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The diffusion of H2 in porous materials is important for applications such as H2 storage by physisorption, H2/D2 separation and heterogeneous catalysis [1]. Developing better materials for these applications requires a detailed understanding of how H2 diffuses in carbon materials with different porous and graphenic structures.
Quasi-elastic neutron scattering (QENS) is well suited for studying H2 diffusion due to H2 large incoherent scattering cross-section, the suitable energy range of neutron spectroscopy for diffusive processes and the deep penetration of neutrons in most materials, that enables to use complex sample environments [2].
In this study, we investigate the self-diffusion of H2 adsorbed in Mo2C derived carbons synthesized at 700 °C, 800 °C and 900 °C (C700, C800, C900) [3,4]. QENS experiments were performed at various H2 loadings and at temperatures between 50–100 K. The porous structure was characterized using N2, CO2, H2 and Ar adsorption, and the graphenic structure was analyzed with wide angle X ray scattering (WAXS). The ultramicropores (pore width w < 7 Å) can be interpreted as pore wall corrugations within larger micro- (w < 20 Å) and mesopores (20 Å < w < 500 Å). C700 shows more corrugated pore walls than C900 which is consistent with WAXS results. QENS reveals two distinct diffusion timescales, corresponding to H2 adsorbed in the first monolayer near the pore wall and H2 in subsequent layers. Activation energies calculated from the diffusion coefficients show that H2 is more strongly confined in C700 than in C900. Thus, more corrugated pores provide stronger confinement of adsorbed H2 than smoother pores.
[1] Mohan, M. et al. Energy Storage 2019, 1, e35.
[2] Bée, M. Quasielastic Neutron Scattering. Adam Hilger, Bristol, 1988.
[3] Koppel, M. et al. Carbon 2022, 197, 359–367.
[4] Koppel, M. et al. Carbon 2024, 219, 118799.