Borohydride Oxidation at a Gold Electrode

The multistage process of borohydride oxidation in an 8 e- reaction to borate at a Au electrode has been studied by means of fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The total irreversibility of this process observed previously is shown to be a result of the presence of very unstable intermediates. CV measurements showed that at least two stages of the process are quasi-reversible, and the presence of a coupled homogeneous chemical reaction was proved by SECM. The rate constant for this reaction as well as the electrochemical kinetic parameters for the first stage of oxidation are evaluated using digital simulation. The adsorption of the electroactive species associated with the first two-electron stage of the oxidation becomes apparent at scan rates higher than 200 V/s. A very small fractional surface coverage (estimated to be less than 0.001) is shown to produce CV waves characteristic of adsorption- rather than diffusion-controlled processes. The second chemical stage of this process is much faster than the first. The oxidation of borohydride at a gold anode is shown to have a different mechanism than that proposed earlier for a platinum electrode. We describe here a study of the electrochemical oxida-tion of borohydride (or tetrahydroborate), BH;, at a gold electrode and the use of ultramicroelectrodes and fast techniques to study this complex process. Electrochemi-cal oxidation of sodium borohydride and its derivatives in aqueous olutions was a subject of several detailed studies as described in a previous review 1. In earlier studies 2-8, car-ried out in the 1950s and 1960s, fast electrochemical tech-niques were not available to study rapid chemical stages and detect unstable intermediates. Interest in this process arises from its possible application as an anodic fuel in a battery, its role in electroless deposition (e.g., of Au), and the challenge of elucidating a complicated (irreversible 8e-) electrode reaction BH ~ + 8 OH---- BO2 + 6H20 + 8 e- 1 Study of aqueous olutions of BH; is complicated by its rapid reaction with water (termed hydrolysis) at pHs below about 12 k 1 BH; + H20-+ BH3OH- + Ha 2 k 2