Palladium and palladium alloy membranes for hydrogen separation and production: History, fabrication strategies, and current performance

Our aim is to survey the state-of-the-art in the development of palladium and palladium alloy membranes for hydrogen separations and hydrogen production. At the time this paper was written, several research groups have developed palladium and/or palladium alloy membranes with pure hydrogen permeances approaching 0.01 mol/m2 s Pa0.5. This includes our work to prepare Pd and Pd alloy composite membranes on stainless steel porous substrates with zirconia diffusion barriers. A 0.93 μm thick pure Pd composite membrane on a Pall AccuSep® stainless steel substrate achieved a pure hydrogen flux of 1.3 mol/m2 s at 400 °C and 1.38 bar differential pressure. The hydrogen flux values we have measured compare favorably to the U.S. DOE 2010 pure hydrogen flux performance target of 0.96 mol/m2 s = 250 ft3(STP)/ft2 h at conditions of 400 °C and a differential pressure of 20 psi or 1.38 bar. Pd alloy composite membranes containing Au and/or Ag show higher hydrogen flux than pure Pd membranes, as expected from the literature. At these same DOE test conditions of 400 °C and 1.38 bar, the pure hydrogen flux for a 2.3 μm thick, Pd95Au5 composite membrane was 1.01 mol/m2 s. Approximately the same pure hydrogen flux of 0.97 mol/m2 s was measured for a thicker, 4.6 μm, Pd80Ag20 composite membrane at 400 °C and 1.38 bar, consistent with the higher permeability of the Pd–Ag alloy compared to pure Pd and Pd–Au. Ideal H2/N2 separation factors of these composite membranes ranged from 337 to over 80,000.