Combustion synthesis of TiFe-based hydrogen storage alloy from titanium oxide and iron

In this paper, we describe the combustion synthesis of a TiFe-based hydrogen storage alloy from Fe and TiO₂ using metallic calcium as the reducing agent and heat source. The effects of hydrogen on the combustion ignition temperature and the hydrogenation properties of the products were examined. In the experiments, Fe, TiO₂, and Ca were mixed with a molar ratio of 1:1:4 and heated in a hydrogen atmosphere until the ignition due to the hydrogenation of calcium. For comparison, the same experiment was performed in an argon atmosphere. The ignition and maximum temperatures in the hydrogen atmosphere were drastically lower than those in the argon atmosphere. According to X-ray diffraction (XRD) analyses, all the peaks of the product combustion-synthesized in a hydrogen atmosphere were due to the TiFe phase, although some peaks of the product synthesized in argon indicated the existence of the TiO phase in addition to the TiFe phase. The product synthesized in hydrogen demonstrated a hydrogen storage capacity of 1.39 mass%, which is equal to that achieved using pure TiFe reagent. Moreover, a fine powdered product was obtained without any pulverization processes. This method creates an innovative production route for TiFe.     

Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures

In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi₅ utilizing the hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the effect of hydrogen pressure on the ignition temperature and the initial activation of hydrogenation. In the experiments, La₂O₃, Ni, and Ca were dry-mixed, and then heated at 0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from bulk to powder with hydrogen pressure. This was probably caused by volume expansion due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The results of this research can be applied as an innovative production route for LaNi₅ without the conventional melting of La and Ni.     

Complex hydrides as solid storage materials: First safety tests

Hydrogen technology requires efficient and safe hydrogen storage systems. For this purpose, storage in solid materials, such as high capacity complex hydrides, is studied intensely. Independent from the actual material to be used eventually, any tank design will combine nanoscale powders of highly reactive material with pressurized hydrogen gas and so far, little is known about the behavior of these mixtures in case of incidents. For a first evaluation of a complex hydride in case of a tank failure, NaAlH₄ (doped with Ti) was investigated in a small-scale tank failure test. 80–100 ml of the material was filled into a heat exchanger tube, and sealed under argon atmosphere with a burst disk. Subsequently, the NaAlH₄ was partially desorbed by heating. When the powder temperature reached 130 °C and the burst disk ruptured at 9 bar hydrogen overpressure the behavior of the expelled powder was monitored using a high speed camera, an IR camera as well as sound level meters. Expulsion of the hydrogen storage material into (dry) ambient atmosphere yields a dust cloud of finely dispersed powder which does not ignite spontaneously. Similar experiments including an external source of ignition (spark/water reacting with NaAlH₄) yield a flame of reacting powder. The intensity will be compared to the reaction of an equivalent amount of pure hydrogen.     

Self-ignition combustion synthesis of oxygen-doped TiFe

This paper describes the Self-Ignition Combustion Synthesis (SICS) of the hydrogen storage alloy Ti_1.15FeO_x (x = 0, 0.024, and 0.050) in a hydrogen atmosphere and the obtained product's hydrogenation properties. Ti, Fe, and α-Fe₂O₃ powders were mixed to produce Ti_1.15FeO_x and uniformly heated to the eutectic temperature 1358 K for self-ignition, which occurred after the hydrogenation and decomposition of Ti. The X-ray diffraction results for the final product had TiFeH_0.06 and Fe₂Ti as the major and minor phases, respectively. The hydrogen storage capacity of the product at a hydrogen pressure of 5 MPa was 1.18–1.50 m%. As the value of x increased, the capacity gradually decreased. However, the initial hydrogenation kinetics of the product improved. In contrast, the equilibrium pressure for the product was almost unaffected. These results demonstrate that SICS is quite effective for producing O-doped TiFe alloys as well as TiFe without rare metal. The procedure offers many benefits of releasing tough activation treatment, minimizing operation time, and saving energy.     

Exergy analysis of self-ignition combustion synthesis for producing rare-earth-based hydrogen storage alloy

A new production system for rare-earth-based hydrogen storage alloys is proposed. We applied self-ignition combustion synthesis (SICS) utilizing hydrogenation heat of metallic calcium. The required primary energy and total exergy loss (EXL) for the production of 1 kg of LaNi₅ alloy with the proposed and conventional systems were evaluated. The results revealed that the production of raw materials accounted for more than 90% of the total EXL in both systems. Specifically, the use of calcium had decisive effects on the total EXL of the system for producing LaNi₅ alloy. The proposed system reduced the total EXL by 14.6 MJ/kg-LaNi₅ as compared with the conventional system. The SICS was remarkably exergy-saving because the heating temperature was decreased by utilizing the hydrogenation heat of calcium and the product absorbed hydrogen without an activation treatment.     

Shock tube study of ethylamine pyrolysis and oxidation

Ethylamine (CH₃CH₂NH₂) pyrolysis and oxidation were studied using laser absorption behind reflected shock waves. For ethylamine pyrolysis, NH₂ time-histories were measured in 2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, and NH₂ and OH time-histories were measured in ethylamine/O₂/argon mixtures. Measurements covered the temperature range of 1200–1448 K, with pressures near 0.85, 1.35 and 2 atm, and fuel mixtures with equivalence ratios of 0.75, 1 and 1.25 in 0.2%, 0.8% and 4% O₂/argon. Simulations using the recent Li et al. mechanism gave significantly shorter ignition delay times and higher intermediate radical species concentrations than the experimental results. The reaction rate constants for the two major ethylamine decomposition pathways were modified in the Li et al. mechanism to improve the prediction of the time-histories of NH₂ and OH in ethylamine pyrolysis. In addition, recommendations from recent studies of ethylamine + OH reactions were implemented. With these modifications, the Modified Li et al. mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay time data.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

Formation and hydrogen storage properties of in situ prepared Mg–Cu alloy nanoparticles by arc discharge

Mg–Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg₂Cu, MgCu₂ and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg₂Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.     

Noncatalytic hydrothermolysis of ammonia borane

Hydrolysis of ammonia borane (AB) is attractive as a chemical method for hydrogen storage. The use of catalysts is, however, usually required. In the present paper, two new methods for releasing hydrogen from AB and water are investigated which do not involve any catalyst. One method is based on combustion of AB mixtures with nanoscale aluminum powder and gelled water. It is shown experimentally that these mixtures, upon ignition, exhibit self-sustained combustion with hydrogen release from both AB and water. The other method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure to avoid water boiling. To clarify the reaction mechanism, isotopic experiments using D₂O instead of H₂O were conducted. It is shown that heating AB/D₂O solution to temperatures 117–170 °C releases 3 equiv. of hydrogen per mole AB, where 2–2.1 equiv. originate from AB and 0.9–1 equiv. from water. The prospects of both methods for hydrogen storage are discussed.     

Self-ignition combustion synthesis of TiFe1−xMnx hydrogen storage alloy

The paper describes the self-ignition combustion synthesis (SICS) of the hydrogen storage alloy TiFe_1?xMn_x (X = 0, 0.1, 0.2, 0.3, and 0.5) in a hydrogen atmosphere, where the hydrogenation properties of the products are mainly examined. In the experiments, the well-mixed powders of Ti, Fe, and Mn in the molar ratio of 1:1-X:X were uniformly heated up to 1473 K, and then were cooled naturally in pressurized hydrogen at 0.9 MPa. All products were successfully synthesized by utilizing the exothermic reaction, which occurred at around 1358 K. The XRD analysis showed that SICS generated TiFe_1?xMn_x in the range of X value from 0 to 0.3. All SICSed products absorbed hydrogen smoothly at 298 K at an initial pressure of 4.1 MPa. Most significantly, TiFe_0.8Mn_0.2 improved the dual plateau property. The results revealed that SICS was quite effective for producing the hydrogen storage alloy TiFe_1?xMn_x.     

Effects of CeO2 additive on redox characteristics of Fe-based mixed oxide mediums for storage and production of hydrogen

Effects of CeO₂ additive to Fe-based mixed oxide mediums with Rh and ZrO₂ for chemical hydrogen storage were investigated in terms of stability and reactivity of the mediums in water splitting oxidation with repeated redox cycles. The mediums with CeO₂ content ranging from 0 to 30 wt% were prepared by co-precipitation method using urea solution as a precipitant. The hydrogen storage and release properties were investigated during repeated isothermal redox cycles at 823 K for reduction with hydrogen and 623 K for oxidation with water vapor under atmospheric pressure. The amount of hydrogen produced by the mediums, both with and without CeO₂, was maintained at an almost constant level over ten repeated redox cycles. However, the oxidation rates of the mediums without CeO₂ were decreased during repeated redox cycles while that increased with increasing CeO₂ contents. Especially, the mediums added with 30 wt% of CeO₂ (FRZC-30) showed high activity and stability for ten redox cycles, the degree of hydrogen storage was almost maintained ca. 1.9 wt% on the basis of total amount of the medium.     

Effect of electrolyte on electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 alloy electrode for hydrogen storage

A series of experiments have been performed to investigate the effects of three electrolytes of different compositions (EO, EA and EM) on the electrochemical characteristics of MmNi_3.55Co_0.72Al_0.3Mn_0.43 hydrogen storage alloy electrode. Electrolytes EA and EM were obtained by adding appropriate amounts of Al₂(SO₄)₃ and MnSO₄ to the original electrolyte EO (6 M KOH + 1 wt% LiOH), respectively. Electrode activation, maximum capacity, cycle life, self-discharge and high-rate discharge characteristics have been studied. It was found that a maximum capacity of about 260 mA h/g has been obtained for the alloy electrodes in all these electrolytes after 5–7 cycles of charging/discharging. The alloy electrodes have a good durability in electrolytes EA and EM, especially after 175 cycles. Using the capacity retention as an indication of self-discharge resistance, almost identical degree of capacity retention (82% after 4 days and 45% after 16 days) has been observed at 298 K, regardless of the electrolytes used. When tested at higher temperature, however, a higher capacity retention (46% after 3 days) at 333 K has been observed for electrodes in electrolyte EA, and about 32% for electrodes in both electrolytes EO and EM. As to high-rate discharge behavior of the results of high-rate discharge tests indicated that about 50% of discharge efficiencies were obtained in the three electrolytes at 333 K by continuous-model high-rate discharge method, at a discharge rate of 7C, and 22% in 298 K. The alloy electrode in electrolyte EM has the best durability, in which about 50% of discharge efficiency at DC = 9C was obtained by step-model high-rate discharge method at 333 K, which was even higher than that at 298 K.     

Investigations on hydrogenation behaviour of CNT admixed Mg2Ni

The aim of the present paper is to report results on hydrogenation behaviour of the new composite material Mg₂Ni: CNT. Admixing of carbon nanotubes (CNT) in storage material Mg₂Ni leads to noticeable enhancement in desorption kinetics as well as storage capacity. We have found that the composite material Mg₂Ni–2 mole% CNT is the optimum material. The Mg₂Ni–CNT composite exhibits hydrogen desorption rate of 5.7 cc/g/min as against 3.0 cc/g/min for Mg₂Ni alone (enhancement of ∼ 90%) and storage capacity of ∼ 4.20 wt% in contrast to ∼3.20 wt% for Mg₂Ni alone (increase of ∼ 31%). Feasible mechanisms for the enhancement of hydrogen desorption kinetics and storage capacity have been put forward.     

Experiments and modeling of propane combustion with vitiation

The chemical species composition of a vitiated oxidizer stream can significantly affect the combustion processes that occur in many propulsion and power generation systems. Experiments were performed to investigate the chemical kinetic effects of vitiation on ignition and flame propagation of hydrocarbon fuels using propane. Atmospheric-pressure flow reactor experiments were performed to investigate the effect of NO_x on propane ignition delay time at varying O₂ levels (14–21 mol%) and varying equivalence ratios (0.5–1.5) with reactor temperatures of 875 K and 917 K. Laminar flame speed measurements were obtained using a Bunsen burner facility to investigate the effect of CO₂ dilution on flame propagation at an inlet temperature of 650 K. Experimental and modeling results show that small amounts of NO can significantly reduce the ignition delay time of propane in the low- and intermediate-temperature regimes. For example, 755 ppmv NOx in the vitiated stream reduced the ignition delay time of a stoichiometric propane/air mixture by 75% at 875 K. Chemical kinetic modeling shows that H-atom abstraction reaction of the fuel molecule by NO₂ plays a critical role in promoting ignition in conjunction with reactions between NO and less reactive radicals such as HO₂ and CH₃O₂ at low and intermediate temperatures. Experimental results show that the presence of 10 mol% CO₂ in the vitiated air reduces the peak laminar flame speed by up to a factor of two. Chemical kinetic effects of CO₂ contribute to the reduction in flame speed by suppressing the formation of OH radicals in addition to the lower flame temperature caused by dilution. Overall, the detailed chemical kinetic mechanism developed in the current work predicts the chemical kinetic effects of vitiated species, namely NO_x and CO₂, on propane combustion reasonably well. Moreover, the reaction kinetic scheme also predicts the negative temperature coefficient (NTC) behavior of propane during low-temperature oxidation.     

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Enhancement of the hydrogen storage characteristics of Mg by reactive mechanical grinding with Ni,Fe and Ti

Mg-10wt%Ni-5wt%Fe-5wt%Ti powder was prepared by reactive mechanical grinding using a planetary ball mill. The Mg-10wt%Ni-5wt%Fe-5wt%Ti powder exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after two hydriding–dehydriding cycles. After the reactive mechanical grinding, the particle size of the powder was reduced, as compared with those of the starting materials. The hydrogen storage properties were measured at temperatures of 473 K, 573 K and 623 K. The activated Mg-10wt%Ni-5wt%Fe-5wt%Ti powder absorbed 5.31 wt% and 5.51 wt% of hydrogen for 5 min and 1 h, respectively, at 573 K under 12 bar H₂. It desorbed 5.18 wt% of hydrogen at 573 K under 1.0 bar H₂ for 1 h. The initial hydrogen absorption rate increased when passing from 473 K to 573 K, but decreased at 623 K. The hydrogen desorption rate increased rapidly with increasing temperature from 473 K to 623 K. The hydrogen storage capacity was about 6.72 wt% at 573 K.     

Evaluation of the hydrogen storage behavior of a LiNH2 + MgH2 system with 1:1 ratio

A one-to-one molar ratio of LiNH₂ to MgH₂ was ball milled and characterized to evaluate the proposed hydrogen storage reaction: LiNH₂ + MgH₂ ⇔ LiMgN + 2H₂. The pressure–composition isotherm shows that less than 3.4 wt.% H₂ is released at a plateau pressure near 20 atm at 210 °C. Furthermore, X-ray diffraction show that the products of the reaction include Li₂Mg₂(NH)₃ rather than LiMgN. Combined thermogravimetric and residual gas analyses reveal that large quantities of ammonia are released from the system.     

Changes in hydrogen storage properties of carbon nano-horns submitted to thermal oxidation

The effect of thermal oxidation on the hydrogen storage properties of carbon nano-horns was investigated by gravimetric and electrochemical methods. The pristine nano-horn sample was oxidised at 673 K in air for different periods (15, 30 and 60 min) and the resulting materials were characterised. The N₂ adsorption experiments reveal a marked increase in the surface area, from 267 m² g⁻¹, for the pristine sample, up to 1360 m² g⁻¹ for the sample oxidised for the 60 min period, and a reduction in the average pore diameter. The gravimetric investigation, conducted at low temperature (77 K) showed an increase in the hydrogen storage, from 0.75 wt% for the pristine sample up to 2.60 wt% for the oxidised material. Reproducible and stable hydrogen storage was found for all the samples examined apart from the sample oxidised for 60 min. For the latter, a decrease in the amount of hydrogen stored between the first and second cycles was found. Electrochemical loading of hydrogen in the samples was performed at room temperature (298 K) in alkaline solution by the galvanostatic charge/discharge technique. The results obtained here however show a much lower hydrogen storage level by the samples as compared to the gas storage method, with a maximum value of 0.124 wt% H₂ and with very little dependence on the thermal oxidation treatment.     

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg20Ni10−xCox (x = 0–4) alloys prepared by melt-spinning

In order to improve the hydriding and dehydriding performances of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂₀Ni_10−xCo_x (x = 0–4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. Thermal stability of the as-spun alloys was researched by DSC. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results showed that no amorphous phase formed in the as-spun Co-free alloy, but the as-spun alloys containing Co showed certain amount of amorphous phase. The hydrogen absorption capacities of the as-cast alloys first increase and then decrease with the variety of Co content. The hydrogen desorption capacities of as-cast and spun alloys rise with increasing Co content. The rapid quenching significantly improved the hydrogenation and dehydrogenation capacities and the kinetics of the alloys. When the quenching rate increased from 0 (as-cast was defined as spinning rate of 0 m/s) to 30 m/s, the hydrogen absorption capacity of the alloys (x = 0) at 200 °C and 1.5 MPa in 20 min rose from 1.39 to 3.12 wt%, and from 1.91 to 2.96 wt% for the alloy (x = 4). The hydrogen desorption capacity of the alloy (x = 0) in 20 min increased from 0.19 to 0.89 wt%, and from 1.39 to 2.15 wt% for the alloy (x = 4).     

Hydrogenation behaviour of La0.5R0.5Ni4.8Al0.1Li0.1 (R = La,Ce, Pr or Nd) alloys

In the present work, we have investigated the hydrogenation properties of La_0.5R_0.5Ni_4.8Al_0.1Li_0.1 (R = Ce, Pr or Nd) alloys under the hydrogen pressure ranging from 0.02 to 5 MPa. The pressure–composition (p–c) isotherms have been measured at four temperatures (293, 313, 333 and 353 K) in order to evaluate hydrogen absorption/desorption abilities of the alloys. For the investigated systems, the enthalpy and entropy of the dehydrogenation process have been calculated using van't Hoff plots.