sustainability

Article Fuel-Cell Electric Vehicles: Plotting a Scientific and Technological Knowledge Map

Izaskun Alvarez-Meaza * , Enara Zarrabeitia-Bilbao , Rosa Maria Rio-Belver and Gaizka Garechana-Anacabe Foresight, Technology and Management (FTM) Group, Department of Industrial Organization and Management Engineering, University of the Basque Country, Pl. Ingeniero Torres Quevedo, 48013 Bilbao, Spain; [email protected] (E.Z.-B.); [email protected] (R.M.R.-B.); [email protected] (G.G.-A.) * Correspondence: [email protected]; Tel.: +34-946-014-245



Received: 23 February 2020; Accepted: 13 March 2020; Published: 17 March 2020


Abstract: The fuel-cell electric vehicle (FCEV) has been defined as a promising way to avoid road transport greenhouse emissions, but nowadays, they are not commercially available. However, few studies have attempted to monitor the global scientific research and technological profile of FCEVs. For this reason, scientific research and technological development in the field of FCEV from 1999 to 2019 have been researched using bibliometric and patent data analysis, including network analysis. Based on reports, the current status indicates that FCEV research topics have reached maturity. In addition, the analysis reveals other important findings: (1) The USA is the most productive in science and patent jurisdiction; (2) both Chinese universities and their authors are the most productive in science; however, technological development is led by Japanese car manufacturers; (3) in scientific research, collaboration is located within the tri-polar world (North America–Europe–Asia-Pacific); nonetheless, technological development is isolated to collaborations between companies of the same automotive group; (4) science is currently directing its efforts towards hydrogen production and storage, energy management systems related to battery and hydrogen energy, Life Cycle Assessment, and greenhouse gas (GHG) emissions. The technological development focuses on technologies related to electrically propelled vehicles; (5) the International Journal of Hydrogen Energy and SAE Technical Papers are the two most important sources of knowledge diffusion. This study concludes by outlining the knowledge map and directions for further research.

Keywords: fuel cell electric vehicle; bibliometric analysis; patent analysis

1. Introduction The increase in energy use and related emissions was generated by a higher demand for heat from the residential and commercial sectors and road transport demand; consequently, road transport greenhouse gas (GHG) emissions increased for the second subsequent year, continuing the upward trend in emissions that started in 2014 [1]. The electrification of mobility is an essential element in a wider strategy for achieving reduced greenhouse gas emissions [2]. During the previous century, the automotive industry transformed society, bringing new technologies to the market that enhanced their internal combustion engine vehicles, such as global electric vehicles, which are known as one of the most hopeful alternatives for lowering transport-sector carbon dioxide emissions [3,4]. An Electric Vehicle (EV) is a road vehicle that includes electric propulsion. With this definition in mind, EVs may include battery electric vehicles (BEV), hybrid electric vehicles (HEV), and fuel-cell electric vehicles (FCEV) [5]. As sustainable products, FCEVs bring hope for solving several mobility-related problems, as they have no local emissions [6]. One of the most promising ways to achieve an ideal zero-emissions replacement is to use cleanly produced electricity from non-fossil fuels, such as hydrogen, using

Sustainability 2020, 12, 2334; doi:10.3390/su12062334 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 2334 2 of 25 fuel-cell technology [7]. Although FCEVs are promising ways to avoid emissions, both technologies are far from being profitable for car manufacturers [6]. In addition, the European Commission [8] set a target for 40% of new cars and vans to be zero- or low-emission vehicles by 2030. Furthermore, in the European Strategic Energy Technology Plan, hydrogen and fuel-cell technologies are identified as the key technologies for achieving GHG reduction targets by 2050 [9,10], and in the European Community Research Program, electromobility is a priority. Here, electric vehicle policy focuses mainly on technology optimization and market development, setting future challenges concerning battery and supercapacitor durability, and charging infrastructure, among others [11]. In turn, the collaborative research and development (R&D) technological projects in Europe are funded by the European Commission [12], and one of the main priorities for transport research and innovation in Horizon 2020 (H2020) is making transport more sustainable [13]; for this reason, the European Commission is promoting clean transport, both for electric vehicles by investing in electromobility initiatives and for FCEVs or hydrogen FCEVs, enabling their commercial development by 2020 [11]. In addition, Edwards et al. [14] performed a roadmap review of deployment status and targets for fuel-cell applications, such as fuel-cell vehicles, and the annual sales forecast was between 0.4–1.87 million during 2020–2025. In order to help all actors involved in clean transport, it is useful to understand how scientific research is evolving and whether it is having an impact on its technological development. Bibliometric techniques open the door to a fuller understanding of the scientific research carried out on FCEVs, differing from a conventional literature review: The bibliometric method supplies an innovative, objective perspective through reliable, quantitative processes, and has been broadly used in scientific research as an analytical tool to provide assistance to scholars with a general comprehension of typical research topics [15]. According to Garousi [16], bibliometric analysis is a well-established method used to measure publications in a scientific research area, and assessing trends and the value of research is becoming increasingly important [17,18]. According to different authors, bibliometrics is approached in various ways and is defined as a research method, or research technique, that allows scientific literature representing extensive global data sets and reliable data to be analyzed and quantitatively measured [19–21]. We can take this a little further and introduce the term “scientometrics”, defined by Nowak [19]: “Scientometrics focuses on the processes occurring in science” [21]. Scientometrics is the quantitative study of research transfer, and its analysis makes it possible to capture and map scientific knowledge [21–23]. Hence, its main objective is to aid the analysis of emerging trends in the knowledge domain [24]; in addition, knowledge mapping and visualization are meaningful fields of scientometrics [25]. In the main databases, such as Web of Science and Scopus, there is no bibliometric analysis of the “FCEV” research field. However, other bibliometric analyses have been carried out in research fields related to fuel-cell technology. Kang et al. [26] define a diffusion model based on bibliometric analysis applied to fuel-cell technologies. Cindrella et al. [27] present a bibliometric analysis about the field of fuel cells in general from 1992 to 2011, offering a comprehensive overview of trending publications, journals, and countries, and identifying research hotspots through keywords. A similar overview was conducted by Yonoff et al. [28], identifying research trends in Proton Exchange Membrane Fuel Cells (PEMFCs). Related to energy and fuel research in China, Chen et al. [29] conducted a bibliometric analysis and, as a result, hydrogen and fuel cells are among the energy research priorities. Bibliometric studies on energy materials related to hydrogen are also relevant, such as the analysis of sodium borohydride (NaBH4) done by Santos and Sequeira [30], as well as on a lithium mineral developed by Agusdinata et al. [31]. Considering that FCEVs are a particular field of EV application, Egbue and Long [32] and Ramirez et al. [33] carried out bibliometric analyses in order to detect the most relevant research points. In turn, Zhao et al. [34] depict a bibliometric analysis for EV charging system reliability to analyze the emerging trends of this active research point, such as in EV batteries. Patents provide an exclusively detailed source of information of inventive activity [35] and increase the use and commercialization of technologies though market transactions [36], promoting the diffusion of knowledge and innovation. According to Griliches [37], patents are one of the most Sustainability 2020, 12, x FOR PEER REVIEW 3 of 27 Sustainability 2020, 12, 2334 3 of 25 the diffusion of knowledge and innovation. According to Griliches [37], patents are one of the most influential proxies for assessing the performance of industry research and development (R&D). For influentialthese reasons, proxies the patent for assessing is an important the performance tool for investigating of industry research a technological and development development (R&D). from For an theseeconomic reasons, perspective the patent [38]. is anAccording important to toolBorgstedt for investigating et al. [6], in a the technological automotive development industry, patents from are an economicthe most common perspective way [ 38to ].protect According intellectual to Borgstedt property, et al. so [ 6it], can in thebe used automotive as innovative industry, output. patents Patent are thedata most analysis common allows way us toto protectascertain intellectual the technologi property,cal state so it of can the be studied used as te innovativechnology, output.defining Patent who, datawhen, analysis where, allowsand what us tois being ascertain developed the technological by mining staterelevant of the data studied from patent technology, documents defining in terms who, when,of technology where, and development what is being [39]. developed In addition, by mining in this relevant case, in data the frommain patent databases, documents such inas termsWeb of technologyScience and development Scopus, there [ 39is ].no In patent addition, data in analysis this case, of in“FCEV” the main research databases, articles; such nevertheless, as Web of Science other andresearch Scopus, papers there related is no patent to pate datants analysis in fuel-cell of “FCEV” technologies research articles;have been nevertheless, developed. other Related research to papersbiohydrogen, related Leu to patents et al. [38] in fuel-cellcarried out technologies a patent data have and been citation developed. analysis. Related With to regard biohydrogen, to fuel cells, Leu etChang al. [38 et] carriedal. [40] studied out a patent the coactivity data and between citation analysis.science and With technology regard to by fuel analyzing cells, Chang patent–paper et al. [40] studiedpairs. Related the coactivity to technology between forecasting, science and Chen technology et al. [41] defined by analyzing a model patent–paper for a patent pairs.strategy Related for fuel- to technologycell technologies forecasting, using the Chen S-curve et al. [method.41] defined a model for a patent strategy for fuel-cell technologies usingIn the demonstrating S-curve method. the evolution of the scientific–technological research of the FCEV domain, this paperIn offers demonstrating a comprehensive the evolution assessment of the of scientific–technological the FCEV research practices research which of thewere FCEV published domain, in thisthe paperScopus o ffdatabaseers a comprehensive from 1999–2019, assessment in order of theto FCEVidentify research the key practices actors in which the weregeneration published and intransmission the Scopus of database knowledge from related 1999–2019, to FCEVs. in order As far to as identify technologica the keyl development actors in the is generationconcerned, andthis transmissionpaper presents of knowledgethe technological related trends to FCEVs. of industrial As far as developments, technological development mainly led in is this concerned, case by thisthe paperautomotive presents industry, the technological in order to establish, trends of amon industrialg others, developments, who leads the mainly research led inand this development. case by the automotiveAll this is done industry, in order in order to draw to establish, the FCEV among technology others, knowledge who leads themap, research to discuss and it development. with the results All thisof other is done scientific–technological in order to draw the research FCEV technology studies, and, knowledge therefore, map, to be toable discuss to predict it with the the future results paths of otherof research scientific–technological trends and foreseeable research scenarios. studies, and, therefore, to be able to predict the future paths of research trends and foreseeable scenarios. 2. Materials and Methods 2. Materials and Methods The research process, adapted from Bildosola et al. [42] with some changes, is based on three The research process, adapted from Bildosola et al. [42] with some changes, is based on three steps steps that are intended to define the scientific research activity profile and the technological profile that are intended to define the scientific research activity profile and the technological profile of FCEVs. of FCEVs. These steps are developed to answer the questions related to who, where, and what is These steps are developed to answer the questions related to who, where, and what is being or has being or has been researched or developed related to scientific literature and patents. Figure 1 shows been researched or developed related to scientific literature and patents. Figure1 shows the research the research approach, revealing that each step has its input and output, creating a flow of approach, revealing that each step has its input and output, creating a flow of information that allows information that allows the set objectives to be achieved. In each step, the specific technique used is the set objectives to be achieved. In each step, the specific technique used is identified. The stages are identified. The stages are developed consecutively, until the scientific and technological profiles are developed consecutively, until the scientific and technological profiles are determined. However, the determined. However, the objective of this research process is to be able to cover any type of emerging objective of this research process is to be able to cover any type of emerging technology or application. technology or application.

Figure 1. Research process step by step.

ProfileProfile generationgeneration isis carried carried out out through through the the first firs threet three steps, steps, and and the the main main tasks tasks to be tocarried be carried out areout explainedare explained below. below. Step 1. Retrieving data and refiningrefining the search. The firstfirst assignment is to generate two specificspecific databases concerningconcerning scientific scientific publications publications and an patentsd patents related related to the emergingto the emerging technology technology analyzed. Sustainability 2020, 12, 2334 4 of 25

Regarding the selection of the scientific database, different studies show that better results are obtained [43,44] by using all of the databases (Scopus, Web of Science (WoS), and Google Scholar (GS)). Nevertheless, a very high percentage of WoS and Scopus citations are normally found in GS; those that are not, called unique citations, present a lower scientific impact than WoS and Scopus citations [45]. Furthermore, the two databases complement each other [43]; however, in this case, Scopus returns more citations than WoS. For this reason, in the case of FCEVs, the specific databases were generated from Scopus as the scientific database and Lens as the patent database. Scopus is one of largest abstract and citation databases of peer-reviewed literature (75 million documents indexed) [46], and it was selected to provide scientific publications. Because blooming technologies meet different approaches, the definition of the search query is very important. In bibliometric search strategies, the balance between recall and precision is very important [47]. However, information scientists usually detect an inverse association between recall and precision [48]. Therefore, the query was built using “fuel-cell vehicle/car/automobile” as author keywords, obtaining a highly precise query. In addition, to achieve greater recall, the search for the same terms based on the index terms is added to the query (see Table1). Index terms are derived from thesauri that Elsevier owns and are added to improve search recall [46]. The data collection time span was established between 1999 and 2019. In addition, according to Hawkins [49], gatekeeping publications, such as Abstracting and Indexing (A&I) publications, are the main way to identify advances of a technology. Therefore, the query is in line with the document type: Journal Article and Conference Proceeding. The main query used in the Scopus database retrieved a total of 2514 articles and conference papers for the defined time span. The patent analysis was carried out using Lens, a complete open-source global patent database and research platform containing the world’s most comprehensive full-text patent collection. The search for patents relating to FCEVs was carried out on the basis of Cooperative Patent Classification (CPC), also explained as the patent’s field of technological application. Specifically, the search is directed by the classification Y02T90/34—fuel-cell-powered electric vehicles—which makes the results of localized patents far more accurate. The Y CPC was created in order to give a technological application coverage to new technologies that are not included in the International Patent Classification (IPC); Y: General tagging of new technological developments. Y02T90: Technologies for climate-change mitigation or adaptation related to transportation—enabling technologies with a potential or indirect contribution to GHG emission mitigation. In addition, the search was limited to the patent priority year (or the year in which the patent was invented) in the period from 1999 to 2019.

Table 1. Search query for fuel-cell electric vehicles adapted to the Scopus database.

Query AUTHKEY(“fuel cell*” W/2 vehicle*) OR AUTHKEY(“fuel cell*” W/2 car) OR AUTHKEY(“fuel cell*” W/2 cars) OR AUTHKEY (“fuel cell*” W/2 automobile*) OR INDEXTERMS (“fuel cell*” W/2 vehicle*) OR INDEXTERMS (“fuel cell*” W/2 car) OR INDEXTERMS(“fuel cell*” W/2 cars) OR INDEXTERMS (“fuel cell*” W/2 automobile*) AND (LIMIT-TO (DOCTYPE, “cp”) OR LIMIT-TO (DOCTYPE, ”ar”)) Source: Own work.

Step 2: Cleaning up the refined database. This second task includes the use of text mining tools. The scientific database and the patent database were imported into the Vantage Point (VP) software [50]. VP works with search results from text databases. VP’s capabilities can be broad after importing raw data from scientific databases. VP includes powerful data cleaning tools based on a thesaurus or fuzzy matching techniques. Furthermore, VP integrates powerful techniques for analyzed data, such as natural language programming, a co-occurrence matrix, Principal Component Analysis (PCA), Social Network Analysis (SNA), clusters, and other capabilities for visualizing data. Scientific publications obtained from the previous step were integrated into the database, and in specific fields, such as authors, affiliations, journals, and authors’ keywords, a fuzzy matching was applied in order to group Sustainability 2020, 12, 2334 5 of 25 the variations of a word (plurals, acronyms, and similar expressions, among others) that convey the same meaning. Step 3: Generating the profile. The profile is divided into two parts: The scientific research profile and theSustainability technological 2020, 12, x FOR profile. PEER REVIEW The scientific profile is based on the literature profile and5 of research 27 community profile, and these describe the research activity in terms of publication trends, academic Step 3: Generating the profile. The profile is divided into two parts: The scientific research profile performance, research topics, and sources of knowledge. The technological profile deals with issues and the technological profile. The scientific profile is based on the literature profile and research relatedcommunity to patent profile, trends, and the these main describe countries the research of jurisdiction, activity in inventors terms of publication and applicants, trends, academic and the main technologicalperformance, fields. research To facilitate topics, and the analysissources of of knowledge. scientific–technological The technological trends, profile an deals analysis with of issues networks was carriedrelated to out, patent visualizing trends, the the main collaborative countries of networks jurisdiction, between inventors countries, and applicants, co-authorship and the networks,main keywordtechnological co-occurrences, fields. To and facilitate collaboration the analysis networks of scientific–technological of applicants and inventors. trends, an For analysis this, startingof fromnetworks the matrices was carried of co-occurrences, out, visualizing created the collabor bothative statically networks and between dynamically countries, in co-authorship the VP software (Searchnetworks, Technololy keyword Inc. co-occurrences, Atlanta, USA), and the collaboration networks were networks generated of applicants and visualized and inventors. through For the this, Gephi softwarestarting [51 ].from the matrices of co-occurrences, created both statically and dynamically in the VP software (Search Technololy Inc. Atlanta, USA), the networks were generated and visualized through 3. Resultsthe Gephi software [51].

3.1. Scientific3. Results Research Profile in FCEVs

3.1.1.3.1. General Scientific Trends Research of PublicationsProfile in FCEVs and Citations The3.1.1.evolution General Trends of publications of Publications and and citations Citations in the last 20 years is represented in Figure2 (on a logarithmicThe basis). evolution Both of ofpublications them increased and citations sharply, in the indicating last 20 years growing is represented academic in interestFigure 2 in(on FCEVs. a Whereaslogarithmic in 1999, basis). there Both were of onlythem 13increased scientific sharply, articles, indicating this number growing grew academic to 171 interest in 2018, in FCEVs. an increase of 1215%.Whereas Between in 1999, the there years were 2005 only and 13 scientific 2006, the arti numbercles, this of number publications grew to jumped 171 in 2018, markedly an increase and then stabilizedof 1215%. with Between a constant the years production, 2005 and 20092006, beingthe numb theer most of publications productive jumped year markedly with 203 and publications. then However,stabilized the with number a constant of citations production, per year 2009 rapidly being the increased most productive from 2005, year and with was 203 overpublications. twenty-two timesHowever, higher in the 2018 number (5081) of citation than ins 2005per year (227). rapidly Hence, increased the scientific from 2005, research and was community over twenty-two is paying moretimes attention higher to in topics 2018 (5081) related than to cleanerin 2005 transport.(227). Hence, In the addition, scientific Figure research2 shows community the results is paying of a less more attention to topics related to cleaner transport. In addition, Figure 2 shows the results of a less precise search of the two terms, displayed independently (query: Fuel cell* and query: Vehicle*/car precise search of the two terms, displayed independently (query: Fuel cell* and query: Vehicle*/car or cars automobile*), resulting in a very high number of publications, with a similar growth and or /cars/automobile*), resulting in a very high number of publications, with a similar growth and continuouscontinuous increase increase of bothof both searches. searches.

100000

14974 16834 12201 10000 9134 10056 9725 10880 70417292 8499 7401 7117 7728 7911 8510 63185663 64376605 6333 6677 6489 4386 4643 50624504 4443 5081 4454 3964 3437 3968 3517 3592 3823 2912 2795 2779 2763 1959 2081 17491776 1562 1293 12651393 1209 1000 904 698669 786 555566 484 608 311 227 203 193 179 174 154 133 143 171 171 145 100 108 123 131 129 120 74 79 91 54 46 51 35 30 18 10 13

2 1

nº of publications citations per year

publications related to fuel cell publications related to vehicle/car/automobile

Figure 2. General trends of publications and citations from 1999 to 2019. Figure 2. General trends of publications and citations from 1999 to 2019. 3.1.2. Academic Performance: Countries, Organizations, and Authors Sustainability 2020, 12, 2334 6 of 25

Sustainability 2020, 12, x FOR PEER REVIEW 6 of 27 3.1.2. Academic Performance: Countries, Organizations, and Authors In total,In total, authors authors from from 61 61countries countries published published papers papers on on FCEVs. FCEVs. The The affiliations affiliations of of authors authors are a goodare indicator a good indicator of the specific of the specific patterns patterns of resear of researchch concentration concentration and and excellence excellence followed followed by by these countriesthese countriesand organizations. and organizations. The most The advantageous most advantageous countries countries in terms in terms of publishing of publishing about about FCEVs (see FCEVsFigure (see 3) are Figure not3 geographically) are not geographically concentrated, concentrated, but are but rather are rather located located in the in thetri-polar tri-polar world world (North America,(North Asia-Pacific, America, Asia-Pacific, and Europe). and Europe). The Thetwo twoprincipal principal countries countries with thethe highest highest number number of of publicationspublications are are the the USA USA (618) (618) and and China China (451). AlmostAlmost half half of of the the publications publications are are from from USA, USA, China China and andJapan Japan (364). (364).

Figure 3. Evolution of the number of articles by country according to publication year, from 1999 Figureto 2019. 3. Evolution of the number of articles by country according to publication year, from 1999 to 2019. In addition, the USA, UK, and Canada stand out as pioneers in scientific research (with regards to the 1999 start date), and they also grew progressively over time. Production in China, despite being In addition, the USA, UK, and Canada stand out as pioneers in scientific research (with regards the country with the second-most publications, began to be relevant in 2002–2003. to the 1999In orderstart date), to identify and they collaboration also grew activities progressi betweenvely over different time. countries, Production an in eff ectiveChina, method despite is being the countrya network with analysis. the second-most Using Gephi publications, for network analysis,began to the be mainrelevant countries in 2002–2003. (countries that have at leastIn order three to collaborative identify collaboration publications) activities with international between collaborationsdifferent countries, were identified an effective and plottedmethod is a network(see Figure analysis.4). The Using size of Gephi the node for represents network theanalysis, number the of connections;main countries consequently, (countries a country that have with at a least threelarger collaborative node, such aspublications) the USA, is more with active internationa in academicl collaborations collaborations inwere the field identified of FCEVs. and The plotted width (see Figureof the 4). connectingThe size of line the represents node represents the cooperative the numb frequency.er of connections; Academic collaborations consequently, between a country China, with a largerGermany, node, such Japan, as and the the USA, USA areis more the most active frequent. in academic In the main collaborations cluster led by thein the USA, field both of European FCEVs. The widthand of North the connecting American countries,line represents as well the as Asian-Pacificcooperative frequency. countries, are Academic collaborating. collaborations The identified between collaborations between countries follow the same pattern of collaboration convergence clubs as those China, Germany, Japan, and the USA are the most frequent. In the main cluster led by the USA, both related to applied science, as defined by Barrios et al. [52]. In addition, international collaborations are European and North American countries, as well as Asian-Pacific countries, are collaborating. The located in a tri-polar world (Europe, North America, and Asia-Pacific); the science powerhouses are identifiedcountries collaborations placed in the centralbetween positions countries of the follow networks, the surrounded same pattern by emerging of collaboration scientificcountries. convergence clubsPossible as those reasons related for to collaborations applied science, may include as defined English by languageBarrios et skills, al. [52]. post-colonial In addition, links, international science collaborationsskill proximity, are economiclocated in proximity, a tri-polar and world international (Europe, students, North America, among others and [53Asia-Pacific);]. the science powerhouses are countries placed in the central positions of the networks, surrounded by emerging scientific countries. Possible reasons for collaborations may include English language skills, post- colonial links, science skill proximity, economic proximity, and international students, among others [53]. Sustainability 2020, 12, 2334 7 of 25 Sustainability 2020, 12, x FOR PEER REVIEW 7 of 27

Figure 4. Academic collaborations among the main countries. Figure 4. Academic collaborations among the main countries. According to organizations, Table2 describes the 10 most active organizations, including the numberAccording of publications, to organizations, their affi liatedTable 2 countries, describes and the their10 most average active citations organizations, per publication. including Thethe mostnumber productive of publications, organization their by affiliated far is Tongji countries, University and their with 134average publications. citations However,per publication. the average The citationsmost productive per publication organization for the by Tsinghua far is Tongji University University (ranked with #2) 134 is publications. four times greater, However, and the for average General Motorscitations (ranked per publication #9), it is for seven the timesTsinghua greater University than for (ranked Tongji University.#2) is four times Therefore, greater, this and organization for General (TongjiMotors Univ.)(ranked needs #9), it to is improve seven ti themes quality greater of than its publicationsfor Tongji Universi to ensurety. thatTherefore, its publications this organization become sources(Tongji Univ.) of knowledge needs to for improve other scientific the quality publications. of its publications In general, to ensure Chinese that organizations its publications have become fewer averagesources of citations knowledge per publication, for other scientific and their publications. publications In are general, concentrated Chinese within organizations a few organizations. have fewer Withaverage the citations exception per of publication, Tongji University, and their the publications National Renewableare concentrated Energy within Laboratory, a few organizations. and the US DepartmentWith the exception of Energy, of Tongji the publications University, fromthe Na thetional most Renewable active organizations Energy Laboratory, do impact and academic the US research.Department Highlighting of Energy, the the presence publications of non-academic from the most organizations active organizations among the do top impact 10 organizations academic (Generalresearch. Motors,Highlighting Ford Motors,the presence and Toyota of non-academ Motors),ic the organizations research results among have the an importanttop 10 organizations impact on the(General science-driven Motors, Ford domain. Motors, and Toyota Motors), the research results have an important impact on the science-driven domain. Table 2. The most active organizations. Table 2. The most active organizations. Organization Publications Country Average Citations Per Publication TongjiOrganization University Publications 134 Country China Average Citations 3.98 per Publication TsinghuaTongji University University 76 134 China China 3.98 16.72 ToyotaTsinghua Motor Corporation University 56 76 JapanChina 16.72 23.34 University of California, Davis 50 USA 15.00 Toyota Motor Corporation 56 Japan 23.34 Argonne National Laboratory 44 USA 17.95 NationalUniversity Renewable of California, Energy Laboratory Davis 44 50 USA USA 15.00 5.48 ArgonneSeoul National National University Laboratory 43 44 South USA Korea 17.95 22.42 NationalFord Renewable Motor Company Energy Laboratory 41 44 USA USA 5.48 11.02 SeoulGeneral National Motors University 35 43 South USA Korea 22.42 28.91 US Department of Energy 35 USA 9.09 Ford Motor Company 41 USA 11.02 General Motors 35 USA 28.91 TheUS analysis Department of the of most Energy productive authors 35 (see Table USA3 ) reveals Minggao 9.09 Ouyang (Tsinghua University, China), Joeri Van Mierlo (Vrije Universiteit, Brussel), and Jianqiu Li (Tsinghua, China) as theThe three analysis researchers of the publishingmost productive the most authors in the (s field.ee Table All 3) of reveals the top Mi authorsnggao haveOuyang at least (Tsinghua fifteen publications;University, China), however, Joeri if Van we Mierlo evaluate (Vrije their Universite citation averageit, Brussel), per and publication, Jianqiu Li important (Tsinghua, di China)fferences as betweenthe three authors researchers appear. publishing For instance, the themost sixth in the author, field. Zuo, All S. of with the 16top publications, authors have has at a lowleast average fifteen numberpublications; of citations however, per if publication we evaluate (2.19), their meaning citation thataverage their per academic publication, work important has little influence differences on otherbetween scientific authors research. appear. In For general, instance, the top the 10 sixth most author, productive Zuo, authors S. with have 16 apublications, high average has number a low of average number of citations per publication (2.19), meaning that their academic work has little influence on other scientific research. In general, the top 10 most productive authors have a high Sustainability 2020, 12, 2334 8 of 25 citations per publication, highlighting the authors from Tsinghua University, whose research work has an important influence on other scientific developments.

Table 3. Top 10 most productive authors.

Average Authors Institution Country Counts Citations per Subject Area a Publication Ouyang, Energy and Tsinghua University China 25 33,20 Minggao Engineering Van Mierlo, Energy and Vrije Universiteit Brussel Belgium 23 25,91 Joeri Engineering Energy and Li, Jianqiu Tsinghua University China 21 36,90 Engineering Energy and Xu, Liangfei Tsinghua University China 18 42,28 Engineering Université de Ravey, Energy and Technologie France 16 15,44 Alexandre Engineering Belfort-Montbéliard Zuo, Shuguang Tongji University China 16 2,19 Engineering Seoul National South Cha, Sukwon 15 17,60 Engineering University Korea Djerdir, Université Bourgogne Energy and France 15 6,67 Abdesslem Franche-Comté Engineering Rathore, Concordia University Canada 15 26,93 Engineering Akshay Kumar National Renewable Wipke, Keith B. USA 15 8,53 Engineering Energy Laboratory a Subject areas are summarized from the selected publications in the established dataset (Scopus), taking into account the most frequent subject areas.

A co-authorship network was generated and plotted by using Gephi, as shown in Figure5, identifying a collaboration network of the top authors (authors with over three publications as co-authorships). The layout of the networks was developed using Force Atlas 2. Each node represents each author, while each link represents the pattern of collaboration. Among the top 10 authors, only the authors affiliated with Tsinghua University (Minggao Ouyang, Jianqiu Li, and Liangfei Xu) have a strong co-authorship between them; the rest of the authors have less intense relationships with other authors who are not in the top 10. The main academic groups can be identified, highlighting that there is no collaboration between them. Five research groups can be identified, whose main authors work in the research field (subject area) of energy and engineering related to FCEVs. Therefore, as far as the research field is concerned, they present few differences, but their locations do: Each group is from a different country or continent (USA, Japan, and Belgium), except for research group 1, in which the authors located in the tri-polar world (France, Canada, USA, China, and South Korea) collaborate, and research group 4, in which authors from China and South Korea collaborate.

3.1.3. Sources and Subject Area Classification Sources with more documents published about FCEVs were identified. The search was limited to journals and conference proceedings, with 60% of the publications published in journals and 40% as conference proceedings. The most productive journal is closely related to hydrogen; however, most of the top journals are related to the automobile industry, electrochemical science, and energy applications. The most notable of the 10 most productive sources was the International Battery, Hybrid, and Fuel-Cell Electric Vehicle Symposium (EVS) organized by the World Electric Vehicle Association (WEVA). The 2514 publications were published in 721 different sources; therefore, the works are not centered around a few journals, but more than 20% were published in two sources. Specially, the International Journal of Hydrogen Energy (271 articles, 10.7%), SAE Technical Papers (257 articles, 10.15%), and Journal of Sustainability 2020, 12, x FOR PEER REVIEW 9 of 27

Sustainability 2020, 12, 2334 9 of 25

Power Sources (104 articles, 4.1%) are the top journals (see Figure6). Moreover, the top journals are

Sustainabilityindexed in 2020 the, 12 Clarivate, x FOR PEER Analytics REVIEW Journal Citation Report with upper quartile rankings. 9 of 27

Figure 5. Co-authorship network.

3.1.3. Sources and Subject Area Classification Sources with more documents published about FCEVs were identified. The search was limited to journals and conference proceedings, with 60% of the publications published in journals and 40% as conference proceedings. The most productive journal is closely related to hydrogen; however, most of the top journals are related to the automobile industry, electrochemical science, and energy applications. The most notable of the 10 most productive sources was the International Battery, Hybrid, and Fuel-Cell Electric Vehicle Symposium (EVS) organized by the World Electric Vehicle Association (WEVA). The 2514 publications were published in 721 different sources; therefore, the works are not centered around a few journals, but more than 20% were published in two sources. Specially, the International Journal of Hydrogen Energy (271 articles, 10.7%), SAE Technical Papers (257 articles, 10.15%), and Journal of Power Sources (104 articles, 4.1%) are the top journals (see Figure 6). Moreover, the top journals are indexed in the Clarivate Analytics Journal Citation Report with upper quartile rankings. FigureFigure 5. 5. Co-authorshipCo-authorship network. network.

3.1.3. 25thSources International and Battery, Subject Hybrid and Area Fuel Cell Classification Electric Vehicle Symposium… 25 24th International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium… 25 Sources with more documents publishedEnergy Policy about FC26EVs were identified. The search was limited to journals and conference proceedings,Applied with Energy 60% of the27 publications published in journals and 40% as conference22nd International proceedings. Battery, Hybrid and FuelThe Cell most Electric Vehicproductivele Symposium… journal is35 closely related to hydrogen; however, most ECS Transactions 42 of the top journals are related to the automobile industry, electrochemical science, and energy World Electric Vehicle Journal 50 applications. The most notable of the 10 most productive sources was the International Battery, Journal of Power Sources 104 Hybrid, and Fuel-Cell Electric VehicleSAE Technical Symposium Papers (EVS) organized by the World Electric257 Vehicle Association (WEVA). TheInternational 2514 Journalpublications of Hydrogen Energy were published in 721 different sources; therefore,271 the works are not centered around a few journals, but more than 20% were published in two sources. Specially, the InternationalFigure 6.JournalA summary of Hydrogen of publications Energy according (271 articles, to the top10.7%), journals. SAE Technical Papers (257 articles, 10.15%), Figureand Journal 6. A summary of Power of Sourcespublications (104 according articles, 4.1%)to the aretop thejournals. top journals (see Figure 6). Moreover,According the to top Scopus, journals the are publications indexed in can the beClarivate categorized Analytics into fourJournal general Citation areas, Report which with are According to Scopus, the publications can be categorized into four general areas, which are upperfurther quartile divided rankings. into 27 major subject areas [43]. With regard to the research fields for the identified publications,further divided they into are 27 categorized major subject into areas a total [43]. of 21 With specific regard areas, to the and research these are fields primarily for the the identified areas of

Engineering25th International (67.3%), Battery, Hybrid Energy and Fuel Cell (40.6%), Electric Vehic Environmentalle Symposium… 25 Science (16.3%), Physics and Astronomy (14.7%), and24th Computer International Battery, Science Hybrid and (7.8%). Fuel Cell Electric Vehicle Symposium… 25 Energy Policy 26 3.1.4. Research Topic Applied Energy 27 22nd International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium… 35 Keywords represent the knowledge hubs of academic studies; therefore, the analysis of keywords ECS Transactions 42 can help to identify the leadingWorld and Electric developing Vehicle Journal topics in50 the research field of FCEVs. A total of 3263 keywords (author keywords) wereJournal of extracted Power Sources from 2514 publications104 by using Vantage Point. The maturity level of those data can beSAE determined Technical Papers by analyzing the first year in which a term in the257 analysis data set appears. In theInternational case of Journal theanalysis of Hydrogen Energy of the authors’ keywords, two growth phases are identified271

Figure 6. A summary of publications according to the top journals.

According to Scopus, the publications can be categorized into four general areas, which are further divided into 27 major subject areas [43]. With regard to the research fields for the identified Sustainability 2020, 12, x FOR PEER REVIEW 10 of 27

publications, they are categorized into a total of 21 specific areas, and these are primarily the areas of Engineering (67.3%), Energy (40.6%), Environmental Science (16.3%), Physics and Astronomy (14.7%), and Computer Science (7.8%).

3.1.4. Research Topic Keywords represent the knowledge hubs of academic studies; therefore, the analysis of keywords can help to identify the leading and developing topics in the research field of FCEVs. A Sustainability 2020, 12, 2334 10 of 25 total of 3263 keywords (author keywords) were extracted from 2514 publications by using Vantage Point. The maturity level of those data can be determined by analyzing the first year in which a term (See Figurein the analysis7). The firstdata phaseset appears. can be In called the case the of growth the analysis phase of betweenthe author thes’ keywords, years 1999–2010, two growth and the secondphases phase, are identified maturity, (See between Figure the7). The years first 2011–2019, phase can be can called be the considered growth phase as a between stage of the continuous years 1999–2010, and the second phase, maturity, between the years 2011–2019, can be considered as a stage downturn and growth, in which the appearance of new study terms is very important. of continuous downturn and growth, in which the appearance of new study terms is very important.

Records Terms

284 Maturity 2007-2019 254 249 236 222 223 212 200 205 Growth 1999-2006 191 155 203 180 160 193 165 179 174 171 171 154 143 145 100 133 131 123 129 120 76 63 91 41 79 31 51 11 46 5 35 30 131999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

FigureFigure 7. Number 7. Number of newof new author’s author’s keywords keywords in any year year vs. vs. the the number number of records of records of that of thatyear. year.

In addition,In addition, the the co-occurrence co-occurrence network network of of authors’authors’ keywords keywords that that co-occur co-occur at least at least five fivetimes times was was plottedplotted by using by using Gephi. Gephi. The The analysis analysis was was carried carried out out using using a dynamic a dynamic matrix matrix of co-occurrences,of co-occurrences, which which made it possible to analyze the evolution of keywords in the period 1999–2019. In order to made it possible to analyze the evolution of keywords in the period 1999–2019. In order to analyze the analyze the current situation and to be able to deduce a future trend of the research topics, Figure 8 currentshows situation the keyword and to networks be able tofor deduce the 2018–2019 a future period. trend In of this the network, research the topics, three Figuremost important8 shows the keywordclusters networks define the for three the 2018–2019 areas of action period. in which In this research network, science the threeis currently most importantworking: FCEVs clusters as definea the threetechnological areas of development action in which to improve research the science environment is currently or reduce working: GHG emissions, FCEVs as batteries a technological and developmentenergy management to improve thesystems, environment and hydrogen or reduce producti GHGon emissions, and storage, batteries with anda loss energy of interest management in systems,research and related hydrogen to the production topic of powertrains. and storage, with a loss of interest in research related to the topic ofSustainability powertrains. 2020, 12, x FOR PEER REVIEW 11 of 27

Figure 8. Authors’ keyword co-occurrence networks: 2018–2019 period. Figure 8. Authors’ keyword co-occurrence networks: 2018–2019 period.

The analysis of these dynamic networks based on the author keywords with the greatest number of co-occurrences allows us to focus on the path followed by research, and also to predict future trends. These research topics have been ongoing for the past twenty years. However, they have evolved until reaching their technological maturity, generating new fields of research related to the main topics. Mainly related to the cluster led by PEMFCs are FC hybrid vehicles, batteries, and energy management systems (EMSs), and to the cluster led by FCEVs: Hydrogen storage/production and Solid Oxide Fuel Cell (SOFC.) As a summary of the analysis in Figure 9, the keyword networks of the periods 1999–2010 (growth period) and 2011–2019 (maturity period) are shown.. Sustainability 2020, 12, 2334 11 of 25

The analysis of these dynamic networks based on the author keywords with the greatest number of co-occurrences allows us to focus on the path followed by research, and also to predict future trends. These research topics have been ongoing for the past twenty years. However, they have evolved until reaching their technological maturity, generating new fields of research related to the main topics. Mainly related to the cluster led by PEMFCs are FC hybrid vehicles, batteries, and energy management systems (EMSs), and to the cluster led by FCEVs: Hydrogen storage/production and Solid Oxide Fuel Cell (SOFC.) As a summary of the analysis in Figure9, the keyword networks of the periods 1999–2010 (growth period) and 2011–2019 (maturity period) are shown.

3.1.5. Source of Knowledge In addition, analysis was carried out to identify the five most cited articles in order to be able to detect the origin of the knowledge (See Table4). These articles were published between 2004–2011; these were years of growth of the scientific development of FCEV technology. However, the results show that China and the USA are the reference countries, and the authors of the most cited articles are affiliated with universities and research entities, such as Harbin Institute of Technology (Wuhan University), Institut National de la Recherche Scientifique, Énergie, Matériaux, and Télécommunications, and Los Alamos National Laboratory. Nevertheless, only one of the most cited articles was made in collaboration between different organizations from different countries (USA and Canada). As far as sources are concerned, the most cited articles were published in IEEE Proceedings, which covers technical developments in electronics, electrical and computer engineering, and computer science, and in Energy and Environmental Science, which is related to energy conversion and storage, alternative fuel technologies, and environmental science. Sustainability 2020, 12, 2334 12 of 25 Sustainability 2020, 12, x FOR PEER REVIEW 12 of 27

Figure 9. Authors’ keyword co-occurrence networks: Growth period (1999–2006) and Maturity period (2007–2019). Figure 9. Authors’ keyword co-occurrence networks: Growth period (1999–2006) and Maturity period (2007–2019).

0 Sustainability 2020, 12, 2334 13 of 25

Table 4. Most cited articles: Where knowledge comes from.

Times Cited Title Publication Year Authors Organizations Country Source Author Keywords Fuel-cell electric vehicle (FCEV) International Research Electric machines The state of the art of electric, Centre for Electric Vehicles, Proceedings of the 1107 2007 Chan, C.C. China Electric vehicle (EV) hybrid, and fuel cell vehicles [5] University of Hong IEEE Modeling Kong/Wuhan University Hybrid electric vehicles (HEV) Jaouen, F. Fuel-cell performance * Proietti, E. Institut National de la Fuel-cell vehicles * Lefèvre, M. Recherche Scientifique, United States metal catalysis * Recent advances in non-precious Chenitz, R. Énergie, Matériaux and metal catalysis for Energy and 1021 2011 Télécommunications/Materials oxygen-reduction reaction in Dodelet, J.-P. Environmental Wu, G. Physics and Applications Science polymer electrolyte fuel cells [54] Cathodes * Chung, H.T. Division, Los Alamos Canada Johnston, C.M. National Laboratory Zelenay, P. Atmospheric science: Cleaning Jacobson, M.Z. Electrolysis * 591 the air and improving health with 2005 Colella, W.G. Stanford University United States Science Hydrogen Fuel-cell vehicles* hydrogen fuel-cell vehicles [55] Golden, D.M. Air quality * Peng, F.Z. University of Michigan Fuel-cell electric vehicle A new ZVS bidirectional DC–DC Li, H. Florida State UniversityIEEE Transactions DC–DC converter 557 converter for fuel cell and battery 2004 Oak Ridge National United States on Power Su, G.-J. Power generation application [56] Laboratory Electronics ZVS Lawer, J.S. University of Tennessee Auxiliary power supply Wagner, F.T. Automotive applications * Electrochemistry and the future General Motors Research Journal of Physical 532 2010 Lakshmanan, B. United States Li-ion batteries * of the automobile [57] and Development Chemistry Letters Mathias, M.F. Hydrogen infrastructure * * These articles only have the Indexed Keywords registered. Times cited: Data according to the Scopus database. Sustainability 2020, 12, x FOR PEER REVIEW 16 of 27 Sustainability 2020, 12, 2334 14 of 25

19 3.2. Technological Profile in FCEVs 3.2. Technological Profile in FCEVs 20 3.2.1. General Trends 3.2.1. General Trends 21 Technological trends were analyzed by using the patent data. For this, Lens.org was used, as it Technological trends were analyzed by using the patent data. For this, Lens.org was used, as 22 is a global patent database that includes European Patent Office (EPO), United States Patent and it is a global patent database that includes European Patent Office (EPO), United States Patent and 23 Trademark Office (USPTO), World Intellectual Property Organization (WIPO), and Australian patent Trademark Office (USPTO), World Intellectual Property Organization (WIPO), and Australian patent 24 data collections. The search was carried out based on patent families. A patent family (PF) is a data collections. The search was carried out based on patent families. A patent family (PF) is a 25 collection of patent applications covering the same or similar technical content. For the period from collection of patent applications covering the same or similar technical content. For the period from 26 1999 to 2019, a total of 1909 PFs were detected. 1999 to 2019, a total of 1909 PFs were detected. 27 First, it is interesting to answer the questions: When did the invention publication take place? First, it is interesting to answer the questions: When did the invention publication take place? 28 Who is inventing it? Who is its beneficiary? What is the framework for commercial foresight? What Who is inventing it? Who is its beneficiary? What is the framework for commercial foresight? What 29 other fields of technological application do these patents cover? The production of patents is low up other fields of technological application do these patents cover? The production of patents is low up 30 until 2000, but in the period from 2001 to 2016, technological development is in constant growth (see until 2000, but in the period from 2001 to 2016, technological development is in constant growth (see 31 Figure 10). It is to be expected that there are not many patents in the years 2018 and 2019 because the Figure 10). It is to be expected that there are not many patents in the years 2018 and 2019 because the 32 search was made by the earliest priority year, and the patent applications are normally published 18 search was made by the earliest priority year, and the patent applications are normally published 18 33 months after the date of filing or the earliest priority date, so patents filed in 2018 and 2019 may not months after the date of filing or the earliest priority date, so patents filed in 2018 and 2019 may not 34 have been published yet. have been published yet.

212 211 178 167 154 155 156 139 140 121 118 112 99 95 95 85 90 66

13 1 9 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

35

36 FigureFigure 10. 10. TheThe number number of of fuel-cell fuel-cell electric electric vehicle vehicle (FCEV) (FCEV) patent patent families families by by publication publication year. year.

37 3.2.2.3.2.2. Inventive Inventive Performance: Performance: Inventors, Inventors, Applicants, Applicants, and and Countries Countries 38 InIn order order to to reflect reflect inventive inventive performance, performance, the the to topp inventors inventors are are from from South South Korea, Korea, Japan, Japan, and and the the 39 USA,USA, and and their their main main applicants applicants are are automotive automotive companies companies (See (See Table Table 5).5).

Table 5. Who is inventing? 40 Table 5. Who is inventing?

NumberNumber of ofPatent Patent Family Family (PFs) (PFs) Inventors Inventors Main Main Applicants Applicants 37 37 KWON KWON SANG SANG UK UK Hyundai Hyundai Motor Motor Co Co LTD LTD 35 35 TABATA TABATA ATSUSHI ATSUSHI Toyota Toyota Motor Motor Corp. Corp. 33 33 BORRONI-BIRDBORRONI-BIRD CHRISTOPHER CHRISTOPHER E E General General Motors Motors Corp. Corp. 33 33 VITALE VITALE ROBERT ROBERT LOUIS LOUIS General General Motors Motors Corp. Corp. 33 33 HIBIKI HIBIKI SAEKI SAEKI Honda Honda Motor Motor Co Co LTD LTD 32 32 CHERNOFF CHERNOFF ADRIAN ADRIAN B B General General Motors Motors Corp. Corp. 32 SHABANA MOHSEN D General Motors Corp. 32 SHABANA MOHSEN D General Motors Corp. 27 KENJI UMAYAHARA Toyota Motor Corp. 27 KENJI UMAYAHARA Toyota Motor Corp. 25 KOTA MANABE Toyota Motor Corp. 25 19 WALTER KOTA MANABE MARKUS Toyota Daimler Motor AG Corp. 19 18WALTER KERETLI MARKUS FAHRI RenaultDaimler SA AG 18 KERETLI FAHRI Renault SA Sustainability 2020, 12, 2334 15 of 25

Sustainability 2020, 12, x FOR PEER REVIEW 17 of 27 The holder of the legal rights and responsibilities on a patent application is called the assignee 41 or applicant.The holder It of can the be legal an individual, rights and aresponsibilities corporation, a on university, a patent aapplication hospital, or is a called government the assignee entity. 42 orTherefore, applicant. if It we can analyze be an individual, the applicants, a corporatio we can seen, a who university, the beneficiary a hospital, of theor a invention government is. Theentity. top 43 Therefore,patent-generating if we analyze organizations the applicants, are presented we can in see Table who6. the Toyota beneficiary Motors Corp.of the (JP)invention has the is. most The PFs,top 44 patent-generating360, Honda Motors organizations (JP) follows withare presented 210 PFs, and, in Table in the 6. third Toyota position, Motors Hyundai Corp. (JP) Motors has the (SK) most with PFs, 175 45 360,PFs. Honda In short, Motors the main (JP) applicantsfollows with are 210 companies PFs, and, from in the the third automotive position, sector Hyundai or auxiliary Motors automotive (SK) with 46 175sector. PFs. In In order short, to determinethe main theapplicants market allocationare comp strategyanies from of companiesthe automotive (what issector the framework or auxiliary for 47 automotivecommercial sector. foresight?), In order the to main determine countries the where market patents allocation are protectedstrategy of were companies identified. (what The is USAthe 48 frameworkdominates withfor commercial 1583 PFs, the foresight?), second most the PFs main come countries from Germany where with patents 1147 PFs,are andprotected Japan haswere the 49 identified.third most The with USA 1095 dominates PFs. with 1583 PFs, the second most PFs come from Germany with 1147 50 PFs, and Japan has the third most with 1095 PFs. Table 6. Main applicants and main jurisdiction countries. 51 Table 6. Main applicants and main jurisdiction countries. Applicants Countries Number of PFsApplicants Applicants Number of PFs JurisdictionCountries Country Number of PFs360 ApplicantsToyota Motors Numb 1583er of PFs Jurisdiction USA Country 360 210Toyota Honda Motors Motors 11471583 GermanyUSA 210 175Honda Hyundai Motors Motors 10951147 JapanGermany 175 160Hyundai Daimler Motors AG 9281095 WIPOJapan GM Global Tech 160 103 Daimler AG 838928 ChinaWIPO Operations Inc. 103 GM Global Tech Operations Inc. 838 China 92 Nissan Motors 552 EPO 92 Nissan Motors 552 EPO 62 General Motors Corp. 413 South Korea 62 57 General Bosch Motors GMBH Corp. 192 413 Canada South Korea 57 56 KiaBosch Motors GMBH Corp. 88192 FranceCanada 56 55 FordKia Motors Global Corp. Tech Llc. 58 88 Great Britain France 55 Ford Global Tech Llc. 58 Great Britain

52 AsAs far far as as technological technological cooperation cooperation between between applic applicantsants and and inventors inventors is is concerned, concerned, an an applicant applicant 53 collaborationcollaboration network network and and co-inventorship co-inventorship network network were were generated generated and and plotted plotted by using by using Gephi, Gephi, as 54 shownas shown in Figure in Figures 11 and 11 Figure and 12 12,, identifying identifying a a collaboration collaboration networknetwork forfor the top applicants applicants and and top top 55 inventorsinventors (inventors (inventors and and applicants applicants with with at at least least five five patents patents in in collaboration). collaboration).

56 Figure 11. Collaboration networks of applicants. 57 Figure 11. Collaboration networks of applicants. Sustainability 2020, 12, 2334 16 of 25 Sustainability 2020, 12, x FOR PEER REVIEW 18 of 27

58 59 Figure 12. CollaborationCollaboration networks of inventors.

60 As far as collaboration between applicants is concerned, the collaborative work takes place 61 between automotiveautomotive sector sector companies companies (from (from the the same same business business group) group) and associated and associated research research centers. 62 centers.This highlights This highlights the relationship the relationship between between Daimler Daimler (Germany), (Germany), Ford Global Ford TechnologyGlobal Technology (USA), and(USA), the 63 andBorroni-Bird the Borroni-Bird working groupworking (top group inventor) (top as inventor) applicants. as Withapplicants. regard With to inventors, regard theto inventors, collaborations the 64 collaborationsare clustered by are country clustered and by applicants. country and applicants.

65 3.2.3. Technology Technology Classification Classification 66 As regards thethe technologicaltechnological fieldsfields where where the the inventions inventions are are being being developed developed (what (what other other fields fields of 67 oftechnological technological application application do thesedo these patents patents cover?), cover? to determine), to determine the approach the approach of R&D of e ffR&Dorts inefforts FCEVs, in 68 FCEVs,the Cooperative the Cooperative Patent Classification Patent Classification (CPC) was (CPC analyzed.) was analyzed. The CPC The is a CPC joint is undertaking a joint undertaking between 69 betweenthe European the European Patent O ffiPatentce (EPO) Office and (EPO) the United and the States United Patent States and Patent Trademark and OTrademarkffice (USPTO) Office to 70 (USPTO)merge their to merge classification their classification systems into systems a single into system, a single which system, has awhich similar has structure a similar to structure that of the to 71 thatInternational of the International Patent Classification Patent Classification (IPC) administered (IPC) administered by the World by Intellectual the World Property Intellectual Organization Property 72 Organization(WIPO) [58], which(WIPO) provides [58], which the inventionprovides the category invention and standardcategory and information standard on information their technological on their 73 technologicalsingularity (World singularity Intellectual (World Property Intellectual Organization Property Organization (WIPO), 2018), (WIPO), enabling 2018), the enabling main key the items main of 74 keyFCEV items inventions of FCEV to inventions be identified. to be identified. In addition to the technological field (Y02T90/34) in which the patent search was carried out, the 75 resultsIn addition display that to the the technological other most common field (Y02T90/34) CPCs for in FCEV which patents the patent are basedsearch on was arrangements carried out, the for 76 resultscontrolling display a combination that the other of batteries most common and fuel CPCs cells (B60L58for FCEV/40) patents with 893 are PFs; based the on electricity arrangements domain for in 77 controllingprocesses related a combination to fuel cells of batteries in motive and systems fuel cells (H01M2250 (B60L58/40)/20) iswith with 893 791 PFs; PFs the (see electricity Table7). domain Within 78 inthe processes top 10 classes, related the to remaining fuel cells mostin motive common systems IPCs are(H01M2250/20) related to fuel is cellswith in 791 electrically PFs (see propelledTable 7). 79 Withinvehicles the (B60L) top and10 classes, to new the technological remaining developmentsmost common for IPCs climate are related change to mitigation fuel cells (Y02T).in electrically 80 propelled vehicles (B60L) and to new technological developments for climate change mitigation 81 (Y02T). 82 Sustainability 2020, 12, 2334 17 of 25

Table 7. Main Cooperative Patent Classifications (CPCs).

Number of PFs CPC Definition PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY-PROPELLED 893 B60L58/40 VEHICLES—Methods or circuit arrangements for monitoring or controlling batteries or fuel cells, specially adapted for electric vehicles—for controlling a combination of batteries and fuel cells ELECTRICITY—BASIC ELECTRIC ELEMENTS—PROCESSES OR MEANS, e.g., BATTERIES, FOR THE DIRECT CONVERSION OF 791 H01M2250/20 CHEMICAL ENERGY INTO ELECTRICAL ENERGY—Fuel cells in motive systems, e.g., vehicle, ship, plane GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE 785 Y02T90/32 CHANGE—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION—Fuel cells specially adapted to transport applications, e.g., automobile, bus, ship PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY-PROPELLED 595 B60L58/33 VEHICLES—Methods or circuit arrangements for monitoring or Controlling batteries or fuel cells, specially adapted for electric vehicles—by cooling PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY—PROPELLED 558 B60L58/30 VEHICLES—Methods or circuit arrangements for monitoring or controlling batteries or fuel cells, specially adapted for electric vehicles—for monitoring or controlling fuel cells PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY-PROPELLED 447 B60L50/72 VEHICLES—Electric propulsion with power supplied within the vehicle—Constructional details of fuel cells specially adapted for electric vehicles GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE 402 Y02T10/7077 CHANGE—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION—Road transport of goods or passengers—on board the vehicle GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE 380 Y02T10/705 CHANGE—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION—Road transport of goods or passengers—Controlling vehicles with one battery or one capacitor only PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY-PROPELLED 376 B60L58/34 VEHICLES—Methods or circuit arrangements for monitoring or controlling batteries or fuel cells, specially adapted for electric vehicles—by heating PERFORMING OPERATIONS; TRANSPORTING—VEHICLES IN GENERAL—PROPULSION OF ELECTRICALLY-PROPELLED 320 B60L1/003 VEHICLES—Supplying electric power to auxiliary equipment of vehicles—to auxiliary motors, e.g., for pumps, compressors

4. Discussion

4.1. Science Research Profile: The Situation within the Scientific Research Community Scientific production related to FCEVs has remained constant for the last ten years. In contrast, the citation of these publications has skyrocketed, so these publications are an important source of Sustainability 2020, 12, 2334 18 of 25 knowledge from other research articles. This frequency of citations is a good indicator of the importance and quality value of the documents [59]. This good indicator opens the door to future research that analyzes whether this research is feeding a new FCEV field of knowledge studies or being directed to other fields of research. In addition, the citation analysis allows us to identify the most influential publications that should be considered as important conveyors of knowledge [60]. In this case, the most cited publication reflects the state-of-the-art of electric vehicles in general, published in a technical source (Proceedings of IEEE). However, the second most cited article investigates a technical procedure in fuel cells and has been developed in collaboration between two research centers in the USA and Canada, published in a scientific journal (Energy and Environmental Science). The main sources of knowledge originate in China and USA, and it should be noted that only the second most cited publication was produced in collaboration between two countries, the USA and Canada. In the analysis of all scientific research, the USA is the leading country, supported by its large automotive industry (Ford, General Motors) and its important research centers (Argonne National Laboratory, National Renewable Energy Laboratory, and US Department of Energy). Followed by China, these countries are placed in the central positions of the networks and their collaborations follow the same patterns as in other applied sciences, the great powers of science being centralized, which in turn collaborate with other countries located in the tri-polar world [52]. Moreover, according to Gui et al. [61], the number of countries that are present in the collaboration network is gradually increasing, creating new connections between countries, which can lead to a shift in the center of gravity, with the countries that so far have not been in the central core being drawn closer to it and, therefore, acquiring a greater capacity for collaboration. This opens the door to future new research on the trends of scientific collaboration networks. The most active organizations are Chinese, most notably the University of Tsinghua, whose results are led mainly by the Ouyang, M. research group, which has the greatest impact, and also represents an important focus on the international collaboration network (see Figure6), which adds greater transfer to their knowledge. The growing trend and the important role of international collaboration in the increase of scientific production in China have been confirmed from several perspectives [62–66]. However, with Tongji University, despite being the most productive, its publications do not have a great impact on the scientific community. A large part of the scientific development relating to FCEVs is being carried out by the automotive industry and research centers. This research marks the preliminaries that, a posteriori, will become the patentable technological applications. Therefore, most scientific publications are garnered from two sources: “International Journal of Hydrogen Energy” and “SAE technical Papers” whose publications have a very technical content, and are therefore aimed at future technological development. Forecasting using a growth curve method is beneficial for estimating the level of technological growth [67]. Therefore, through the technological S-Curve method, Chen et al. [41] conclude in their study published in 2011 that the three main fuel-cell technologies (PEMFC, SOFC, and Direct Methanol Fuel Cell (DMFC)/Direct Alcohol Fuel Cell (DAFC) are in their maturity phase. The same conclusion that is obtained in this study, the analysis of new terms defined by year allows us to conclude that the growth curve has been in its state of maturity since 2010. The temporal analysis of the research topics defines two main fields of research: Hydrogen and its productive management and storage (where SOFC technology is included), and the development of the energy-electrical part for the propulsion of these vehicles (where PEMFC technology is included). In this case, DMFC is not an important research topic, so it does not appear in the network. However, it seems that it is not enough to put this type of vehicle onto the market, and that perhaps it needs governmental support with adequate policies to ensure implementation, among other things. Having identified the most influential organizations and authors in scientific developments related to FCEVs, and in order to optimize the new research paths, it would be useful to identify the research topics of these actors by analyzing the networks. Furthermore, in order to identify similarities between Sustainability 2020, 12, 2334 19 of 25 authors and to map the scholarly structure [68,69], which would favor the creation of new collaborative synergies, it would be of interest to carry out a co-citation analysis.

4.2. Technological Profile: The Situation within the Industrial Applications According to classical innovation theory [70,71], scientific publications will hopefully begin to decline in favor of a rise in technological activity or patents, moving from the research phase to the prototype and then to commercialization. As this case study shows, science reaches its fullness in 2009 and then begins to retreat; in turn, the largest number of patents published is produced a posteriori, during the period 2014–2016, but applied some eighteen months earlier, representing inventions or prototypes developed during the period 2012–2014. China is one of the biggest research powers, and this knowledge is being developed in universities, but it is not as formidable in the innovation process. Although new innovation policies in China were predicted to convert it into a major patent-producing power [72], in the case of FCEVs, this has not happened. Technological advances are still concentrated in universities, contrary to what happens in the main producer countries (USA, Japan, and South Korea) whose technological developments are being carried out in the automotive industry. The collaborative networks also indicate an important degree of relationship between car manufacturers and their research centers, moving towards a field of application relating to powertrain technologies and sustainability. Nonetheless, promising future vehicle propulsion technology is rather vague. There are still a number of technical barriers holding FCEVs back from the marketplace. Despite the scientific and technological progress made in recent years, this has not been enough to put a marketable product on the market. According to Borgstedt et al. [6], an influential factor may be the high degree of technological complexity, as well as the infrastructures necessary for refueling stations, plus the great advances made in other technologies related to Internal Combustion Engine Vehicles (ICEVs) and Hybrid Electric Vehicles (HEVs). After observing, among other things, certain coincidences between the increases in scientific–technological development and initiatives that stimulate innovation, such as the zero-emission vehicle legislation or the Kyoto protocol, and that the abrupt fall in the number of patents has occurred at times of unstable investment or financial uncertainty, Haslam et al. [72] recommend helping the transition occur: Developing new indicators related to the Kyoto protocol, “design complexity” analysis to identify technical bottlenecks, re-assessing the triple-helix innovation that promotes disruptive technologies, and analyzing new methods for producing hydrogen. The results of this research allow us to affirm that the scientific community has continued to investigate sustainability policies linked to topics such as zero-emission vehicles and greenhouse gas emissions, as well as the production of hydrogen. However, it is also true that most work has continued to be directed at topics more closely linked to the technological development of FCEVs, such as elements directly linked to the propulsion of this type of vehicle, such as PEMFC, SOFC, and DMFC technologies, among others. Likewise, bibliometric and patent data analyses respond to a model that has reached fullness, a technological development carried out, relatively, in international collaboration and that is beginning to regress at the technological level (patents), despite being in a more stable financial situation; even so, we have not reached the commercial goal. Therefore, the discussion now is, has this research been directed judiciously enough to obtain the necessary technological capacity to put FCEVs on the market? Is this destroying a great technological advance that would otherwise provide significant headway in climate change? Do we need new policies that favor its entry into the market? Relating to the interactions between science and technology, it is important to identify the link between a patent and academic research. A patent document cites both patents and scientific articles, which serve to justify technological development. These scientific articles cited in patents are also known as Non-Patent Literature (NPL), and are defined by the applicant and, in some cases, by the patent examiners, depending on the country. Furthermore, these references become the direct relationship between technological and scientific knowledge, and, therefore, NPL is considered an important indicator of the scientific impact on technological development (patents), making it possible Sustainability 2020, 12, 2334 20 of 25 to measure the transfer of knowledge between science and technological developments [73–76]. This opens the way for future research to analyze the research areas covered by the NPL of identified FCEV patents, and thus to identify the knowledge gaps between science and technology.

5. Conclusions A wide range of technologies linked to a more sustainable transport sector are being developed. Some of them have managed to penetrate the global market, as is the case of the hybrid vehicle and battery electric vehicle, and others, however, are still in the pipeline, as is the fuel cell vehicle. Therefore, this bibliometric and patent data study allows us to see what has happened in the last twenty years. The data obtained have made it easier to identify who the main agents involved in both scientific and technological research are, as well as to reveal their collaborative relationships. At the same time, in order to ascertain where the scientific–technological development has been heading, the main research topics and technological fields have been identified. Its evolution has allowed us to conclude that an important research path has been established, opening new horizons over the years, which has given way to technological development intrinsically linked to technological development related to the propulsion of electric vehicles, but that has failed to reach the market. Providing solutions to this problem, in addition to those already discussed in this paper, future research frameworks related to detecting bottlenecks occurring for commercial development may be proposed: Studying the research programs led by the European Commission (CORDIS) and the US Government (National Science Foundation (NSF)) concerning FCEVs, making a predictive model of technologies in order to forecast their future, performing forecasting studies to study the lifecycle stage of a specific technology, or deeper analyses into which technologies have not reached a state of maturity and to detect if they are key. Overall, the above topics summarize the main research into FCEV technology, which advances our understanding of this knowledge. As a result, an integrated knowledge map is presented in Figure 13. Sustainability 2020, 12, 2334 21 of 25

Sustainability 2020, 12, x FOR PEER REVIEW 23 of 27

214 215 Figure 13. A knowledge map of FCEV. Figure 13. A knowledge map of FCEV. Sustainability 2020, 12, 2334 22 of 25

Author Contributions: Conceptualization, I.A.-M. and R.M.R.-B.; methodology, I.A.-M., E.Z.-B., and G.G.-A.; software, I.A.-M.; validation, I.A.-M., R.M.R.-B., and E.Z.-B.; formal analysis, I.A.-M.; investigation, I.A.-M. and G.G.-A.; data curation, I.A.-M.; writing—original draft preparation, I.A.-M.; writing—review and editing, I.A.-M.; visualization, I.A.-M.; supervision, E.Z.-B., G.G.-A., and R.M.R.-B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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