Segregation and H₂ Transport Rate Control in Body-Centered Cubic PdCu Membranes

Abstract

The H2 permeation of a supported 2 microm thick Pd48Cu52 membrane was investigated between 373 and 909 K at DeltaP=0.1 MPa. The initial H2 flux was 0.3 mol.m(-2).s(-1) at 723 K with an ideal H2/N2 selectivity better than 5000. The membrane underwent a bcc-fcc (body-centered cubic to face-centered cubic) phase transition between 723 and 873 K resulting in compositional segregation. After reannealing at 723 K the alloy layer reverted to a bcc structure although a small fcc fraction remained behind. The mixed-phase morphology was analyzed combining X-ray diffraction with scanning electron microscopy-energy-dispersive spectroscopic analysis (SEM-EDS) measurements, which revealed micrometer-scale Cu-enriched bcc and Cu-depleted fcc domains. The H2 flux JH2 of the fcc Pd48Cu52 single phase layer prevailing above 873 K could be described by an Arrhenius law with JH2=(7.6+/-4.9) mol.m(-2).s(-1) exp[(-32.9+/-4.5) kJ.mol(-1)/(RT)]. The characterization of the H2 flux in the mixed-phase region required two Arrhenius laws, i.e., JH2=(1.35+/-0.14) mol.m(-2).s(-1) exp[(-10.3+/-0.5) kJ.mol(-1)/(RT)] between 523 and ca. 700 K and JH2=(56.1+/-9.3) mol.m(-2).s(-1) exp[(-25.3+/-0.6) kJ.mol(-1)/(RT)] below 454 K. The H2 flux exhibited a square root pressure dependence above 523 K, but the pressure exponent gradually increased to 0.77 upon cooling to 373 K. The activation energy and pressure dependence in the intermediate temperature range are consistent with a diffusion-limited H2 transport, while the changes of these characteristics at lower temperatures indicate a desorption-limited H2 flux. The prevalence of desorption as the permeation rate-limiting step below 454 K is attributed to the pairing of an extraordinarily high hydrogen diffusivity with a marginal hydrogen solubility in bcc PdCu alloys. These result in an acceleration of the bulk diffusion rate and a deceleration of the desorption rate, respectively, allowing the bulk diffusion rate to surpass the desorption rate up to relatively high temperatures.

The hydrogen permeability of Pd–Cu based thin film membranes in relation to their structure: A combinatorial approach

“…The observed mixed bcc/fcc region for our thin films is much larger than expected from the literature [26,28,36] and ranges from at least 29.5 at% Pd to 67 at% Pd at 483 K. This is in agreement with a single composition measurement of Pd 66 Cu 34 by Pan et al [37]. This larger mixed phase region is possibly caused by the phase segregation process when the PdeCu alloy thin films undergo a rapid quenching to substrate i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 2 temperature and thereby crossing the bcc/fcc phase boundary [36].…”

“…7ag. Identification of the fcc-diffraction peaks is done with the database of the JCPDS-International Centre for Diffraction Data [27], whereas the bcc-peaks were obtained from Yuan et al [28], and the lattice parameter data for different Pd concentrations as tabulated by Subramanian and Laughlin [26].…”

“…Yuan et al showed that upon heating and cooling down a Pd 47.3 Cu 52.7 based membrane (i.e. bcc-> bcc/fcc-> bcc), a mixed bcc/fcc phase is observed in the final state [28]. Unfortunately, the crystal structure of the membrane is not often analysed (neither before or after such a heat cycle) which makes a clear comparison between an actual thick layer membrane and our thin films difficult [3,28,42,44].…”

“…The fast decrease in permeability (isothermic) on the Curich side of the composition Pd 48 Cu 52 has been attributed to the decreasing hydrogen solubility and diffusivity as function of increasing Cu concentration [45,46] whereas the decreasing permeability on the Pd-rich side is due to the order-disorder transition and a rapidly increasing fraction of the fcc phase which is less permeable [28]. As a result, Yuan et al argues that the highest permeability should occur in bcc samples with a composition bringing it close to the phase boundary at any given temperature [28].…”

Preparation Technique of a Single Pd Membrane Using Electroless Plating for Hydrogen Permeation

“…Palladium membranes are promising for use in pure hydrogen production because of their high hydrogen flux and almost complete hydrogen selectivity [1,2], and several alloys are expected to be useful [3][4][5]. Properties required for purified hydrogen production are not only purity but also the flow rate for improvement in productivity.…”