Thermodynamic aspects of hydrogen electrosorption into Pd–Rh alloys

• Hydrogen sorption in Pd–Rh alloys was studied by cyclic voltammetry and chronoamperometry.
• Thermodynamic parameters of hydrogen absorption were determined.
• Thermodynamic stability of the β-phase decreases with Rh addition.
• Hydrogen absorption becomes less exothermic with Rh addition.
• Absorbed hydrogen probably occupies both Pd and Rh neighboring interstices.

Pd–Rh alloys were electrodeposited potentiostatically from baths containing a mixture of aqueous solutions of PdCl2, RhCl3 and HCl in various proportions. The thickness of the deposited layers was ca. 1 μm. The processes of hydrogen electrosorption were studied in 0.5 M H2SO4 solution in the temperature range of 283–328 K, using chronoamperometry and cycling voltammetry. Thermodynamic parameters of hydride formation and decomposition (Gibbs energy — ΔGα → β, ΔGβ → α, enthalpy — ΔHα → β, ΔHβ → α and entropy — ΔSα → β, ΔSβ → α) were calculated from the potentials of the α → β and β → α phase transitions. The absolute values of Gibbs energy and enthalpy distinctly decrease with increasing bulk content of Rh, while the entropy values are only slightly affected by the alloy bulk composition. It means that with increasing bulk content of Rh the hydride phase formation in Pd–Rh alloys becomes less exothermic and the hydride itself becomes thermodynamically less stable. Absorbed hydrogen probably occupies both Pd and Rh neighboring interstices.