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 (Fe_xZr₁−_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 Fe_xZr₁−_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 Fe_xZr₁−_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.