Gas hydrates offer great potential in storage, desalination and capturing gases. Among the existing gas hydrates, clathrate hydrates are typical non-stoichiometric structures stabilized by gases like H2, CH4 and CO2. With the growing need for decarbonization and energy transition towards renewable energy, the current studies on gas hydrates have focused on their natural occurrence, environmental impact, and potential applications in facilitating energy transition.
Although clathrates have several structures, whose formation depends on various factors like temperature and pressures, sI, sII and sH are the most common. Among those common three structures, sI clathrate possess the highest volumetric and gravimetric capacity. Unlike other clathrates, sI is unable to accommodate organic solvent thermodynamic promoters and is often encapsulated single gas species under high pressures. The cage occupancy and inter-cage transport mainly depend on the stability of the pre-occupied cages as well as the diffusion ability of the guest gases into specific cages. Despite the significant amount of studies on the properties and applications of sI clathrate, the current experimental and computation data fail to accurately evaluate the gas clathrate stability and are not appropriate for estimating cage occupancy. Therefore, in-depth knowledge of the inter-cage transport and diffusion mechanism at the molecular scale level requires accurate estimation of the flexible transitions.
Recently, hydrogen clathrates have become attractive for green mass storage, emerging as a potential and cheap replacement for metal-organic frameworks. However, the pressure-temperature conditions required to synthesize pure hydrogen sII clathrates are demanding and challenging to scale up. Several approaches, such as using thermodynamic hydrate promoters such as Tetrahydrofuran, have been proposed to address this issue but also have various limitations such as drastic reduction of hydrogen storage capacity.
Herein, Ahmed Omran -a researcher in ENSICAEN- and Professor Valentin Valtchev from Normandie University in collaboration with Dr. Nikolay Nesterenko from TotalEnergies One Tech Belgium explored the possible use of CH4 or CO2 as thermodynamic promoters for hydrogen diffusion and high capacity storage in sI clathrate. They studied the cage occupancy of diffusion mechanism in the single (H2), mixed (CH4-H2) and (CO2-H2) sI clathrates which have been rarely investigated experimentally. Using accurate Ab initio computational studies, the authors also investigated the interactions between the binding energy, stability diffusion and storage capacity of CO2 and CH4 in sI clathrate containing hydrogen. After confirming the accuracy of calculations by comparing their structural data to experimental ones, the authors showed the important impact of the dispersion forces associated with the adsorption of CH4, H2 and CO2 in sI clathrates. The work is currently published in the International Journal of Hydrogen Energy.
The research team revealed that the presence of CH4 and CO2 facilitated the diffusion of hydrogen into the small and large cages and enhanced the thermodynamic stability and storage capacity in sI clathrates. The sI gas hydrate stability of CO2 was higher, followed by that of CH4 and finally H2. While H2 exhibited multiple cage occupancies, the stability of the H2 clathrate was improved in both H2-CH4 and H2-CO2 systems with heterogenous occupancy of gases in the same cage. The hydrogen diffusion activation energy from the doubly occupied mixed system to the small and large cages was calculated to be 0.685 and 0.181 eV, respectively. Additionally, the gravimetric storage, volumetric storage and molecular hydrogen content in H2-CH4 binary sI clathrate reportedly reached 1.8 kW h/L, 2.0 kW h/kg and 5.0 wt% wt, respectively.
In summary, a comprehensive study was reported to evaluate the feasibility of sI clathrate for hydrogen storage in the presence of CH4 and CO2. The study findings proved the feasibility of direct transition mechanism of hydrogen through a fully relaxed structure. Moreover, the combinations of methane and hydrogen in sI clathrate were in good agreement with existing computational and experimental data and their energy content were comparable to the US Department of Energy (DOE) targets for hydrogen storage. In a statement to Advances in engineering, Professor Valentin Valtchev, the lead and corresponding author explained that their study advanced our knowledge into the promising properties of sI methane clathrate as a promising material for transportation and storage of hydrogen.
Omran, A., Nesterenko, N., & Valtchev, V. (2022). Ab initio mechanistic insights into the stability, diffusion and storage capacity of sI clathrate hydrate containing hydrogen. International Journal of Hydrogen Energy, 47(13), 8419-8433.