Support Effects on Oxygen Reduction Reaction Activity of Pt Supported on SnO2

Metal oxides are gaining growing interest as support materials for Pt catalysts in polymer electrolyte fuel cells (PEFCs). They are expected to have higher stability in the oxidative electrochemical environment of a PEFC cathode compared to carbon supports of standard Pt/C catalysts [1]. In this context, SnO₂ is one of the most promising candidates. Several studies could already demonstrate the improved durability of SnO₂ supported Pt cathode catalysts compared to Pt/C but their oxygen reduction reaction (ORR) activity is usually moderate. Doping with Antimony or Niobium improves the poor conductivity of the SnO₂ and simultaneously increases the catalytic activity of the supported Pt catalyst [2]. Strong metal-support interactions (SMSI) as well as conductivity effects are suggested to explain this observation. It remains unclear, however, which material properties are stimulating SMSI and how much does the support conductivity influence the ORR activity of Pt based catalysts.  

We have prepared un-doped as well as Ta, Nb, W and Pt doped SnO₂ thin film electrodes by reactive magnetron sputtering [3]. Using these SnO₂ films as Pt support enabled us to separate between influences coming from the support conductivity and other support related effects. The surface composition of the doped SnO₂ thin films was evaluated using X-ray photoemission spectroscopy. The binding energies of Sn 3d_5/2, W 4f_7/2, Pt 4f_7/2, Nb 3d_5/2 and Ta 4d_5/2 are corresponding to SnO₂, WO₃, PtO, Nb₂O₅ and Ta₂O₅ showing that the dopants are present in the highest oxidation state with exception for Pt. Electrical properties of the doped thin films were analyzed by four-point probe measurements. For all dopants an increase of the conductivity by 1-3 orders of magnitude compared to the un-doped SnO₂ prepared under the same conditions could be observed and is a reliable indication for the homogeneous distribution of the dopant in the SnO₂ lattice. The highest conductivities of approximately 7*10³ Scm^-1 were obtained for Ta_0.01Sn_0.99O₂ and Nb_0.021Sn_0.979O₂.

Electrochemical characterizations in 0.1 M HClO₄ were carried to evaluate the ORR activity of the prepared Pt/SnO₂ catalysts. An advance H_upd method [4] has been applied for the determination of the electrochemical active surface area (ECSA) of the oxide supported Pt catalysts. Using this method reproducible ECSA values of 30±1 m²g^-1 were found for all catalysts having the same Pt loading of 2 µgcm^-2. Among the supports studied, Pt deposited on Nb_0.017Sn_0.983O₂ displayed the highest catalytic ORR activity which is superior compared to our Pt/GC catalysts even though this support does not show the highest conductivity. Pt supported on doped SnO₂ films having the highest conductivities showing moderate ORR activities comparable to Pt/GC and Pt/undoped SnO₂. Using these results we can demonstrate the influence of the dopant towards the catalytic activity of supported Pt catalysts and developed structure-reactivity relationships which indicate that the dopant is not only modifying the conductivity of the support but also triggers the activity of the supported catalyst.

Acknowledgment

The authors thank Umicore GmbH and Co KG and the Competence Center for Energy and Mobility Switzerland (CCEM) for financial support within the project DuraCat.

References

[1]           A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal. 2, 864 (2012).

[2]           K. Katinuma, Y. Chino, Y. Senoo, M. Uchida, T. Kamino, H. Uchida, S. Deki, M. Watanabe, Electrochim. Acta 110, 316 (2013).

[3]           A. Rabis, D. Kramer, E. Fabbri, M. Worsdale, R. Kötz, T.J. Schmidt, J. Phys. Chem. 118, 11292 (2014).

[4]           T. Binninger, E. Fabbri, R. Kötz, T.J. Schmidt, J. Electrochem. Soc. 161, H121 (2014).