The Effect of Surface Composition On the Hydrogen Permeability of Pd77Ag23 Membranes

Title: The effect of surface composition on the hydrogen permeability of Pd₇₇Ag₂₃ membranes

We studied the effect of surface composition and roughness on hydrogen permeability of Pd₇₇Ag₂₃ membranes. We found that after 24hours annealing in a vacuum environment (1x10^-4 Torr) at 683K, silver segregates on the surface of the membrane yielding a surface composition of Pd:Ag=65:34 compared to the untreated sample with Pd:Ag=72:27. The hydrogen permeability at 683K with upstream hydrogen pressure of 200kPa is 7.86x10^-9 mol/msPa^0.5, decreases by 33% compared to the untreated membranes (1.08x10^-8mol/msPa^0.5). Additionally, we found that the surface roughness increases after vacuum annealing at 683K; however, this change in roughness does not promote the overall hydrogen permeability. After hydrogen permeation tests, we found that the surface composition ratio recovers to approximately that of the original sample Pd:Ag=72:28 and the hydrogen permeability also recovers to 1.3x10^-8 msPa^0.5. After twenty four hours of annealing in air the surface composition become Pd:Ag=84:16. Our X-ray photoelectron spectroscopy measurements indicate that some of the Pd has transformed into Palladium oxides. The hydrogen permeation test afterwards shows a promotion of permeability of 1.27x10^-8 msPa^0.5(17% increase compared to vacuum annealing samples) and after test Pd and Ag ratio becomes Pd:Ag=79:21. By changing the surface ratio through vacuum annealing and air treatment, the hydrogen permeability changed.  Our results indicate the first time that higher Pd/Ag ratio on Pd₇₇Ag₂₃ membrane surface could improve hydrogen permeability at 683K which is usually considered diffusion-dominated.

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