Hydrogen in novel solid-state metal hydrides

The transportation demands in our growing hydrogen (H2) economy requires robust storage systems. The current commercially implemented technology in fuel cell cars relies on the well-established technology of hydrogen gas compressed to 350–700 bar, depending on the application. The compressed gas tanks in use today are bulky and cost intensive. To address this challenge, material-based storage is one of the long-term alternatives considered and constitutes the focus of this talk.Material-based storage is broadly defined as hydrogen bound to solid materials, with its binding strength varying from physisorption to porous materials, such as zeolites and metal organic frameworks, to chemisorption in (complex) metal hydrides. The ultimate targets set by the U.S. Department of Energy for this technology include a system gravimetric capacity of 6.5 wt% and volumetric capacity of 40 g/L at 100 bar, operating temperatures ranging between -40 C and +40 C and adsorption/desorption timescales of < 5 min. Storage in the form of physisorbed or chemisorbed hydrogen has guided the materials research, which metal-organic framework and (complex) metal hydrides being the most promising materials classes. A variety of these materials have met one or more of the targets, but it has remained elusive for a single material system to meet all these stringent requirements. Nano-encapsulation and low-concentration chemical additives have previously been employed separately to overcome such challenges. Functionalization via atomic layer deposition (ALD), however, offers unique characteristics that make it suitable for both, nano-encapsulation and “doping” with low-concentration additives. This deposition technique has sub-monolayer thickness control, is highly conformal in high-surface area materials and is self-limiting, i.e., once the gas-phase precursor reacts with the available surface sites, the surface reactions stop.In this presentation, we will showcase examples where ALD, more generally vapor-phase functionalization, on (complex) metal hydrides and organic frameworks has improved the material properties for H2 storage. In our first study, Al2O3 was deposited on magnesium borohydride, Mg(BH4)2, at room temperature using trimethylaluminum (TMA) and water. From our findings, encouraging initial results were obtained: the H2 desorption temperature was lowered by 60–120 ˚C, the desorbed gravimetric H2 capacity at temperatures < 250 ˚C was doubled, and the desorption kinetics increased by a factor of ~6 compared to uncoated Mg(BH4)2. However, hydrolysis reactions caused by residual surface -OH groups from the ALD water-pulse degraded the sample substantially. Through this study, the use of TMA was observed to be highly reactive with the Mg(BH4)2 surface species, leading to the strategy of exposing the sample only to TMA. The relative mole fraction of the vapor-phase additive can be precisely controlled with the number of TMA pulses and pulse duration. By tuning these parameters, we show in our second study that 10 pulses of TMA at ambient conditions were able to decrease the H2 desorption temperature by ~100 ˚C while retaining <95 % of its H2 capacity. We applied this approach with other additives such as BBr3, TiCl4 and tetrahydrofuran, and demonstrate that this unique approach opens the door to a new class of molecular additives and catalysts which cannot easily be introduced with conventional mechano-chemical or solvent-based techniques for (complex) metal-hydrides. In the case of metal organic frameworks, ALD is a promising technique to functionalize the pores with metal atoms or functional groups able to tune the gas selectivity and binding energy, for which ~15 kJ/mol has been established as the optimal value for H2 storage in sorbent materials.Vapor-phase techniques such as ALD have numerous benefits over other functionalization tools for H2 storage materials opening innumerable opportunities to the field. To exploit these opportunities, the current limitations on room temperature and water-less ALD processes needs to be overcome which will greatly expand the possibilities of encapsulation and incorporation of additives for organic frameworks and (complex) metal hydrides.

Metal hydrides are considered a promising means of hydrogen storage mainly because they store hydrogen at low pressure while having volumetric densities comparable to liquid hydrogen (Reilly, 1979). Furthermore, the release of hydrogen in metal hydrides being an endothermic process, this makes them inherently safe. In these lectures, we will discuss how the hydrogen reacts with metals. More specifically, we will see how hydrogen diffuse into the metal and forms a solid solution or a metal hydride. The first lecture treats the basic phenomena of hydrogen-metal systems such as solid solution of hydrogen in metals and formation of metal hydrides with particular emphasis on their structure and thermodynamics. The location of hydrogen in the crystal structure of the host metal will also be reviewed. In the second lecture, the effect of alloying, the electronic structure of metal hydride alloys and kinetics of hydrogen in metals will be studied. In the third lecture, we will see three specific metal hydrides; palladium, niobium and magnesium. Finally, the last lecture will have for object the nanocomposite system Mg-Nb and the possible hydrogen desorption mechanism of this system.

Application of machine learning to thermal management of solid-state hydrogen storage: A comprehensive review

HyCARE project, supported by the Clean Hydrogen Partnership of the European Union, deals with a prototype hydrogen storage tank using a solid-state hydrogen carrier. Up to 40 kilograms of hydrogen are stored in 12 tanks at less than 50 barg and less than 100&deg;C. The innovative design is based on a standard 20-foot container including 12 TiFe-based metal hydride (MH) hydrogen storage tanks, coupled with a thermal energy storage in phase change materials (PCM). This article aims at showing the main risks related to hydrogen storage in a MH system and the safety barriers considered, based on HyCARE&rsquo;s specific risk analysis. Regarding the TiFe MH material used to store hydrogen, experimental tests showed that the exposure of the MH to air or water did not cause spontaneous ignition. Furthermore, an explosion within the solid MH cannot propagate due to internal pore size. Additionally, in case of leakage, the speed of hydrogen desorption from the MH is self-limited, which is an important safety characteristic since it reduces the potential consequences from the hydrogen release. Regarding the integrated system, the critical scenarios identified during the risk analysis were explosion due to release of hydrogen inside or outside the container, internal explosion inside MH tanks due to accidental mix of hydrogen and air and asphyxiation due to inert gas accumulation in the container. The identification phase of risk analysis identified the most relevant safety barriers already in place and recommended additional ones, if needed, which were later implemented to further reduce the risk. The main safety barriers identified were material and component selection (including the MH selected), safety interlocks, safety valves, ventilation, gas detection and safety distances. The risk management process, based on risk identification and assessment, contributed to coherently integrate inherently safe design features and safety barriers.

Transition metal hydrides (MH) are an attractive class of materials for several energy technologies. Primary benefits include their large volumetric storage capacity (often exceeding that of liquid hydrogen) and capability to absorb and desorb hydrogen for hundreds of cycles. In this thesis, we set out to understand two of the thermodynamic inefficiencies of MH: the pressure hysteresis associated with hydrogen absorption and desorption and the corrosion and dissolution of high capacity MH alloys in high pH electrolyte environments. The volume change associated with hydriding transitions can exceed 10%, and a macroscopic nucleation barrier resulting from coherency strains has been proposed as the origin of the pressure hysteresis. We investigated this hypothesis for the palladium-hydrogen system. The hysteresis and phase transformation characteristics of bulk and nanocrystalline PdH were characterized with coupled in situ X-ray diffraction and pressure composition isotherm measurements. Size effects are observed in the total hydrogen uptake and hydrogen solubility in the hydride phases. Experimentally determined hysteresis energies were found to be comparable to the misfit strain between the Pd and PdH phases and much larger than the energy for dislocation formation. Theoretical predictions of pressure hysteresis overestimate the experimentally measured hysteresis, and we suggest methods of accommodation which could explain the discrepancy. Finally, we propose that an effect of the nucleation barrier is to split the coherent spinodal phase diagram and introduce directionally dependent phase boundaries. We report a successful development of Ti29V62-xNi9Crx (x = 0, 6, 12) body-centered cubic (BCC) MH electrodes for MH batteries by addressing vanadium corrosion and dissolution in potassium hydroxide electrolytes. The effectiveness of a limited oxygen environment and vanadate ion addition against corrosion are compared to the effects of Cr substitution. By identifying oxygen as the primary source of corrosion and eliminating oxygen with an Ar-purged cell, the Cr-free alloy electrode achieved a maximum capacity of 594 mAh/g, double the capacity of commercial AB5 MH electrodes. With modified coin cells suppressing oxygen evolution, the cycle stability of the Ti29V62Ni9 alloy electrode was greatly improved with either vanadate ion additions to the electrolyte or Cr-substitution in the alloy. Both approaches lead to reversible capacity of 500 mAh/g for 200 cycles.

A general trend for the hydrogenation reactivity of metal hydride(s): 3c–4e bridging hydride &gt; 2c–2e terminal hydride &gt; 3c–2e bridging hydride.

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

The widespread implementation of the hydrogen economy demands access to materials with a high weight percentage of hydrogen. Mg-based hydrogen storage alloys have become a research hot-spot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, high decomposition temperature of Mg-based hydrideslimits their practical usage. Hydrogen is lightest of all elements, typically it need large volumes or high pressures to store appreciable amount of hydrogen.To overcome these challenges research activities are currently going on metal hydrides and complex hydrides based on transition metal. The main challenges come when endothermic decomposition enthalpy is considered. It becomes difficult to release the hydrogen at ambient thermodynamic conditions. On the other hand, the meta-stable hydrides are characterized by a low reaction enthalpy and a decomposition reaction that is thermodynamically favourable under the ambient conditions. The good kinetics along with evolution of hydrogen around the room temperature possessed by these materials offer much promise for underground storage and it can be used as a grid energy.The present research specifically focus on transition metal based complex hydrides such as NaMgMnH6 and NaMgFeH6 for hydrogen storage and especially predicted the crystal structure of the same using VASP simulation package. ABCX6 and AB2X6 were taken as reference composition A,B,C and X were substituted with Na, Mg, transition metals(Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and H, respectively.From the ICSD database search the corresponding structural variants were considered as trial structure to predict the ground state structure of these newly designed compounds. using the predicted ground state structure for these compounds, important properties like formation energy, hydrogen site energy, partial density of states(pDOS), electron density(ED), Mulliken effective charge(MEC), Bader electron charge(BEC), Born effective charge, ELF and COHP were calculated. the calculated properties of these compounds were compared with the well known reference compounds. We found that both NaMgMnH6 and NaMgFeH6 are stable with negative formation energy and als othey are semiconductors with the band gap value of 0.51eV and 1.01 eV, respectively. from the calculated electronic structure we found that both these materials are having indirect band behaviour. Analysing the electronic density of states, charge density and various charges mentioned above the chemical bonding behaviour was established as iono-covalent in nature. from the force as well as stress minimisation considering 53 structural variants as input the ground state structure, its space group, equilibrium lattice parameters and atom positions are listed. Also from the energy vs. volume curve for the ground state structure, the equilibrium volume, bulk modulus and pressure derivatives were found out. Among NaMgMnH6 and NaMgFeH6 our spin polarised calculation show that NaMgMnH6 is possesing spontaneous magnetic polarisation with substantial magnetic moment at the Mn site. Among the considered paramagnetic, ferromagnetic, as well as A-, C-,and G- antiferromagnetic configurations the G type antiferromagnetic configuration is found to be the ground state for NaMgMnH6.

Metal hydride (MH) actuators have a simple structure and a number of features that make them attractive for use in rehabilitation engineering and assistive technology. The MH actuator provides a high power-to-weight ratio, high-strain actuation, human-compatible softness, and noiseless operation, while being environmentally benign. On the other hand, there remain technical challenges to be overcome to improve the MH actuator regarding its speed of operation and energy efficiency, given the low heat conductivity of the MH powder that is used as the pressure generator for soft actuation. To overcome the issues of low heat conductivity and the handling of MH powder, we developed an MH paper, which is a special paper incorporating MH powder and carbon fiber, for use as a new pressure-generating element for a soft MH actuator system. In addition, the basic properties and structure of the proposed MH paper were investigated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and several thermodynamic experiments. The results of these experiments showed that the hydrogen absorption and desorption rates of the MH paper were significantly higher than those of the MH powder around room temperature.

Capturing multiphysics effects in hydride moderated microreactors using MARM

Because of its low density, storage of hydrogen in the gaseous and liquids states possess technical and economic challenges. One practical solution for utilizing hydrogen in vehicles with proton-exchange fuel cells membranes is storing hydrogen in metal hydrides. Magnesium hydride (MgH2) remains the best hydrogen storage material due to its high hydrogen capacity and low cost of production. Due to its high activation energy and poor hydrogen sorption/desorption kinetics at moderate temperatures, the pure form of MgH2 is usually mechanically treated by high-energy ball mills and catalyzed with different types of catalysts. These steps are necessary for destabilizing MgH2 to enhance its kinetics behaviors. In the present work, we used a small mole fractions (5 wt.%) of metallic glassy of Zr70Ni20Pd10 powders as a new enhancement agent to improve its hydrogenation/dehydrogenation behaviors of MgH2. This short-range ordered material led to lower the decomposition temperature of MgH2 and its activation energy by about 121 °C and 51 kJ/mol, respectively. Complete hydrogenation/dehydrogenation processes were successfully achieved to charge/discharge about 6 wt.%H2 at 100 °C/200 °C within 1.18 min/3.8 min, respectively. In addition, this new nanocomposite system shows high performance of achieving continuous 100 hydrogen charging/discharging cycles without degradation.

Electrocatalytic dechlorination of 2,4-dichlorophenoxyacetic acid using nanosized titanium nitride doped palladium/nickel foam electrodes in aqueous solutions

The main ways measuring the parameters that characterize processes of the gas-phase and electrochemical hydrogenation of intermetallic compounds—promising materials for metal hydride hydrogen tanks and chemical power sources—are listed and discussed. The general principles of the mathematical treatment of pertinent experimental data and ways of determining the effective diffusion coefficient of hydrogen in metal hydrides are described. It is shown that sorption/desorption isotherms can be constructed for metal−hydrogen systems on the basis of electrochemical measurements.

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.

The recent advances on the effects of microstructural refinement and various nano-catalytic additives on the hydrogen storage properties of metal and complex hydrides obtained in the last few years in the allied laboratories at the University of Waterloo (Canada) and Military University of Technology (Warsaw, Poland) are critically reviewed in this paper. The research results indicate that microstructural refinement (particle and grain size) induced by ball milling influences quite modestly the hydrogen storage properties of simple metal and complex metal hydrides. On the other hand, the addition of nanometric elemental metals acting as potent catalysts and/or metal halide catalytic precursors brings about profound improvements in the hydrogen absorption/desorption kinetics for simple metal and complex metal hydrides alike. In general, catalytic precursors react with the hydride matrix forming a metal salt and free nanometric or amorphous elemental metals/intermetallics which, in turn, act catalytically. However, these catalysts change only kinetic properties i.e. the hydrogen absorption/desorption rate but they do not change thermodynamics (e.g., enthalpy change of hydrogen sorption reactions). It is shown that a complex metal hydride, LiAlH4, after high energy ball milling with a nanometric Ni metal catalyst and/or MnCl2 catalytic precursor, is able to desorb relatively large quantities of hydrogen at RT, 40 and 80 °C. This kind of behavior is very encouraging for the future development of solid state hydrogen systems.