Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media. Part I: Bulk electrodes
Résumé
We studied the borohydride oxidation reaction (BOR) by voltammetry for BH4− concentrations between 10−3 M and 0.1 M NaBH4 in 0.1–1 M NaOH for bulk polycrystalline Pt, Ag and alloyed Pt–Ag electrocatalysts. In order to compare the different electrocatalysts, we measured the kinetic parameters and the number of electrons exchanged (faradic efficiency). BOR on bulk Pt is more efficient when the concentration of NaBH4 increases (~3e− in 1 mM and ~6e− in 10 mM BH4−/0.1 M NaOH). BOR on Pt can occur both in a direct pathway and in an indirect pathway including hydrogen generation via heterogeneous hydrolysis of BH4− and subsequent oxidation of its by-products (e.g. BH3OH− and H2). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH (~2e− at pH 12.6 and 6e− at pH 13.9 at 25 °C). The BOR kinetics is faster for Pt than for Ag (iPt=0.02 A cm−2 iAg=1.4 10−7 A cm−2 at E=−0.65 V vs. NHE in 1 mM NaBH4/0.1 M NaOH, 25 °C) both as a result from Pt high activity regarding the BH4− heterogeneous hydrolysis and subsequent HOR, above −0.83 V vs. NHE and following direct oxidation of BH4− or BH3OH− below −0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH4− concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect.