Journal of Power Sources 158 (2006) 735–739 Short communication Oxygen reduction reaction on nanosized manganese oxide particles dispersed on carbon in alkaline solutions M.L. Calegaro ∗ , F.H.B. Lima, E.A. Ticianelli Instituto de Qu´ ımica de S ˜ ao Carlos, Universidade de S ˜ ao Paulo, Avenida Trabalhador Sancarlense, 400 Parque Arnold Schmidt, CP 780 13560-970 S˜ ao Carlos, SP, Brazil Received 23 June 2005; received in revised form 18 August 2005; accepted 31 August 2005 Available online 16 November 2005 Abstract In this work a manganese oxide dispersed on a high surface area carbon powder at two ratios (Mn y O x /C) was evaluated as catalyst for the oxygen reduction reaction (ORR) in alkaline solutions. An impregnation method was used to prepare the catalysts, starting from a manganese nitrate precursor solution containing the carbon powder (Vulcan XC-72). A thin porous coating rotating disk electrode was employed to collect the experimental ORR polarization data and the results obtained were analyzed with the application of the flooded-agglomerate/thin film model for the catalyst layer. Results indicated that the reaction is sensitive to the manganese oxide to carbon ratio on the catalyst. The catalyst containing lower Mn y O x /C ratio tended to follow the peroxide pathway (2e - mechanism) forming peroxide ions. When the Mn y O x content is increased the complete reduction of oxygen to hydroxide ions pathway (the 4e - mechanism) takes place into some extent at low potentials due to the occurrence of a catalytic disproportionation of HO 2 - . © 2005 Elsevier B.V. All rights reserved. Keywords: ORR; Alkaline solutions; Flooded-agglomerate/thin film model; Manganese oxide catalysts 1. Introduction One of the most studied processes in electrochemistry is the cathodic reduction of molecular oxygen, either in alkaline or in acid solutions, due to the importance of this reaction for fuel cells [1] and metal-air batteries [2]. The most effective catalyst used to promote the oxygen reduction reaction (ORR) kinetics is a material formed by platinum-based particles highly dispersed on a high surface area substrate (normally a carbon powder). However, Pt-based catalysts are very expensive, and additionally suffer gradual decrease in activity. These aspects have moti- vated the development of alternative materials that have been successfully employed on the ORR, particularly in the construc- tion of cathodes for alkaline fuel cells and metal-air batteries [3,4]. The main challenge related to the development of high active catalysts for the ORR is to obtain a material capable of achieving the complete reduction process, where a four-electron transfer ∗ Corresponding author. Fax: +55 16 3373 9952. E-mail address: calegaro@iqsc.usp.br (M.L. Calegaro). per O 2 is involved. However, some of the catalysts reported in the literature accelerate a two-electron reduction of O 2 to produce H 2 O 2 . Manganese oxide is among the most widely used catalyst for the ORR in alkaline metal-air batteries [5–8]. One of the functions of this oxide is to perform the decomposition of hydrogen peroxide, which is formed during the electrochemical reduction of oxygen [8], by a disproportionation mechanism conducting the ORR to follow the complete reduction pathway, where a 4e - transfer per oxygen molecule is obtained. The objective of the present work is to further demon- strate the efficiency of a manganese oxide as catalyst for the ORR in alkaline solutions (1.0 mol L -1 NaOH). In order to obtain high specific area, the catalyst was obtained in the form of a highly dispersed powder supported on carbon fol- lowing an impregnation method as suggested by Lamminen et al. [9]. The method involves the use of a nitrate precursor (Mn(NO 3 ) 2 ·4H 2 O), which is thermally decomposed to produce very small metal oxide particles. Since the resulting material is a non-stoichiometric metal oxide (see details in Section 2) it will be represented as Mn y O x /C. This catalyst was characterized by XRD (X-ray diffraction) measurements, while cyclic voltam- metry and steady-state polarization techniques were employed 0378-7753/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2005.08.048