metal hydride-based system for the storage of renewables energies is presented, giving evidence of about 50 kg of hydrogen stored in metal hydrides. A small-scale hydrogen refuelling station developed to provide hydrogen for a fuel cell driven drone will be described. The Life Cycle Assessment (LCA) methodology to evaluate the environmental impacts associated with developed systems is also shortly described. Finally, main open challenges will be outlined, suggesting possible approaches for their overcoming.

State of the Art in Development of Heat Exchanger Geometry Optimization and Different Storage Bed Designs of a Metal Hydride Reactor.

The efficient operation of a metal hydride reactor depends on the hydrogen sorption and desorption reaction rate. In this regard, special attention is paid to heat management solutions when designing metal hydride hydrogen storage systems. One of the effective solutions for improving the heat and mass transfer effect in metal hydride beds is the use of heat exchangers. The design of modern cylindrical-shaped reactors makes it possible to optimize the number of heat exchange elements, design of fins and cooling tubes, filter arrangement and geometrical distribution of metal hydride bed elements. Thus, the development of a metal hydride reactor design with optimal weight and size characteristics, taking into account the efficiency of heat transfer and metal hydride bed design, is the relevant task. This paper discusses the influence of different configurations of heat exchangers and metal hydride bed for modern solid-state hydrogen storage systems. The main advantages and disadvantages of various configurations are considered in terms of heat transfer as well as weight and size characteristics. A comparative analysis of the heat exchangers, fins and other solutions efficiency has been performed, which makes it possible to summarize and facilitate the choice of the reactor configuration in the future.

A metal hydride compressor for a small scale H2 refuelling station

The use of hydrogen as energy vector implies the development of the necessary infrastructures for hydrogen handling. A two-stage metal hydride compressor was developed and integrated in a small-scale hydrogen refuelling station at prototype level. In this work, the compression on site of green hydrogen using metal hydrides is exhaustively presented taking into account all the necessary aspects to bridge the gap between the laboratory and the real application. In particular, all aspects for the setting up of a metal hydride compressor, i.e. selection and characterization of selected metal hydrides, sizing of the plant components, design and the tests aimed to optimize the working performances are presented and deeply discussed, including energy consumption and efficiency necessary to build up a commercial system. The compressor employs two commercial alloys, i.e. a La0.9Ce0.1Ni5 from Labtech in the first stage and the Hydralloy-C5 from GfE in the second one. Working between room temperature for absorption and 150 °C for desorption, the hydrogen produced by the electrolyser at 28 bar is compressed up to 250 bar, resulting in a compression ratio of about 9. The metal hydride compressor has a final power consumption of 614 W, of which 85 W are linked to the hydrogen sorption reactions, while other contributions come from the pumps involved in the plant and the dissipations. The compressor presents an isentropic efficiency of about 11% for less than 1 kg of powder in each stage and an average H2 flowrate of 104 Nl/min is observed. The performances of the plant were optimized and maintained for a working time of about 245 h.

Nanointerface Engineering of Metal Hydrides for Advanced Hydrogen Storage

With global efforts to relieve the formidable impact of climate change, hydrogen is considered a viable replacement for fossil fuels without intermittency concerns of other renewable sources. Hydrogen storage plays a pivotal role in the implementation of hydrogen economy, coupling hydrogen production with fuel cell technologies. Storing hydrogen in the form of solid-state hydride materials has been studied as a future hydrogen storage technology for enabling a safe, energy-efficient, and high-energy-density system. However, hostile thermodynamic and kinetic properties of each hydride material result in insufficient hydrogen storage performance for practical applications, such as sluggish hydrogen absorption or desorption, high dehydrogenation temperatures, and sometimes limited reversibility; thus, these kinetic and thermodynamic characteristics need to be thoroughly understood depending on each hydride material. Among various strategies, nanostructuring has been regarded as a general approach to tackling such limitations regarding thermodynamic and kinetic characteristics of hydride materials. In particular, the formation of nanosized hydrides within a nanostructured scaffold─also known as nanoconfinement─is of great potential for advanced hydrogen storage because it can additionally leverage host–guest interactions at the nanointerfaces of hydride materials and scaffolds. In this context, the active tuning of such nanointerfaces brings about additional thermodynamic or kinetic changes in hydrogen sorption reactions compared to the unmodified nanoconfined hydride composites, holding great promise for tailored strategies for each metal hydride. In this Perspective, we summarize the major thermodynamic and kinetic barriers of each metal hydride and highlight the recent progress in overcoming such limits, mainly focusing on nanointerface engineering in nanoconfined metal hydrides. Further, we provide our insight and current challenges in understanding the underlying mechanisms of the interaction at the nanointerface, whereby the noticeable technological leaps can be emulated in practical systems.

Solid-State Hydrogen Storage Systems and the Relevance of a Gender Perspective

This paper aims at addressing the exploitation of solid-state carriers for hydrogen storage, with attention paid both to the technical aspects, through a wide review of the available integrated systems, and to the social aspects, through a preliminary overview of the connected impacts from a gender perspective. As for the technical perspective, carriers to be used for solid-state hydrogen storage for various applications can be classified into two classes: metal and complex hydrides. Related crystal structures and corresponding hydrogen sorption properties are reviewed and discussed. Fundamentals of thermodynamics of hydrogen sorption evidence the key role of the enthalpy of reaction, which determines the operating conditions (i.e., temperatures and pressures). In addition, it rules the heat to be removed from the tank during hydrogen absorption and to be delivered to the tank during hydrogen desorption. Suitable values for the enthalpy of hydrogen sorption reaction for operating conditions close to ambient (i.e., room temperature and 1–10 bar of hydrogen) are close to 30 kJ·molH2−1. The kinetics of the hydrogen sorption reaction is strongly related to the microstructure and to the morphology (i.e., loose powder or pellets) of the carriers. Usually, the kinetics of the hydrogen sorption reaction is rather fast, and the thermal management of the tank is the rate-determining step of the processes. As for the social perspective, the paper arguments that, as it occurs with the exploitation of other renewable innovative technologies, a wide consideration of the social factors connected to these processes is needed to reach a twofold objective: To assess the extent to which a specific innovation might produce positive or negative impacts in the recipient socioeconomic system and, from a sociotechnical perspective, to explore the potential role of the social components and dynamics in fostering the diffusion of the innovation itself. Within the social domain, attention has been paid to address the underexplored relationship between the gender perspective and the enhancement of hydrogen-related energy storage systems. This relationship is taken into account both in terms of the role of women in triggering the exploitation of hydrogen-based storage playing as experimenter and promoter, and in terms of the intertwined impact of this innovation in their current conditions, at work, and in daily life.

Development of Ca–Mg–H2–ZrCl4 composite for hydrogen storage applications

Both CaH2 and MgH2 are good candidate for the development of hydrogen storage materials because of their high hydrogen storage capacity. However, both the hydrides are quite stable thermodynamically and required high temperature for hydrogen sorption process. The MgH2–CaH2 composite could show the favourable hydrogen sorption reaction because of Ca–Mg intermetallic formation. The idea motivated to perform the experiments starting with these metal hydrides. It has been found that the hydrogen sorption reaction kinetics improved substantially. The dihydrogen product has shown a few intermetallic of magnesium and calcium. The hydrogen sorption temperature and pressure of the alloy was remarkably improved by the doping with ZrCl4 as a catalyst. The activation energy and the thermodynamic parameters of un-catalyzed and catalyzed alloy were studied. Present studied indicated that the CaH2–MgH2–ZrCl4 could be a potential candidate for the mobile hydrogen storage system.

Impact of Surfactants and Stabilizers on Palladium Nanoparticle–Hydrogen Interaction Kinetics: Implications for Hydrogen Sensors

Surfactants and stabilizers are always present on the surfaces of colloidal nanocrystals due to their critical function in promoting selective facet growth and since they are essential to prevent aggregate formation in solution. After synthesis, however, the presence of these molecules on the surface of a nanocrystal is problematic because they potentially significantly alter the nature of the interaction with the environment, which is critical for sensor or catalysis applications. Here, we quantitatively scrutinize this effect experimentally for the four most common stabilizers in Pd nanoparticle synthesis: cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide (TOAB), cetyltrimethylammonium chloride (CTAC), and poly(vinylpyrrolidone) (PVP). We use the surface-catalyzed hydrogen sorption and hydride formation reaction in Pd as a model system, due to its high relevance for hydrogen sensors. Specifically, we map in detail the (de)hydrogenation kinetics of arrays of nanofabricated Pd nanodisks in the presence of the surfactants and benchmark it with an uncoated Pd reference. As the key results, we find that the cationic surfactants significantly decelerate the (de)hydrogenation surface reaction, with the amplitude of deceleration mediated by the interplay between the halide-ion–Pd surface interaction strength and surfactant surface density. In contrast, a polymeric PVP coating is found to significantly accelerate hydrogen sorption. For the Pd-based hydrogen sensor application, our findings thus provide important insights for the appropriate choice of a surfactant to minimize the negative impact on hydrogen sorption kinetics and thus hydrogen detection response/recovery times. In a wider perspective, our results dramatically show how nanoparticles can attain different properties depending on what types of surfactants and stabilizers are present on their surface and how critical the quantitative understanding of their impact is for a specific application.

Cover Feature: Unconventional Approaches to Hydrogen Sorption Reactions: Non‐Thermal and Non‐Straightforward Thermally Driven Methods (ChemPhysChem 10/2019)

Cover Feature: Unconventional Approaches to Hydrogen Sorption Reactions: Non‐Thermal and Non‐Straightforward Thermally Driven Methods (ChemPhysChem 10/2019)

Influence of Defects on the Stability and Hydrogen-Sorption Behavior of Mg-Based Hydrides.

This review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH2 based materials. By providing an insight on metal-hydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH2 stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH2 that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well-defined concentration of defects (Frankel pairs) at the surface and sub-surface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball-milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg- and Mg2 Ni-based alloys: Equal-Angular-Channel-Pressing and Fast-Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H-sorption kinetics have been improved satisfactorily without any additive for mass H-storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast-Forging was used for a "quasi-instantaneous" synthesis of Mg/Mg2 Ni-based composites. Hydrogenation of the as-received almost bi-phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg2 Ni on one side to MgH2 /Mg2 NiH4 on the other side via the rather stable a-Mg2 NiH0.3 , acting as in-situ catalyser.

Unconventional Approaches to Hydrogen Sorption Reactions: Non-Thermal and Non-Straightforward Thermally Driven Methods.

In the last decades, a broad family of hydrides have attracted attention as prospective hydrogen storage materials of very high gravimetric and volumetric capacity, fast H2 -sorption kinetics, environmental friendliness and economical affordability. However, constraints due to their high activation energies of the different H2 -sorption steps and the Gibbs energy of their reaction with H2 has led to the need of high thermal energy to drive H2 uptake and release. High heat leads to significant degradation effects (recrystallization, phase segregation, nanoparticles agglomeration…) of the hydrides. In this context, this short review aims to summarize alternative non-thermal methods and non-straightforward thermally driven methods to overcome the previous constraints. The phenomenology lying behind these methods, i. e. tribological activation, sonication, and electromagnetic radiation, and the effect of these processes on hydrogen sorption properties of hydrides are described. These non-usual approaches could boost the capability of the next generation of solid-hydride materials for hydrogen conversion in energy sector, in mobile devices and as hydrogen reservoirs.

XPS study on the vanadocene-magnesium system to understand the hydrogen sorption reaction mechanism under room temperature

Magnesium surface dope with vanadium has been considered as one of the best material for the hydrogen storage application. Optimization of vanadium concentration doping is important to retain the hydrogen storage capacity above 6 wt %. Vanadocene, bis(η5-cyclopentadienyl) vanadium, with the formula V(C5H5)2, commonly abbreviated as Cp2V is considered as the best precursor for the vanadium to dope the optimized concentration of vanadium over the Mg surface. The vanadium doping over the magnesium has been studied by X-ray photoelectron spectroscopy (XPS) in details. The vanadium doped Mg has been found to be hydrogenated even at 0 °C under nominal hydrogen pressure (above plateau pressure). Besides, the dehydrogenation temperature was also found to be below 200 °C which is remarkable less as compared to pure magnesium, and comparable to the best catalysts for Mg-MgH2 system reported till date.

Influence of high thermal conductivity addition on PCT-isotherms of hydrogen storage alloy

High reaction heat of an interaction between intermetallic compounds and hydrogen along with low thermal conductivity of fine dispersed metal hydride powders lead to crisis phenomena in metal hydride hydrogen storage facilities. Thus investigations of composite materials with improved heat transfer are of great interest for development of hydrogen storage technologies. A composite material was made of powders of activated La0.9Ce0.1Ni5-alloy and copper. PCT-isotherms of hydrogen sorption and desorption was measured and compared for the composite and the pure alloy at temperature of 313, 333 and 353K. Values of ΔH and ΔS of the hydrogen sorption and desorption reactions were calculated, and temperature and pressure relaxation in the pure alloy and the composite material beds were studied. A significant difference between hydrogen storage behaviour of the pure alloy and the composite has been revealed.

Reactive Hydride Composite of Mg2NiH4 with Borohydrides Eutectic Mixtures

The development of materials showing hydrogen sorption reactions close to room temperature and ambient pressure will promote the use of hydrogen as energy carrier for mobile and stationary large-scale applications. In the present study, in order to reduce the thermodynamic stability of MgH2, Ni has been added to form Mg2NiH4, which has been mixed with various borohydrides to further tune hydrogen release reactions. De-hydrogenation/re-hydrogenation properties of Mg2NiH4-LiBH4-M(BH4)x (M = Na, K, Mg, Ca) systems have been investigated. Mixtures of borohydrides have been selected to form eutectics, which provide a liquid phase at low temperatures, from 110 °C up to 216 °C. The presence of a liquid borohydride phase decreases the temperature of hydrogen release of Mg2NiH4 but only slight differences have been detected by changing the borohydrides in the eutectic mixture.

Hydrogen - A sustainable energy carrier

Hydrogen may play a key role in a future sustainable energy system as a carrier of renewable energy to replace hydrocarbons. This review describes the fundamental physical and chemical properties of hydrogen and basic theories of hydrogen sorption reactions, followed by the emphasis on state-of-the-art of the hydrogen storage properties of selected interstitial metallic hydrides and magnesium hydride, especially for stationary energy storage related utilizations. Finally, new perspectives for utilization of metal hydrides in other applications will be reviewed.

Thermodynamic Characterization of the Hydrogen Sorption Reaction of Lithium Silicides

In the modern society new energy storage systems have become a basic need not only as a mobile energy source but also in the context of the global energy transition. Currently, the dominating anode material in lithium ion batteries is Graphite. It´s main drawback is the limited storage capacity and the low energy density (372 mAh g-1). Therefore, Silicon seems to be an attractive alternative for the next generation anode material. It offers an extremely large theoretical specific energy density (Li17Si4: 4054 mAh g-1) and low discharge potentials. Furthermore, lithium silicides show high reversible hydrogen storage capacity and may be used as safe hydrogen storage material [1,2]. For the application of lithium silicides in lithium ion batteries fundamental knowledge of the physicochemical properties of the various phases is essential. Especially accurate thermodynamic data is crucial for the understanding of the Li-Si phase diagram, phase transitions and electrochemical equilibria as well as for any rational design of materials for battery applications. Numerous investigations of the binary Li-Si system were reported in literature, however, only few of them provided reliable thermodynamic data. Therefore, we recently reported experimental heat capacities and entropies of the five lithium silicides Li17Si4, Li16.42Si4, Li13Si4, Li7Si3 and Li12Si7 [1,3]. For thermodynamic calculations the enthalpies of formation are also required but the uncertainty of available data in literature is about 10 %, which is unacceptable for reliable equilibrium calculations. Motivated by this situation we recently focused our activity on the accurate determination of the standard enthalpies of formation ΔFH°(298K) of the lithium silicides Li17Si4, Li13Si4, Li7Si3 and Li12Si7. For this purpose, the hydrogen absorption and desorption equilibria at three different temperatures (450°C, 475°C and 500°C) were investigated using a Sieverts type apparatus. Two different sample types were used for the measurements: (i) a stoichiometric mixture of lithium hydride and silicon and (ii) pure silicide. The determination of the enthalpy of formation was performed by applying the van´t Hoff relation. This approach resulted in uncertainties of at least 10 %. This unsatisfying result directed our attention to an alternative evaluation method. Combining the hydrogen equilibrium pressure peq(H2) from the sorption measurements with our precise heat capacity and entropy data of the lithium silicides finally provided values for the standard enthalpies of formation with an error of less than 2 %. Utilizing the complete set of experimental thermodynamic data, the Li-Si phase diagram can now be calculated (CALPHAD) with excellent accuracy. [1] D. Thomas, M. Zeilinger, D. Gruner, R. Hüttl, J. Seidel, A. U. Wolter, T. F. Fässler, F. Mertens, The Journal of Chemical Thermodynamics 2015, 178–190. [2] M. T. McDowell, S. W. Lee, W. D. Nix, Y. Cui, Adv. Mater. 2013, 25, 4966–4985. [3] D. Thomas, M. Abdel-Hafiez, T. Gruber, R. Hüttl, J. Seidel, A. U. B. Wolter, B. Büchner, J. Kortus, F. Mertens, J. Chem. Thermodyn. 2013, 64, 205–225.

Thermodynamic Assessment of the Hydrogen Sorption Reaction in the System Li-Si-H

Recently, extensive studies using silicon anodes in lithium ion batteries (LiB) have shown that the Li-Si-system is an attractive alternative to the currently dominating graphite anode in view of costs and specific capacity. Currently the main obstacle of silicon is the poor charge/discharge cycling stability and capacity fading due to the large volume variation during charging/discharging. However, remarkable progress has recently been achieved by using thin films of silicon or silicon nanowires to relieve the mechanical stress [1]. It is indisputable that future progress in lithium silicide applications will require fundamental knowledge of the physicochemical properties of the various lithium silicide phases. Especially accurate thermodynamic quantities are crucial for the understanding as well as modelling of phase, reaction and electrochemical equilibria. In contrast to the numerous phase diagram studies reported in the literature, only few reliable thermodynamic quantities were available. Motivated by this situation, we recently reported experimental heat capacities and entropies of the lithium silicides Li17Si4, Li16.42Si4, Li13Si4, Li7Si3 and Li12Si7 [2,3]. Presently, the standard molar enthalpies of formation of the lithium silicides seem to be one of the most unreliable fundamental data because available values scatter by about 10 % which is unacceptable in view of reliable thermodynamic equilibrium calculations. This contribution is focused on the accurate experimental determination of the standard enthalpies of formation DFH°(298) of the lithium silicides Li17Si4, Li13Si4, Li7Si3 and Li12Si7. For this purpose the hydrogenation/dehydrogenation equilibria in the system Li-Si-H were investigated at three different temperatures (450 K, 475 K, 500 K) using a Sievert’s type apparatus. The measurements based on two different sample types: (i) pure silicide and (ii) a stoichiometric mixture of lithium hydride and silicon. The calculation of the standard enthalpies of formation by using the slopes of the classical van’t Hoff plots results in uncertainties of at least 10 %. Due to this unsatisfying large error, an alternative evaluation method is demonstrated linking the hydrogen equilibrium pressures peq(H2) from the sorption measurements with our precise heat capacity and entropy data of the lithium silicides. The obtained values for the standard enthalpies of formation show strongly reduced uncertainties of less than 2%. Finally, it will be shown that the phase diagram of the Li-Si-system can now be calculated (CALPHAD) with excellent quality using a complete set of experimental thermodynamic data. [1] M. T. McDowell, S. W. Lee, W. D. Nix, Y. Cui, Adv. Mater. 2013, 25, 4966–4985. [2] D. Thomas, M. Abdel-Hafiez, T. Gruber, R. Hüttl, J. Seidel, A. U. B. Wolter, B. Büchner, J. Kortus, F. Mertens, J. Chem. Thermodyn. 2013, 64, 205–225. [3] D. Thomas, M. Zeilinger, D. Gruner, R. Hüttl, J. Seidel, A. U. Wolter, T. F. Fässler, F. Mertens, The Journal of Chemical Thermodynamics 2015, 178–190.

ChemInform Abstract: Surface Reactions Are Crucial for Energy Storage

AbstractReview: 43 refs.

Hybrid aqueous capacitors with improved energy/power performance

This work reports on a high-voltage, hybrid capacitor involving two separate redox reactions. Aqueous solutions of Mg(NO3)2 and KI have been used for negative and positive electrode, respectively. Adjusting pH=2 for electrode (+) with KI solution and modifying Mg(NO3)2 solution to pH=9 for negative side play a crucial role for a stable long-term operation of capacitor at enhanced voltage. A benefit from such a construction is a pseudocapacitive contribution from hydrogen sorption reaction on the negative electrode and high iodine/iodide activity on the positive electrode, enhancing the energy with no remarkable impact on the power profile. Proposed solution allows a high voltage (1.8V) to be reached and thereby high power and energy performance (~20Wh/kg at 1kW/kg) to be obtained. High long-term stability has been confirmed by floating and galvanostatic tests.

Surface Reactions are Crucial for Energy Storage

Reactions between gas molecules, e.g. H2 and CO2 and solids take place at the surface. The electronic states and the local geometry of the atomic arrangement determine the energy of the adsorbate, i.e. the initial molecule and the transition state. Here we review our research to identify the surface species, their chemical state and orientation, the interaction with the neighbouring molecules and the mobility of the adsorbed species and complement the experimental results with thermodynamic modelling. The role of the Ti was found to be a bridge between the charged species preventing the individual movement of the ions including charge separation. The Ti has no catalytic effect on the hydrogen sorption reaction in borohydrides. The physisorption of molecular hydrogen is too weak at ambient temperature to reach a significant hydrogen storage density. The addition of a hydrogen dissociation catalyst to a nanoporous material with a large specific surface area may potentially enable the spillover of hydrogen atoms from the metal catalyst to the surface of the porous material and chemisorb on specific sites with a much higher binding energy compared to physisorption. The intercalation of alkali metals in C60 fullerenes increases the interaction energy of hydrogen with the so-called metal fullerides significantly. Sterical diffusion barriers by partial oxidation of the surface of borohydrides turned out to redirect the reaction path towards pure hydrogen desorption and suppress the formation of diborane, a by-product of the hydrogen evolution reaction from borohydrides previously undetected. The combination of a newly developed gas controlling system with microreactors allows us to investigate complex reactions with small quantities of nano designed new catalytic materials. Furthermore, tip-enhanced Raman spectroscopy (TERS) will allow the investigation of the reactions locally on the surface of the catalyst and the near ambient pressure photoelectron spectroscopy enables analysis of the surfaces in ultra-high vacuum and in situ interaction with the adsorbates i.e. while the reaction takes place. This brings us in a unique position for the investigation of the heterogeneous reactive systems. The mechanism of the Ti catalysed hydrogen sorption reactions in alanates was recently established based on spectroscopic investigations combined with thermodynamic analysis of the transition states.

In-situ synchrotron X-ray diffraction investigation of the hydriding and dehydriding properties of a cast Mg–Ni alloy

In previous research, it has been demonstrated that the hydrogenation kinetics of Mg–Ni alloys is significantly improved by elemental Na doping. To clarify the underlying mechanisms in terms of crystallographic phase changes during the reactions, in-situ hydrogen absorption and desorption as a function of temperature under constant hydrogen atmospheres of 2MPa and 0.2MPa, respectively, was studied. The experiments were performed at the Powder Diffraction beamline of the Australian Synchrotron facility with a gas flow cell and a hot air blower. This study showed that the hydrogenation of Na-doped Mg–Ni alloys can be identified at a temperature as low as 260°C via an interface-controlled nucleation and growth mechanism without any prior activation whilst the dehydrogenation occurred at about 370°C. The sequence of phase transformations associated with these reactions as well as associated expansion properties of individual phases during hydrogen sorption reactions as provided by the high resolution X-ray diffraction data are discussed.