Alloying effects on the hydrogen-storage capability of Pd-TM-H (TM = Cu, Au, Pt, Ir) systems

Pressure-composition isotherms and the magnetic susceptibilities of Pd-TM-H (TM = Cu, Au, Pt, and Ir) systems were measured at ambient temperature, and the effects of alloying between Pd and transition metals on the hydrogen storage capability of these Pd-TM alloys were investigated by considering their electronic band structures. All of the magnetic susceptibilities for the Pd-TM-H systems decreased linearly with hydrogen uptake. For the Pd-Cu alloy, the magnetic susceptibility was nearly zero at the terminal composition of hydrogen in the plateau region obtained from the pressure-composition isotherm, and the terminal composition decreased with increasing Cu substitution. These results indicated that the hydrogen-storage capability was proportional to the amount of unoccupied d states in the electronic band structure of the Pd-Cu alloy. The Pd-Au-H system exhibited substantially the same behavior as the Pd-Cu-H system. For the Pd-Pt and Pd-Ir alloys, the magnetic susceptibility at the terminal composition of hydrogen in the plateau exhibited a finite positive value, indicating that the unoccupied d states in the Pd-Pt and Pd-Ir alloys were not filled when the maximum quantity of hydrogen was stored in the alloys. These finite magnetic susceptibilities at the terminal composition of hydrogen in the plateau region were explained by the structural modification of the unoccupied d states in the electronic band structures due to alloying.