Driving Innovation and Sustainability: The synergy between Fuel-Cell Hydrogen Vehicles and Smart Mobility

INTRODUCTION: To set the stage by highlighting the convergence of hydrogen vehicles and smart mobility as a significant development in transportation. It discusses the potential of hydrogen vehicles powered by fuel cells to address environmental challenges and emphasizes the role of smart mobility initiatives in optimizing transportation systems. OBJECTIVES: To analyze the main research themes related to fuel-cell hydrogen vehicles and smart mobility, focusing on productivity, impact, and content. METHODS: Using bibliometric methodologies, techniques and tools, this research analyzes the main research themes, pursuant to its productivity, impact and contents according to the literature available in Scopus. To this end, all the documents related to fuel-cell hydrogen vehicles and smart mobility were retrieved and analyzed (610 publications, with 19,494 cites, from 1992 to 2023) using VOSviewer. RESULTS: In terms of bibliometric performance, the volume of literature pertaining to research on hydrogen vehicles and smart mobility has exhibited a significant surge in recent years (1992–2023). Given the substantial number of publications and citations garnered in this domain, it is anticipated that interest will continue to escalate, thereby bolstering other knowledge domains such as sustainable development, climate change, fuel cell technologies, connectivity, and beyond. CONCLUSION: The importance of various factors in the context of hydrogen vehicles and smart mobility cannot be overstated. Sustainable development stands as a guiding principle, ensuring that advancements in transportation align with environmental preservation and societal well-being.


Introduction
The world faces numerous challenges in terms of mobility, encompassing a range of complex issues that impact society, the environment, and economic development.One of the most pressing challenges is congestion, particularly prevalent in densely populated urban areas.This congestion results in environment.Reducing greenhouse gas emissions from the transportation sector is therefore imperative for mitigating the effects of climate change and improving air quality [5].
Safety is also a major concern in the realm of mobility, with traffic accidents causing numerous fatalities and injuries worldwide each year.Inadequate infrastructure, insufficient enforcement of traffic laws, and unsafe driving behaviors all contribute to the high incidence of accidents, particularly in developing regions where road safety measures may be lacking [6].
Furthermore, infrastructure deficiencies hinder overall mobility efficiency.Aging transportation infrastructure and inadequate public transit systems limit access to reliable and affordable transportation options, especially for marginalized communities and those living in rural areas.This lack of accessibility exacerbates socioeconomic disparities and limits opportunities for economic and social mobility [7].
Energy dependence on finite fossil fuels poses additional challenges, including risks to energy security and economic stability.Transitioning to renewable energy sources and alternative fuels is crucial for reducing reliance on nonrenewable resources and mitigating the environmental impacts of transportation [8].
Technological advancements, such as the rise of autonomous vehicles and shared mobility services, present both opportunities and challenges for mobility.While these innovations hold the potential to improve efficiency and reduce emissions, they also raise concerns about data privacy, cybersecurity, and equitable access to transportation services [9].
Addressing these mobility challenges requires a comprehensive and integrated approach that combines sustainable urban planning, investment in infrastructure, promotion of alternative transportation modes, and adoption of innovative technologies.Collaboration among governments, industries, and communities is essential to develop and implement effective solutions that promote equitable, safe, and environmentally sustainable mobility systems for all [10][11][12].
In this way, hydrogen vehicles and their technologies represent an innovative and promising response to the aforementioned mobility challenges.Firstly, hydrogen vehicles offer a clean and sustainable transportation alternative by utilizing fuel cells that convert hydrogen into electricity, emitting only water vapor as a byproduct.This significantly reduces greenhouse gas emissions and air pollution, addressing the pollution and climate change issues associated with internal combustion vehicles [13,14].
Moreover, hydrogen vehicles can help alleviate congestion and improve transportation efficiency by providing greater range and shorter refueling times compared to conventional electric vehicles.This helps reduce reliance on fossil fuels and encourages the adoption of more efficient and sustainable mobility in urban and suburban settings.In terms of safety, hydrogen vehicles also offer advantages as their hydrogen storage and distribution systems are designed to meet stringent safety standards.Additionally, the infrastructure required for hydrogen production, storage, and distribution can be integrated with renewable energy systems, providing a more resilient and decentralized energy source [15,16].
However, there is a challenge in understanding which lines of research, development, and innovation are currently being pursued to grasp the efforts and thus generate relevant progress in various domains.It is crucial to identify ongoing initiatives and emerging trends across different fields to inform strategic decision-making, resource allocation, and collaborative endeavors.By comprehensively mapping out research and innovation landscapes, stakeholders can identify synergies, address gaps, and leverage opportunities for impactful advancements.This holistic understanding enables the cultivation of a dynamic ecosystem of knowledge exchange, interdisciplinary collaboration, and transformative innovation, driving sustainable development and societal progress forward [17][18][19][20][21][22].
In this respect, the current research is dedicated to scrutinizing the evolution of fuel-cell hydrogen vehicles and smart mobility, alongside key related themes in existing literature, employing bibliometric methodologies, techniques, and tools.Initially, the research quantifies crucial performance indicators, including published documents, citations received, prominent journals, leading authors, and geographic distribution of publications, among others.Subsequently, leveraging bibliometric analysis software grounded in bibliographic networks, the investigation delves into the realms of scientific knowledge development associated with the delineations of Fuel-cell hydrogen vehicles and smart mobility within a specified timeframe [23][24][25][26][27][28].

Methodology and dataset
Bibliometric methodologies, tools and techniques are indispensable for assessing the impact and evolution of knowledge domains such as hydrogen vehicles and smart mobility.They enable the measurement of academic, scientific, and productive quality, identifying key trends and contributors in knowledge development.Bibliometrics comprises performance analysis and scientific relationship mapping.Performance analysis evaluates publication impact through citations and temporal evolution, while relationship maps visualize interconnections among documents, themes, and authors, revealing hidden relationships between scientific concepts of interest.
To examine the relationship between these concepts, VOSviewer was employed alongside a comprehensive search in the widely recognized Scopus database.The query used is: TITLE-ABS-KEY("fuel cell vehicle*" OR "fuel-cell vehicle*" OR "f-cell vehicle*" OR "fuel cell electric vehicle*" OR "fuel-cell electric vehicle*" OR "fuel-cell hybrid electric vehicle*" OR "fuel cell-based electric vehicle*" OR "fuel cell-based hybrid electric vehicle*" OR "hydrogen vehicle*") AND TITLE-ABS-KEY("sustain*" OR "smart mobility" OR "intelligent transportation system*" OR "intelligent transportation system*" OR "intelligent transport*") AND (LIMIT-TO(DOCTYPE, "ar") OR LIMIT-TO(DOCTYPE, "cp") OR LIMIT-TO(DOCTYPE, "re")) AND (LIMIT-TO(LANGUAGE , "English")).
This query yielded a total of 610 publications, with 19,494 falling within the timeframe spanning from 1992 to 2023.Further refinement of the corpus was conducted, restricting it to English-language publications of article, proceeding, and review types.The citations associated with these publications were also incorporated into the analysis, with data collection extending up to December 13, 2023.
To enhance data quality, a de-duplication process was implemented, consolidating synonymous meanings and concepts under a unified label.For instance, terms like "fuelcell vehicle" "f-cell vehicle" and "fcv" were merged into a single category termed "fuel cell vehicle".
Bibliometric relationship maps serve as valuable tools for organizing and analyzing scientific information, facilitating the visualization and classification of complex topics.Relevant themes present in at least 15 publications were identified and grouped to unveil emerging application fields, subsequently ranked by significance and represented with distinct colors.
It is crucial to acknowledge that the relationship between themes can be internal or external to the groups, reflecting how one theme may influence others.This relationship should be assessed considering its frequency, as the balance between occurrences and connections indicates the theme's relevance to research in the field.

Publications and Citations
Interest in researching the realms of smart mobility and hydrogen vehicles is burgeoning not only within the academic and scientific communities but also among businesses.This surge in interest stems from the recognition of the pressing need to address transportation challenges such as congestion, pollution, and energy dependence.Academia and research institutions are driven by a desire to explore innovative solutions and advance knowledge in these areas, aiming to develop sustainable and efficient mobility systems.Similarly, businesses are motivated by the potential for market growth, innovation, and competitive advantage in providing solutions for smart mobility and hydrogenpowered transportation.Additionally, there is a growing awareness of the economic and environmental benefits associated with investing in these fields, further fueling interest and engagement from both academic and business sectors.Considering the observed development pattern, it is reasonable to anticipate that this positive trend will persist.However, it's important to note that the total citations of publications over the past three years have displayed a downward trend, albeit not reflecting an actual decline.As highlighted by Wang [29], publications tend to accrue the most citations in preceding years due to the time lag between publication and referencing.It's estimated that a publication typically requires between 3 to 7 years to achieve its peak citation count.Taking into account the performance and impact results, it is possible to say that, the synergy between fuel-cell hydrogen vehicles and smart mobility is still growing in terms of quantity and quality and consolidating its position as a reference point for improving an organization's competitiveness.

Science mapping analysis of the synergy between fuel cell hydrogen vehicles and smart mobility
Subsequently, an examination is presented regarding the science mapping and the latent connections among key themes within the primary research domains associated with the convergence of fuel-cell hydrogen vehicles and smart mobility.This examination comprises two interrelated components: an analysis of the content delineated in the published articles and a conceptual evolution map.The former facilitates the identification of emerging themes within the corpus of fuel-cell hydrogen vehicles and smart mobility, elucidating their principal concepts and their contribution to the establishment of the field over the analyzed period.Meanwhile, the latter illustrates the progression of these themes and the interconnections among them throughout the entire duration of analysis.

Analysis of the content of the articles published related to the synergy between fuel cell hydrogen vehicles and smart mobility.
Figure 2 presents a network visualization map depicting the synergy between fuel-cell hydrogen vehicles and smart mobility spanning from 1992 to 2023.The identified research themes have been categorized into three clusters based on their relevance and relationships.Additionally, within the network visualization map, these research themes are depicted as spheres, with the size of each sphere proportional to the number of associated publications.The lines connecting these spheres represent the common elements covered by these themes.
A total of 68 core themes were identified within the synergy between hydrogen vehicles and smart mobility.These themes concentrate 3,129 occurrences and 19,396 links between them.As previously mentioned, to understand their relationship and position in the development of knowledge, they were classified into three clusters: Sustainable development (Green, 24 themes, 1,105 occurrences and 7,335 total link strengths), Hydrogen technologies (Red, 24 themes, 1,053 occurrences and 6,431 total link strengths) and New energy systems (Blue, 20 themes, 1,034 occurrences and 5,630 total link strengths).In this background, the Table 2, Table 3 and Table 4 present the performance indicators (number of occurrences and total links strength) of the themes within each cluster.The Sustainable development cluster (Table 2) are inherently linked to hydrogen technologies and smart mobility, offering crucial alternatives to fossil fuels and addressing associated environmental challenges such as greenhouse gas emissions.This cluster cover themes such as: greenhouse gas emissions, alternative fuels, environmental impact, sustainability, among others.
In this regard, it is possible to comment that, the use of hydrogen-powered fuel cell vehicles presents a clean and sustainable alternative to internal combustion vehicles, emitting only water as a byproduct.This drastic reduction in greenhouse gas emissions contributes to mitigating climate change and improving air quality.Additionally, smart mobility, including technologies like autonomous vehicles, shared transportation services, and advanced traffic management systems, optimizes resource use and reduces traffic congestion, thereby decreasing greenhouse gas emissions and enhancing air quality in urban environments.By replacing fossil fuels with hydrogen as a transportation fuel, dependence on non-renewable resources is reduced, fostering a transition towards a cleaner and more sustainable economy.Furthermore, the use of hydrogen produced from renewable sources, such as wind or solar energy, can lead to an even more sustainable transportation system by completely eliminating greenhouse gas emissions associated with fossil fuel production and use.
In summary, hydrogen technologies and smart mobility offer innovative and sustainable solutions to address environmental and economic challenges associated with fossil fuels and greenhouse gas emissions.By promoting their adoption and development, progress can be made towards a cleaner, more efficient and sustainable future of transportation.In the same way, the themes within Hydrogen technologies cluster (Table 3) are closely intertwined with hydrogen as a fuel, its storage systems and infrastructure, the utilization of renewable energies, and commercial applications.
Firstly, hydrogen plays a crucial role as a fuel in sustainable mobility, with the ability to be produced from renewable sources like solar or wind energy through electrolysis processes.This makes it an appealing option for reducing greenhouse gas emissions and lessening dependence on fossil fuels in transportation.Ensuring a reliable and safe supply of hydrogen for various applications, including vehicles, relies on essential hydrogen storage systems.These encompass various technologies such as compression, liquefaction, and storage in metal hydrides, each presenting its own set of advantages and technical challenges.
In this background, the development of hydrogen infrastructure, comprising refueling stations and distribution systems, is vital to facilitate the widespread adoption of hydrogen vehicles.Expanding this infrastructure globally is crucial to ensuring the accessibility and convenience of hydrogen as a fuel.
Integrating renewable energies into hydrogen production further enhances its sustainability and reduces its carbon footprint.Utilizing clean energy sources to generate hydrogen establishes a renewable fuel cycle that significantly contributes to mitigating climate change and conserving natural resources.
On the other hand, commercially, hydrogen and smart mobility technologies offer promising prospects across sectors like transportation, energy, and industry.Opportunities include the advancement of hydrogen vehicles, the establishment of hydrogen infrastructure, and the provision of smart mobility services, among others.Lastly, the New energy systems cluster (Table 4) is related closely with various areas, including fuel cells and their different types, batteries, energy management, and various types of vehicles.
Fuel cells are central to hydrogen vehicles, converting hydrogen into electricity for propulsion.Various types exist, such as proton exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC), each with distinct advantages and applications.These technologies facilitate efficient and clean electricity generation, crucial for sustainable mobility.Moreover, batteries are integral to smart mobility, notably in hybrid and electric vehicles.They store energy for electric motors and can complement fuel cells, enhancing energy efficiency.Effective energy management optimizes propulsion system performance, regardless of fuel source.Smart mobility leverages advanced energy management technologies to maximize vehicle autonomy and efficiency.Finally, hydrogen and smart mobility technologies are adaptable to diverse vehicle types, including cars, buses, trucks, trains, and drones.These vehicles address various transportation needs, from urban mobility to long-distance cargo and passenger transport.
Together, hydrogen and smart mobility technologies have the potential to provide innovative and sustainable solutions to address the mobility and energy challenges of the 21st century.By effectively leveraging these technologies, we can move towards a future of transportation that is cleaner, more efficient, and more sustainable.The discussed topics highlight the pivotal role of hydrogen and smart mobility technologies in addressing pressing energy and mobility challenges.From reducing greenhouse gas emissions to creating new business opportunities, these innovations offer promising solutions for a cleaner, more efficient, and sustainable transportation future.By prioritizing the advancement and adoption of these technologies, we can strive towards a more resilient and environmentally friendly society.

Conceptual evolution map related to the synergy between fuel cell hydrogen vehicles and smart mobility.
Utilizing the bibliometric methodology outlined in this study, which employs VOSviewer, evolution maps are employed to unveil the developmental trends within the field across the analyzed periods through graphical interaction.The evolution network visualization map illustrating the synergy between fuel-cell hydrogen vehicles and smart mobility from 1992 to 2023 is depicted in Figure 3.
The development of hydrogen vehicles and smart mobility has been an evolutionary process that has involved multiple areas of research and technology.It all began with research on hydrogen as a fuel and the improvement of combustion engines.Hydrogen has been recognized as a promising alternative fuel due to its high energy density and its ability to produce zero emissions when combined with oxygen in a fuel cell.This makes it an attractive option for reducing greenhouse gas emissions and dependence on fossil fuels in transportation.

Figure 3. Evolution network visualization map of the synergy between Fuel-cell hydrogen vehicles and smart mobility from 1992 to 2023
As research on hydrogen as a fuel progressed, so did the need to improve combustion engines to make them more efficient and compatible with this new energy source.Research was conducted in areas such as fuel-air mixture optimization, reduction of internal friction, and improvement of thermal efficiency to maximize the performance of hydrogen vehicles.
Simultaneously, the development of hydrogen vehicles and smart mobility was also driven by advances in communication and digitization technologies.The integration of information and entertainment systems, driver assistance, and internet connectivity in vehicles allowed for a safer, more comfortable, and connected driving experience.This not only improved the user experience but also laid the foundation for the future integration of autonomous driving technologies and connected vehicles.
Additionally, the focus on environmental protection and reducing environmental impact has been a central aspect in the development of hydrogen vehicles and smart mobility.Increasingly strict regulations on exhaust emissions and environmental standards have driven the adoption of cleaner and more sustainable technologies in the transportation sector.Hydrogen vehicles, by producing zero local emissions and not generating atmospheric pollutants during operation, have stood out as a viable solution to address these environmental challenges.
Recently, the focus has shifted towards advances in renewable energies and the utilization of new propulsion systems in hydrogen vehicles and smart mobility.With growing concerns about climate change and the need to reduce greenhouse gas emissions, research into clean and renewable energy sources, such as solar, wind, and hydroelectric, for sustainable hydrogen production has intensified.Additionally, new propulsion systems, such as solid-state fuel cells and high-energy-density hydrogen storage systems, are being developed and adopted to improve the efficiency and autonomy of hydrogen vehicles.
Lastly, connectivity between vehicles, infrastructures, and users has become a crucial research area in the development of hydrogen vehicles and smart mobility.The implementation of vehicle-to-vehicle (V2V), vehicle-toinfrastructure (V2I), and vehicle-to-everything (V2X) communication technologies allows for more efficient communication and coordination between the different elements of the mobility ecosystem, enhancing road safety, reducing traffic congestion, and optimizing transportation flow.Furthermore, the growing adoption of shared mobility services and smart transportation platforms is transforming how people move and utilize transportation resources in modern cities.
In summary, the development of hydrogen vehicles and smart mobility has been driven by a wide range of research and technological advancements, from initial research on hydrogen as a fuel to recent developments in renewable energies, propulsion systems, and connectivity.These advancements are transforming the transportation sector and offering innovative and sustainable solutions to address current and future mobility challenges.

Conclusions
This study represents the inaugural bibliometric investigation into hydrogen vehicles and smart mobility, delineating its principal concepts and identifying key themes and associated research domains.Over 610 original research articles were meticulously scrutinized and analyzed employing VOSviewer.
In terms of bibliometric performance, the volume of literature pertaining to research on hydrogen vehicles and smart mobility has exhibited a significant surge in recent years .Given the substantial number of publications and citations garnered in this domain, it is anticipated that interest will continue to escalate, thereby bolstering other knowledge domains such as sustainable development, climate change, fuel cell technologies, connectivity, and beyond.
It's noteworthy that the expansion of the hydrogen vehicles and smart mobility field, along with its primary components, has facilitated its integration into the energy and automotive industries.Nevertheless, these industrial sectors are perpetually evolving, prompting the quest for synergies among various approaches to hydrogen vehicles and smart mobility.
In this background, the synergy between hydrogen vehicles and smart mobility comprises a total of 68 core themes.These themes have been categorized into three clusters: Sustainable development (Green), Hydrogen technologies (Red), and New energy systems (Blue), covering themes such as: sustainable development, fossil fuels, greenhouse gas emissions, internal combustion engines, energy policy, hydrogen fuels, hydrogen storage system, renewable energy source, sustainable mobility, hydrogen production, economic and social effects, fuel cell stack, energy management strategies, energy storage systems, electric batteries, fuel cell system, among others.
The importance of various factors in the context of hydrogen vehicles and smart mobility cannot be overstated.Sustainable development stands as a guiding principle, ensuring that advancements in transportation align with environmental preservation and societal well-being.Fossil fuels, long the backbone of transportation, are being reconsidered due to their finite nature and contribution to greenhouse gas emissions, necessitating a shift towards cleaner alternatives.The focus on reducing greenhouse gas emissions underscores the urgency to transition away from internal combustion engines, notorious for their environmental impact, towards more sustainable propulsion technologies such as fuel cells.
Energy policy plays a pivotal role in shaping the trajectory of hydrogen vehicles and smart mobility, as government regulations and incentives influence adoption rates and infrastructure development.Hydrogen fuels and their storage systems are central to enabling the widespread use of fuel cell vehicles, offering a viable alternative to traditional gasoline or diesel.Leveraging renewable energy sources for hydrogen production further enhances sustainability, reducing reliance on finite resources and minimizing environmental harm.
The concept of sustainable mobility encompasses various aspects, from reducing emissions and congestion to promoting accessibility and equity in transportation.Economic and social effects are significant considerations, as the transition to hydrogen vehicles and smart mobility can impact industries, job markets, and societal behaviors.Fuel cell stacks serve as the heart of hydrogen vehicles, converting hydrogen into electricity with remarkable efficiency.
Effective energy management strategies are essential for optimizing the performance and range of hydrogen vehicles, ensuring seamless integration into existing transportation networks.Energy storage systems, including electric batteries and fuel cell systems, play complementary roles in providing power and extending the driving range of vehicles.
In summary, these factors collectively shape the landscape of hydrogen vehicles and smart mobility, guiding innovation and policy decisions towards a more sustainable and efficient future of transportation.By addressing each element comprehensively, stakeholders can work towards realizing the full potential of hydrogen vehicles and smart mobility in transforming the way we move and interact with our environment.
Finally, future research in the field of hydrogen vehicles and smart mobility holds vast potential for further advancements and innovations.One crucial area of investigation could focus on optimizing the performance of hydrogen fuel cell systems, aiming to enhance durability, reduce costs, and improve energy conversion efficiency.Additionally, there is a need for research into expanding hydrogen refueling infrastructure to support the widespread adoption of hydrogen vehicles, including identifying optimal locations for refueling stations and exploring decentralized hydrogen production methods.
Further advancements in energy storage technologies, such as solid-state hydrogen storage and novel battery chemistries, could significantly improve energy density, charging speed, and overall lifespan of energy storage systems in hydrogen vehicles.Moreover, exploring innovative methods for renewable hydrogen production, such as electrolysis powered by renewable energy sources or photoelectrochemical water splitting, could further reduce greenhouse gas emissions associated with hydrogen production.
Integration of hydrogen vehicles into renewable energy grids, development of intelligent transportation systems for optimizing traffic flow and reducing congestion, and comprehensive lifecycle assessments to evaluate environmental and societal impacts are also crucial areas for future research.Additionally, investigating the development of supportive policy frameworks and regulatory incentives, understanding consumer attitudes and and promoting interdisciplinary collaboration across sectors will be essential to drive the successful deployment and adoption of hydrogen vehicles and smart mobility solutions.
By addressing these research priorities, stakeholders can contribute to advancing the state-of-the-art in hydrogen vehicles and smart mobility, ultimately paving the way for a more sustainable, efficient, and equitable transportation future.

Figure 1 .Figure 1
Figure 1.Distribution of publications and citations related to the synergy between fuel-cell hydrogen vehicles and smart mobility by year (1992-2023)

Figure 2 .
Figure 2. Network visualization map of the synergy between Fuel-cell hydrogen vehicles and smart mobility from 1992 to 2023

Table 2 .
Most relevant themes included in the

Table 3 .
Most relevant themes included in the

Table 4 .
Most relevant themes included in the New