Electrochemical impedance spectroscopy study of borohydride oxidation reaction on gold—Towards a mechanism with two electrochemical steps

Abstract

This paper focuses on the mechanism and kinetics of the borohydride oxidation reaction (BOR). This reaction was studied by quasi-steady-state hydrodynamic cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on a gold rotating disk electrode (RDE). The best operating conditions to obtain stable and reproducible experimental data were first surveyed: only (i) electrolyte solutions much more diluted than practical DBFC anolyte ( e.g. 0.01 M NaOH + 5 × 10 −5 M NaBH 4) and (ii) polarization of the gold electrode strictly below the region of gold oxide formation, are compatible with quasi-steady-state measurements. Both this diluted solution and a more classical one (0.1 M NaOH + 10 −3 M NaBH 4) were eventually investigated, the latter for comparison purposes with relevant literature data. From the modeling of the CV and EIS experimental data and consecutive parametric identification, a simplified reaction pathway was proposed, the theoretical behavior of which agrees with the experimental findings. Whatever the composition of electrolyte used, this pathway includes at least two electrochemical steps (EE) for the BH 4 − oxidation. Using the expression of the faradaic current density, concentration impedance and electrode impedance for the redox EE reaction, the theoretical steady-state hydrodynamic CV and EIS spectra can be calculated. The obtained simulated plots are representative to the corresponding experimental CV and EIS traces. However, the values of the kinetic parameters calculated from such modeling are weak, suggesting that the redox EE is still a simplification of the actual borohydride oxidation reaction pathway on gold in diluted sodium borohydride solutions.