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

Abstract

The activation of oxygen in fuel cells and generally in electrocatalytic processes is of paramount importance to make such technologies marketable. For this reason it has been extensively studied for the last fifty years [1]. It is known that platinum-based binary alloys are excellent catalysts for the ORR (oxygen reduction reaction), which occurs at the cathode of PEMFCs (proton exchange membrane fuel cells). Beyond the lowering of the platinum loading in the electrode catalytic layers, alloyed Pt-based materials lead to the improvement of the catalytic activity [2], to the increase of the tolerance towards poisoning species and, for some authors, to durability enhancement of the catalysts under fuel cell working conditions [3]. PtxM1-x bimetallic materials (M = Co, Fe, Ni and Cr) have been extensively studied in order to correlate their catalytic activity and their electrochemical stability towards the ORR to their composition, crystallographic and electronic structures. Recent works have clearly shown that commercial Pt3Co/C nanoparticles were not totally stable during PEMFC operation even if they were more stable than monometallic Pt/C catalysts, they suffered compositional changes at the nanoscale.Other noble metals were also considered for the activation of the ORR in acidic medium, such as palladium and gold. Amongst pure metals, palladium leads to the highest activity towards the ORR, just after platinum. A review paper of Antolini [4] cited numerous publications explaining that palladium-based alloys or core-shell structures could lead to activities towards the ORR higher than those observed on pure palladium, and almost comparable to those recorded on pure platinum. Pd-Co and Pd-Fe are the most cited metallic alloys because they are the most active, in agreement with DFT (Density Functional Theory) calculations. Recently, PtPd alloys with very low Pt atomic ratio led to remarkable fuel cell electric performances [5]. Bimetallic PtxAu1-x catalysts have also led to excellent stability and activity under PEMFC working conditions, making this kind of materials a serious alternative candidate to platinum as cathode electrocatalysts in a PEMFC.In comparison, few studies have been carried out on the behavior of trimetallic catalysts towards the ORR. PtFeNi nanoparticles showed excellent electrocatalytic performances under PEMFC working conditions [6]. The research group of Manthiram studied Palladium-based trimetallic catalysts. They obtained activities towards the ORR comparable or even slightly higher to that of a Pt/C catalyst using Pd–Co–Mo/C and Pd–Co–Au/C materials. Although a few studies on trimetallic catalyst activity and stability is available in the literature, several trends could be drawn : (i) alloying platinum with a less noble metal seems to be beneficial for the ORR in terms of activity and tolerance, (ii) the interactions between two different noble metals such as Pt-Pd, Pt-Au and Au-Pd seems to enhance the durability and the catalytic activity towards the ORR with respect to pure metals and (iii) platinum seems unavoidable in order to achieve the best possible activity from the catalyst in an acidic environment. So in the present work, the synthesis and characterization of carbon supported Pt, Pd and Au based binary and ternary catalysts (PtxPdyAuz/C) are described as well as their systematic study (activity and selectivity) towards the ORR. The aim of this work is to allow defining a relevant catalyst formulation (composition and atomic ratio) for enhanced ORR performance. Acknowledgement: The financial support of the European Commission under the FP7 Fuel Cells and Hydrogen Joint Technology Initiative grant agreement FP7-2012-JTI-FCH-325327 for the SMARTCat project is gratefully acknowledged.[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] M. T. Paffet, J. G. Beery, S. Gottesfeld, Oxygen reduction at Pt0.65Cr0.35, Pt0.2Cr0.8 and roughened platinum J. Electrochem. Soc. 135 (1988) 1431-1436.[3] X.Yu, S.Ye. Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC: Part II: Degradation mechanism and durability enhancement of carbon supported platinum catalyst, J. Power Sources 172 (2007) 145-154.[4] E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915-931.[5] M. Mougenot, A. Caillard, P. Brault, S. Baranton, C. Coutanceau, High Performance Plasma Sputtered PdPt Fuel Cell Electrodes with Ultra Low Loading, Int. J. hydrogen Energy 36 (2011) 5429-8434.[6] B. Fang, B. N. Wanjala, J. Yin, R. Loukrakpam, J. Luo, X. Hu, Jo. Last, C.-J. Zhong, Electrocatalytic performance of Pt-based trimetallic alloy nanoparticle catalysts in proton exchange membrane fuel cells, Int. J. Hydrogen Energy 37 (2011) 4627–4632