Pressure Composition Isotherm Curves Research Articles

Design of multicomponent alloys with C14 laves phase structure for hydrogen storage assisted by computational thermodynamic

Design methods with predictive properties modeling are paramount tools to explore the vast compositional field of multicomponent alloys. The applicability of an alloy as a hydrogen storage media is governed by its thermodynamic properties, which can be represented by pressure-composition-temperature (PCT) diagrams. Therefore, the prediction of PCT diagrams for multicomponent alloys is fundamental to design alloys with optimized properties for hydrogen storage applications. In this work, a strategy to design C14-type Laves phase multicomponent alloys for hydrogen storage assisted by computational thermodynamic is presented. Since electronic and geometrical factors play an important role in the formation and stability of multicomponent Laves phase, valence electron concentration (VEC), atomic radius ratio (rA/rB), and atomic size mismatch (δ) are initially considered to screen a high number of compositions and find alloy systems prone to form Laves phase structure. Then, CALPHAD method was employed to investigate the phase stability of alloys of the (Ti, Zr or Nb)(V, Cr, Mn, Fe, Co, Ni, Cu, or Zn)2 system, resulting in 440 alloys prone to solidify as C14 Laves phase structure. In addition, we present a thermodynamic model, which allowed calculating the PCT diagrams of the suggested C14 Laves phase alloys based solely on the alloy's composition. For these calculations, the entropy and enthalpy of hydrogen solution in the C14 Laves phase were modelled considering that hydrogen solid solution occurs only at the A2B2-type interstitial sites of the C14 Laves phase structure. Therefore, the room temperature PCI diagrams of 440 C14 Laves phase multicomponent alloys were calculated. Experimental pressure-composition-isotherm (PCI) diagrams of six C14 Laves phase alloys were compared against the calculated ones. Although the PCI curve shapes were not perfectly predicted for some alloys, the order of magnitude of the equilibrium pressure for all the tested alloys were well predicted. The results show that C14 Laves phase multicomponent alloys within a wide range of equilibrium pressure at room temperature can be obtained, being promising candidates for different hydrogen storage applications, such as room temperature tanks, hybrid tanks and Ni-metal hydrides batteries.

Effects of Stoichiometry on the H2 -Storage Properties of Mg(NH2 )2 -LiH-LiBH4 Tri-Component Systems.

The hydrogen desorption pathways and storage properties of 2 Mg(NH2 )2 -3 LiH-xLiBH4 samples (x=0, 1, 2, and 4) were investigated systematically by a combination of pressure composition isotherm (PCI), differential scanning calorimetric (DSC), and volumetric release methods. Experimental results showed that the desorption peak temperatures of 2 Mg(NH2 )2 -3 LiH-xLiBH4 samples were approximately 10-15 °C lower than that of 2 Mg(NH2 )2 -3 LiH. The 2 Mg(NH2 )2 -3 LiH-4 LiBH4 composite in particular began to release hydrogen at 90 °C, thereby exhibiting superior dehydrogenation performance. All of the LiBH4 -doped samples could be fully dehydrogenated and re-hydrogenated at a temperature of 143 °C. The high hydrogen pressure region (above 50 bar) of PCI curves for the LiBH4 -doped samples rose as the amount of LiBH4 increased. LiBH4 changed the desorption pathway of the 2 Mg(NH2 )2 -3 LiH sample under a hydrogen pressure of 50 bar, thereby resulting in the formation of MgNH and molten [LiNH2 -2 LiBH4 ]. That is different from the dehydrogenation pathway of 2 Mg(NH2 )2 -3 LiH sample without LiBH4 , which formed Li2 Mg2 N3 H3 and LiNH2 , as reported previously. In addition, the results of DSC analyses showed that the doped samples exhibited two independent endothermic events, which might be related to two different desorption pathways.

Pressure–composition isotherm curves of a Ti–H system above the eutectoid temperature

The pressure–composition isotherm (PCI) curves of a Ti–H system were measured over the range 350–550°C in a high-vacuum Sievert's apparatus. At 350°C, the hypo- and hyper-eutectoid Ti–H alloys were found to be stable at hydrogen pressures of 0.9 and 2.2Pa, respectively. This study successfully provides full PCI isotherm curves over the range 350–441°C, previously absent from the literature. These PCI curves improve our understanding of the hypo- and hyper-eutectoid phase transitions in the Ti–H phase diagram in terms of the thermodynamic properties, kinetics, phase boundaries and plateau pressures.

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Hydrogen absorption/desorption behavior of Mg50La20Ni30 bulk metallic glass

In this work, Mg50La20Ni30 bulk metallic glass (BMG) was prepared and its hydrogen absorption/desorption behavior was studied. The amorphous structure was found to be retained after gaseous hydrogenation. The Tg and Tx of the hydrogenated Mg50La20Ni30 BMG was 561 K and 619 K respectively, much higher than the corresponding value of 463 K and 504 K of the unhydrogenated sample. Mg50La20Ni30 BMG absorbed 0.73 and 1.85 wt% hydrogen within 1 h at 313 K and 423 K respectively, which was higher than that of ball-milled Mg2Ni alloy. Mg50La20Ni30 BMG exhibited an equilibrium hydrogen absorption plateau with a pressure of 0.07 MPa in pressure–composition isotherm curve at 423 K. It suggested that Mg-based BMGs are promising materials for hydrogen storage applications.

Synthesis and hydrogen absorption kinetics of V4Cr4Ti alloy

V4Cr4Ti alloy is synthesized by aluminothermy process followed by electron beam refining. Hydrogen absorption characteristics of the alloy have been evaluated by measuring the pressure composition isotherm (PCIT) at 57 °C temperature. Two plateau pressures are observed in the PCIT curve. Substantial decrease in the hydrogen absorption capacity of the alloy as compared to vanadium has been recorded. Hydrogen absorption kinetics of the alloy was investigated in the temperature range of 200–500 °C. Three-dimensional diffusion appears to be the rate controlling step of the hydrogen absorption. The apparent activation energy was calculated as 0.16 eV/atom-hydrogen.

Hydrogen Storage Properties of Mg-BCC Composite

MgH2 with 10 wt% Ti0.4Mn0.22Cr0.1V0.28 (termed BCC for its body-centered cubic structure) nanocomposite was fabricated by ball milling using different ball-to-powder weight ratios. The X-ray diffraction patterns make it clear that pure Mg powder is partly transformed to MgH2, while by adding the BCC, its hydriding becomes complete. The scanning electron microscope images showed that the BCC particles were uniformly dispersed on the surface of the Mg particles. Differential scanning calorimetry traces of the samples showed that the addition of the BCC obviously decreases the desorption temperature, and an additional decrease is observed from increasing the ball-to-powder weight ratio. The hydriding/dehydriding and the pressure-composition isotherm curves indicate significant improvement in the absorption/desorption kinetics and the hydrogen storage capacity of MgH2 from both adding the BCC and increasing the ball-to-powder weight ratio. The results indicate that the BCC acts as a medium that facilitates hydrogen absorption during hydrogenation on Mg, thus improving hydrogen storage capacity and absorption/desorption kinetics.

Self-ignition combustion synthesis of TiFe 1− xNi x in hydrogen atmosphere

This paper describes the self-ignition combustion synthesis ( SICS) of the hydrogen storage alloy TiFe 1− x Ni x ( x = 0, 0.5, 0.1, 0.2, and 0.4) in a hydrogen atmosphere and the hydrogenation properties of the obtained products, in which well mixed powder of Ti, Fe, and Ni was uniformly heated to the eutectic temperature of the Ti–Fe system (1358 K) for self-ignition. TiFe 1− x Ni x was successfully obtained through five complex continuous steps. The activation behavior of TiFe improved considerably after Ni substitution. As x increased, the pressure–composition–isotherm curves shifted drastically to the low-pressure region. In addition, the hysteresis factor, HF, decreased, and the plateau flatness, PF, increased. These results also demonstrate that SICS offers many benefits of minimizing operating time, saving energy, releasing activation treatment and supplying the product of TiFe 1− x Ni x alloys similar to those produced by ingot metallurgy.

Improvement of Mg–Al alloys for hydrogen storage applications

In the context of energy carrier, storage of hydrogen is one of the key challenges for research today. The group of Mg-based hydrides stands as a promising candidate for competitive hydrogen storage with high reversible hydrogen capacity. The present studies report encouraging results for Mg–Al system with Nb 2O 5 additive catalyst. Three nominal compositions [(Mg x Al 100− x )−99 + (Nb 2O 5)−1 mol.%], X = 100 (Mg), X = 39 ( β-Mg 2Al 3) and X = 70 (Mg + γ-Mg 17Al 12) have been investigated for their hydrogen equilibrium pressure of absorption and desorption reactions, from 250 °C to 400 °C. Decomposition of initial Nb 2O 5 phase was shown after long annealing of samples, with reduction into other oxides. Moreover, the hydrogenation of Mg–Al alloys leads to separate initial phase(s) through several stages with the formation of MgH 2 hydride and metallic Al-fcc as the final products. This behavior is clearly visible from multi-plateaux of pressure–composition isotherm curves. In the case of the X = 70 compound, the reaction was accomplished through three reversible transformation steps. The hydrogen weight capacity of 4.7 wt.% has been found at 250 °C under reproducible conditions.

Rietveld analysis of neutron powder diffraction of Mg 6Pd alloy at various hydriding stages

The evolution of the crystal structure of Mg 6Pd alloy, synthesized by ball milling, was investigated by simultaneous Rietveld refinement of neutron and X-ray powder diffraction. Samples with different deuterium contents were measured, corresponding to reaction end-products of proposed hydrogenation step. After full hydrogenation, Mg 6Pd alloy transforms to MgPd alloy and MgD 2. Increases in lattice parameters of MgPd alloy agrees well with measured hydrogen capacities. There are some evidences that at each hydrogenation step in magnesium alloys, magnesium atoms with high values of thermal parameters are the ones that will form magnesium hydride upon hydrogenation. Magnesium hydride phases presented a high level of strain which could be related to the important hysteresis in the pressure-composition isotherm curve.

Microstructure and some dynamic performances of [formula omitted][formula omitted] hydrogen storage electrode alloys

Microstructure and some dynamic performances of Ti 0.17 Zr 0.08 V 0.34 RE 0.01 Cr 0.1 Ni 0.3 ( RE = Ce , Dy) hydrogen storage electrode alloys have been investigated using XRD, FESEM–EDS, ICP-MS and EIS measurements. The alloy is composed of V-based solid solution phase with a dendritic shape and a continuous C14 Laves phase with a network shape surrounding the dendrite. Pressure–composition isotherm curves indicate that the alloy with Dy addition has a lower equilibrium hydrogen pressure and a wider plateau region. The alloy electrode with Dy addition has higher discharge capacity, while the alloy electrode with Ce addition has better activation and higher cycle stability. The alloy electrode with Ce addition has better electrochemical activity with higher exchange current density ( 127.5 mA g - 1 ) , lower charge transfer resistance ( 1.37 Ω ) and lower apparent activation energy ( 30.5 kJ mol - 1 ) . The capacity degradation behavior for the alloy electrode is attributed to two main factors: one is the dissolutions of V and Zr element to KOH solution, and another is the larger charge transfer resistance which increases with increasing cycle number. Hydrogen diffusion coefficient is a rate-limiting factor at higher discharge current, while exchange current density is the rate-limiting factor at lower discharge current, both hydrogen diffusion coefficient and exchange current density are responsible for the poor high-rate dischargeability of the alloy in this case.

Analysis of hydride formation for hydrogen storage: Pressure–composition isotherm curves modeling

Abstract A thermodynamic model of a partially open two-phase system initiated by Schwarz and Khachaturyan is extended for the hydrogen storage investigation. A particular attention is paid to the modeling of the evolution of hydrogen content absorbed in the solid solution as a function of the pressure of hydrogen gas (for isothermal external conditions), e.g. PCI curves. The extension concerns the description of anhysteretic PCI curves with plateau or slope. Finally, the experimental data including anhysteretic behavior and also hysteretic one are fairly described by our modeling.

Crystallographic and thermodynamic study of La 0.55Y 0.45Ni 5–H 2, a candidate system for hydrogen buffer tanks

Electrical vehicles (EV) powered by a fuel cell will need a buffer tank to supply hydrogen especially for cold start and/or to overcome the delayed response time in the hydrogen supply system (reformer, dehydrogenation, etc.). To allow hydrogen supply at low temperature, the crystallographic and thermodynamic properties of the system La 0.55Y 0.45Ni 5– H 2 have been investigated. The pressure–composition–isotherm (PCI) curves have been carried out between 253 and 318 K. The compound La 0.55Y 0.45Ni 5 can deliver 5.4 H/f.u. (1.3 wt.%) under 0.35 MPa at 253 K. At room temperature, between 0.6 and 3.3 H/f.u., a well-defined plateau is observed in the PCI curve. However, for capacity values larger than 3.3 H/f.u., a step is observed followed by a sloppy domain. Investigation by neutron diffraction at various states of charge has allowed determination of the La 0.55Y 0.45Ni 5– D 2 phase diagram. The existence of an intermediate γ phase has been established. The crystal structures of the α , γ and β phases have been determined. The occurrence of the γ phase could significantly increase the cycle life of the hydride by decreasing the constraints which are generated during absorption/desorption cycling as previously observed in other LaNi 5-type systems.

Hydrogen absorption properties of Li–Mg–N–H system

Isothermal hydrogen absorption properties of the ball milled mixture of 3Mg(NH 2) 2 and 8LiH after dehydrogenation at 200 °C under high vacuum were investigated at two different temperatures of 150 and 200 °C. The pressure–composition isotherm (PCT) curve at 200 °C revealed a two-plateaus-like behavior, while the PCT curve at 150 °C showed a single-plateau-like behavior. The hydrogenated phases were composed of LiH and Mg(NH 2) 2 under 9 MPa at 200 °C, while those were observed as mixed phases of LiH and LiNH 2 at 150 °C without any trace of Mg(NH 2) 2 in XRD measurements. These results indicate that there are two-step hydrogenation processes corresponding to high and low pressures at 200 °C, but the kinetics at 150 °C is too slow to proceed with the second hydrogenating step at high pressure region.

Neutron diffraction study of the deuterides of the over-stoichiometric compounds LaNi 5+ x

The deuterides of three intermetallic compounds LaNi 5+ x with x=0, 0.2 and 0.4 have been prepared and analyzed by neutron powder diffraction at two different deuterium concentrations. On one hand, the crystallographic properties of the α and the β phases have been studied with the two phases in equilibrium on the pressure plateau characteristic for the phase transition. On the other hand, the β phase has been studied as a single phase in the solid solution domain. Crystal structures were refined using the Rietveld method. Crystal symmetry, lattice parameters and deuterium sites and occupancy parameters are reported for all the different phases. Anisotropic line profile analysis has been used to characterize the strains induced by deuterium absorption in the various regions of the Pressure–Composition–Isotherm curves. Results are compared for the different values of x and related to the cycle life properties for each compound.

In situ neutron-diffraction study of deuterium desorption . from LaNi 5+x ( x ?1) alloy

Structural changes during deuterium desorption from highly over-stoichiometric LaNi5.95 alloy have been monitored by in situ powder neutron diffraction and related to its pressure–composition–isotherm (PCI) at room temperature. The PCI curve exhibits no clear plateau pressure and a maximum capacity of 4.9 D-atoms per unit cell (D/u.c.) at 10 MPa. However, structural investigations reveal a phase transition from β-deuteride to α-solid solution with a reduced α+β coexistence region, which extends from 1.4 to 3.5 D/u.c. This transition exhibits variable α- and β-lattice volumes and involves a minor lattice expansion of 3.5 vol. %. It was found that the required volume expansion to host D-atoms in the β-phase is as high as 4.8 A3/D, which could be related to the reduced H-storage capacity in over-stoichiometric LaNi5+x alloys.

The Effect of Annealing on the Discharge Characteristics of a Zr‐V‐Mn‐Ni Hydrogen Storage Alloy

In order to improve the discharge performance of the alloy electrode, the alloy was annealed at 1000°C for 12 h. It is shown that the total hydrogen storage capacity is unchanged, but the plateau pressure is increased in the lower hydrogen concentration range of the pressure composition isotherm curve after annealing. The annealed alloy has a higher rate capability and a higher discharge capacity than the as‐cast one. In order to analyze the above phenomena, the reaction surface area and surface reaction kinetics were examined by Brunauer, Emmett, and Teller method (BET) and electrical impedance spectroscopy (EIS) analyses, respectively. The measurement of the fully activated electrode surface area by BET analysis shows that there is little difference between as‐cast and annealed alloy. The higher rate capability is due to the decrease of charge‐transfer resistance for the hydrogenation reaction in the KOH electrolyte after annealing. It is well known that the Ni component plays a role in improving the electrocatalytic activity of the alloy electrode. The microstructures of the as‐cast and annealed alloys were investigated by scanning electron microscope and energy‐dispersive spectroscopy analyses. The second phase (Zr‐Ni phase) is widely distributed in as‐cast alloy, but this second phase disappears after annealing, resulting in the homogenizing of the alloy and an increased Ni content in the matrix. The annealed alloy has a higher exchange current density than any single phase of as‐cast alloy. Consequently, it is concluded that the improved rate capability of the annealed alloy is due to increased Ni content in the matrix resulting from the disappearance of the secondary phase.

In situ neutron diffraction study of solid gas desorption of non-stoichiometric AB 5 type hydrides

In situ neutron diffraction studies during in beam solid gas desorption have been carried out on both stoichiometric LaNi 4CuD y and non-stoichiometric LaNi 5CuD y deuterides. The first compound exhibits a two phase behaviour typical of the β to α transformation in agreement with the plateau pressure observed in the pressure composition isotherm curve. The second one shows only one phase typical of a solid solution. The continuous cell volume variation observed for the non-stoichiometric LaNi 5Cu deuteride is assumed to be the origin of its better cycle life when used as negative electrode material in NiMH batteries.