Direct Borohydride Fuel Cells (DBFC)

Direct Borohydride Fuel Cells are unique in that they do not rely on pure hydrogen or on hydrogen derived from hydrocarbon fuels. Borohydride is the fuel that supplies these cells and there is no actual production of hydrogen gas that takes place. They are one of the newest and most experimental forms of fuel cell. Like metal hydride fuel cells, DBFCs also show potential for the fact that borohydride can be used to store hydrogen.

Function and Reactions of DBFCs

It is somewhat misleading to say that borohydride supplies hydrogen in these cells because the production of hydrogen never actually occurs. Like other alkaline fuel cells, oxygen reacts with water and electrons at the cathode to produce hydroxide ions, which migrate across the electrolyte layer. At the anode, however, the reaction is different.

At the anode, sodium borohydride reacts directly with hydroxide ions to produce water, electrons, and sodium metaborate (NaBO2). There is no intermediate step in which hydrogen gas is produced, which is the case in all other fuel cells except the Zinc-air fuel cell. The electrons travel back to the cathode through an external circuit where they are used in the cathode reaction again.

Benefits of DBFCs

Direct borohydride fuel cells do not rely on platinum catalysts for oxidation and, because hydrogen production is completely side-stepped, they are more efficient than other alkaline fuel cells. In fact, they are capable of operating efficiencies as high as 70% without catalysts at temperatures of 70 C. They also have very high power densities. These features make them attractive for transportation purposes.

Drawbacks of DBFCs

In an odd twist, one of the drawbacks of a DBFC is that they do produce hydrogen in small quantities and it must be removed from the cell. The production of gaseous hydrogen in these cells is considered an inefficiency of the system. The hydrogen can be pumped out to a hydrogen fuel cell, increasing the efficiency of the entire system, but also increasing the complexity and decreasing reliability.

Currently, sodium metaborate cannot easily be converted into sodium borohydride, making the chemical reaction in these cells irreversible. Techniques are being developed in an attempt to reverse the reaction, but they may impact the overall efficiency of the cell due to their requirement for electricity. Current recycling efficiency is less than 1%, meaning only 1% of the sodium metaborate is converted back into sodium borohydride. The cost of sodium borohydride is currently $50 per kilogram, making recycling essential if these cells are to be economically viable.