Hydrogen sorption by Pd 77 Ag 23 metallic membranes. Role of hydrogen content, temperature and sample microstructure P. Millet a, *, R. Ngameni a , C. Decaux b , S.A. Grigoriev c a Institut de Chimie Mole ´culaire et des Mate ´riaux, UMR CNRS n  8182, Universite ´ Paris Sud 11, ba ˆt 410, 91405 Orsay Cedex, France b ADEME, 2, square La Fayette, B.P. 406e49004, Angers Cedex 01, France c Hydrogen Energy and Plasma Technology Institute of Russian Research Center “Kurchatov Institute”, Kurchatov sq., 1, 123182 Moscow, Russia article info Article history: Received 14 April 2010 Received in revised form 23 June 2010 Accepted 27 June 2010 Available online 12 August 2010 Keywords: Hydrogen Permeation Palladium alloys Impedance abstract Permeation across metallic membranes is a process used in the industry for purifying hydrogen. In conventional technology, a few tens of micrometers thick metallic membranes made of palladium alloys are used in the 400e600  C temperature range, using a driving force of several bars for enhanced kinetics. In stationary conditions of flow, the diffusion-controlled transport of atomic hydrogen across the membrane is usually rate-determining. When thin (sub-micron thick) membranes are used, surface rate contributions become more significant. To optimize permeation performances, there is therefore a need for separately measuring surface and bulk rate contributions. In this communication, we report on the kinetics of hydrogen permeation across Pd 77 Ag 23 metallic membranes using pneumato-chemical impedance spectroscopy. The role of different operating parameters (temperature, surface state, membrane microstructure) on the kinetics of permeation is analyzed and discussed. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Utilization of H 2 /O 2 PEM fuel cells in the automotive industry in place of internal combustion engines, raises the still unsolved problem of efficient hydrogen storage: pressurized vessels are potentially dangerous and metal hydrides suffer from inappropriate energy densities [1]. On-board catalytic reforming of liquid bio-fuels such as ethanol offers an alter- native solution to direct hydrogen storage [2] but a purification step is required to feed fuel cells. In the industry, hydrogen can be purified by permeation across palladium-based metallic membranes in the 400e600  C temperature range. For application in the automotive industry, the temperature range of operation must be extended down to subzero values and thin (sub-micron thick) membranes are required in order to meet acceleration requirements and cost considerations. In the micron- and sub-micron-thick ranges, surface rate contributions are relatively more important than for thicker membranes. Also, the side of the membrane facing the gas reformer is prone to severe surface corrosion and this can significantly alter the overall permeation kinetics. Finally, lattice strains and stresses induced by hydrogen transport can also have a deleterious effect [3e6]. Therefore, it is necessary to analyze in detail the mechanism of hydrogen permeation, and to separately measure surface and bulk rate contributions to optimize permeation processes. As reported elsewhere [7,8], permeation mechanisms can be analyzed in the frequency domain, to obtain information on the different steps involved in the overall mechanism and to measure individual rate parameters: surface resistances related to hydrogen dissociation (upstream side) and recombination (downstream side) processes, and bulk H diffusion * Corresponding author. Tel.: þ33 1 6915 4812. E-mail address: pierre.millet@u-psud.fr (P. Millet). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 36 (2011) 4262 e4269 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.109