ZrO₂Based Oxygen Reduction Catalysts for PEMFCs: Towards a Better Understanding

Abstract

The oxygen reduction reaction (ORR) activity of ZrO 2 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 ZrO 2 based ORR catalysts exhibit an activity gap of ca. two orders of magnitude compared to the DOE target of 300 A/cm 3 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.

Communication—Synergistic Effect on the Activity of ZrO2-Fe as PGM-Free ORR Catalysts for PEMFCs

“…While we could not exclude the possibility that we may not have found the optimum synthesis conditions, we hypothesized that the ≈10-fold lower activity of our catalyst compared to Ota et al most active catalysts might be due to the presence of iron impurities. 5 In principle, this would be consistent with the commonly stated hypothesis that the active sites in ZrO 2 based ORR catalysts are due to the presence of oxygen vacancies or uncoordinated metal sites at the oxide surface, 6,7 so that doping of the oxide structure with aliovalent metals could enhance the ORR activity. Therefore, we have chosen to examine the effect of iron as a substituent, particularly since it is both ORR-active by itself [8][9][10][11] and is known to be able to substitute Zr in the ZrO 2 structure.…”

“…Figure 2a shows the activities of the various catalysts in 0.1 M HClO 4 at 1600 rpm under pure O 2 , whereby the current density range (<0.5 mA/cm 2 ) was far below the limiting current densities for the 4-electron reduction to H 2 O (≈6 mA/cm 2 ) and the 2-electron reduction to H 2 O 2 (≈3 mA/cm 2 ), so that the shown ORR activities reflect the kinetic mass activities of the catalysts. It shows that the ZrO 2 -Fe/KB graph catalysts (0.1, 0.36, 1.0, and 10 wt% Fe) have a much higher ORR activity in comparison to pure 0.36 wt% Fe/KB graph , 12 wt% ZrO 2 /KB graph , and our previously published 13 wt% ZrO 2 /KB, 5 clearly indicating the strong synergistic effect of Fe on the ORR activity of ZrO 2 . The highest ORR activities are observed for the ZrO 2 -Fe/KB graph catalysts with 0.36 and 1 wt% Fe (≡ Fe/Zr atomic ratios of 0.02 and 0.08), which show ≈200 mV lower overpotentials compared to pure ZrO 2 at −1 A/g cat , whereby their mass activities of ≈0.15-0.20 A/g cat at 0.8 V RHE are very close to the best ZrO 2 /carbon catalysts reported so far.…”

“…The procedure to determine the ORR activity and H 2 O 2 yield of the catalysts is reported in Ref. 5,18,19. The only difference was in the ink preparation, here done by dispersing the catalysts in pure N,N-dimethylformamide (≥99.8%, Fisher Scientific) by sonication in a cold-bath for 1 h. After this, a Nafion solution (5 wt% Nafion) was added to obtain a Nafion/carbon mass ratio of ≈1/4 and the mixture was further sonicated for 5 min.…”

Synthesis optimization of carbon-supported ZrO2 nanoparticles from different organometallic precursors

“…16. In particular, sample ZrOPc/KB 900°C/PO (which is one of the most interesting catalyst on the electrochemical point of view [26]) shows the measured spectra with Gaussian-Lorentzian fits and a Shirley type background. The deconvolution of the Zr 3d spinorbit doublet is in agreement with the existence of Zr 4?…”

“…Our final aim was to check the ORR activity of the supported nanoparticles and find a possible correlation between their activity and size-structure. We have reported the electrochemical results in an electrochemistry-oriented journal [26].…”