Enhancement in ORR Activity of Pt/Pd/C Catalyst by Removal of Small Size Pd Core Particles

Introduction

PEFCs are attractive energy source because their total well-to-wheel efficiency is higher than that of internal combustion engine [1]. However, the usage of Pt cathode catalyst must be reduced for widespread commercialization. Core-shell structured catalyst is a strong candidate for the reduction, and it has been reported that the ORR activity of a Pt monolayer (Pt_ML) formed on Pd single crystal and on Pd NPs core can be enhanced [2, 3]. So far, we have prepared carbon supported Pd core (Pd/C) by an impregnation and the resulting Pd NPs have a wide size distribution. It was found that the ORR activity of Pd core-Pt shell structured catalyst (Pt/Pd/C) was declined when the catalyst was synthesized using Pd/C core with a smaller mean diameter [4]. In this study, we developed a method removing small Pd NPs to improve the ORR activity of Pt/Pd/C catalyst. The method uses equilibrium potentials of Cu²⁺/Cu and of O₂/H₂O (Cu-air treatment), which mimics potential cycling using a potentiostat. The ORR activity of the Pt/Pd/C catalysts synthesized using Pd/C core after the Cu-air treatment was investigated.

Experimental

A Pd precursor was impregnated on a carbon support and thermally reduced under N₂ atmosphere. 300 mg of the Pd/C core (Pd mean diameter: 3.8 nm, 34 wt.%) was dispersed in 0.1 M H₂SO₄ containing 10 mM CuSO₄ and stirred at 30°C for 5 h with and without coexistence of a metallic Cu sheet under air bubbling (Cu-air and air treatments, respectively). The treated Pd/C core was re-dispersed in 0.05 M H₂SO₄ containing 10 mM CuSO₄ and stirred for 5 h at 5°C with coexistence of a Cu sheet under Ar bubbling to form Cu shell on the Pd core surface (Modified Cu-UPD method [5]). Then, the Cu mesh was removed and K₂PtCl₄ was added at 5°C to replace Cu shell with Pt, obtaining Pt/Pd/C catalyst. The Pt/Pd/C catalyst was characterized with TG, XRD, XRF, TEM and CV. ORR activity of the catalyst was evaluated with the RDE technique in O₂ saturated 0.1M HClO₄ at 25°C. Accelerated durability tests (ADTs) were performed using rectangular potential cycling (0.6 V 1.0 V vs. RHE) in Ar saturated 0.1 M HClO₄at 80°C for 10,000 cycles.

Results and Discussion

Figure 1 shows TEM images of Pd/C cores. The mean diameter of the Pd/C core after the Cu-air treatment increased slightly (3.8 → 4.2 nm). On the contrary, the mean diameter after the air treatment showed little change. Table 1 summarizes the mean diameters and Pd loadings of the Pd/C cores. The Pd loading after the Cu-air treatment decreased by 30 % from that of the pristine Pd/C core, while the loading decreased by only 7 % after the air treatment. These results suggest that the smaller Pd core NPs were removed by the Cu-air treatment. Figure 2 depicts CV of the original Pd/C core. The onset potential for Pd oxide formation was ca. 0.75 V vs. RHE and the oxide was reduced at ca. 0.50 V vs. RHE. During the Cu-air treatment, the Cu²⁺/Cu equilibrium potential (ca. 0.3 V) is applied to the Pd/C core when the core contacts with the metallic Cu sheet and the potential for O₂/H₂O (ca. 1.0 V) is applied when the core is in the solution. On the contrary, only ca. 1.0 V is applied to the core in the air treatment. The increase in mean diameter and the decrease in Pd loading of the Pd/C core after the Cu-air treatment are considered to be caused by repeated potential cycling across the redox potentials of Pd.

ORR mass activity of Pt/Pd/C catalysts is summarized in Fig. 3. There are little differences in the initial ORR mass activities among the Pt/Pd/C catalysts, whereas the Pt/Pd/C catalysts synthesized using the Pd/C core after the Cu-air treatment exhibited higher ORR activity after the ADT at 80°C. These results imply that the removal of the small size Pd NPs by the Cu-air treatment is an effective method to enhance the ORR activity of Pt/Pd/C catalyst, especially the activity after the ADT at 80°C.

Acknowledgement

This work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.

References

[1] B. G. Pollet et al., Electrochim. Acta, 84, 235 (2012).

[2] J. Zhang et al., J. Phys. Chem. B, 108, 10955 (2004).

[3] J. Zhang et al., Angew. Chem. Int. Ed., 44, 2132 (2005).

[4] N. Aoki et al., 54^thBattery Symposium, Abstract 2H18, Osaka, Japan (2013).

[5] Y. Ikehata et al., 224^thECS Meeting, Abstract #1497,

San Francisco, USA (2013)