Comments regarding the non-miscible solvent microcapillary method for superoxide detection in aqueous electrolytes

Certain aspects of the nanoelectrochemical method reported by Zhou et al. (J. Am. Chem. Soc., 137, 6517 (2015)) for the detection of solution phase superoxide, O₂ ^.− (aq), generated by the oxygen reduction reaction, ORR, on polycrystalline Pt in aqueous electrolytes, have been critically assessed. Experiments performed under conditions similar to those employed by these authors have shown that upon formation of the liquid-liquid interface, as required by this technique, benzotrifluoride, BTF, undergoes partial dissolution into the aqueous phase and subsequent adsorption on the Pt electrode. As evidenced by data collected with a Pt|Pt rotating ring disk electrode, this effect induces significant changes in the kinetics and mechanism associated with the ORR on the otherwise bare electrode, a factor that might limit the overall utility of this tactic as a reliable tool for elucidating detailed pathways involved in this important redox process. Also included in this communication is a mathematical model that allows for the concentration of O₂ ^.− (aq) next to the disk of a RRDE to be determined during the ORR based on the magnitude of the current collected with a Au ring specifically functionalized to detect exclusively O₂ ^.− (aq), using data reported earlier in our laboratories for a glassy carbon disk electrode as a model system.