Nanometric Fe-substituted ZrO2 on Carbon Black: a Novel PGM-Free ORR Catalyst for PEMFCs
Authors: Piana Michele , Madkikar Pankaj , Menga Davide , Harzer Gregor S. , Mittermeier Thomas , Siebel Armin , Wagner Friedrich E. , Merz Michael , Schuppler Stefan , Nagel Peter , Gasteiger Hubert A. 
Published: February 2020

The independence from Platinum-Group-Metal (PGM) is one of the major goals towards the commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs), due to the limited availability of PGM and their considerable loading on the Oxygen-Reduction-Reaction (ORR), increasing their cost contribution to the overall system. On the other hand, the challenges for PGM-free ORR catalysts are an activity approaching that of Pt and a long-term operational stability in the strong acidic environment of a PEMFC. Low-cost FeNC catalysts fulfill the high activity requirement, but they still face fast degradation in acidic environment [1, 2]. On the contrary, pure valve-metal oxides as non-PGM ORR catalysts need a significant improvement in ORR activity and H2O yield but they are very promising in terms of stability during PEMFC operation [3, 4].

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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Figure 1