Supercritical Hydrogen Adsorption in Nanoporous Carbons: Understanding Confinement Effects Through Neutron Scattering

Abstract

The transition to a hydrogen-based energy economy is gaining momentum as industries and governments seek sustainable alternatives to fossil fuels. However, the efficient storage of hydrogen remains a critical challenge, particularly for mobile applications requiring high energy density and compact storage solutions. Among the various hydrogen storage methods, physisorption in nanoporous materials has emerged as a promising approach, offering high storage capacities under cryogenic conditions without the need for extreme pressures. This thesis investigates the potential of nanoporous carbons as hydrogen storage materials, focusing on their structural characteristics, hydrogen adsorption behavior, and the densification of hydrogen in nanopore confinement. The first part presented explores the synthesis of bio-derived activated carbons from orange peels and used tea leaves, highlighting their potential as sustainable hydrogen storage materials. By employing a simple chemical activation procedure, the resulting nanoporous carbons exhibit surface areas exceeding 2100 m²/g and pore volumes beyond 1.5 cm³/g, key factors influencing hydrogen adsorption capacity. The systematic evaluation of pore structure and chemical composition reveals that ultramicropores (< 0.7 nm) dominate hydrogen uptake at low pressures, while supermicropores (0.7–2 nm) and mesopores (>2 nm) enhance adsorption at higher pressures. Statistical correlation analysis confirms that the micropore volume and the total surface area are the strongest predictors of storage performance, whereas the influence of heteroatom concentration on storage performance requires further studies. These insights provide a framework for optimizing activated carbons for hydrogen storage applications. In the second part, in-situ neutron scattering is utilized to directly probe the spatial distribution of hydrogen in nanoporous carbons. Small-angle neutron scattering (SANS) measurements at cryogenic temperatures reveal a pore-size-dependent densification, where hydrogen confined in the smallest pores approaches or even exceeds its bulk solid density. The study establishes a hierarchical contrast model describing density evolution across different pore classes, confirming that confinement in ultramicropores (< 0.7 nm) leads to extreme densification. The findings underscore the importance of strong confinement effects on hydrogen adsorption behavior, providing crucial experimental evidence to support the development of next-generation storage materials. The third part integrates atomistic simulations to complement experimental observations, offering deeper insight into the confinement-dependent adsorption mechanisms. Molecular simulations in realistic atomistic models of nanoporous carbons reveal preferred adsorption sites, showing that highly defective regions enhance local hydrogen density. The comparison between simulated and experimental scattering data suggests that while deuterium (D₂) follows simulated adsorption trends, hydrogen (H₂) shows clustering effects, likely due to specific spin isomer interactions that were not considered in the interaction potentials. These results highlight the complexity of hydrogen adsorption in nanoporous carbons and emphasize the need for refined atomistic models incorporating surface chemistry and spin effects in the interaction potentials. This thesis advances the understanding of hydrogen adsorption in nanoporous carbons, demonstrating the potential of bio-derived materials for sustainable hydrogen storage applications. The integration of experimental techniques and atomistic modeling provides a comprehensive perspective on the role of pore size, surface area, and chemical composition in optimizing hydrogen uptake. The insights gained contribute to the ongoing development of high-performance hydrogen storage systems, bridging the gap between fundamental adsorption mechanisms and practical applications.