# On Green Hydrogen Generation Technologies: A Bibliometric Review
###### Abstract  
Green hydrogen, produced by water electrolysis with renewable energy, plays a crucial role in the revolution towards energy sustainability, and it is considered a key source of clean energy and efficient storage. Its ability to address the intermittency of renewable sources and its potential to decarbonize sectors that are difficult to electrify make it a strategic component in climate change mitigation. By using a method based on a bibliometric review of scientific publications, this paper represents a significant contribution to the emerging field of research on green hydrogen and provides a detailed review of electrolyzer technologies, identifying key areas for future research and technology development. The results reflect the immaturity of a technology which advances with different technical advancements, waiting to find the optimal technical solution that allows for its massive implementation as a source of green hydrogen generation. According to the results found in this article, alkaline (ALK) and proton exchange membrane (PEM) electrolyzers seem to be the ones that interest the scientific community the most. Similarly, in terms of regional analysis, Europe is clearly committed to green hydrogen, in view of the analysis of its scientific results on materials and electrolyzer capacity forecasts for 2030.  
green hydrogen; sustainability; renewable energy resources; sustainable development; bibliometric review; electrolyzers; technologies +
Footnote †: journal: _Sci._  
Footnote †: journal: _Sci. * It can be used directly in internal combustion reciprocating internal combustion engines, requiring relatively minor modifications if boosted to a moderately high pressure, as well as in turbines and boilers. * It can be used in hydrogen/oxygen fuel cells to directly produce electricity. Again, the only product is H2O. * It can also be used in fuel cells with higher temperatures to generate electricity and heat simultaneously (as a cogeneration plant). However, despite its advantages, the widespread adoption of hydrogen still poses significant technical challenges. Large-scale production of clean hydrogen is expensive, and its efficient storage and transport require advanced technologies. In addition, the infrastructure for its use in applications such as transportation is still under development in many places. The hydrogen production process can be carried out from various energy sources. Several types of hydrogen can be distinguished (Figure 1), each with its unique characteristics and specific applications depending on its generation technology [11]:  
* Green hydrogen is produced by a water electrolysis process using electricity from renewable sources, such as solar or wind energy. This water electrolysis process is completely clean, with no CO2 emissions [12; 13];
* Gray hydrogen is obtained from natural gas reforming, a process that emits CO2. Although it is neither sustainable nor clean by itself, CO2 capture technologies can reduce its emissions [14; 15];
* Blue hydrogen is similar to gray in terms of its production from natural gas but differs in that CO2 emissions are captured and stored rather than released into the atmosphere. This makes it a more sustainable option [16; 17];
* Pink hydrogen is also known as synthetic hydrogen, and it is produced by electrolysis of water using nuclear energy. Although it is considered clean, its large-scale production presents regulatory and safety challenges [18];
* Yellow hydrogen is produced from the electrolysis of water using concentrated solar power (CSP) [19];
* Black hydrogen is generated from the combustion of coal; therefore, it is extremely polluting [20];
* Brown hydrogen is similar to black in terms of CO2 emissions, and it is based on the combustion of lignite to generate hydrogen [21];
* Orange hydrogen is produced from emissions or waste from other sectors [22];
* White hydrogen is the one stored in subway reservoirs [23]. Figure 1: Hydrogen production methods, feedstock/electricity sources, and technology production.