Building upon the Koutecky-Levich Equation for Evaluation of Next-Generation Oxygen Reduction Reaction Catalysts

Abstract

The performance of oxygen reduction reaction (ORR) catalysts has been substantially improved over the past several decades. These catalysts are evaluated for electrochemical activity in a rotating disk electrode (RDE) assembly using an oxygen saturated liquid electrolyte. Koutecky-Levich (K-L) analysis provides a simple and effective method to extract electrokinetic information by correcting for mass transport effects. We propose extensions to the K-L analysis to address some of the simplifying assumptions made during its derivation. In particular, we demonstrate that decreased concentrations of surface reactants contribute to measured overpotentials in a Nernst fashion, and can lead to an underestimation of catalytic activity. By applying a Nernst overpotential correction in conjunction with K-L analysis, more accurate measurements of the intrinsic reaction kinetics under mass transport limited conditions are possible. As the K-L method assumes a reaction order of unity, we also consider kinetic reaction order deviation from unity due to measurement conditions. We show that examining the dependence of the reaction order on overpotential can provide a straightforward technique to probe blocking effects of surface absorbents. We propose that these extensions to the K-L method can allow for increased versatility of the RDE technique for extracting electrokinetic parameters for ORR catalysts.