Synthesis and characterisation of metal-organic frameworks based hydrogen storage materials

Abstract

Hydrogen gas is one of the most prominent and efficient energy carriers for several energy conversion and storage applications. To explore hydrogen-based economy with full potential, efficient hydrogen storage materials need to be developed. The recent improvement in the development of metal-organic frameworks (MOFs) structure (mono-bi metallic, hybrid composite and core-shell structure) attracts wide attention for solid-state hydrogen storage and its application. Hence in this thesis, MOF structures such as core-shell ZIFs structure (ZIF-8@ZIF-67 and ZIF-67@ZIF-8), hybrid MOFs composite (carbon-MIL-101, RHA-MIL-101, and AC-MIL-101) and alkali metal doped hybrid MOFs composites are synthesised and their H2 gas sorption properties are investigated. In this direction, core-shell ZIF-8@ZIF-67 and ZIF-67@ZIF-8 based zeolitic imidazole frameworks are synthesised by solvothermal method using seed-mediated methodology. TEM-EDXS line scan, elemental mapping, XPS, and ICP-AES analysis were probed to confirm the formation of core-shell structure with the controlled Co:Zn elemental composition of ~0.50 for both the core-shell ZIF frameworks. The synthesised core-shell ZIF-8@ZIF-67 and ZIF-67@ZIF-8 frameworks exhibited enhanced H2 (2.03 wt% and 1.69 wt%) storage properties at 77 K and 1 bar, which is ca. 41% and 18%, respectively higher than the parent ZIF-8. Notably, remarkably enhanced H2 storage properties shown by both the core-shell ZIFs over the bimetallic Co/Zn-ZIF and the physical mixture of ZIF-8 and ZIF-67, clearly evidenced the unique structural properties (confinement of porosity) and elemental heterogeneity due to the core-shell morphology of the outperforming core-shell ZIFs. Along with the remarkably enhanced H2 storage capacities exhibited by the core-shell ZIFs, they also displayed improved CO2 capture behavior. Hence, we demonstrated here that the controlled structural features endorsed by rationally designed porous materials might find high potential for H2 storage applications. For development of hybrid MOFs composite structure, a sustainable methodology was explored to synthesise carbon-MIL-101 hybrid composites by advantageously inducing in situ hydrothermal carbonization (HTC) of glucose during the synthesis of MIL-101. By tuning the content of glucose, carbon-MIL-101 hybrid composites with varying carbon content were synthesised. The HTC of glucose and incorporation of carbon in MIL-101 was confirmed by probing 13C NMR, TEM, XPS and Raman investigations. The microporosity of composites can be fine-tuned by optimizing the carbon loading. Consequently, the carbon-MIL-101 hybrid composites with optimized pore size, high pore volume, and surface area conferred enhanced H2 uptake properties (by ca. 11% as compared to MIL-101) at 77 K and 1 bar. The noteworthy enhancement in H2 uptake for the synthesised carbon-MIL-101 hybrid composites endorsed the potential of the studied methodology to design hybrid MOF composites with tuned porosity for the H2 storage application. Another hybrid composite of MIL-101 with silica-rich rice husk ash (RHA) was synthesised to explore such materials for improved low-pressure hydrogen storage applications, compared to the well-explored carbon-based composites of MIL-101. RHA-MIL-101 was prepared by in situ incorporation of RHA in MIL-101 during the synthesis, under hydrothermal conditions. The incorporation of RHA in MIL-101 was confirmed by PXRD, FTIR, TGA, SEM, EDXS, and N2 adsorption and desorption isotherms studies. The as-synthesised RHA-MIL-101 composite displayed enhanced BET surface area (8.6% compared to bare MIL-101), whereas AC-MIL-101 showed an enhancement of 12.7% in BET surface area compared to the bare MIL-101. Hydrogen uptake properties of these materials were evaluated at 77 K and 1 bar. Despite that RHA-MIL-101 exhibited lower surface area as compared to AC-MIL-101, the hydrogen uptake capacities of RHA-MIL-101 reached an enhanced value of 1.54 wt%, which is higher than the bare MIL-101 (1.40 wt%) and AC-MIL-101 (1.48 wt%) by 9.1% and 5.7%, respectively, and is comparable with most of the reported carbon incorporated MOFs. The observed improved hydrogen uptake properties were attributed to the bifunctional properties of the synthesised RHA-MIL-101, abundance of silanol bonds of RHA (which shows high affinity towards H2 molecules) and tuned porous properties of RHA-MIL-101. In continuation to the previous research work, a simple methodology to enhance the hydrogen uptake properties of RHA incorporated MIL-101 (RHA-MIL-101) by controlled doping of Li+ ions were also explored. Hydrogen gas uptake of Li-doped RHA-MIL-101 was found to be significantly higher (up to 72%) compared to the undoped RHA-MIL-101, where the content of Li+ ions doping greatly influenced the hydrogen uptake properties. We attributed the observed enhancement in hydrogen gas uptake of Li-doped RHA-MIL-101 to the favorable Li+ ion to H2 interactions and the co-operative effect of silanol bonds of silica-rich rice-husk ash incorporated in MIL-101. The obtained results implied that the developed MOFs structures have the potential for enhanced hydrogen gas storage, and various adsorption based applications. Keywords: Core-shell frameworks, ZIF-8, ZIF-67, MIL-101, carbon-MIL-101 hybrid composite, HTC of glucose, Hydrothermal condition, Silica-rich composite, Rice husk ash, Lithium, Adsorption, Hydrogen storage, CO2 capture