Enhanced CO Tolerance and Durability of PtRu/C By the Modification with Metallic Ruthenium Nanosheets

PtRu/C is presently used as the anode catalyst for residential polymer electrolyte fuel cells. Anode catalysts with increased CO tolerance are desired since trace amounts of CO residing in reformates poisons the catalysts. In addition, the loss of performance of the catalyst due to gradual dissolution of Ru is known to occur due to start/stop cycles. Thus efforts have been made to develop CO tolerant electrocatalysts with high durability. In an earlier study, the authors showed that the addition of oxide nanosheets to carbon supported PtRu/C catalysts can enhance the CO tolerance and durability ¹⁾. Here we have studied catalyst activity and durability of PtRu/C catalysts modified with Ru metal nanosheets towards the hydrogen oxidation reaction in the presence of CO.

RuO₂ nanosheet was derived via exfoliation of layered H_0.2RuO_2.1×nH₂O through a process reported previously ^2,3). The RuO₂ nanosheet modified PtRu/C catalyst (RuO₂ns-PtRu/C) was synthesized following our previous recipe ¹⁾. Heating the RuO₂ns-PtRu/C under gas-flow-controlled condition (10% H₂ + 90% N₂, 250 ml min^-1) at 150°C at a rate of 5^oC min^-1 for 2 hours yielded the modified catalyst with Ru metal nanosheets (Ru metal ns-PtRu/C). The catalyst morphology was characterized by the high-resolution scanning electron microscopy (HR-SEM; Hitachi, S-5000, accelerated voltage of 20 kV) and the transmission electron microscopy (TEM; JEOL, JEM-2010, accelerated voltage of 200 kV). Electrochemical measurements were conducted with a rotating disk electrode (Nikko Keisoku). The working electrode was prepared by depositing 5.5 mg-carbon cm^‒2 of the catalyst ink on a mirror-polished glassy carbon rod (6 mm in diameter) and vacuum dried at 60°C for 30 min. A carbon fiber (Toho Tenax Co., HTA-3K) was used as a counter electrode, and a reversible hydrogen electrode (RHE) was used as a reference electrode. Catalytic activity was measured by the chronoamperometry at 20 mV vs. RHE in 0.1 M HClO₄ (25^oC) with 300 ppm CO/H₂. Accelerated durability test (ADT) in 300 ppm CO/H₂was conducted by 3000 square-wave potential stepping between 0.05 and 0.4 V vs. RHE with a holding time of 3 sec at each potential.

The Ru metal nanosheets remain a lateral size ranging from submicrometer to micrometer of the RuO₂ nanosheets. The binding energy of the Ru 3p peak for Ru metal ns-PtRu/C is lower compared to PtRu/C and RuO₂ns-PtRu/C, indicating that the RuO₂ nanosheets successively metallized. The metallic Ru nanosheets have poor hydrogen oxidation reaction (HOR) activity. The HOR current at 0min of Ru metal ns-PtRu/C was comparable to Pt₂Ru₃/C, indicating that the metallic Ru nanosheets do not inhibit the H₂ diffusion. HOR current of all catalyst gradually decreases due to adsorption of CO on the catalyst surface. The HOR activity in CO/H₂ at 20 mV vs. RHE (quasi-steady state current after 60 min) was PtRu/C ≈ Pt₂Ru₃/C << Ru metal ns-PtRu/C. This indicates that the Ru metal ns-PtRu/C is hardly poisoned by CO. The durability of Ru metal ns-PtRu/C after ADT of 3000 potential steps was higher compared to Pt₂Ru₃/C, PtRu/C and RuO₂ns-PtRu/C. The surface Ru composition of PtRu/C and Ru metal ns-PtRu/C was not change by the durability test, suggesting that the degradation of catalytic activity for PtRu/C and Ru metal ns-PtRu/C is not due to dissolution of Ru atoms. Enhanced of the durability of PtRu/C is attributed to improved CO tolerance by the modification with Ru metal nanosheets.

This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

1. D. Takimoto, T. Ohnishi, and W. Sugimoto, ECS Electrochem. Lett., 4, F35 (2015).

2. W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003).

3. K. Fukuda, H. Kato, J. Sato, W. Sugimoto, and Y. Takasu, J. Solid State Chem., 182, 2997 (2009).