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

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 Fe_0.07Zr_0.93O_1.97 is also in agreement with the most likely reaction paths from DFT calculations. In addition, Tafel and Arrhenius analyses are performed on Fe_0.07Zr_0.93O_1.97 using both RRDE and PEMFC data at various temperatures. [ABSTRACT FROM AUTHOR]