ZrO₂ based oxygen reduction catalysts for PEMFCs: Towards a better understanding

The oxygen reduction reaction (ORR) activity of ZrO₂ based, carbon-supported nanoparticles is not conclusively reported in literature. This study examines the dependence of the ORR activity on the used precursors as well as on the heat-treatment atmosphere and temperature. We further determine the ORR activation energy and the ORR mechanism. Various precursors containing Zr and/or N were employed in the synthesis, and the ORR activity was measured by rotating (ring) disk electrode (R(R)DE) voltammetry in both acidic and alkaline electrolyte as well as by measurements in a single-cell polymer electrolyte membrane fuel cell (PEMFC) configuration. We show that even the most active ZrO₂ based ORR catalysts exhibit an activity gap of ca. two orders of magnitude compared to the DOE target of 300 A/cm³ for PGM-free ORR catalysts, thus requiring further development. Our RRDE analysis suggests a primarily 2-electron ORR mechanism in the case of the tested catalysts in acid, which in turn provides a consistent temperature dependence between RDE and PEMFC experiments, allowing also for a mechanistic (re-) interpretation of experimental results in the literature.