Overview of hydrogen storage technologies
Hydrogen storage technologies are pivotal in harnessing hydrogen as a clean energy carrier. Currently, high-pressure gas storage and cryogenic liquid storage dominate the field, each with its own trade-offs in terms of efficiency, safety, and cost. Additionally, metal hydrides are used in some niche applications. We will examine these technologies in comparison with the novel potential offered by solid-state storage based on reticular materials.
Compressed gas storage, the most prevalent method, is widely utilized in on-board fuel storage, ground storage, and gas transportation applications. Hydrogen is stored at pressures of 200–700 bar in all-steel, aluminum, or plastic-lined carbon composite pressure vessels. Traditional steel vessels, which operate at 200–250 bar, are durable but extremely heavy, reducing their viability for applications in mobility and negatively impacting the economics of gas delivery due to limited payload capacity. Steel vessels are also susceptible to hydrogen embrittlement over time. Plastic-lined carbon composite vessels, operating at 350–700 bar, are lighter and more suitable for transport applications, though they come with high costs due to their materials and expensive balance of plant at elevated pressures. Consequently, the high cost of pressure vessels, gas compression, and management of high-pressure safety present significant challenges to the scalability of compressed gas storage.
Cryogenic liquid storage, another prevalent method, stores hydrogen at temperatures below −253°C, achieving a higher energy density. This method is preferred for large-scale storage and long-distance transportation, but it poses substantial safety, technical, and economic challenges. Liquefaction is energy-intensive, amounting to 40–50% of the energy content of hydrogen. Liquefaction equipment is capital-intensive, requiring a project scale that may not be met by many distributed ‘green hydrogen’ generation projects. Furthermore, hydrogen’s low boiling point leads to unavoidable boil-off, resulting in continuous energy losses approaching 30%. Safety and specialized infrastructure are also critical challenges due to risks such as leaks, frostbite, and explosion hazards. Given these complexities, cryogenic storage is typically limited to large-scale stationary industrial applications, where its high costs and infrastructure requirements pose a barrier to widespread use.
Lastly, metal hydride storage offers a way to store hydrogen by chemically bonding it with metal alloys, enabling storage at low pressures and near-ambient temperatures in a solid state. Despite these advantages, metal hydride systems are generally heavy and require very high amounts of heat to release hydrogen, making their operation more complex, energy-consuming, and therefore inefficient. This weight and the associated thermal management needs limit their use, confining them primarily to stationary applications or niche uses, such as in forklifts. Thus, while metal hydride systems offer distinct advantages in specific use cases such as material handling, their limitations make them unsuitable for broader hydrogen mobility and gas transport solutions.
Reticular material-based solid-state hydrogen storage
Reticular materials such as metal-organic frameworks – invented by Omar Yaghi, Professor of Chemistry at the University of California, Berkeley, and co-founder of H2MOF – offer a promising alternative to traditional hydrogen storage technologies. Reticular materials are crystalline materials composed of metal ions or clusters coordinated with organic ligands, forming porous structures with extremely high surface areas. Nano-engineered with atomic precision, these reticular materials are ideally suited for solid-state hydrogen storage because they can trap hydrogen molecules within their pores at low pressures and ambient temperatures, providing significant advantages in safety, efficiency, and scalability.
How reticular materials store hydrogen
MOFs are unique in their ability to adsorb and store large amounts of hydrogen within their porous networks at low pressures. This adsorption occurs at a molecular level, where hydrogen molecules are trapped in the high surface area of the reticular material. By tuning the chemical composition and pore structure of the reticular material, it is possible to optimize hydrogen storage capacity.
- Physical adsorption: Hydrogen is stored via weak van der Waals forces within the pores of the reticular material, allowing for easy release under controlled conditions.
- Low-pressure storage: Reticular materials can store hydrogen at pressures as low as 30 bar, reducing the need for heavy and costly high-pressure vessels.
The ability of reticular materials to adsorb hydrogen at low pressures makes them highly efficient compared to compressed gas and cryogenic storage, offering potential breakthroughs in efficient hydrogen storage, transportation, and handling across various applications.
Is hydrogen storage with reticular materials safe?
Safety is a critical advantage of hydrogen storage based on reticular materials. Unlike compressed gas storage, which involves high-pressure vessels that pose significant safety and jurisdictional challenges, reticular materials operate at low pressures and near-ambient temperatures. Hydrogen stored in reticular materials is adsorbed within the material, reducing the risk of rapid release of gas or violent combustion in the case of a fire or accidental impact.
- Low pressure, low risk: Since reticular materials can store hydrogen at much lower pressures than compressed gas, they eliminate many of the safety and permitting concerns associated with high-pressure systems.
- Comparison: Compressed gas storage involves storing at pressures of up to 700 bar, with the energy content of the compressed gas being equivalent to tons of explosives in some cases. Cryogenic hydrogen storage faces similar risks from rapid boil-off vapor release and vulnerability to mechanical impacts and penetration.
Reticular materials, storing hydrogen in solid, ‘concrete-like’ form, by contrast, provide an inherently safer solution for both stationary and mobile hydrogen storage applications, with no need for the elaborate and expensive safety mechanisms required by high-pressure or cryogenic systems.
Cost efficiency of reticular materials
Hydrogen storage based on reticular materials has the potential to dramatically lower the costs associated with hydrogen storage and transportation. The high costs of compressing hydrogen gas to 700 bar or liquefying it to cryogenic temperatures, combined with the significant energy losses of 15–45%, are major economic hurdles for current physically based storage technologies. Material-based storage, leveraging, for instance, metal hydrides, is slightly more cost-efficient than their physically based counterparts, but still suffers from an extreme weight penalty and costly energy consumption at the point of release. In contrast, reticular material-based storage offers significant savings:
- Reduced capital expenditures: Because reticular material-based storage vessels can store hydrogen at low pressures as it is generated, for instance, by an electrolyzer, hydrogen producers will not need to invest in expensive compression equipment such as multi-stage oil-free diaphragm compressors and associated high-pressure balance of plant. As for the reticular materials themselves, they can potentially absorb low-pressure hydrogen directly from an electrolyzer. High-capacity storage systems based on reticular materials can store hydrogen on the ground and also help with transporting bulk hydrogen cost-effectively to off-taker sites.
- Reduced operating expenditures: Hydrogen transport using traditional steel pressure vessels is expensive since only about 200–300 kg of hydrogen can be transported per trailer. The high logistics cost is reflected in the high cost of delivered high-pressure hydrogen. Liquid hydrogen allows for more efficient transport of hydrogen; however, the delivered hydrogen remains extremely expensive due to the high cost of liquid hydrogen, tankers, and boil-off losses. Reticular material-based low-pressure storage and transportation have the potential to dramatically lower the cost of delivered hydrogen. The cost of high-pressure compression is avoided, and the transportation logistics costs are significantly reduced due to the high capacity of the material.
- Scalability: The modular nature of storage systems based on reticular materials allows for scalability across different sectors, ranging from small-scale decentralized green hydrogen generation projects to industrial power generation, and vehicle refueling applications, as well as safe and economical on-board fuel storage.
Energy density and storage capacity
One of the key performance metrics for hydrogen storage technologies is energy density. Reticular materials exhibit impressive hydrogen storage capacities relative to their weight and volume due to their high surface areas and tunable pore sizes.
The porous structure of reticular materials allows for the densification of hydrogen within their framework at much lower pressures than those required for compressed gas storage. In fact, thanks to the properties of these novel reticular materials, the hydrogen storage density on both the gravimetric and volumetric levels is competitive with that of a high-pressure tank at 700 bar, despite the fact that the reticular technology operates at much lower pressures and temperatures.
By adjusting the composition of the reticular materials, researchers can optimize them for specific hydrogen storage needs. Armed with the deep insight of Sir Fraser Stoddart – a Nobel laureate in 2016 for the design and synthesis of molecular machines – and a co-founder of H2MOF, we are in an excellent position to achieve this.
This flexibility allows hydrogen storage systems based on reticular materials to be tailored for specific applications, from stationary grid storage to bulk gas transportation and on-board hydrogen storage in mobility applications.
Conclusion
Solid-state hydrogen storage based on reticular materials represents a significant leap forward in the hydrogen value chain, offering a safe, cost-effective, and scalable alternative to traditional storage methods. By operating at low pressures and near-ambient temperatures, reticular materials overcome many of the limitations of compressed gas, cryogenic liquids, and metal hydrides, making them an attractive solution for hydrogen storage in stationary applications, gas transportation, and mobile applications.
As the technology continues to mature, reticular materials will play a critical role in enabling the widespread adoption of hydrogen as a clean energy carrier, unlocking new possibilities for transportation, industrial storage, and grid applications. Their unique properties, combined with the flexibility of their design, make storage systems based on reticular materials one of the most promising developments in the field of hydrogen storage.
