Hydrogen permeation through a Pd/Ta composite membrane with a HfN intermediate layer

• Dense HfN layer retarded the reaction between the Pd coating and Ta substrate at 873 K.
• HfN was permeable for hydrogen, although the permeability was smaller than that of Ta.
• Hydrogen solubility in HfN was higher than that in Pd and comparable with that in Ta.
• The small permeability of hydrogen in HfN was attributed to small diffusivity.

Dense hafnium nitride (HfN) layers were prepared between Pd protection films and a Ta substrate in a composite hydrogen separation membrane to prevent a reaction between the Pd and the substrate at high temperatures. No significant reduction in hydrogen permeation rate was observed for the membrane with 50-nm-thick HfN layers at 873 K through at least 35 h, whereas the specimen without HfN layers rapidly deteriorated within 5 h. Hydrogen permeability of the former specimen was 4 × 10−9 mol m−1 s−1 Pa−0.5 at 873 K at steady state. This value was smaller than the initial permeability of Pd-covered Ta before deterioration by an order of magnitude. The measurements of pressure–composition isotherms by using a HfN powder specimen showed that the hydrogen solubility in HfN was sufficiently high and comparable with the solubility in Ta. Therefore, the low permeability observed with the HfN intermediate layers was ascribed to low hydrogen diffusivity in HfN.