Bibliometric Analysis of the Mass Transport in a Gas Di ﬀ usion Layer in PEM Fuel Cells

: The growth trend of publications in the ﬁeld of Proton Exchange Membrane Fuel Cell (PEMFC) was analyzed using bibliometric techniques to the identiﬁcation of the areas with signiﬁcant development and the orientations that have guided the research on energy cells. This study extracted the data from Scopus and Web of Science (WoS) databases to compare the bibliometric indicators of the published productions. In spite of bibliometric analysis advantages to knowing about the trends in a study area, this research requires methods to support the investigation process through the selection of a relevant bibliographic portfolio. This study applied the Methodi Ordinatio that provides a new approach to achieve it. A proposed list of the articles ranked by InOrdinatio is presented to compose the ﬁnal portfolio. The obtained results in the research sub-theme of the Mass Transport in Gas Di ﬀ usion Layer (GDL) conﬁrm the complexity in the study area by presenting erratic patterns of exponential growth. United States, China, and Japan are the leading countries on PEMFC publications. These countries have in common a strong spending by the business sector for R&D, and their gross domestic product is greater than 2%.


Introduction
The use of fossil fuels for energy generation damage the environment due to the high emission of pollutants such as CO 2 , NO 2 and SO 2 [1,2]. Therefore, there are worldwide efforts to develop alternative energy sources to reduce the greenhouse effect and other problems derived from air pollution [3][4][5][6].
Hydrogen is a zero-emissions potential vector of energy that has been studied over the last 30 years. In this sense, alternative technologies have been developed in the generation of electricity using hydrogen like the so-called Proton Exchange Membrane Fuel Cells (PEMFC) [7,8].
Fuel cells are devices that produce electricity through the electrochemical reaction of oxidation, where the used fuel is hydrogen, and the oxidant is pure oxygen or air. The main elements of a PEMFC are the cathode, the anode and the electrolyte membrane. Hydrogen fuel is provided to the anode, and the oxygen is transferred across the cathode. The oxidation of hydrogen produces protons and electrons. Protons pass over the electrolyte membrane, and electrons are transported to the cathode through an external circuit. During the reaction process, the fuel cell produces electricity, water, and heat [9][10][11]. Research on the issue of cells have become popular because they have demonstrated operator was determined as ("Fuel Cell *") AND (PEM OR "Proton Exchange Membrane" OR "Polymer Electrolyte Membrane"). A total of 30,820 articles were found in Scopus and 31,874 in WoS.
The analysis of the recurrence of keywords in the published papers can be used to know the research trends in certain themes of the PEMFC. Through the tool VOSviewer [36,37] the map of co-occurrence of text was made ( Figure 1). The words "Cell", "Membrane", "Catalyst" and "Model" were the most recurrently appearing in the titles and summaries of the publications. This analysis confirmed that GDL and mass transport topics appeared among the first 50 most used keywords, and support this research. The analysis of the recurrence of keywords in the published papers can be used to know the research trends in certain themes of the PEMFC. Through the tool VOSviewer [36,37] the map of cooccurrence of text was made ( Figure 1). The words "Cell", "Membrane", "Catalyst" and "Model" were the most recurrently appearing in the titles and summaries of the publications. This analysis confirmed that GDL and mass transport topics appeared among the first 50 most used keywords, and support this research. The selection of a bibliographic portfolio to support the mass transport in a gas diffusion layer in PEM fuel cell research was conducted using the Methodi Ordinatio. Figure 2 shows the nine phases of this methodology.  The selection of a bibliographic portfolio to support the mass transport in a gas diffusion layer in PEM fuel cell research was conducted using the Methodi Ordinatio. Figure 2 shows the nine phases of this methodology. The analysis of the recurrence of keywords in the published papers can be used to know the research trends in certain themes of the PEMFC. Through the tool VOSviewer [36,37] the map of cooccurrence of text was made ( Figure 1). The words "Cell", "Membrane", "Catalyst" and "Model" were the most recurrently appearing in the titles and summaries of the publications. This analysis confirmed that GDL and mass transport topics appeared among the first 50 most used keywords, and support this research. The selection of a bibliographic portfolio to support the mass transport in a gas diffusion layer in PEM fuel cell research was conducted using the Methodi Ordinatio. Figure 2 shows the nine phases of this methodology. The next sequence describes the implementation of Methodi Ordinatio to achieve the objective of this study: 1. Phase 1. The intention of research: The purpose of this research is "Mass Transport in a Gas Diffusion Layer in PEM Fuel Cells". The next sequence describes the implementation of Methodi Ordinatio to achieve the objective of this study: 1.
Phase 1. The intention of research: The purpose of this research is "Mass Transport in a Gas Diffusion Layer in PEM Fuel Cells". 2.
Phase 2. Preliminary research with keywords: The first approach for the search was "PEM" AND "Fuel Cell" AND "Mass Transport" AND "Gas Diffusion Layer". This test aids in defining a set of keywords for the final search through analysis of relevant words in the articles. 3.
Phase 4. Final search in the databases: The string search was comprised of words using wildcard symbols and Boolean operators. Final keywords query was defined as: ("Fuel Cell *") AND ("PEM" OR "Proton Exchange Membrane OR Polymer Electrolyte Membrane") AND ("GDL" OR "Gas Diffusion Layer") AND ("Mass Transfer" OR "Transport Phenomena" OR "Mass Transport") AND ("Model *" OR "Simulation"). The results obtained were exported in csv and bibText format for later analysis with tools such as VOSviewer and reference manager Mendeley. A total of 363 articles were found in Scopus and 416 in WoS. 5.
Phase 5. Filtering procedures: A gross portfolio was obtained in the previous phase. Afterward, a filtering procedure is necessary to obtain the most relevant articles. Figure 3 shows the four filters applied: (i) Eliminate duplicates, remaining 525. (ii) Keep only with works categorized as article, 413 remain. (iii) The research period was set up from 2008 to 2019. After the filter, 327 articles were left. (iv) Apply the index InOrdinatio, items with an InOrdinatio equal or greater than 72 were selected to comprise the final portfolio. 6.
Phase 6. Identification of the impact factor, the year of publication, and the number of citations: Through the results extracted from both databases (WoS, Scopus), the year of publication and the number of citation criterion was acquired. The impact factor of the journal was obtained from the Clarivate Analytics Incites Journal Citation Reports [38] or the Scopus Source List [39] web site. 7.
Phase 7. Ranking the papers using the InOrdinatio: After the phases 1 to 6 were conducted, the Methodi Ordinatio applies the index InOrdinatio using the Equation (1) presented by Pagani et al. (2015). This coefficient considers the total of citations ( Ci), the normalized impact factor (IF/1000), a weighting factor (∝) whose value is set by the researcher between a range 1 to 10, the research year, and the publication year to ranking the articles.
where: Dynamics". After analyzing them, a set of keywords was defined for the final search ( Figure 3). WOS and Scopus were selected as databases. 4. Phase 4. Final search in the databases: The string search was comprised of words using wildcard symbols and Boolean operators. Final keywords query was defined as: ("Fuel Cell *") AND ("PEM" OR "Proton Exchange Membrane OR Polymer Electrolyte Membrane") AND ("GDL" OR "Gas Diffusion Layer") AND ("Mass Transfer" OR "Transport Phenomena" OR "Mass Transport") AND ("Model *" OR "Simulation"). The results obtained were exported in csv and bibText format for later analysis with tools such as VOSviewer and reference manager Mendeley. A total of 363 articles were found in Scopus and 416 in WoS. 5. Phase 5. Filtering procedures: A gross portfolio was obtained in the previous phase. Afterward, a filtering procedure is necessary to obtain the most relevant articles. Figure 3 shows the four filters applied: i) Eliminate duplicates, remaining 525. ii) Keep only with works categorized as article, 413 remain. iii) The research period was set up from 2008 to 2019. After the filter, 327 articles were left. iv) Apply the index InOrdinatio, items with an InOrdinatio equal or greater than 72 were selected to comprise the final portfolio. 6. Phase 6. Identification of the impact factor, the year of publication, and the number of citations: Through the results extracted from both databases (WoS, Scopus), the year of publication and the number of citation criterion was acquired. The impact factor of the journal was obtained from the Clarivate Analytics Incites Journal Citation Reports [38] or the Scopus Source List [39] web site. 7. Phase 7. Ranking the papers using the InOrdinatio: After the phases 1 to 6 were conducted, the Methodi Ordinatio applies the index InOrdinatio using the Equation (1) presented by Pagani et al. (2015). This coefficient considers the total of citations (∑ ), the normalized impact factor As the goal of this paper is the use of bibliometric techniques and inOrdinatio equation to evidence the support of these tools finding an adequate portfolio with the most pertinent articles on the study of "GDL and Mass Transport" fields; the results of the phase nine will be published in a future paper which presents the findings for the theme.

Results and Discussion
The obtained results show the tendency growth in PEMFC; this section describes a comparison of the last ten years of the publication percentage of the mass transport in gas diffusion layer. Likewise, a relevant suggested portfolio was obtained through the index InOrdinatio.

Bibliometric Analysis of PEM Fuel Cells
The goal of the first search was researching articles containing "PEM" and "Fuel cell(s)" in the title, abstract, and keywords published until 2019. The total of published articles found between the years 1970 to 2019 was 30,856 in Scopus, and 31,901 in WOS. The first article indexed in Scopus was published in the Society of Automotive Engineers (SAE) Technical Papers in 1969. The highest scientific production years were 2011 with 2231 publications in Scopus and the year 2018 with 2244 in WoS. Clearly, Figure 4 shows the ascending growth of the number of papers with a sigmoidal behavior for the two databases where it can be observed the stages defined by the Price Law (precursors, exponential, linear, collapse) [40].  During the 50 years of scientific production on the subject of PEMFC, 71,458 authors have contributed to the generation of 31,901 publications according to the results generated in the WOS query. Figure 5 shows the growth trend of the scientific population between the period of 1969 and 2009, reaching the highest amount in 2014 with 10,256 authors. This behavior demonstrates the boom in the issue of fuel cells, considering that in recent years, the maturity in knowledge has been reached by presenting more gradual growth.  Table 1 shows the list of the 30 countries with the highest contribution in the research of the PEMFC; United States, China, and Japan together have almost 50% of the production of the articles worldwide generated. The United States ranks the first place with a total of 6871 publications, followed by China with 5284 and in the third place Japan with 2431. The results are expected since national policies supported the investment made in research and development in these countries to obtain sufficient funds for the growth of fuel cells during these years [23,[41][42][43]. Likewise, the investment expenditure in the first three leading countries represents more than 2% of the gross domestic product [44]. During the 50 years of scientific production on the subject of PEMFC, 71,458 authors have contributed to the generation of 31,901 publications according to the results generated in the WOS query. Figure 5 shows the growth trend of the scientific population between the period of 1969 and 2009, reaching the highest amount in 2014 with 10,256 authors. This behavior demonstrates the boom in the issue of fuel cells, considering that in recent years, the maturity in knowledge has been reached by presenting more gradual growth.  During the 50 years of scientific production on the subject of PEMFC, 71,458 authors have contributed to the generation of 31,901 publications according to the results generated in the WOS query. Figure 5 shows the growth trend of the scientific population between the period of 1969 and 2009, reaching the highest amount in 2014 with 10,256 authors. This behavior demonstrates the boom in the issue of fuel cells, considering that in recent years, the maturity in knowledge has been reached by presenting more gradual growth.  Table 1 shows the list of the 30 countries with the highest contribution in the research of the PEMFC; United States, China, and Japan together have almost 50% of the production of the articles worldwide generated. The United States ranks the first place with a total of 6871 publications, followed by China with 5284 and in the third place Japan with 2431. The results are expected since national policies supported the investment made in research and development in these countries to obtain sufficient funds for the growth of fuel cells during these years [23,[41][42][43]. Likewise, the investment expenditure in the first three leading countries represents more than 2% of the gross domestic product [44].  Table 1 shows the list of the 30 countries with the highest contribution in the research of the PEMFC; United States, China, and Japan together have almost 50% of the production of the articles worldwide generated. The United States ranks the first place with a total of 6871 publications, followed by China with 5284 and in the third place Japan with 2431. The results are expected since national policies supported the investment made in research and development in these countries to obtain sufficient funds for the growth of fuel cells during these years [23,[41][42][43]. Likewise, the investment expenditure in the first three leading countries represents more than 2% of the gross domestic product [44].  States  6871  4627  16  Malaysia  346  205  2  China  5284  7334  17  Brazil  332  263  3  Japan  2431  1745  18  Mexico  316  261  4  South Korea  2339  2239  19  Australia  302  262  5  Canada  2120  1686  20  Switzerland  299  262  6  Germany  1939  1558  21  Thailand  292  210  7  France  1502  1130  22  Russian Federation  281  295  8  India  1211  993  23  Singapore  266  221  9  Taiwan  1143  971  24  Sweden  221  195  10  Italy  1125  886  25  Greece  217  165  11 United The worldwide distribution of leader countries is presented in Figure 6. The Asian continent is the highest publisher with 14,220 items, Europe is the continent with more participating countries, and South America and Africa have the lowest contribution in publications and publisher countries. The worldwide distribution of leader countries is presented in Figure 6. The Asian continent is the highest publisher with 14,220 items, Europe is the continent with more participating countries, and South America and Africa have the lowest contribution in publications and publisher countries.

Mass Transport in a Gas Diffusion Layer Bibliographic Portfolio
Once the magnitude of the research in the PEMFC topic was determined through bibliometric techniques, two queries were applied to find the Mass Transport in a Gas Diffusion Layer gross portfolio following the Methodi Ordinatio. The searching string used in the Scopus was: ( Table 2 shows the number of papers discarded with the four criteria established in the Ordinatio Method phase five. After the filtering process was executed, 39 most relevant articles remained. Table 3 presents the ranking list with an InOrdinatio index α = 7 of the final portfolio. Two additional columns were included in this table: The position of the articles with α = 5 and α = 10.    Table 3 shows how the position of the articles changes in the ranking list, depending on the alpha used in the InOrdinatio equation. An alpha near to one (α = 1) generates portfolios with classic articles, but if recent papers are more important for the research, then the value of alpha should be closer to 10. One example is the case of Ramasamy´s article [50] which has the 6th position with an alpha equal to 7, but its position changes to 54 applying an alpha equal to 10. In addition, the researchers have the option to select a variable number of articles for the index generated by InOrdinatio and comprise the final portfolio.
The list of 39 articles presented in this study is a no restrictive proposal. Its functionality is to support research and suggest the application of a multicriteria method like InOrdinatio to achieve a relevant and pertinent bibliographic portfolio. The list helps the researchers in their investigation process during the systematic analysis of literature, making the work easier.

Relevant Subjects in the Development of "GDL and Mass Transport" Research
In this section, a bibliometric study was executed based on the data of 412 articles corresponding to the bibliographic portfolio after applying filter number two. It presents quantitative information to complement knowledge about the development of the research area.
The analysis of keyword co-occurrence through VOSviewer software displays the most frequent topics used in mass transport in gas diffusion layer research. Figure 7 shows the themes developed over the years. The keywords "computational fluid dynamics", "models", "simulation" demonstrate the tendency of applying computational resources to support new studies in the last years. researchers have the option to select a variable number of articles for the index generated by InOrdinatio and comprise the final portfolio. The list of 39 articles presented in this study is a no restrictive proposal. Its functionality is to support research and suggest the application of a multicriteria method like InOrdinatio to achieve a relevant and pertinent bibliographic portfolio. The list helps the researchers in their investigation process during the systematic analysis of literature, making the work easier.

Relevant Subjects in the Development of "GDL and Mass Transport" Research
In this section, a bibliometric study was executed based on the data of 412 articles corresponding to the bibliographic portfolio after applying filter number two. It presents quantitative information to complement knowledge about the development of the research area.
The analysis of keyword co-occurrence through VOSviewer software displays the most frequent topics used in mass transport in gas diffusion layer research. Figure 7 shows the themes developed over the years. The keywords "computational fluid dynamics", "models", "simulation" demonstrate the tendency of applying computational resources to support new studies in the last years.
A test with 1676 index keywords was realized. Only 140 (8.35%) of keywords met the threshold of five or more occurrences, 40 (2.38%) were used four times, 83 (4.95%) keywords appeared thrice, and 1413 (84.30%) keywords were used once or twice. Over 25 years of study from 1994-2019, 932 authors participated on publications on mass transport in gas diffusion layer. Figure 8 co-authorship map displays 106 (11.37%) authors with production on at least three papers. There are 34 clusters of collaboration and only one of them is composed of the largest number of elements in the groups with nine items. Likewise, 10 clusters are comprised of one author, and the most extensive collaboration network of authors consists of 51 authors corresponding to 5.47% of the sample. This behavior reveals a lack of interrelationship between the researcher groups around the world working on the theme.
The most cited author is Jeff Gostick, with six articles and 632 cites, while the most productive author is Yan Wei-Mon with 12 articles and 278 citations. A test with 1676 index keywords was realized. Only 140 (8.35%) of keywords met the threshold of five or more occurrences, 40 (2.38%) were used four times, 83 (4.95%) keywords appeared thrice, and 1413 (84.30%) keywords were used once or twice.
Over 25 years of study from 1994-2019, 932 authors participated on publications on mass transport in gas diffusion layer. Figure 8 co-authorship map displays 106 (11.37%) authors with production on at least three papers. There are 34 clusters of collaboration and only one of them is composed of the largest number of elements in the groups with nine items. Likewise, 10 clusters are comprised of one author, and the most extensive collaboration network of authors consists of 51 authors corresponding to 5.47% of the sample. This behavior reveals a lack of interrelationship between the researcher groups around the world working on the theme. A total of 412 papers classified as articles were published during a period of 25 years. Table 4 shows the annual publications in the sub-theme of the gas diffusing layer beginning in 1994. However, there are seven years without the appearance of a new investigation document. This behavior reveals a scarce growth of the research area through the years, and the tendency has practically remained between 22 to 32 papers produced since the year 2006.  1  2011  22  2001  2  2012  29  2003  4  2013  18  2004  13  2014  24  2005  12  2015  26  2006  26  2016  28  2007  28  2017  26  2008  25  2018  29  2009  34  2019  32  2010  33  TOTAL  412 United States, China, and Canada are the most significant contributors to the Mass Transport in Gas Diffusion Layers in PEMFC with 90, 63 and 51 articles respectively. Additionally, they are the most cited, showing their high impact on the research. Around the world, only 44 (22.68%) countries have been conducting studies through the period of 1994 to 2019. The co-authoring collaboration between countries is shown in the network map of Figure 9, where, once again, China, United States and Canada present the greatest partnerships with other countries. The most cited author is Jeff Gostick, with six articles and 632 cites, while the most productive author is Yan Wei-Mon with 12 articles and 278 citations.
A total of 412 papers classified as articles were published during a period of 25 years. Table 4 shows the annual publications in the sub-theme of the gas diffusing layer beginning in 1994. However, there are seven years without the appearance of a new investigation document. This behavior reveals a scarce growth of the research area through the years, and the tendency has practically remained between 22 to 32 papers produced since the year 2006.  1994  1  2011  22  2001  2  2012  29  2003  4  2013  18  2004  13  2014  24  2005  12  2015  26  2006  26  2016  28  2007  28  2017  26  2008  25  2018  29  2009  34  2019  32  2010  33   TOTAL  412 United States, China, and Canada are the most significant contributors to the Mass Transport in Gas Diffusion Layers in PEMFC with 90, 63 and 51 articles respectively. Additionally, they are the most cited, showing their high impact on the research. Around the world, only 44 (22.68%) countries have been conducting studies through the period of 1994 to 2019. The co-authoring collaboration between countries is shown in the network map of Figure 9, where, once again, China, United States and Canada present the greatest partnerships with other countries.  Table 5 shows a comparison of the last ten years of the publication percentage of the mass transport in gas diffusion layer and the PEMFC area. It reveals the low contribution of articles again through the years on this subarea.

Leading Journals of PEM Fuel Cell Publications
This study analyzed which journals are the most commonly used by the researchers to disclose their works. The 31,901 documents of the WoS query for the PEMFCs were published in a total of 1676 journals. Journal of Power Sources is the leading journal in publications with 3640 papers, followed by the International Journal of Hydrogen Energy with 3203 documents. Table 6 lists the number of publications of the most influential journals according to WoS for each of the two search levels (PEMFC, Mass Transport in GDL). The Journal of Power Sources, the  Table 5 shows a comparison of the last ten years of the publication percentage of the mass transport in gas diffusion layer and the PEMFC area. It reveals the low contribution of articles again through the years on this subarea.

Leading Journals of PEM Fuel Cell Publications
This study analyzed which journals are the most commonly used by the researchers to disclose their works. The 31,901 documents of the WoS query for the PEMFCs were published in a total of 1676 journals. Journal of Power Sources is the leading journal in publications with 3640 papers, followed by the International Journal of Hydrogen Energy with 3203 documents. Table 6 lists the number of publications of the most influential journals according to WoS for each of the two search levels (PEMFC, Mass Transport in GDL). The Journal of Power Sources, the International Journal of Hydrogen Energy, and the Journal of the Electrochemical Society are located in the first three places, being thus preferred by authors for the publication of their works. The analysis of the most influential journals in the PEMFC field was generated based on the citations ( Figure 10) using the VOSviewer bibliometric mapping tool. The Journal of Power Sources appears again in the first place, which allows us to confirm the importance of this journal because it took the preference of the authors to select it as the first publication option. International Journal of Hydrogen Energy, and the Journal of the Electrochemical Society are located in the first three places, being thus preferred by authors for the publication of their works. The analysis of the most influential journals in the PEMFC field was generated based on the citations ( Figure 10) using the VOSviewer bibliometric mapping tool. The Journal of Power Sources appears again in the first place, which allows us to confirm the importance of this journal because it took the preference of the authors to select it as the first publication option. According to the classification performed by Claritive analytics in the Journal Citation Reports of the year 2018, [38,82] the Journal of Power Sources appears with an impact factor of 7.467 points, while the International Journal of Hydrogen Energy with 4.084 and the Journal of the Electrochemical Society with 3.12 points. It is important to mention that these journals are not the ones that have the highest impact factor in the ranking, but they are the main ones in terms of the number of the received citations. According to the classification performed by Claritive analytics in the Journal Citation Reports of the year 2018, [38,82] the Journal of Power Sources appears with an impact factor of 7.467 points, while the International Journal of Hydrogen Energy with 4.084 and the Journal of the Electrochemical Society with 3.12 points. It is important to mention that these journals are not the ones that have the highest impact factor in the ranking, but they are the main ones in terms of the number of the received citations.
The impact factor is one of the five criteria that Methodi Ordinatio considers to ranking the articles, and it does not have a high weight in the inOrdinatio equation. Nevertheless, the researchers decided to publish in a journal based on the impact factor as a guarantee of visibility and seriousness in the investigation process. The analysis of bibliometric information provides the support to make a decision for the best options to release the results of the research work.

Conclusions
This study provided an overview analysis of the characteristics of PEM fuel cell literature using bibliometric methods to retrieve information from two major databases: Scopus and Web of Science. Bibliometrics cannot asses the research quality directly, but it is a sensor of the area of productivity since it provides indicators as the number of worldwide publications, the most cited articles, the most referenced authors, and the leading publisher countries.
Over 50 years, a total 31,901 PEMFC documents were published, the first article of PEMFC was in 1969. During this period, the growth trend has displayed the precursor, exponential, and linear phases established by the Price Law. The leading countries to productivity in this area are the United States, China, and Japan. These countries have, in common, strong spending by the business sector for R&D, and their gross domestic product is greater than 2%. There is a broad difference between the amounts spent in purchasing power parity dollars of first countries in respect to the others.
The qualitative nature of the papers is not easy to know by bibliometric indicators that allow responding to why these areas have not achieved a plateau in knowledge. However, it alerts us about the changes that occur in terms of the amount of productivity. Therefore, researchers need to determine the relevance of new issues to achieve the evolution of knowledge towards the innovation in this area.
The influence of a publication on the scientific community has been related frequently to the journal impact factor. Nevertheless, this factor is not enough to evaluate the relevance of an article that can be selected to compose the bibliographic portfolio for new research. Nowadays, the scientific population requires new strategies to facilitate selection criteria.
In regard to this situation, the methodology Methodi Ordinatio is an adequate framework that allows us to rank articles with more critical rigor, as it takes into account three article selection criteria (impact factor, year, and number of citation), and provides an organized list of relevant publications. The researchers can decide the importance degree for the year of publication by pondering alpha. This study used the Methodi Ordinatio to demonstrate the present benefits in initiating new research and decreasing the time gap to comprise a bibliographic portfolio.