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The Hydrogen on Demand™ System

Millennium Cell has devised a compact and portable way to store hydrogen (H₂) for many practical purposes including propelling a car. It is based on liquid water salt solutions at room temperature, is easily accessed without energy input, is stable in air, is not too heavy, and not too big.

The Millennium Cell Hydrogen on Demand™generator uses the following chemical reaction.

NaBH4 + 2 H2O —› 4 H2 + NaBO2

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Sodium borohydride (NaBH₄), a salt, is dissolved in water where it stays until gaseous hydrogen (H₂) is needed. This is the left side of the reaction. When H₂ is desired, the NaBH4 solution is pumped over a catalyst. This is the arrow in the center of the reaction marked "cat". The H₂ gas comes out, and leaves behind sodium borate (NaBO₂), another salt, which remains dissolved in water. This is the right side of the reaction. To stop the left to right progression of the H₂ generation reaction, the pump stops and the solution is kept from the catalyst. Without the catalyst, the H₂ generation does not occur.
Some important points about the above chemistry:

• The solution of borohydride dissolved in water is non-flammable.
  
• The reaction is easily controlled via the catalyst and reactor configuration.
  
• Half of the hydrogen comes from the borohydride. The other half comes from water.
  
• The catalyst can be reused many times.
  
• Sodium borate is a common, non-toxic household item; it is used in detergents.
  
• Sodium borate can be recycled into new sodium borohydride.

These points are all critical if the goal is to have a practical transportation fuel. Most important is the idea that on board a car, borohydride fuel will be stored in a tank just like the ones cars use today. The tank can be made out of plastic, molded to match the shape of an automobile chassis, and will be essentially the same size as the standard gas tank. The rest of the system is reasonably compact. According to the ideas discussed above the fuel needs to be in contact with the catalyst. To accomplish this task in a car the fuel is pumped to a chamber containing the catalyst. The chamber releases all of the H₂ from the borohydride in one pass, enough to power the car, and the remaining borate goes to a spent fuel tank. When H₂ is no longer desired, the pump is shut off, isolating the catalyst from the fuel. By turning the pump on and off, the hydrogen flow is easily controlled. Increasing and decreasing the rate of pumping can also affect a much finer control of the H₂ generation rate.

While cars are very interesting, Hydrogen on Demand™ can also be employed in off board applications. Small systems have been designed which do not even require pumps, using only pressure differences to move fuel to and away from a fixed catalyst. All in all, Hydrogen on Demand™ is proving to be a very flexible method of supplying hydrogen to fuel cells and engines of various sizes, without any emissions.