# On Green Hydrogen Generation Technologies: A Bibliometric Review
## 4 Discussion
At the same time, these words are the ones with the highest total link strength; therefore, they appear to a greater extent together with other relevant keywords. Figure 9: International comparison of different variables around green hydrogen (data collected from Scopus database in January 2024 and from the Global Hydrogen Review 2023 [38]). On the other hand, when carrying out the same analysis of the documents that were published in Stage II (2011-2023), it is observed that the most used keywords are \"hydrogen production\" and \"solar power generation\" (Table 6). All words in this period have a reduced percentage of occurrence compared to the results obtained in the period of 1993-2010. This is due to the greater number of keywords that were identified in Stage II compared to those identified in Stage I. Of the 10 most used words in both periods, 7 are repeated in both periods: hydrogen, hydrogen production, hydrogen storage, solar energy, solar power generation, electrolysis, and fossil fuels. These results demonstrate that since the beginning of research in green hydrogen, there has been a trend on the part of the scientific community toward hydrogen, electrolysis as a method of generating green hydrogen, as well as solar energy and fossil fuels as the sources of electrical energy generation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Keyword** & **Occurrences** & **Occurrences (\%)** & **Total Link Strength** \\ \hline hydrogen production & 142 & 51.3\% & 2405 \\ solar power generation & 132 & 47.7\% & 2278 \\ electrolysis & 115 & 41.5\% & 2056 \\ solar energy & 80 & 28.9\% & 1446 \\ hydrogen & 70 & 25.3\% & 1338 \\ hydrogen storage & 61 & 22.0\% & 1234 \\ fossil fuels & 60 & 21.7\% & 1210 \\ renewable energies & 50 & 18.1\% & 895 \\ water electrolysis & 49 & 17.7\% & 918 \\ renewable energy & 43 & 15.5\% & 718 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Most frequently used keywords in Stage II (2011–2023). Figure 10: Keyword trends (1993–2010 and 2011–2023). Source: developed by the authors in VOS Viewer software 1.6.16. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Keyword** & **Occurrences** & **Occurrences (\%)** & **Total Link Strength** \\ \hline hydrogen & 16 & 64.0\% & 349 \\ solar energy & 15 & 60.0\% & 385 \\ renewable energy resources & 11 & 44.0\% & 221 \\ electrolysis & 10 & 40.0\% & 194 \\ fuel cells & 9 & 36.0\% & 221 \\ hydrogen production & 9 & 36.0\% & 176 \\ solar power generation & 8 & 32.0\% & 224 \\ hydrogen storage & 7 & 28.0\% & 245 \\ electricity & 6 & 24.0\% & 226 \\ fossil fuels & 6 & 24.0\% & 198 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Most frequently used keywords in Stage I (1993–2010). The large-scale production of green hydrogen faces major challenges that must be overcome [50; 56]. Among these challenges, it is possible to identify the following (Table 7): (i) limited infrastructure; (ii) low electrolysis efficiency; (iii) high technical complexity in storage and transport; (iv) economic feasibility; and (v) environmental impact, as the technological development of green hydrogen implies some environmental impact, as well as the consumption of sometimes scarce material resources. After the first search in the Scopus database (Table 1), additional search fields were added to identify the importance of these challenges for the scientific community. In view of the results obtained in this bibliometric review, it can be seen that these challenges, which are identified by some authors, are also relevant for the rest of the scientific community, with aspects related to them appearing in more than 50% of the obtained results. The first of these challenges (Table 7) is the limited infrastructure for green hydrogen production, which poses significant challenges that must be addressed to realize its potential as a sustainable energy vector [57]. First, the lack of large-scale production facilities hinders the ability to meet the growing demand for green hydrogen.