Pressure Composition Temperature Research Articles

A comparative study on effect of microwave sintering and conventional sintering on properties of Nd–Mg–Ni–Fe 3O 4 hydrogen storage alloy

The hydrogen storage samples of Nd–Mg–Ni–Fe 3O 4 alloy were prepared by microwave sintering (MS) and conventional sintering (CS) methods, respectively. Their phase structures, morphologies, hydrogen storage properties were intensively studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure–composition–temperature (PCT). XRD and SEM analysis results show that the microwave sintered Nd–Mg–Ni–Fe 3O 4 alloy has multiphase structure involving Mg and homogeneous grains, whereas the alloy prepared by CS has Mg 41Nd 5 phase and coarse grains. The alloy prepared by MS can release 85% of the saturated hydrogen capacity at 573 K in 600 s and its characteristic reaction time ( t c) is less than 2900 s, while the alloy prepared by CS releases less than 70% of the absorbed hydrogen at 573 K within 1300 s and its t c is more than 3000 s. It is found that the alloy prepared by MS not only has high hydrogen capacity, but also better dehydriding kinetic property than the alloy prepared by CS.

Copper oxide-decorated porous carbons for carbon dioxide adsorption behaviors

Copper oxide-loaded porous carbons (PCs) for high efficient carbon dioxide capture were prepared. Copper oxides were loaded onto porous carbons by a postoxidation method involving copper electroplated PCs at 300 °C in an air stream. Additionally, porous carbons were prepared from ion-exchangeable polymeric resin by a chemical activation method. The microstructure of the copper oxide/PCs was characterized by XRD, and the formation of copper oxides after the postoxidation process was confirmed by XPS. The carbon dioxide adsorption behaviors were evaluated by a PCT (pressure–composition–temperature) apparatus at 298 K and 1.0 atm. It was found that the presence of copper oxides significantly led to an increase in the carbon dioxide adsorption capacity of the carbons. Copper oxide nanoparticles have electron-donor features, resulting in the enhancement of adsorption capacity of carbon dioxide molecules, which have an electron acceptor feature.

Hydriding behavior in Zr-based AB 2 alloy by gas atomization process

The hydriding characteristics of Zr-based AB 2 alloy produced by gas atomization have been investigated during its absorption–desorption reaction with hydrogen gas. Its gas-phase hydrogenation properties are different from those of specimens prepared by conventional methods. For the particle morphology of the as-cast and gas-atomized powders, it can be seen that the mechanically crushed powders are irregular, while the atomized powder particles are spherical. In PCT (Pressure–Composition–Temperature) measurements, for the gas-atomized particles smaller than 50 μm, the hydrogen storage capacity is dramatically decreased and the hysteresis loop becomes larger than that of the gas-atomized particles larger than 50 μm. In addition, the increase of jet pressure of gas atomization results in the decrease of hydrogen storage capacity and the slope of plateau pressure significantly increases. TEM and EDS studies showed the increase of jet pressure in the atomization process accelerated the phase separation within grain of the gas-atomized alloy, which brought about a poor hydrogenation property. In the measurements of hydrogen absorption–desorption kinetics, the improvement of desorption kinetics of gas-atomized AB 2 alloys was mainly caused by the higher plateau pressure, which is attributed to the smaller grain size and higher site energy for hydrogen in the gas-atomized alloys.

Hydrogen Storage Properties of Nanosized MgH2−0.1TiH2 Prepared by Ultrahigh-Energy−High-Pressure Milling

Magnesium hydride (MgH(2)) is an attractive candidate for solid-state hydrogen storage applications. To improve the kinetics and thermodynamic properties of MgH(2) during dehydrogenation-rehydrogenation cycles, a nanostructured MgH(2)-0.1TiH(2) material system prepared by ultrahigh-energy-high-pressure mechanical milling was investigated. High-resolution transmission electron microscope (TEM) and scanning TEM analysis showed that the grain size of the milled MgH(2)-0.1TiH(2) powder is approximately 5-10 nm with uniform distributions of TiH(2) among MgH(2) particles. Pressure-composition-temperature (PCT) analysis demonstrated that both the nanosize and the addition of TiH(2) contributed to the significant improvement of the kinetics of dehydrogenation and hydrogenation compared to commercial MgH(2). More importantly, PCT cycle analysis demonstrated that the MgH(2)-0.1TiH(2) material system showed excellent cycle stability. The results also showed that the DeltaH value for the dehydrogenation of nanostructured MgH(2)-0.1TiH(2) is significantly lower than that of commercial MgH(2). However, the DeltaS value of the reaction was also lower, which results in minimum net effects of the nanosize and the addition of TiH(2) on the equilibrium pressure of dehydrogenation reaction of MgH(2).

Electrochemical properties and hydrogen storage mechanism of perovskite-type oxide LaFeO3 as a negative electrode for Ni/MH batteries

Abstract Perovskite-type oxide LaFeO3 powder was prepared using a stearic acid combustion method. Its phase structure, electrochemical properties and hydrogen storage mechanism as negative electrodes for nickel/metal hydride (Ni/MH) batteries have been investigated systematically. The results of X-ray diffraction (XRD) analysis show that both the calcined powder and the charged/discharged samples after 10 cycles have orthorhombic structures. The discharge capacity, whose maximum value appeared at the first cycle, is 530.3 mA h g−1 at 333 K and increases with an increase in temperature. The discharge capacity decreases distinctly during the first three cycles and then stays steady at about 80 mA h g−1, 160 mA h g−1 and 350 mA h g−1 at 298 K, 313 K and 333 K, respectively. The hydrogen storage mechanism is studied by XRD, X-ray photoelectron spectroscopy (XPS) and mass spectrometry (MS), coupled with pressure–composition–temperature (PCT) methods. Hydrogen atoms may be intercalating into the oxide lattice and forming a homogeneous solid solution during the charging process.

A thermodynamic study of the hydrogenation of the pseudo-binary Mg 6Pd 0.5Ni 0.5 intermetallic compound

Hydrogenation thermodynamic properties of a new pseudo-binary Mg 6Pd 0.5Ni 0.5 intermetallic compound (IMC) are presented. This IMC disproportionates reversibly on hydrogenation into MgH 2, Mg 5Pd 2 and Mg 2NiH 4 compounds. Pressure–Composition–Temperature measurements in the temperature range 590–650 K have been acquired. The enthalpy and entropy of the hydrogenation reaction are respectively, −63 ± 3 kJ/molH 2 and −114 ± 4 J/K molH 2. They agree well with those calculated from the enthalpy and entropy of formation of the several phases involved in the disproportionation reaction. In particular, the Miedema model has been used to calculate the standard enthalpy of formation of the several Mg–Pd IMC's, ranging from −21.8 to −61.5 kJ/mol atoms for Mg 6Pd and MgPd, respectively. As for the standard entropy changes, an inverse relationship between entropy and density was used to calculate the formation entropy of the Mg–Pd IMC's.

Reversible hydrogen storage in titanium-catalyzed LiAlH 4–LiBH 4 system

We have investigated the hydrogen storage properties of the LiAlH 4–LiBH 4 system, both un-doped and doped with titanium based catalysts. It was found that TiF 3 exhibited the superior catalytic effects in terms of enhancing the hydriding/dehydriding kinetics and reducing the dehydrogenation temperature of the LiAlH 4–LiBH 4 system. Compared to the un-doped LiAlH 4–LiBH 4 system, the onset temperatures of the 5 mol% TiF 3-doped sample for the first and second dehydrogenation steps were decreased by 64 and 150 °C, respectively. X-ray diffraction patterns of the dehydrogenated samples revealed that the produced Al from LiAlH 4 could react with B from the decomposition of LiBH 4 to form AlB 2 and LiAl compounds. Pressure-composition-temperature (PCT) and van’t Hoff plots made it clear that the decomposition enthalpy of LiBH 4 in the TiF 3-doped LiAlH 4–LiBH 4 system is decreased from 74 kJ/(mol of H 2) for the pure LiBH 4 to 60.4 kJ/(mol of H 2). The dehydrogenation products of the TiF 3-doped LiAlH 4–LiBH 4 sample can absorb 3.76 and 4.78 wt.% of hydrogen in 1 h and 14 h, respectively, at 600 °C and under 4 MPa of hydrogen. The formation of LiBH 4 was detected by X-ray diffraction in the rehydrogenated sample.

Thermodynamics of Li–N–H system for hydrogen storage: A theoretical and experimental study

Abstract In the present work, electronic structure, chemical bonding and thermal stability of Li–N–H system for hydrogen storage were calculated by a first principle approach. On the basis of the thermal analysis of this system, pressure–composition–temperature (PCT) isotherm measurements of hydrogen desorption were performed and analyzed. The theoretical and experimental enthalpies of this system were calculated as −75.67 and −69.17 kJ/mol of H2, which agree well with other corresponding findings of −73.6 and 66.1 kJ/mol of H2, respectively. The theoretical and experimental values of desorption enthalpies in this study are reasonably agreeable with each other.

Improvement of the LiAlH4−NaBH4 System for Reversible Hydrogen Storage

The hydrogen storage properties of a combined LiAlH4−NaBH4 system have been investigated. It was found that there is a mutual destabilization between the LiAlH4 and the NaBH4. Two major dehydrogenation steps with hydrogen capacities of 3.1 wt % and 5.2 wt % were observed for the system with a molar ratio of LiAlH4:NaBH4 = 1:1 at around 250 and 600 °C, respectively. The onset dehydrogenation temperatures for the first and the second steps in the LiAlH4−NaBH4 mixture were decreased to 95 and 450 °C, respectively, which are 30 and 50 °C lower than those of bare LiAlH4 and NaBH4, respectively. Furthermore, doping with TiF3 significantly decreases the hydrogen release temperature compared to the undoped system. The TiF3 doping can further decrease the onset temperatures of the first and second steps to 60 and 300 °C. The presence of TiF3 also destabilizes the NaH phase, which leads to an increased desorption capacity. Pressure−composition−temperature (PCT) and van’t Hoff plots illustrate that the decomposition enthalpy of NaBH4 in the TiF3-doped LiAlH4−NaBH4 system is decreased from 106.8 kJ/(mol of H2) for pure NaBH4 to 68.16 kJ/(mol of H2). In addition, about 4 wt % hydrogen can be reversibly stored by the dehydrogenated product, and the formation of NaBH4 was detected by X-ray diffraction in the rehydrogenated sample.

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 3. Empirical formula

In Part 1 and Part 2 of this series of papers, the pressure–concentration–temperature (PCT) isotherms hysteresis was found to be closely related to the axial ratio a/ c for both simple ternary and more complicated multi-element C14 Laves phase based alloys. Furthermore, the particle pulverization rate, which is the major determining factor in the duration of metal hydride electrode cycling, was found to correlate well with PCT hysteresis. In the current Part 3, we discuss an empirical equation which was developed to predict the PCT hysteresis of battery alloys through the study of the lattice constant ratios of a series of ZrCr 2-based ternary alloys. The empirical formula can then be used to estimate the pulverization rate of metal hydride electrode. To fit the empirical formula, an equivalent number of outer shell electrons for some non-transition metals was calculated from the axial ratio of ZrCr 1.8M 0.2 ternary alloys, where M is an element from the group of Al, Si, Ga, Ge, and Sn. Other factors, such as the amount of substitution, the difference in A and B element electronegativities, atomic size, and the choice of A element, were also investigated.

Study on hydrogen storage properties of LiAlH 4

LiAlH 4 doped with 5 mol% Ti, Ni, Ce(SO 4) 2 and LaCl 3 additives has been studied by pressure–composition–temperature (PCT) experiment and X-ray diffraction. Doping with Ce(SO 4) 2 induced the largest decrease in the temperature of hydrogen release about 38 °C, significantly, the amount of hydrogen release had only a slight increase. Comparatively, doping with Ti and LaCl 3 decreased the temperature of hydrogen release, and markedly decreased the amount of hydrogen release. The study on rehydrogenation showed that doping with Ti caused the most amount of rehydrogenation, however, doping with Ce(SO 4) 2 induced the least adsorption amount. The result obtained by XRD spectra for LiAlH 4 doped with 5 mol% Ti heated at 270 °C and adsorped at 180 °C under ∼8 MPa hydrogen pressure indicated slow composition of LiH, Al and H 2 into Li 3AlH 6.

Enhanced hydrogen storage performance of LiBH 4–Ni composite

The hydrogen storage properties of LiBH 4 ball milled with various ratios of Ni powders were investigated. It was found that Ni addition improved the dehydrogenation of LiBH 4, resulting in the majority of hydrogen released from LiBH 4 below 600 °C. According to van’t Hoff equation, the desorption enthalpy of LiBH 4–Ni system was calculated to be Δ H = −60.76 kJ mol −1 H 2, which is similar to that of the pure LiBH 4. Pressure–composition–temperature (PCT) results revealed that Ni addition accelerate the dehydrogenation rate of LiBH 4. Furthermore, the LiBH 4–Ni system could be reversed partly at 600 °C and 10 MPa hydrogen pressure.

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 2. Applications in NiMH batteries

This part of our series of papers reports our efforts to predict sealed cell cycle life from lattice constant a/ c ratio and/or PCT hysteresis based on correlations established from our previous studies reported in Part 1. In the case of Ti 21Zr 12.5V 14Ni 21.5− x Cr 10Mn 21Sn x alloys, as x is increased the a/ c ratio increases and the PCT hysteresis decreases resulting in extended cycle life for the battery. This is in agreement with the results from Part 1. In the case of Ti 18Zr 15.5V 14Ni 24.2− x Cr 10Mn 18− y Sn 0.3Co x Al y alloys, the a/ c ratio trend was not observed due to existence of more than one C14 phase, but the correlation between PCT hysteresis and cycle life was maintained. In the case of Ti 21Zr 12.5V 10Ni 40.2Cr 8.5− x Mn 5.6− y Co 1.5+ x+ y Al 0.4Sn 0.3 alloys, the cycle life was dominated by the corrosive nature of the alloy affected by the Cr content. As a consequence, predictions from a/ c ratio and PCT hysteresis can only be applied to the degree of pulverization in gas phase cycling and not to the sealed cell cycle life.

Synthesis mechanism and properties of Mg–Mg2Ni composite hydrogen storage alloy produced by hydriding combustion synthesis

Hydriding combustion synthesis (HCS) has been regarded as an innovative process to produce magnesium based hydrogen storage alloys. In the present paper, a Mg–Mg2Ni composite hydrogen storage alloy was prepared by the HCS process under moderate conditions. Phase composition, synthesis mechanism and hydrogen storage properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure composition temperature (PCT). The results indicated that the HCS product was composed mainly of MgH2, Mg2NiH4 and Mg2NiH0·3. A gas–solid reaction mechanism played an important role in the HCS process. The hydriding activity of the as synthesised product was very high so that it absorbed 4·61 mass% hydrogen in the first hydriding process without any activation treatment and the maximum hydrogen storage capacity achieved was 5·24 mass%. Moreover, the hydriding rate was excellent. For example, it could absorb >3·20 and 4·05 mass% hydrogen within 4 min in the first and forth hydriding/dehydriding cycle respectively. The relationships between the plateau pressure and temperature were: lgP(0·1 MPa)=−4250·5/T+7·9635 (hydriding), lgP(0·1 MPa)=−4125·4/T+7·6502 (dehydriding) for the lower plateau, and lgP(0·1 MPa)=−2894·6/T+5·9656 (hydriding), lgP(0·1 MPa)=−3855·9/T+7·3435 (dehydriding) for the higher plateau.

Hydrogen storage in a CaH 2/LiBH 4 destabilized metal hydride system

A study was done to determine if the destabilized hydride system, CaH 2/LiBH 4, would be suitable for hydrogen storage. When several additives, TiCl 3, V 2O 5, TiF 3 and TiO 2 were ball milled with the CaH 2/LiBH 4 mixture, TGA analysis showed that all of the mixtures released hydrogen in the 400–450 °C range with the desorption temperatures being in the order TiF 3 < TiO 2 ≤ TiCl 3 < V 2O 5. Kinetic modeling measurements showed that the reaction is controlled by diffusion. An attempt was also made to lower the reaction temperature by using CaNi 5H 4 as a destabilizing agent instead of CaH 2. TGA analysis showed that the CaNi 5H 4 had virtually the same effect on the reaction temperature as CaH 2 and the added weight of the nickel in CaNi 5H 4 reduced the overall weight percentage of hydrogen released from the system. In a further attempt to reduce the reaction temperature, a ternary system consisting of CaH 2, LiBH 4 and LiNH 2 was tested for hydrogen desorption. TGA analyses showed that the ternary system released all of its hydrogen at a temperature that was about 100 °C lower than the CaH 2/LiBH 4 mixture. However, attempts to do pressure composition isotherm analyses and temperature programmed desorption measurements showed that the release of hydrogen from the ternary mixture occurs irreversibly. It is likely that the reaction products are very stable thus, preventing the reverse reaction from occurring.

Electrochemical PCT isotherm study of hydrogen absorption/desorption in AB 5 type intermetallic compounds

The enthalpy (Δ H) and entropy (Δ S) of hydride formation/decomposition could be determined either experimentally or theoretically based on models proposed in the literature. The experimental pathway includes gas/solid-phase measurement of pressure–composition–temperature (PCT) isotherms at different temperatures. This measurement is followed by plotting of van't Hoff dependences and evaluation of the Δ H and Δ S from their slopes and intersects, respectively. In this study we demonstrate the applicability of electrochemical PCT isotherm measurements as an advanced method for thermodynamic analysis of hydrogen adsorption/desorption process. Experimentally this is done by electrochemical charging/discharging of an electrode, prepared from AB 5 type alloy with MmNi 4.6Co 0.6Al 0.8 composition (Mm – mischmetal). In addition, the hydride formation as a result of the electrochemical charging is independently confirmed by ex-situ XRD diffraction. Our work demonstrates that not only the electrochemical approach is a viable alternative of PCT gas/solid-phase measurement but it also represents a safer, cost-effective and faster protocol than its hydrogen gas–solid phase equivalent.

Hydrogenation study of suction-cast Ti 40Zr 40Ni 20 quasicrystal

Suction casting was predicted to be an usable method for improving the hydriding kinetics of Ti/Zr-based icosahedral quasicrystals (IQCs) in our previous work. To further determine it, a suction-cast Ti 40Zr 40Ni 20 IQC alloy was used for hydrogenation studies by Pressure Composition Isotherm (PCI) and Temperature Programmed Desorption (TPD) techniques. The results showed that, this alloy absorbed hydrogen rapidly with obvious hydrogen pressure plateau and some reversibility, however, displayed very limited hydrogen capacity (about 0.7 wt.%) and low equilibrium pressure. After several hydrogenation/dehydrogenation cycles, the IQC structure transformed into two hydride phases, ZrH 2− x and one unknown, both of which decomposed at above 600 °C, suggesting high thermo-stability for them. On the whole, indeed the suction-casting method can increase the hydrogen absorption rate of Ti/Zr-based IQCs, however, the hydrogenation properties of the Ti 40Zr 40Ni 20 IQC alloy still need a mighty advancement.

Interaction of hydrogen with the β-Al 3Mg 2 complex metallic alloy: Experimental reliability of theoretical predictions

In this paper we report on some hydrogenation characteristics of β-Al 3Mg 2. Pressure–composition–temperature (PCT) absorption curves were obtained in the temperature range 573–658 K up to 4 MPa of H 2-pressure. During the H-absorption process, β-Al 3Mg 2 disproportionates into MgH 2 and the solid solution of Al(Mg). This reaction is reversible at the temperatures studied. The enthalpy and entropy of the reaction were determined from the Van’t Hoff plot as −62(4) kJ/mol H 2 and −122(6) J/K mol H 2, respectively. A detailed analysis of the sample microstructure at several stages of the absorption and desorption processes has been carried out. The microstructure can be explained by hydrogen diffusion and the disproportionation/recombination reactions of the β phase.

Influence of temperature on the electrochemical characteristics of MmNi 3.03Si 0.85Co 0.60Mn 0.31Al 0.08 hydrogen storage alloys

The heat of hydride formation is a crucial parameter in characterizing a hydrogen storage alloy for battery applications. Novel AB 5-type, non-stoichiometric, lanthanum-rich MmNi 3.03Si 0.85Co 0.60Mn 0.31Al 0.08 (Mm: Misch metal) hydrogen storage metal hydride alloy electrodes are prepared. Electrochemical hydrogen absorption/desorption and electrochemical impedance measurements are carried out at various temperatures in conjunction with sintered nickel hydroxide positive electrodes. The specific capacity of the prepared metal hydride electrodes decreases from 283 mAh g −1 at 303 K to 213 mAh g −1 at 328 K. Electrochemical pressure–composition–temperature (PCT) isotherms are constructed from galvanostatic discharge curves and the change in enthalpy ( Δ H e ° ) and the change of entropy ( Δ S e ° ) of the metal hydride alloy electrodes are evaluated as −41.74 kJ mol −1 and 146.28 J mol −1 K −1, respectively. Kinetic parameters are obtained by fitting the electrochemical impedance spectrum performed at different temperatures. The charge-transfer resistance decreases with temperature, whereas exchange current density and diffusion coefficient parameters increase with temperature. It is concluded that the deterioration in capacity is due to enhanced surface activity at higher temperatures.

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La 2Ni 7–H(D) 2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg) 2Ni 7 alloys. Substitution of La in La 2Ni 7 by Mg to form La 1.5Mg 0.5Ni 7 preserves the initial Ce 2Ni 7 type of the hexagonal P6 3/ mmc structure and leads to contraction of the unit cell. The system La 1.5Mg 0.5Ni 7–H 2 (D 2) was studied using in situ synchrotron X-ray and neutron powder diffraction in H 2/D 2 gas and pressure–composition–temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi 4 slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La 2Ni 7, the unit cell of La 1.5Mg 0.5Ni 7D 9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δ a/ a=7.37%) and along [001] (Δ c/ c=9.67%). In contrast with La 2Ni 7D 6.5 where only LaNi 2 layers absorb hydrogen atoms, in La 1.5Mg 0.5Ni 7D 9.1 both LaNi 5 and LaMgNi 4 layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg) 2Ni 2, (La,Mg)Ni 3, Ni 4, tetragonal pyramidal La 2Ni 3 and trigonal bipyramidal (La,Mg) 3Ni 2 interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD 6 and a 16-vertex polyhedron LaD 16 around La. The interatomic distances are in the following ranges: La–D (2.28–2.71), Mg–D (2.02–2.08), Ni–D (1.48–1.86 Å). All D–D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the Δ H and Δ S of hydrogenation/decomposition; Δ H H=−15.7±0.9 kJ (mol H) −1 and Δ S H=−46.0±3.7 J (K mol H) −1 for H 2 absorption, and Δ H H=16.8±0.4 kJ (mol H) −1 and Δ S H=48.1±1.5 J (K mol H) −1 for H 2 desorption.