The effects of ruthenium-oxidation states on Ru dissolution in PtRu thin-film electrodes
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The effects of ruthenium (Ru)-oxidation states were investigated on Ru dissolution from PtRu thin-film electrodes, with the 200 cycles between 0.4 and 1.05 V (versus normal hydrogen electrode). The Ru-oxidation states of the PtRu thin films were systematically modified by an anodic (oxidation) treatment. The anodic-treated PtRu electrodes, whose methanol-oxidation activity was similar to untreated electrodes before the 200 cycles, showed a remarkable decrease in methanol oxidation after the cycles, because of the Ru dissolution from the PtRu surface. The results suggest that the Ru-oxide species are the origin of Ru dissolution in the PtRu alloy. I. INTRODUCTION
Direct-methanol fuel cells (DMFCs) have attracted considerable attention as power sources for portable electronic devices because of their high energy density, easy handling of liquid, low operating temperatures, etc.1–3 In general, the platinum-ruthenium (PtRu) alloy is currently considered to be the most promising DMFC anode catalyst for methanol oxidation.4–7 Recently, the stability of electrocatalysts has received a great deal of attention, because the electrode performance can degrade after the long-term operation of fuel cells, due to the dissolution of the noble metal-based catalysts and the aggregation of catalyst nanoparticles.8 In particular, Piela et al.9 reported on DMFCs that Ru from the PtRu anode can cross through the proton exchange membrane (Nafion) and settle on the Pt cathode. This Ru crossover, which originates in Ru dissolution from the PtRu anode, can degrade the performance of all the major components of the fuel cell. It has been reported that the anode potential of a DMFC can vary from 0.4 to 1.0 V [versus normal hydrogen electrode (NHE)] during on-off operations, and the Ru dissolution is caused by cycling between the reduced and oxidized states of Ru [thermodynamically, metallic Ru oxidizing to Ru(OH)3, and RuO22H2O, respectively, at 0.74 and 0.94 V versus NHE].10 Accordingly, our group has recently investigated the effects of a metal-phosphate coating layer on the block of Ru crossover.11 However, the origin of Ru dissolution from the PtRu alloy is not completely elucidated. Therefore, it is important to identify the mechanisms of Ru dissolution from PtRu-alloy anodes. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0331
2762
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 19 Mar 2015
The oxophilic Ru metals as pair sites of Pt metals allow the water dissociation for the promotion of methanol activation, and the adsorption of OH species on the Ru sites facilitates the oxidation of CO on adjacent Pt sites.12 The oxophilic properties of Ru also allow the easy formation of oxide species, and Ru oxides are present on the PtRu alloy surface at a significant fraction.13 Enhanced proton transport in the hydrous Ru oxides (RuO2H2O or RuOxHy) in Pt–RuO2 has been reported.14 Recently, it was also reported that the mixture of Pt metal and hydrous Ru
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