## 3 Results  
### Bibliometric Analysis  
#### 3.1.1 General Overview  
The evolution curve of the publications related to the subject (n = 1584) and the total number of citations per year reveal three periods/trends between 1957 and 2023 (Figure 1). The trend was constant in the period between 1957 and 2000, when it started to increase steadily until 2015. Between 2016 and 2023, the growth was exponential. The Kyoto Protocol (1997) seems to be linked to the start of the increase in publications, while COP 15, the Copenhagen Agreement (2009), and COP 21, the Paris Agreement (2015), had a huge impact. The countries that have most contributed to this area are the United States (19.3%), China (17.3%), Germany (13.5%), the United Kingdom (9.22%), France (8.27%), and Australia (6.19%) (see Supplementary Materials, Figure S1 for more information and Supplementary Materials, Figure S2 for the trends). Figure 1: Evolution of publications over time (trend 1: 1970–2000, trend 2: 2001–2015, trend 3: 2016–2023). Major events about climate mitigation decisions. (1) IPCC first report 1990; (2) United Nations Framework Convention on Climate Change, Rio de Janeiro; (3) COP 3, The Kyoto Protocol; (4) COP 15, The Copenhagen Agreement; (5) COP 21, The Paris Agreement; (6) COP 25, Conference in Madrid; (7) COP 26, Glasgow Pact. Cluster 1. Storage in porous media (red color): This cluster primarily focuses on hydrogen-rock-fluid interactions in porous reservoirs (i.e., saline aquifers and depleted hydrocarbon reservoirs). Cluster 2. Storage in salt caverns (yellow color): The second cluster explores salt caverns as the most important geological structures for temporary H\({}_{2}\) storage and the associated physical and chemical processes. Cluster 3. Decarbonization and climate change (green color): This cluster considers hydrogen storage and production as key strategies to energy transition, decarbonization, and climate change mitigation. Cluster 4. Hydrogen industry and technology (blue color): This cluster covers the integration of advanced technologies and infrastructure essential for the production, storage, and distribution of green hydrogen and e-fuels and how electricity generated from renewable sources, such as solar and wind energy, can be efficiently converted and stored as gas. It is important to note that these clusters form an integral framework that summarizes research on the temporary storage of H\({}_{2}\) in rock formations. Although they have been divided into specific groups for analysis, it is evident that these groups are not independent, and there is a significant interrelation among them. Key terms related to laboratory-scale experimentation, for example, emerge in different clusters: experimental study (Cluster 2), porosity, wettability, H\({}_{2}\)-prime-rock interaction, diffusion, etc. (Cluster 1), pH, methane (Cluster 3). The studies within these groups overlap and complement each other, demonstrating the complexity and interconnectedness of the aspects involved in H\({}_{2}\) storage in rock formations. This collaboration between the groups is essential for a comprehensive understanding of the viability of this sustainable energy solution and for effectively addressing long-term UHS challenges. Figure 2: Cluster network based on authors’ keywords (see Section 2). Cluster 1—storage in porous media; cluster 2—storage in salt caverns; cluster 3—decarbonization and climate change; cluster 4—hydrogen industry and technology. LCA (life cycle assessment); CCS (carbon capture and storage). Analyzing the research trends, the most used keywords are energy storage (13.76%), carbon capture and storage (CCS) (12.18%), and salt cavern (7.83%). The keywords used for the longest periods are carbon dioxide emissions (10 years), electricity (8 years), and experimental study (8 years), and the current trends are wettability, cushion gas, molecular simulation, and energy transition (Figure 3; for more data see Supplementary Materials, Figure S3).