Hydrogen sorption and diffusion in coals: Implications for hydrogen geo-storage

A full-scale hydrogen economy requires a bulk energy storage system to store the excess energy as a buffer and to fulfill the demand constantly. Hydrogen sorption capacity and diffusion behavior in coal quantify its potential to become a well storage candidate from geological formations. In this study, the sorption and diffusion behaviors of eight coals across the major coalfields in the United States in terms of their ranks, fixed carbon content, vitrinite reflectance, vitrinite or huminite content, O/C ratio, and H/C ratio were measured and analyzed. The sorption data shown that all eight coals have considerable sorption capacities, among which the LvB coal has the maximum adsorption capacity of ∼ 1.17 mmol/g, followed by An coal with the maximum hydrogen adsorption capacity of ∼ 0.95 mmol/g and the SemiAn coal of approximately 0.82 mmol/g. The fixed carbon content and typically the O/C ratio are obviously correlated with the maximum hydrogen adsorption capacity in coals, which may be attributed to the oxygen-containing functional groups. Hydrogen has superior diffusive gas deliverability defined by hydrogen effective diffusivity. The effective diffusivities of hydrogen in SemiAn coal decreases from ∼ 0.00156 to ∼ 9.26 × 10-41/s with pressure increases from ∼ 2.45 MPa to ∼ 10.07 MPa, among which the lowest diffusivity at ∼ 10.07 MPa is even ∼ 4 times higher than that of CH4 at equivalent pressure. This is a vantage for a promising field candidate of hydrogen storage in coal with maximized injectivity. The results reveal that the sorption and diffusion behaviors of hydrogen in different rank coals towards the depleted coalbed methane formation with injected hydrogen can serve as a geological “H2-Battery”.