Influence of Metal Oxide Incorporation on the Structure, Surface Features, and Hydrogen Storage Behavior of ZIF‑8

Zeolitic imidazolate framework-8 (ZIF-8), a type of metal-organic framework (MOF), is considered a promising material for cryogenic hydrogen storage due to its high surface area, well-defined microporous structure, and strong thermal stability. In this study, pure ZIF-8 and several composite materials containing iron oxide (Fe₃O₄), nickel oxide (NiO), and a combination of both were synthesized using a methanol-assisted method. The aim was to understand how adding these metal oxides affects the structure and hydrogen storage performance. X-ray diffraction and nitrogen adsorption-desorption analyses showed that the characteristic structure of ZIF-8 was preserved in all samples. The pure ZIF-8 had a high surface area of about 2088 m² g^-1 and a crystallite size of around 43 nm. Hydrogen adsorption measurements under cryogenic conditions revealed a maximum storage capacity of 1.140 wt % for the pure material. The addition of Fe₃O₄ and NiO caused slight decreases in hydrogen uptake, which can be explained by partial pore blocking balanced by the formation of new adsorption sites. Increasing the amount of NiO led to a more noticeable reduction in both surface area and hydrogen capacity, whereas Fe₃O₄ better maintained the porous structure of ZIF-8. Overall, the results show that hydrogen storage performance depends on both the textural properties and the interactions between the surface and hydrogen molecules, with Fe₃O₄ being a more suitable additive than NiO for preserving performance.