# On Green Hydrogen Generation Technologies: A Bibliometric Review
## 4 Discussion
In the different electrolyzer technologies, the most relevant differences are found in (Figure 11) (i) the separator located between the two electrodes and its function; and (ii) the way of introducing the initial product (H\({}_{2}\)O) and segregating the obtained products (H\({}_{2}\) and O\({}_{2}\)). These differences drastically influence the way in which chemical reactions take place both at the anode and the cathode. In the case of an alkaline electrolyzer (ALK), water enters the cathode and separates into hydrogen (H\({}^{+}\)) and hydroxide ions (OH\({}^{-}\)). These hydroxide ions travel through the electrolyte to the anode, where they are reduced to form gaseous oxygen molecules. The efficiency of this process is influenced by several factors, such as the quality of the materials used, the temperature, and the pressure. Although the ALK electrolyzer has traditionally been more suitable for stationary applications due to its slower response to changes in load, continuous improvements are expanding its viability in more dynamic applications:  
\[\text{Cathode : }2\text{H}_{2}\text{O}\ (1)+2\text{e}^{-}\rightarrow\text{H}_{ 2}(\text{g})+2\text{OH}^{-} \tag{2}\] \[\text{Anode : }2\text{OH}^{-}\ (\text{aq})\rightarrow\
icefrac{{1}}{{2}} \text{O}_{2}(\text{g})+\text{H}_{2}\text{O}(\text{l})+4\text{e}^{-}\]  
Figure 11: Schematic diagram of operating principle of different electrolyzer technologies to obtain green hydrogen. In the case of proton exchange membrane (PEM) electrolyzers, water is injected into the anode, where it is split into protons (H+) and oxygen molecules (O2). The protons (H+) pass through the membrane to the cathode to generate hydrogen (H2):  
\[ \begin{array}{c}\text{Anode}:\;\text{H}_{2}\text{O}\;(\text{l})\to 2\text{H}^{+}( \text{g})+\
icefrac{{1}}{{2}}\,\text{O}_{2}+2\text{e}^{-}\\ \text{Cathode}:\;2\text{H}^{+}(\text{g})+2\text{e}^{-}\to\text{H}_{2}\end{array}  \tag{3}\]  
Finally, in the case of solid oxide electrolysis cell (SOEC) electrolyzers [65], as in ALK electrolyzers, water enters the cathode and is split into hydrogen (H+); however, unlike ALK, oxygen anions (O2-) are produced at the cathode. These anions pass through the solid oxide or ceramic electrolyte to produce oxygen (O2) molecules at the anode:  
\[ \begin{array}{c}\text{Cathode}:\;\text{H}_{2}\text{O}\;(\text{l})+2\text{e}^ {-}\to\text{H}_{2}(\text{g})+\text{O}^{2-}\\ \text{Anode}:\;\text{O}^{2-}\to\
icefrac{{1}}{{2}}\,\text{O}_{2}(\text{g})+2 \text{e}^{-}\end{array}  \tag{4}\]  
ALK electrolyzers are the most widely used at present [63]. In view of the results obtained in the present research (Figure 12), the alkaline electrolyzer (ALK) and proton exchange membrane (PEM), which both had over 90 occurrences, meaning that they appear in more than 40% of the results obtained, are the ones that generate the most interest in the scientific community. Finally, solid oxide electrolytic cell (SOEC) electrolyzers are only investigated in a minority of studies, obtaining only 21 results, therefore appearing in less than 10% of the results obtained. The need for a sufficiently sized and safe transportation and storage network is essential to ensure a successful transition (Table 7). Among the various storage techniques, the injection of green hydrogen into depleted fossil hydrocarbon reservoirs [65], as well as storage as a compressed gas and processing the hydrogen into another more stable substance, such as ammonia, are positioned as viable techniques. The cost of green hydrogen has decreased significantly, making it competitive with other technologies (Table 7). The cost of H2 production from PEM electrolyzers is around 9 USD/kgH2, and it is also the most environmentally friendly electrolysis process, emitting the lowest GWP per kg H2 produced (1.0 kg CO2/kg H2) [62]. However, many researchers question the role of hydrogen in decarbonizing economies and propose as an alternative to see a huge role for gas [67].