The aim of this contribution is to combine the intrinsic high ORR activity of Fe with the stability of valve-metal oxides, via partial substitution of Zr4+ with Fe3+ in the ZrO2 structure [5]. The formation of a solid solution of oxides of the two aliovalent-metals should also provide oxygen vacancies or uncoordinated metal sites at the oxide surface, hypothesized to be correlated to an increased ORR activity of valve-metal oxides [6, 7].
In this study, to obtain Fe-substituted ZrO2 we used a homogeneous mixture of the two metals at the molecular level, by employing two soluble organometallic precursors, i. e., zirconium (IV) tetra-tert-butyl dichloro phthalocyanine (ZrCl2Pc(tBu)4) and iron(II) tetra-tert-butyl phthalocyanine (FePc(tBu)4). Graphitized Ketjen-Black carbon is first impregnated with the precursors and then heat-treated as described in [8, 9]. XRD and TEM characterization show that our catalysts consist of ZrO2 particles of about 3 nm (Fe-substituted) and 7 nm (pure ZrO2). Mössbauer spectroscopy reveals that the Fe moieties are isolated and in Fe3+ electronic configuration at high spin, typical of an oxidic environment. This is confirmed by soft XAS data at the Fe L-edge and XPS data, pointing to the desired FexZr1-xO2-δ phase.
We already published an evaluation of the ORR mass activity of FexZr1-xO2‑δ in comparison to Fe-only and ZrO2-only catalysts, using a thin-film rotating (ring) disk electrode (RRDE) setup [9]. Using the preferred catalyst Fe0.07Zr0.93O2-δ, the variation of its mass activity and selectivity as a function of the loading is further discussed, with a H2O2 yield even lower in comparison to an FeNC catalyst [10]. The catalyst mass activity and its Arrhenius analysis in a single PEMFC at ≈0.4 mgcat/cm² is compared to the RDE results (Figure 1) [11]. The Arrhenius analysis resulted in an ORR activation energy of 16 kJ/mol (RDE) and 18 kJ/mol (PEMFC) at 0.4 VRHE, significantly lower than our best pure-ZrO2 catalyst reported (21 kJ/mol from RDE and 29 kJ/mol from PEMFC data) [4]. Stability tests and loading optimization of nanometric FexZr1-xO2-δ will be the future outlook.
Acknowledgements: This work was supported by the Bayerische Forschungsstiftung (Project ForOxiE², AZ 1143-14).
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