# On Green Hydrogen Generation Technologies: A Bibliometric Review
## 3 Results  
This section shows the results that were obtained in the performed bibliometric analysis (Phase II, Figure 3). The first study identified in Scopus on green hydrogen was conducted by [PERSON] et al. [37] in 1992. This article questioned the photovoltaic technology as a source of efficient electric power generation and for the first time raised the possibility of connecting a H\({}_{2}\) storage system to a photovoltaic installation. Figure 4 below shows the annual trend of publications on this topic, generated from the Scopus sample of 302 results (Figure 5). Two clear periods are observed in this publication trend: Stage I, from 1993 to 2010; and Stage II, from 2011 to 2023. This trend change after 2011 is also in line with the evolution proposed by the International Energy Agency (IEA) to achieve a global production of around 8 million tons of hydrogen per year in 2030 from low-emission sources, which means obtaining around 350 thousand tons in 2023 [38; 39]. In terms of annual productivity, between 1993 and 2010, the interest in green hydrogen by the scientific community was very low, with the results/year indicator being 0 in most years of this period. The year 2001 stand out when the values were four results/year, respectively. From 2011 onwards, there was a slight interest on the part of the scientific community, based on the results obtained, which were close to 10 results/year in the period of 2011-2017. From the year 2017, an exponential increase of interest in green hydrogen occurred in the scientific community, going from 10 results per year in the year 2010 to almost 70 results per year in the year 2023. This means an average of 32.5 results per year in the period of 2017-2023. Of the 302 results obtained, 237 were published in journals (Table 2), while 32 were published in conference proceedings, 24 in books, and 9 in book series. Therefore, 89% of the results obtained in the search used the first two source types. The most influential source in Scopus on green hydrogen is the _International Journal of Hydrogen Energy_ with 27 (8.94% of the total) of the obtained results published in it. The second most influential source is _Energies_, with 19 results (6.29% of the total). The podium is completed by the _Chemical Society Review_ with 12 results (3.97% of the total). The rest of the sources have several obtained results, all less than 10. It can be seen, therefore, that green hydrogen, as a research area, is very diversified, with no preferential sources when it comes to publishing relevant results in the scientific community. Of the 302 results obtained, Table 3 below shows the results obtained by the most relevant sources. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Source Type** & **Number** & **Percentage** \\ \hline Journal & 237 & 78.48\% \\ Conference Proceeding & 32 & 10.60\% \\ Book & 24 & 7.95\% \\ Book Series & 9 & 2.98\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Green hydrogen result distribution by source type (1993–2023) (data collected from Scopus database in January 2024). Figure 5: Green Hydrogen results by co-authorship network (1993–2023) (data collected from Scopus database in January 2024). Source: developed by the authors in VOS Viewer software 1.6.16. On the other hand, the 302 articles are divided into 21 areas, with the Top 5 introduced in Table 4. The category \"Energy\" is the main category, with 169 associated results (55.96% of the total). The second category with the most associated results is \"Engineering\" with 109 results (36.09% of the total). And the third place goes to \"Chemistry\" with 65 results (21.52% of the total). Note that some results may have more than one subject area. Energy and Engineering account for more than 90% of the results. Figure 4 below shows the co-authorship network of authors. The eleven most representative clusters of the network were identified.