FUEL CELL TECHNOLOGIES PROGRAM
Hydrogen Storage
Developing safe, reliable, compact, and cost-effective hydrogen stor- age technologies is one of the most technically challenging barriers to the widespread use of hydrogen as a form of energy. To be competitive with conventional vehicles, hydrogen- powered cars must be able to travel more than 300 mi between fills. This is a challenging goal because hydrogen has physical characteristics that make it difficult to store in large quantities without taking up a significant amount of space.
Where and How Will Hydrogen can be stored on the surfaces of solids by adsorption. In adsorption,
Hydrogen be Stored? hydrogen associates with the surface of a material either as hydrogen molecules (H2) Hydrogen storage will be required or hydrogen atoms (H). This figure depicts hydrogen adsorption within MOF-74. onboard vehicles and at hydrogen production sites, hydrogen refueling sta- tions, and stationary power sites. Possible particularly within the size and weight high-volume manufacturing methods for approaches to storing hydrogen include: constraints of a vehicle. those materials and components.
• Physical storage of compressed A light-duty fuel cell vehicle will carry hydrogen gas in high pressure tanks approximately 4-10 kg of hydrogen on Research Directions (up to 700 bar); board (depending on the size and type Reducing our dependence on foreign of the vehicle) to allow a driving range oil for transportation is a key driver for • Physical storage of cryogenic hydro- of more than 300 mi, which is generally using hydrogen as a form of energy. gen (cooled to -253°C, at pressures regarded as the minimum for widespread Hydrogen storage research, therefore, of 6-350 bar) in insulated tanks; and public acceptance. Drivers must also is focused primarily on technologies be able to refuel at a rate comparable • Storage in advanced materials — and systems used onboard a vehicle. to the rate of refueling today’s gasoline within the structure or on the surface Scientists in government, industry, and vehicles. of certain materials, as well as in academia are working to improve the weight, volume, and cost of current the form of chemical compounds Using currently available high-pressure hydrogen storage systems, as well as that undergo a chemical reaction to tank storage technology, placing a identify and develop new technologies release hydrogen. sufficient quantity of hydrogen onboard that can achieve similar performance, at a vehicle to provide a 300-mile driv- a similar cost, as gasoline fuel storage ing range would require a very large What Are the Challenges? systems. tank — larger than the trunk of a typi- Hydrogen has a very high energy content cal automobile. Aside from the loss of Compressed gas and liquid by weight (about three times more than cargo space, there would also be the hydrogen tanks gasoline), but it has a very low energy added weight of the tank(s), which could content by volume (liquid hydrogen is reduce fuel economy. Low-cost materials Traditional compressed hydrogen gas about four times less than gasoline). This and components for hydrogen storage tanks are much larger and heavier than makes hydrogen a challenge to store, systems are needed, along with low-cost, what is ultimately desired for light-duty vehicles. Researchers are evaluating FUEL CELL TECHNOLOGIES PROGRAM
light-weight, safe, composite materials cylindrical tanks, which do not package to capacity, the uptake and release of that can reduce the weight and volume of well in a vehicle. hydrogen (i.e., kinetics), management of compressed gas storage systems. heat during refueling, cost, and life cycle Materials-based storage impacts. Liquefied hydrogen is denser than gas- eous hydrogen and thus it contains more Hydrogen atoms or molecules bound For More Information tightly with other elements in a com- energy in a given volume. Similar sized More information on the Fuel Cell pound (or potential storage material) may liquid hydrogen tanks can store more Technologies Program is available at make it possible to store larger quanti- hydrogen than compressed gas tanks, http://www.hydrogenandfuelcells.energy. ties of hydrogen in smaller volumes at but it takes energy to liquefy hydrogen. gov. However, the tank insulation required to conditions that are within the practical prevent hydrogen loss adds to the weight, operational boundaries of a polymer elec- volume, and costs of liquid hydrogen trolyte membrane (PEM) fuel cell. tanks. Researchers are also studying a Scientists are investigating several hybrid tank concept that can store high- different kinds of materials, including pressure hydrogen gas under cryogenic metal hydrides, adsorbent materials, and conditions (cooled to around -120 to chemical hydrides, in addition to iden- -196°C) — these “cryo-compressed” tifying new materials with potential for tanks may allow relatively lighter weight, favorable hydrogen storage attributes. more compact storage. Hydrogen storage in materials offers Gasoline tanks used in cars and trucks great promise, but additional research is today are considered conformable and required to better understand the mecha- take maximum advantage of available nism of hydrogen storage in materials vehicle space. Researchers are evaluating under practical operating conditions and concepts for conformable high-pressure to overcome critical challenges related hydrogen tanks as an alternative to
Hydrogen can be stored in different forms In tanks... And in materials... Hydrogen can be stored on the surfaces of solids (by adsorption) or within solids Compressed Gas (by absorption). In adsorption (A), hydrogen attaches to the surface of a mate-
rial either as hydrogen molecules (H2) or hydrogen atoms (H). In absorption (B), hydrogen molecules dissociate into hydrogen atoms that are incorporated into the solid lattice framework – this method may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and at temperatures close to room temperature. Finally, hydrogen can be strongly bound within molecular structures, as chemical compounds containing hydrogen atoms (C, D). Density increases from A to D. A) Surface Adsorption B) Intermetallic Hydride
Cryogenic Liquid
C) Complex Hydride D) Chemical Hydride
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