Concentration ratio of [OH−]/[BH4−]: A controlling factor for the fuel efficiency of borohydride electro-oxidation

The fuel efficiency of borohydride electro-oxidation on carbon-supported Au and Ag electrodes was found to be highly dependent on the concentration ratio of [OH−]/[BH4−] in the solution. Near-8e reactions occurred when [OH−]/[BH4−] ≥ 5. However when [OH−]/[BH4−] was smaller than 5, hydrogen gas was evolved and the fuel efficiency was reduced. Only 3e reaction stoichiometry was obtained at [OH−]/[BH4−] = 1. Detailed cyclic voltammetry (CV) studies revealed that both the Au/C and Ag/C showed different anodic waves in varied NaOH–NaBH4 solutions. The CV analysis results suggest that BH3OH−, an intermediate possibly produced by homogeneous hydrolysis, is responsible for the electrochemical reaction at [OH−]/[BH4−] = 1. Comparison of CV voltammograms in borohydride solutions with that in the H2-bubbled NaOH solution suggests that borohydride electro-oxidation on Au or Ag electrode is through direct BH4− oxidation rather than through a hydrogen ionization mechanism. It is concluded that there exists an inherent competition between two oxidizing species of OH− and H2O during borohydride electro-oxidation, that is, if accessible OH− ions are not sufficient for each BH4− to accomplish the 8e electro-oxidation, part of BH4− will react simultaneously with H2O to generate hydrogen.