Sebastian Stock1,2,Malina Seyffertitz1,Bruno Demé2,Viviana Cristiglio2,Nikolaous Kostoglou3,Oskar Paris1
Montanuniversität Leoben, Chair of Physics1,Institute Laue Langevin2,Montanuniversität Leoben, Chair of functional materials and materials systems3
Sebastian Stock1,2,Malina Seyffertitz1,Bruno Demé2,Viviana Cristiglio2,Nikolaous Kostoglou3,Oskar Paris1
Montanuniversität Leoben, Chair of Physics1,Institute Laue Langevin2,Montanuniversität Leoben, Chair of functional materials and materials systems3
When produced with renewable energy, molecular hydrogen (H<sub>2</sub>) is considered an environmentally beneficial alternative to carbon-rich fossil fuels. However, one of the major challenges to the widespread use of H<sub>2</sub> as a fuel is its efficient and secure storage. Physisorption of molecular H<sub>2</sub> in high-surface area nanoporous materials is regarded as an alternative volume-efficient solution as compared to high-pressure tanks [1]. Due to the low binding energy between the gas molecules and solid surfaces, it is associated with quick adsorption/desorption kinetics and complete reversibility. H<sub>2</sub> adsorbs preferentially within very small, sub-nanometer-sized pores due to their overlapping potentials from opposing pore walls [2,3]. Experimental observations of the underlying mechanisms during physisorption can be facilitated by neutron scattering techniques, as neutrons strongly interact with the hydrogen nucleus. In this study, we employed in-situ small angle neutron scattering (SANS) during gas sorption to investigate the physisorption of H<sub>2</sub> and D<sub>2</sub> in a micro-/mesoporous activated carbon material at cryogenic temperatures. The SANS measurements were carried out with the high resolution diffractometer D16 at the Institute Laue-Langevin, which allowed covering a wide range of the scattering vector. The SANS data indicate that H<sub>2</sub> fills the small micropores at very low pressures, and with increasing pressure a densification of the adsorbed gas in the micropores, as well as a filling of the larger mesopores takes place. Using established models for the scattering from nanoporous carbons [4], with strong emphasis towards different contrast scenarios to analyze the scattering data [5,6], allowed to quantify the density evolution as a function of pressure within the two distinct pore size regimes [7]. The strong interaction of neutrons with the hydrogen nucleus and the different interaction with D<sub>2</sub> enabled us to probe the density of gas molecules in their confined spatial arrangement, as well as to evaluate isotope effects .Our results suggest that the isotope substitution of H<sub>2</sub> with D<sub>2</sub> has a significant effect on the adsorption behaviour in nanoporous carbons, with the capacity of D<sub>2</sub> being higher than the one of H<sub>2</sub> due to a preferred adsorption of D<sub>2</sub> in even smaller pores.<br/><br/>[1] D.P. Broom, C.J. Webb, G.S. Fanourgakis, G.E. Froudakis, P.N. Trikalitis, M. Hirscher, Concepts for improving hydrogen storage in nanoporous materials, Int. J. Hydrogen Energy. 44 (2019) 7768–7779. https://doi.org/10.1016/j.ijhydene.2019.01.224.<br/>[2] J.C. Palmer, A. Llobet, S.H. Yeon, J.E. Fischer, Y. Shi, Y. Gogotsi, K.E. Gubbins, Modeling the structural evolution of carbide-derived carbons using quenched molecular dynamics, Carbon N. Y. 48 (2010) 1116–1123. https://doi.org/10.1016/j.carbon.2009.11.033.<br/>[3] Y. Gogotsi, C. Portet, S. Osswald, J.M. Simmons, T. Yildirim, G. Laudisio, J.E. Fischer, Importance of pore size in high-pressure hydrogen storage by porous carbons, Int. J. Hydrogen Energy. 34 (2009) 6314–6319. https://doi.org/10.1016/j.ijhydene.2009.05.073.<br/>[4] P. Debye, H.R. Anderson, H. Brumberger, Scattering by an inhomogeneous solid. II. the correlation function and its application, J. Appl. Phys. 28 (1957) 679–683. https://doi.org/10.1063/1.1722830.<br/>[5] C.J. Gommes, Small-angle scattering and scale-dependent heterogeneity, J. Appl. Crystallogr. 49 (2016) 1162–1176. https://doi.org/10.1107/S1600576716007810.<br/>[6] L. Ludescher, R. Morak, S. Braxmeier, F. Putz, N. Hüsing, G. Reichenauer, O. Paris, Hierarchically organized materials with ordered mesopores: adsorption isotherm and adsorption-induced deformation from small-angle scattering, Phys. Chem. Chem. Phys. 22 (2020) 12713–12723. https://doi.org/10.1039/d0cp01026j.<br/>[7] L. He, Y.B. Melnichenko, N.C. Gallego, C.I. Contescu, J. Guo, J. Bahadur, Investigation of morphology and hydrogen adsorption capacity of disordered carbons, Carbon N. Y. 80 (2014) 82–90. https://doi.org/10.1016/j.carbon.2014.08.041.