A neutron boost towards a clean energy future

The global need to transition to a sustainable and clean energy future has brought hydrogen to the forefront as a promising clean energy carrier. Efficient and safe storage is a key challenge and hydrogen storage in activated carbons is a promising option. Investigating the fundamental interactions between hydrogen and nanoporous carbon at the atomic level can provide vital information for enhancing the storage efficiency of these materials.

Neutron scattering is sensitive to hydrogen, thus opening the possibility for many different insights. This paper reports on a SANS (Small-Angle Neutron Scattering) study using ILL’s highly versatile neutron diffractometer D16 to investigate hydrogen and deuterium adsorption in nanoporous activated carbon cloth as a function of (micro)pore size. These findings will contribute to a better understanding of the processes involved and will influence the design of materials for efficient hydrogen storage devices working at realistic cryogenic conditions and low pressures.

Neutron diffraction is a powerful and often unique tool for looking inside things. In a recently published study, neutron scattering measurements performed at ILL are used to investigate hydrogen (H2) and deuterium (D2) storage in nanoporous activated carbon cloth.The goal is to help gain a more comprehensive and quantitative understanding of how the hydrogen density achieved evolves with pore size variation for the two isotopes, H2 and D2 , at realistic cryogenic conditions and low pressures.

Hydrogen is the lightest and most abundant element in the universe. It can be obtained from various sources, including water and biomass. It also has the highest energy per mass of any fuel, and an environmentally friendly nature if produced using renewable energy sources. Hydrogen can play an important role in decarbonizing industries that are difficult to electrify and can be used in clean and highly efficient fuel cells with applications ranging from portable electronic devices to vehicles.

While one kilogram of hydrogen contains more energy than one kilogram of any other fuel, it may take a lot more space. The boiling point of hydrogen at normal atmospheric pressure is −252.8°C, which results in a very low density and thus a very low energy per unit volume at atmospheric temperatures. In other words, while the energy per mass content of hydrogen is very high, the energy content per volume is particularly low. For storage, hydrogen is usually liquefied or put under high pressure. Nevertheless, compressed gas and liquid hydrogen storage methods have limitations in terms of energy densities and also raise safety concerns.

Hydrogen can also be stored within solids (by absorption) or on the surface of solids (by adsorption). Porous activated carbon materials have an impressive record of applications and hydrogen storage in such materials has emerged as a promising option. In this case, the storage mechanism is physisorption, or physical adsortion: gas molecules simply stick onto the surface of the solid. Investigating the fundamental interactions between hydrogen and nanoporous carbon at the atomic level and the effects of hydrogen confinement within the pore space is essential.

The neutron scattering measurements were conducted using ILL’s instrument D16, a highly versatile neutron diffractometer for the study of partially ordered structures. D16 applications cover a very large number of fields: biology, soft condensed matter, chemistry and materials science, in particular porous materials. The monochromatic neutron beam and flexible beam geometry can be adapted to various sample shapes, sizes and scattering geometries. In the present case, they were optimised for small-angle neutron scattering (SANS) and a neutron wavelength of 4.47 Å was used.

The results obtained show that hydrogen adsorbate density in ultramicropores approaches and can even exceed the bulk solid hydrogen density at 77 K (-196°C) and ambient pressure. The significance of ultramicropores in determining storage capacity is thus highlighted. These findings contribute to better understanding the confinement effects on hydrogen densification.

While inelastic neutron scattering and computer simulations can help us understand the dynamics of hydrogen molecules within nanoporous carbons, SANS is useful in investigating the density distribution of molecules in such systems at different length scales. In fact, scientists have only just begun to exploit this potential, which can provide vital contributions to enhancing storage efficiency and influence the design of hydrogen storage devices.

Bruno Demé, D16 instrument responsible scientist, explains that “The experiment was performed before the recent upgrade of D16,” and reveals that “there is more to come in this project with results obtained in 2023 shortly after the new D16 commissioning.

Reference: S. Stock, M. Seyffertitz, N. Kostoglou et al. “Hydrogen densification in carbon nanopore confinement: Insights from small-angle neutron scattering using a hierarchical contrast model” Carbon 221 (2024) 118911.

ILL Contact: Bruno Demé