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Boron-Based Batteries

Millennium Cell has patented processes to directly extract electricity from certain boron-based compounds. These processes are based on direct electro-oxidation without release of hydrogen. For borohydride anions, the chemistry of this reaction is as follows:

BH₄- + 8 OH- —› BO₂- + 6 H₂O + 8 e- (1)

This reaction can take place at an electrode inside a typical disposable battery. On the right side of the equation, electrons are shown (as e-). The electrons move through an external circuit and provide electrical power to a wide range of electronic devices. To complete the electrical circuit, another electrode, called a cathode, must accept the electrons. Air can be a cathode. It uses oxygen as a final place for the electrons released from borohydride. The chemical equation at an air cathode is written:

8 e- + 4 H₂O + 2 O₂ —› 8 OH- (2)

The eight electrons that are shown on the left side of this reaction originate from the right side of the reaction (1). Thus, the circuit is complete and reactions (1) and (2) form the battery chemistry.

BH₄- + 2 O₂ —› BO₂- + 2 H₂O (3)

In an actual sodium borohydride battery, all of the negatively charged borohydrides (BH₄-) and all of the negatively charged borates (BO₂-) are balanced with positive sodium atoms (Na+) and the final reaction is:

NaBH₄ + 2 O₂ —› NaBO₂ + 2 H₂O (4)

This is a very energetic reaction, and also the components are very light in weight. For these reasons Millennium Cell is developing prototype batteries with promising performance characteristics.
Millennium Cell is also developing prototype batteries from a family of boron-based compounds called borides. A representative example is titanium diboride which, when coupled with an air electrode, has the overall reaction:

2 TiB₂ + 5 O₂ —› 2 TiO₂ + 2 B₂O₃ (5)

The reasons why this battery is so promising are tabulated below in a comparison of Free Energy:

Free Energy = n · F · E

Where n is the number of electrons (in moles) exchanged in the electrochemical reaction, F is a number called Faraday's constant, and E is the battery voltage.

Battery Material	n (moles of electrons)	F (C / moles of electrons)	E (V)	Free Energy (kJ)
Zinc	2	96485	1.25	241
Sodium Borohydride	8	96485	1.24	957
Titanium Diboride	10	96485	1.8	1,737

Zinc is the metal typically used in commercial batteries. In comparison, boron-based batteries are potentially several times better than zinc batteries. The reason is the number of electrons exchanged per reaction. Real life is never as simple as calculations show, however, and the example boron-based battery chemistries from the table suffer some negative factors. The good news is that even when these things are factored in, boron-based batteries can still last twice as long as traditional batteries.

Borohydride Fuel Cell

Millennium Cell has also patented a borohydride fuel cell. This is a device that contains the best aspects of the borohydride battery and the Hydrogen on Demand™system. The hydrogen generator stores a great deal of energy in an easily handled liquid. The borohydride battery is a high efficiency energy conversion device. The borohydride fuel cell is a high efficiency energy conversion device using an energy dense, easily handled liquid fuel.

The overall electrochemistry of a borohydride fuel cell is the same as for the borohydride battery.

NaBH₄ + 2 O₂ —› NaBO₂ + 2 H₂O (1)

The main difference between borohydride fuel cells and borohydride batteries is that fuel cells generate electricity and batteries store it. In the case of a battery, all of the borohydride fuel it has available is present in the system at the beginning. When that fuel is spent, the battery is dead. On the other hand, a fuel cell stores its fuel away from the electrode, ideally in a form that is easily replenished. As long as the fuel is supplied to the fuel cell, it will continue to operate. Even if a fuel cell runs out of fuel, it can be restarted again when new fuel is provided.

The chemistry at the electrode for the borohydride fuel is

BH₄- + 8 OH- —› BO₂- + 6 H₂O + 8 e- (2)

The cathodic (air) electrode reaction is

8 e- + 4 H₂O + 2 O₂ —› 8 OH- (3)

which draws its fuel directly from the oxygen in air.

The advantage of using the borohydride battery chemistry in a fuel cell instead of PEM fuel cell chemistry (½H₂ + O₂ —›H₂O) is the possibility of extracting more usable energy from the same amount of fuel. In other words, more energy is obtained from the same fuel, and the borohydride fuel cell can be refilled and used over and over again.