NaBH4 as a Hydrogen Storage Material in Solution

The high stability of NaBH4 in alkaline solutions offers an alternative for the delivery of hydrogen for PEM fuel cells. Half of the four hydrogen molecules released originate from water which increases the storage capacity of the system. 

Accordingly to Eq. (5.14), four moles of hydrogen are produced from one mole of NaBH4 [52]. 

Alternatively, NaBH4 could be used in borohydride fuel cell systems, where the borohydride is directly oxidized [53]. Such systems have a slightly higher potential of = 1.64 V, compared to the PEM fuel cell with hydrogen gas as energy source. The reaction at the cathode is described as follows (Eq. (5.15))  

The product of the decomposition is, like in the Millennium Cell system, sodium metaborate. The system shows the same problems for the regeneration of NaBH4 from an aqueous solution of metaborate as described below. One advantage of the direct borohydride fuel cell systems is that platinum as catalyst is not needed. Unfortunately, depending on the temperature of the solution, some hydrogen gas is produced in a side reaction. However, this hydrogen can be piped out or can be used as additional fuel in a subsequent PEM fuel cell. 

---