Generally, Pd membranes used for H-2 purification are deposited on porous stainless steel (SS) or porous ceramics. These membranes are thick ( > 5 mu m) because of the large pore size of the support used; hence, such thick membranes are not cost-effective. In this study, cost-effective, viable ultrathin Pd membranes, which were deposited on a polymer substrate, e.g., polybenzimidazole-4,4'-(hexafluoroisopropylidene) bis(benzoic acid) (PBI-HFA), with a thickness less than 700 nm, were prepared by vacuum electroless plating (VELP). The estimated thickness and effective permeation area of the Pd/PBI-HFA membranes were 130-656 nm and 8.3 cm(2), respectively. An optimum Pd/PBI-HFA membrane was deposited after two times activation (A2). Because of the adequate grain size of Pd crystals and thickness, Pd/PBI-HFA exhibited better hydrogen permselectivity than the samples prepared by one and three times activation. Furthermore, the polymer surface treated by the CO2 plasma (CO2) and oxidation by H2O2 (H) leads to the enhanced selectivity performance of the final Pd/PBI-HFA membrane, caused by the enhanced adhesion between Pd and PBI-HFA. Gas permeation properties of H-2, N-2, CO2, and CO were evaluated between 35 and 200 degrees C and pressure differences between 4 and 8 bar. Pd films fabricated by A2 -CO2 methods exhibited superior performance, as well as excellent alpha(H2/N2) and alpha(H2/CO2), permselectivities of 41.4 and 22.3, respectively, at 150 degrees C and 8 bar, and proved to be impermeable to carbon monoxide (CO). It is a 48% increase in the alpha(H2/N2) permselectivity and fourfold increase in the alpha(H2/CO2)., permselectivity from bare PBI-HFA membrane. In this study, an effective Pd layer was deposited on PBI-HFA under optimum Pd electroless plating conditions by controlling the interface adhesion strength and distribution of Pd seeds on the substrate.(C) 2017 Elsevier B.V. All rights reserved.