Electrochemically Quantifying Oxygen Reduction Selectivity in Nonaqueous Electrolytes

Understanding the factors that govern the selectivity of the oxygen reduction reaction (ORR) has motivated the study of ORR in nonaqueous media, in which the proton donor can be controlled and molecular catalysts can be solubilized. Rotating ring-disc electrode (RRDE) voltammetry is a powerful method for quantifying ORR selectivity but requires that the ring electrode catalyze H2O2 oxidation at transport-limited rates. While the potentials and ring materials required to satisfy this requirement have been examined for aqueous electrolytes, they have not been systematically investigated for nonaqueous electrolytes despite the growing use of RRDE in these media. Herein, we report that conventional Pt ring electrodes and typical RRDE protocols for nonaqueous media do not give rise to mass-transport-limited H2O2 oxidation kinetics. Instead, H2O2 oxidation is strongly activation-controlled on Pt surfaces with kinetics that depend on the electrolyte and solvent. We identify Au as a superior catalyst for nonaqueous H2O2 oxidation and show that transport-limited H2O2 oxidation can be accessed on high-surface-area Au rings under certain electrolyte conditions. Additionally, we develop a protocol for calibrating and correcting for sluggish H2O2 oxidation kinetics. We show that these two methods provide improved estimates of H2O2 selectivity for ORR catalysis in certain nonaqueous electrolytes.