RuO2 Nanosheet as a Durability Enhancing Additive to Pt/C Catalyst: Model Electrode Studies

The loss of the electrochemically active Pt surface area due to the dissolution and coalescence of Pt nanoparticles during longtime operation is one of the major obstacles for wide-spread commercialization of polymer electrolyte fuel cells. A number of nanostructured oxides have been proposed as an additive to Pt/C catalyst to minimize the loss of activity. As an additive, it is desirable that the oxide phase is electrically conductive and stable in acidic environment. We have suggested the use of RuO₂ nanosheet (RuO₂ns) as an additive to enhance the properties of Pt-based electrocatalysts [1-3] RuO₂ns is a 2-dimensional RuO₂ nanocrystal which is synthesized by chemical exfoliation of a layered ruthenic acid (H_0.2RuO_2.1· 0.9H₂O) [4]. Here we report model electrode studies were performed to explore and clarify the mechanism of enhanced durability of Pt/C modified by RuO₂ns [5,6]. The electrostatic interaction between RuO₂ns and dissolved Pt ions and the strong metal-support interaction (SMSI) between nanosheets and metallic Pt are identified by model electrode studies.

   The electrostatic interaction between Pt ions and RuO₂ns was investigated via model electrode studies by dipping RuO₂ns/HOPG model electrodes into a solution containing electrochemically dissolved Pt ions in sulfuric acid. Spontaneous adsorption of hydrated Pt ions on RuO₂ns were detected by STM, while no adsorption was observed on the HOPG substrate. The amount of hydrated Pt ions adsorbed on RuO₂ns estimated from the STM images and showed a close match to the estimated saturated adsorption amount of cationic Pt ions on the surface of RuO₂ns assuming purely electrostatic interaction. After electrochemical reduction, AFM image showed the presence of Pt nanoparticles on the RuO₂ns surface. The reduced Pt nanoparticles showed ORR activity towards oxygen reduction reaction. The electrostatic interaction between Pt ions and RuO₂ns should facilitate trapping and re-deposition of the Pt ions on RuO₂ns, thereby addition of nanosheet should impede loss of Pt during potential cycling in practical supported catalysts. Similar electrostatic interaction between TiO₂ nanosheets (TiO₂ns) and dissolved Pt ions was also observed, thus TiO₂ns may be an alternative material to substitute the precious Ru oxide.

   The strong metal-support interaction (SMSI) between RuO₂ns and metallic Pt was investigated by using a model electrode consisting of vacuum deposited Pt on RuO₂ns coated on HOPG with sub-monolayer coverage. AFM images showed that Pt formed a well-defined, 2D over-layer on the RuO₂ns surface, whereas Pt aggregated and formed 3D islands on HOPG. In-situ EC-AFM images showed that deposited Pt on the HOPG surface readily dissolved and migrated with potential cycling, while no such phenomena could be observed on RuO₂ns. The driving force for this different behavior is considered to be related to the stronger adsorption of Pt on RuO₂ns compared to carbon, which may explain the enhanced durability of practical Pt/C catalysts modified with RuO₂ns. Similar SMSI behavior was observed for TiO₂ns, although the interaction between Pt and TiO₂ns was weaker than RuO₂ns.

This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan and a grant in-Aid for Excellent Graduate Schools, of the MEXT, Japan.

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[6] Q. Liu, K. S. Lokesh, C. Chauvin, W. Sugimoto, J. Electrochem. Soc.,161, F259 (2014).