Binary and Ternary Catalysts Based on Pt, Au and Ni for the Oxygen Reduction Reaction

Abstract

Proton exchange membrane fuel cells (PEMFC) are currently considered as an alternative to fossil fuels for transportation, stationary, aeronautics applications1… However, the activation of oxygen reduction reaction (ORR) at the cathode of PEMFC has been and remains a key challenge to make this technology marketable. Indeed, oxygen reduction reaction has a low kinetics and requires catalyst to be activated. Platinum is known to be the most active electrocatalyst for this reaction. The disadvantages of platinum based catalysts are their high cost and low durability. Then several studies were carried out for the last decades in order to develop new catalysts both to improve the catalytic activity and the durability, and to reduce the platinum loading at the cathode. Studies indicated that structural and electronic effects involved in Pt alloys or core shell structures (with metals such as Au, Ni, Fe, Co) enhance the catalytic activity 2, 3. Recent works on Pt3M (M= Ni, Fe, Co) showed considerable improvement of catalytic activity toward the ORR4. It was also shown that the durability could significantly be improved by alloying Pt with Au. This contribution presents a comparative study of the ORR on monometallic, alloyed binary and ternary nanocatalysts based on Pt, Ni and Au. Water-in-oil microemulsion method has been implemented for synthesizing Pt/C, Au/C, PtxM1-x/C (M= Ni, Au) and PtxNiyAuz/C nanocatalysts supported on a carbon black Vulcan XC-72 support. Nanocatalysts were comprehensively characterized by atomic absorption and thermogravimetric analysis in order to determine bulk chemical compositions and metal loadings. The morphology of catalysts and the particle sizes were determined by transmission electron microscopy (TEM). X-ray diffraction (XRD) studies allowed determining their microstructure (crystallite size and alloy formation). The surface composition was determined by X-Ray photoelectron spectrometry (XPS) and electrochemical analyses. The catalytic activity and the selectivity of the nanocatalysts toward the ORR have been determined by rotating disc electrode (RDE) and rotating ring disc electrode (RRDE) in O2-saturated 0.1 M HClO4 electrolyte. It was shown that the catalytic activity of PtxAu1-x/C nanocatalysts is maintained up to 50 at % of gold in the alloy. The addition of Ni to Pt allows improving the catalytic activity in comparison to that recorded on pure Pt/C catalyst (figure 1), with an optimum for a Pt3:Ni1 ratio. Hence, different ternary PtxNiyAuz/C catalysts were synthesized keeping the Pt:Ni ratio 3:1, and their catalytic activities were determined, as well as their selectivity towards the 4-electron process. [1] C. Coutanceau, S. Baranton, “Advanced Processes in Catalytic Oxidation - Oxygen Activation in Fuel Cells and Electrocatalytic Processes”, in “From Laboratory Studies to Industrial Applications", D. Duprez and F. Cavani (Eds.), Imperial College Press, London, 2014. [2] F. H. B. Lima, J. Zhang, M. H. Shao, K. Sasaki, M. B. Vukmirovic, E. A. Ticianelli, R. R. Adzic, J. Phys. Chem. C 2007, 111, 404-410. [3] M. T. Paffet, J. G. Beery, S. Gottesfeld, J. Electrochem. Soc. 135 (1988) 1431. [4] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 1999, 146, 3750. Acknowledgement: This work was made under the framework of the SMARTCat project funded by the European Commission under the FP7 Fuel Cells and Hydrogen Joint Technology Initiative grant agreement FP7-2012-JTI-FCH-325327. Figure 1