Effects of thin film Pd deposition on the hydrogen permeability of Pd60Cu40 wt% alloy membranes

Pd–Cu alloys have great potential as hydrogen separation membranes due to their relatively low cost and excellent durability compared to commercial dense metal membranes such as Pd and Pd–Ag alloys. At a composition of Pd60Cu40 wt% the body centred cubic (bcc) phase possesses the highest hydrogen permeability of the Pd–Cu alloy system. Furthermore, a Pd–Cu membrane containing a Pd-rich Pd–Cu fcc phase exhibits improved resistance to hydrogen sulphide (H2S) contamination.

The basis of this study was to modify the surface composition of bcc Pd60Cu40 wt% membranes through deposition of a Pd thin film onto one side to produce a stable Pd-rich fcc layer and to investigate its effects on hydrogen permeability. The hydrogen permeability of two as-received Pd60Cu40 wt% membranes (Membranes 1 and 2) was measured in addition to two Pd coated Pd60Cu40 wt% membranes (Membranes 3 and 4) for comparison. A Pd-rich Pd–Cu fcc phase with an approximate composition of Pd70Cu30 wt% was formed as a result of Cu interdiffusion between the Pd–Cu bulk membrane and the Pd thin film during hydrogen permeability testing. This new phase was responsible for the reduction in hydrogen permeability shown in the Samples 3 and 4.

Using variable temperature in-situ X-ray diffraction(XRD), it was observed that Cu interdiffusion occurred from the Pd–Cu bulk membrane and into the Pd thin film between 300 and 600 °C under 445 kPa of flowing helium. Under 445 kPa of flowing hydrogen the Pd thin film readily formed the β-palladium hydride (β-PdH) phase at room temperature and gradually formed the α-interstitial hydrogen solid solution (α-PdH) phase at around 200 °C with Cu interdiffusion also occurring between 300 and 600 °C.