The Prediction of the Hydriding Thermodynamics of Pd-Rh-Co Ternary Alloys

A dilute solution model (with respect to the substitutional alloying elements) has been utilized to accurately predict the hydride formation and decomposition thermodynamics and hydrogen storage capacities of dilute Pd-Rh-Co alloys. The effect of varying the rhodium and cobalt compositions on the thermodynamics of hydride formation and decomposition and hydrogen capacity of several palladiumrhodium-cobalt ternary alloys has been investigated using pressure-composition (PC) isotherms. Alloying in the dilute regime (<10 at. pct) causes the enthalpy for hydride formation to linearly decrease with increasing alloying content. Cobalt has a stronger effect on the reduction in enthalpy than rhodium for equivalent alloying amounts. Also, an increasing alloying content of cobalt reduces the hydrogen storage capacity. The plateau thermodynamics are strongly linked to the lattice parameters of the alloys. A nearlinear dependence of the enthalpy of hydride formation on the lattice parameter was observed for both the binary Pd-Rh and Pd-Co alloys, as well as for the ternary Pd-Rh-Co alloys. The Pd-5Rh-3Co (at. pct) alloy was found to have similar plateau thermodynamics to a Pd-10Rh alloy; however, this ternary alloy had a diminshed hydrogen storage capacity relative to Pd-10Rh.