Leucine-Enhanced sII Hydrate Kinetics for HydrogenStorage

Hydrogen (H₂) is increasingly recognized as a key component in the transition to a low-carbon, sustainable future, thanks to its high energy density and clean combustion, which produce no pollutants or greenhouse gases. Storing H₂ in solid-hydrate form (Solid-HyStore) offers significant advantages, including the ability to operate at moderate pressures and temperatures, unlike compressed H₂ storage, and lower energy consumption compared to liquid H₂ storage. However, the slow kinetics of H₂ hydrate formation present a significant barrier to the large-scale adoption of this storage method. In this study, we introduce l-leucine, an environmentally benign kinetic promoter, in combination with the thermodynamic promoter, 1,3-dioxolane (DIOX), to accelerate the kinetics of hydrogen hydrate formation. Our research focuses on the influence of l-leucine on mixed H₂-DIOX hydrates, examining both the kinetics and morphology of hydrate formation. We found that the optimal combination of 1.0 wt % l-leucine with 5.56 mol % DIOX significantly enhances hydrate formation, achieving a maximum volumetric hydrogen uptake of 34.40 (±1.86) v/v at 270.5 K and 12.5 MPa. Notably, the time required to reach 90% of the maximum hydrogen uptake (<i>t</i>₉₀) was drastically reduced from 804.33 min in pure DIOX to 194.67 min with the inclusion of 1.0 wt % l-leucine. Structural confirmation via Raman spectroscopy revealed that DIOX molecules consistently occupy the 5¹²6⁴ cages of the sII hydrate, while hydrogen is enclathrated in the smaller 5¹² cages, supporting the cage occupancy pattern of the mixed H₂-DIOX hydrates. Our study also highlights the importance of compounding driving forces such as initial pressures, gas–liquid ratios, and thermodynamic promoter (DIOX) concentrations in determining a more favorable environment for hydrate nucleation and growth and the overall kinetics of mixed H₂-DIOX hydrate formation. Insights gained in this study offer the potential of strategic promoter combinations to overcome kinetic limitations, thereby advancing the feasibility of hydrate-based hydrogen storage.