Hydrogen Production from Sodium Silicide Powder; Prospects for On-Board Generation
14 July 2006
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| 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.
Resources:
“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
Sodium Silicide and Alkali Metal-Silica Gel for Convenient Hydrogen Production
FST Energy: A Cassette-Based System for Hydrogen Storage and Delivery
Sounds a lot like some other systems I've seen using Magnesium or Aluminum. At least there are no rare elements used. Would love to see a stable compound like this that uses Lithium which is 3 times lighter.
Posted by: N | 14 July 2006 at 04:38 PM
What's the EROEI? How many btus needed to make a lb of sodium silicide?
Posted by: tom deplume | 14 July 2006 at 05:21 PM
IANAC, but this does not make sense to me. Sodium takes a lot of energy to produce, silicon thakes quite a bit. The reaction proposed releases a lot of heat. My guess is that would not make energetic or economic sense.
Posted by: Robert Schwartz | 14 July 2006 at 06:16 PM
If a kg of hydrogen is equivalent to one gallon of gasoline which weighs less than .4 kg and:
0.095 kg H2 per 1 kg fuel
That would make the equivalent weigh more than 10 kg. Seems a bit heavy for the power.
Posted by: sjc | 14 July 2006 at 10:38 PM
It takes a huge amount of energy to produce silicon, weighing in at around 13 MWh/tonne (based on the reduction of SiO2 with carbon in an electric furnace at 1700 deg. C). I wrote a piece about this recently: ergobalance.blogspot.com
Producing sodium is also fairly energy intensive, and therefore, on the huge scale required to be of any significance, I think the energy costs will prove prohibitive. Ulf Bossel has stated that the hydrogen fuel cell powered car is not a viable option anyway, and we should use cells that can burn actual fuel (i.e. gas or oil) rather than making hydrogen which is NOT a fuel but an energy carrier, being made by consuming primary fuels.
I am not convinced there is any real advance here.
Posted by: Chris Rhodes | 15 July 2006 at 12:32 AM
I share your thoughts, Chris.
Posted by: John W. | 15 July 2006 at 09:09 AM
Sounds a lot like some other systems I've seen using Magnesium or Aluminum.
If you're going to use aluminum, it would make much more sense to use aluminum-air batteries (with replaceable aluminum electrodes), and skip the whole hydrogen middleman.
Posted by: Paul Dietz | 15 July 2006 at 03:08 PM
Fuel cells probably are viable, just not the hydrogen fuel cells which the fossil interests are using as a diversion for research funds and a delaying tactic. The problem to be overcome is the activation energy required to break up oxygen molecules; doing this near room temperature requires expensive precious-metal catalysts. The viable fuel cells are:
Posted by: Engineer-Poet | 16 July 2006 at 10:11 AM
The problem to be overcome is the activation energy required to break up oxygen molecules; doing this near room temperature requires expensive precious-metal catalysts.
I don't believe this is the case, EP. The air cathodes in commercial zinc-air cells are made of things like manganese dioxide, teflon, and nickel. Now, granted, these are alkaline cells, so if it were rechargable one would have to worry about carbonate formation.
Posted by: Paul Dietz | 17 July 2006 at 07:48 AM
As mentioned, the main source for hydrogen production is water. Today, the most common method of production is electrolysis. Electrolysis decomposes the water molecules to its components (hydrogen and oxygen) by passing a strong electric current through it. This process is relatively simple but it requires a significant amount of electricity and thus is currently considered to be too expensive for large scale production of hydrogen.
Breaking the water molecules by simply heating them is not practical, since it requires achieving temperature above 2,500°C (4,530F). Many years ago it was discovered that it is possible to use pure zinc to extract the oxygen from water, therefore releasing hydrogen. This process can be done in the much lower temperature of 350°C (662F).
Posted by: Mike S. | 12 August 2006 at 08:22 AM