Available online at www.sciencedirect.com Journal of Power Sources 179 (2008) 42–49 Effect of the catalyst composition in the Pt x (Ru–Ir) 1-x /C system on the electro-oxidation of methanol in acid media K.I.B. Eguiluz, G.R. Salazar-Banda, D. Miwa, S.A.S. Machado, L.A. Avaca ∗ Instituto de Qu´ ımica de S ˜ ao Carlos, Universidade de S ˜ ao Paulo, C.P. 780, 13560-970 S˜ ao Carlos, SP, Brazil Received 23 October 2007; received in revised form 18 December 2007; accepted 19 December 2007 Available online 27 December 2007 Abstract The effect of variations in the composition for ternary catalysts of the type Pt x (Ru–Ir) 1-x /C on the methanol oxidation reaction in acid media for x values of 0.25, 0.50 and 0.75 is reported. The catalysts were prepared by the sol–gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectroscopy (AAS) and energy dispersive X-ray (EDX) analyses. The nanometric character (2.8–3.2 nm) of the sol–gel deposits was demonstrated by XRD and TEM while EDX and AAS analyses showed that the metallic ratio in the compounds was very near to the expected one. Cyclic voltammograms for methanol oxidation revealed that the reaction onset occur at less positive potentials in all the ternary catalysts tested here when compared to a Pt 0.75 –Ru 0.25 /C (E-Tek) commercial composite. Steady-state polarization experiments (Tafel plots) showed that the Pt 0.25 (Ru–Ir) 0.75 /C catalyst is the more active one for methanol oxidation as revealed by the shift of the reaction onset towards lower potentials. In addition, constant potential electrolyses suggest that the addition of Ru and Ir to Pt decreases the poisoning effect of the strongly adsorbed species generated during methanol oxidation. Consequently, the Pt 0.25 (Ru–Ir) 0.75 /C composite catalyst is a very promising one for practical applications. © 2007 Elsevier B.V. All rights reserved. Keywords: Methanol oxidation; Direct methanol fuel cells; Electrocatalysis; Sol–gel 1. Introduction The need for more efficient energy conversion systems is a strong reality for two important reasons, namely, the future shortage of fossil fuel sources as well as the urgency in reduc- ing the contamination levels produced by the use of those fuels in urban centers. In this sense, fuel cells are very promising energy sources due to the high efficiency of the electrochem- ical combustion in comparison with the chemical combustion thus minimizing the formation of by-products that pollute our planet. Among the different systems under investigation, the use of methanol as the fuel has been the subject of numerous stud- ies since considerable advances have been achieved using that material [1–9]. Although the use of methanol as a fuel is attractive in terms of its theoretical energy density (6.09 kWh kg -1 ) and theoret- ical efficiency (96.7%), high overpotentials at both the anode ∗ Corresponding author. Tel.: +55 16 3373 9943; fax: +55 16 3374 2565. E-mail address: avaca@iqsc.usp.br (L.A. Avaca). and the cathode reduce the cell potential to 0.51 V and the over- all efficiency to ∼41%. Moreover, the formation of CO poisons the Pt-based anode and further reduces the overall efficiency to ∼27%, thereby making CO poisoning one of the major limita- tions for the technological development of direct methanol fuel cells (DMFCs) [10]. Among the precious metals, Pt shows the highest activ- ity for the electro-oxidation of methanol but the performance of pure Pt electrodes is not very satisfactory due to the for- mation of strongly adsorbed intermediates. Efforts to reduce the amount of adsorbed CO are centered on the use of co- catalysts and, to date, the addition of ruthenium into the platinum catalyst has yielded the best reported results [11–13]. When binary catalysts are used in DMFCs, the beneficial effect of the second metal M in Pt–M is attributed to a bi- functional mechanism originally proposed by Watanabe and Motoo [2]. There, Pt sites serve to adsorb and dehydrogenate the methanol molecules while M provides nucleation sites for OH ads formation, but the Pt sites become blocked by adsorbed CO making the overall reaction fairly slow. Then, the reaction between CO ads(Pt) and OH ads(M) accelerates again 0378-7753/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2007.12.070