Palladium (Pd) has high solubility and diffusivity of hydrogen and shows therefore great promise as a membrane material for medium to high temperature hydrogen separation (250–550 °C). In pure Pd, however, an α-to-β-hydride phase transition may occur in hydrogen below about 290 °C, and only a few cycles through this transition make the material brittle and must be avoided. By alloying Pd with different elements, the phase transition can be suppressed, and the majority of the work related to Pd-alloy membranes applies particularly alloys with 20–30 wt.% Ag and 40 wt.% Cu. The drawback is, however, that these alloys are to various degrees prone to poisoning by CO- and sulfur-containing gases leading to reduced H₂ flux or even to a complete membrane failure. Research therefore currently focuses on developing more advanced ternary or quaternary alloys that may be needed to improve the mechanical, thermal, and chemical stability of Pd-based membranes.

The main problems with membrane...