Quasi-elastic Neutron Scattering Investigation of the Hydrogen Surface Self-Diffusion on Polymer Electrolyte Membrane Fuel Cell Catalyst Support Ole-Erich Haas* Department of Chemistry, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway Jean Marc Simon Institut Carnot de Bourgogne, UMR 5209 CNRS-UniVersite ´ de Bourgogne, F-21078 Dijon Cedex, France Signe Kjelstrup Department of Chemistry, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway Astrid Lund Ramstad Department of Chemistry, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway Peter Fouquet Institut Laue-LangeVin, BP 156, 38042 Grenoble Cedex 9, France ReceiVed: September 26, 2007; In Final Form: December 10, 2007 Quasi-elastic neutron scattering (QENS) measurements have been performed to investigate the surface self- diffusion of hydrogen molecules. A monolayer of molecular hydrogen was adsorbed on a carbon material commonly used in polymer electrolyte membrane fuel cells, called XC-72. QENS spectra were recorded at the time-of-flight spectrometer IN5 at Institut Laue-Langevin (ILL) in Grenoble at 40, 50, 60, and 70 K. By using the Chudley & Elliott model for jump diffusion, we found the diffusion coefficient at each temperature. The logarithm of the diffusion coefficient was plotted versus the inverse of the temperature to give the coefficient in the Arrhenius equation. From this, we can estimate the diffusion at higher temperatures as well. Our observed diffusion follows the same trend as that found for hydrogen molecules on the surface of single- walled carbon nanotubes and Grafoil. Introduction Our motivation is to learn more about the gas transport through the backing electrode and catalyst layer in the polymer electrolyte membrane fuel cell (PEMFC). The PEMFC is in the most basic configuration made of two backing electrodes separated by a polymer electrolyte membrane (PEM). For a full description, see the work of J. Larminie and A. Dicks and P. Costamagna et al. 1,2 The purpose of the polymer electrolyte membranes is to conduct H + ions between the two electrodes. The electrodes are made of carbon paper or cloth that is electrically conducting, highly porous, and lets H 2 and O 2 gas diffuse to the catalytic layers located close to the PEM. In the catalytic layers, the reaction sites are located at noble metal catalyst, often Pt or Pt alloy, which must be in contact with both the ionic and the electric conductor. This catalytic layer is a few micrometers thick and the noble metal catalysts are supported on electrical conducting carbon black particles, as XC-72. 3-5 The noble metal particles have radii of only a few nanometers, typically 2-10 nm as demonstrated by Stevens et al. 6 Because the noble metal particles must be in contact with both the ionomer and the carbon, the pore sizes surrounding them are small. The purpose of this work is to examine the theoretical hypothesis made by Meland et al. 7 that access to the three-phase contact line in porous gas electrodes is via the interfaces between the homogeneous phases that surround the contact line. It is therefore important to know whether surface diffusion takes place down the pore or crevice to access the Pt particle. This surface diffusion is in contradiction to the accepted view of reactant transport in the PEM fuel cell today. But the recent work of Meland et al. gave an indication of H 2 adsorbing and diffusing along the carbon surface to the catalyst particles. 7 Their study is based on electrochemical impedance spectroscopy. They gave a theoretical estimate of the surface diffusion constant for H 2 on carbon black based on their findings. In recent years, a lot of work has been done on the adsorption of H 2 in carbon materials. The main purpose of these investiga- tions is the possibility of using it as a storage material in mobile applications. 8-16 Especially carbon nanomaterials have been tested after their introduction. Because neutron scattering is an excellent probe to examine hydrogen properties, this technique has been used extensively. 17-22 Only two studies give a diffusion coefficient for H 2 on carbon. Bienfait et al. and Narehood et al. 21,22 have reported surface self-diffusion constants for H 2 on oriented graphite and single-wall nanotubes. Carbon blacks have not been subject to the same dynamical studies with neutron scattering techniques, and to our knowledge * Corresponding author. E-mail: oleerich@chem.ntnu.no. Tel: 0047 735 50 208 / 0047 920 21 938. Fax: 0047 735 50 877. 3121 J. Phys. Chem. C 2008, 112, 3121-3125 10.1021/jp077715+ CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008