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, experimental and 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. Figure 1 shows the reaction current, I₁, I₂ and I₃, evaluated with different analysis methods. With the conventional Damjanovic-Hsueh approach (left), the evaluated reaction currents are very noisy and do not correspond to the measured current. After the experimental and mathematical modification (middle), the noise in the analysis results are much reduced. Furthermore, by considering the quasi-four electron reduction, the overestimation of the four-electron pathway can be avoided (right). In the most overestimated case, the percentages contribution of four-electron pathway over the Fe/N/C catalyst was estimated as 85% with the modified Damjanovic approach, 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.

Figure 1. The comparison of conventional Damjanovic-Hsueh approach, modified Damjanovic approach and newly proposed model for ORR over Fe/N/C catalyst.