Hydrogen is an appealing energy carrier because its combustion produces only water vapor and it possesses the highest energy content per unit of mass of any fuel. However, hydrogen gas presents a significant challenge: its extremely low volumetric energy density at ambient temperature and pressure. This means a large volume is required to store the energy equivalent of a small tank of liquid fuel. To make hydrogen viable for mobile and stationary applications, its volume must be drastically reduced, leading to the development of three primary storage pathways.
Storing Hydrogen as a Highly Compressed Gas
The most mature and widely adopted method for hydrogen storage involves compressing the gas to extremely high pressures. This forces more hydrogen molecules into a smaller container volume. Compressed hydrogen storage systems in vehicles typically operate at two main pressure levels: 350 bar (5,000 psi) and the more energy-dense 700 bar (10,000 psi). Higher pressure translates to a greater driving range for fuel cell vehicles, making 700 bar systems the norm for passenger cars.
These high pressures necessitate the use of specialized, robust, and lightweight containers. The most advanced are Type III and Type IV tanks, which use carbon-fiber composite materials wound around a liner. Type IV tanks, featuring a polymer liner and an outer shell made entirely of carbon fiber composite, are the preferred choice for vehicles due to their superior strength-to-weight ratio. A significant limitation of this method is the substantial energy required to compress the hydrogen gas, which represents a thermodynamic loss in the overall energy cycle.
Storing Hydrogen as a Cryogenic Liquid
An alternative physical method to achieve high storage density is to cool hydrogen into its liquid state. This process requires reducing the temperature to the extremely low liquefaction point of hydrogen, approximately -253°C (-423°F). The resulting liquid hydrogen (LH2) offers a higher volumetric energy density than 700 bar compressed gas, making it attractive for applications demanding maximum range or large-scale bulk storage.
Liquefaction is an energy-intensive process, consuming a significant portion of the hydrogen’s total energy content. Liquid hydrogen storage systems are also subject to “boil-off.” Even with advanced thermal insulation, heat leaks into the cryogenic tank, causing the liquid to vaporize and increase internal pressure. To prevent overpressure, the resulting gas must be periodically vented, meaning fuel is gradually lost over time, especially when a vehicle is parked for an extended period.
Storing Hydrogen Within Solid Materials
Material-based storage is an advanced, non-physical approach that avoids the high pressures and extreme cold of the other two methods. This technique involves storing hydrogen by chemically or physically bonding it within a solid material. The goal is to create a system that can absorb hydrogen under moderate conditions and release it on demand, potentially offering high volumetric density and improved safety.
Metal Hydrides
Metal hydrides involve hydrogen atoms reversibly absorbed into the crystal lattice of an alloy. Heat is required to break the chemical bond and release the hydrogen gas for use. Metal hydrides offer excellent volumetric density, sometimes surpassing that of liquid hydrogen. Because the hydrogen is chemically bound and only released upon heating, this method provides improved safety. However, the materials can be heavy, leading to low gravimetric efficiency, and the speed of the absorption and release process can be slow.
Adsorbents and Chemical Hydrides
Adsorbent materials, such as Metal-Organic Frameworks (MOFs), are highly porous structures with vast internal surface areas. These materials physisorb (weakly bind) hydrogen molecules to their surfaces, often requiring cryogenic or near-cryogenic temperatures to maximize uptake. Chemical hydrides, like sodium borohydride, react with hydrogen or water to release the gas. This reaction is often irreversible or requires a complex, energy-intensive process to regenerate the spent material. Research focuses on optimizing these materials to achieve high storage capacity at ambient temperatures and pressures.
Comparison of Storage Methods
Compressed gas storage is the most commercially ready, offering a good balance of refueling speed and density, though it requires significant energy for compression. Liquid hydrogen provides the highest volumetric density but is limited by high liquefaction energy costs and the loss of fuel due to boil-off. Solid-state storage, using materials like metal hydrides, offers the potential for high volumetric density and greater safety by chemically binding the gas. These systems are currently limited by the heavy weight of the storage material and the slow kinetics of hydrogen release and uptake. Ultimately, the choice of storage method depends on the application, balancing the need for compact volume, light weight, system complexity, and overall energy efficiency.