The study of the effects of partial substitution of transition elements in zirconium-based alloys and the hydrogen storage properties, were carried out. The alloys were prepared by arc melting the constituent pure elements under argon atmosphere of 0.5 bar. Hydrogen studies were performed in the ranges 0.0001≤ P(bar)≤ 30 and 30≤ T(°C)≤ 100 using a high and low pressure hydrogen absorption/desorption unit based on pressure reduction method. The analysis indicated the crystallization of alloy in C14 hexagonal structure as confirmed from the powder x-ray diffraction studies, taken with Fe K α radiation at room temperature.

Sundara Ramaprabhu

Department Of Physics

Narayanaswami Mani

Activation energy

Alloying

Gas absorption

GAS FUEL STORAGE

High pressure effects

Isotherms

Temperature distribution

Thermodynamic properties

Transition metals

X ray diffraction

Zirconium alloys

C14-TYPE HEXAGONAL

P-C ISOTHERMS

PRESSURE RANGES

THERMODYNAMICS OF DISSOLVED HYDROGEN

ZR-BASED ALLOYS

Hydrogen

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Journal of Hydrogen Energy

The hydrogen absorption behavior of Laves phase Ho1-xTixCo2 (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho1-xTixCo2-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed. © 2007.

Journal of Physics and Chemistry of Solids

Thermodynamic and kinetic properties of Ho1-xTixCo2-hydrogen system

The heat of hydride formation is a crucial parameter in characterizing a hydrogen storage alloy for battery applications. Novel AB5-type, non-stoichiometric, lanthanum-rich MmNi3.03Si0.85Co0.60Mn0.31Al0.08 (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 He °) and the change of entropy (Δ Se °) 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. © 2008 Elsevier B.V. All rights reserved.

Journal of Power Sources

Influence of temperature on the electrochemical characteristics of MmNi3.03Si0.85Co0.60Mn0.31Al0.08 hydrogen storage alloys

The hydrogen diffusion studies in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2) have been performed in the hydrogen solid solution phase (α-phase) by measuring the time dependence of hydrogen pressure during the hydrogen absorption process. The measurements were carried out between 500 and 700 °C, and pressures 50, 70 and 90 mbar, respectively, and the diffusion coefficient data are described by an Arrhenius relation. Thermodynamic parameters namely partial enthalpy and the chemical potential of dissolved hydrogen have been determined from the pressure composition isotherm. The dependence of diffusion coefficient, activation energy and the chemical potential of the dissolved hydrogen on the alloy content in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2) have been discussed. © 2006.

Journal of Alloys and Compounds

Interstitial hydrogen diffusion in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2)

The influence of Ti on the structural and hydrogen absorption-desorption properties of Laves phase Ho1-xTixCo2 (x = 0-0.6) alloys have been studied using X-ray diffraction (XRD), pressure-composition (PC) isotherms, differential scanning calorimetry (DSC) and thermogravimetry (TG). The crystalline nature and lattice expansion of hydrogenated alloys at different hydrogen concentrations have been reported. In addition, the coexistence of two hydride phases at intermediate hydrogen concentrations has been identified from the XRD. The changes in plateau pressure and hydrogen absorption-desorption capacity upon increasing Ti content have been discussed. The preferential occupation of different types of interstitial sites has been identified from the DSC studies of hydrides. The complete desorption temperature of hydrogen has been obtained from the TG studies of hydrides. © 2007 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Structural and hydriding-dehydriding properties of Ho1-xTixCo2 alloys

The hydriding kinetics of cubic Laves phase Ho1-xMmxCo2 (x = 0, 0.1 and 0.2) have been investigated in the pressure range 100-1000 mbar and in the temperature range 100-200 °C. The experimental results are interpreted from the time dependent hydrogen absorption curves using Johnson-Mehl-Avrami model. The existence of three different phase regions, α-phase, (α + β)-phase and β-phase, have been identified from hydriding kinetic reactions and corresponding reaction rate constants. The dependence of the reaction rate constants on hydriding pressure and temperature in the (α + β) region have been discussed and the equilibrium pressures for hydrogen absorption have been calculated. The mechanism of hydriding reactions in different phase regions has been analyzed, which proceeds quite differently in different phase regions. The activation energy in the (α + β)-phase region is 13 ± 0.6, 22 ± 2 and 28 ± 3 kJ/mol H for Ho1-xMmxCo2 (x = 0, 0.1 and 0.2), respectively. © 2006 Elsevier B.V. All rights reserved.

Kinetics of hydrogen absorption in Ho1-xMmxCo2 alloys

Hydrogen absorption isotherms for the CaCu5 hexagonal structured alloys MmNi3.8Al0.4Fe0.4Co 0.4, MmNi3.5Al0.5Fe0.5Co 0.5, MmNi3.4Mn0.4Al0.4Fe 0.4Co0.4, MmNi3.368Mn0.333V 0.333Al0.3Fe0.333Co0.333 and MmNi3Mn0.333V0.333Al0.333Fe 0.333Co0.333Cu0.333 with Fe and Co in the mole ratio 1:1 have been obtained in the temperature and pressure ranges 30 ≤ T/°C ≤ 100 and 0.1 ≤ P/bar ≤ 30 using a high pressure unit based on the pressure reduction method. The powder X-ray diffractograms of the unannealed alloys show the formation of single phase. The lattice constants and the unit cell volume of these alloys increase with substitution of Mn, V, Cu, Fe, Al and Co at the Ni site in MmNi5. The hydrogen absorption isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied. The hydrogen absorption studies show no marked change in the plateau slope due to the presence of Fe and Co in 1:1 mole ratio. The maximum hydrogen intake capacity (r = nH/n f.u.) is around 4.75 in MmNi3.8Al0.4Fe 0.4Co0.4 at 30 °C and at 30 bar. The dependence of the thermodynamics of dissolved hydrogen in these alloy hydrides on the hydrogen concentration shows the different phase regions α, α+β and β seen in the hydrogen absorption isotherms. At any particular temperature investigated, the chemical potential of dissolved hydrogen in these alloy hydrides decreases with the substitution of Mn, V, Cu, Fe, Al and Co at the Ni site, which has been correlated with the increase in the volume of the unit cell of the alloys. The desorption isotherms of MmNi 3.8Al0.4Fe0.4Co0.4 at 30 and 50 °C show that the hysteresis is very small with free energy loss per cycle about 0.4 kJ/mol H at 30 °C. The kinetics of hydrogen absorption at 30, 50, 75 and 100 °C have been studied for MmNi3.8Al 0.4Fe0.4Co0.4. The different phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms. The average activation energies Ea in the α, (α+β) and β phases are found to be 0.38 eV, 0.32 eV and 0.054 eV, respectively. The diffusion coefficient at 303 K in MmNi 3.8Al0.4Fe0.4Co0.4-H is about 2.55 × 10-10 cm2 s-1. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo any structural transformation upon hydrogenation. © 2003 Elsevier B.V. All rights reserved.

Effect of substitutional elements on hydrogen absorption properties in Mm-based AB5 alloys

Hydrogen absorption pressure-composition isotherms have been investigated for the ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys having the C14 type hexagonal structure in the temperature and pressure ranges 30≤T≤100°C and 0.1≤P≤40 bar using a pressure reduction method. Powder X-ray diffractograms show that the lattice constants and the unit cell volume of ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys decrease with increasing Ni content. The P-C isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied and it was found that the plateau pressure and plateau slope at any given temperature increase with increasing Ni content except for ZrMnFe0.5Ni0.5, where the plateau slope is found to be small due to the atomic ordering in the alloy due to presence of Fe and Ni in a 1:1 molar ratio. The desorption isotherms of ZrMnFe0.5Ni0.5 at 30 and 50°C show that the hysteresis is very small. The dependence of the thermodynamics of dissolved hydrogen in ZrMnFe1 - xNix-H (x=0.2, 0.4, 0.5 and 0.6) systems in the temperature range 30-50°C on the hydrogen concentration have shown the existence of α, α+β and β phase regions as seen in the isotherms. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo a structural transformation upon hydrogenation. In ZrMnFe0.5Ni0.5, the kinetics of hydrogen absorption during activation and after several cycles at 30 °C have been studied and compared. © 2002 Elsevier Science B.V. All rights reserved.

Hydrogen storage properties of ZrMnFe1 - x Nix (x=0.2, 0.4, 0.5 and 0.6) alloys

Hydrogen absorption isotherms have been obtained for the C14-type hexagonal-structured alloys ZrMnFe1-xCox (x = 0.2, 0.4, 0.5 and 0.6) in the temperature and pressure ranges 30 ≤ T/°C ≤ 100 and 0.1 ≤ P/bar ≤ 40 using a high-pressure unit based on the pressure reduction method. The powder x-ray diffractograms show that the lattice constants and the unit-cell volume for ZrMnFe1-xCox (x = 0.2, 0.4, 0.5 and 0.6) alloys decrease with increase of XCo. The hydrogen absorption isotherms show the presence of a single plateau region (α + β) in the temperature and pressure ranges studied and it is found that the plateau pressure and plateau slope at any given temperature increase with increase of the XCo-Content, except in the case of ZrMnFe0.5Co0.5 wherein the hydrogen absorption plateau slope is found to be smaller due to the ordering of the alloy. The dependence of the thermodynamics of the dissolved hydrogen in ZrMnFe1-xCox-H (x = 0.2, 0.4, 0.5 and 0.6) systems in the temperature range 30-50 °C on the hydrogen concentration shows the different phase regions (α, α + β, β) seen in the hydrogen absorption isotherms. The powder x-ray diffractograms of the hydrides of ZrMnFe0.8Co0.2 show that the formation of different phases is manifested in the isotherms without there being any structural change and the inclusion of hydrogen in the interstitial sites expands the unit-cell volume by about 21%. Comparison of the hydrogen absorption kinetics of ZrMnFe1-xCox with that of the corresponding Ni alloys in similar experimental conditions shows that the kinetics is slow in the ZrMnFe1-xCox alloys.

Journal of Physics Condensed Matter

Study of hydrogen absorption by ZrMnFe1-xCox (x = 0.2, 0.4, 0.5 and 0.6) alloys

The hydrogen absorption pressure-composition-isotherms of the PuNi3 type rhombohedral Tb1-xZrxFe3 (x=0.1, 0.2, 0.3) alloys are investigated by Sievert's apparatus, in the temperature and pressure ranges 75≤T(°C)≤150 and 0.1≤P(bar)≤25, respectively, with a view to study the dependence of hydrogen solubility and the plateau pressure on Zr content. The P-C isotherms show the presence of a single plateau region in the temperature and pressure ranges studied and it is found that the plateau pressure at any given temperature increases with increase of Zr content along with a reduction in the hydrogen capacity. The absorption of hydrogen is accompanied by a maximum of 17.4% expansion in the unit cell volume of the host material without any change in PuNi3 type structure as evidenced by the X-ray diffraction studies on the alloy-hydrides. The presence of single and two phase regions as seen in the P-C isotherms are confirmed from the powder X-ray diffractogram of the alloy-hydrides with appropriate hydrogen concentration. The powder X-ray diffractograms on the alloy-hydrides with maximum hydrogen concentration further show the absence of hydrogen induced amorphization (H1A) upon hydrogenation.

Hydrogen absorption characteristics in the Tb1-xZrxFe3 (x=0.1, 0.2, 0.3) system

The hydrogen absorption and the kinetics of hydrogen absorption are studied on the C15 type Laves phase pseudo-binary alloy Zr0.2Ho0.8Fe2 in the temperature and pressure ranges RT ≤ T ≤ 150 °C (RT = room temperature) and 0 ≤ P ≤ 70 bar respectively. The maximum hydrogen concentration of 8.1 hydrogen atoms per formula unit is attained at the equilibrium pressure of 67 bar at RT, which is the highest value observed so far in Zr-based C15 Laves phase alloys. From the dependence of thermodynamics of dissolved hydrogen on the hydrogen concentration, the two-phase region in the isotherm corresponding to the equilibrium pressure of about 67 bar at RT is identified as the γ-phase region. This is further confirmed through the X-ray powder diffractogram of the hydrogenated samples on the two-phase regions and from the kinetics of hydrogen absorption. The relative partial molar enthalpy (ΔH̄H) and entropy (AS̄H) of hydrogen at infinite dilution are found to be in the ranges -(3-13) kJ (mol H)-1 and -(14-33) J K-1 (mol H)-1 respectively. The unit cell volume expansion (ΔV/V) upto 26% is found at the maximum hydrogen concentration at RT without change in its cubic Laves phase structure. The average activation energies (Ea) in the (α + β) and β phases are found to be 24 kJ mol-1 and 2 kJ mol-1 respectively.

Journal	International Journal of Hydrogen Energy
ISSN	03603199
Open Access	No