A BIBLIOMETRIC STUDY ON RESEARCH TRENDS IN HYDROGEN PRODUCTION FROM SOLAR SOURCES BASED ON SCOPUS

Purpose: To carry out a bibliographic study reviewing scientific works related to hydrogen and thermal/photovoltaic solar energy being published within Brazil and globally. Method/design/approach: The Bibliometrics R tool was applied to analyse document data extracted from the Scopus database. Results and conclusion: Over three thousand documents about hydrogen with photovoltaic energy were found, published in 1,200 journals. Near 6,600 documents were catalogued for the general issue of hydrogen and solar energy. The number of publications per year continuously grew from 1975 to 2022, reaching 200 in recent years. Research implications: The theme related to hydrogen and photovoltaic solar energy still needs to gain space in Brazil, as the number of publications is modest compared with other countries. The present work shows that, despite the increasing attention that this theme is receiving, a lot of potential remains unexplored. Originality/value: This study seeks to map and identify the main published works on hydrogen production from solar-photovoltaic and solar-thermal sources. The study focuses on the global as well as the Brazilian scenario and uses the Scopus database in association with the Bibliometrix R tool to analyse the reported data.


INTRODUCTION
The present work deals with a bibliometric analysis conducted from publications that relate solar energy, both photovoltaic and thermal, with the production of hydrogen.The method of bibliometric analysis is widely used to detect the state of a given field of research, and is capable of using quantitative and statistical analyzes to describe patterns of publications in a given period.
Scientific mapping is becoming an essential academic activity, as it is possible to observe the trends of the publications on the subject addressed, and Bibliometrix is an open source tool to perform a comprehensive mapping analysis of the scientific literature.Some articles have been used to support the bibliometric methodology to be used, such as Cuccurullo (2017), Donthu, Kumar, Mukherjee, Pandey and Lim (2021) and locallyBortoluzzi, De Souza and Furlan (2021).
Hydrogen is an energy carrier that has great potential in the quest for reducing and, in the long term, eliminating the emission of carbon dioxide by the economies, serving as a means of storing renewable energy, and mainly as a fuel in sectors difficult to electrify, such as heavy transport, production of chemicals, steel and iron.However, for hydrogen to play an important role in decarbonization, its production cannot come from fossil fuels that involve carbon emission (Cabral et al., 2014).
Hydrogen can enable renewable energy to make a greater contribution to the global energy matrix, with the potential to mitigate negative effects arising from the variability of some renewable sources, such as solar energy.Hydrogen also represents one of the options with the greatest potential for cheaper energy storage with the development of new technologies, with storage capacity for days, weeks and even months (Amaral, 2021).
In the light of the growing interest in the theme, this work aims to search, in the Scopus database, documents related to the production of green hydrogen and solar energy to identify the publications in the world and Brazilian scenarios, showing the most cited documents, the authors who are publishing the most in the area, the institutions that are most developing research related to the theme, and also showing what can still be explored.To do so, the Bilbiometrix R tool was used to compile the data and to carry out the analysis.3

SEARCH METHOD
The present work was based on a bibliometric research, during which key words were inserted into the Scopus database, with files containing all the information related to the documents found.After the keywords were entered, and once the metadata files were generated by the Scopus database, in .bibformat, the tool was used Bibliometrix R for the analysis itself.
With the graphs generated during the analysis of the data, it was possible to visualize how the researches and publications are doing in the area of green hydrogen related to photovoltaic solar energy and to identify the poles that are working most in this area, and also to know what is still little researched.It was also possible to identify the authors who most publish on this subject, the most cited works, the countries and centers of greatest relevance, among other points.The keywords considered initially were "hydrogen" and "photovoltaic", where "and" was used as a boolean criterion in the list of all descriptors, and later "hydrogen".With an emphasis on the characterization of the national context, a new search was also carried out in the Scopus database, where only documents related to Brazil were filtered, that is, in documents that contained at least one of their authors linked to the Brazilian institutions.
For this study, a search was carried out on the Scopus database in November 2022, using the methodology suggested by Donthu et al. (2021), together with the parameters listed in Table 1, where 3,503 documents were obtained for the search considering "hydrogen" and "photovoltaic", and 6,669 for the search considering "hydrogen".After generating the two files separately with the documents found, through a programming in R, removed the duplicate files, leaving with 9,320 documents that were unified in a single .bibfile to be read by Biblioshiny, where were generated charts and figures that could be analyzed, and whose general information, is in Table 2, while Table 3 shows the information considering only the documents that are linked to Brazilian institutions or researchers, which after removing the duplicate documents, was left with 96 articles.
Table 2 shows that the set to be analyzed is made up of 9,320 documents that are distributed in 1,232 newspapers, which were published in the period between 1947 and already with some of 2023.Table 3 shows that there are 96 documents connected with Brazilian institutions or with Brazilian researchers in the data set to be analyzed and that they are distributed in 60 newspapers, over a period between 1985 and 2022, showing that the potential of this theme can be explored more effectively.

RESULTS AND DISCUSSIONS
The present work analyzed publications considering a global context and, later, considering the national context of Brazil.

Annual scientific production
The set of 9,320 analyzed documents was published in the period between 1947 and already considering the first articles of 2023, however in the years 1947, 1955, 1958, 1961, 1963, 1967, 1970 has only one article, and in the years 1948 -1954, 1956 -1957, 1 960, 1962, 1964, 1966, 1968 -1969, 1970 -1971 there are no published articles.In the year 1973 appear 2 documents and in the year 1974 appear 5 documents Fig. 1a shows the quantity of articles per year, from 1975 to the present time around the world, while Fig. 1b shows the quantity in Brazil.One can clearly perceive the tendency of an increase in the growth of research and publication in this area, showing that it is a current theme, that is, a motor for the development of the area, but Brazil still has very few publications related to the subject.
With regard to Brazil, it is clear that there are few studies dealing with photovoltaic energy and hydrogen, which can be considered an area of study that is still incipient and that can be better explored.2023) which addresses a system that considers energy, cooling, heating and hydrogen energy as utility systems for urban and industrial areas and renewable solar energy is supplied to the system using the solar photovoltaic system for electrical generation and the solar thermal collector for heat generation.In the work of Zhang et al. (2023) a renewable energy usage model, including photovoltaic module, electrolyzer module and fuel cell module, is developed to simulate the performance of the hydrogen production system and power generation.Already in the work of Izadi, Shahafve, Ahmadi and Hanafizadeh (2023) a hybrid system of renewable energy, using photovoltaic panels, with hydrogen energy storage is simulated to cover the power demand of sections and wards of a hospital that attended COVID-19 patients.Subsequently, the modeled system was optimized with a genetic neural network algorithm to calculate the optimal amount of demand energy from the network, CO2 emission, oxygen capsules and cost rate.
In the paper by Chen, Yiliang, Hongxia, Yujie and Xiongwen (2023) integrated solar energy, heating and hydrogen systems based on photovoltaic (PV), photothermal (PT) and photocatalysis for hydrogen production (PH) is proposed and investigated.The work of Khanmohammadi, Razi, Delpisheh and Panchal (2023) addresses thermodynamic modeling and multi-objective optimization of a solar-based multigeneration system that produces hot water, heating, cooling, hydrogen and fresh water using a humidification and dehumidification unit (HDH).
Considering the national context, among the most recent articles one can cite the work of Gabriel et al. (2022), whose focus is the numerical simulation of an on-grid hybrid system to meet small residential demands, using natural gas and solar energy.The system includes a natural gas reformer, a fuel cell, photovoltaic (PV) panels and batteries connected to the grid.The work by Restrepo, Izidoro, Násner, Venturini and Lora (2022) analyzes the production of renewable hydrogen using concentrated solar energy considering a hypothetical production plant located in Guamaré, Brazil.In the work of Macedo and Peyerl (2022), the economic viability of renewable hybrid systems for the production and storage of hydrogen in the Brazilian electricity sector was verified, based on economic cost analyzes of two wind and solar photovoltaic plants in Brazil. In Nadaleti, De Souza, and De Souza (2022), the authors sought to evaluate the excess energy potential of solar and nuclear plants installed in Brazil for the production of green and purple hydrogen using high and low temperature electrolysis methods.

Most relevant sources
Figure 2 shows the 20 most relevant publication vehicles when considering the total set of documents analyzed, in which the International Journal of Hydrogen Energy appears first with 1,179 documents published in it, followed by Solar Energy with 186 articles published.
In the case of Brazil, Figure 3 shows that the International Journal of Hydrogen Energy continues to appear in first place with 17 publications in the area.
Figures 2 and 3 show which magazines are publishing the most in the area of hydrogen related to solar energy and photovoltaics.This can help in choosing a journal to be researched and where it can eventually be published.Among the articles found in the journal International Journal of Hydrogen Energy, the 10 most cited are in Table 4. 7 Considering the national context, among the articles found in the International Journal of Hydrogen Energy, one can cite the work of Palhares, Vieira and Damasceno (2018) that designed a cylindrical electrolytic cell and the system was connected to a 20W photovoltaic panel to produce hydrogen.The work of Micena, Llerena-Pizarro, De Souza and Silveira (2020) proposes a project of Hydrogen Refueling Station (HRS) powered by a photovoltaic plant to supply the taxi fleet of a Brazilian city considering different scenarios and assuming that vehicles powered by hydrogen.The work of Esteves et al. (2015) analyzes the feasibility of ammonia production, using the Haber-Bosch process with hydrogen from a solar photovoltaic source.

Most important affiliations
The world's institutions that are working most, producing publications with the theme in question, appear in Figure 4, with emphasis on the University of California.
In the specific case of Brazil, the University of São Paulo leads with 18 articles, followed by the Federal University of Santa Catarina with 5 articles, as shown in Table 5.

Countries of the corresponding authors
Figure 5 shows the amount of work related to the country of the corresponding authors.China leads with 2,613 articles, followed by the United States with 973 and Japan with 424.

Most cited articles in Brazil and the world
Table 6 shows 20 articles, among the most cited, and the respective total number of citations of each, considering the world context of the research carried out.
The work of Wang et al. (2009) shows an abundant material, polymeric carbon nitride, which has the potential to produce hydrogen from water under light irradiation.The work of Lewis and Nocera (2006) addresses the scientific challenges involved in capturing and converting solar energy and then storing energy as hydrogen.Carrasco et al. (2006) presented new trends in power electronics for the integration of wind and photovoltaic (PV) power generators and a review of the appropriate storage system technology used for the integration of intermittent sources.According to Zou, Ye, Sayama, and Arakawa (2001), the photocatalytic division of water into hydrogen and oxygen using solar energy is a potentially clean and renewable source of hydrogen fuel, and efforts have been invested in the development of photocatalysts capable of using less energetic visible light.According to Carmo, Fritz, Mergel and Stolten (2013), hydrogen is considered the best means to store energy from and intermittent renewable energy sources, and PEM electrolysis provides a solution for hydrogen production and is suitable to combine with solar source.And Grätzel (2009) states that capturing solar energy and converting it into electricity or chemical fuels, such as hydrogen, at low cost and using abundantly available raw materials, remains a major challenge, so his research aims to identify and provide solutions to some efficiency problems.
The work of Cook et al. (2010) addresses semiconductor-based photocatalysis which attracts great attention due to its ability to directly use solar energy for the production of solar fuels such as hydrogen.Low, Yu, Jaroniec, Wageh and Al-Ghamdi (2017) talk about the challenges related to photoelectrochemical cells (PEC) that offer the ability to convert electromagnetic energy from our largest renewable source, the sun, into chemical energy stored by dividing water into molecular oxygen and hydrogen.Maeda and Domen (2010) described the state of the art and challenges regarding photocatalizers, as the division of water to form hydrogen and oxygen using solar energy in the presence of semiconductor photocatalysts has long been studied as a potential means of clean, large-scale fuel production.In the paper by Bak, Nowotny, Rekas and Sorrell (2002) the authors considered the generation of hydrogen from water using solar energy, where the work focused on the issues related to materials in the development of high-efficiency photoelectrochemical cells (PECs).10 In Table 7, 10 articles were listed among those with the highest number of citations, linked to Brazilian institutions or authors.
Among the works with more citations that have authors linked to Brazilian institutions, one can cite the work of Khosravi, Koury, Machado and Pabon (2018) whose goal was to define and evaluate an off-grid hybrid renewable energy system with hydrogen storage, combining solar and wind energy, hydrogen production unit and fuel cell.Silva, De Oliveira and Severino (2010) have evaluated the use of a photovoltaic fuel cell system for the supply of electricity in an isolated community in the Amazon region, analyzing the hydrogen storage system.And in the work of Da Silva et al. (2005), the technical and economic feasibility for the implantation of a hypothetical hydrogen production plant, powered by electricity generated by wind and solar sources and conventional hydroelectricity, was studied through the potential of to the northeast of Brazil, from where the produced hydrogen would be exported.

Key words
To better visualize the keywords found in the document set, TreeMap was used.With it, it is possible to perceive which themes have been most explored and those that still need to be studied more.Figure 6 shows TreeMap for the set of searched words considering the world context.The larger the frame occupied by each word, the greater the number of results found and their relevance.

Figure 1 .
Figure 1.Quantity of items per year in the global (a) and national (b) scenarios Source: Scopus base, Bibliometrix tool (2022)

Figure 2 .
Figure 2. Most relevant sources with their respective numbers of publications in the researched area Source: Scopus base, Bibliometrix tool (2022)

Figure 3 .
Figure 3.Most relevant sources with their respective numbers of Brazilian publications in the area Source: Scopus base, Bibliometrix tool (2022)

Figure 4 .
Figure 4. Most relevant institutions and their quantities of articles produced Source: Scopus base, Bibliometrix tool (2022)

Table 2 -Key information on the data generated by Bibliometrix Returns TRUE on success or FALSE on failure.
Source: Scopus base, Bibliometrix tool (2022)

Table 3 -
Main information on the data generated by Bibliometrix linked to Brazilian institutions or researchers Returns

Table 6 -
The most cited articles worldwide related to the considered research