Borohydride Oxidation on Pt-Based Electrodes: Evidence of Residence Time Effect on the Reaction Onset and Faradaic Efficiency

Abstract

The borohydride oxidation reaction (BOR) was studied on Vulcan XC72-supported Pt active layers by cyclic voltammetry in 0.1 M NaOH / 10−3 mol L-1 NaBH4. The BOR onset potential and the number of electrons (ne-) exchanged per BH4- anion (faradaic efficiency) were investigated for active layers of different thickness and/or compositions, to determine whether the residence time of reaction intermediates does influence the reaction. The results indicate that the ne- value exchanged in the BOR strongly depends on the thickness of the active layer. While thin (ca. 0.5 µm-thick) Pt/C active layers yield ne- < 4, thick layers (approximately 3 µm) yield ne- ≈ 8, which can be ascribed to the sufficient residence time of the molecules formed within the active layer (H2 by heterogeneous hydrolysis or BOR intermediates). This puts into evidence that not only the nature of the electrocatalyst is important to reach high BOR efficiency, but also the structure/thickness of the active layer.