As a high-calorific, carbon-free, and pollution-free "ultimate energy source," the key to promoting hydrogen energy lies in reducing costs across the entire industry chain. Hydrogen storage and transportation is a crucial link, involving the storage, transportation, and distribution of hydrogen, playing a decisive role in the commercialization and large-scale development of the hydrogen energy industry. Currently, the cost of hydrogen storage and transportation accounts for over 30% of the end-use cost of hydrogen. In recent years, the domestic substitution of hydrogen storage cylinder materials and the rise of solid-state hydrogen storage have brought new development prospects to the hydrogen storage and transportation industry.

　　The three hydrogen storage routes of gas, liquid, and solid have different characteristics and are suitable for different scenarios.

　　The main storage methods for hydrogen are gaseous hydrogen storage, liquid hydrogen storage, and solid-state hydrogen storage. Gaseous hydrogen storage technology is relatively mature, mainly achieved through high-pressure hydrogen storage cylinders; liquid hydrogen storage and solid-state hydrogen storage technologies have not yet been commercialized, but there has been continuous technological progress in recent years, with development potential.

　　High-pressure gaseous hydrogen storage refers to compressing hydrogen into a high-pressure resistant container or underground gas storage under high pressure, with the hydrogen storage capacity being directly proportional to the storage pressure. The most commonly used containers are currently gas tanks and on-board hydrogen storage cylinders, with pressures typically classified into 35 MPa and 70 MPa levels. Gaseous hydrogen storage is the most widely used of all current hydrogen storage methods. The main factor currently hindering the further development of high-pressure hydrogen storage in China is that some materials, processes, and components of hydrogen storage cylinders still rely on imports, and the performance of domestic substitutes still has a gap.

　　Liquid hydrogen storage includes two technical routes: cryogenic liquid hydrogen storage and organic liquid hydrogen storage. The basic principle of cryogenic liquid hydrogen storage is to compress and cool hydrogen to -253℃ to liquefy it and store it in a cryogenic insulated container, with a liquid hydrogen density of 70.78 kg/m³. This technology has advantages such as large storage capacity, high purity, small footprint, and fast filling, but the low liquefaction temperature of hydrogen leads to high energy consumption in the liquefaction process, with liquefying 1 kg of hydrogen requiring 4-10 kilowatt-hours of electricity, and requiring special antifreeze, pressure-resistant, and highly insulated hydrogen storage containers, resulting in high overall costs. Therefore, its application is limited to a few scenarios such as aviation and has not been extended to the civilian field.

　　Organic liquid hydrogen storage utilizes the reversible chemical reaction between hydrogen and an organic medium to achieve hydrogen storage and release. Alkenes, alkynes, and aromatic hydrocarbons are currently common organic liquid hydrogen storage materials. This technology has advantages such as high stability, good safety, high hydrogen storage density, and recyclable hydrogen storage medium, but it has problems such as high dehydrogenation temperature, low efficiency, and high energy consumption. Due to the high cost, both liquid hydrogen storage routes are still in the research and demonstration stage.

　　Solid-state hydrogen storage is a recent highlight, divided into physical adsorption hydrogen storage and chemical hydrogen storage. Physical adsorption hydrogen storage refers to adsorbing hydrogen molecules onto the surface of a solid through van der Waals forces. Storage materials include carbon-based materials, inorganic porous materials, and metal-organic framework compounds. The main disadvantage is that most physical adsorption materials can only achieve a certain hydrogen storage density at low temperatures, with very low hydrogen absorption at room temperature and pressure. Chemical hydrogen storage materials mainly include metal hydrides, coordination hydrides, and chemical hydrides. Among them, metal hydrides have become a research focus due to their high hydrogen storage capacity and good cycle stability. Solid-state hydrogen storage can be completed near room temperature and pressure, combining advantages such as high volumetric hydrogen storage density, reversibility, high cycle life, good safety, and high hydrogen purity, making its application prospects promising.

　　Type IV cylinders offer better performance and cost, but replacing Type III cylinders still takes time.

　　Hydrogen storage cylinders are the key carriers for high-pressure gaseous hydrogen storage and are also the most commercialized aspect of hydrogen storage and transportation. Currently, domestic on-board hydrogen storage cylinders are mainly 35 MPa Type III cylinders, which have been mass-produced; Type IV hydrogen storage cylinders are continuously making research progress, have been included in national standards, and are accelerating development, becoming a key area of focus in the next stage.

　　The main difference between the two is that the inner liner material of Type III cylinders is aluminum alloy, while that of Type IV cylinders is high-composite plastic; the commonality is that both are externally made of omnidirectional wound carbon fiber composite materials.

　　Light weight: The weight-to-capacity ratio of Type III cylinders is approximately 0.98, while that of Type IV cylinders is approximately 0.74. The smaller the weight-to-capacity ratio, the lighter the hydrogen storage cylinder is for the same volume, thus improving the effective payload.

　　High hydrogen storage capacity: The hydrogen storage density of Type IV cylinders is above 6%, while that of Type III cylinders is around 4%. The greater the hydrogen storage density, the more hydrogen is stored in the same weight of hydrogen storage cylinder.

　　Long lifespan: The inner liner of Type IV cylinders is made of plastic, making it less prone to fatigue failure, and the service life can be extended to 15 years.

　　Low risk of hydrogen embrittlement: The inner liner of Type IV hydrogen storage cylinders usually uses high-composite plastics, which are more corrosion-resistant than the metal inner liner of Type III cylinders, effectively avoiding stress corrosion and hydrogen embrittlement.

　　Type IV hydrogen storage cylinders are not only superior in performance but also lower in cost than Type III hydrogen storage cylinders. According to recent calculations by the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, the cost of 35 MPa and 70 MPa Type III hydrogen storage cylinders is US$3084 and US$3921 respectively, while the cost of 35 MPa and 70 MPa Type IV hydrogen storage cylinders is US$2865 and US$3486 respectively, which are 7% and 11% lower respectively.

　　There are three main reasons for this:

　　Inner liner material: The inner liner of Type IV cylinders uses high-composite plastics, such as polyamide (PA) or high-density polyethylene (HDPE). Compared with the metal aluminum inner liner of Type III cylinders, it has a lower price per unit mass and is easier to mold, thus reducing the overall manufacturing cost.

　　Winding process: The plastic inner liner of Type IV cylinders does not require heat treatment and is easier to mold. Compared with the metal inner liner of Type III cylinders, its winding process is simpler, reducing manufacturing costs.

　　Material savings: Carbon fiber is the main cost component of hydrogen storage cylinders. Type IV cylinders can improve the utilization rate of carbon fiber composite materials in design, reducing material usage and thus lowering costs. As the production efficiency of winding equipment and the accuracy of current designs improve, the cost advantage of Type IV cylinders will become even more apparent.

　　On May 23, 2023, China's national standard "Carbon Fiber Fully Wrapped Cylinders with Plastic Inner Liners for Compressed Hydrogen in Vehicles" was released and officially implemented on June 1, 2024, marking a significant step forward in the standardization of Type IV cylinders and promising to accelerate the industrialization process.

　　However, this does not mean that the process of Type IV cylinders replacing Type III cylinders will be immediate.

　　At the 2024 International Hydrogen Energy and Fuel Cell Vehicle Conference in June this year, Li Huasheng, Deputy General Manager of China National Materials Group Corporation (002080.SZ), stated that under a working pressure of 35 MPa, large-capacity Type III cylinders have a greater cost advantage compared to Type IV cylinders. For vehicle types such as buses, urban logistics vehicles, and special vehicles that do not have high requirements for lightweight and range, the 35 MPa Type III cylinder solution can basically meet operational needs, and the performance advantages of Type IV cylinders are not obvious. However, under a working pressure of 70 MPa, the advantages of lightweight and low cost of Type IV cylinders become apparent. Heavy-duty trucks have higher requirements for lightweight and range, and large-capacity, high-pressure Type IV cylinder products may also have more opportunities in the heavy-duty truck field.

　　In addition, some industry analysts have pointed out that the insufficient sealing reliability between the plastic inner liner and the metal joint of Type IV cylinders, the relatively low stiffness of the plastic inner liner, and its higher sensitivity to temperature still need further solutions.

　　Based on this judgment, Type III and Type IV cylinders will form a differentiated competitive relationship in the future, rather than a complete replacement.