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Estimation of the Inherent Kinetic Parameters for Oxygen Reduction over a Pt-free Cathode Catalyst by Resolving the Quasi-Four-Electron Reduction Azhagumuthu Muthukrishnan, and Yuta Nabae J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07905 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016
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The Journal of Physical Chemistry
Estimation of the Inherent Kinetic Parameters for Oxygen Reduction over a Pt-free Cathode Catalyst by Resolving the Quasi-Four-Electron Reduction Azhagumuthu Muthukrishnan and Yuta Nabae Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 S8-26, Ookayama, Meguro-ku, Tokyo 152-8552, Japan KEYWORDS. Non-Precious-Metal, Oxygen Reduction, Fuel Cell, RRDE and Fe/N/C Catalyst
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
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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.
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INTRODUCTION The oxygen reduction reaction (ORR) in acidic media is a very important reaction considering its involvement in polymer electrolyte fuel cells (PEFCs). In particular, the ORR over a Pt-free cathode catalyst is strongly desired for the commercialization of PEFCs. Since Jasinski discovered the catalytic activity of cobalt phthalocyanine for the ORR1 and Jahnke et al. reported the heat treatment of cobalt dibenzotetraazaannulene (CoTAA)2, numerous attempts have been made to develop non-precious-metal (NPM) cathode catalysts by pyrolysis of precursors that contain transition metals (mainly Fe or Co), a nitrogen source, and a carbon source, which has led to promising fuel cell performances3–10. However, such Fe/N/C catalysts require further improvement in terms of the catalytic activity and durability, and understanding the mechanism of the ORR over these Fe/N/C catalysts is essential to design a new class of catalysts. In the ORR, the four-electron reduction pathway produces H2O and releases a relatively large free energy (1.23 V vs. RHE), which is suitable for fuel cell applications, while the twoelectron reduction pathway produces H2O2 at a lower potential (0.7 V). Some of the H2O2 produced could be further reduced to H2O, which would result in quasi-four-electron reduction11. Typical rotating disk electrode (RDE) voltammetry with Koutecky-Levich (KL) analyses and rotating ring-disk electrode (RRDE) voltammetry for the Fe/N/C catalysts tend to exhibit relatively high electron numbers close to four5,8,12–20. However, these analyses cannot resolve the quasi-four-electron reduction and the experimental data do not guarantee an actual four-electron pathway over such NPM catalysts. As a slightly detailed approach, the estimation of k1, k2 and k3 (Figure 1a) by application of the Damjanovic model21,22 has been performed for several NPM
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catalysts23–25; however, these analyses also struggle with the quasi-four-electron pathway problem. The objective of this study is to establish a novel analysis method to handle the ORR that includes both the four-electron and two-electron pathways. The Damjanovic approach is helpful to understand the trend of the reaction scheme, but the analysis results tend to be very noisy because the kinetic parameters are evaluated from the slope and intercept values of fittings to the experimental data at various rotation speeds. In this study, several equations have been derived for the analysis of experimental data from a single rotation speed, rather than fitting the data at various rotation speeds. To evaluate the rate constant of the H2O2 reduction (k3) more accurately, H2O2 voltammetry was conducted separately and the obtained data were combinatorally analyzed with those from the ORR experiments. Furthermore, the effect of catalyst loading was carefully investigated to correct the quasi-four-electron pathway and evaluate the real proportion of the currents. The voltammograms for the H2O2 reduction reaction (HPRR) and ORR over a Fe/N/C catalyst were measured and the data were analyzed using both the conventional Damjanovic method and the newly developed method.
THEORY Conventional analysis with the Damjanovic model. The Damjanovic model was originally proposed in 196621, and the analysis method with actual experimental data was reported by Hsueh et al. in 198322. The concept and theory of the conventional method are hereby briefly summarized to highlight the difference from the newly modified method.
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In the mathematical treatment by Hsueh et al., the mass balances for O2(surf) and H2O2(surf) are respectively described as: ܼଵ ߱ଵ/ଶ ൫ܿைమ , − ܿை∗ మ ൯ − ሺ݇ଵ + ݇ଶ ሻܿை∗ మ = 0 ݇ଶ ܿை∗ మ − ൫݇ଷ + ܼଶ ߱ଵ/ଶ ൯ܿு∗ మ ைమ = 0
(1)
(2)
where ω (s-1) is the rotation speed, k1, k2 and k3 (cm s-1) are the rate constants, ܿమ,ୠ and ܿ∗ మ (mol
cm-3) are the bulk and surface concentrations of O2, ܿୌ∗ మ మ is the surface concentration of H2O2, and ܼଵ = 0.62ܦమ ߥ ିଵ/ and ܼଶ = 0.62ܦୌమ మ ߥ ିଵ/ (cm s-1/2) are the parameters calculated ଶ/ଷ
ଶ/ଷ
from the diffusion coefficients of O2 and H2O2 and the kinetic viscosity of the electrolyte. The disk current, ID (A), and the ring current, IR (A), are respectively described as: ܫୈ = 2ܨܣൣሺ2݇ଵ + ݇ଶ ሻܿை∗ మ + ݇ଷ ܿୌ∗ మ మ ൧
= ୖܫ2ܼܰܨܣଶ ܿୌ∗ మ మ ߱ଵ/ଶ
(3)
(4)
where A (cm2) is the surface area of the disk electrode, F (C mol-1) is the Faraday constant, and N is the collection efficiency. Combining Eqs. 2-4 yields: ூీ ே ூ
= ቀ1 + 2 ቁ + భ మ
ೖ ଶቀଵା భ ቁయ ೖమ
మ
߱ ିଵ/ଶ
(5)
Here, N is placed on the left side to use the values measured at each rotation speed. Assuming IR
