Direct borohydride fuel cell

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Direct borohydride fuel cells (DBFCs) are a subcategory of alkaline fuel cells that use a solution of sodium borohydride for fuel. The advantage of sodium borohydride over conventional hydrogen in an alkaline fuel cell is that the highly alkaline fuel and waste borax prevents poisoning of the fuel cell from carbon dioxide (CO₂) in the air.

Sodium borohydride could potentially be used in more conventional hydrogen fuel cell systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by catalytic decomposition of the borohydride:

NaBH₄ + 2H₂O → NaBO₂ + 4H₂

Direct borohydride fuel cells decompose and oxidize the borohydride directly, side-stepping hydrogen production and even producing slightly higher energy yields:

Cathode: 2O₂ + 4H₂O + 8e^- => 8OH^- E⁰=.4 V

Anode: NaBH₄ + 8OH^- => NaBO₂ + 6H₂O + 8e^- E⁰=1.24 V Total E⁰=1.64 V

DBFCs could be produced more cheaply than a traditional fuel cell because they do not need expensive platinum catalysts. In addition, they have a higher power density. Unfortunately, DBFCs do produce some hydrogen from a side step reaction of NaBH₄ with water heated by the fuel cell. This hydrogen can either be piped out to the exhaust or piped to a conventional hydrogen fuel cell. Either fuel cell will produce water, and the water can be recycled to allow for higher concentrations of NaBH₄.

After releasing its hydrogen and being oxidized, NaBO₂ or borax is produced. Borax is a common detergent and soap additive and is relatively non-toxic. Borax can be hydrogenated back into borohydride fuel by several different techniques, some of which require nothing more than water and electricity or heat. These techniques are still in active development.

Sodium borohydride costs US$50 per kg, but with borax recycling and mass production projected prices for the fuel are as low as US$1/kg.

References

The Direct Sodium Borohydride Fuel Cell for Unmanned Underwater Vehicle Application (pdf)

MERIT reseach on DBFC

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