Quantifying intrinsic kinetics of electrochemical reaction controlled by mass transfer of multiple species under rotating disk electrode configuration

Precise measurements and accurate analysis of reaction processes are the prerequisite for correctly understanding the key factors that control kinetics of electrode reaction. Rotating disk electrode systems are widely used to study the kinetics of reactions that may suffer from insufficient mass transfer. The commonly used Koutecky-Levich equation is only applicable for reactions affected by the mass transfer of single reactant. In this work, we propose a general method for estimating the intrinsic kinetics of reactions hampered by the mass transfer of more than one reactant. Its application is demonstrated by the kinetic study of oxygen reduction reaction (ORR) at Pt (111) in un-buffered solutions with medium pH, where the mass transfer of both O-2 and H+ are slow. Our analysis reveals that the kinetic rate constant at activation overpotential around 300 mV (similar to 0.9 V vs RHE) increases for ca. three orders of magnitude with the increase of solution pH from 1 to 3. Possible origins for such pH effect are discussed.