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

Abstract

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.

Carbon Supported Multi-Branch Nitrogen-Containing Polymers as Oxygen Reduction Catalysts

“…value was k1 >> k2~k3, which indicated that the predominant pathway was the direct four-electron ORR pathway with water as the final product [41,42]. Another phenomenon was the value of k2 < k3, which meant that H2O2 produced in the two-electron pathway was immediately transformed to H2O.…”

Electrocatalytic Oxygen Reduction at Multinuclear Metal Active Sites Inspired by Metalloenzymes

“…As the reaction mecha- nism on CcO is discussed, copper ions could interact with oxygen that is bonded to iron sites and then activate it for efficient O−O bond cleavage [103]. Further experimental studies including spectroscopic measurements and kinetic analysis [104] and theoretical studies [97] are needed to gain insights into the synergistic effect and reaction mechanism on Fe/Cu/N/C electrocatalysts.…”

Morphology-Controlled Nitrogen-Containing Polymers as Synthetic Precursors for Electrochemical Oxygen Reduction Fe/N/C Cathode Catalysts

“…While Ohsaka and co-workers tried to apply the Damjanovic-Hsueh approach [21,26], which involves detailed RRDE analysis by plotting of the obtained parameters against the rotation speeds, such analysis relying on rotation speeds suffered from huge noise [27,28]. To address these concerns, Nabae and co-workers recently developed a new model (Figure 2b) and method for RRDE analysis [29]. This new method avoids overestimating the contribution of the four-electron reduction reaction by studying the different loading densities of the catalysts and extrapolating the obtained parameters to null loading density.…”

“…The advantages associated with using nitrogencontaining polymers for controlling catalyst morphology have been studied in detail by our research group and are discussed in the next section. I s refers to the current obtained via the series reaction in the catalyst layer matrix and γ is the catalyst loading density [29].…”

“…To address these concerns, Nabae and co-workers recently developed a new model ( Figure 2b) and method for RRDE analysis [29]. This new method avoids overestimating the contribution of the four-electron reduction reaction by studying the different loading densities of the catalysts and extrapolating the obtained parameters to null loading density.…”