Borohydride Electro-Oxidation on Gold Investigated by Electrochemical Quartz Crystal Microbalance

Abstract

The quartz crystal microbalance technique coupled with cyclic voltammetry (referred to as electrochemical quartz crystal microbalance (EQCM)) was employed to study the oxidation of BH4− on Au in alkaline media (0.1 M and 2 M NaOH, respectively) using a newly designed vertical cell to minimize the interference by evolved H2. The electrode potential was scanned between −0.7 and 0.7 V vs. SHE with scan rates in the range of 0.01 V s−1 and 1 V s−1. The relative mass changes of the Au resonator-electrode were interpreted in combination with density functional theory (DFT) modeling and pertinent experimental results from the literature. The potential scan rate dependent mass change profiles revealed: i) weak adsorption of BH4− below −0.2 V leading to high anodic overpotential, ii) accumulation of reaction intermediates on the surface between approximately −0.2 V to 0.3 V (scan rate dependent), and iii) Au surface hydroxide and oxide assisted oxidation of BH4− and possibly of other active species present such as BH3OH− formed by hydrolysis.