## 3 Results
### Bibliometric Analysis
#### 3.1.1 Performance Analysis  
Performance analysis is very useful to evaluate the productivity and impact of the articles included in the review. Figure 2 and Table 2 show the number of publications per year classified by HVC stages and the number of citations per year. An upward trajectory in annual publication volume is observed between 2019 and 2025, with a marked increase in the growth rate commencing in 2024. This trend is largely driven by increased CET funding initiated in 2020, as a response to the COVID-19 crisis and the EU's recognition of the strategic vulnerability posed by its reliance on natural gas. In this context, major programs such as NextGenerationEU (2020) and RePowerEU (2022) were launched, positioning renewable hydrogen as a central pillar. NextGenerationEU allocated approximately \(\copyright\)250 billion to accelerate investments in renewable energy, clean technologies, and energy efficiency. RePowerEU further established ambitious targets of achieving 10 million tons of domestically produced renewable hydrogen and importing an additional 10 million tons annually by 2030 [51]. The research carried out during the first years of these programs has begun to be published from the year 2024, since the number of publications for the period 2024-2025 represents 52% of the total number of publications for the review period. In terms of the stages of the value chain, the production stage represents 48% of the total number of publications, followed by end-uses (18%) and storage (12%). This distribution across value chain stages provides context for the analysis in Figure 3, which presents the percentage breakdown of publications according to the geospatial techniques outlined in Table 1. Seventy-nine percent of the total number of publications corresponds to the application of GIS. Therefore, by analyzing Figures 2 and 3 together, it can be concluded that GIS has a significant impact on HVC specifically in the production and end-use stages, as will be discussed in detail in Section 3.2. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **2019** & **2020** & **2021** & **2022** & **2023** & **2024** & **2025** \\ \hline \hline \multirow{3}{*}{Cross-cutting issues} & 0 & 0 & 1 & 3 & 1 & 9 & 7 \\  & End-uses & 2 & 7 & 3 & 4 & 2 & 9 & 5 \\ \cline{1-1}  & Transportation & 1 & 1 & 0 & 1 & 5 & 1 \\ \cline{1-1}  & Storage & 3 & 0 & 2 & 1 & 7 & 5 & 4 \\ \cline{1-1}  & Production/Transportation & 0 & 1 & 2 & 0 & 0 & 0 & 1 \\ \cline{1-1}  & Production/Storage & 1 & 1 & 0 & 0 & 0 & 1 & 0 \\ \cline{1-1}  & Production & 4 & 2 & 9 & 15 & 10 & 25 & 20 \\ \cline{1-1}  & Number of citations & 716 & 728 & 1900 & 859 & 923 & 1525 & 698 \\ \hline \end{tabular}
\end{table} 
Figure 2: Number of publications and citations per year on the application of geospatial techniques to HVC. Table 3 shows the annual distribution of publications classified by publisher and journal. Elsevier and Multidisciplinary Digital Publishing Institute (MDPI) dominate the publication landscape, contributing 80.8%, and 9.6% of the total reviewed articles, respectively. Although the studies are distributed across 34 journals, nearly half (49.7%) were published in the _International Journal of Hydrogen Energy_ (78 articles) and _Applied Energy_ (10 articles). Figure 3: Percentage distribution of the number of publications classified according to the geospatial techniques established in Table 1.