Storage is one of the Grand Challenges for using hydrogen in transportation, and researchers are rising to it.

While storage of compressed gaseous hydrogen or liquefied hydrogen in high-pressure or cryogenic tanks is one option, finding an advanced solid-state or liquid chemical material that can be filled with, hold and then release hydrogen as needed is another.

There are numerous factors that go into finding an effective solid storage medium: the amount of hydrogen it can hold, the energy cost for “filling it” with hydrogen, the environmental impact of byproducts (if any), the number of fill and discharge cycles without degradation, the rate at which it can fill and discharge, stability, the ease of manufacturing and, of course, the cost.

Work on solid hydrogen storage falls into two basic categories:

• Metal and chemical hydrides

• Carbon and nanoscale materials

Very broadly, a hydride is a binary chemical compound between hydrogen and another element. (e.g., nickel metal hydride).

A metal hydride decomposes when heated, releasing the hydrogen. For transportation, this is a great theoretical solution. But finding a viable commercial implementation of this requires identifying a compound with sufficient adsorption capacity operating under realistic temperature ranges for a vehicle.

An illustrative schematic of a metal hydride storage system for vehicles is to the right.

A chemical hydride slurry or solution can also be used. Here, the hydrogen in the hydride is released through a reaction with water. Chemical hydride systems are irreversible and require thermal management and regeneration of the carrier to recharge the hydrogen content.

Sodium borohydride, used in DaimlerChrysler’s concept Natrium and a Samsung concept scooter (earlier post) is an example of a chemical hydride.

Carbon and nanoscale material look to a different approach—essentially building molecular cages that can hold the hydrogen molecules.

The DOE has set research targets for the capacity of hydrogen solid storage, measured as the percentage weight hydrogen released.

At the meeting of the American Physical Society in Los Angeles this week, more than 35 presentations described different research into all of these areas.

Although much of the focus was on hydrides, a number of presentations were on new nanomaterials.

• Researchers from NREL presented two different papers on their work with organo-metallic fullerenes (carbon buckeyball (C₆₀) complexes with Iron or Scandium) in creating a solid nanostorage material, some of which achieved hydrogen storage of 8.7 wt%. (Abstract) (Abstract)

• Researchers from Pacific Northwest National Laboratory (PNNL)are investigating ammonia borane (NH₃BH₃)and polyammonia borane (-NH₂BH₂-) within a scaffold of mesoporous silica templates. This family of molecules demonstrates hydrogen capacities of > 12 wt.%. The use of the scaffolding appears to increase H₂ production and decrease borazine formations (undersirable in a hydrogen feed). (Abstract)

• Researchers from UC Berkeley described their work with a glassy material (boron oxide) as a pathway to finding new classes of materials for hydrogen storage that can hold hydrogen at ambient conditions through physisorption. (Abstract)

• Researchers from Air Products and Chemicals presented their work on understanding the storage dynamics of single-walled carbon nanotubes (SWNT) and determining potential approaches to increasing that storage capacity. (Abstract)

Among the numerous presentations on work with hydrides, GM researchers presented two papers describing their new quaternary hydride Li₃BN₂H₈. This material (which can be produced relatively inexpensively via ball milling) releases more than 10 wt% hydrogen at temperatures greater than 250ºC—a storage capacity and operating temperature very attractive for cars.

Their preliminary work indicates, however, that the hydrogen release is not easily reversed; they have yet to achieve rehydriding (refilling the material with hydrogen). (Abstract, abstract).

A full listing of the APS March 2005 program is available here.

Solid progess, so to speak.

Resources:

• DOE EERE Hydrogen Storage

• Metal Hydrides: Transition Metal Hydride Complexes