Keywords

1 Introduction

Hydrogen has become one of the most promising clean and sustainable energy [1] due to its non-carbon emissions, which could make great contributions to the construction of global carbon neutral energy system. Hydrogen can be produced from various resources including fossil fuels [2,3,4], biomass [5], biological sources [6], and water electrolysis [7]. In contrast, electrolysis is the most economical method for large-scale production of high-purity (> 99.99%) hydrogen, which could be further classified into alkaline water electrolysis (AWE), solid oxide electrolysis (SOE), and proton exchange membrane (PEM) water electrolysis.

SOE is easy to achieve high producing efficiency [8], but the technology is still trapped in the laboratory stage and lacks of industrial application. AWE has become a well matured technology for hydrogen production up to the megawatt range, and constitutes the most extended electrolytic technology at a commercial level worldwide [9]. But its disadvantages including low partial load range and limited current density [10] limit the promotion of this technology and the enthusiasm of relevant academic research. PEM by contrast, as a novel electrolysis method, has the advantages of high flexibility, compact equipment and high current density [11]. With the trial of non-noble metal catalysts utilization [12], more and more important achievements were accomplished in recent years, and in summary, the disadvantages of PEM are gradually overcome and the number of published papers related to PEM electrolysis is rapidly increasing (as summarized in Sect. 3.1).

Over the past two decades, more than 400 articles are published on the R&D of PEM electrolyzer, and the development road map of PEM electrolyzer over the years has been gradually clear. Thus, the purpose of this paper is to analyze the modification of research hot-spots in the field of PEM electrolyzer based on the publications, in order to offer beneficial inspirations and new frontiers for relevant scholars.

2 Methods

2.1 Data Collection

Relevant articles are identified from the Web of Science Core Collection (WoSCC) database. To avoid issues due to daily updates of the database, we conducted our search and downloaded relevant items on January 3, 2023, Tuesday (Fig. 1). We used search formulae as TS = (“PEM Electrolyzer”). The search terms were selected from the Science Citation Index Expanded (SCI-EXPANDED). A total of 444 publications were retrieved. After excluding 33 publications including early access, proceeding paper, corrections, meeting abstract, 411 articles were included in the final data set. Full records and cited refernces were exported as plaint text files for further analyses.

Fig. 1.
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Screenshot of web of science searching

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The obtained data were imported into Citespace software (version 6.1.6) for deduplication. The number of publications, citations, countries, authors, journals, highly cited references, keywords, and other data were extracted by several independent researchers (JH Li, ZF Deng, H Tan, L Li) for data recheck. The H-index and average citations per items (ACI) utilized in the paper are calculated by ZW Bai based on information at WoS Online.

2.2 Data Analyses

Data collected from WoSCC needed to be further analyzed to accurately acquire the impact of each part. Citespace (version 6.1.6) and VOSviewer were introduced to bibliometric analysis and visual display. Microsoft Excel (version 16.58) was used to summarize relevant information in tables. Flow chart of publication filtering and analysis was shown in Fig. 2.

Fig. 2.
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Flow chart of publication filtering and analysis

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3 Results

The above mentioned 411 samples published in 99 Journals were completed by 1382 authors from 60 countries/regions and affiliated to 506 organizations. These contents would be further described and analyzed through Publication and Citation, Regions and Countries, Authors, Journals, and Keywords, respectively. Details are shown as following.

3.1 Publication and Citation

As can be seen from Fig. 3, the total publications was relatively flat before 2017, and then the numbers displayed a sharply increasing trend since 2018. Almost 73.6% of total were published between 2018 and 2022. Citations almost rose straightly after 2018 too. It can be seen that the PEM Electrolytic is a research hot-spots indeed. Relevant research increases year by year.

Fig. 3.
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Trends of publications and citations

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The top 10 most-cited articles were listed in Table 1 in order to outcome the most influential paper in this field. The most-cited paper [13] is written by Barbir, F with 708 times cited, followed by papers [14,15,16] which has been cited for 307 times, 294 times, and 286 times, up to now, respectively.

The first mentioned one [13] is the first published paper related to PEM electrolyzer which discussed several possible applications for PEM electrolyzer including grid connected hydrogen generation, grid peak shaving, and power storage cooperated with fuel cell. Furthermore, several specific issues including size, operation, and efficiency are discussed in the paper. The second article [14] in the table should be paid specially attention. Because it published recently and been cited for 43.9 times annually. This paper is mainly study about the active and highly stable catalysts and several experiments and analysis were cleverly made. The fourth publication [16] in Table 1 is the most-cited article in 2022 which is mainly focus on the Energy and exergy analysis of a PEM electrolyzer system, and offered better understanding of the characteristics of PEM electrolyzer plant. This paper represents the boom of PEM electrolyzer system related research.

Table 1. High cited references in PEM electrolyzer
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3.2 Publication and Citation

Figure 4 displays the distribution of total publications about PEM electrolyzer. Iran (88), China (61), USA (59), Germany (52), Canada (50), Turkey (50), Italy (32), France (27), South Korea (14), Japan (12) as the top 10 most productive countries up till 2022. These countries are mainly located in Asia, Europe and North America. Data in Fig. 4 is not directly equal to value in Fig. 3, because some articles are finished through global cooperation and will be duplicated statistics.

Fig. 4.
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Note Color relates to the paper released (Note Taiwan belongs to China; England, Northern Ireland, Scotland and Wales belongs to UK.)

Geo visualization of research distribution of PEM electrolyzer.

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3.3 Authors

Tabel 2 summaries the top 10 authors in terms of the number of publications. Dincer, Ibrahim from University of Ontario was the most productive author with 22 publishments up to 2022. His outstanding investigation has significantly increased the number of related papers published from Canada, accounting for about 44%. In the same way, Iran, as the most productive country mentioned in Sect. 3.2, also owes its outstanding scientific research contributions to research teams led by Afshari, Ebrahim and Baniasadi, Ehsan. As can be seen, the rise of an outstanding scholar (team) has a significant impact on the research status of the country in relevant fields. While it is also important to find that as the second largest publication country, China, do not have any outstanding researcher being recorded in Table 2. It is considered as up to now, this topic has been greatly attracted by Chinese researchers but no one has become the academic leader.

Table 2. Most productive authors in PEM Electrolyzer.
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3.4 Journals

There were 99 journals used to publish PEM electrolyzer related papers, in which the International Journal of Hydrogen Energy contributed the most articles with total publications of 147 (33.1%), and Energy Conversion and Management ranked second (36, 8.1%), followed by Journal of Power Sources (23, 5.2%) (Fig. 5).

Fig. 5.
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Pie chart visualization of the top journals of PEM electrolyzer

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3.5 Keywords

VOS viewer which could be used to classified the similar keywords was introduced, 411 publications contained 1891 keywords, and 180 left after merging of similar items. Heat map of Keywords (Fig. 6) highlights the top keywords including PEM Electrolyzer, Performance, Energy, Hydrogen Production, and Optimization. These keywords related to the deep research around PEM electrolyzer especially on catalyst and system performance.

Fig. 6.
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Heat map of keywords in 2022

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4 Discussion

4.1 Countries and Authors

Figure 7 demonstrates the annual publications of top 10 most productive countries since 2014. Iran, China and USA are the top 3 countries who contributed the most publications as previous shown in Fig. 4. Iran has far more publications than any other country (20%), most of which are finished in recent 5 years [17,18,19,20,21]. Iran remains the most producer in recent five years, until been surpassed by China (23) in 2022.

Fig. 7.
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Trends of publications of the top 10 countries/regions

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Author cooperation analysis revealed inter-author connection networks. In the PEM Electrolyzer research area, Dincer Ibrahim (22), Friedrich K Andreas (14), Ahmadi, Pouria (12) are the top 3 authors with the most paper publications. And it can be seen from Fig. 8, which summarised the authors with more than 4 papers published, as some of these authors have quiet close stable team cooperation. Dincer, Ibrahim and Ahmadi, pouria used to co-work on the energy analysis and optimization of electrolyzer included system since 2013 [22,23,24,25]. Friedrich, Wang, Abouatallah, and bazylak are together engaged in the characteristics investigation of gas diffusion layer and bipolar of PEM electrolyzer [26,27,28,29,30,31].

Fig. 8.
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Notes the size of the nodes reflects the strength of their cooperation.

Network visualization map of co-authorship.

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4.2 The Changing of Keywords

Fig. 9.
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(Notes the size of the nodes reflects the strength of their frequency)

Analysis of keywords for PEM electrolyzer

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The keywords clarification through these papers are summarized as Fig. 9. It could be seen as three cluster are summarized with different colors, System type (Exergy Analysis, Multi-objective Optimization, et al.), component type (Two Phase Flow, Oxygen Evolution Reaction, et al.), and modeling type (Simulation, Design, et al.), which may related to system, component, and electrolyzer, respectively. Furthermore, the annual keywords in Fig. 10b clearly shows that the keywords on the right are newer, which emphasize that the research focus shifted from system into component gradually. This phenomenon is predictable, as technical research should gradually modification from application demonstration to theoretical verification.

Moreover, the verification of the keywords are summarized as Fig. 10. It is partial similar with the result presented in Fig. 9. Not only the component study related keywords, such as Modelling and Degradation [32,33,34,35] shows increment these years, but also the Hybrid System related keywords, such as Geothermal and Photovoltaic [21, 36,37,38], show a boom increment these years. As for the System Optimum Analysis and Oxygen Evolution Reaction, these keywords fail to present a continuous research trend and seem suspended.

Fig. 10.
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(Notes the size of the nodes reflects the strength of their frequency)

Cluster timeline of co-cited reference

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4.3 Recent Research Trends

To analyze the subtle changes in research over recent years, the heat map of keywords till 2020 is drawn(as shown in Fig. 11) to compare with Fig. 6. The hot keywords related to Performance and Optimization are slightly decreased during 2020–2022, which are replaced by publications about Modelling and Two Phase Flow. This trend modification is consistent with the research topic timeline as shown in Fig. 10.

Fig. 11.
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Heat map of keywords in 2020

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Moreover, several outstanding authors are chosen for case analysis for the trend modification on researching area in PEM electrolyzer. The most prolific authors in the area are summarized in Table 3 as basic data, and their latest publications during 2020–2022 are utilized for special analysis.

Table 3. Most productive authors in PEM electrolyzer during 2020–2022
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Based on the chosen publications, it should be mentioned that Aimy and Kieran are closely cooperated [32, 39, 40] around porous transport layers research. Meanwhile, Mojtaba and Ehsanolah were deeply co-worked [41,42,43] on electrolyzer related hybrid system investigation. Ibrahim is one of the greatest researchers who has made a significant contribution on PEM electrolyzer utilizing renewable energy [44, 45]. Marcelo was devoted to improve the electrolysis efficiency through novel components and operating conditions [46, 47]. These researchers have carried out their previous research directions in the past three years.

Damien engaged in electrolyzer modeling and experimental verification for several year. His latest publication on ‘Journal of Processes’ in 2021 turn to study electrolyzer related testing applications [48]. This slightly variation maybe signify the further development of electrolysis related technology in application level.

Journal statistics help researchers choose the most suitable journal for their work. The publication Journal clusters show a clearly verification for the chosen publications, changing from ‘Journal of International Journal of Hydrogen Energy’ into other Journals instead, such as Energies and sustainable energy technology, which can easily be seen in Fig. 12.

Fig. 12.
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Analysis of publication journals

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However, even the weight of ‘International Journal of Hydrogen Energy’ among the articles significantly decreased, this Journal is still the mainly journal for publishing work about PEM electrolyzer for most researchers. Data analysis indicates that some Journals, such as Sustainable Energy Technologies, Journal of Power Sources, Energies, are becoming more attractive to the related research. This phenomenon maybe related to the transfer of the research topics of the journals, so that gives authors more choices (Fig. 13).

Fig. 13.
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(Notes the size of the nodes is weighted by publications)

Overlay visualization of journals co-occurrence year

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5 Conclusion

Based on the bibliometric analysis of the 411 PEM Electrolyzer publications released between 2005 and 2022, it is found that the research has increased explosively with the Iran being the leading producer, especially in recent 5 years. China lacks top scholars and core leader in the field. Most researchers have stable cooperative relationships, and continues their research direction in recent years. Component and hybrid system related study are investigation hot-spots recently. Furthermore, keywords including two phase flow and geothermal hybrid systemare more concerned than ever. More journals are joining the publications about hydrogen, but the status of the main journal (International Journal of Hydrogen Energy) is still unshakable.

Overall, electrolysis is a novel production method for clean energy. Bibliometrics is a useful tool to discover the current research hot-spots and future trends of a field. PEM electrolyzer related analysis in this paper clearly exhibits the research topic modification and research trend in recent years. This paper is hoping to offer beneficial inspirations and new frontiers for relevant scholars.