Fuel cells use hydrogen to produce electricity. They are a favorite powerplant for future energy-efficient automobiles. However hydrogen is difficult to store, typically requiring liquification and heavy, high pressure cryogenic tanks. Millennium Cell is a company researching boron-based power cells, and specifically liquified sodium borohydride as a safe, high-density storage medium for hydrogen, for long-distance transport, or as a fuel for cars. Their liquid fuel is safe, non-combustible, and recyclable.

Millennium Cell

My inquiries have led me to this website about the chemist who got the nobel prize for borane chemistry, and who discovered the surprising property of sodium borohydride: that although reacts violently with water, will not react with water if it is stabilized with a base compound (alkaline)...

Herbert C. Brown Lecture

It got me thinking and this is how I see it...The video provided on Millennium Cell's website stated that their solution contained:

75 percent water
20 percent sodium borohydride
5 percent sodium hydroxide

I assumed that these proportions are based on mass of solute. So 100 grams of liquid solution would contain:

4.170 moles water
0.530 moles sodium borohydride
0.125 moles sodium hydroxide

These amounts of solute would create approximately 102.1 millilitres of solution. Assuming the alkalinity of sodium hydroxide stabilizes the solution, then the addition of a catalyst would cause the reaction:

Ruthenium
catalyst
NaBH4 + 2H2O --------> NaBO2 + 4H2

Note that the original solution has a water-to-sodium borohydride molar ratio of 7.87-to-1. This means there is already extra water in the solution, making the quantity of dissolved sodium borohydride the limiting factor in how much hydrogen is produced. For each mole of NaBH4 the reaction yields eight moles of hydrogen, or 4H2. This 100-gram sample solution should yield 4.24 moles of atomic hydrogen, which will form into 4.24 grams of diatomic gaseous hydrogen.

At STP this amount of hydrogen will occupy 2.377 Litres volume, and will burn with oxygen in the air. When burned this amount of hydrogen will liberate 255,990 Joules of energy. So lets compare the final energy densities, in million Joules per kilogram, of Millennium Cell's "Hydrogen On Demand" to other various fuels and electric batteries:

Liquid fuels:
1. Liquid hydrogen = 2.41 MJ/kg (0.64 MJ/gal)
2. NaBH4 solution = 2.56 MJ/kg (9.491 MJ/gal)
3. Gasoline = 47.5 MJ/kg (130.827 MJ/gal)

Electric batteries:

4. Lead-acid battery = 0.792 MJ/kg
5. Nickel-cadmium battery = 0.1584 MJ/kg
6. Silver-Zinc battery = 0.396 MJ/kg
7. Sodium-sulfur battery = 0.792 MJ/kg
8. Lithium-Sulfur battery = 0.792 MJ/kg
9. Iron-titanium hydride battery = 2.124 MJ/kg
10.Magnesium hydride with Ni catalyst battery = 8.280 MJ/kg

I am by no means an expert in chemistry, and maybe I've made an error, but my high school chemistry suggests Millennium Cell's "hydrogen on demand" fuel has approximately the same energy density per unit mass as liquid hydrogen. However it has nearly 15-times the energy density per unit volume, is liquid at room temperature & pressure, and appears to be superior to every electric battery except the solid metal-hydride battery. Gasoline is still far superior in every respect.