Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part I. BH4 electro-oxidation on Au and Ag catalysts

The direct oxidation of sodium borohydride in concentrated sodium hydroxide medium has been studied by cyclic and linear voltammetry, chronoamperometry and chronopotentiometry for silver and gold electrocatalysts, either bulk and polycrystalline or nanodispersed over high area carbon blacks. Gold and silver yield rather complete utilisation of the reducer: around 7.5 electrons are delivered on these materials, versus 4 at the most for platinum as a result of the BH₄⁻ non-negligible hydrolysis taking place on this latter material. The kinetic parameters for the direct borohydride oxidation are better for gold than for silver. A strong influence of the ratio of sodium hydroxide versus sodium borohydride is found: whereas the theoretical stoichiometry does forecast that eight hydroxide ions are needed for each borohydride ion, our experimental results prove that a larger excess hydroxide ion is necessary in quasi-steady state conditions. When the above-mentioned ratio is unity (1 M NaOH and 1 M NaBH₄), the tetrahydroborate ions direct oxidation is limited by the hydroxide concentration, and their hydrolysis is no longer negligible. The hydrolysis products are probably BH₃OH⁻ ions, for which gold displays a rather good oxidation activity. Additionally, silver, which is a weak BH₄⁻ oxidation electrocatalyst, exhibits the best activity of all the studied materials towards the BH₃OH⁻ direct oxidation.Finally, carbon-supported gold nanoparticles seem promising as anode material to be used in direct borohydride fuel cells.