# On Green Hydrogen Generation Technologies: A Bibliometric Review
###### Abstract
A simple indicator of the interest that is shown in the different hydrogen generation technologies is the number of research papers including the term as the focus of the research (Figure 2). By searching in the Scopus database for related articles based on their title, abstract, or keywords (data collected in January 2024), the words of the different hydrogen generation technologies in the period between 1950 and 2023 reveal a clear reality. The number of results obtained for green hydrogen in this period is higher than 33,000, which means a rate of results close to 450 per year, while in the case of blue hydrogen technology, the number slightly exceeds 22,000 results, which means a rate of results close to 300 per year. For the rest of the technologies, only black and white hydrogen slightly exceed 10,000 results in the period of 1950-2023, with an obtained rate of results of less than 100 per year. For the rest of the hydrogen generation technologies, the results are even lower. In the period of 1950-2023, the results obtained for green hydrogen account for 35% of the total obtained results for all hydrogen generation technologies, while those obtained for blue hydrogen account for 23% of the total results. These two hydrogen generation technologies generate more than 50% of the research results obtained by the nine analyzed hydrogen generation technologies. Based on the results obtained, green hydrogen has become a key pillar for the scientific community in the search for a cleaner and more sustainable energy grid. Its production is closely linked to renewable energy sources [24], which makes it highly desirable in the fight against climate change and the reduction in carbon emissions [25]. It has a wide variety of applications. It can be used as a fuel in fuel cells for electric vehicles, enabling clean, zero-emission mobility. It is also used in industrial applications to power production processes and as a stationary energy source to back up the power grid at times of high demand. In addition, it can be a source of clean heat for industrial and residential applications. The production of green hydrogen is based on a process called water electrolysis. This process involves the decomposition of water (H\({}_{2}\)O) into its basic components, hydrogen (H\({}_{2}\)) and oxygen (O\({}_{2}\)), using electricity. The electricity used in this process comes exclusively from renewable energy sources, such as solar panels or wind turbines [26]. This means that no greenhouse gases are emitted during the production of green hydrogen, making it a truly clean and sustainable energy source [27]. Figure 2: Results of the Scopus database search (data collected from the Scopus database in January 2024). Electrolyzers are the fundamental components for using the electrical energy that is generated by renewable sources and subsequently generating hydrogen [28]. Currently, there are different types of electrolyzers, depending on their size and function, among which the following can be highlighted:  
* Alkaline electrolyzers: these electrolyzers usually use an aqueous solution of potassium hydroxide as the electrolyte, and they are remarkable for their energy efficiency and low manufacturing cost [29; 30];
* Proton exchange membrane (PEM) electrolyzers: these electrolyzers are very popular, and many modern electrolyzers are built with this technology. The electrolyte is a thin ion-conducting solid membrane used in place of the aqueous solution [31];
* Solid oxide electrolytic cells (SOECs): these are basically a corresponding fuel cell operating in reverse [32]. In this context, the objective of the present research is to analyze the state of the specialized literature on green hydrogen, its main generation technologies, as well as its fundamental applications. For this purpose, this work develops a bibliometric and technical review of the different green hydrogen generation technologies, as well as their applications.