Sodium borohydride, also known as sodium tetrahydridoborate, has the chemical formula NaBH4. It is a selective specialty reducing agent used in the manufacture of pharmaceuticals, intermediates and fine chemicals. It is a yellow solid, usually encountered as a powder. It melts at 36°C (97°F) and is stable (when dry) up to approximately 300°C (570°F). It is soluble in cold water, but reacts with hot water.
Fuel cells
It is also used in experimental fuel cell systems as a means of storing hydrogen. As a fuel it is less flammable and less volatile than gasoline but much more corrosive. It is relatively environmentally friendly because it will quickly degrade into inert salts when released into the environment. The hydrogen can be regenerated for a fuel cell by catalytic decomposition of the aqueous borohydride solution:
- NaBH4 + 2H2O → NaBO2 + 4H2
Direct Borohydride Fuel Cells (DBFC ) decompose and oxidize this fuel directly, side-stepping hydrogen production and even producing slightly higher energy yields:
- Fuel Cell's Cathode: 2O2 + 4H2O + 8e- => 8OH- E0=.4v
- Fuel Cell's Anode: NaBH4 + 8OH- => NaBO2 + 6H2O + 8e- E0=1.24v Total E0=1.64v
DBFCs could be produced more cheaply (because they do not need expensive platinum catalysts) and provide higher power output per mass than a traditional fuel cell. 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 relatively inert and non-toxic; it is a common detergent and soap additive. The borax waste can be pumped into a waste tank or bladder inside the fuel tank.
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.
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