|Hydrogen production by the two SiGNa Chem materials. Click to enlarge.|
SiGNa Chem has developed a promising method for using highly reactive alkali metals to produce different types of strong reducing agents and convenient sources for hydrogen. The latter application could provide an on-board mechanism for hydrogen storage and production for vehicles.
The company has achieved hydrogen production of 9.5 wt% (0.095 kg H2 per 1 kg fuel) by inserting a stable form of sodium into water. The DOE 2015 target for H2 storage in vehicles is 9 wt%, although that target is based on total system weight, not just fuel weight.
Alkali metals—Group 1 elements such as lithium (Li), sodium (Na) and potassium (K)—are highly reactive, and are especially known for their violent reactions with water, the byproducts of which are metal hydroxides, hydrogen and heat.
SiGNa Chem absorbs sodium in silica gel (porous silicon dioxide), or reacts it directly with elemental silicon to create sodium silicide powders that then react rapidly and controllably with water to produce hydrogen.
The silicide material—NaSi—would be the material of choice for a vehicle application, as it provides greater power density per unit of weight than the silica gel (Na-SG). It is the NaSi material that produces the 9.5 wt% hydrogen.
An example of the reaction that produces the highest yield per unit mass is:
2NaSi (s) + 5H2O (l) → Na2Si2O5 (aq.) + 5H2 + Heat (~175 kJ/mol)
The reaction produces five moles of hydrogen, almost instantaneously, from the reduction of five moles of water and only two moles of NaSi.
Sodium silicide is labelled a pyrophoric material—a material that ignites spontaneously, and thus required special storage and handling. SiGNa’s NaSi powder, however, is easily handled and used, does not react with dry oxygen, and absorbs moisture from air slowly without ignition. They react rapidly, but controllably.
FST Energy, a company working on developing a fuel cassette-based system to store and releases H2 at optimum efficiencies, is evaluating SiGNa Chem’s NaSi material for its cassettes.
“Alkali Metals Plus Silica Gel: Powerful Reducing Agents and Convenient Hydrogen Sources”; James L. Dye, Kevin D. Cram, Stephanie A. Urbin, Mikhail Y. Redko, James E. Jackson, and Michael Lefenfeld; J. Am. Chem. Soc., 127 (26), 9338-9339, 2005. DOI: 10.1021/ja051786+ S0002-7863(05)01786-5