Estimation of the Inherent Kinetic Parameters for Oxygen Reduction over a Pt-Free Cathode Catalyst by Resolving the Quasi-Four-Electron Reduction

Understanding
the kinetics of the oxygen reduction reaction (ORR) for fuel cell
applications is quite important but difficult because the four-electron
pathway is often overestimated by including a quasi-four-electron
pathway that consists of the formation and reduction of H2O2. To solve this problem, here we demonstrate a novel
analysis method with experimental data over a Pt-free Fe/N/C cathode
catalyst. In this study, H2O2 voltammetry was
conducted separately to evaluate the rate constant of the H2O2 reduction more accurately, and the obtained data were
combinatorally analyzed with those from the ORR experiments. First,
mathematical modification of the conventional Damjanovic approach
was performed, and then the effect of the catalyst loading density
was carefully studied by utilizing a novel reaction model with consideration
of the quasi-four-electron pathway to avoid overestimation of the
four-electron pathway kinetic parameters. In the most overestimated
case, the percentages contribution of four-electron pathway over the
Fe/N/C catalyst was estimated as 85% with the conventional Damjanovic
model, while that from the newly proposed model is 51%. This method
will be applicable for many other cathode catalysts and will facilitate
understanding the nature of the ORR.