Thermochemical transformations in 2MNH2–3MgH2 systems (M = Li or Na)

Thermochemical reactions between alkali metal amides and magnesium hydride taken in 2:3 molar ratios have been investigated using pressure-composition-temperature, X-ray powder diffraction and residual gas analysis measurements. The thermally induced reactions in both title systems are stoichiometric and proceed as a following solid state transformation: 2MNH₂ + 3MgH₂ → Mg₃N₂ + 2MH + 4H₂↑. A total of 6.45 wt.% of hydrogen is released by the 2LiNH₂–3MgH₂ system beginning at 186 °C, and a total of 5.1 wt.% H₂ is released by the 2NaNH₂–3MgH₂ system starting at 130 °C. Combined structure/property investigations revealed that the transformation in the lithium containing system proceeds in two steps. In the first step, lithium amide reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen. In the second step, reaction between Li₂Mg(NH) and MgH₂ leads to the formation of the Mg₃N₂ nitride, lithium hydride and additional gaseous hydrogen. The transformation in the sodium containing system appears to proceed through a series of competing solid state processes with formation of Mg(NH₂)₂ and NaMgH₃ intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH₂–3MgH₂ system at 395 °C.     

Hydrogen-storage property characterization of Mg–15 wt%Ni–5 wt%Fe2O3 prepared by reactive mechanical grinding

Mg–15 wt%Ni–5 wt%Fe₂O₃ (Mg155) was prepared by reactive mechanical grinding (RMG). Mg155 exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after only two hydriding–dehydriding cycles. The activated Mg155 absorbed 5.06 and 5.38 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 12 bar H₂. It desorbed 1.50 and 5.28 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 1.0 bar H₂. The initial hydrogen absorption rate decreased, but the hydrogen desorption rate increased rapidly with an increase in temperature from 563 K to 603 K. The rate-controlling step for the dehydriding reaction in a range from F ? 0.20 to F ? 0.75 is considered to be the chemical reaction at the Mg hydride/α-solid solution interface. The absorption and desorption PCT curves exhibited two plateaus at 573 K. The hydrogen-storage capacity of the activated Mg155 was about 6.43 wt% at 573 K.     

NH3 adsorption and dissociation on a nanosized iron cluster

We employed spin-polarized density functional theory to study the bonding and dissociation of NH₃ and its fragment on a nanosized icosahedral Fe₅₅ cluster. The site-preference investigations, suggest that for NH₃, only the interaction perpendicular to the cluster is favorable (−0.37 eV < B.E.(NH₃) < +0.05 eV). Stable geometries of N and H on the high symmetry adsorption site of Fe₅₅ have been calculated as well. Both of these atoms have similar behavior: only the hollow or top sites are stable. Possible dissociation paths of the NH₃ to atomic nitrogen and hydrogen were identified. The calculated lowest reaction barrier for the overall process is 1.48 eV. The rate limiting step is the first hydrogen removal from the NH₃. Our results suggest that the catalytic activity of iron surfaces towards ammonia-like molecules is enhanched when the metal is in the nanostructured phase.     

The role of differently distributed vanadium nanocatalyst in the hydrogen storage of magnesium nanostructures

Using a glancing angle (co)deposition technique, ∼4.6 at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E_a^a = 35.3 ± 0.9 kJ/mol H₂) and hydrogen desorption (E_a^d = 38.9 ± 0.3 kJ/mol H₂) than the V-decorated Mg sample when deposited at the same deposition angle of θ = 70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples.     

Syntheses of alkali-metal carbazolides for hydrogen storage

Metalorganic hydrides are a new class of hydrogen storage materials. Replacing the H of N–H or O–H functional groups using metal hydrides have been recently reported, which substantially improved the dehydrogenation properties of heteroaromatic organic hydrides by lowering their enthalpies of dehydrogenation (ΔH_d), enabling dehydrogenation at much lower temperatures. Among the reported metalorganic hydrides, lithium carbazolide and sodium carbazolide appear to be the most attractive hydrogen storage/delivery material owing to its high hydrogen capacity (>6.0 wt%) and ideal ΔH_d. Nevertheless, the interaction of carbazole and corresponding metal hydride to form metallo-carbazolide is a multistep process involving intensive ball milling and high temperature treatment, where the interaction was not investigated in detail. In this paper, both alkali metal hydrides and amides were employed to react with carbazole to synthesize corresponding carbazolides, aiming to broaden and optimize the synthetic method and understand the reaction mechanism. Our experimental results showed that around one equivalent of H₂ or NH₃ could be released from the reactions of carbazole and corresponding hydrides or amides, respectively. Instrumental spectroscopic analyses proved that metallo-carbazolides were successfully synthesized from all precursors. It is found that the alkali metal amides (i.e., LiNH₂ and NaNH₂) with stronger Lewis basicities as metal precursors could synthesize the metallo-carbazolides under milder conditions. Furthermore, quasi in situ nuclear magnetic resonance results revealed that alkali metal could replace H (H–N) gradually, donating more electrons to carbazole ring. Additionally, the solubilized alkali cation may unselectively interact with π-electron of aromatic systems of both carbazole molecules and carbazolide anions via electrostatic cation-π interactions.     

Reactive ceramics of CeO2–MOx (M=Mn,Fe, Ni,Cu) for H2 generation by two-step water splitting using concentrated solar thermal energy

The addition of MO_x (M: di- or tri-valent transition metal ion) into cerium dioxide (CeO₂) enhanced the ability of CeO₂ for the oxygen (O₂)-releasing reaction at lower temperature and swift hydrogen (H₂)-generation reaction. CeO₂–MO_x (M=Mn, Fe, Ni, Cu) reactive ceramics having high melting points were synthesized with the combustion method from their nitrates for solar H₂ production. The prepared CeO₂–MO_x samples were solid solutions between CeO₂ and MO_x with the fluorite structure through the X-ray diffractometry measurement. Two-step water-splitting reactions with CeO₂–MO_x reactive ceramics proceeded at 1573–1773 K for the O₂-releasing step and at 1273 K for the H₂-generation step by irradiation of infrared image furnace as a solar simulator. The amounts of O₂ evolved in the O₂-releasing reaction with CeO₂–MO_x increased with an increase in the reaction temperature. The amounts of H₂ evolved in the H₂-generation reaction with CeO₂–MO_x systems except for M=Cu were more than that of CeO₂ system after the O₂-releasing reaction at the temperatures of 1673 and 1773 K. The amounts of H₂ evolved in the H₂-generation reaction with CeO₂–MnO and CeO₂–NiO systems were more than those of CeO₂–Fe₂O₃, CeO₂–CuO and CeO₂ systems after the O₂-releasing reaction at the temperature of 1573 K. The amounts of evolved H₂ after the O₂-releasing reaction at the temperature of 1773 K in cm³ per gram of CeO₂–MO_x were 0.975–3.77 cm³/g. The O₂-releasing reaction at 1673 K and H₂-generation reaction at 1273 K with CeO₂–Fe₂O₃ proceeded with repetition of 4 times stoichiometrically.     

The desorption kinetics of the Mg(NH2)2 + LiH mixture

The kinetics of hydrogen desorption of the Mg(NH₂)₂ + LiH mixture has been studied by measuring desorption rates at various temperatures. A desorption kinetic model based on the Gauss-diffusion equation derived from Fick's second law is proposed to interpret the dehydriding reaction. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are carried out to assist the foundation of the model. Results show that the kinetic model obtained can basically describe the curvature of the experimental data and the dehydriding activation energy can be represented by the diffusion activation energy (104.3 KJ/mol) of H^δ+ in the matrix. These indicate that the dehydrogenation can be described by H^δ+ diffusing through the product layer between reactants. Based on the results, the methods of exploring suitable dopants to create more vacancies in the matrix and activating the N–H with an electromagnetic field are suggested to improve the desorption kinetics.     

Bimetallic NixCo10-x/CeO2 as highly active catalysts to enhance mid-temperature ammonia decomposition: Kinetics and synergies

Catalytic ammonia decomposition is an attractive method to generate hydrogen at mid-temperatures (<700 °C) but must incorporate precious metals (Pd, Ru, etc.) to ensure high reactivity. Developing Ni-based catalysts to decompose ammonia can enhance its prospect for hydrogen generation. However, the catalytic activity of Ni is hardly satisfactory at mid-temperatures. In this work, we show the bimetallic Ni_xCo_10-x/CeO₂ towards mid-temperature NH₃ decomposition, with the metal loading of Ni and Co tuned. Kinetics study demonstrates that the NH₃ decomposition reaction follows the Temkin Pyzhev mechanism and the synergy between Ni and Co can decrease the reaction orders regarding NH₃ and increase the reaction orders regarding H₂. Mechanistic results indicate that the recombinative N desorption limits the reaction rate. The synergy between Ni and Co can simultaneously decrease the energy barriers of the recombinative N desorption and mitigate the H₂ poisoning effect. Therefore, Ni_7.5Co_2.5/CeO₂ displays both high ammonia conversion (96.96%) and hydrogen formation rate (1947.9 mmol/(g_cat.h)) at 650 °C. We hope the mechanism in this work can be used to guide the design of inexpensive catalysts to decompose ammonia at mid-temperatures.     

Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling

This work deals with the assessment of the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. Samples are prepared from Mg_xFe (x: 2, 3 and 15) elemental powder mixtures via low energy ball milling under hydrogen atmosphere at room temperature. The highest yield is obtained with Mg₁₅Fe after 150 h of milling (90 wt% of MgH₂). The thermodynamic characterization carried out between 523 and 673 K shows that the obtained Mg–Fe–H hydride systems have similar thermodynamic parameters, i.e. enthalpy and entropy. However, in equilibrium conditions, Mg₁₅Fe has higher hydrogen capacity and small hysteresis. In dynamic conditions, Mg₁₅Fe also shows better hydrogen capacity (4.85 wt% at 623 K absorbed in less than 10 min and after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability in comparison to the other studied compositions. From hydrogen uptake rate measurements performed at 573 and 623 K, the rate-limiting step of the hydrogen uptake reaction is determined by fitting particle kinetic models. According to our results, the hydrogen uptake is diffusion controlled, and this mechanism does not change with the Mg–Fe proportion and temperature.     

Crystal structure and cyclic properties of hydrogen absorption–desorption in Pr2MgNi9

We investigated the crystal structure and cyclic hydrogen absorption–desorption properties of Pr₂MgNi₉. The structural model is based on the PuNi₃-type structure; the Mg atom is assumed to substitute for the Pr site in an MgZn₂-type cell. The refined lattice parameters were determined from X-ray diffraction. A wide plateau region was observed in the P–C (pressure composition) isotherm at 298 K. The maximum hydrogen capacity reached 1.12 H/M (1.62 mass%) under a hydrogen pressure of 2.0 MPa. After 1000 hydrogen absorption–desorption cycles, the hydrogen capacity was superior to that of LaNi₅ (82%). Anisotropic lattice strain occurred in the hydriding process. The anisotropic peak-broadening vector was determined to be <001>. The calculated anisotropic lattice strains of the initial cycle and after 1000 cycles were far smaller than those of LaNi₅.     

Improved hydrogen storage properties of LiBH4 doped Li–N–H system

Remarkable improvement of hydrogen sorption properties of Li–N–H system has been obtained by doping with a small amount of LiBH₄. The starting and ending temperatures of hydrogen desorption shift to lower temperatures and the release of NH₃ is obviously restrained by 10 mol% LiBH₄ doping. The kinetics of hydrogen desorption and absorption of Li–N–H system became faster by the addition of LiBH₄. About 4 wt.% H₂ can be released within 30 min and ∼4.8 wt.% H₂ can be reabsorbed within 2 min by LiBH₄ doped sample at 250 °C, while only 1.44 wt.% H₂ is released and 2.1 wt.% is reabsorbed for pure Li–N–H system. The quaternary hydride (LiNH₂)_x(LiBH₄)_(1−x) formed by the reaction between LiBH₄ and LiNH₂ may contribute to the enhancement of the hydrogen sorption performances by yielding a ionic liquid phase and transferring LiNH₂ from solid state to molten state with a weakened N–H bond.     

Synthesis and decomposition reactions of metal amides in metal–N–H hydrogen storage system

The synthesis and decomposition properties of some metal amides M(NH₂)_x such as LiNH₂, NaNH₂, Mg(NH₂)₂ and Ca(NH₂)₂ were investigated, which play important roles for designing a new family of metal–N–H hydrogen storage systems. Both the gas chromatographic examination and X-ray diffraction measurement indicated that the reaction between alkali or alkaline earth metal hydride MH_x (such as LiH, NaH, MgH₂ and CaH₂) and gaseous NH₃ could quickly proceed at room temperature by ball milling and the corresponding metal amides were easily synthesized in high quality. The kinetics of these kind of reactions is faster in the order of NaH > LiH > CaH₂ > MgH₂, which is consistent with the inverse order of electronegativity of those metals, i.e. Na < Li = Ca < Mg. The thermal decomposition properties indicated that both Mg(NH₂)₂ and Ca(NH₂)₂ decomposed and emitted NH₃ at lower temperature than LiNH₂.     

The effects of ball milling and nanometric nickel additive on the hydrogen desorption from lithium borohydride and manganese chloride (3LiBH4 + MnCl2) mixture

A mixture of [3LiBH₄ + MnCl₂] was processed by high energy ball milling in ultra-high purity hydrogen gas for 0.5 and 1 h. The XRD patterns of milled powders show the sole diffraction peaks of LiCl. The reaction occurring during milling of [3LiBH₄ + MnCl₂] seems to have all characteristics of the metathesis-type reactions occurring between borohydrides (LiBH₄ and NaBH₄) and metal chlorides (MCl_n) induced in a solid state by a mechano-chemical activation synthesis (MCAS). Under pressure of 0.1 MPa H₂ (atmospheric) the ball milled [3LiBH₄ + MnCl₂] mixture is able to desorb ∼4.0 wt.% H₂ at 100 °C within 21,000 s and ∼4.5 wt.% H₂ at 120 and 200 °C within 8000 s and 4000 s, respectively. The addition of n-Ni with SSA = 60.5 m²/g allows desorption of ∼3.7wt.%H₂ within 8,700 s at 100 °C. This is one of the highest H₂ desorption capacities obtained for a complex hydride at 100 °C under atmospheric pressure of H₂ taking into account the fact that the microstructure contains some amount of a useless LiCl constituent. The activation energy of hydrogen desorption for a ball milled undoped [3LiBH₄ + MnCl₂] is ∼102 kJ/mol and ∼98 and 92 kJ/mol after doping with 5 wt.% of nanometric Ni having specific surface area (SSA) of 9.5 and 60.5 m²/g, respectively. After volumetric desorption from 100 to 450 °C the XRD patterns show only LiCl. The n-Ni additive slightly lowers the total quantity of desorbed H₂. Re-absorption tests, under pressure of 10 MPa H₂ at 200 °C, show that the system is, most likely, irreversible. Flammability studies show that the ball milled [3LiBH₄ + MnCl₂] mixture can be ignited by scraping the cylinder walls with a metal tool as well when it is thrown and dispersed in air in a powder form. It also reacts violently in contact with water and a nitric acid.     

Compressed hydrogen production via reaction between liquid ammonia and alkali metal hydride

Ammonia NH₃ is recognized as one of the attractive hydrogen H₂ carriers because it has a high hydrogen content of 18 mass% and it is easily liquefied under about 1 MPa of pressure at a room temperature. NH₃ can react with alkali metal hydrides and generate H₂ even at room temperature, resulting that metal amides are formed as reaction products. The H₂ generation is exothermic reaction, and it is not effectively prevented by H₂ partial pressure in a closed system as thermodynamic properties. In this work, we demonstrated the production of compressed H₂ by the reaction between liquid NH₃ and lithium hydride LiH in a closed pressure vessel, where liquid NH₃ would realize better kinetic properties for the reaction with metal hydride than gaseous NH₃. Actually, more than 12 MPa H₂ was obtained within several hours.     

Effect of cations (Na, Ca, Fe) on the conductivity of a Nafion membrane

It is known that trace amounts of cations have a detrimental effect on the liquid-phase conductivity of perfluorosulfonated membranes at room temperature. However, the conditions used were very different from typical fuel cell conditions. Recent research has shown the impact of conductivity measurement conditions on NH₄⁺ contaminated membranes. In this study, the impact of nonproton-containing cations (Mⁿ⁺ = Na⁺, Ca²⁺, and Fe³⁺) on Nafion membrane (N-211) conductivity was investigated both in deionized (DI) water at room temperature (∼25 °C) and in the gas phase at 80 °C under conditions similar to in a PEMFC. These conductivities were compared with those of Nafion membranes contaminated with NH₄⁺ ions. Under the same conditions, the conductivity of a metal cationic-contaminated membrane having the same proton composition (yH⁺m) was similar, but slightly lower than that of an NH₄⁺-contaminated membrane. The conductivity in the purely H⁺-form of N-211 was more than 12 times greater than the Mⁿ⁺-form form at 25 °C in DI water. At 80 °C, the gas-phase conductivity was 6 times and 125 times greater at 100%RH and 30%RH, respectively. The quantitative results for conductivity and activation energy of contaminated membranes under typical fuel cell conditions are reported here for the first time.     

Hydrogen release by thermolysis of ammonia borane NH3BH3 and then hydrolysis of its by-product [BNHx]

Ammonia borane (AB) is one of the most attractive hydrides owing to its high hydrogen density (19.5 wt%). Stored hydrogen can be released by thermolysis or catalyzed hydrolysis, both routes having advantages and issues. The present study has envisaged for the first time the combination of thermolysis and hydrolysis, AB being first thermolyzed and then the solid by-product believed to be polyaminoborane [NH₂BH₂]_n (PAB) being hydrolyzed. Herein we report that: (i) the combination is feasible, (ii) PAB hydrolyzes in the presence of a metal catalyst (Ru) at 40 °C, (iii) a total of 3 equiv. H₂ is released from AB and PAB-H₂O, (iv) high hydrogen generation rates can be obtained through hydrolysis, and (v) the by-products stemming from the PAB hydrolysis are ammonium borates. The following reactions may be proposed: AB → PAB + H₂ and PAB + xH₂O → 2H₂ + ammonium borates. All of these aspects as well as the advantages and issues of the combination of AB thermolysis and PAB hydrolysis are discussed.     

Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells

Thermodynamic calculations have been made to predict the stability of solid oxide fuel cell (SOFC) anode materials when exposed to hydrogen sulphide (H₂S) in hydrogen (H₂) over a range of partial pressures of sulphur (pS₂) and oxygen (pO₂) representative of fuel cell operating conditions. The study focussed on the behaviour of nickel and ceria, which form the basis of nickel–gadolinium-doped ceria (Ni-CGO) anodes, often used as an active layer within SOFCs. The reaction of Ni with sulphur is predicted to become more favourable as temperature and hydrogen partial pressure (pH₂) decrease. Ceria is shown to become increasingly non-stoichiometric (CeO_n, n < 2) as pO₂ decreases and temperature increases, and it is predicted that its reaction with sulphur becomes more favourable under these conditions.     

Hydrogen storage properties and reaction mechanisms of K2Mn(NH2)4–8LiH system

Hydrogen storage properties of K₂Mn(NH₂)₄–8LiH were investigated by considering its de/re-hydrogenation properties and reaction mechanisms. Experimental results show that the dehydrogenated K₂Mn(NH₂)₄–8LiH can be almost re-hydrogenated completely at 230 °C and 50 bar of H₂ with a hydrogenation rate more than 1.0 wt%/min. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and FTIR investigations reveal that during ball milling K₂Mn(NH₂)₄ reacts with LiH to form LiNH₂ and K–Mn-species1 which is probably a K–Mn-containing hydride. The ball milled sample releases hydrogen in a multi-step reaction with the formation of K₃MnH₅ and K–Mn-species2 as intermediates and Li₂NH, Mn₃N₂ and MnN as final products. The full hydrogenated products are LiH, LiNH₂, and K–Mn-species2. The K–Mn-species2 may play a critical role for the fast hydrogeneration. This work indicates that transition metal contained amide-hydride composite holds potentials for hydrogen storage.