Hydrogen Horizons: A Bibliometric Review of Trends in Diverse Emission Sectors

: Recognizing the future and present challenges facing humanity, the United Nations has developed 17 Sustainable Development Goals. To achieve these goals, it is necessary to understand energy sources and the origin of their raw materials. Therefore, the role of hydrogen in the future energy balance is being discussed more and more frequently. The aim of this study is to use bibliometric analysis to determine the effectiveness of hydrogen use in different sectors to achieve sustainable development goals. This work identifies the most cited articles, as well as the most productive journals, countries, and institutions. This work provides insight into the current state of hydrogen sustainability research in six different areas. The reviewed sectors include energy, industry, transport, agriculture, commercial, and residential. The results show that the energy sector shows the greatest interest in sustainable development, followed by industry and transport.


Introduction
The problem of global warming was first raised in the 1980s, when it was determined that carbon dioxide increases in the Earth's atmosphere, which, in turn, contributes to climate change [1].At present, the negative impact of human activity is becoming more and more visible, prompting us to both change our habits and look for better solutions in the usual systems.The improvement in people's quality of life, together with the overall population growth, also contributes to the greater consumption of resources, including in the energy sector [2].Future research on energy system planning will increasingly assess the possible contribution of hydrogen to decarbonization goals.
Global energy consumption has grown exponentially in recent decades, from 8,588.9 million tons (Mtoe) in 1995 to 13,147.3Mtoe in 2015 [3].The total world demand for primary energy is expected to be 17,487 Mtoe in 2040 [4] compared with the global primary energy consumption in 2021 of 595.15 EJ or 14,214.9Mtoe [5].
Green hydrogen and sustainable biomass are significant decarbonization options for businesses looking to reduce carbon emissions while contributing to energy security.Concerns over energy security, particularly in Europe, are putting pressure on policymakers to speed up the deployment of alternative energy sources, with renewables, energy efficiency, and hydrogen at the top of the list [6].
According to the findings, the possible range of global hydrogen consumption is increasing, with estimates ranging from 73 to 158 Mt in 2030, 300 Mt in 2040, and 568 Mt in 2050, depending on the scenario provided [7].
The United Nations 2030 Agenda for Sustainable Development, approved by all UN member states in 2015, presents a shared blueprint for peace and prosperity for people and the planet, both today and in the future.The 17 sustainable development goals (SDGs), which are further broken into 169 specific targets that can be used to accomplish the various SDGs, are depicted in Figure 1.Clean energy is one of the key drivers of the SDGs.As an energy vector and one of the cleanest energy carriers, hydrogen may also be directly related to the different SDGs [8].The goal of this research is to apply bibliometric analysis to establish how well sustainable development goals can be reached through the use of hydrogen and how sustainability is discussed in different sectors.

Previous Research
The search for the title ("hydrogen OR h2" AND SDG OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") resulted in 5180 publications in the time period from 1975 to 2023 (August 15th) in the SCOPUS database, while adding keywords (Bibliometr* OR "Bibliometric Analysis" OR "Bibliometric map" OR "Research Trends" AND sector*) to the SCOPUS database to find similar studies resulted in eight publications.The selection process involved choosing results that were written in the English language, with a focus on articles and review papers.Table 1 presents a comprehensive overview of the articles that were identified.Table 1.A summary of the prior review articles.

Title Year Sectors Discussed SDGs Mapped by SCOPUS Source
Optimized energy management schemes for electric vehicle applications: A bibliometric analysis towards future trends 2021 Mainly transport, power sector 7, 9, 13, 17 [9] Selected aspects of sustainable mobility reveals implementable approaches and conceivable actions 2021 Mainly transport, power sector 7, 8, 11, 13 [10] Management of hydrogen mobility challenges: A systematic literature review 2023 Mainly transport, power sector, commerce sector 9, 11, 17 [11] Global research trends on anaerobic digestion and biogas production from cassava wastewater: A bibliometric analysis 2022 Mainly industry, agriculture 6, 7, 8, 9, 17 [12] Applications of P-graph to carbon management: A minireview 2022 Industry, power sector, agriculture 7, 9, 13, 17 [13] Analysis of the scientific production on the green hydrogen theme published in scientific journals indexed by EBSCO 2023 Power sector, industry, transport 7, 8, 9, 17 [14] Use of patents as a tool to map the technological development involving the hydrogen economy 2019 Transport, industry 7, 13 [15] A systematic review on green hydrogen for off-grid communities-technologies, advantages, and limitations 2023 Power sector, industry 7, 17 [16] Three out of the eight discussed research topics mostly related to the mobility sector.Md.Sazal Miah et al. conducted a bibliometric analysis and investigation of optimized energy management schemes (EMSs) for electric vehicle (EV) applications [9].The researchers emphasized the various challenges and issues associated with EMSs in EVs.These challenges include power distribution concerns, thermal management problems related to batteries, battery storage life cycle and aging issues, policy and regulation challenges, power electronics controller and converter issues, such as switching loss, high ripple current, voltage stress, high voltage gain, high impedance, optimization integration, and complex control techniques.The discourse brought attention to the annual shortfall in achieving the SDGs in the transportation industry, as well as the related apprehensions regarding the environmental consequences and human welfare.Another study regarding hydrogen mobility was presented by Davide Calandra et al. [11].The authors presented a comprehensive scientific roadmap outlining key concerns for managers and policymakers aiming to expand the utilization of hydrogen in the transportation sector.Their roadmap was constructed based on an analysis of 233 papers sourced from the SCOPUS database, all of which were focused on the subject matter at hand.The primary issues identified were those pertaining to expenses, specifically high costs, insufficient investments, and the absence of well-defined business models.The authors of the study underscored the importance of fostering strategic alliances to facilitate collaborative efforts beyond the realm of public transportation.Additionally, the authors advocated for the dissemination of public education initiatives aiming to enhance awareness and acceptance of hydrogen technology among the general populace.Suprava Chakraborty et al. examined studies about sustainable mobility in the last two decades [10].Their article explores the role of sustainable mobility in relation to the attainment of the SDGs.One of the main concerns raised in this context is the diversity of interests and expectations among various stakeholders.The authors believe that all parties must have a shared understanding of the larger goal of creating a transportation system that is intended for the future.Additionally, they recognize the necessity of achieving this goal.Furthermore, this bibliometric study elucidates the need to advocate for transportation services that are efficient, safe, and convenient.They also highlight the importance of reducing car traffic in metropolitan areas and encouraging a low transportation demand.According to the authors, the limited supply of resources for battery production has caused people to think about hydrogen as a possible fuel for environmentally friendly transportation.
Amanda L.M. dos Santos et al. conducted a review analysis aiming to investigate global scientific research trends in the anaerobic treatment of wastewater resulting from cassava processing, one of the world's most consumed vegetable roots [12].Although the study did not explicitly address SDGs, it did show an increasing interest in this process, including the recovery of by-products such as hydrogen.However, their results noted that cassava's circular economy production chain is still limited to large-scale companies, and that further research into this subject, including economic-financial, and environmental viability analyses, is required.
Migo-Sumagang M.V. et al. used the P-graph framework and presented a bibliometric analysis of decarbonization and carbon management networks including biohydrogen and carbon capture and storage technologies in their research [13].The growing interest in biohydrogen was observed in the work.
Henrique César Melo Ribeiro and Rosany Corrêa, in their review, discussed green hydrogen production and how this topic was described in scientific journals indexed by EBSCO.This study presents and verifies that green hydrogen offers opportunities for economic growth and job creation [14].Juan P. Viteri's research concerned the potential of hydrogen to serve as an affordable, dependable, and sustainable form of energy for the world's poorest and presented an extensive bibliometric analysis of green hydrogen to address SDG 7: affordable and green energy [16].
Tiago Sinigaglia et al. conducted a patent-based review study to ascertain the technological evolution of the hydrogen economy.The majority of patents were found to be related to the mobility sector, specifically vehicle and fuel cell technologies.Fuel station-related patents were followed by hydrogen production-and storage-related patents.The number of patent licenses has decreased since 2012; this might be due to a loss of interest among scientists.Based on research, Toyota and Honda are clearly leading in the car industry [15].

Research Outline
Examining the issue of hydrogen sustainability in different sectors, we rigorously adhere to the organized literature review process.First, we state our research question in the introduction section.Then, in the methodology section, we outline the literature review protocol and present the research sample.Finally, we pair critical analysis and the identification of future research directions in the results, discussion, and conclusions sections.

Materials and Methods
Nees Jan Van Eck and Ludo Waltman, two scientometric experts at Leiden University in the Netherlands, developed a scientometric and knowledge graph analysis tool called VOSviewer [17,18], which was used in this study to assess publications in the field of hydrogen sustainability in different sectors.In recent years, VOSviewer has been widely recognized and used in the fields of scientometrics and knowledge graphs because of its extremely simple, user-friendly interface design and trustworthy results.
VOSviewer provides a mapping method that is similar to the multidimensional scaling method, with a more explicit representation of the results.The objective function of VOSviewer is expressed in mathematical notation by Equation (1) [19].
where   denotes the position of node i and ‖  −   ‖ denotes the Euclidean distance between nodes i and j in a two-dimensional space, and sij represents the similarity between two nodes, i and j, which is multiplied by dividing the number of times that nodes i and j appear at the same time by the number of times that nodes i and j appear.The constraint of Equation ( 2 where n represents the total number of nodes in the network.VOSviewer specifically provides a method for visualizing knowledge graph density graphs, which is the most distinctive feature compared with other knowledge graphs.The density map () for node x = (x1, x2) is calculated as follows: where : [0, ∞) → [0, ∞) denotes the kernel function, which is nonincreasing and is represented using the Gaussian function () = exp(− 2 ) , ℎ denotes kernel width, and  ̅ denotes the point density, that is, the average distance between two nodes [18,20].
A good literature review is the basis for knowledge development, so extensive survey research was carried out to provide the content.A thorough approach was used to analyze and gather the most trusted and relevant papers for the study.This study examined hydrogen sustainability in six key sectors: energy, transportation, industry, agriculture, residential, and commercial, as shown in Figure 2. Performance analysis included an analysis of the number of publications, the number of citations, types and research areas, and highly cited publication analysis.Scientific mapping analysis included citation, co-authorship, burst detection, co-occurrence, and timeline analysis.The discussion section includes an analysis of popular issues, trends, and challenges.
The SCOPUS database was used on 15 August 2023, to investigate how sustainable development goals can be achieved through the use of hydrogen energy in various sectors.The SCOPUS database was chosen as the data source because it provides access to a huge number of indexed articles and is a powerful database with extensive sorting, ranking, and refinement options [21].To understand how different sectors are researching the impact of hydrogen on the SDGs, we looked at the energy/power, industrial, transport, agriculture, commercial, and residential sectors (Section 3.4).The following request was used to obtain common insights: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems" AND TITLE-ABS-KEY (sector AND industry) OR TITLE-ABS-KEY (sector AND power OR energy) OR TITLE-ABS-KEY (sector AND transport) OR TITLE-ABS-KEY (sector AND agriculture) OR TITLE-ABS-KEY (sector AND commercial) OR TITLE-ABS-KEY (sector AND residential)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re) OR LIMIT-TO (DOCTYPE, cp)) AND (LIMIT-TO (LANGUAGE, English)).
Consistent with the literature [22,23], the search string adopted Boolean operators in addition to pertinent terms such as "Bibliometr*", which includes the words "bibliometric" and "bibliometry".

Performance Analysis
This section analyzes the performance of publications in terms of annual indicators, publication types, research fields, and highly cited articles.

Annual Indicators of Publications
Figure 3a,b display the annual publication and citation of hydrogen sustainability research, respectively.As shown in Figure 3a, the first related publication, entitled "Prospects for sustainable energy in Atlantic Canada", was published in 1991.In addition to this publication, three more publications were published in the 20th century, two of them in 1997 and one in 1999.The number of publications is generally on the rise but fluctuates slightly.Before 2010, there were less than 20 research papers per year on the sustainable development of hydrogen energy in different sectors.Since then, this field of research has gained considerable importance.The number of publications increased more than 12 times from 2011 to 2022, and over the past five years it has grown from 54 publications in 2019 to 213 publications in 2022.The number of publications in the middle of 2023 almost reached the total number of publications in 2022, an indicator of growing interest.
Similarly, a growing trend is visible in Figure 3b regarding the number of citations in the last 15 years.The first citation of research on this study topic appeared in 1997.Figure 3b shows that, since 2008, citation numbers grew relatively slowly before 2016.In the time between 2016 and 2019, growth was moderate but, since 2019, the number of citations rapidly increased, from 2110 in 2019 to 6742 in 2022.It is evident that the hydrogen sustainability topic has received increasing scholarly attention in recent years.

. Types and Research Areas of Publications
In the SCOPUS database, all publications can be classified according to their different types.It should be noted that some documents are of two or more types.
Most (605 or 59.4%) of the obtained publications were articles, followed by review articles (214 or 21.0%) and proceedings papers (199 or 19.5%).Each publication covers at least one subject area.Figure 4 presents the top 10 research areas.As can be seen from Figure 4, the most popular fields of study are energy (28.2%), engineering (16.8%), and environmental science (13.5%), followed by fields such as social sciences, physics and astronomy, chemistry, computer science, mathematics, business administration, management, and accounting.

Highly Cited Publications
A publication's citation count is a key metric for assessing its impact because it shows that the publication is well-known and relevant.Table 2 shows the 10 most cited works in descending order of citation count.The "SDG" column shows which sustainable development goals are covered in the most cited publications according to the Elsevier SDG data mapping.This mapping was built by Elsevier data science teams and uses extensive keyword queries, supplemented by machine learning, which, in turn, provides document mapping to SDGs with very high precision.It is available on the SCOPUS website under the description of each article in the "Sustainable Development Goals 2023" section.The most cited study, by David Martin A. et al., covers catalytic methods for the production of biofuels from aqueous carbohydrates separated by biomass pretreatment and hydrolysis.This study discusses the importance of hydrogen in biorefining and explores numerous methods for effectively controlling its usage to promote a shift from the dependency on fossil fuels [24].The study authored by Seyed Ehsan Hosseini et al. provides a comprehensive examination of contemporary hydrogen manufacturing methodologies that use renewable and sustainable energy sources.The researchers concluded that the primary obstacles hindering the broad commercialization of solarbased hydrogen generation are the poor efficiency of the solar-to-hydrogen system and the high cost of photovoltaic cells.They concluded that since electricity costs affect hydrogen pricing, solar and wind energy should be four times cheaper than commercial power prices for the manufacture of carbon-free hydrogen at a competitive price with fossil fuels [25].
The study of P. Preuste et al. discusses the importance of hydrogen in the field of biorefining and investigates numerous techniques for efficiently controlling its use, fostering a shift from the dependency on fossil fuels.They found that technological readiness varies according to field.For stationary energy storage systems, liquid organic hydrogen carrier (LOHC) technology has been demonstrated to be feasible in commercial demonstrations, and cost will determine its commercial viability.Other interesting solutions, such as supplying hydrogen to hydrogen filling stations or direct-LOHC-fuel cell applications, require extensive basic and applied research [26].Serrano E. et al. presented a review of the most recent advances in nanotechnology for sustainable energy production, storage, and use, concluding that solar, hydrogen, and new-generation batteries and supercapacitors are the most significant examples of nanotechnology's contributions to the energy sector [27].According to Singh S. et al., practically any energy source may be used to make hydrogen, and hydrogen can be used as fuel for almost any application that uses fossil fuel.Their paper studies current breakthroughs in the fields of hydrogen generation, storage, transport, and delivery, as well as the environmental and safety concerns associated with its usage as an energy carrier.Onboard hydrogen storage was identified as one of the major technological challenges related to the use of hydrogen as a vehicle fuel and political-economic strategies and money allocation were mentioned in the article as the most necessary tools to promote hydrogen energy use in daily life [28].Wilberforce T. et al. also presented an overview, focusing on the use of hydrogen in fuel cell technologies, and emphasizing the advantages and disadvantages of these technologies [29].M. Voldusnd et al. presented an overview of technologies for hydrogen production from fossil fuels with CO2 capture technologies, concluding that, at the stage of transition technology, hydrogen production with CO2 capture could be a promising solution [30].Kovač A. et al. also addressed the energy transition problem, concluding that hydrogen plays an important role in this process.In their work, they point to positive examples in other countries and highlight the barriers that stand in the way of the development of hydrogen technologies [32].Puga A.V.'s article views photocatalytic hydrogen production as a sustainable, viable, and efficient process, with particular emphasis on the role of biorefinery products, agricultural residues, and industrial or municipal waste in hydrogen production [31].Giovanni N. et al., in their 2014 article, compared the technical and environmental indicators of hydrogen with those of fossil fuels and concluded that renewable hydrogen, as a low-emission fuel, has a low environmental impact, which would positively affect the global environment in the long term.Technical deficiencies and barriers were identified [33].
Figure 5 shows a summary of the SDGs mapped in the top ten most cited publications.The results show that all the most cited publications discuss solutions that could achieve Goal 7 (affordable and clean energy).Behind this SDG is Goal 13 (climate protection) and Goal 9 (industry, innovation, and infrastructure).Other SDGs mentioned included Goal 8 (decent work and economic growth), Goal 11 (sustainable cities and communities), Goal 12 (responsible consumption and production), and Goal 17 (partnering for the goals).In Table 3, the 10 journals are ranked according to number of publications.As can be seen, the most productive journal is the International Journal of Hydrogen Energy, with 91 relevant publications, followed by Energies and Renewable and Sustainable Energy Reviews.The journal with the most citations (945) and the highest number of average

Analysis of the Countries/Regions, Institutions, and Authors
This section evaluates publications from three perspectives: author, institution, and country.First, a citation analysis identifies the most important institutions, countries, and authors.The analysis also considers the most productive countries, the most productive institutions, and a collaborative analysis by the authors.NP: the number of publications; NC: the number of citations.

Citation Analysis
Table 4 demonstrates that the United States, India, and Germany are the countries that show the highest levels of interest in this subject, because each of these countries produced more than 100 publications.On the other hand, comparing the citation data, it can be seen that the most influential countries with the highest citation rates are the U.S., Italy, India, and the United Kingdom.Figure 6 displays co-authorship among the 29 nations where at least 15 articles were produced.

Keyword Analysis
Keyword analysis helps identify key themes and topics, trends, and patterns, as well as knowledge gaps.To locate research hotspots, boundaries, and trends, this part conducts co-occurrence analysis, the burst detection analysis of keywords, and timeline visualization.This process uses the VOSviewer visualization tool.

Co-Occurrence Analysis
The examination of terms' co-occurrences makes it easier to comprehend the research hotspots in each topic.Using VOSviewer, this section analyzes the co-occurrence of keywords in publications.A keyword must appear at least 23 times before it is considered present (out of 8589 keywords, 111 did so).Figure 7 displays the network of keywords' co-occurrences.Two connected nodes indicate that two keywords appear in the same publication.Nodes are used to represent keywords.The more frequently the keyword appears, the larger the node.The number of keyword co-occurrences increases with link width.Information about each cluster and its top five keywords, ranked by the total link strength, is shown in Table 5. Cluster analysis shows that the keywords can be divided into four clusters.The first (red) includes terms that describe various raw materials and fuels (e.g., biomass, bioenergy, fossil fuel, biogas, biodiesel), hydrogen production technologies (e.g., gasification, electrolysis, steam reforming), and keywords characterizing the climate (e.g., climate change, greenhouse gas, carbon dioxide).The second (green) cluster describes keywords related to the use of hydrogen (e.g., hydrogen technologies, hydrogen storage, commerce), policy-related keywords (e.g., energy policy, energy security, decarbonization, hydrogen economy, economic and social effects, investments).The third (blue) cluster largely affects mobility (transportation sector, fuel economy, fuel cells, combustion engines, etc.).The fourth (yellow) cluster includes keywords related to analysis (cost-benefit analysis, economic analysis, decision-making).Notably, the keywords with an average publication year later than 2021 are "decarbonisation" (2021.97),"decarbonization" (2021.92),"power" (2021.86),"energy transitions" (2021.83),"energy transition" (2021.75),and "current" (2021.97).These results demonstrate that scientists are addressing the challenges of decarbonization and exploring innovative solutions to energy transitions, including hydrogen utilization, for a sustainable future.
The top 15 keywords with the highest co-occurrence rate are shown in Figure 9.As can be seen from the figure, although hydrogen is a long-studied topic, the issue of hydrogen storage is a hot topic at present, alongside energy discussions.In addition, the top 15 keywords with the most occurrences are depicted in Table 6.These keywords are related to the sustainability topic ("sustainable development" and "sustainability"), hydrogen technologies ("hydrogen", "hydrogen production", "hydrogen storage", "fuel cells"), emissions ("greenhouse gasses", "gas emissions", "carbon dioxide"), and energy sources ("fossil fuels", "renewable energy", "renewable energies", "renewable energy sources", "hydrogen fuels"), as well as energy policy, showing the measures that should be taken for hydrogen development.
The average publication year shows that hydrogen, as well as sustainability, have been discussed for some time.The most relevant of the top 15 keywords were related to renewable energy and hydrogen storage (Table 6).This section describes how the topic of sustainability is represented in the keywords' analysis.Among the determined keywords, the following keywords related to sustainability were identified: "sustainable development", "sustainability", "sustainable energy".
Figures 10 and 11 show how the found keywords relate to the topic of sustainability.Thus, it was determined that the keyword "sustainable development" is most associated with keywords describing emissions, such as "carbon dioxide" and "greenhouse gases", as well as with the term "fossil fuels" and hydrogen-related keywords such as "hydrogen", "hydrogen fuels", "fuel cells".The keyword "sustainability" is most associated with energy-related keywords ("energy", "alternative energy", "renewable energy"), as well as with "hydrogen", "sustainable development", and "environmental impact"."Sustainable energy", however, is not only connected to the keywords "hydrogen", "sustainable development", and "hydrogen production", but also to "energy policy" and "energy conservation".Terms related to sustainability are mostly associated with concepts such as "hydrogen", "hydrogen production", "energy", and "energy policy".With these settings, the term "SDG" does not appear in the keyword list.To determine the co-occurrence network of keywords related to SDG, 4000 most used keywords were taken.Figure 12 shows the results obtained by setting the number of allowed line connections to the maximum (10,000).The analysis of keywords showed that, when talking about SDGs, the most frequently mentioned words are "green hydrogen", "biomass", and "ammonia", which also appeared in Figure 11.

Sector Analysis
This section will compare the results for six sectors, power, industry, transport, agriculture, commercial, and residential, by analyzing the co-occurrence of keywords in publications and timeline visualization using VOSviewer.

Power Sector
To gain insight into the stage of hydrogen development in the energy sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (SDG OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND energy OR power)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).
This request corresponded to 735 publications (23 August 2020).The condition was set that keywords must appear at least 18 times.Out of 6505 keywords, 104 keywords corresponded to this request.Figures 13 and 14 show the visualization of keyword cooccurrence.According to Figure 13, keywords were divided into six different clusters.A summary list of their information is shown in Table 7.These findings show that research focusing on hydrogen sustainability is diverse in the power sector.This includes research into hydrogen production and storage, its role as a clean energy carrier, and its potential use in fuel cells and power generation.Thus, from the keywords used, it can be concluded that such topics as sustainable development, climate change mitigation, greenhouse gas emissions reduction, and a transition away from fossil fuels are very prominent in the power sector, and hydrogen's contribution to solving these problems is actively being studied.According to Figure 13 and Table 7, the most relevant clusters with the highest mean years are P1 and P4, which describe hydrogen production, storage, renewable energies (P1), and decision-making-describing keywords (P4).
Nine out of ten of the most cited publications are described in Section 3.1.3.Apart from the articles described above, the most citations about the power sector were found in articles that performed technical analyses of the available renewable alternatives, including hydrogen [34][35][36], as well as many articles that were focused on the connection between the power sector and the transport sector [35,37,38].
The theme of sustainable development appears in the following keywords: "sustainable development", "sustainable energy", "sustainability", "sustainable energy", and "sustainable energy systems".The most frequently mentioned keywords, with 1541 and 100 mentions, were "sustainable development" and "sustainability", respectively (see Figure 15).Similar to 3.3.3,the term "SDG" does not show up in the keyword list in the section containing these parameters.To determine the co-occurrence network of keywords related to SDGs, all the found keywords were taken.Figures 16 and 17 show the results obtained by setting the number of allowed line connections to the maximum (10,000).

Industry Sector
To gain an insight into the stage of hydrogen development in the industry sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND industry)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).
This request corresponded to 264 publications (23 August 2020).The condition was set that keywords must appear at least eight times; out of 3357 keywords, 91 keywords corresponded to this request.Figures 18 and 19 show the co-occurrence of keywords.In Figure 18, keywords are divided into five different clusters.A summary list of their information is shown in Table 8.Renewable energy resources 2020.5 Climate change As shown above, each cluster represents a specific aspect of research related to sustainable development and bioenergy, emissions, hydrogen technologies, and climate change.From Figure 19, it can be concluded that most keywords whose average year is prior to 2022 are depicted in yellow color and include the keywords "decarbonization", "energy systems", and "green hydrogen".The concept of sustainability was evident through the usage of terms such as "sustainability", "sustainable development", and "sustainable energy".The initial pair of keywords exhibited greater total link strength values (304 and 520, respectively), indicating a more detailed representation in Figure 20.Based on the analysis of Figure 18, it is clear there is a significant overlap between the two mentioned keywords.Furthermore, these keywords also exhibit a substantial degree of overlap with emission-characterizing keywords, as well as fossil and renewable fuels, and hydrogen and its production.The co-occurrence network was expanded to include all discovered keywords to analyze "SDG" and "SDGs" keyword co-occurrences (Figures 21 and 22).

Transport Sector
To gain an insight into the stage of hydrogen development in the transport sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND transport)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).
This request corresponded to 221 publications (23 August 2020).The condition was set that keywords must appear at least seven times; out of 2505 keywords, 103 keywords corresponded to this request.Figures 23 and 24 show the keywords' co-occurrence.In Figure 23, the found keywords are divided into five different clusters.A summary list of their information is shown in Table 9. Hydrogen fuels 2019.52 Gas emissions 2017.43 Transport sectors The most cited papers not only focused on hydrogen as a solution to decarbonizing the transport sector [29,31,32,39] but also discussed the current state of the art and the deployment of hydrogen fueling stations [40,41], examined the existing International Energy Agency Hydrogen Agreement support program for hydrogen development [42], compared fuel cell technologies with other alternatives [43][44][45][46], and examined hydrogen fuel sustainability [47,48].
In this section, under the selected conditions, SDGs do not appear as keywords, but the theme of sustainability appears in the keywords "sustainable development", "sustainable transport", "sustainable mobility", and "sustainability".The connection between the keywords "sustainable development" and "sustainability" and other keywords is shown in Figure 25.In the transport sector, the keywords "sustainability" and "sustainable development" are largely related to fuel cells, and they appear together with the use of terms "renewable resources", "hydrogen production", and keywords describing emissions (e.g., carbon dioxide, greenhouse gases).
A co-occurrence network was built using all keywords and 10,000 connecting lines to examine the linkages that are shown to be in contact with the SDGs (Figure 26).The terms "sustainable development goal" and "sustainable development goals", both of which featured in two articles and had a total link strength of 24, were discovered when publishing such a network (for comparison, the keyword total link strength shown in Table 9 varied from 510 to 2206).According to Figure 26, the terms "fossil fuels", "sustainable development", "hydrogen storage", and "decarbonization" have the strongest relationships with the terms "sustainable development goals" and "sustainable development aim".The terms "sustainable development aim", "current", "environmental effect", and "energy source" are also related to the term "sustainable development goal".The attainment of SDGs in the transport industry just lately began to be considered, as seen by the fairly recent (2023.0)average publication date of both keywords.

Agriculture Sector
To gain an insight into the stage of hydrogen development in the agriculture sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND agriculture)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).
This request corresponded to 38 publications (23 August 2020).The condition was set that keywords must appear at least three times; out of 1050 keywords, 65 keywords corresponded to this request.Figures 27 and 28 show a visualization of keyword cooccurrence.According to Figure 27, keywords were divided into four different clusters.A summary list of their information is shown in Table 10.An analysis of the most cited articles showed that the article that was cited the most focused on the utilization of biogas and renewable hydrogen as catalysts for sustainable development [49].It delved into an analysis of existing and potential sources of feedstock for the generation of these gases, with particular emphasis on agricultural waste and wastewater.Other highly cited publications discussed biofuel production and the importance of alternative fuels in future transport systems [46], as well as discussing energy generation and hydrogen production via the gasification process [50].
The most recent works discussed the contribution to ecology and the achievement of SDGs by microalgae and the hydrogen produced by these [51,52], as well as plant-growthpromoting endophytic bacterium and how they were further tested for other plantgrowth-promoting attributes, such as hydrogen cyanide [53].The role of biogas in achieving a hydrogen economy was also investigated [54].
In this section, under the selected conditions, SDGs do not appear as keywords, but the theme of sustainability appears in the keywords "sustainable development" and "sustainability".The connection between these keywords with other keywords is shown in Figure 29.Remarkably, within the realm of agriculture, the term "hydrogen" does not emerge as one of the most prominent keywords in conjunction with the term "sustainable development".However, the discussion includes ideas such as the circular economy, recycling, and bioenergy.Based on these findings, it can be inferred that "sustainable agriculture" exhibits a stronger correlation with biomass as compared with hydrogen.It is important to acknowledge that a particularly robust association exists between the keyword in question and the term "nonhuman".
A co-occurrence network made up of all keywords and 10,000 link lines was developed to study the connections that are seen in relation to SDGs (Figure 30).The terms "sustainable development aim" and "sustainable development goals" were discovered when building such a network; they both appeared in two articles and had total link strengths of 153 and 97, respectively (for comparison, the keyword total link strength shown in Table 10 varied from 188 to 864). Figure 31 shows that the keyword "sustainable development goal" has the strongest connection with the keywords "circular economy", "chemical contamination", "bioenergy", "nitrogen", "biodegradability", "alternative energy", and "biofuel production".On the other hand, the keyword "sustainable development goals" has the strongest connection with the keywords "biosorption", "bioremediation", "bioenergy", "biodiesel", "biodegradability", "ammonium nitrate", "biofuel production", and "aquaculture".

Commercial Sector
To gain insight into the stage of hydrogen development in the commercial sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND commercial)) AND (LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).
This request corresponded to only 65 publications (23 August 2020).The condition was set that keywords must appear at least three times; out of 997 keywords, 74 keywords corresponded to this request.Figures 32 and 33 show a visualization of keyword cooccurrence.In Figure 32, keywords are divided into five different clusters.A summary list of their information is shown in Table 11.The results show that keywords from the second cluster were used most recently, and the most relevant keywords related to hydrogen are "hydrogen storage" and "hydrogen economy" (see Figure 33).
In C1 and C2, the topic of sustainability appears in the keywords with the strongest total link strength, with "sustainable development" in cluster C1 and "sustainability" in cluster C2.Sustainable development goals do not appear to be one of the keywords with applicable conditions.Figure 34 shows the keywords associated with sustainable development in the commerce sector.In order to investigate the connections observed in contact with SDGs, a cooccurrence network was created, which consisted of all keywords and represented 10,000 connection lines (Figure 35).When creating such a network, the keyword "sustainable development goal" was found, the total link strength of which was 41 (for comparison, the total link strength of the keyword shown in Table 11 varied up to 426).In the commercial sector, there was only one article with the keyword "sustainable development goal".Dang B.T. et al.'s article investigated the current application of seaweed waste for composting and biochar production.Hydrogen was mentioned in the article as a potential use of biochar, as biochar may have been used for hydrogen storage purposes.

Residential Sector
To gain an insight into the stage of hydrogen development in the residential sector, the following request was used: (TITLE-ABS-KEY (hydrogen OR h2) AND TITLE-ABS-KEY (sdg OR "sustainable development goal*" OR sustainability OR sustainable OR "sustainable development" OR "sustainability assessment" OR "sustainable energy" OR "sustainable energy systems") AND TITLE-ABS-KEY (sector AND residential)) AND   In Figure 36, keywords are divided into four different clusters.The summary list of their information is shown in Table 12.This study shows that there are very few targeted residential area studies on the application of hydrogen.Like other industries, after analyzing the timeline of keywords (Figure 37), it can be concluded that "gas emissions" and "hydrogen storage" are the most relevant topics, the mean year of which corresponds to 2023.Similarly, it can be concluded that topics of decarbonization, electric vehicles, and climate change are currently the most topical.
The theme of sustainable development appears in the keywords "sustainable development" and "sustainability", which are shown in Figure 38.As in other sectors, "sustainable development" is mostly associated with "hydrogen storage", "greenhouse gases", "fuel cells", and "gas emissions".In this sector, the economic sector is prominently divided, which describes keywords such as "costs", "investments", "economic analysis", and "cost benefit analysis".However, the keyword "sustainability" is most associated with keywords such as "hydrogen", "greenhouse gases", "fuel cells", "climate change", "housing", "emission control", and "performance assessment".Of all the obtained keywords, it was found that the individual term SDG or SDGs does not appear in a string of other keywords.

Discussion
By identifying important areas of research interest and the connections between them, this keyword analysis aids in presenting a thorough overview of the major themes in a particular dataset.The results show a rapid increase in interest in this topic, which increased more than 12 times in the period from 2011 to 2022.A similarly rapid increase in citations was also observed.
The sustainability of hydrogen is most popular in the energy, environmental science, and engineering research areas.The most productive journal on the discussed topic is the International Journal of Hydrogen Energy, followed by Energies and Renewable and Sustainable Energy Reviews.The last one showed the most productive results when comparing the average number of citations per publication.
Most articles on this topic were written in the US, India, and Germany, while an analysis of institutions indicated that, despite the fact that Yildiz Technical University contributed the most research to this topic, with five articles retrieved, it corresponds to only 0.5% of all publications, which indicates that the study of sustainability is decentralized.The most influential authors are Dincer, I., who contributed twelve articles, followed by Agrawal, R. (eight articles), Delgass, W.N. (six articles), and Ribeiro, F.H. (six articles).
A comparison of studies on sustainable development by sector concluded that the energy sector was the most discussed sector, followed by industry and transport.The number of publications that met the selected conditions in the agriculture, commercial and residential sectors was small, at-38, 65, and 16, respectively.The keywords "sustainable development goal" or "sustainable development goals" appeared much less often than the keywords "sustainable development" or "sustainability", which indicates that although the topic of sustainability is widely studied, the number of studies that analyze the achievement of the SDGs is not too high.Both keywords ("SDG" and "SDGs") are relatively young, as their average publication date was after 2022.
In the general performance analysis (Section 3.3), it was determined that keywords can be grouped into the following groups that describe various raw materials and fuels (e.g., biomass, bioenergy, fossil fuel, biogas, biodiesel):

•
Keywords related to the use of hydrogen (e.g., hydrogen technologies, hydrogen storage, commerce).
These keywords also appeared, to an extent, in the finer comparison of industries.
The time trend analysis showed that "the youngest" keywords with the most current average publication year are "decarbonization", "gas emissions", "energy systems", "energy transitions", "power", "green hydrogen", and "hydrogen storage".This indicates that the issue of decarbonization is likely becoming more and more relevant, which could also contribute to the increase in interest in the field of energy transition.The issue of hydrogen energy storage remains very relevant.
Overall, this paper shows that the use of hydrogen is being explored to achieve sustainable development goals and, despite the relatively limited number of publications, interest in the field is growing rapidly and there is no reason to believe it will decline soon.This study can provide researchers who are new to the topic of hydrogen sustainability with a quick overview of the research status and trends.

Conclusions
In response to the anticipated and existing difficulties confronting humanity, the United Nations has formulated a set of 17 sustainable development goals.In order to attain these objectives, it is important to comprehend the energy sources and the provenance of their primary constituents.There is growing debate regarding the possible contribution of hydrogen to the future energy balance.The aim of this study is to provide a bibliometric analysis assessing the effectiveness of the use of hydrogen in different sectors in achieving the SDGs and to provide an overview of how sustainability is discussed in different sectors.This study identifies the most cited publications, as well as the most productive journals, countries, and organizations.This study analyzes the current state of research on hydrogen sustainability in six sectors.The areas of study include energy, industry, transport, agriculture, commercial and residential sectors.The results show that the energy sector shows the greatest interest in sustainable development, followed by industry and transport.
The work has several limitations; for example, it is known that to obtain a more accurate understanding of research reviews, it is important to explore ways to combine data from different databases.The limitations of the work include the fact that only the SCOPUS database was examined.Researching other databases could provide additional insight into the researched topic.Another limitation is that the topic of SDGs and sustainability can also be discussed in other publications that are not tied to any specific sector.Another limitation is that, due to the diverse range of techniques employed in the production of green hydrogen, it is likely that many scientific papers related to this topic may not incorporate the particular terms "hydrogen" or "H2" within their abstract, title, or keywords.A potential increase in the number of articles might be achieved by supplementing the keywords with terms on all established techniques for the production of green hydrogen.

Figure 1 .
Figure 1.Sustainable development goals of the United Nations.

Figure 3 .
Figure 3. Annual publication and citation rates: (a) the average number of publications; (b) the average number of citations.

Figure 4 .
Figure 4. Distribution of publications by the most popular research areas.

Figure 5 .
Figure 5. SDGs mapped in the top 10 most cited publications.

Figure 6 .
Figure 6.Co-authorship among the 29 nations where at least 15 articles were produced.As shown in Table 4, Yildiz Technical University contributed the most research on this topic, with five retrieved articles.Research on this topic is arguably institutionally fragmented, as the institutions with the most publications account for only a small fraction of all publications.The most influential authors are Dincer, I., who contributed twelve articles, followed by Agrawal, R. (eight articles), Delgass, W.N. (six articles), and Ribeiro, F.H. (six articles).

Figure 7 .
Figure 7.The network of keywords that appeared at least 23 times.

3. 3
.2. Timeline Visualization Topics and keywords evolve over time, and research in related fields is constantly changing.Timeline visualizations show the development of research topics and published results over time.The co-occurrence overlay of keywords is shown in Figure 8.This shows the temporal distribution of keywords in different clusters, where the average publication year of a keyword determines its color shade.The darker the color, the earlier the average year of publication.

Figure 8 .
Figure 8.The overlay of keywords that appeared at least 15 times.

Figure 9 .
Figure 9.The overlay visualization of the top 15 keywords with the strongest citation bursts.

Figure 10 .Figure 11 .
Figure 10.An overlay of the keyword "sustainable development" with the strongest links.

Figure 12 .
Figure 12.The overlay visualization of keywords (a) "sustainable development goals" and (b) "sustainable development goal" with the strongest links.

Figure 13 .
Figure 13.Co-occurrence network generated by VOSviewer in the power sector.Cluster analysis.

Figure 14 .
Figure 14.Co-occurrence network generated by VOSviewer in the power sector.Time trend analysis.

Figure 15 .
Figure 15.Keywords that are connected to (a) "sustainability" and (b) "sustainable development" in the power sector.

Figure 16 .
Figure 16.The overlay visualization of the keyword "sustainable development goal" (power sector).

Figure 17 .
Figure 17.The overlay visualization of the keyword "sustainable development goals" (power sector).

Figure 18 .
Figure 18.Co-occurrence network generated by VOSviewer in the industry sector.Cluster analysis.

Figure 19 .
Figure 19.Co-occurrence network generated by VOSviewer in the industry sector.Time trend analysis.

Figure 20 .
Figure 20.Keywords that are connected to (a) "sustainable development" and (b) "sustainability" in the industry sector.

Figure 21 .
Figure 21.The overlay visualization of the keyword "sustainable development goal", along with the strongest links (industry sector).

Figure 22 .
Figure 22.The overlay visualization of the keyword "sustainable development goals" (industry sector).

Figure 23 .
Figure 23.Co-occurrence network generated by VOSviewer in the transport sector.Cluster analysis.

Figure 24 .
Figure 24.Co-occurrence network generated by VOSviewer in the transport sector.Time trend analysis.

Figure 25 .
Figure 25.Keywords that are connected to (a) "sustainable development" and (b) "sustainability" in the transport sector.

Figure 26 .
Figure 26.The overlay visualization of keywords (a) "sustainable development goals" and (b) "sustainable development goal" in the transport sector.

Figure 27 .
Figure 27.Co-occurrence network generated by VOSviewer in the agriculture sector.Cluster analysis.

Figure 28 .
Figure 28.Co-occurrence network generated by VOSviewer in the agriculture sector.Time trend analysis.

Figure 29 .
Figure 29.Keywords that are connected to (a) "sustainable development" and (b) "sustainable agriculture" in the agriculture sector.

Figure 30 .
Figure 30.Overlay visualization of the keyword "sustainable development goals" in the agriculture sector.

Figure 31 .
Figure 31.Overlay visualization of the keyword "sustainable development goal" in the agriculture sector.

Figure 32 .
Figure 32.Co-occurrence network generated by VOSviewer in the commerce sector.Cluster analysis.

Figure 33 .
Figure 33.Co-occurrence network generated by VOSviewer in the commerce sector.Time trend analysis.

Figure 34 .
Figure 34.Keywords that are connected to the keyword "sustainable development" in the commerce sector.

Figure 35 .
Figure 35.Overlay visualization of the keyword "sustainable development goal" in the commercial sector.

(
LIMIT-TO (DOCTYPE, ar) OR LIMIT-TO (DOCTYPE, re)) AND (LIMIT-TO (LANGUAGE, English)).This request corresponded to only 16 publications (23 August 2020).The condition was set that keywords must appear at least two times; out of 33 keywords, 221 keywords corresponded to this request.Figures36 and 37show a visualization of keyword cooccurrence.

Figure 36 .
Figure 36.Co-occurrence network generated by VOSviewer in the residential sector.Cluster analysis.

Figure 37 .
Figure 37. Co-occurrence network generated by VOSviewer in the residential sector.Time trend analysis.

Figure 38 .
Figure 38.Keywords that are connected to (a) "sustainable development" and (b) "sustainability" in the residential sector.

Table 2 .
The 10 most cited works in descending order of citation count.

Table 3 .
Top 12most-productive journals that study the application of hydrogen to achieve the SDGs in particular emission sectors.
NP: the number of publications; NC: the number of citations; ANC: the average number of citations per publication.

Table 4
lists the top 10 most cited countries/regions, institutions, and authors of publications and is sorted in ascending order by the number of citations.

Table 4 .
The top 10 most cited countries/regions/institutions/authors of publications by the number of publications.

Table 5 .
Cluster information summary list.

Table 6 .
The top 15 keywords with the biggest total link strength.

Table 7 .
Power sector cluster information summary list.

Table 8 .
Industry sector cluster information summary list.

Table 9 .
Transport sector cluster information summary list.

Table 10 .
Agriculture sector cluster information summary list.

Table 11 .
Commercial sector cluster information summary list.

Table 12 .
Residential sector cluster information summary list.