PCT Curves Research Articles

Effect of hydrogen absorption/desorption cycling on hydrogen storage properties of a LaNi3.8Al1.0Mn0.2 alloy

The effect of long-term hydrogen absorption/desorption cycling up to 3500 cycles on the hydrogen storage properties of LaNi3.8Al1.0Mn0.2 alloy was investigated. The pressure-composition (PC) isotherms for absorption/desorption and the absorption kinetics were measured at 433 K, 453 K and 473 K. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu5 type structure and kept this structure even after 3500 cycles, but the diffraction peaks were broadened. The degree of peak broadening was increased with increase of the cycle number, but exhibiting a maximum after initial activation. The shapes of PCT curves after 300, 2000 and 3500 cycles were similar to that after initial activation. It was found that the alloy subjected to 300 cycles did not exhibit significant changes in hydrogen storage capacity, but the long-term cycling up to 2000 and 3500 cycles resulted in obvious decrease in hydrogen storage capacity. The degradation of the hydrogen capacity might be resulted from the formation of the irreversible sites and more stable hydride phase, though no new phase was found after absorption/desorption cycling from XRD pattern as shown in Fig. 6 because of the limitation of XRD analysis sensibility. The hydrogen absorption kinetics after 300 cycles was deteriorated but improved again after 2000 and 3500 cycles compared with that of after initial activation. The changes in hydrogenation properties of the alloy induced by cycling were discussed by considering the crystal structure, lattice strain and pulverization of the sample.

Hydrogen absorption properties of Zr(V 1− xFe x) 2 intermetallic compounds

The Zr(V 1− x Fe x ) 2 ( x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V 1− x Fe x ) 2 alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV 2-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V 1− x Fe x ) 2 alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V 1− x Fe x ) 2 alloys.

The effect of chromium (III) oxide (Cr 2O 3) nanopowder on the microstructure and cyclic hydrogen storage behavior of magnesium hydride (MgH 2)

High energy ball milling in a Fritsch P6 planetary ball mill was used for the synthesis of the nanocomposite of MgH 2 with 10 wt.% of Cr 2O 3 nanopowder catalyst (particle size <50 nm). After ball milling for 1 h the resulting grain size of β-MgH 2 was equal to ∼30 nm. Subsequently, the nanocomposite was subjected to 150 desorption/absorption cycles at 325 °C. A gradual loss of hydrogen storage capacity upon cycling from ∼5.2 wt.% after 1 cycle to ∼4.6 wt.% after 150 cycles is observed on respective PCT curves obtained at 325 °C after every 25 cycles. The TPD curves temperature maxima are gradually shifted to a higher temperature range with increasing number of cycles which may indicate a lowering of the overall rate of transformation. Thorough microstructural investigations using such techniques as X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) of the samples after 150 cycles of desorption/absorption show profound microstructural changes such as sintering of individual powder particles into agglomerates, increase in grain/crystallite size (>100 nm), segregation of the Cr 2O 3 particles to the interfaces between the sintered Mg particles and some reduction of Cr 2O 3 to Cr and MgO. All these microstructural changes seem to be responsible for the reduction of hydrogen storage capacity upon cycling and lowering the overall rate of transformation.

A facile two-step synthesis and dehydrogenation properties of NaAlH 4 catalyzed with Co–B

Co–B doped NaAlH 4 is successfully synthesized by two-step synthesis process. The first activation step is milling NaH/Al powder and Co–B mixtures under Ar atmosphere. The second step is milling in a lower hydrogen pressure atmosphere. XRD patterns and FTIR spectrum demonstrate that NaAlH 4 is completely formed after 15 h milling in Ar atmosphere following by 40 h milling in 2 MPa H 2 atmosphere. PCT curves of as-prepared NaAlH 4 show that it can release hydrogen at a low temperature of 90 °C. The activation energy value calculated by Arrhenius equation is only 67.95 kJ mol −1. Moreover, the formation mechanism of NaAlH 4 is also discussed.

Hydrogen-storage property characterization of Mg–15 wt%Ni–5 wt%Fe 2O 3 prepared by reactive mechanical grinding

Mg–15 wt%Ni–5 wt%Fe 2O 3 (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 H 2. It desorbed 1.50 and 5.28 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 1.0 bar H 2. 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.

Phase Structure and Hydrogen Storage Characterization of the As-Cast Mg-10Ni-2Mm Alloy

The microstructural revolution of non-hydrogenated and hydrogenated Mg-based Mg-10Ni-2Mm alloy was studied. PCT and H-absorption/desorption kinetics were performed to evaluate the hydrogen storage properties. Storage capacities of 4.75, 5.03 and 5.27wt.%H for the alloy were obtained at 300, 325 and 350°C, respectively. The phases in the hydrogenated samples are mainly MgH2 and Mg2NiH4. Two absorption/desorption plateau existed in the PCT curves at each isothermal temperature. The values of ΔH and ΔS of the Mg2NiH4-formation was respectively -61.5 kJ/mol H2 and -118.6 J/mol H2 K which is lower compared with literature values. The kinetics of the H-absorption/desorption reactions for the alloy was improved by increasing the temperature. The alloy at 350°C showed the best kinetics performance of the H-absorption/desorption among the three temperatures. It is suggested that metallic particles and Mm may be mainly responsible for the improvement of the H-absorption/desorption kinetics, and Ni for the enhancement of hydrogen absorption capacity of the alloys.

Structures and Properties of TiMn2-5x(V4Fe)x(x=0.30, 0.35) Hydrogen Storage Alloys

Abstract Structures and electrochemical properties of TiMn2-5x(V4Fe)x(x=0.30,0.35) alloys were investigated. The results of XRD analysis show that the alloys are mainly composed of body centered cubic (bcc) phase, whose lattice parameters are enhanced with the increase of V and Fe content. The SEM images of the alloys show that there are some colonies in the matrix phase. The proportion of colony to the matrix decreases with increasing of V and Fe content. The electrochemical measurements show that TiMn0.25(V4Fe)0.35 alloy has unfavorable activation performance until temperature reaches 327 K, while TiMn0.50(V4Fe)0.30 alloy shows better activation performance, but passivation phenomenon was found in the process of charging at low temperature, which can be eliminated by heating to high temperature. The PCT curves of alloys show that the TiMn0.50(V4Fe)0..30 alloy has a good comprehensive performance, with a favorable plateau pressure at room temperature and wide plateau. The enthalpy and entropy in hydrogen desorption of TiMn0.50(V4Fe)0..30 alloy were also calculated and they are −36.1 kJ/mol, −126.9 J/(mol·K), respectively.

La4MgNi19水素吸蔵合金のMg蒸気圧制御におけるアニール条件と水素吸蔵特性

In La-Mg-Ni system hydrogen storage alloys, which have been attracting attention as a promising negative electrode material for Ni-MH secondary batteries, there are many phases with a variously layered stacking structure. When subjected to annealing at high temperature, the composition of these alloys becomes inhomogeneous due to the volatilization of Mg. It is therefore difficult to prepare a single phase by the normal annealing procedure. We carried out high-temperature annealing processing in which we controlled the Mg vapor pressure of La4MgNi19 alloy using a temperature-gradient furnace. By controlling the Mg vapor pressure under the sealing atmosphere, we successfully obtained homogeneous La4MgNi19. We prepared a PMg-T diagram of the phases formed for La4MgNi19 composition, and found the optimum annealing conditions for the La4MgNi19 phase. The optimum annealing conditions obtained were between T=1123 K, PMg=0.4 kPa and T=1223 K, PMg=2 kPa. However, we found the coexistence of 3R-type and 2H-type structures, which are polytypes of La4MgNi19. The lattice constants of the 3R type were a=0.5031 nm, c=4.826 nm, and those of the 2H type were a=0.5032 nm, c=3.216 nm. The hydrogen storage characteristics of the annealed alloy were obtained by measuring the PCT curves at 333 K. The PCT curves showed an almost flat plateau, PH2=0.1 MPa, H/M=1.1, 1.5 mass%H2, and good cycle characteristics. It was confirmed that the dehydrogenated alloy retained the same structure but an expansion of 1 to 2% was observed in the c-axis.

Effect of vanadium content on electrochemical properties of la-based AB 5-type metal hydride electrodes

Commercial La–Ni–Al–Co–Mn–V hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure and electrochemical properties. It is found that for the stoichiometric ‘B’-rich compound, single phase with CaCu 5-type exists. However, for B-poor compounds, there is principally a CaCu 5-type phase with a small amount of V-rich type phase and the amount of V-rich phase reduces with vanadium. With the increase of V y⩽0.1 content, hydrogen storage capacity is enhanced, whereas when y=0.2–0.3 it is decreased. The discharge capacity and cyclability are increased considerably by addition of vanadium in the range 0.02–0.1 with a maximum value at about 0.02%. The decrease of capacity for high V content was also correlated with the amount of V-rich phase. The V-rich phase is consisted of La 0.1Ni 2.6Al 0.2Co 2.0Mn 0.6V 1.3. The improvement of kinetics is due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This can be explained because the improvement of capacity for alloys with low V content is due to better kinetics. These alloys have been subjected to analysis by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (PCT curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.

A study on hydrogen adsorption of polymer protected Pt nanoparticles

Monodispersed Pt nanoparticles protected with poly(N-vinyl-2-pyrrolidone) were synthesized from H 2 PtCl 6 by the methanol reduction. The structure of obtained nanoparticles was shown to be analogous to that in bulk crystals, and the particle size was determined to be 3.2 ±0.7 nm from the observation of TEM images. Hydrogen adsorption into particles was investigated by observing PCT curves repeatedly measured at 303 K. From the result of PCT measurements, the amount of absorbed hydrogen in the present size-controlled nanoparticles was found to be more than that in Pt black.

Compositional dependence of hydriding properties of vanadium alloys at low hydrogen pressures

The alloying effects have been investigated experimentally on the hydriding properties of vanadium at low hydrogen pressures. The PCT curves for the β phase (V2H or VH) are measured using an electrochemical method. It is found that the logarithm of the plateau pressure for the V2H and the VH phases change almost linearly with the amount of alloying element in vanadium metal. Also, when the Ti/Cr compositional ratio is fixed at 1, the equilibrium hydrogen pressure increases for the V2H phase but decreases for the VH phase with increasing total content of Ti and Cr. However, both the V2H and the VH phases become unstable with decreasing Ti/Cr compositional ratio, so that the PCT curve shifts towards the lower H/M side and the second plateau region for the VH2 phase existing at high hydrogen pressures spreads to some extent, which leads to the improvement in the effective hydrogen capacity of the alloy.

Protium Absorption and Desorption Properties of bcc Ti-Fe Solid Solution Alloys Stabilized by Mo Addition

The present study investigates the effect of Mo addition to the TiFe alloys on a stabilization of bcc phase and their protium absorption and desorption properties. Ti 0.5 Fe 0.5 alloys with more than 10mol% Mo had CsCl phase and bcc phase, which show single plateaus in the PCT curves and were easily hydrogenated without homogenization. The bcc phase has larger lattice parameter than that of the CsCl phase. TiFe-10mol% Mo annealed alloys with the Ti/Fe ratio of more than 2 had a bcc phase as a main phase and CsCl as a minor one. The alloys had 3 mass% of the maximum protium capacity and desorbed protium only 1 mass%. The phases of the hydrogenated alloy were fcc and orthorhombic structure, which may be caused by di-hydrides of bcc solid solution and mono-hydrides of CsCl phase, respectively. It is found that the Mo addition to TiFe alloys could stabilize bcc phase and enhances the hydrogenation reaction of the disordered bcc Ti-Fe-Mo phase similar to Ti-Cr-X bcc alloys.

Alloying Effects on the Hydriding Properties of Niobium Metal

Niobium metal forms two kind of hydrides, NbH and NbH 2 , by hydrogenation, and the corresponding plateau region appears at low and high hydrogen permeable regions in the PCT curve of the Nb-H system. However, alloying effects have never been investigated in this system. In the present study, alloying effects were firstly investigated with various binary Nb-3 mol%M and Nb-5 mol%M alloys. Here M's were 4d transition metals, Zr, Mo, Ru, Rh and Pd, all of which were soluble in bcc Nb. The stability of each hydride was found to be varied with the addition of a small amount of alloying elements into niobium metal. For example, the β (NbH) phase became most unstable by the addition of Ru, whereas δ (NbH 2 ) phase became most unstable by the addition of Mo. These data will be useful for the design of hydrogen permeable niobium-based membranes.

Effect of V and Zr on the electrochemical properties of La-based AB 5-type metal hydride electrodes

In this paper, investigation has been carried out to find the effects of V- or Zr-addition and fluorination on the discharge capacity of AB 5-type alloy with a multicomponent La–Ni–Al–Co–Mn–M y (M=V, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3) system. Spectral studies have been made by EDS and SEM, and the crystal structure was also determined by XRD. In order to calculate the hydrogen storage capacity, the pressure–composition isotherms (PCT curves) were utilized. The metal hydride electrode was also used as a galvanostatic cycling test. As the amount of added V or Zr is increased, the portion of the second phase is also increased gradually, but the crystal structure, CaCu 5-type Laves phase, is not changed. But, in the matrix, the main component is Ni, in one of the major phases the main component is La and in another minor phase the main component is V or Zr. It has been confirmed that the addition of V or Zr causes improvement in the discharge capacity (Zr, y≤0.05; V, y≤0.1) and the cyclability (Zr, y≤0.05; V, y≤0.1) up to 200 cycles. However, adding more than y=0.2 has some significant effect on these properties. Generally, V-containing alloys may have many second phases of a round shape appearing in dispersed form in the matrix, while Zr-containing alloys’ second phases appear with a plate or needle-like shape. La-rich particles precipitated along boundaries of the second phases which can act as sites of preferred hydrogen absorption, to increase the discharge capacity. In addition, many micro-cracks are formed around the La-rich particles at the surface of the La-rich particle itself. Therefore, such second phases are considered as catalytic sites for hydrogen penetration, surface reaction, initial activation and change transfer properties.

Effect of Ti and Zr additions on the characteristics of AB 5-type hydride electrode for Ni–MH secondary battery

AB 5-type intermetallic compounds were prepared by arc-melting in an argon atmosphere. The composition of a stoichiometric compound ( LMNi 3.6 Al 0.4 Co 0.7 Mn 0.3) M y ( LM= La -rich mischmetal , M=Ti, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3 ) with a hexagonal CaCu 5 structure was varied by stoichiometric and nonstoichiometric addition of Ti or Zr. The alloy surface was analyzed by energy-dispersive spectroscopy and scanning electron microscopy and the crystal structure was characterized by X-ray diffraction. To determine hydrogen storage capacity, the pressure composition isotherms (PCT curves) were utilized. Metal hydride electrode was characterized by galvanostatic cycling test. It was found that the addition of Ti ( y=0.05) or Zr ( y=0.02) improves the activation, discharge capacity and cycle life property. But, the alloy with Ti addition ( y⩾0.2), containing a Zr addition ( y⩾0.2), is found to have bad discharge capacity and cycle life. The kinetic properties are additionally increased by the formation of a Ti- or Zr-rich second phase. The improvement in discharge capacity caused by increasing the Ti or Zr content (0.1⩾ y) in the alloy is attributed to active sites for the electrochemical reaction. Also, they seem to be strongly related with the shapes and amounts of the second phases.

Influence of carbon on the electrode characteristics of MgNi prepared by mechanical alloying

The influence of carbon addition on the characteristics of MgNi alloy prepared by mechanical alloying and used as metal hydride electrode has been studied. Our results indicate that, despite its low proportion, carbon addition has a major deleterious effect on the electrode performance, i.e. the initial discharge capacity decreases exponentially from 522 to 332 mA h g−1 with carbon content increasing from 0 to 3.5 wt.%. This exponential decay cannot be only explained by a decrease in the H solubility due to the dissolution of C atoms in the hydrogen interstitial sites. The cycle life of the electrode is not influenced by the carbon content. The particle morphology and the amorphous structure of the material are unmodified by the carbon addition. In addition, our results indicate that the hydrogen diffusivity in MgNi is unchanged by carbon addition (DH/a2=3.2×10−5 s−1). In contrast, the exchange current density I0 decreases from 92 to 62 mA g−1 with increasing carbon content in the alloy. This results indicate that carbon limits the charge-transfer reaction at the surface of the alloy, which must have an influence on the initial discharge capacity of the alloy. In addition, the electrochemical PCT curves reveal that the slopes of the isotherms become steep with C addition indicating a wider distribution of energy levels for hydrogen. This is considered as the major reason for the observed decrease in the initial discharge capacity of the MgNiCx alloys with increasing x.

The effect of Zr substitution for Ti on the microstructures and electrochemical properties of electrode alloys Ti 1− xZr xV 1.6Mn 0.32Cr 0.48Ni 0.6

The effect of Zr substitution for Ti on the microstructures and electrochemical properties of the electrode alloys Ti 1− x Zr x V 1.6Mn 0.32Cr 0.48Ni 0.6 ( x=0.2,0.3,0.4,0.5) has been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectroscope analyses that all the alloys consist of a C14 Laves main phase with hexagonal structure and a V-based solid solution secondary phase with b.c.c. structure. A small amount of TiNi-based third phase with b.c.c. structure has been found precipitated in the C14 Laves main phase in addition. With the increase in the amount of Zr substitution, the lattice parameters of the main phase and secondary phase are found increased and decreased, respectively. The electrochemical PCT curves indicate that the maximum hydrogen absorption capacity [H/M] max decreases with increasing Zr substitution, which may be attributed to the decrease in the content of V-based solid solution. The maximum discharge capacity and high rate dischargeability of the alloy electrodes both decrease, while the cyclic durability increases with the increasing amount of Zr substitution.

Alloying Effects on the Phase Stability of Hydrides Formed in Vanadium Alloys

The alloying effects on the stability of vanadium hydrides have been investigated systematically for binary V-M and ternary V-Ti-M alloys, where M's are various alloying elements. The PCT curves for the β phase (V 2 H or VH) are measured at low hydrogen pressures using an electrochemical method. On the other hand, for the y phase (VH 2 ), the PCT curves are measured at high hydrogen pressures using an ordinary Sieverts-type apparatus. It is found that the phase stability of vanadium hydrides is affected strongly by the presence of a small amount of elements added to vanadium metal. The alloying effects on the hydride stability are different between the y phase and the β phase. For example, the y phase becomes most unstable when the group 8 elements in the periodic table, Fe, Ru and Os, are added to vanadium metal. On the other hand, the stability of the β phase changes monotonously following the order of elements in the periodic table. Also, the efficiency of alloying element in modifying the hydride stability is found to be different between the β 1 phase (V 2 H) and the β 2 phase (VH). In addition, it is shown that the logarithm of the plateau pressures for the β 1 and the γ phases change almost linearly with the amount of alloying element in the alloy. Using this linear relationship for the y phase, the second plateau pressures of ternary V-Ti-Ru and V-Ti-Cr alloys can be estimated quantitatively in the wide compositional range.

Protium Absorption Properties of Ti-Cr-V-Mn Alloys in Low Pressure Regions

The present study aims to investigate the effects of addition of Mn to Ti–Cr–V alloys on their protium absorption properties and to find the design criteria for increasing effective protium capacity. The effects of V and Mn content of the alloys on the protium absorption properties of heat-treated Ti–Cr–V–Mn alloys in low pressure regions were studied at various temperatures, and it was found that increasing Mn content and decreasing V content unstabilize protoride in low pressure region. The PCT curves from low pressure plateau region (10 −1 –10 5 Pa) to high pressure plateau region (10 5 –10 7 Pa) shifted to the side of low protium concentration. It was found that the addition of Mn or decreasing V content in the alloys is effective for increasing effective protium capacity.

Hydrogenation Behavior in Ti-Cr Alloys with Compositions around TiCr&lt;SUB&gt;2&lt;/SUB&gt; Laves Phase

Hydrogenation behavior in Ti–Cr alloys with compositions around TiCr2 Laves phase has been examined by the tritium radioluminography and the PCT measurement. All of the specimens have been two-phase structure of BCC phase and Laves phase. The ratio of the BCC phase to the Laves phase has been increased as increasing the heat treatment temperature. The Ti-rich BCC phase and Cr-rich BCC phase have been formed in the Ti50Cr50 alloy and the Ti30Cr70 alloy, respectively. It is found that hydrogen easily penetrates into the Laves phase in the initial stage of hydrogen penetration. In the PCT measurements, Ti50Cr50 alloy absorbed the largest amount of hydrogen. The maximum protium storaging capacity of 3.5 mass% was obtained in the Ti50Cr50 specimen heat treated at 1400 ◦ C at the measured temperature of 150 ◦ C. The plateau of PCT curves in Ti50Cr50 alloy and Ti30Cr70 alloy appeared at low hydrogen pressure and high hydrogen pressure region, respectively. These plateaus are attributed to the Ti-rich BCC phase and the Cr-rich BCC phase, respectively.