Carbon supported Pt–Cr alloys as oxygen-reduction catalysts for direct methanol fuel cells E. ANTOLINI 1 , J.R.C. SALGADO 1 , L.G.R.A. SANTOS 1 , G. GARCIA 1 , E.A. TICIANELLI 1 , E. PASTOR 2 and E.R. GONZALEZ 1, * 1 Instituto de Quı´mica de Sa ˜o Carlos, USP, C.P. 780 Sa ˜o Carlos, SP 13560-970, Brazil 2 Departamento de Quı´mica Fı´sica, Universidad de La Laguna, 38071, La Laguna, Espan ˜a (*author for correspondence +55-0-16-3373-9952, e-mail: ernesto@iqsc.usp.br) Received 27 July 2005; accepted in revised form 13 September 2005 Key words: electrocatalysis, methanol tolerance, oxygen reduction, PEM fuel cell, Pt–Cr alloy Abstract Electrocatalytic activities of various carbon-supported platinum–chromium alloy electrocatalysts towards oxygen reduction in 1 mol l )1 H 2 SO 4 and in 1 mol l )1 H 2 SO 4 /1–3 mol l )1 CH 3 OH, were investigated by means of rotating disc electrode experiments and in solid polymer electrolyte direct methanol fuel cells. The activity of these elect- rocatalysts for methanol oxidation was evaluated using cyclic voltammetry. It was found that Pt 9 Cr/C prepared by reduction with NaBH 4 exhibits the lowest activity for methanol oxidation and the highest activity for oxygen reduction in the presence of methanol, in comparison to commercial Pt/C, Pt 3 Cr/C and PtCr/C electrocatalysts. 1. Introduction Direct methanol fuel cells (DMFCs) are promising electrochemical energy converters for a variety of appli- cations because of the system simplicity. The liquid-feed system does not require any fuel processing equipment and can be operated even at room temperatures. Another advantage of the DMFC is the fact that it does not require complex humidification and heat management modules, as in the hydrogen fed proton exchange membrane (PEM) fuel cell system, because the dilute methanol water mixtures entering the DMFC provide the necessary humidification and heat control. These advantages allow the DMFC to be customized for use in portable electronic devices [1]. The crossover of methanol from the anode to the Pt- based oxygen cathode is one of the major practical problems limiting the performance of the direct meth- anol fuel cell [2–6]. The mixed potential, which results from the oxygen reduction reaction and the methanol oxidation occurring simultaneously, reduces the cell voltage, generates additional water and increases the required oxygen stoichiometric ratio. To overcome this problem, a common action is to test the activity for the oxygen reduction reaction (ORR) in the presence of methanol of some platinum alloys with first-row transition metals which present a higher activity for the ORR than pure platinum in low temperature fuel cells operated on hydrogen, and use them as DMFC cathode electrocatalysts [7–12]. The improvement in the ORR electrocatalysis has been ascribed to different factors such as changes in the Pt–Pt interatomic distance [13], the surface area [14] and, particularly, the Pt electronic configuration [15]. Toda et al. [15] proposed a new mechanism for the enhancement of the ORR on Pt–M alloys, based on an increase of d-electron vacan- cies of the thin Pt surface layer caused by the underlying alloy. On the basis of their model, such an increase of 5d vacancies leads to an increased 2p electron donation from the oxygen to the surface Pt atoms, resulting in an increased O 2 adsorption and a weakening of the O–O bond, enhancing in this way the kinetics of the ORR. In a work with X-ray absorption spectroscopy (XAS), Mukerjee et al. [16] explained the enhanced electrocatal- ysis of Pt-based alloys on the basis of the interplay between the electronic (Pt d-vacancy) and geometric factors (Pt coordination number) and their effect on the chemisorption behaviour of OH species from the electrolyte. The ensemble effects where the dilution of the active component with the catalytically inert metal changes the distribution of active sites, open different reaction pathways [17], and can explain the enhanced metha- nol-resistance of the Pt-based alloys. The dissociative chemisorption of methanol requires the existence of several adjacent Pt ensembles [18, 19]. Thus the presence of atoms of the second metal around Pt active sites could block methanol adsorption on Pt sites due to a dilution effect. Consequently, methanol oxidation on the binary-component electrocatalyst is reduced. On the Journal of Applied Electrochemistry (2006) 36:355–362 Ó Springer 2005 DOI 10.1007/s10800-005-9072-0