NaBH₄ – Sodium tetrahydridoborate:

A superb compound for storing hydrogen for use in small fuel cells.

Sodium tetrahydridoborate is not exactly a well-known chemical, but it is not particularly hazardous or expensive. It is a yellow substance, usually a powder. It is flammable, because of the high hydrogen content, and toxic. However, it is neither more toxic nor more flammable than other regularly used fuels, such as methanol or gasoline.

It is particularly suitable for use in our mini fuel cell, because when dissolved in water or an alkaline solution, in the presence of a platinum catalyst (which is on the anode) the NaBH₄ reacts, giving up its hydrogen:-

NaBH₄ + 2H₂O à NaBO₂ + 4H₂

This is a very effective system, since we are getting hydrogen out of the water as well! We get eight hydrogen atoms (four molecules) from just four atoms of hydrogen in the NaBH₄. The hydrogen is formed right there on the anode, and is thus immediately used by the fuel cell.

However, that is not all. The platinum catalyst on the anode also promotes the "direct" NaBH₄ fuel cell reaction. This does not involve the production of hydrogen, and gives a slightly higher voltage. The anode reaction is:-

NaBH₄ + 8OH^- à NaBO₂ + 6H₂O + 8e^-

In either case we get eight electrons for each fuel molecule, a superb state of affairs! The result is that just 20 mg, or 2 cubic millimetres, 0.02 cm³ of NaBH₄ will be enough fuel to operate a single cell at 100 mA for about 1 hour!

We are selling a small bottle of sodium tetrahydridoborate, containing 10g, for £6 (=$9). This will give over 200 hours running, and storing such a small quantity poses no particular hazards. Most of the cost here is the packaging, and if larger quantities are required, they can be obtained at much lower cost (per g) from most of the major chemical suppliers, such as Alfa Aesar. This is sold as product code HS03

HS03 - Small bottle containing 10g of sodium tetrahydridoborate (NaBH₄), enough for over 200 hours typical usage.

Note. The Millennium Cell Corp. in the USA is developing commercial power sources of various types that use sodium tetrahydridoborate.