Organo-Metallic Chemical Deposition of Metal Oxide Supported Electrocatalysts for ORR with High Noble Metal Utilization

Oxygen electrocatalysis is a key enabling technology for a sustainable future. The oxygen reduction reaction (ORR) has received extensive research attention as it is central to the development of several renewable energy technologies such as fuel cells and metal-air batteries [1]. Governed by sluggish kinetics, this reaction requires expensive noble metal based catalysts that require an efficient utilization in regard of the economic competitiveness of the respective technologies.

In the case of the polymer electrolyte fuel cell (PEFC), Pt supported on high surface area carbon is at present the state-of-the-art material. However, the carbon support is thermodynamically unstable at high oxidative potentials leading to Pt detachment and severe fuel cell performance losses [2]. This presents a major hurdle to the widespread availability of commercial applications. In order to overcome the problem of carbon support corrosion, chemically robust, conductive metals oxides in their thermodynamically favorable oxidation states have been shown to be a promising alternative [3]. However, conventional wet-chemical methods of Pt nanoparticle deposition on high-surface-area support materials yield much lower specific Pt surface areas when applied to metal oxide supports as compared to the results for carbon supports. The resultant reduction in Pt utilization thwarts the performance of such metal oxide supported Pt catalysts.

In this study, we have developed an organo-metallic chemical deposition method to produce Pt nanoparticles supported on several TiO₂ and SnO₂ based materials with a high Pt utilization for PEFC applications. The resultant Pt particle size distribution and dispersion on the support was determined by transmission electron microscopy. The Pt metal loading was determined by inductively-coupled plasma optical emission spectroscopy.

The ORR activity and durability of the metal oxide supported Pt nanoparticles was tested by cyclic voltammetry (CV) in oxygen saturated 0.1 M HClO₄ using the thin-film rotating disk electrode technique [4]. Here, a known aliquot of catalyst ink is drop-coated onto a glassy carbon disk as the working electrode. The Pt utilization of the catalyst was evaluated in terms of the electrochemically active surface area (ECSA) by hydrogen underpotential deposition [5]. For the durability studies, CV measurements were performed over 1000 potential cycles between 0.5 and 1.5 V versus a hydrogen reference electrode (RHE) using a scan rate of 50 mV s^-1 at room temperature. The performance and stability of the prepared catalysts for the ORR will be discussed.

Acknowledgements

We thank the South African Department of Science and Technology for financial support in the form of HySA/Catalysis Centre of Competence programme funding and HySA/Catalysis student bursary. The financial assistance of the National Research Foundation (DAAD-NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the DAAD-NRF. Financial support from the Competence Center for Energy and Mobility (CCEM) Switzerland and Umicore AG & Co. KG is greatly acknowledged.

References

[1] A. Rabis, P. Rodriguez and T. J. Schmidt, ACS Catalysis, 2, 864 (2012).
[2] T. J. Schmidt, in, F. N. Büchi, M. Inaba and T. J. Schmidt Editors, Springer Science and Business Media LLC, New York (2009).
[3] E. Fabbri, A. Rabis, R. Kotz and T. J. Schmidt, Physical chemistry chemical physics : PCCP, 16, 13672 (2014).
[4] T. J. Schmidt, H. A. Gasteiger, G. D. Stäb, P. M. Urban, D. M. Kolb and R. J. Behm, Journal of The Electrochemical Society, 145, 2354 (1998).
[5] T. Binninger, E. Fabbri, R. Kötz and T. J. Schmidt, Journal of The Electrochemical Society, 161, H121 (2014).