Global Research Trends in Catalysis for Green Hydrogen Production from Wastewater: A Bibliometric Study (2010–2024)

Abstract

By turning a waste stream into a clean energy source, green hydrogen generation from wastewater provides a dual solution to energy and environmental problems. This study presents a thorough bibliometric analysis of research trends in the field of green hydrogen generation from wastewater between 2010 and 2024. A total of 221 publications were extracted from Scopus database, and VOSviewer (1.6.20) was used as a visualization tool to identify influential authors, institutions, collaborations, and thematic focus areas. The analysis revealed a significant increase in research output, with a peak of 122 publications in 2024, with a total of 705 citations. China had the most contributions with 60 publications, followed by India (30) and South Korea (26), indicating substantial regional involvement in Asia. Keyword co-occurrence and coauthorship network mapping revealed 779 distinct keywords grouped around key themes like electrolysis, hydrogen evolution reactions, and wastewater treatment. Significantly, this work was supported by contributions from 115 publication venues, with the International Journal of Hydrogen Energy emerging as the most active and cited source (40 articles, 539 citations). The multidisciplinary aspect of the area was highlighted by keyword co-occurrence analysis, which identified recurring themes including electrolysis, wastewater treatment, and hydrogen evolution processes. Interestingly, the most-cited study garnered 131 citations and discussed the availability of unconventional water sources for electrolysis. Although there is growing interest in the field, it is still in its initial phases, indicating a need for additional research, particularly in developing countries. This work offers a basic overview for researchers and policymakers who are focused on promoting the sustainable generation of green hydrogen from wastewater.

1. Introduction

Produced through water electrolysis powered by renewable energy (e.g., wind or solar), green hydrogen has an extremely low or negligible carbon footprint during its manufacture and usage [1,2]. Regarded as a vital decarbonization tool, it serves as a clean energy carrier for hard-to-abate sectors—including industry, transportation, power generation, and heating—where direct electrification alone is impractical [3]. Green hydrogen also acts as an energy storage solution, stabilizing renewable energy grids by balancing supply and demand. It provides a way to convert excess renewable power into a storable and transportable energy carrier, enhancing grid flexibility [3,4].

Despite its potential, green hydrogen faces several challenges such as high production costs, the need for substantial new infrastructure, and the lack of established laws and standardized regulations to properly define and validate its ‘green’ credentials [5]. To overcome these challenges and make green hydrogen economically competitive with fossil fuels in the years to come, advancements in electrolyze technology, storage options, and international collaboration are crucial [4,5]. Green hydrogen plays a significant role in future energy systems, promoting energy independence and advancing climate goals simultaneously due to the growing R&D investment [6].

Green hydrogen production from wastewater presents a viable method to address the shortage of water and energy [7]. It can be produced from treated wastewater using a variety of techniques, including electrolysis [8], microbial electrochemical cells [9], and photoelectrochemical processes [10]—often driven by renewable energy sources such as solar power, rather than depending on the limited supply of freshwater [11,12]. Integrating hydrogen production with wastewater valorization achieves several benefits, including clean energy generation, reduced environmental impacts, and improved circular resource utilization [13]. Furthermore, producing hydrogen from wastewater can contribute to the decarbonization of the wastewater sector, particularly when combined with net-zero objectives and on-site renewable energy [14]. Utilizing wastewater is more economical and environmentally friendly than using potable water or desalinated water according to economic calculations. In addition, the oxygen by-product of electrolysis can be beneficial to wastewater treatment operations [14]. The multi-benefit of generating clean energy while treating wastewater makes green hydrogen a critical area for research and industrial deployment. While challenges remain in scaling and system optimization, its combined environmental advantages create compelling value for sustainable development [2,6,15]. Figure 1 depicts the fundamental concept of green hydrogen generation from wastewater.

Bibliometric studies on the production of green hydrogen from wastewater are crucial because they systematically map the evolution, trends, and research hotspots in this rapidly expanding field [16,17]. In addition, these studies can help researchers, policymakers, and industry stakeholders understand the status and future direction of the field. Furthermore, bibliometric studies emphasize areas of rapid advancement, indicate gaps or insufficiently researched themes that require more research, and highlight the most significant technologies, major contributions, and worldwide collaborations [16]. To direct future research paths and funding priorities, bibliometric studies offer a quantitative and visual overview of the research landscape through the analysis of publication patterns, citation networks, and topic clusters [17]. Additionally, they aid in evaluating the compatibility with circular economy and sustainability objectives and comparing the efficiency and maturity of various wastewater-based hydrogen production techniques [18]. Furthermore, bibliometric analysis is a potent instrument for encouraging researchers to collaborate internationally and across disciplines. It can spur innovation, coordinate scientific endeavors with the Sustainable Development Goals (SDGs), and offer evidence-based backing for policymaking by charting research trends and knowledge gaps [17]. To sum up, bibliometric studies facilitate continuous research and development while underscoring their significance, making them a favored methodology across diverse scientific fields. They are also essential for expanding scientific understanding, guiding efficient resource allocation, and supporting the switch to sustainable energy systems that use green hydrogen produced from wastewater [18,19].

In recent years, numerous bibliometric analyses have investigated research trends in green hydrogen, encompassing renewable sources and technologies such as water electrolysis, biomass, and photoelectrochemical systems. Nevertheless, most of these works either use bibliometric analysis with little visual or network mapping or adopt a broad analytical scope. The current work, on the other hand, provides a targeted and comprehensive bibliometric examination of green hydrogen production from wastewater specifically within the 2010–2024 timeframe. It sets itself apart by mapping transnational collaborations, co-authorship networks, and keyword co-occurrence clusters. For example, Odoi-Yorke et al. [15] conducted an extensive mapping of hydrogen generation from wastewater, whereas Shi et al. [16] examined global trends in wastewater-to-energy research. Broader studies, including that by Islam et al. [17], have also examined wastewater as a feedstock in conjunction with other hydrogen production methods. Nevertheless, most of these works either uses bibliometric analysis with little visual or network mapping or adopt a broad analytical scope. The current work, on the other hand, provides a targeted and comprehensive bibliometric examination of green hydrogen production from wastewater specifically within the 2010–2024 timeframe. It sets itself apart by mapping transnational collaborations, co-authorship networks, and keyword co-occurrence clusters.

The study has two primary objectives: mapping the historical evolution and growth patterns in the field of green hydrogen and highlighting the most influential authors and institutions’ contributions. The analysis additionally evaluates the extent of international collaboration and interdisciplinary integration across these studies. The scope encompasses literature from 2010 to 2024, incorporating journal articles and conference papers from Scopus database. Through systematic data collection and network analysis techniques, this research provides both quantitative metrics and qualitative insights regarding technological trends, research hotspots, and global cooperation patterns in this emerging sustainable energy field.

2. Results and Discussion

2.1. The Characteristics of Research Publications

The bibliometric assessment of publications demonstrates a varied scholarly output concerning the production of green hydrogen from wastewater The initial dataset comprised 358 documents, with research articles forming the predominant category (255, 71.2%), followed by review papers (42, 11.7%). Additional contributions include conference papers (38, 10.6%), book chapters (11, 3.1%), books (2, 0.6%), and other review documents (10, 2.8%). Figure 2 shows a pie chart illustrating the distribution of 358 document types before filtering.

For this study, only primary research articles and conference papers were prioritized for in-depth examination, resulting in a filtered dataset of 221 documents. This focused methodology emphasizes the value of empirical studies and current academic exchange in developing sustainable pathways for green hydrogen generation from wastewater. The distribution pattern reflects both the maturity of research in this specialized area and the scientific community’s preference for peer-reviewed, data-driven studies.

2.2. Trend of Annual Publications

Figure 3 demonstrates the progressive publication trends, which exhibit notable dynamics in research productivity from 2015 to 2024. No output from 2010 to 2014 are presented, as these papers were omitted throughout the screening process for failing to meet the inclusion requirements. Annual publication volume serves as a key indicator of research evolution, while citation analysis reflects scholarly impact [15]. With a modest production of two publications in 2015 and a noteworthy citation count of 170, the field demonstrated early signs of interest. However, between 2016 and 2019, there was a significant fall, with only one publication annually, receiving 41, 24, 11, and 17 citations, respectively. This decrease indicates various underlying factors. During this period, the integration of wastewater treatment and green hydrogen production represented a nascent and interdisciplinary field, requiring expertise in both environmental engineering and advanced energy systems, which limits the pool of researchers and contributes to the modest publication output [20]. Significantly, 2020 witnessed a total lack of publications in this domain. The extensive interruption caused by the COVID-19 pandemic, which had a major influence on worldwide research activity, postponed experimental work and redirected research priority into health-related fields, is one reasonable explanation for this discrepancy [21]. Furthermore, the pandemic significantly impacted access to research facilities and grants, restricted international mobility, and intensified existing challenges such as securing funding, particularly affecting emerging multidisciplinary fields such as hydrogen and wastewater [22]. A sustained upward trajectory emerged in 2021 (11 articles and 401 citations), increasing to 21 articles in 2022 with 461 citations. In 2023, the number increased to 61 articles and 842 citations, reaching its peak in 2024 (122 articles and 705 citations). The apparent 2024 citation decline likely reflects incomplete data rather than diminished impact, as it usually takes months or more for new research to be read, cited, and indexed in scholarly databases. As a result, many articles published in 2024 could not have had enough time to accrue citations even in 2025. Overall, the trends demonstrate increasing scientific recognition of wastewater significance in the green hydrogen production domain, suggesting both expanding research investment and the field’s maturation.

2.3. Countries Distribution

As shown in

Table 1. The top 10 countries by publication count.

Nation	Cluster	TLS	Publications	Citations
China	3	2240	60	791
India	2	1921	30	319
South Korea	2	1924	26	453
Saudi Arabia	2	1845	18	278
Spain	1	601	15	124
United States	3	569	15	165
Germany	1	465	15	174
United Kingdom	1	620	13	168
Brazil	2	234	10	87
Italy	1	106	10	158

, the bibliometric analysis of leading countries in wastewater-to-green hydrogen research provides vital insight into the contributions of various countries in this crucial field. The information includes key statistics, incorporating publication counts and citation metrics, indicative of each nation’s dedication to the advancement of knowledge in this domain. China stands as the primary contributor, with a total of 60 publications and 791 citations. This significant production demonstrates China’s strong research skills and its pioneering position in the investigation of the utilization of wastewater treatment for green hydrogen generation. India comes in second place with 30 publications and 319 citations, demonstrating its substantial influence in the field of research.

Another significant participant is South Korea, which has generated 453 citations and 26 publications. This suggests that tackling the technical issues related to wastewater treatment for green hydrogen production is a major area of research interest. With 18 publications and 278 citations, Saudi Arabia comes in fourth, demonstrating its increasing interest in using wastewater as a source to produce green hydrogen.

Spain, Germany and Italy have contributed 15, 15, and 10 articles and 124, 174, and 158 citations, respectively, to the European green hydrogen community, indicating their dedication to sustainable techniques. The United States provides another perspective, with 15 publications and 165 citations. Even with 13 articles and 168 citations, the United Kingdom demonstrates a growing interest in this area. Another growing contribution is noticed in Brazil, contributing with 10 publications, and a citation count of 87.

The presence of China, India, and South Korea suggests that these countries have developed research infrastructures and a high level of academic engagement. Meanwhile, the small participation of Arab nations points that more attention in this research domain is needed. These results highlight how crucial international cooperation is, especially between Arab and European countries, in enabling the sharing of information and techniques required to create sustainable and efficient wastewater treatment systems to produce green hydrogen. In the end, these collaborations may handle the intricate problems associated with such domain and improve environmental sustainability by utilizing the advantages of each nation.

With a distinction between Single Country Publications (SCP) and Multiple Country Publications (MCP), Figure 4 displays the distribution of scientific publications by the nations of the corresponding authors and their collaboration patterns. Orange bars denote SCP, whereas dark blue bars signify MCP. It is significant that, while the ratios differ by nation, some of them maintain a balance between domestic publications (SCP) and international ones (MCP). Brazil, for example, exhibits a definite preference for domestic collaborations (SCP), whereas nations such as Saudi Arabia continue to maintain a larger share of national publications (MCP). This analysis shows the number of scientific publications by nation as well as trends in research cooperation, demonstrating the various publication strategies used by each country. A strong MCP rate indicate that a nation’s research system is developed, globally interconnected, and receptive to collaboration [23]. Countries exhibiting robust international co-authorship typically generate research with elevated scientific impact, as indicated by citation counts and field-weighted citation metrics [24]. For instance, China has the highest MCP rate, which is reflected in its citation count.

Both developed and developing nations have made substantial contributions to the topic, illustrating a diverse and inclusive global research environment. This analysis not only emphasizes the competitiveness of national research efforts, but also the growing importance of international collaboration.

Figure 5 emphasizes the importance of international networks for promoting information sharing about the application of wastewater in green hydrogen production. In this network, nodes symbolize countries, with larger nodes signifying greater research output, while lines reflect collaborative connections between nations. China manifests as a prominent node with numerous connections, indicative of its significant publication output and vigorous international interactions. Because this field of study is multifaceted, worldwide cooperation is crucial for tackling difficult issues, including the need for effective resource recovery and water shortages. Nations that are leaders in this area usually have reputable research institutes, which allow them to significantly enhance green hydrogen generation using wastewater technology.

China’s capacity to efficiently utilize local natural resources, including wind, solar, and biomass, as well as its deliberate investments in cutting-edge wastewater treatment technology, are key factors in the country’s dominance in green hydrogen research output [25]. This makes China a major contributor to the generation of creative methods for treating wastewater for green hydrogen generation. In addition, India has also considered its abundant natural resources, especially high solar insolation and growing focus on renewable energy integration [26]. Case studies maximizing hydrogen output from wastewater treatment plants in industrial complexes show how South Korea’s wastewater treatment infrastructure is up to date and becoming more integrated with biogas and hydrogen generation [27].

In conclusion, these countries’ collaborative efforts demonstrate how vital it is to combine resources and knowledge in the pursuit of effective wastewater treatment methods that may result in advancements in green hydrogen generation technology.

2.4. Co-Citation Network of Authors

One effective bibliometric method for mapping academic literature is author co-citation analysis which identifies important conceptual frameworks and thematic relationships within a certain field of study by looking at authors who are frequently cited together. In this study, a thorough co-citation network analysis was conducted using VOSviewer, focusing on the application of wastewater treatment for the production of green hydrogen. The findings highlight well-known authors in this field along with their impact on citations and publication production.

The researchers with the highest citation counts are highlighted in

Table 2. Co-occurrence map of 2010–2024 researcher collaboration.

Author	Total Citations
Amorim, Filipa	131
Catarino, Justina	131
Di Berardino, Santino	131
Gírio, Francisco	131
Lopes, Tiago F	131
Picado, Ana	131
Ponce de Leão, T	131
Rangel, C.M.	131
Simoes, Sofia G	131
Chandrasekhar K.	112

.

According to the analysis, several well-known researchers have made substantial contributions to the body of knowledge regarding the treatment of wastewater for the generation of green hydrogen. The researchers with the most publications are highlighted in the table’s first section. This section indicates that despite the authors not being very extensive, their research has yielded fundamental knowledge that underpins more general developments in the field. A prominent group of researchers, on the other hand, stands out with a combined citation count of 131, demonstrating their significant impact on the field’s advancement. Following closely behind is Chandrasekhar K., whose work has received 112 citations, highlighting his significant influence on the direction of contemporary research.

These scholars are emerging contributors to wastewater conversion for the generation of green hydrogen, based on the modest citation network. While their publication outputs and citation metrics remain limited in scope, they represent early intellectual foundations in this niche research area. The early stage of wastewater-derived green hydrogen research is reflected in this preliminary co-citation analysis, which shows emerging collaboration patterns rather than well-established networks. Although they are not yet comprehensive, the relationships that have been found showed promise at the multidisciplinary intersections of several fields. The study provides a foundation for monitoring the field’s evolution, as interest in this sustainable technology persists, where even the current modest citation trends may indicate future research trajectories.

2.5. Leading Journals Publishing Research Papers in Green Hydrogen and Wastewater Treatment

Table 3. The top 6 journals by TLS.

Source Journal	TLS *	Documents	Citations
International Journal of Hydrogen Energy	65	40	539
Chemical Engineering Journal	37	9	147
ACS Applied materials and interfaces	27	5	81
Journal of Cleaner Production	18	6	229
Electrochemical Acta	9	5	23
Energies	6	5	29

* TLS: Total Link Strength.

illustrates the top 6 principal contributing sources of scientific literature on green hydrogen and wastewater treatment, highlighting the quantity of articles and total citations within each journal.

The International Journal of Hydrogen Energy is the leading journal, with 40 papers and an impressive citation count of 539, highlighting their significance in the domain. With a total of 9 documents accumulating 147 citations, Chemical Engineering Journal comes next. The Journal of ACS Applied Materials and Interfaces garnered 81 citations despite the relatively low number of publications (5). Journal of Cleaner Production (6), Electrochemical Acta (5), and Energies (5) are some noteworthy journals that exhibit a moderate number of articles, with a total citation number of 229, 23, and 29, respectively.

Because of their scientific value and capacity to draw in significant research, these journals are essential for the spread of knowledge about wastewater-to-green hydrogen. These sources’ dominance suggests that information and technical advancement are concentrated in a few publications, which can direct scholars to the best avenues for publishing and consulting significant works in this field. Figure 6 illustrates a mapping visualization of the top 6 journals. This distribution implies that research on wastewater and green hydrogen is constantly growing and diversifying, involving a range of methodologies and technical applications.

The cumulative evolution of scientific publications in several specialist journals in this field is depicted in

Table 4. Publication statistics on wastewater-to-green hydrogen.

Source	2015	2016	2017	2018	2019	2020	2021	2022	2023	2024
International Journal of Hydrogen Energy	1	0	1	1	0	0	3	1	4	29
Chemical Engineering Journal	0	0	0	0	0	0	1	0	4	4
ACS Applied materials and interfaces	0	0	0	0	0	0	0	0	2	3
Journal of Cleaner Production	0	0	0	0	0	0	2	0	3	1
Electrochemical Acta	0	0	0	0	0	0	0	1	0	4
Energies	0	0	0	0	0	0	0	2	0	3

.

Research on green hydrogen synthesis from wastewater has been widely disseminated by prestigious journals. Given its importance to the area, the International Journal of Hydrogen Energy is the most active contributor. In recent years, other important journals have also consistently contributed, such as the Journal of Cleaner Production, ACS Applied Materials & Interfaces, and the Chemical Engineering Journal. Their continuous publication activity demonstrates the significance of interdisciplinary efforts in tackling sustainability and environmental concerns as well as the expanding scholarly interest in these topics.

This distribution suggests that researchers should concentrate on a small number of important publications if they are looking for important material and the most recent advancements in green hydrogen and renewable energies. As extremely important information centers for the scientific community, these main sources focus on the most important discoveries and developments. This focus also implies that to optimize the influence and exposure of their research, writers should take these fundamental sources into account while developing their publication strategy.

2.6. Most Influential Publications

Citation analysis is an essential tool for understanding the intellectual climate of a given academic field because it shows how one publication cited others [28]. Researchers can find fundamental works within a field by using this analytical method. Citation analysis of academic publications concentrating on wastewater-to-green hydrogen was conducted as a part of this investigation.

Table 5. The top 19 most-cited papers.

First Author	Year	Document Title	Journal	Citations	Ref.
Simoes, S. G.	2021	Water availability and water usage solutions for electrolysis in hydrogen production	Journal of Cleaner Production	131	[28]
Kadier, A.	2015	Hydrogen gas production with an electroformed Ni mesh cathode catalysts in a single-chamber microbial electrolysis cell (MEC)	International Journal of Hydrogen Energy	112	[29]
Scheepers, F.	2021	Temperature optimization for improving polymer electrolyte membrane-water electrolysis system efficiency	Applied Energy	101	[30]
Woods, P.	2022	The hydrogen economy—Where is the water?	Energy Nexus	72	[31]
Li, L.	2024	Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst	ACS Nano	63	[32]
Bonacina, C. N.	2022	Assessment of offshore liquid hydrogen production from wind power for ship refueling	International Journal of Hydrogen Energy	60	[33]
Cinti, G.	2015	SOFC fuelled with reformed urea	Applied Energy	58	[34]
Musa Ardo, F.	2022	A review in redressing challenges to produce sustainable hydrogen from microalgae for aviation industry	Fuel	55	[35]
Alsalme, A.	2022	Fabrication of S-scheme TiO2/g-C3N4 nanocomposites for generation of hydrogen gas and removal of fluorescein dye	Diamond and Related Materials	53	[36]
Winter, L. R.	2022	Mining Nontraditional Water Sources for a Distributed Hydrogen Economy	Environmental Science and Technology	48	[37]
Yin, C.	2023	Heterostructured NiSe2/MoSe2 electronic modulation for efficient electrocatalysis in urea assisted water splitting reaction	Chinese Journal of Catalysis	47	[38]
Yao, M.	2021	Solar-driven hydrogen generation coupled with urea electrolysis by an oxygen vacancy-rich catalyst	Chemical Engineering Journal	46	[39]
Kannaiyan, K.	2023	Perspectives for the green hydrogen energy-based economy	Energy	42	[40]
Yang, X.	2023	Engineering In Situ Catalytic Cleaning Membrane Via Prebiotic-Chemistry-Inspired Mineralization	Advanced Materials	42	[41]
Yin, Z.-H.	2023	Revealing the In Situ Evolution of Tetrahedral NiMoO4 Micropillar Array for Energy-Efficient Alkaline Hydrogen Production Assisted by Urea Electrolysis	Small Structures	41	[42]
Rico-Oller, B.	2016	Photodegradation of organic pollutants in water and green hydrogen production via methanol photoreforming of doped titanium oxide nanoparticles	Science of the Total Environment	41	[43]
Chauhan, D.	2023	Alkaline electrolysis of wastewater and low-quality water	Journal of Cleaner Production	38	[44]
Krishnan, S.	2024	Prospective LCA of alkaline and PEM electrolyser systems	International Journal of Hydrogen Energy	37	[45]
Zhang, B.	2023	Photoelectrochemical conversion of plastic waste into high-value chemicals coupling hydrogen production	Chemical Engineering Journal	36	[46]

highlights the top 20 most-cited papers in this field.

It also highlights the research that has made major contributions to the field. The citation counts of these seminal works are ranked in

Table 6. Catalyst Types in Wastewater-to-Green Hydrogen Research.

Catalyst	Ref.
Nickle based catalysts	[30]
Amorphous RuO2-coated NiO nanosheet	[33]
TiO2/g-C3N4 S-scheme heterojunctions	[37]
NiSe2/MoSe2 heterostructures	[39]
oxygen vacancy-rich catalysts	[40]
NiMoO4 micropillar arrays	[43]
doped titanium oxide nanoparticles	[44]
Photoelectrochemical catalysts	[47]

, offering important insights into the approaches taken and the major discoveries that have largely influenced the current discussion about the use of wastewater treatment in green hydrogen generation. A better understanding of the research that has shaped present practices and future directions in the field can be observed by looking at these highly cited works.

For example, the research conducted by Simoes, S. G. et al. [28] offers a thorough methodology for evaluating the suitability of water sources for electrolysis-based green hydrogen production, integrating social, environmental, and economic factors into a single decision-making process. It also examined five different water sources: groundwater, surface water, seawater, public water, and reused wastewater. It concluded that public water is typically the best choice for green hydrogen because of its consistent quality, pretreatment needs, and existing infrastructure. Nevertheless, the research shows that when treatment and transportation costs are reduced, especially in areas with limited water supplies or when using renewable energy, alternative sources like seawater and recycled water can become feasible. The study of Kadier, A. et al. [29] illustrated the feasibility of producing hydrogen in single-chamber microbial electrolysis cells (MECs) utilizing an electroformed nickel (Ni) mesh cathode as a more affordable substitute for platinum. The Ni mesh cathode maintained good stability and a Faradaic efficiency of 60–75% while producing 80–90% of the hydrogen produced by platinum cathodes. Significant cost reduction (about 50 times less expensive than platinum) and superior corrosion resistance in comparison to pure Ni foils are two of the main benefits. In addition, the study by Scheepers, F. et al. [30] shows that temperature modification can greatly improve the performance of polymer electrolyte membrane (PEM) water electrolysers, underscoring the significance of modeling the total system efficiency of these systems. According to the study, the applied cell voltage determines the ideal operating temperature. Proper temperature control not only increases efficiency but also guards against safety hazards brought on by hydrogen crossing. The work offers useful insights for maximizing PEM electrolyser performance while upholding safety regulations by methodically examining these correlations. It also offers helpful assistance for both present applications and upcoming system designs. Woods, P. et al. in their study [31] stated that green hydrogen, made via renewable-powered water electrolysis, is a key to decreasing water scarcity challenges. Wastewater offers a scalable solution—Sydney’s treated effluents alone could meet Australia’s 2030 hydrogen demand while producing valuable oxygen byproducts to offset costs ($3/kg H₂) and improve water treatment. Unlike energy-intensive desalination, wastewater is climate-resilient and avoids competing with drinking supplies, positioning water utilities as critical enablers of sustainable hydrogen [32].

When it comes to high-efficiency bifunctional electrocatalyst, a study by Li, L. et al. [32] reported the development of an amorphous RuO₂-coated NiO nanosheet catalyst (a-RuO₂/NiO) that achieves exceptional hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) performance, requiring only 1.372 V to reach 10 mA cm⁻² in urea electrolysis. The amorphous/crystalline interfaces create oxygen vacancies that generate spin-polarized electrons, accelerating reaction kinetics. Density Functional Theory (DFT) studies confirm these interfaces enhance charge transfer and optimize intermediate adsorption via d-band center tuning, demonstrating a novel design strategy for efficient bifunctional electrocatalysts. Furthermore, Bonacina, C. N. et al. in their study [33] investigated a plant that produces liquefied green hydrogen offshore for ship refueling. The facility includes an electrolyze stack for producing hydrogen, a water treatment unit for producing demineralized water, a wind farm for producing sustainable power, and a hydrogen liquefaction plant for storing and distributing hydrogen to ships.

Another magnificent study was conducted by Cinti, G. et al. [34] proved urea as a promising hydrogen-carrier fuel for solid oxide fuel cells (SOFCs). Through nickel-catalyzed thermal decomposition, urea generates hydrogen-rich streams for SOFC operation. Experimental tests achieved >40% efficiency, while modeling projected 60% efficiency, highlighting urea’s potential for sustainable hydrogen production and utilization in high-temperature fuel cell systems while enabling waste valorization. Musa Ardo, F. et al. [35] in their article discussed the pre-treatment techniques used to increase the rates of hydrogen production from microalgae as well as the many mechanisms and approaches used to produce hydrogen from microalgae. Moreover, Alsalme, A. et al. [36] fabricated a successful S-scheme Heterojunctions with ultrasonic features to efficiently break down fluorescein dye and produce green hydrogen. Winter, L. R. et al. [37] revealed that, in comparison to water electrolysis, the energy and expenses required to treat unconventional water sources, such as saltwater, industrial and resource extraction wastewater, and municipal wastewater, are insignificant. For the urea-assisted water splitting processes, Yin, C. et al. in their study [38] a NiSe₂/MoSe₂ heterostructure catalyst with tuned interfacial electron redistribution and urea adsorption energies via a powerful built-in electric field proved to be successful. A study conducted by Yao, M. et al. [39] offered a way to securely produce green hydrogen utilizing solar energy while addressing the environmental problems presented by urea. In another study by Kannaiyan, K. et al. [40], a knowledge gap was addressed by illustrating the possibilities of the hydrogen economy from multiple perspectives while discussing the challenges and potential.

The study by Yang, X. et al. [41] described a bioinspired method for creating superhydrophilic hierarchical MnO₂ nanocoatings on hydrophobic polymer membranes using amino malononitrile (AMN)/Mn2+-mediated mineralization. In addition, Yin, Z.-H. et al. [42] demonstrated effective and steady hydrogen production using the manufactured electrodes in a real-world cell powered by a 1.5 V commercial dry battery. This suggests that the proposed technique might be used to rationally design highly active electrocatalysts to produce green hydrogen. Furthermore, Rico-Oller, B. et al. [43] reviewed the possibility of a process by which hybrid materials photo-catalytically reform methanol to produce hydrogen.

According to a study by Chauhan, D. et al. [44], advanced treated water and wastewater can both benefit from the production of green hydrogen when impurities are eliminated, marking it as one potential for the hydrogen economy. Moreover, Krishnan, S. et al. [45] in their study discussed a sustainable photoelectrochemical approach to recycling waste PET plastic into green hydrogen and compounds with added value. And lastly, the study by Zhang, B. et al. [46] identified the ideal operating conditions for wastewater electrolysis to maximize hydrogen production.

2.7. Catalyst Trends in Wastewater-to-Green Hydrogen Research

Catalysts are essential for facilitating effective hydrogen synthesis from wastewater. An examination of the most-cited research identifies several predominant groups of catalysts that have arisen in the last decade, summarized in Table 6.

• Transition Metal-Based Catalysts:

Nickel is the most extensively researched non-precious catalyst, especially in microbial electrolysis cells (MECs) and alkaline electrolysis. Kadier, A. et al. [29] proved that electroformed nickel mesh cathodes may achieve 80–90% of the hydrogen output of platinum electrodes, while ensuring substantial stability and decreasing costs by nearly fiftyfold.

2.

Noble Metal-Modified Catalysts:

Despite their high cost, modifications with noble metals markedly improve catalytic activity. Li, L. et al. [32] documented an amorphous RuO₂-coated NiO nanosheet catalyst including oxygen vacancies, which exhibited superior hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) performance at a minimal operating voltage

3.

Heterojunction and Composite Catalysts:

TiO₂/g-C₃N₄ S-scheme heterojunctions [37] and NiSe₂/MoSe₂ heterostructures [39] demonstrate potential in improving charge separation and reaction kinetics under photo- and electrochemical circumstances. These systems facilitate elevated current densities and decreased overpotentials.

4.

Multifunctional Catalysts for Integrated Wastewater Treatment:

A significant trend is the development of catalysts that concurrently decompose contaminants (e.g., dyes, urea, plastics) and produce hydrogen. Examples are oxygen vacancy-rich catalysts for urea electrolysis [40], NiMoO₄ micropillar arrays for urea-assisted alkaline electrolysis [43], and doped titanium oxide nanoparticles for methanol photoreforming [44]. Zhang, B. et al. [46] expanded this concept to encompass plastic trash, integrating hydrogen production with the synthesis photoelectrochemical catalysts.

The longevity of catalysts in actual wastewater is significantly constrained by fouling, heavy metals, and variable pH levels. Although nickel-based and heterojunction catalysts show potential, their long-term stability and scalability remain unproven [48,49,50]. This gap highlights contradictions between laboratory-scale performance and real-world applicability.

2.8. Primary Research Areas of the Application of Wastewater to Green Hydrogen Production

One effective technique for mapping research topics and monitoring the development of research fronts within a field is co-occurrence analysis. Examining how keywords or concepts occur together in scientific literature, it makes it possible to identify new themes, research gaps, and the organization of scientific domains [47].

This section examined Scopus data using a minimum keyword occurrence threshold of 5, consequently extracting 21 keyword strings from 779 author keywords. The keywords that met or surpassed this threshold are shown in

Table 7. Top 21 keywords from the research published on green hydrogen and wastewater treatment.

Keyword	Cluster	TLS	Occurrence
Green Hydrogen	1	35	59
Hydrogen	2	23	27
Hydrogen Production	1	12	18
Water Electrolysis	1	18	17
Oxygen Evolution Reaction	3	13	16
Electrolysis	2	17	14
Hydrogen Evolution Reaction	3	12	14
Wastewater Treatment	1	15	12
Wastewater	2	16	10
Water Splitting	2	11	10
Renewable Energy	3	10	9
Photocatalysis	1	8	7
Urea Oxidation Reaction	3	8	7
Circular Economy	1	6	7
Green Hydrogen production	1	6	7
Biohydrogen	1	3	6
Electrocatalysis	3	8	5
Biogas	1	7	5
Hydrogen Energy	3	6	5
Microalgae	2	6	5

. It is crucial to note that the authors’ keywords were the main focus of the investigation. Based on occurrence, the top 21 keywords are arranged in descending order.

Terms like “Electrolysis,” “Electrocatalysis,” and “Water Electrolysis” are essential to this research as they describe the core processes and technologies that enable the sustainable production of hydrogen from water using renewable energy. Keywords like “Urea Oxidation Reaction,” “Hydrogen Evolution Reaction,” and “Water Splitting“ are used to emphasize the key electrochemical processes and strategies that enable efficient, cost-effective, and sustainable hydrogen generation.

The Keywords “Biogas”, “Microalgae”, “Biohydrogen” are dominant in research on sustainable energy production, particularly in the context of renewable biofuels. These terms represent interconnected concepts and technologies that are shaping the future of clean energy. Furthermore, the references to “Circular Economy” and “Renewable Energy” are directly related to the production of green hydrogen from wastewater, establishing a sustainable system that promotes decarbonization, minimizes waste, and recovers resources. The creation of green hydrogen from wastewater is an example of how these ideas combine to solve energy and environmental issues.

The concept of “Wastewater” and “Wastewater Treatment” indicates the potential for innovative technologies that utilize wastewater as a medium for producing hydrogen as well as a feedstock. This method promotes sustainability and resource recovery by treating and valorizing wastewater in addition to producing clean hydrogen fuel.

The VOSviewer visualizations provided deeper insights into the structure of the research domain. In the co-authorship and co-occurrence maps, node size represents the frequency of occurrence (e.g., number of publications for authors or number of appearances for keywords), while link thickness indicates the strength of relationships (e.g., collaboration intensity or co-occurrence frequency). Nodes with similar patterns are grouped into clusters, which highlight distinct thematic or collaborative communities within the field. For example, one cluster centered on “wastewater treatment” reflects environmental and engineering perspectives, while another cluster around “green hydrogen” highlights renewable energy and sustainability approaches.

In the density visualization, areas with warmer colors (yellow/red) indicate high research concentration and frequent connections, whereas cooler colors (blue/green) indicate less explored topics. This allows identification of research hotspots (e.g., “green hydrogen production from wastewater”) as well as underexplored areas that could represent future research opportunities.

Overall, these terms describe a system that turns wastewater from a waste product into a source of clean energy, with the help of renewable energy sources and the concepts of the circular economy guaranteeing a sustainable and effective outcome. This integrated strategy provides a single, scalable solution for waste management, sustainable energy generation, and illustrates a holistic, sustainable approach to green hydrogen production. In this regard, a network diagram showing the co-occurrences of terms in research publications on wastewater-to-green hydrogen is presented in Figure 7.

Figure 7 illustrates a network diagram where nodes represent different elements, with their shapes and positions reflecting the probability of co-occurrence. The keyword co-occurrence analysis identifies three distinct clusters, each color-coded and associated with specific research themes in wastewater and green production. The colored nodes correspond to these thematic groups, highlighting key disciplines within the field. Node size represents term frequency, while the thickness of the connecting lines indicates the strength of relationships between terms.

3. Methods

3.1. Data Source and Search Strategy

This study employs a systematic literature review. Although the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines are commonly used in meta-analyses, this review did not adopt PRISMA, as no meta-analysis was conducted [28]. Bibliometric analysis is well-known as scientific specializations and is a crucial component of research assessment methodology, particularly in the practical and scientific domains [51]. It acts as an essential instrument in systematic literature reviews, enabling the development of a thorough and reproducible database [52]. The ability of bibliometric analysis to provide an integrated perspective of research sectors, outputs, organizations, and trends is responsible for its broad adoption and success [1,53]. This approach is very effective in analyzing enormous academic data, clarifying the correlation between journal citations, and providing insights into established or emerging topics of research [51].

In this study, bibliometric analysis was applied to look at developments and trends in green hydrogen production from wastewater. To analyze the research trends, a bibliometric analysis was conducted on the retrieved publications. The bibliographic data was exported from the Scopus database in RIS and CSV formats. The analysis focused on both performance indicators (publication output, citations, authorship) and science mapping (collaboration and thematic structures). The unit of analysis included documents (to assess annual publication trends and citation counts), authors (to identify most productive and influential researchers), institutions and countries (to evaluate geographical distribution and collaboration networks), and keywords (to explore thematic evolution and research hotspots).

The bibliometric data was processed using VOSviewer for network visualization of co-authorship, co-citation, and keyword co-occurrence. The latest publications on the production of green hydrogen in the period between 2010 and 2024 were retrieved from the Scopus database. The Scopus database was selected due to its huge collection of pertinent published papers and thorough coverage of all disciplines of study [54]. The literature search was performed utilizing the following query string and keywords (TITLE-ABS-KEY (“Green Hydrogen”) AND (“Wastewater Treatment” OR “waste water treatment” OR “Sewage treatment”)) AND PUBYEAR > 2010 AND PUBYEAR < 2025 AND (LIMIT-TO (LANGUAGE, “English”)) AND (LIMIT-TO (DOCTYPE, “ar”) OR LIMIT-TO (DOCTYPE, “re”)) AND (LIMIT-TO (SRCTYPE, “j”) OR LIMIT-TO (SRCTYPE, “p”)).

Although Scopus offers comprehensive coverage for bibliometric analysis, it fails to encompass all global research. Incorporating additional databases like Web of Science would mitigate potential indexing bias and give a more comprehensive perspective.

Figure 8 demonstrates the search strategy used for this study. The data collection process started with the extraction and organization of records from the Scopus database, with results saved in a comma-separated Values (CSV) file format. To guarantee data quality during selection, multiple filtering steps were performed. First, duplicate entries were identified and removed. Strict rules were applied next for including journal articles and conference papers published in English discussing green hydrogen production from wastewater. Conference reviews, non-peer-reviewed works, and publications not directly related to green hydrogen were excluded. After this initial filtering, the remaining records were examined in detail. This involved checking publication sources, author information as well as author keywords. Then, an evaluation of each document’s quality was executed by reading titles and abstracts. Any records with missing information or errors were removed. Through this careful selection method, a final dataset containing exactly 221 qualified English language-based journal articles and conference papers suitable for analysis was considered.

3.2. Visualization Procedures

VOSviewer is a specialized software tool designed for constructing and visualizing bibliometric networks, making it highly effective for bibliometric analysis as it enables systematic mapping and visualization of scholarly literature networks [55]. Unlike generic statistical programs, it employs the Visualization of Similarities (VOS) mapping technique to generate accurate and interpretable network maps of co-authorship, co-citation, and keyword co-occurrence relationships [55]. VOSviewer was used due to its proficiency in processing enormous network datasets and its powerful text-mining functionalities [56]. This tool facilitates the detection of thematic linkages and research trends by constructing bibliometric networks that graphically represent associations among relevant publications [57]. A distinctive feature of VOSviewer is its adaptive labeling system, which dynamically adjusts to analytical parameters, ensuring clear visualization of co-occurrence patterns [19]. This study examined several fundamental dimensions: distribution across academic journals, author keywords, and geographical distribution of publications. These elements collectively offer a systematic framework for understanding the research domain’s structure and evolution, forming essential components of rigorous bibliometric analysis [58]. The investigation specifically evaluated quantitative indicators such as publication count, citation impact, and total link strength (TLS). These measures serve as critical proxies for evaluating scholarly influence and research dissemination patterns within the scientific community [59].

Following the execution of our systematic search protocol, the query identified a final dataset of 221 scholarly research outputs, comprising exclusively peer-reviewed journal articles and conference papers. These publications were disseminated across 115 distinct academic sources, reflecting contributions from international researchers affiliated with 635 institutions spanning 61 countries. The total citation counts of 2672 underscores the substantial academic influence of this corpus. Subsequent bibliometric examination of author keywords identified 779 unique keyword entries, revealing the field’s extensive conceptual diversity. A summary of key bibliometric findings can be seen in

Table 8. Summary of key bibliometric results (2010–2024).

Description	Findings
Publications (articles and Conference Papers)	221
Authors	1198
Countries	61
Publication venues	115
Authors’ Affiliation	635
Author keywords	779
Total citations	2672

.

4. Future Work

The bibliometric study indicates that research on the integration of wastewater for green hydrogen production remains at its early stage and encounters different technological, economic, and regional obstacles. The longevity of catalysts in actual wastewater is significantly constrained by technological factors, as fouling, heavy metals, and variable pH levels diminish hydrogen output and elevate overpotentials [48]. Although nickel-based and heterojunction catalysts show potential, their long-term stability and scalability remain unproven [49,50]. The elevated expense of electrolysers and noble-metal catalysts hinders widespread use, and techno-economic models hardly account for the twin advantages of hydrogen generation and wastewater remediation [60].

Future studies should aim to develop the field by taking the following steps:

• Broadening the geographic scope, particularly in poor nations, to tailor solutions to varying wastewater properties and resource limitations.
• Implementing pilot and demonstration projects to produce reliable data on cost, efficiency, and durability in real-world situations.
• Developing multifunctional, fouling-resistant catalysts that not only improve durability, but also reduce dependence on precious metals.
• Fostering interdisciplinary collaboration by combining skills from engineering, microbiology, chemistry, and the water–energy domains.
• Promoting policy frameworks and incentives that facilitate dual benefit technologies for energy recovery and wastewater remediation.

By addressing these obstacles and broadening research involvement across many locations, wastewater-to-green-hydrogen systems might progress from promising laboratory investigations to scalable, commercially feasible solutions for sustainable energy and resource management.

5. Conclusions

This bibliometric analysis has provided a thorough overview of global research trends in green hydrogen production from wastewater between 2010 and 2024. The substantial increase in publications and citations, especially in the last five years, indicates a growing interest in this novel approach to sustainable energy among academics and industry. Leading contributors have been China, India, and South Korea, but the data also shows growing involvement from Europe, suggesting a wider range of international cooperation. “Water electrolysis” and “electrocatalysis” concepts were recognized as key research areas in the study. The keyword and co-citation analyses highlighted a multidisciplinary approach that connected “environmental science”, “engineering”, and “renewable energy technologies”. Even with significant advances, the area is still in its early stages. Future studies should concentrate on converting laboratory-scale discoveries into economically feasible solutions, particularly in developing nations where the most pressing energy and water issues exist. Furthermore, fostering cross-sectoral partnerships and enhancing international collaboration would be necessary to remove current technological and policy-related barriers. Eventually, green hydrogen generation from wastewater offers a viable route to resource recovery, environmental sustainability, and decarbonization. In addition to mapping the existing environment, this study provided a basis for directing future investigations and policymaking initiatives in this nascent field.