Hydrogen storage is a key enabling technology for the advancement of fuel cell power systems in transportation, stationary, and portable applications. For transportation, the overarching technical challenge for hydrogen storage is how to store the amount of hydrogen required for a conventional driving range (>300 miles), within the vehicular constraints of weight, volume, efficiency, safety, and cost. Durability over the performance lifetime of these systems must also be verified and validated and acceptable refueling times must be achieved.
Low cost, energy efficient off-board storage of hydrogen will also be needed throughout the hydrogen delivery system infrastructure. For example, storage is required at hydrogen production sites, hydrogen refueling stations, and stationary power sites. Temporary storage may also be required at terminals and/or intermediate storage locations. Requirements for off-board bulk storage are generally less restrictive than on-board requirements; for example, there may be no or less restrictive weight requirements, but there may be volume or "footprint" requirements.
The DOE hydrogen storage program element will focus primarily on developing on-board storage materials and technologies.
The objectives of the DOE Hydrogen Storage activity are:
Possible approaches to hydrogen storage include physical storage via compression or liquefaction, and storage in materials via reversible sorption processes or chemical reaction.
Pressurized Storage Tanks
Compressed H2 tanks [5000 psi (~35 MPa) and 10,000 psi (~70 MPa)] have been certified worldwide according to ISO 11439 (Europe), NGV?2 (U.S.), and Reijikijun Betten (Iceland) standards and approved by TUV (Germany) and The High-Pressure Gas Safety Institute of Japan (KHK). Tanks have been demonstrated in several prototype fuel cell vehicles and are commercially available. Composite, 10,000-psi tanks have demonstrated a 2.35 safety factor (23,500 psi burst pressure) as required by the European Integrated Hydrogen Project specifications.
Advanced lightweight pressure vessels, with minimum permeation losses, have been designed and fabricated. These vessels use lightweight bladder liners that act as inflatable mandrels for composite overwrap and as permeation barriers for gas storage. These tank systems have demonstrated 12wt% hydrogen storage at 70 MPa (~10,000 psi).
Liquid tanks are being demonstrated in hydrogen-powered vehicles and a hybrid tank concept combining both high-pressure gaseous and cryogenic liquid storage is being studied. These hybrid insulated pressure vessels are lighter than hydrides, more compact than ambient-temperature pressure vessels, and require less energy for liquefaction and have less evaporative losses than liquid hydrogen tanks.
Storage in Materials
In absorptive hydrogen storage, hydrogen is absorbed directly into the bulk of the material. In simple crystalline metal hydrides, this absorption occurs by the incorporation of atomic hydrogen into interstitial sites in the crystallographic lattice structure.
Adsorption may be subdivided into physisorption and chemisorption, based on the energetics of the adsorption mechanism. Physisorbed hydrogen is more weakly energetically bound to the material than is chemisorbed hydrogen. Sorptive processes typically require highly porous materials to maximize the surface area available for hydrogen sorption to occur, and to allow for easy uptake and release of hydrogen from the material.
The chemical reaction route for hydrogen storage involves displacive chemical reactions for both hydrogen generation and hydrogen storage. For reversible hydrogen storage chemical reactions, hydrogen generation and hydrogen storage take place by a simple reversal of the chemical reaction as a result of modest changes in the temperature and pressure. Sodium alanate-based complex metal hydrides are an example. For irreversible hydrogen storage chemical reactions, the hydrogen generation reaction is not reversible under modest temperature/pressure changes, so that storage requires larger temperature/pressure changes or alternative chemical reactions. Sodium borohydride is an example.
Currently, three classes of materials are being investigated:
Metal Hydrides (High and Low Temperature)
Conventional high capacity metal hydrides require high temperatures (300°-350°C) to liberate hydrogen, but sufficient heat is not generally available in fuel cell transportation applications. Currently existing low temperature hydrides, however, suffer from low gravimetric energy densities and require too much space on board or add significant weight to the vehicle. Researchers are developing low-temperature metal hydride systems that can store 3-5 wt% hydrogen. Alloying techniques have been developed that result in high-capacity, multi-component alloys with excellent kinetics, albeit at high temperatures. Additional research is required to identify alloys with appropriate kinetics at low temperatures.
Various pure or alloyed metals can combine with hydrogen, producing stable metal hydrides. The hydrides decompose when heated, releasing the hydrogen. Hydrogen can be stored in the form of a hydride at higher densities than by simple compression. Using this safe and efficient storage system depends on identifying a metal with sufficient adsorption capacity operating under appropriate temperature ranges.
Alanates are considered to be the most promising of the complex hydrides studied to date for on-board hydrogen storage applications. They have been the focus of extensive research to increase the storage capacity of the materials, extend the durability and cycle lifetime and uptake and release reproducibility. A thorough thermodynamic and kinetic understanding of the alanate system is needed in order to serve as the basis for systematically exploring other complex hydride systems. In addition, engineering studies must be initiated to understand the system level issues and to facilitate the design of optimized packaging and interface systems for on-board transportation applications.
An approach for the production, transmission, and storage of hydrogen using a chemical hydride slurry or solution as the hydrogen carrier and storage medium is being investigated. There are two major embodiments of this approach. Both require some degree of thermal management and regeneration of the carrier to recharge the hydrogen content. Significant technical issues remain regarding the regeneration of the spent material and whether regeneration can be accomplished on-board. Life cycle cost analysis is needed to assess the costs of regeneration.
In the first embodiment, a slurry of an inert stabilizing liquid protects the hydride from contact with moisture and makes the hydride pumpable. At the point of use, the slurry is mixed with water and the consequent reaction produces high purity hydrogen.
2LiH + 2H2O ® 2LiOH + 2H2
An essential feature of the process is recovery and reuse of spent hydride at a centralized processing plant. Research issues include the identification of safe, stable, and pumpable slurries and the design of the reactor for regeneration of the spent slurry.
The second, and most advanced, embodiment is sodium borohydride. The sodium borohydride is combined with water to create a non-toxic, non-flammable solution that produces hydrogen when exposed to a catalyst.
NaBH4 + 2H2O + catalyst à 4H2 + NaBO2
When the sodium borohydride solution and catalyst are separated, the solution stops producing hydrogen. After being in contact with the catalyst, the fuel is spent and goes into a waste tank. This waste is recyclable into new fuel.
The borohydride system has been successfully demonstrated on prototype passenger vehicles such as the Chrysler Natrium.
Adsorption of hydrogen molecules on activated carbon has been studied in the past. Although the amount of hydrogen stored can approach the storage density of liquid hydrogen, these early systems required low temperatures (i.e., liquid nitrogen). Subsequent work showed that hydrogen gas can condense on carbon structures at conditions that do not induce adsorption within a standard mesoporous activated carbon.
Carbon materials present a long-term potential for hydrogen storage and several carbon nanostructures are being investigated with particular focus on single-wall nanotubes(SWNTs). However, the amount of storage and the mechanism through which hydrogen is stored in these materials are not well-defined. Fundamental studies are directed at understanding the basic reversible hydrogen storage mechanisms and optimizing them.
Therefore, a coordinated experimental and theoretical effort is needed to characterize the materials, to understand the mechanism and extent of hydrogen absorption/adsorption, and to improve the reproducibility of the measured performance. These efforts are required to obtain a realistic estimation of the potential of these materials to store and release adequate amounts of hydrogen under practical operating conditions.