## 4 Conclusions  
An in-depth study of the adsorption and diffusion mechanism of hydrogen in low-rank coals is of great significance for optimizing the technological path of geological hydrogen storage and improving the efficiency of hydrogen storage. It was found that the high specific surface area and porosity in the coal structure will provide more effective adsorption sites, which is favorable for hydrogen storage. The presence of oxygen-containing functional groups will strengthen the adsorption of hydrogen in the coal structure, making it more difficult for hydrogen to diffuse. The specific conclusions are as follows:  
(1) During the construction of the coal's molecular structure, it was found that the aliphatic carbons in coal primarily exist as branched chains linked to aromatic rings in the form of alkyl groups, side chains, or bridge carbons. The lower the metamorphic degree of the coal, the longer the alkyl side chains in the molecular structure. The oxygen atoms in both coal structures mainly exist as oxygen-bound aliphatic carbons and oxygen-substituted aromatic carbons, with the C-O structures in LR@1 and LR@2 predominantly consisting of single bonds.  
(2) Based on DFT calculations, it was determined that the head region of the H\({}_{2}\) molecule tends to undergo electrophilic and nucleophilic reactions with the oxygen-containing functional groups in the molecular structures of both coals, forming stable adsorption structures. Additionally, the branched structures of aliphatic carbons and the multi-aromatic ring structures of aromatic carbons provide more effective adsorption sites, thereby enhancing the adsorption capacity of low-rank coal structures for H\({}_{2-}\).  
(3) According to the simulation calculations of the micropore model, it was found that LR@2 has a higher specific surface area and porosity in its micropore structure compared with LR@1, with values of 2402.60 m\({}^{2}\)/g and 26.905%, respectively. The high specific surface area and porosity in the coal structure improve the coal's ability to adsorb H\({}_{2}\). Additionally, the presence of oxygen-containing functional groups in low-rank coal  
Figure 6: Adsorption/diffusion analysis of H\({}_{2}\) by two lignite microporous structures, LR@1 and LR@2 (**a**) specific surface area and porosity of coal, (**b**) H\({}_{2}\) adsorption capacity, (**c**) diffusion coefficient of H\({}_{2}\), (**d**) microporous models, (**e**) gas diffusion models.  
strengthens the interaction between the micropore structure and H\({}_{2}\), making H\({}_{2}\) migration more difficult.  
Data curation, H.S.; Formal analysis, J.J.; Funding acquisition, J.J.; Investigation, S.F.; Methodology, X.G., J.J., H.S. and H.T.; Software, S.F. and H.S.; Validation, S.F. and H.S.; Writing--original draft, X.G. and S.F.; Writing--review and editing, X.G., S.F., J.J., H.S. and H.T. All authors have read and agreed to the published version of the manuscript.  
This research is conducted with financial support from the National Natural Science Foundation of China (No. 52174183 and 52374203).  
The relevant data of this manuscript are obtained from field measurements and tests, which are real and effective.  
The authors declare that there are no conflicts of interest in the publication of this paper.