How THF Tunes the Kinetics of H2–THF Hydrates? A Kinetic Study with Morphology and Calorimetric Analysis

Hydrogen (H2) is a clean energy that holds great promise as a sustainable alternative to fossil fuels. H2 storage has gained increasing research interest in recent decades. Hydrate-based H2 storage technology in the presence of thermodynamic promoters is promising for large-scale H2 storage due to the mild storage conditions required, the nonexplosive nature, and the easy recovery of the stored H2. However, sluggish H2 hydrate formation kinetics and low H2 storage capacity pose major challenges for large-scale applications. In this study, we aim to elucidate the tuning effect of tetrahydrofuran (THF) no more than its stoichiometric concentration (5.56 mol %) on H2–THF hydrate kinetics based on systematically designed kinetic experiments with morphology observation at macroscale complemented with microscale characterization of the synthesized H2–THF hydrates. 3.5 mol% THF yielded a superior H2 gas uptake of 2.36 v/v compared to 5.56 mol % THF. The H2–THF hydrate morphology transited from slurry-like in the aqueous phase to plate-like at the gas–liquid interface with increasing THF concentration (CTHF). Single H2 molecules were identified to be enclathrated in the 512 small cages of the H2–THF sII hydrate for all CTHF based on spectroscopic analysis. THF and binary H2–THF hydrates were identified, and the ratio of the THF hydrate to the H2–THF hydrate increased with increasing CTHF based on calorimetric analysis. Higher H2 gas uptake achieved with CTHF less than 5.56 mol % was closely linked to the morphology of the H2–THF hydrate, which in turn was controlled by CTHF. The coexistence and the ratio of THF and H2–THF hydrates under various CTHF first reported in our study suggested that optimizing CTHF was key in achieving high H2 gas uptake. These findings provide insights for understanding the tuning effect of H2–THF hydrates at multiscales and guide the optimization of thermodynamic promoter concentrations in future large-scale hydrate-based H2 storage applications.