A preliminary study of sodium octahydrotriborate NaB3H8 as potential anodic fuel of direct liquid fuel cell

Theoretically, sodium octahydrotriborate NaB3H8, solved in alkaline aqueous solution, is a potential boron-based liquid anodic fuel of direct liquid fuel cell, since the B3H8− anion could be oxidized (B3H8− + 20OH− → 3BO2− + 14H2O + 18e−) in a reaction ideally involving 18 electrons. Hence, we focus on this material. We firstly synthesize NaB3H8 and fully characterize it (NMR, FTIR, Raman, XRD, TGA, DSC), then study its stability in protic solutions by NMR, and lastly investigate its oxidation with the help of electrochemical techniques. The diffusion coefficient for the B3H8− anion is calculated (3.7·10−5 cm2 s−1) in order to assess the coulombic efficiency for the oxidation on platinum and gold electrodes. It is found effective numbers of electrons of 5.2 and 10.1 (out of a theoretical total of 18 electrons) respectively. Neither platinum nor gold smooth surfaces enable the complete oxidation of B3H8−, which is explained mainly by the occurrence of heterogeneous hydrolysis (i.e. hydrogen evolution), but also by the difficulty to break the B-B bonds (difficult to oxidize) present in the B3H8− anion. Yet, these first results confirm that the B3H8− anion has a potential as anodic fuel, provided the proper electrocatalyst is isolated: this is discussed herein.