Effects of iron-phosphate coating on Ru dissolution in the PtRu thin-film electrodes
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The effects of FePO4 nanoscale coating on PtRu thin films were investigated on the block of Ru crossover. Ru dissolution was examined by the accelerated-potential cycles between 0.4 and 1.05 V. The results showed that Ru dissolution from FePO4-coated PtRu surface was inevitable due to the direct contact between the PtRu surface and aqueous electrolyte. However, the FePO4 coating layer on PtRu thin-film electrodes effectively retained the dissolved Ru species, thus preventing the dissolved Ru species from diffusing into the electrolyte. Moreover, the retained Ru species within the FePO4-coating layer were redeposited onto the PtRu surface during the cycling in the fresh electrolyte.
I. INTRODUCTION
The direct-methanol fuel cell (DMFC) is a promising alternative to conventional batteries for powering portable electronic devices.1–3 In general, the platinumruthenium (PtRu) alloy is currently considered 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 long-term operation due to the dissolution of noble metal-based catalysts and the aggregation of catalyst nanoparticles.8 Although metallic Ru is stable in the DMFC anode operating potential range, it is dissolved abnormally in practical operations. In particular, Piela et al. reported that ruthenium in the DMFC anode crossed from the PtRu anode, through the proton exchange membrane (Nafion), to the Pt cathode.9 The Ru crossover originating from Ru dissolution can deteriorate the performances of PtRu anode, membrane, and Pt cathode. Therefore, it is important to block the Ru crossover. Our group has recently studied the dissolution mechanisms of Ru from the bare PtRu surface.10 It has been reported that the electrochemical stabilities were enhanced by metal-phosphate coating on LiCoO2.11–13 One of the main reasons for the enhanced stability was the diminished Co dissolution through the prevention of direct contact between the electrode and carbonate-based electrolyte by the nanoscale metalphosphate coating. Moreover, nanoporous structures of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0013 140
J. Mater. Res., Vol. 24, No. 1, Jan 2009
amorphous (or poor crystalline) FePO4 allowed effective transfer of proton, water, and methanol to catalyst surface.3 In this study, a thin-film geometry was used to focus on the intrinsic effects of an iron-phosphate coating for the potential block of Ru crossover, instead of carbon–PtRu–FePO4 nanocomposite powders.
II. EXPERIMENTAL SECTION
The PtRu thin-film alloy was deposited on indium-tin oxide (ITO) coated glass (Samsung Corning, Cheonan, Korea) by radio frequency (rf) magnetron sputtering using Pt and Ru targets.3 The FePO4 thin films were sputter-coated on the PtRu thin-film alloy using a FePO4 target. The effect of the FePO4 coating was examined by varying the thickness of FePO4 from 5 to 20 nm. The FePO4-coated PtRu thin-fil
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