Nanometric Fe-Substituted ZrO2 on Carbon Black as PGM-Free ORR Catalyst for PEMFCs

Pankaj Madkikar,Stefan Schuppler,Hubert A Gasteiger,Thomas Mittermeier,Davide Menga,Michele Pavone,Michael Merz,Armin Siebel,Friedrich E Wagner,Peter Nagel,Michele Piana,Gregor S Harzer,Ana Belén Muñoz-García

doi:10.1149/2.0041907jes

Abstract

In this contribution, we demonstrate the presence of high-spin Fe3+ in Fe-substituted ZrO2 (FexZr1−xO2−δ), as deduced from X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and 57Fe Mössbauer spectroscopy measurements. The activity of this carbon-supported FexZr1−xO2−δ catalyst toward the oxygen reduction reaction (ORR) was examined by both rotating (ring) disk electrode (R(R)DE) method and single-cell proton exchange membrane fuel cells (PEMFCs). DFT calculations suggest that the much higher ORR mass activity of FexZr1−xO2−δ compared to Fe-free ZrO2 is due to the enhanced formation of oxygen vacancies: their formation is favored after Zr4+ substitution with Fe3+ and the oxygen vacancies create potential adsorption sites, which act as active centers for the ORR. H2O and/or H2O2 production observed in RRDE measurements for the Fe0.07Zr0.93O1.97 is also in agreement with the most likely reaction paths from DFT calculations. In addition, Tafel and Arrhenius analyses are performed on Fe0.07Zr0.93O1.97 using both RRDE and PEMFC data at various temperatures.

Highlights

To cite this article: Pankaj Madkikar et al 2019 J
The metal content in ZrO2, Fe0.17Zr0.83O1.91, and Fe0.07Zr0.93O1.97 (2nd, 3rd, and 4th row) is only approximately half of the nominally expected value. This discrepancy is likely due to the carbon produced by decomposition of the Zr precursor, which is significantly larger in comparison to that produced in the Fe sample, since the amount of FePc(t-Bu)[4] is ≈70 times lower than the ZrCl2Pc(t-Bu)[4]
While we had already reported on the high ORR activity of carbon-supported nanometric Fe-substituted ZrO2 in a previous communication,[27] it remained unclear what the nature of the active site(s) would be, how Fe is coordinated in this catalyst, and how it would perform in a single-cell PEMFC

Summary

Introduction

To cite this article: Pankaj Madkikar et al 2019 J. From our DFT-based results, the much higher mass activity of Fe-substituted ZrO2 with respect to ZrO2 can be ascribed to the different surface patterns and, potential oxygen (intermediate) adsorption sites.

Results

Conclusion