I. Introduction

　　Against the backdrop of global efforts to achieve carbon peaking and carbon neutrality, hydrogen energy, as a zero-carbon emission clean energy solution, has attracted widespread attention. In particular, high-pressure gaseous hydrogen storage technology has become a research focus due to its importance in the storage and transportation of hydrogen energy. Among them, Type IV hydrogen storage cylinders using plastic liners and carbon fiber overwrap technology show great potential. However, traditional plastic liner manufacturing methods have welding defects, which may lead to the failure of hydrogen storage cylinders under high pressure. Therefore, developing integrated liner molding technology and technological innovations in molding efficiency, liner quality, and energy utilization are crucial to improving the efficiency and safety of hydrogen energy storage and transportation.

　　II. Classification of On-board Hydrogen Storage Cylinders

　　In on-board hydrogen storage systems, according to the structural characteristics and materials used in the hydrogen storage cylinders, they can be roughly divided into four types: Type I to Type IV. Type I hydrogen storage cylinders are mainly made of metal materials, but because metal materials are susceptible to hydrogen embrittlement, their use is limited, and the hydrogen storage density and service life are relatively low. With technological advancements, Type II hydrogen storage cylinders enhance their pressure resistance by adding fiber material wrapping to the metal cylinder body, thereby improving hydrogen storage density and operating pressure. Type III hydrogen storage cylinders use a lightweight aluminum liner and are fully wrapped with fiber materials, which not only further improves the hydrogen storage density and working pressure but also enables hydrogen to be used in a wider range of fields. Type IV hydrogen storage cylinders represent a major breakthrough in hydrogen storage technology. They use plastic materials as the liner and are fully wrapped with fiber materials on the outside. This not only significantly reduces the weight of the cylinder but also effectively avoids the problem of metal hydrogen embrittlement in the liner, significantly improving the hydrogen storage density. These different types of hydrogen storage cylinders have their own characteristics. The evolution from all-metal to plastic liners reflects the continuous progress of hydrogen storage technology in safety, economy, and application breadth.

　　III. Latest Developments at Home and Abroad

　　1. Overseas Technological Developments

　　In response to climate change, the world is actively developing hydrogen storage technology. Toyota Motor Corporation of Japan has been developing fuel cell vehicles since 1992 and launched the Mirai series equipped with Type IV hydrogen storage cylinders in 2015. This series of hydrogen storage cylinders maintains excellent fatigue resistance and thermal stability while reducing weight, achieving a hydrogen storage density of 5.7 wt%. The Type IV hydrogen storage cylinder launched by QUANTUM in the United States uses a "TriShield" design to improve impact resistance. The one-piece liner avoids welds, reducing the risk of hydrogen leakage. These technological advancements have not only promoted the application of hydrogen energy in the transportation sector but also provided strong support for the realization of a low-carbon economy.

　　The Type IV hydrogen storage cylinder developed by DaimlerChrysler in Germany has a nominal pressure of 70 MPa, allowing a hydrogen fuel cell vehicle carrying two such hydrogen storage cylinders to achieve a cruising range of 700 kilometers. In addition, Faurecia has successfully reduced the overall weight of the hydrogen storage cylinder by 20% by improving the liner material and has become a supplier to many well-known automobile manufacturers such as Hyundai Motor, SAIC Motor, and Citroen. These technological advances have not only promoted the commercialization process of hydrogen vehicles but also provided an effective solution for achieving long-distance, low-carbon emission transportation.

　　2. Domestic Technological Developments

　　In China, with the clear proposal of the national carbon peaking and carbon neutrality strategic goals, the development of Type IV hydrogen storage cylinder technology has ushered in new opportunities and challenges. Driven by policies, it is expected that the number of hydrogen energy vehicles will increase significantly by 2035, which constitutes a strong impetus for the Type IV hydrogen storage cylinder market. Although China is currently slightly behind the international market in the development and application of Type IV hydrogen storage cylinders, domestic enterprises are accelerating their R&D efforts and have made a series of progress. However, the lack of core technologies, plastic-metal connection technology, and material supply remain the main obstacles hindering the development of Type IV hydrogen storage cylinders in China. In the face of these challenges, domestic enterprises and research institutions need to strengthen technological innovation and cooperation to promote the breakthrough and industrialization process of Type IV hydrogen storage cylinder technology, supporting the healthy development of China's hydrogen energy and fuel cell vehicle industry. For example, Tianhai Industry Co., Ltd. already has a complete Type III hydrogen storage cylinder production line and testing center and has independently developed Type IV hydrogen storage cylinders in recent years. These hydrogen storage cylinders have been applied to heavy-duty trucks and have achieved a weight reduction of 30% compared to Type III hydrogen storage cylinders. China National Materials Co., Ltd. has also invested heavily in the R&D of Type IV hydrogen storage cylinders and has achieved the domestic production of carbon fiber, breaking through the mass production of 385L hydrogen storage cylinders and putting them into sales. Aoyang Technology Co., Ltd. has developed a Type IV hydrogen storage cylinder that solves the problem of connecting and sealing the plastic liner with the metal valve seat. In addition, CIMC Enric has cooperated with Hexagon to build a Type IV hydrogen storage cylinder production line, while Fengchen Hydrogen Energy Technology Co., Ltd. has cooperated with Steelhead to develop the Chinese Type IV hydrogen storage cylinder market. Although domestic hydrogen storage cylinder manufacturers have signed cooperation agreements with foreign companies, the lack of core technologies still exists, which is not conducive to the country's long-term development strategic goals. The development of Type IV hydrogen storage cylinders in China still faces many difficulties, such as the connection strength problem of the plastic-metal heterogeneous interface, the monopoly problem of high-performance liner materials, and the prohibition of high-modulus carbon fiber, which are "bottleneck" problems that need to be urgently solved.

　　IV. Progress in Liner Molding Technology for Hydrogen Storage Cylinders

　　The key to Type IV hydrogen storage cylinders and their liner design lies in achieving a hollow structure between the metal valve seat and the plastic liner. High-density polyethylene or nylon is commonly used as the base material. Research focuses include optimizing the processing technology to improve the mechanical and barrier properties of the liner, ensuring dimensional accuracy, and strengthening the bonding strength between the metal valve seat and the plastic liner, which is crucial for improving the overall performance of the hydrogen storage cylinder.

　　The molding processes for Type IV hydrogen storage cylinder liners include injection molding-welding, blow molding, and rotational molding. Different molding processes have their own advantages and disadvantages.

　　Injection molding and welding are widely used in the manufacturing of type IV hydrogen storage tank liners, both domestically and internationally, due to their high maturity and reliability. This method is particularly suitable for smaller vehicle-mounted hydrogen storage tanks, but for larger tanks, such as those used in heavy-duty trucks and hydrogen stations, extrusion molding technology needs to be combined to meet the production requirements of the end caps and cylinders. Although injection molding and welding provide high precision and dimensional stability, technical challenges still exist in connecting the plastic liner to the metal valve seat. On the other hand, blow molding and rotational molding offer the possibility of one-piece molding, especially rotational molding, which also allows for the online connection of the metal valve seat and the plastic liner, showing greater development potential and application value, although it currently faces challenges in molding precision and cycle time. The development and application of these technologies are of great significance to improving the performance and reducing the cost of type IV hydrogen storage tanks. Currently, some vehicle-mounted hydrogen storage tank plastic liners from Toyota in Japan, Hyundai in South Korea, ILJIN Composites, and NPROXX in Germany are formed by injection molding and welding. Because blow molding cannot achieve online connection of the metal valve seat structure and the plastic liner, the valve seat structure needs to be subsequently assembled onto the main body of the plastic liner, and this molding process has high assembly requirements. For example, some vehicle-mounted hydrogen storage tank plastic liner products from Quantum, General Motors, Impco, and Hexagon Lincoln in the United States are blow-molded. In addition, DSM in the Netherlands has achieved blow molding of nylon liners for low melt stiffness and complex nylon materials. Rotational molding can not only achieve one-piece molding of the end cap and cylinder, but also can achieve online connection of the metal valve seat structure and plastic, such as some liner products from Quantum in the United States and CEA in France, which are manufactured by rotational molding. However, the existing rotational molding process for plastic liners has low molding precision, long molding cycle, and poor liner quality, and the rotational molding process for liners still has great development potential and application value.

　　V. Analysis of the Vehicle-Mounted Hydrogen Storage Market

　　As a storage container for high-pressure gaseous hydrogen, hydrogen storage tanks have seen continuously high-speed growth in domestic market demand, thanks to policy support and technological advancements. Classified into types I to IV, with the development of various models, the demand for vehicle-mounted hydrogen storage tanks in China in 2022 is expected to reach 70,000 units, with a market size of approximately 2.42 billion yuan. It is expected that by 2025, with the promotion and expanded application of hydrogen fuel cell vehicles, the market size will significantly increase to 10.15 billion yuan, showing the huge development potential and market prospects of the hydrogen storage tank industry.

　　VI. Summary and Outlook

　　The development of integrated molding technology for type IV vehicle-mounted hydrogen storage tanks and their liners demonstrates the potential of hydrogen energy in achieving zero-carbon emission goals. Currently, this technology faces challenges such as the connection strength between the plastic and metal interfaces, the monopoly of high-performance liner materials, and the ban on high-modulus carbon fibers. Injection welding, blow molding, and rotational molding each have their advantages and disadvantages, but they all play an important role in improving the performance and reducing the cost of type IV hydrogen storage tanks. In particular, injection welding molding is widely used in the manufacturing of small vehicle-mounted hydrogen storage tanks, while rotational molding shows greater development prospects due to its one-piece molding potential and its ability to connect metal valve seats and plastic liners online, although it faces challenges in molding precision and cycle time.

　　Looking to the future, the domestic and international demand for type IV hydrogen storage tanks continues to grow, especially against the backdrop of the promotion and expanded application of hydrogen fuel cell vehicles. It is expected that by 2025, the market size of vehicle-mounted hydrogen storage tanks in China will significantly increase, showing the huge development potential and market prospects of the industry. To meet the challenges of technological and market development, the focus should be on addressing current technological barriers, promoting innovation in molding processes, strengthening domestic and international cooperation, and optimizing the material supply chain. These efforts will help to further break through type IV hydrogen storage tank technology, support the healthy development of the hydrogen energy and fuel cell vehicle industry, and accelerate the achievement of carbon neutrality goals.