Effect of Nanosheet Size on Activity and Durability of RuO 2 Nanosheet Pt/C Catalyst

Saida

Sugimoto

2014

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A series of composite electrocatalysts composed of RuO 2 nanosheets and carbon supported Pt (RuO 2 ns-Pt/C) has been synthesized by mixing different amounts of commercial 50 wt% Pt/C with RuO 2 ns derived from exfoliation of layered K 0.2 RuO 2.1 · nH 2 O. The oxygen reduction activity and stability of the different electrocatalysts has been evaluated as a function of the nanosheet content in the composite electrocatalysts. An increase in initial activity was observed for composite electrocatalyst with RuO 2 /Pt<0.4. After conducting accelerated durability tests, the RuO 2 ns-Pt/C composite electrocatalyst with RuO 2 /Pt = 0.3 exhibited a 25% higher mass activity toward oxygen reduction than the pristine Pt/C electrocatalyst. Carbon supported platinum and platinum-based alloy electrocatalysts are commonly used for the cathode catalyst in polymer electrolyte membrane fuel cells.1 Improved initial electrocatalytic activity and suppression of loss in electrocatalytic activity during fuel cell operation are major obstacles that must be overcome for wide-spread commercialization. It has been suggested that alloys such as Pt with Co, Ru, Fe, Cr or Ni lead to higher oxygen reduction reaction activity due to favorable Pt-Pt distance, electronic structure or Pt crystal orientation.2-12 Unfortunately, the promoter metal can often easily dissolve from the alloy leading to loss in catalytic activity 13 and in some cases damage the polymer electrolyte membrane.14 One way to overcome these drawbacks is to use electrochemically stable oxides as additives. Some metal oxides have been suggested to help oxygen dissociation, promote interaction with Pt, and improve the ionic mobility. 15,16 Metal oxide supported Pt electrocatalyst based on TiO 2 , SnO 2 , RuO 2 or TiO 2 -RuO 2 have also been developed. [17][18][19][20] While most of these composite electrocatalysts show improved durability compared to Pt/C, very few of these electrocatalyst show higher or even comparable initial activity. RuO 2 is one of the few oxides that is an exception, possessing good electronic conductivity and high electrochemical stability within the hydrogen and oxygen evolution region. 21,22 The addition of RuO 2 has been shown to increase the activity toward oxygen reduction by improving the wettability of the catalyst layer due to self-humidification. [23][24][25] The addition of RuO 2 has also been reported to catalyze the oxidation of water, therefore providing protection from other redox active species at high cathode potential. 26 An enhancement in the electronic structure through interaction between Pt and RuO 2 has also been suggested. 27 We have reported the use of RuO 2 nanosheets (RuO 2 ns) with thickness of ∼1 nm as an efficient co-catalyst.22,28 RuO 2 ns has been shown to enhance activity and durability of Pt/C as an anode 29,30 as well as cathode catalyst. 31 In an earlier study, it was shown that the durability of commercial Pt/C (20 mass% of Pt) with ultrasmall Pt (1-1.5 nm) could be improved without sacrificing initial activity. 32Smalle...

“…31 In an earlier study, it was shown that the durability of commercial Pt/C (20 mass% of Pt) with ultrasmall Pt (1-1.5 nm) could be improved without sacrificing initial activity.…”

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confidence: 99%

Influence of the RuO₂Nanosheet Content in RuO₂Nanosheet-Pt/C Composite Toward Improved Performance of Oxygen Reduction Electrocatalysts

Saida

Sugimoto

2014

Self Cite

“…We consider that the ORR activity shown by the reduced Pt nanoparticles on nanosheet contributes to the higher ORR activity retention rate in practical catalyst. As discussed in our previous papers, [10][11][12] RuO 2 nanosheet modified Pt/C electrocatalyst shows enhanced durability compared to non-modified Pt/C. Certainly, the degradation rate of carbon blacks used in practical catalysts and HOPG used in this model electrode study should be quite different due to the difference in degree of graphitization.…”

Section: Resultsmentioning

confidence: 79%

Model Electrode Studies of the Electrostatic Interaction between Electrochemically Dissolved Pt Ions and RuO₂Nanosheets

Sannegowda

et al. 2013

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Model electrodes consisting of ruthenium oxide nanosheets coated on freshly cleaved highly oriented pyrolytic graphite (RuO 2 nanosheet/HOPG) were prepared to investigate the electrostatic interactions between RuO 2 nanosheets and electrochemically dissolved Pt ions. The RuO 2 nanosheet/HOPG model electrode was dipped into a solution containing dissolved Pt ions generated by potential cycling a Pt working electrode in sulfuric acid electrolyte. Scanning tunneling microscopy revealed preferential adsorption of Pt ions on the nanosheets as island-like deposits, while no such deposits were observed on HOPG. This shows the strong electrostatic interactions between the positively-charged Pt ions and negatively-charged nanosheet. The calculated amount of Pt ions adsorbed was 0.93 × 10 6 atoms μm −2 , which agreed with the theoretical saturated adsorption amount of Pt ion on RuO 2 nanosheet of 0.96 × 10 6 atoms μm −2 . All of the Pt ions could be electrochemically reduced to Pt nanoparticles showing activity toward the oxygen reduction reaction. Platinum supported on carbon (Pt/C) is widely used as a cathode catalyst in polymer electrolyte fuel cells because of its high oxygen reduction reaction (ORR) activity. The loss of electrocatalytic activity during fuel cell operation is a detrimental factor to the useful lifetime of commercial polymer electrolyte fuel cell systems. Hence, there is a strong demand to improve the durability of electrocatalyst to realize the wide-spread commercialization of polymer electrolyte fuel cells. Numerous studies have clarified that dissolution, migration and/or sintering of platinum nanoparticles on carbon are vital degradation factors of the cathode catalyst. [1][2][3] Oxides that are stable under acidic and oxidizing conditions have been suggested to enhance the durability of Pt as cathode catalysts. For example, SnO 2 has been proposed as an alternative support to replace carbon to enhance the durability of Pt catalyst due to its resistance to corrosion. 4 TiO 2 added to Pt/C was suggested to anchor platinum particles, preventing agglomeration and coalescence during durability testing. 5,6 Carbon supported Pt covered with a thin layer of SiO 2 has been shown to exhibit high stability during potential cycling in H 2 SO 4 electrolyte. 7,8 The foundation of the increase in durability due to the addition of these oxides is not well understood. In addition, due to the poor conductivity of these oxides, the original properties of Pt/C are often inevitably obstructed, which includes the loss of initial electrochemical surface area (ECSA) and ORR activity with the addition of oxides.Contrary to most other oxide systems, RuO 2 nanostructures possess excellent electronic conductivity and electrochemical stability, making them an ideal additive that would not obstruct electrode kinetics. Indeed, we and others have found that the combination of RuO 2 nanostructures and Pt nanoparticles can enhance ORR properties. For example, incorporation of carbon-supported RuO 2 (RuO 2 /C) to Pt was found to a...

“…We have recently shown that the durability of commercial Pt supported on carbon black can be enhanced with the addition of RuO 2 nanosheet. 17,18 Model electrode studies have elucidated that there is a strong electrostatic interaction between RuO 2 nanosheets and dissolved Pt ions derived electrochemically.19 This strong electrostatic interaction between ionic Pt species in solution and the nanosheets is considered to decrease the loss of dissolved Pt species into electrolyte due to migration.In this study, we present experimental evidence of a strong metalsupport interaction between RuO 2 nanosheet and metallic Pt, which may be another reason for the enhanced durability of RuO 2 nanosheet modified Pt/C. A model electrode system was employed to evaluate the difference in the adsorption strength of metallic Pt on RuO 2 nanosheet and carbon surfaces.…”

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Evidence of Strong Metal-Support Interaction between Pt and Crystalline RuO₂Nanosheets by In-Situ AFM

Sugimoto

2014

Self Cite

The enhancement in durability of Pt nanoparticles modified by nanostructured RuO 2 was studied using a model electrode consisting of vacuum deposited Pt on single crystalline RuO 2 nanosheets coated on highly oriented pyrolytic graphite (HOPG) surface with sub-monolayer coverage. Atomic force microscopy images showed that Pt on HOPG aggregated and tended to form 3-dimensional islands. On the other hand, Pt formed a well-defined, 2-dimensional over-layer on the RuO 2 nanosheet surface. In-situ atomic force microscopy images showed that deposited Pt on the HOPG surface readily dissolved and easily migrated with potential cycling in sulfuric acid, while no such phenomena could be observed on the RuO 2 nanosheets. The results indicate that RuO 2 nanosheet has a strong affinity toward Pt, namely strong metal-support interaction for Pt, which can be considered as one of the reasons for the enhanced durability of Pt/C modified by RuO 2 10-12 as corrosion-resistant support for Pt nanoparticles have been proposed to minimize the loss of activity. As an additive or support, it is desirable that the oxide phase is electrically conductive and stable in acidic environment. RuO 2 with electronic conductivity comparable to or higher than most carbonaceous materials, and with high resistance to corrosion seems to be an ideal material. Since RuO 2 is a precious metal oxide, it is essential that the oxide phase is used in a nanostructured form so as to reduce the content in catalyst. In addition, density functional theory has predicted that Pt grows in a 2-dimensional fashion due to the strong adsorption strength on RuO 2 (110), suggesting that the presence of RuO 2 in the catalyst layer may enhance the activity for the oxygen reduction reaction. 12 We have suggested the use of a highly crystalline RuO 2 nanomaterial, namely RuO 2 nanosheet 13 as an additive to enhance the properties of Pt-based electrocatalyts.14-19 RuO 2 nanosheet is a 2-dimensional RuO 2 nanocrystal which is synthesized by chemical exfoliation of a layered ruthenic acid (H 0.2 RuO 2.1 · 0.9H 2 O). 13 The 2-dimensional nanosheet has high surface/bulk ratio due to the ultimately thin thickness of ∼1 nm and retains the original properties of the bulk oxide such as excellent electronic/protonic conductivity and electrochemical stability. We have recently shown that the durability of commercial Pt supported on carbon black can be enhanced with the addition of RuO 2 nanosheet. 17,18 Model electrode studies have elucidated that there is a strong electrostatic interaction between RuO 2 nanosheets and dissolved Pt ions derived electrochemically.19 This strong electrostatic interaction between ionic Pt species in solution and the nanosheets is considered to decrease the loss of dissolved Pt species into electrolyte due to migration.In this study, we present experimental evidence of a strong metalsupport interaction between RuO 2 nanosheet and metallic Pt, which may be another reason for the enhanced durability of RuO 2 nanosheet modified Pt/C. A model electrode system w...