1 Automotive revolution
1.1 Automotive electrification
The automotive industry is a vital component of modern transportation, and advancements in technology are rapidly transforming traditional modes of travel. Innovations such as artificial intelligence (AI), Big Data, smart sensors, and renewable energy sources are boosting the efficiency, safety, and comfort of transportation. Additionally, intelligent transportation systems have the potential to reduce greenhouse gas emissions, promoting environmental protection and energy security.
Climate change is a major challenge facing humanity, and the transportation sector is a significant contributor to greenhouse gas emissions, as highlighted in Fig.1 [
1–
4]. In the coming decades, clean energy sources like solar photovoltaics and wind power are expected to become the dominant energy source. Electrification of transportation is crucial for achieving sustainable travel, and infrastructure development is key to this transition. The establishment of solar and wind power plants, as well as charging stations, is necessary to ensure the stable growth of sustainable energy. The electric vehicle market is rapidly expanding, and as such, the number of charging stations must also increase to meet the demand. Innovations such as vehicle-to-home, vehicle-to-vehicle, and vehicle-to-grid are being explored to improve energy distribution, grid stability, and reduce energy waste [
5]. Effective energy management will optimize electricity distribution, promoting energy sharing between various electrical terminals to promote grid balance, and traditional electrical equipment can also serve as a source of power, feeding back energy to the grid or other devices.
The development of a suitable rechargeable energy storage system poses a critical challenge to automotive electrification, with the power battery having undergone over a century of development since the rechargeable lead-acid battery was invented in 1859 [
6]. However, the degraded performance of batteries in low-temperature environments, their high cost and low energy density, and the risk of explosions continue to limit their overall promotion in electric vehicles (EVs). Meanwhile, fuel cell electric cars offer a unique power source that relies on the most abundant element in the universe, hydrogen, which reacts electrochemically to generate electricity to power the vehicle, making it distinct from battery-powered or plug-in hybrid cars that operate using different mechanisms [
7]. A comprehensive comparison between power battery and hydrogen fuel cell is given in Tab.1.
1.2 Intelligence-connected transportation
The transportation sector has undergone a significant transformation due to the automotive revolution. One of the key areas of development in this revolution is intelligent transportation [
8]. Intelligent transportation systems are built on communication and data exchange between different platforms, and sensors play a critical role in obtaining, collecting, and processing information from the surroundings. The accuracy of data from sensors is crucial to the success of the intelligent transportation system. Sensors can be installed on transportation or infrastructure, which are essential for collecting information that is transferred to control systems for further analysis or actions. The intelligent-connected transportation system has three stages.
The first stage is dynamic perception, which relies on a large number of sensors and strong data analysis through technologies such as AI. In-vehicle sensors monitor the state and environmental information of the vehicle, while in-road sensors collect real-time traffic data. The data from sensors can be analyzed by Big Data and AI to solve congestion and improve efficiency.
The second stage is active management, which is based on automated means to sense, transmit, and apply information. The data collected from in-vehicle and in-road sensors can be processed and analyzed to help manage transportation and optimize paths. Big Data can offer active traffic management and active command and control.
The last stage is intelligent and connected, which is the overall development trend of the intelligent transportation system in the future. The development of vehicle-road cooperation puts forward corresponding demands for infrastructure intelligence. The sensors are added and updated in transportation and infrastructure, and the data will be summarized and analyzed together. Fig.2 shows the case of intelligence-connected transportation based on EVs. The critical point is that the information of every node is traceable and can be extracted.
Self-driving vehicles will play a key role in transportation, relying on intelligent computer systems to perform driving tasks independently, replacing human drivers in transport. Although self-driving vehicles encounter difficulties mainly because the key technology has not yet broken through, manufacturing costs are too high, and there are no corresponding laws and regulations, these problems will be solved soon. Eventually, self-driving vehicles will become a major player in transportation.
1.3 Smart transportation system
The smart transportation system (STS) is a cutting-edge service designed to cater to human travel needs, with its core focus being mobility as a service (MaaS). STS leverages the power of the Internet of Things (IoT) to connect multiple modes of transportation and create an intelligent traffic network. As urbanization continues to grow, STS can help mitigate traffic congestion issues. The key elements of STS include travel needs, travel carriers, travel routes, and travel conditions. When integrated, these four elements create a seamless and intelligent travel experience. The emergence of digital humanistic networks has brought about a significant revolution in the automotive industry. These networks act as the missing link between the energy, information, and transportation networks, driving the development of intelligent systems capable of evolving and upgrading over time. The integration of these four networks and four flows (4N4F) leads to an integrated industrial ecology that can better promote and release the enormous potential of digital productivity [
9]. As technology and science continue to improve, more efficient and comfortable modes of transportation will emerge, ushering in a new era of smart transportation systems.
At the heart of 4N4F lies a commitment to people-centered, sustainable development, and the harmonious coexistence between humanity and nature [
10]. This foundation is built upon the integration of four networks (energy, transportation, information, and humanities) and four streams (energy, information, material, and value) forming the basis for the core objectives of the project. Central to the theory behind 4N4F is the concept of deep integration between the human, information, and physical worlds, emphasizing the interconnectedness of these domains and the importance of collaboration and cooperation in achieving shared goals. The humanities network serves as the superstructure, while the energy, transportation, and information networks serve as the economic foundation. This integration reflects the new productivity and new production relations in the digital economy. Although challenges exist, such as the lack of complete explainability of Big Data algorithms, the benefits of the digital humanistic network revolution for the automotive industry are significant. Through intelligent systems and 4N4F integration, the industry can become more efficient, productive, and sustainable. As the energy industry transitions from Big Data intelligence to cognitive intelligence, it is crucial to overcome the barrier from traditional perception intelligence to future decision intelligence. This can be challenging for most high-safety industries due to the incomplete explainability of Big Data algorithms. However, STS and 4N4F integration provide the automotive industry with an opportunity to optimize its potential and drive a more sustainable future.
2 Hydrogen energy for carbon neutrality
2.1 Overview of carbon neutrality
Carbon neutrality has been identified as the most pressing mission for the world to tackle. More than 130 countries across the globe have proposed a timeframe to achieve carbon neutrality. Developed nations have set their sights on achieving carbon neutrality by 2050, while China has pledged to achieve this target by 2060 [
11]. Despite these commitments, carbon emissions of the world continue to soar at a staggering rate, with a year-on-year growth rate of 5.27% (equivalent to 37.12 Bt) in 2021.
It is worth noting that China has surpassed the United States since 2006 as the largest contributor of CO
2 emissions, accounting for a massive 30.90% of the CO
2 emissions (equivalent to 11.47 Bt) in the world by 2021. Urgent and immediate action is required to reduce carbon emissions in order to achieve carbon neutrality targets (as illustrated in Fig.3). This action must be tailored to individual countries and based on sound scientific research in order to be effective. In 2019, power generation (which includes electricity and heat) and transportation were responsible for over two thirds of total emissions globally and have been the main contributors to emissions growth since 2010. Power generation was the largest emitting sector, accounting for 42.75% of global emissions (equivalent to 15.76 Bt) in 2019. The transportation sector followed closely behind, accounting for 22.30% of global emissions (equivalent to 8.22 Bt) in the same year. In China, power generation was also the largest emitting sector, accounting for 55.56% (equivalent to 5.59 Bt) of national emissions in 2019. The transportation sector was the third largest contributor to carbon emissions, accounting for 8.96% (equivalent to 0.90 Bt) of national emissions in the same year, with 80% of these emissions coming from road transport [
12]. To achieve carbon emission reductions and meet carbon neutrality targets (as illustrated in Fig.4), switching to electrification in the power generation and transportation sectors is considered a promising strategy. An energy and automotive revolution in these sectors can facilitate this transition.
The reliance of various sectors on fossil fuels for both power generation and transportation is well-known. However, if electricity production is shifted from fossil fuels to renewable sources, the electrification of mobility could serve as a promising option for decarbonizing the transport sector [
13]. This is why the electrification and automotive revolution in transportation are crucial in achieving carbon neutrality. EVs have significant advantages over internal combustion engine vehicles (ICEVs) in terms of reducing carbon emissions due to their zero tailpipe CO
2 emissions [
14]. Scholars have also confirmed the potential of EVs in reducing carbon emissions [
15], with EVs emitting 80% less CO
2 than diesel and 81% less than petrol. Battery-powered EVs (BEVs) and plug-in hybrid EVs (PHEVs) are the most popular EV models, with varying levels of powertrain system electrification.
Many governments have identified transport electrification as a key strategy to reduce carbon emissions [
16]. France, Norway, Netherlands, and UK have launched definite bans on fuel-powered vehicle sales soon [
17], making the deployment of EVs a feasible means to promote electrification and automotive revolution [
12]. To meet the increasing public demand for vehicles in China and reduce carbon emissions in the transport sector, the authorities in China have been providing policy and technical support to promote EVs. After entering commercial markets in the first half of the decade, electric car sales have soared, with only 17 thousand electric cars on the roads in the world in 2010, and by 2021, that number had grown to 16.2 million, with 48.15% of them in China. At least 17 countries had more than 100 thousand electric cars on the road, with 20 countries reaching market shares above 1% [
18]. Meanwhile, the stock of EVs continues growing with a 57.28% year-on-year growth rate in 2021, as shown in Fig.5.
In April 2012, the Chinese government approved the Development Plan for Energy Conservation and the New Energy Vehicle Industry (2012–2020). The plan aimed to achieve a cumulative production and sales of 500 thousand EVs and PHEVs by 2015 and more than 2 million production capacity and over 5 million cumulative production and sales of the vehicles by 2020. Currently, China’s EV sales have increased by 73.33% in 2021. Carbon neutrality is especially critical for China. China has set targets to achieve carbon peak by 2030 and carbon neutrality by 2060. However, this poses a significant challenge given the limited time frame, and the process requires significant restrictions to energy development. The 4N4F theory, proposed by Prof. C. C. Chan [
19], can help address this challenge by providing a practical pathway for carbon neutrality through the development of regional intelligent energy control centers, EVs, and new energy industries represented by photovoltaic buildings. This system can efficiently convert waste energy into useful energy and promote carbon neutralization. The 4N4F theory is a powerful tool that links the social, cyber, and physical worlds, embedding a correlation between energy, information, and human behavior. It provides new opportunities for clean energy, including the incremental space for natural gas power generation, wind power and photovoltaic, and the promotion of grid integration for energy infrastructure.
2.2 Production of hydrogen energy
The world is facing dual pressures from energy shortages and environmental degradation, highlighting the urgent need for sustainable and clean energy development. According to the latest
World Energy Statistics Yearbook 2022, published by British Petroleum, global primary energy demand is projected to grow by 5.8% year-on-year in 2021. Furthermore, global gas demand will increase by 5.3%, coal consumption by over 6%, and oil consumption by 5.3 million barrels per day [
20]. In response, hydrogen energy has emerged as a promising solution as it can achieve zero emissions and pollution from development to utilization [
21]. Hydrogen energy has numerous advantages. First, it has a high calorific value of 142351 kJ/kg, approximately three times the calorific value of gasoline. Second, it has good combustion properties, a wide combustible range when mixed with air. Next, it is non-toxic, and the water generated by combustion can continue to produce hydrogen for repeated recycling (partial loss of water during electrolysis process). Finally, the use of a wide range of forms and formats, which can occur in gaseous, liquid, or solid metal hydrides, can be adapted to the requirements of storage, transport and different application environments.
Fig.6 illustrates the hydrogen energy chain, which comprises upstream, midstream, and downstream components. The upstream component involves the preparation of hydrogen, with main technologies including thermochemical reforming of conventional energy sources and electrolysis of water [
22]. The midstream encompasses the storage and transportation of hydrogen, utilizing technologies such as low-temperature liquid, high-pressure gaseous, and solid material hydrogen storage. The downstream involves the application of hydrogen, which has potential uses in various aspects of conventional energy sources, including transportation, industrial fuels, and power generation, through the construction of hydrogen stations and fuel cell technology [
23]. The quality of hydrogen plays a crucial role in the effective operation of the entire chain. Currently, fossil energy hydrogen production and electrolytic water hydrogen production are the most commercialized and scalable hydrogen production technologies, as described below.
2.2.1 Hydrogen production by fossil fuels
At present, the more established technologies for hydrogen production from fossil fuels include coal-based hydrogen production and natural gas-based hydrogen production, which produce hydrogen through chemical pyrolysis or gasification. The primary process for hydrogen production from coal involves mixing coal with oxygen or steam, converting it into a mixture of H
2 and CO at high temperature, and then obtaining high-purity hydrogen products through water–gas shift (WGS), acid gas removal, and hydrogen purification [
24]. The Co–Mo catalyst system is currently utilized in coal gasification hydrogen production systems as a wide temperature sulfur-resistant conversion process, characterized by its excellent sulfur resistance and broad temperature range.
Coal hydrogen production technology has a long history and is characterized by mature technology and low cost. However, this solution has the following disadvantages: serious greenhouse gas emissions, and extremely high input costs as the scale of hydrogen production rises and high maintenance costs later on. In addition, hydrogen production from natural gas is another method of hydrogen production, which uses natural gas to react with water vapor to generate hydrogen. The technical solutions used are steam reforming (SRM), the partial oxidation method (POM), autothermal reforming (ATR), and the catalytic cracking method (CCD) [
25]. However, each option has its shortcomings due to its limitations. For example, SRM catalysts are prone to deactivation and have high greenhouse gas emissions, while the CCD technology is not mature enough. In summary, hydrogen production from fossil energy sources needs to be based on SRM and the synergistic development of the remaining technological solutions, with the help of technological breakthroughs in the development of highly active catalysts and the improvement of reactors, reflecting the comprehensive advantages of efficiency and economy, which is the trend in the development of hydrogen production technology from natural gas.
2.2.2 Hydrogen production by electrolytic
The natural volatility and intermittency of photovoltaic and wind power sources present a significant challenge to the safe and stable operation of the power grid, particularly when connected to the grid with large capacity, leading to the growing problem of wind and solar energy curtailment [
26]. However, through the process of hydrogen electrolysis, the abundant electricity from wind and solar energy curtailment can be fully utilized, conserving electricity resources and significantly reducing the cost of hydrogen production. Moreover, local consumption of renewable energy through hydrogen electrolysis is crucial for achieving important economic and social benefits, while also mitigating the impact of their volatility on the power grid [
27]. By doing so, this approach can alleviate the key issues that limit the promotion of hydrogen production from electrolytic water, such as its high energy consumption and associated high costs.
Various projects related to renewable hydrogen production are underway globally. For instance, German companies RWE and Vattenfall are collaborating on a pilot project that aims to develop two 14 MW turbines by 2025. By 2035, the project is expected to produce an estimated 1 Mt of hydrogen annually [
28]. Shell Nederland and Shell Overseas Investments are constructing Holland Hydrogen I, which will become Europe’s largest renewable hydrogen project by 2025. Japan has also taken the lead in the field, publishing a
Strategic Roadmap for Hydrogen Energy and Fuel Cells, which outlines a three-stage development target for 2025, 2030, and 2040 [
29,
30]. China has been actively exploring new ways to consume wind power since 2014 when it launched a project on the research and demonstration of direct hydrogen production from the wind power and fuel cell power generation system technology [
31]. In 2017, the largest wind power hydrogen production project of the world, the Hebei-Guyuan Wind Power Hydrogen Production Station, was successfully launched, providing a foundation for the scale and industrialization of wind power hydrogen production [
32]. In 2021, the first MW-level solid polymer electrolytic water hydrogen production and fuel cell power generation demonstration project was constructed in Lu’an, a prefecture-level city in western Anhui province, making it the first MW-level hydrogen energy storage power station in China. This project marks the first application of MW-level in the grid.
At the technical level, the researcher proposes a robust control method for the output current of direct-current/direct-current (DC/DC) converters that takes into account the power requirements of the electrolytic cell and the output current of the photovoltaic (PV) array [
33], with more accurate regulation performance, better stability margins and faster dynamic response than those in Refs. [
34,
35]. Fang’s team [
36] discussed the impact of wind and electric waves on the efficiency and safety of hydrogen production in electrolytic cells, and in combination with the formation of a hybrid energy storage system using supercapacitors, proposed an adaptive control strategy for alkaline electrolytic cells. In 2021, Perry Hydrogen Energy developed a single alkaline water electrolysis hydrogen production equipment with a hydrogen production capacity of 2000 Nm
3/h using silicon rectification instead of the original silicon-controlled rectifier method, reducing harmonic interference and reactive power loss to the power grid during device operation, and effectively eliminating high harmonic pollution to the power grid [
37]. In 2022, Cockerell Jingli Hydrogen in Suzhou, Jiangsu province and several companies jointly developed a 1300 Nm
3/h electrolytic cell that was one-third smaller than the original size, and constructed a new nickel-based multi-element alloy catalytic layer, effectively improving the working current density, making the current density of the internationally commercialized large-scale water electrolysis hydrogen production equipment reach 6000 A/m
2 for the first time [
38].
Overall, the renewable energy electrolysis technology for hydrogen production has developed rapidly, but there are still problems such as low hydrogen production efficiency and high energy consumption. Meanwhile, utilizing photovoltaic and wind energy to obtain hydrogen cheaply is an effective way to promote the application of hydrogen energy. However, there are still many issues that need to be addressed, such as high-power hydrogen production power supplies that adapt to wide power fluctuations, low-cost, ultra-high-power PEM electrolytic cells, and stable and reliable renewable energy complementary energy management systems.
2.2.3 Comparative analysis of hydrogen production technologies
Tab.2 presents a comparative analysis of various hydrogen production technologies discussed earlier. Although fossil fuel-based hydrogen production methods offer high production rates, low cost, and a mature technology, they still have greenhouse gas emissions. Therefore, the development focuses on the transition from gray hydrogen to blue hydrogen. Water electrolysis is the primary method for large-scale production of green hydrogen, but it has limited development due to its high energy consumption and cost. Future research should prioritize reducing electricity prices, improving the efficiency of electrolytic water hydrogen production, and lowering the cost of hydrogen production by utilizing renewable energy. Combining renewable energy sources with hydrogen production is the inevitable trend for the future of electrolytic hydrogen production.
2.3 Transportation of hydrogen energy
According to Ref. [
39], the transportation of hydrogen energy can be classified into two forms: distributed and centralized. In the case of distributed production, hydrogen is produced on a small scale in local areas where it is also consumed. This approach can eliminate the need for expensive transportation and distribution infrastructure. Conversely, centralized production involves the production of hydrogen in a different location from where it will be consumed. Although centralized production can benefit from economies of scale and lower the unit cost of hydrogen production, it requires significant investment in transportation infrastructure, which can be costly [
40,
41]. As the hydrogen energy industry continues to develop, the advantages of centralization will become more apparent. However, it is important to establish the necessary transmission infrastructure before realizing these benefits.
Hydrogen transportation is divided into two primary modes: tunnel transportation and road transportation. However, to achieve long-distance and large-scale transportation of hydrogen, pipeline transportation plays a crucial role. Currently, China is in the initial stages of developing its hydrogen infrastructure, with only a hydrogen gas tunnel network spanning approximately 300–400 km. The longest hydrogen tunnel in China is the Baling–Changling, starting from Ulanqab City, Inner Mongolia Autonomous Region and ending at Yanshan Petrochemical in Beijing, which spans about 42 km with a pressure of 4 MPa [
42,
43]. In comparison, the United States and Europe have already built hydrogen pipelines spanning 2400 and 1500 km, respectively. Tab.3 presents a comparative analysis of the primary hydrogen transmission methods. Compressed hydrogen is typically transported via road, with a small transportation volume, short distance, and relatively high cost. Meanwhile, the transportation distance of liquid hydrogen via road is relatively long, and the transportation volume is more significant than that of compressed hydrogen. In general, the focus of future hydrogen transportation development will be on liquid hydrogen transportation, given the accelerated construction of liquid hydrogen storage and hydrogenation stations.
2.4 Application of hydrogen energy
Hydrogen energy finds its primary applications in hydrogen internal combustion engines and fuel cell technology. Compared to hydrogen internal combustion engines, the fuel cell technology has a higher efficiency and greater development potential. Today, the fuel cell technology is widely used in three areas, i.e., transportation, stationary, and portable applications [
44]. Fuel cell vehicles are being actively promoted and are a key area for hydrogen energy applications. It is anticipated that this technology will further expand to downstream areas related to energy, such as automotive, power generation, and energy storage.
Currently, two common types of hydrogen fuel cells are the proton exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC). Although PEMFC is suitable for automotive use in terms of performance, its high cost remains a significant challenge [
45,
46]. The membrane electrode in PEMFC is one of the key components, consisting of a proton exchange membrane, a catalyst, and a diffusion layer. However, this type of membrane is prone to chemical degradation, and high temperatures lead to poor proton conductivity and increased costs. In this regard, domestic and foreign research institutions, including DuPont Company, Allard Company in the United States, and Dongyue Group in China, have made numerous attempts and achieved remarkable results [
47]. However, the use of platinum as the catalyst accounts for 36% of the total cost of fuel cells, making it a primary reason for the high cost of PEMFC. Thus, the development of affordable alternative materials is a key factor for the future development of PEMFC. On the other hand, SOFC is a solid-state power generation device composed of an anode, cathode, electrolyte, sealing material, and connecting material, where the electrolyte is the core component. Leading enterprises in this field include Bloom Energy of the United States and Mitsubishi Heavy Industries of Japan [
48]. Nevertheless, most other countries that have explored the field of SOFC are still in the stage of laboratory research and prototype development and have not yet formed commercial SOFC systems.
Japan and South Korea are leading the way in terms of the global mass production scale and market commercialization share of hydrogen fuel cell vehicles. From 2015 to 2018, the global sales of hydrogen fuel cell vehicles doubled each year. Currently, there are four models that have achieved mass production internationally, such as the Toyota Mirai, Honda Clarity Fuel Cell, Hyundai ix35FCEV, and Hyundai NEXO. In China, the sales volume of fuel cell vehicles in 2018 was 1527, including 1418 passenger cars and 109 trucks. The Blue Book on Infrastructure Development of China’s Hydrogen Energy Industry predicts that by 2030, the number of fuel cell vehicles in China will reach 2 million, marking a rapid development period for fuel cell vehicles in China [
49,
50].
In summary, there is currently a considerable gap between China, Japan, and South Korea, and the gap also exists in most countries involved in this field. To narrow the gap specific arrangements are as follows: strengthen the improvement of the hydrogen fuel cell vehicle industry chain and establish industry-university-research alliances to promote the research and development of the technology; strengthen the investment in research and development to reduce the cost of battery production, which is also needed; and strengthen policy leadership and promote the improvement of hydrogen energy infrastructure.
2.5 Challenges of hydrogen energy
The hydrogen energy industry, both domestically and abroad, is still in its early stage and faces numerous challenges, which can be summarized as follows: First, there is a lack of ideal industrialized production of hydrogen from renewable energy for power generation. Compared to fossil energy production, the competitiveness of renewable hydrogen is poor, and progress in the hybrid technology of hydrogen and renewable energy is slow. Second, the infrastructure development is lagging, resulting in a significant spatial mismatch between hydrogen production and consumption. Next, many standards for hydrogen energy technology are incomplete, such as technical standards related to hydrogen quality, storage and transportation, and hydrogen station design, with few relevant references. Finally, key materials and equipment components for hydrogen energy, such as large-capacity hydrogen storage equipment, high-performance fuel cells, and electrolytic cells resistant to power fluctuations, have many challenges, including demanding requirements, complex processes, and high costs.
3 Conclusions
The automotive revolution is rapidly transforming the industry and people’s daily lives. It has brought about significant advancements in the vehicle technology, driving experience, and safety. The development of electric and hybrid vehicles, intelligence-connected transportation systems, and alternative fuels has paved the way for a more sustainable future, reducing the impact of the industry on the environment. In addition, hydrogen energy has emerged as a promising solution for achieving carbon neutrality, particularly in the transportation and energy sectors. Its versatility and abundance make it an attractive alternative to fossil fuels, and its production can be entirely carbon-free with the use of renewable energy sources. In conclusion, the automotive revolution and hydrogen energy have enormous potential for creating a sustainable future. However, it requires a collective effort to ensure that these innovations are developed and implemented in a way that benefits society and the environment as a whole. By working together toward carbon neutrality, a better and more sustainable future can be created for generations to come.
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