Quasi-elastic neutron scattering measurements together with equilibrium molecular dynamic simulations were performed to investigate the surface interaction between hydrogen molecules and a carbon material (XC-72) commonly used in polymer electrolyte membrane fuel cells. Half a monolayer of molecular hydrogen was adsorbed onto the carbon material at 2K. Quasi-elastic neutron scattering spectra were recorded at the time-of-flight spectrometer IN5 at 40, 45, 50, 60, 70, 80 and 90K (table 13). Simultaneously the pressure was measured as a function of time to monitor the equilibrium surface coverage at each temperature. By using the Chudley-Elliott model for jump diffusion the diffusion coefficient at each temperature was found. At 350K, the temperature function was extrapolated to a self-diffusion coefficient of 2.3 x 10-7m2/s. Graphite in contact with hydrogen molecules was simulated by using equilibrium molecular dynamics simulations. The system was simulated for temperatures of 70 to 350K, in 20K intervals and for five numbers of H2 molecules NH2) 50, 100, 150, 200 and 300. The graphite was made of 9 sheets of graphene in a sandwich. The surface self-diffusion was found from the mean-square displacement, and the values from equilibrium molecular dynamic simulations were the same order of magnitude as the experimental values at 90K, but systematically higher, probably due to the ideal surface. From equilibrium molecular dynamic simulation, the average time between adsorption and desorption events on the surface was also calculated. This was used to find the mean displacement of the hydrogen molecules between adsorption and desorption. This result showed that H2 molecules could move 80Å at ambient temperatures and pressures, along the surface. Using these data, it was concluded that catalyst support material contributed to reactant transport.

Surface Self-Diffusion and Mean Displacement of Hydrogen on Graphite and a Pem Fuel Cell Catalyst Support. Haas, O.E., Simon, J.M., Kjelstrup, S.: Journal of Physical Chemistry C, 2009, 113[47], 20281-9

Table 13

Surface H2 diffusivity on graphene