Changes in ORR Activity and Surface Structure of Pt/Pd/C Catalyst with Accelerated Durability Test

Introduction

The usage of the expensive Pt cathode catalyst in PEFCs must be reduced for their worldwide commercialization. Core-shell structured catalyst, in which Pt monolayer (Pt_ML) is formed on the other metal core surface, is one of the key technologies for the reduction of the Pt usage. Furthermore, it has been shown that the ORR activity of Pt_ML formed on Pd core NPs is enhanced [1, 2]. Conventionally, the Pt_ML has been formed on a core surface through Cu under potential deposition (Cu-UPD) followed by Pt replacement reaction. However, the Cu-UPD requires a precise potential control and weight of treated core on glassy carbon (GC) electrode is extremely small (several tens mg). Recently, we have developed a new method for the Pt shell formation which needs no precise potential control and is suitable for scale-up synthesis toward the core-shell structured catalyst (modified Cu-UPD/Pt replacement method [3]). We further demonstrated that the ORR mass activity of a carbon supported Pd core/Pt shell structured catalyst (Pt/Pd/C) synthesized by the modified Cu-UPD/Pt replacement method is drastically enhanced with an accelerated durability test (ADT) performed at 80°C [3]. TEM observation revealed that the mean diameter of the catalyst decreased and the morphology changed into a spherical shape after the ADT associated with oxidative dissolution of the Pd core. It was presumed that the ORR activity enhancement after the ADT was caused by a rearrangement of the surface Pt atoms and a decrease in Pt atoms with lower coordination numbers. In this study, we tried to correlate the change in ORR activity with the surface micro-structural change in the Pt shell using Bi³⁺cation as a probe, which specifically is adsorbed on the Pt (111) face [4].

Experimental

Pt/Pd/C catalyst was synthesized with the modified Cu-UPD/Pt replacement method [3]. Pd/C core and Pt/Pd/C catalyst were characterized with TG, XRF, XRD, TEM and CV. ORR activity of the catalyst was measured by RDE technique in O₂ saturated 0.1 M HClO₄ at 25°C. ADT was performed using a rectangular wave potential cycling (0.6 V (3 s)-1.0 V (3 s) vs. RHE) in Ar saturated 0.1 M HClO₄ at 80°C for 10,000 cycles. The Pt has a fcc crystallographic structure and the (111) face is energetically the most stable. Since Bi³⁺ is specifically adsorbed on the (111) face of Pt [4], we selected the cation as a probe for an evaluation of the (111) ratio on the surface. A GC electrode casted with the Pt/Pd/C catalyst was immersed into 0.05 M H₂SO₄ containing 0.5 mM Bi₂O₃ to be adsorbed. After washing the GC electrode with DI water, CV was recorded in Ar saturated 0.05 M H₂SO₄ at 25°C. The amount of the adsorbed Bi³⁺was converted into the surface Pt atomic ratio of the (111) face [4].

Results and Discussion

Figure 1 demonstrates the change in ORR specific activity of a commercial carbon supported Pt catalyst (Pt/C, TEC10E50E, TKK) and Pt/Pd/C catalyst before and after the ADT.  The specific activity of both catalysts increased after the ADT; however, the enhancement of Pt/Pd/C catalyst was much higher than that of Pt/C as reported previously [3]. Figure 2 depicts changes in CVs of Pt/C and Pt/Pd/C catalysts probed by Bi³⁺ before and after the ADT. Initially, the Pt/C catalyst showed Bi³⁺ adsorption wave at ca. 0.62 V vs. RHE (emphasized by dotted pink rectangular), which is assigned to adsorption on the Pt(111) terrace. However, the adsorption wave was not observed for the initial Pt/Pd/C catalyst. After the ADT, the Bi³⁺adsorption wave on the Pt/C catalyst decreased, while the wave appeared on the Pt/Pd/C catalyst.

Table 1 summarizes the ratio of the Pt(111) area to the total one for each catalyst. The ratio was calculated using the Bi³⁺ adsorption and H_UPDwaves [4]. It should be noted that the surface Pt(111) ratio on the Pt/Pd/C catalyst increased after the ADT. Although the decrease in the ratio for the Pt/C catalyst after the ADT has not been clarified, it is strongly suggested that the surface Pt rearrangement during the ADT influences the ORR activity of the Pt/C and Pt/Pd/C catalysts.

Acknowledgement

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

References

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

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

[3] H. Daimon et al., 224^thECS Meeting, Abstract #1404, San Francisco, USA (2013).

[4] F. J. Vidal-Iglesias et al., Phys. Chem. Chem. Phys., 10, 1395(2008).