Analysis of proton exchange membranes for fuel cells based on statistical theory and data mining

Summary Fuel cells (FCs) have attracted widespread attention as a highly efficient, clean, and renewable energy conversion technology. Proton exchange membrane (PEM), as one of the core components of FCs, plays a crucial role, and a comprehensive summary of its development is essential for promoting rapid progress in the field of sustainable energy. This article provides a comprehensive review of the development status and research trends of PEMs over the past twenty-eight years, based on statistical analysis and data mining techniques. Price, sustainability, stability, and compatibility issues are the main challenges faced by current PEMs used in FCs research. The current research focuses mainly on the characterization, performance optimization, enhancement mechanisms, and applications of PEMs in FCs. This review provides a systematic summary of PEM materials, serving as a valuable reference for the development, application, and promotion of new PEM materials in FCs.


INTRODUCTION
7][8] FCs have advantages such as high energy density, rapid startup, flexibility, and reliability, making them suitable for various applications, [9][10][11][12][13] including transportation, portable power sources, power supply, and industrial applications.Additionally, FCs can be combined with other energy technologies, 14,15 such as solar and wind energy, to form hybrid energy systems, further improving energy utilization efficiency and sustainable development.4][25] The PEMFCs consist of multiple layers, [26][27][28][29] with a proton exchange membrane (PEM) in the middle, catalyst layers on both sides, and gas diffusion layers further outward.These five layers form a membrane electrode assembly, with bipolar plates on both sides of the membrane electrode assembly, as shown in Figure 1.The electrochemical reactions occur in the PEMFC by introducing hydrogen gas at the anode for oxidation and oxygen or air at the cathode for reduction, generating water with the assistance of catalysts.The reactions occurring in the PEMFC can be explained as follows: Anode : H 2 / 2H + + 2e À (Equation 1) Cathode : Overall : 3) Most current research works are primarily focused on FCs themselves, membrane electrodes, anode catalyst layers, and catalyst materials. 30The PEMs are one of the key components of PEMFCs, and their critical role cannot be overlooked. 31They separate hydrogen and oxygen gases in the electrochemical reaction and facilitate proton conduction, resulting in the generation of electric current. 32,33The PEMs effectively provide proton conduction pathways while preventing electron transport, thereby enabling the normal functioning of FCs.6][47] These membranes find wide applications, particularly in FC vehicles and portable electronic devices. 48FC vehicles, as sustainable transportation solutions, 49 heavily rely on PEMFCs. 50,51The high proton conductivity and stability of PEMs are crucial for the performance and reliability of FC vehicles. 52Additionally, PEMs can be used in portable electronic devices 53,54 such as mobile phones and laptops to provide sustainable power sources.To enhance the efficiency, stability, and reliability of FCs, in-depth research on PEMs is necessary.The performance of PEMs directly affects the output power and lifespan of FCs.Therefore, by gaining a deep understanding of the structure and properties of PEMs, their design and preparation methods can be improved to enhance proton conductivity and stability. 55Furthermore, studying PEMs can also explore new materials and technologies to address challenges in the field of FCs.Simultaneously, optimizing the fabrication processes and methods of PEMs can reduce costs, improve production efficiency, and promote the commercial application of these membranes.
Based on this, the overall overview of research on PEMs for FCs is shown in Figure 2.This study provides a systematic bibliometric analysis of PEM-FC literature over the past 28 years, aiming to identify the background and frontiers of PEM-FC research.In contrast to existing literatures, [56][57][58] this article emphasizes using mapping networks to characterize the intrinsic relationships of PEM research, revealing the multiple factors that need to be considered in developing high-performance PEMs.To distinguish between ''proton exchange membranes for fuel cells'' and ''proton exchange membrane fuel cells,'' this article defines their abbreviations as PEM-FC and PEMFC, respectively.The research work in this article is expected to arouse strong interest among researchers in the field of energy, particularly FCs, and provide answers and insights to the following questions.
(1) What is the status and background of PEM-FC research globally?(2) Which countries or institutions have authors who are focusing on or researching PEM-FC?(3) Who are the authoritative or leading researchers in the field of PEM-FC?(4) Which journals have published research results related to PEM-FC?Which journal is the most important in this field?(5) What are the most recognized and classic articles in the field of PEM-FC research?Based on this, the topic words determined in this article are ("proton exchange membrane* for fuel cell*") or ("proton exchange membrane* in fuel cell*") or ("proton exchange membrane* of fuel cell*") or ("proton exchange membrane* on fuel cell*") or ("proton exchange membrane* used in fuel cell*").

Selection criteria
Based on the research motivation and research questions in introduction, the selection criteria for this article are as follows.
(1) The publication date of the selected papers is from January 1, 1995, to December 31, 2022.The search was conducted on December 4, 2022.It is important to note that, based on the topic determination, the first relevant paper was published in 1995. 602) Document types such as meeting abstracts, proceeding papers, book chapters, and corrections are excluded.
(3) Only articles in English are selected, and articles in other languages are excluded.

Data saving
Based on the topic determination and selection criteria, the relevant literature data on PEM-FCs are saved.The record content includes full records and cited references.These data are crucial for subsequent statistical analysis and provide the original basis for scientometric analysis.

Statistical analysis
Through the aforementioned method, many literatures have been downloaded and relevant information within the articles has been extracted.Subsequently, statistical analysis is conducted to summarize certain indicators in the field, presenting various changes in the research field in the form of data and charts, such as development history, current research status, and research hotspots.Statistical methods enhance the research significance of the topic to some extent.VOSviewer is a literature mapping visualization software [61][62][63] used to construct various networks, such as citation, bibliographic coupling, institutional collaboration, co-citation, or co-authorship relationships. 64It focuses on the visualization of scientific knowledge and provides text mining capabilities to construct and visualize co-occurrence relationships of important terms (keywords) extracted from many scientific literatures. 65Therefore, this article conducts statistical analysis of the literature on PEM-FCs using VOSviewer.
In the statistical analysis of the literature, relationships such as co-authorship, co-citation, co-occurrence, clustering of authors, national collaborations, keyword co-occurrence, and bibliographic co-citation can be represented by circles and lines, where circles represent the objects of our study and lines represent the associations between them.To avoid repetition, this article focuses on the analysis of keyword mapping.Keywords represent the core content of the research, and, in the analysis of keyword co-occurrence, analyzing the distribution of keywords can effectively reflect the research hotspots in the field. 66When two identical keywords are found in the selected papers, they are considered as a co-occurrence.Based on the co-occurrence matrix, a mapping graph of the literature is constructed, and the most critical step is to calculate the similarity matrix and map the similarity matrix.The co-occurrence matrix is normalized, and the similarity measurement is represented by the strength of association.Assuming two keywords can be represented as i and j, their similarity s ij can be expressed as 4) where c ij represents the number of links between keyword i and keyword j, and w i and w j represent the total number of link occurrences where keywords i and j appear together.In the mapping graph, the distance between keyword i and keyword j reflects their similarity.A shorter distance indicates a higher similarity, while a longer distance indicates a lower similarity.The idea of cluster analysis is to minimize the weighted sum of squared Euclidean distances between all pairs of items, 61 which can be solved by minimizing the value of V in the following Equation 2: V ðx 1 ; .; 6) where x i represents the position of the keyword in the mapping graph, n represents the number of items in the mapping, and ||,|| represents the Euclidean norm.The higher the similarity between two keywords, the higher is their weight.

RESULTS AND DISCUSSION
PEMs play a significant role in FCs.Based on the research methodology described in the analysis approaches section, a comprehensive exploration of the literature data on PEMs used in FCs over the past 28 years was conducted, resulting in a total of 301 published papers.This section will provide a systematic statistical analysis of the publication papers, authors, institutions, countries, academic journals, research fields, and research hotspots.
Trends in the research literature

Journal and discipline
Analyzing the sources of publications is an effective method to reveal the influence of academic journals.By comprehensively analyzing various indicators of publications, it is possible to dynamically track the dissemination of academic journals worldwide and periodically assess the most influential academic journals, providing a new reference dimension for evaluating the social impact of journals.In this study, publication quantity ratio, average citation count, impact factor, article influence score, and scientific fields were used as evaluation indicators to analyze the sources of publications on PEM-FC.A total of 301 papers were included in 119 academic journals, and Table 1 shows the relevant information of the top 15 active journals, which account for 50.17% of the total publications.There are six journals with more than 10 papers published, which are considered active journals in PEM-FC research.Among them, the journal "Journal of Membrane Science" dominates the importance of this research to a great extent, with 32 papers and a percentage of 10.63%.It is followed by "International Journal of Hydrogen Energy" (16, 5.32%) and "Journal of Applied Polymer Science" (16, 5.32%).In terms of average citation count, "Macromolecules" (139.19) ranks first with an absolute advantage, followed by "Polymer" (69.78), "Electrochimica Acta" (67.17), "Fuel Cells" (66.33), and "Journal of Membrane Science" (50.56).However, according to the citation indicator in 2021, the top three journals are "Journal of Materials Chemistry A" (1.85), "Journal of Membrane Science" (1.84), and "Journal of Power Sources" (1.44).In terms of impact factor and article influence score, "Journal of Materials Chemistry A" (14.511, 2.094) is the most active and influential journal, followed by "Journal of Power Sources" (9.794, 1.423) and "Journal of Membrane Science" (10.53, 1.089).It is worth noting that the article influence score is also an evaluation indicator, which normalizes the eigenfactor score according to the cumulative size of the cited journal across the prior five years.For example, an article influence score greater than 1.00 for Journal of Materials Chemistry A indicates that each article in the journal has above-average influence.Different journals have different dissemination power and social impact, and different indicators yield different evaluation results for journals.Therefore, these indicators need to be combined and further research is needed to optimize the current evaluation system.Essential Science Indicators (ESI) is a fundamental analysis and evaluation tool for measuring scientific research performance and tracking scientific development trends based on literature records collected in the WOS database.In this study, based on ESI, the publication journals of PEM-FC research were analyzed in terms of related scientific disciplines.Figure 5 shows the statistical analysis of the 301 papers in different scientific disciplines.From the figure, it can be observed that the research on PEM-FC is mainly distributed in three category groups: chemistry (189, 62.79%), materials science (60, 19.93%), and engineering (29, 9.63%).Compared to the other two categories, chemistry shows a very active phenomenon, indicating that researchers have paid great attention to the study of PEM-FC in the field of chemistry.Further analysis of the discipline classification reveals that the research on PEM-FC mainly covers 11 disciplines, with polymer science (44.19%) being the most active field among all disciplines, which is closely related to the polymeric properties of PEM.Researchers are enthusiastic about exploring its intrinsic characteristics.

Co-authorship analysis
Collaborative analysis can help researchers understand the academic relationships and contributions among different countries, institutions, and scholars.It is crucial for gaining insights into the development trends and academic influence of scholarly research.This section will provide a distribution and collaborative network analysis of the countries, institutions, and authors of PEM-FC papers.An analysis of PEM-FC-related papers was conducted based on the geographical locations of the published papers and authors to determine the contributions and distribution proportions of different countries.Out of the 301 literature sources obtained earlier, it was found that there were 48 countries involved.Figure 6 displays the distribution of these papers across different countries.The different colors represent the top 23 countries in terms of publication volume, accounting for 47.92% of the total number of countries.Countries with a publication count of more than 10 papers are primarily distributed in Asia (61.79%) and North America (20.27%), possibly due to the higher consumption of hydrogen energy in these regions.Africa, on the other hand, only has one country, South Africa, with a publication count of only 3 papers (1%).China (95 papers, 31.56%) has the highest number of PEM-FC-related publications among all countries, followed by the United States (50, 16.61%), India (24, 7.97%), and Japan (23, 7.64%).
Evaluating the scientific research capabilities of countries through the construction of a collaborative network is an effective visualization method.Based on this, this study conducted a collaborative network analysis of the countries involved in PEM-FC research to provide some guidance for countries interested in this research topic.Figure 7 presents the collaborative network graph among different countries, where the size of the nodes represents the publication volume of each country, the colors represent different clusters, and the width of the lines represents the strength of collaboration between countries.From the graph, it can be observed that China has a strong collaborative relationship with other countries in the field of PEM-FC, especially with the United States.As shown in Figure 7, the collaborative exchanges among the 29 countries will promote the development of the global economic market.In the future, this strong collaborative effort will continue to promote peace and stability worldwide, while ensuring energy diversification, security, and cleanliness.With the commercialization of FCs, the market prospects are vast.FCs will become the fourth generation of power generation after thermal power, hydropower, and nuclear power, triggering a green revolution in new energy and environmental protection.However, at present, there are relatively few countries involved in PEM-FC research, and collaboration between countries is also lacking.Therefore, further strengthening international cooperation research on the main components of FCs, such as PEMs, is needed in the future.

Institutional distribution and institutional cooperation networks
Through statistical analysis, it was found that the 301 papers on PEM-FC research were mainly from 375 academic institutions.Table 2 shows the main academic institutions according to the number of published papers.These core institutions mainly come from China (71 publications, 12.33%), the United States (9 publications, 1.56%), Japan (8 publications, 1.39%), and Russia (7 publications, 1.22%).In terms of article citations, Jilin University ranked first with a total citation count of 1,391, followed by the National Research Council Canada (941) and Virginia Polytechnic Institute & State University (811).However, in terms of unit citation, the number of papers published by the National Research Council Canada and Virginia Polytechnic Institute & State University is not particularly prominent, but their average citation counts reached 188.20 and 166.20, respectively, which are roughly three times that of Jilin University, which has the most publications.This indicates that their academic achievements have received attention from scholars around the world and have had a positive impact on society.In terms of link numbers, the Chinese Academy of Sciences has the most links with 24, indicating that it has the most cooperation with other institutions in PEM-FC research.Jilin University, the Russian Academy of Sciences, and Sun Yat-sen University have 11 links each.
Collaboration between institutions is an effective way to promote academic communication, which can enhance academic influence, enable researchers' research results to receive more attention and recognition, and help improve academic status.By establishing a visual network of institutions, the collaborative relationship between institutions can be presented.Figure 8 shows the collaboration network map between different institutions.The nodes in the network represent different institutions, and their size depends on the number of publications of the institution.The curves represent the strength of the connection between different institutions, and the color in the visualization network represents different cluster classifications.Generally, institutions of the same color have good cooperation, while the connectivity between different colors is relatively sparse.According to the different colors, the institution's collaborative network is divided into 13 clusters, and small clusters that are not related to each other have been removed from the network.The largest cluster (red, 10 institutions) is led by the Russian Academy of Sciences.The second cluster (blue, 7 institutions) is mainly composed of institutions in China.The second and third clusters around Jilin University and Wuhan University of Technology are mainly composed of institutions in China.It is worth noting that the Chinese Academy of Sciences plays a pivotal role in international cooperation in PEM research.In terms of connection strength, the collaboration   of academic literature as well as to identify new research directions and trends.In this section, the citation relationships of the papers, journals, and authors in the research field of PEM-FC are analyzed to facilitate a full understanding of the overall context for those interested in PEM-FC.To specifically compare the co-citation relationships of cited literature, Table 4 lists the information of the most co-cited articles, which are the most frequently cited literature.In addition to the author, publication year, and DOI number, the table also compares the country, institution, journal, citation count, and link strength.Publications from the USA and Germany have the most citations, followed by Canada and China.These cited literatures belong to different institutions, indicating that the research topic has received wide attention from scholars around the world.Journal of Membrane Science and Chemical Reviews are the most popular journals for PEM-FC research.The literatures Hickner et al. 69 and Kreuer 70 have the highest citation count and link strength, and they are important literature in the knowledge system of this research field, which is consistent with the analysis results in Figure 10.They review and systematically summarize various PEMs used for FCs, analyze the transport properties and swelling behavior of different membranes, and determine the key areas for future research.Further, based on the research objectives, main findings, and limitations summarized in Table 4, we can easily understand the research background of PEM-FC, which provides significant guidance for more in-depth studies in this field.

Cited sources
Journal co-citation analysis is a social network analysis method for studying academic literature, which can establish connections between various journals and explore the influence and contribution of academic literature.By analyzing the citation relationships of journals, important journals in the research field can be identified, hot issues and research directions in the academic community can be traced, existing knowledge networks can be explored, and the most influential journals can be identified.The cited sources of 301 research papers come from 1,580 journals.Setting the minimum citation frequency of a journal to 10, Figure 11 shows the visualized co-citation relationship of 157 journals.The connections between nodes indicate the citation links between journals.Journal of Membrane Science leads the entire green cluster, and its node is the largest; its co-citation relationship with other journals is also the closest.The most active journals shown in Table 5, Journal of Membrane Science, has a co-citation frequency and link strength of 1,873 and 147,153, respectively, indicating that it is the core journal in the PEM-FC field.Journal of Power Sources and Macromolecules rank second and third, respectively, in terms of citation volume.The organic combination relationship between these journals reveals the interdependence and cross-knowledge system in the PEM field.According to the classification method of ESI on subject categories, there are 11 chemistry, 2 materials science, 1 engineering, and 1 physics.It can be seen that the co-citation analysis of journals can determine the disciplinary properties and scientific structure of journals, identify the core areas of disciplines, and effectively evaluate academic journals.

Cited authors
Author co-citation analysis can reflect the academic influence of an author and help us understand the academic level and development trends of their research, providing accurate reference for other researchers.The development of a discipline relies heavily on the support of outstanding scholars.Figure 12 shows the top 15 authors with the highest co-citation counts.Many authors are from the United States, followed by Canada and China.Klaus-Dieter Kreuer ranks first with 170 citations and a link strength of 3,412, followed by Kenji Miyatake and Michael A. Hickner.These authors have made significant contributions to FCs, driving scientific and technological advancements, and content and key information, but also reveals the potential relationships between keywords, including both related and relatively independent relationships, providing insightful information such as research topics and hotspots.And cluster analysis is a multivariate data analysis method that aims to group similar observational units together, forming different clusters.It helps people better understand the patterns and features in a dataset, transforming messy data structures into useful information and knowledge.By employing cluster analysis, people can quickly identify patterns and trends in large amounts of data, uncovering the underlying patterns behind the data, and providing important evidence for decision-making.Additionally, the differences and similarities between different clusters can be clearly expressed, providing new entry points and ideas for further exploration.After merging and eliminating 631 keywords used in the research of PEM-FCs, 558 keywords remained.By selecting keywords with a cooccurrence frequency greater than 4, it was found that a total of 42 keywords reached the threshold.A correlation score was calculated for these keywords, and the most relevant keywords were selected to create a map, as shown in Figure 14A.Each circle represents a keyword, and the size of the circle represents the frequency of occurrence of that keyword.The strength of the connections between these keywords is represented by the width of the lines connecting the circles.Based on the co-occurrence relationships between keywords, it is evident that Figure 14A is divided into five clusters, represented by red, green, blue, yellow, and purple.
In the red cluster, there are 13 keywords, with "proton exchange membrane" appearing most frequently, with a total link strength of 222.The main keywords also include fuel cells, ionomers, phase separation, poly(arylene ether ketone), poly(arylene ether), poly(ether ketones), poly(ether sulfones), polyelectrolytes, polymer electrolytes, proton transfer, sulfonation, and composites.These clustered keywords mainly relate to research on PEMs and FCs, emphasizing the importance of PEMs in FC applications.Ref. [83][84][85][86] provide comprehensive explanations on this point.The green cluster has the second highest number of keywords, primarily focusing on proton conductivity.This cluster consists of keywords such as durability, ion exchange capacity, mechanical properties, methanol permeability, microphase separation, Nafion, polymer electrolyte fuel cells, polymer electrolyte membranes, and water uptake.These keywords are related to PEMs and their relevant properties, highlighting the necessity of understanding PEM performance.This can be seen in ref. [87][88][89][90][91] .The blue cluster mainly includes 8 keywords: composite membrane, conductivity, copolymers, crosslinking, high temperature PEM, morphology, polybenzimidazole, and thermal stability.Some of these technical terms indicate that the blue cluster is related to material modification and enhancement mechanisms, 92 focusing on improving the performance of PEMs.Relevant research can be found in ref. [93][94][95][96][97][98] .The yellow and purple clusters consist of 6 and 5 keywords, respectively, focusing on the preparation processes and methods of PEMs, as well as the development of emerging materials such as polyoxadiazoles, 99 sulfonated PI, 100,101 MoS 2 -NiO-Co 3 O 4 filled chitosan, 102 and branched polymer materials. 103In summary, the different clusters represent research on different aspects of PEMs, including materials, properties, mechanisms, and optimization.This lays the foundation for their application in FCs.
Figure 14B is a time-varying deduction of the mapping network of PEMs.The color of the keywords in the figure represents the average year of publication.It can be observed that earlier research by most scientists mainly focused on the unique properties of the exchange membrane itself, such as sulfonation, poly(arylene ether ketone), polyelectrolytes, copolymers, morphology, and water uptake.Subsequently, there was a widespread study of PEMs and PEMFCs, such as PEM, ionomers, FCs, conductivity, electrospinning, and high-temperature PEM.These studies illustrate the important role and related mechanisms of PEMs in FCs.In recent years, based on the research foundation of PEMs, people have begun to focus on the development of more efficient and stable new PEMs, further emphasizing their practical significance in sustainable energy applications and environmental protection.Relevant terms include metal-organic frameworks, polybenzimidazole, polymer electrolyte membranes, chitosan, and mechanical properties.The research hotspots in different periods also reflect the process of people's understanding of PEMs changing.By analyzing the inherent relationships of keywords, the study reveals their role in enhancing FC performance and enhancing strategies.
In summary, the performance of PEMs has a significant impact on FC performance and application.With the development of science and technology, some requirements have been proposed for PEMs used in FCs, which are as follows.(1) High proton conductivity: the proton conductivity of the PEM directly affects the efficiency of the FC.High proton conductivity can ensure that protons can be quickly transported within the membrane, 104 thereby improving the output power and efficiency of the cell.(2) Low electronic conductivity: lower electronic conductivity can prevent electron leakage and reduce the loss of electrochemical reactions.(3) Good chemical stability: PEMs should have good chemical stability to avoid corrosion or degradation in FCs. 105,106Poorly chemically stable PEMs may be affected by fuel and oxygen, leading to membrane degradation, affecting the life and stability of FCs.(4) Appropriate thermal stability: FCs generate high temperatures during operation, so PEMs need to have good thermal stability to avoid failure at high temperatures. 107(6) Good mechanical strength and flexibility: PEMs need to withstand the vibration and deformation of the FC system while maintaining the continuity of proton conduction channels. 114,115 (7) Good durability: poorly durable PEMs may be affected by factors such as oxidation, degradation, and corrosion, 116,117 leading to membrane failure, affecting the life and performance stability of FCs.Therefore, PEMs with good performance can ensure the high efficiency, long life, and stable operation of FC systems.Future development will inevitably require the intelligent and sustainable development of materials.

CHALLENGES AND POTENTIAL DEVELOPMENTS IN PEMs
The PEM is a core component of FCs and directly affects their performance and stability.Therefore, research on PEM has significant theoretical and practical significance.Figure 15 presents the main challenges and development directions in the research of PEM-FCs.
Currently, the main challenges faced by PEMs are listed as follows: (1) Price and sustainability of PEMs The price of PEMs is an important challenge that hinders the commercial application of FC technology.Traditional PEM materials, such as polytetrafluoroethylene (PTFE), are expensive and have high manufacturing costs.One solution to this problem is to develop low-cost alternative materials, such as polystyrene and PEEK.These materials have lower prices and can be obtained through simple manufacturing processes.Additionally, improving the lifespan and stability of PEMs can reduce the maintenance and replacement costs of FC systems, thereby lowering overall costs.
(2) Stability of PEMs in high-temperature and high-humidity environments The PEMs are prone to degradation and failure in high-temperature and high-humidity environments, which limits the operating temperature and humidity range of FCs.Developing PEM materials with better thermal stability and moisture resistance is key to addressing this issue.For example, organic-inorganic hybrid materials exhibit good thermal and humidity stability and can be used to prepare PEMs with higher operating temperature and humidity ranges.Furthermore, optimizing the structure and morphology of PEMs, such as increasing crosslinking and adjusting porosity, can enhance their stability.
(3) Compatibility of PEMs with other components The PEMs need to have good contact and compatibility with other components, such as electrodes, to ensure the normal operation of FC systems.However, compatibility issues between different materials can lead to poor contact and hindered electron conduction.Developing PEM materials with good compatibility is crucial.For example, adjusting the surface properties of PEMs, such as introducing specific functional groups and altering the material's hydrophilicity, can improve their compatibility with other components.
The performance and stability of PEMs directly impact the efficiency and lifespan of FCs.Therefore, improving the proton conductivity, durability, and stability of PEMs, as well as developing new PEM materials, is an important research direction in the field of PEMs.Specific research directions for the future can be explored in the following aspects: (1) Enhancement of proton conductivity of PEMs Proton conductivity is one of the key indicators of PEMs, directly determining the power output and efficiency of FCs.Currently, some commonly used PEMs already have high proton conductivity, but there are still limitations.Therefore, researchers are actively searching for new proton exchange groups and improving the conduction mechanisms to enhance the proton conductivity of PEMs.For example, introducing new proton exchange groups such as phosphoric acid groups and boronic acid groups can enhance the proton conductivity of PEMs.Additionally, optimizing the structure and morphology of PEMs, such as adjusting the porosity of the membrane and increasing the concentration of proton exchange groups, can also improve proton conductivity.
(2) Improvement of durability and stability of PEMs During long-term use, PEMs can be affected by factors such as oxidation, hydrolysis, and dehydration, leading to performance degradation or even failure.Therefore, improving the durability and stability of PEMs is a key focus of research.One approach is to improve the material and structure of PEMs to enhance their resistance to oxidation, hydrolysis, and dehydration.Another approach is to develop new PEM materials with better stability, such as organic-inorganic hybrid materials and polymer composites.These materials exhibit improved thermal stability and durability, which can enhance the lifespan and stability of PEMs.
(3) Development of novel PEM materials Currently, commonly used PEM materials include PTFE, polystyrene, and PEEK.However, these materials still have limitations in terms of proton conductivity, durability, and stability.Therefore, researchers are actively developing novel PEM materials.For example, organic-inorganic hybrid materials combine the advantages of organic polymers and inorganic materials, exhibiting high proton conductivity and stability.Polymer composites, by combining the PEM with other functional materials such as nanoparticles and carbon nanotubes, can further enhance the performance and functionality of PEMs.
(4) Optimization of preparation processes and methods for PEMs The preparation processes and methods of PEMs have a significant impact on their performance and stability.Therefore, optimizing the preparation processes and methods for PEMs is a key aspect of research.For example, using different polymerization methods, adjusting reaction conditions, and controlling the degree of crosslinking can influence the structure, morphology, and distribution of proton exchange groups in PEMs.Additionally, employing new preparation methods such as solution impregnation and membrane casting can improve the efficiency and consistency of PEM preparation.

Figure 3 .
Figure 3. Schematic diagram of the process of analysis approaches

Figure 4 .
Figure 4. Trend of scientific and technical papers published on PEM-FCs from 1995 to 2022

Figure 6 .
Figure 6.Geographic distribution of papers on PEM-FC in different countries

Figure 7 .
Figure 7. Network map of national partnerships for the study of PEM-FC

Figure 9 .
Figure 9. Collaborative network diagram of the authors on the PEM-FC study

Figure 10 .
Figure 10.Visualization of the relationship between cited references

Figure 11 .
Figure 11.Visualization of relationships for journal co-citation analysis

Figure 13 .
Figure 13.Visualization of the relationship between authors in the co-citation analysis

Figure 15 .
Figure 15.Challenges and development directions for PEMs used in FCs Figure4illustrates the research output in the field of PEM-FCs from 1995 to 2022.It can be observed that, from 1995 to 2002, only four relevant research papers were published, indicating a relatively low level of interest and recognition in the research value of PEM-FCs during this period, which can be referred to as the initiation phase.In the following years, a growing interest in PEMs for FCs emerged, and, by 2009, the number of papers reached 19, representing an increase of 15 papers compared to 2003, nearly quadrupling the research output.This phase can be referred to as the growth period.After the initiation and growth phases, an average of 15.75 papers were published annually from 2010 to 2017, indicating a relatively stable development period.In 2018, the number of papers suddenly jumped to a higher level, reaching a peak of 26, which is the highest value in recent years.Subsequently, these years can be defined as the second development phase.After experiencing two rapid development phases, the topic of PEM-FCs has gained widespread attention among researchers, and it is believed that, in the coming years, continuous development and significant research achievements will be made.It should be noted that the relatively low number of published papers in 2022 is due to the absence of data for December (the search for this topic was conducted on December 4, 2022, and the database displayed the last updated data as November 30, 2022).

Table 1 .
Sources of relevant publications on PEM-FC research Figure 5. Distribution of relevant disciplines in PEM-FC research

Table 2 .
Academic institutions with the highest number of published papers on PEM-FC

Table 3 .
Top 15 authors of published papers