Fuel cells use hydrogen to produce electricity. They are one of the favored power plants for future energy-efficient automobiles. However hydrogen is difficult to store, typically requiring liquification under high pressure inside heavy cryogenic tanks. Millennium Cell is a company researching boron-based power cells, and specifically liquefied 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 into how sodium borohydride (a hypergolic compound) could mix safely with water 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 it reacts violently with water, it will not react with water that is stabilized with a base compound (alkaline).

Herbert C. Brown Lecture

Millennium Cell is using sodium borohydride as a storage medium for hydrogen. To determine the energy density of Millennium Cell’s “Hydrogen on Demand” fuel, let’s first evaluate the properties of gaseous hydrogen and liquid hydrogen.

At room temperature & pressure, one mole of hydrogen will occupy a certain volume as determined by the Ideal Gas Law:

PV = nRT

where,
P = standard atmospheric pressure = 101,300 Pascals
V = standard molar volume (to be determined)
n = number of moles of substance
R = universal gas constant = 8.3144 J/(mole - deg.K)
T = room temperature = 298 deg. K

At STP one mole of hydrogen gas (H2) occupies 24.458945 Liters volume, and has a mass of 2 grams. Therefore the density of hydrogen gas at STP is 0.000081768 g/mL.

Hydrogen burns in pure oxygen to form water in the following reaction:

2H2 + O2 ---> 2H2O + 483 kJ

Therefore the energy density of hydrogen gas is 241.5 kJ/mol, or 9.874 kJ per Liter. The published density of liquid hydrogen is 0.07 g/mL, which translates into 70 moles per Liter. For combustion with oxygen the corresponding energy density of liquid hydrogen is therefore 16,905 kJ per Liter, or 120,750 kJ/kg.

The video provided on Millennium Cell's website stated that their solution contained:

75 percent water
20 percent sodium borohydride
5 percent sodium hydroxide

It is 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 milliliters of solution. Since the alkalinity of sodium hydroxide stabilizes the solution, then the addition of a catalyst would cause the reaction:

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

There is already excess water in the solution so the quantity of sodium borohydride is the limiting factor in how much hydrogen is produced. For each mole of NaBH4 the reaction yields four moles of hydrogen gas, or 4H2. This 100-gram sample solution should therefore yield 2.12 moles, or 4.24 grams of hydrogen gas.

At STP this amount of hydrogen will occupy 52.342 Liters volume. When burned in pure oxygen it will liberate 511.98 kJ of energy. This gives a energy density for sodium borohydride of 5014.5 kJ per Liter, or 5119.8 kJ/kg. So lets compare the final energy densities, in million Joules per kilogram, of Millennium Cell's "Hydrogen On Demand" fuel to other various fuels and electric batteries:

1. Nickel-cadmium battery = 158 kJ/kg
2. Silver-Zinc battery = 396 kJ/kg
3. Lead-acid battery = 792 kJ/kg
4. Sodium-sulfur battery = 792 kJ/kg
5. Lithium-Sulfur battery = 792 kJ/kg
6. Solid Iron-titanium hydride = 2,124 kJ/kg
7. Liquid Sodium borohydride = 5,120 kJ/kg (5014.5 kJ per Liter)
8. Solid Magnesium hydride = 8,280 kJ/kg
9. Liquid hydrogen = 120,750 kJ/kg (16,905 kJ per Liter)
10.Gasoline = 47,500 kJ/kg (34,561 kJ per Liter)

However the fuels will be used in different types of engines with different efficiencies. A gasoline engine has 20 percent efficiency, and a hydrogen fuel cell has 40 percent efficiency. Taking the most promising fuels and adjusting them for engine efficiency yields a better result. Fuels 1 thru 4 are to be used with a fuel cell, while gasoline is to be used in an internal combustion engine:

1. Solid Iron-titanium hydride = 850 kJ/kg
2. Liquid Sodium borohydride = 2,048 kJ/kg (2006 kJ per Liter)
3. Solid Magnesium hydride = 3,312 kJ/kg
4. Liquid hydrogen = 48,300 kJ/kg (6762 kJ per Liter)
5. Gasoline = 9,500 kJ/kg (6912 kJ per Liter)

These results suggest Millennium Cell's "hydrogen on demand" would require a fuel tank 3.45-times larger than an equivalent gasoline, or liquid hydrogen tank. Liquid hydrogen is not considered a viable automotive fuel because the storage vessel is very expensive, as would be the re-fuelling station. Gasoline is still superior in terms of energy density and convenience. However Millennium Cell’s sodium borohydride fuel produces clean energy without releasing any pollution, and is totally renewable. It is also a liquid at room temperature, is non-flammable & non-toxic, and recyclable, providing a similar level of convenience as gasoline.

I think I like Millennium Cell's fuel. But I will remain undecided until I can compare the price per liter to that of gasoline. But the quality is high. I think they will win.