Effect of substitutional elements on hydrogen absorption properties in ZrMnFe 0.5Ni 0.5 and ZrMnFe 0.5Co 0.5

Abstract

The effect of partial substitution of transition elements (M=V, Cr, and Al) in ZrMnFe 0.5Ni 0.5 and ZrMnFe 0.5Co 0.5 systems on the lattice parameter, hydrogen storage properties such as plateau pressure, plateau slope and thermodynamic of dissolved hydrogen have been studied. The powder X-ray diffractograms of ZrMnFe 0.45Co 0.45Ni 0.1, ZrMnFe 0.5Co 0.25Ni 0.25, ZrMn 0.95V 0.05Fe 0.5Ni 0.5, ZrMn 0.9V 0.1Fe 0.5Ni 0.5, ZrMn 0.7V 0.15Cr 0.15Fe 0.5Ni 0.5, ZrMn 0.95V 0.05Fe 0.5Co 0.5, ZrMn 0.5Cr 0.5Fe 0.5Co 0.5, ZrMn 0.45Cr 0.45V 0.1Fe 0.5Co 0.5, ZrMn 0.5Cr 0.5Fe 0.45Co 0.45V 0.1, ZrMn 0.55Cr 0.3V 0.15Fe 0.5Co 0.5, ZrMn 0.5Cr 0.25V 0.25Fe 0.5Co 0.5, ZrMn 0.25Cr 0.5V 0.25Fe 0.5Co 0.5 and ZrMn 0.25Cr 0.25V 0.25Al 0.25Fe 0.5Co 0.5 alloys show the formation of C14 single phase and the lattice constants and the unit cell volume of these alloys are found to increase with substitution of V and Cr at the Mn site in ZrMnFe 0.5Ni 0.5 and ZrMnFe 0.5Co 0.5. The hydrogen absorption isotherms of these alloys using high pressure hydrogen absorption/desorption setup in the temperature and pressure ranges 30⩽T (° C)⩽100 and 0.001⩽P ( bar)⩽30 have been obtained. The maximum hydrogen intake capacity is around 3.5 hydrogen atoms per formula unit at room temperature in ZrMn 0.5Cr 0.25V 0.25Fe 0.5Co 0.5. The plateau pressure, at any particular investigated temperature, decreases with increase of V content. The plateau slope increases when the ordered structure is disturbed by the substitution of other chosen elements at the B site. The plateau pressures in these alloy-hydrides at the temperature investigated depend mainly on their unit cell volume. 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 as seen in the hydrogen absorption isotherms. The chemical potential of dissolved hydrogen (Δ μ H) decreases with the substitution of V and Cr at Mn site in ZrMnFe 0.5Ni 0.5 and ZrMnFe 0.5Co 0.5, which is attributed to the increase in the volume of the unit cell of the alloys. The desorption isotherms of ZrMn 0.5Cr 0.25V 0.25Fe 0.5Co 0.5 show a free energy loss per cycle of about 0.89 kJ mol −1 H at 50°C. The powder X-ray diffractograms of the hydrides of ZrMn 0.95V 0.05Fe 0.5Ni 0.5 and ZrMn 0.25Cr 0.5V 0.25Fe 0.5Co 0.5 show the different phases exists in the isotherms with out any structural change with the inclusion of hydrogen in the interstitial sites with a volume expansion of about 19%. The kinetics of hydrogen absorption at 30°C, 50°C and 75°C have been studied for ZrMn 0.95V 0.05Fe 0.5Co 0.5. The different phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms. The average activation energies E a obtained from kinetics of hydrogen absorption in α+ β and β phase are 205±6 and 540±10 meV , respectively. The diffusion coefficient in β phase at 30°C in ZrMn 0.95V 0.05Fe 0.5Co 0.5-H is about 6.9×10 −11 cm 2 s −1 .