Hydrogen permeance studies in ordered ternary Cu–Pd alloys

Abstract

Considerable attention has recently been given to bimetallic systems such as Cu–Pd alloys as hydrogen storage material. These alloys are attractive alternatives to pure Pd membranes because of their sulphur tolerance and high permeability to hydrogen. The density functional theory based full potential linearized augmented plane wave and projector augmented wave methods have been implemented to understand the phenomenon of hydrogen diffusion in Cu–Pd alloys. The AuCu3 structure of CuPd3 and Cu3Pd allows intercalation of hydrogen atoms in its octahedral sites. The lattice parameters and volume of the unit cell expand on hydrogenation, without phase change.We have studied the formation and stability of hydrides of the ternary alloys. The heats of formation support the hydrogenation of CuPd3 alloy whereas the endothermic reactions indicate that hydrogen will be absorbed in the Cu3Pd alloy at the expense of energy. The phonon dispersion relations of the hydrides of CuPd3 exhibits instability due to the Pd–H bonds which leads to distortion of Pd-octahedra in the unit cell. Softening of acoustic modes is not observed in the Cu3Pd alloy; instead the optical modes due to H–H bonds have very high vibrational frequencies.The permeance of H atoms in the alloy can be quantified from the extent of hybridization between H s-states, Cu and Pd d-states. From the density of states, the movement of the H s-band and centre of d-bands towards the Fermi energy level EF was observed. Both the factors indicated that the monohydride of CuPd3 was most stable. The temperature dependent Hall and Seebeck coefficients reflect the modifications in the electronic structure due to formation of hydrides. At ambient temperatures, the free charges carriers (electrons) are found to increase in the monohydride of Cu3Pd, whereas it decreases in the monohydride of CuPd3. The electronic component of specific heats γ obtained from low temperature specific heats is found to decrease in the hydrides of both the alloys, which implies an increase in the lattice contribution through the electron-phonon coupling. From the optical properties, it is possible to monitor the hydrogen uptake in the alloys by studying the variations in reflectance, absorption coefficients in the visible regions.The electronic and dynamical studies suggest that the alloy with excess Pd has potential applications as hydrogen storage materials. The monohydride of CuPd3 appears to be appropriately stable for applications such as electrode material in rechargeable batteries.