Change in ORR Activity of PtPd/C Alloy and Pt/Pd/C Core-Shell Catalysts with Accelerated Durability Test

Introduction

Core-shell structured catalyst is a promising candidate for the decrease in usage amount of an expensive Pt cathode catalyst for PEFCs. Adzic and his co-workers demonstrated that carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) exhibits higher ORR performance relative to that of a commercial Pt/C catalyst [1] and that the Pd core dissolves out during an accelerated durability test (ADT) using potential cycling [2]. We have recently reported that ORR activity of the Pt/Pd/C catalyst synthesized with a modified Cu under potential deposition (Cu-UPD) and a Pt replacement processes is enhanced with the ADT performed at 80°C [3, 4]. After the ADT, morphology of the Pt/Pd/C catalyst changed into spherical shape and mean diameter of the catalyst decreased associated with an oxidative dissolution of the Pd core. It was presumed that the enhancement of the ORR activity was caused from decrease in number of low-coordinated Pt atoms by rearrangement of the surface Pt atoms with the ADT. Interestingly, the core-shell structure was retained even after the ADT. In this study, carbon supported PtPd alloy catalyst (PtPd/C) was prepared through one-pot synthesis and change in the ORR activity of the catalyst by the ADT at 80°C was investigated in comparison with that of the Pt/Pd/C catalyst.

Experimental

Pt/Pd/C catalyst was synthesized with modified Cu-UPD and Pt replacement processes [3]. Carbon supported Pd core (Pd/C, 4.2 nm, 30 wt.%) was dispersed 0.05 M H₂SO₄ containing 0.01 M CuSO₄ and stirred under Ar atmosphere with co-existence of Cu mesh, by which Cu shell was formed on the Pd core surface. Then, the Cu mesh was removed and K₂PtCl₄ was added to replace the Cu shell with the Pt one, giving the Pt/Pd/C catalyst. PtPd/C alloy catalyst was prepared with one-pot reduction method. H₂PtCl₆ and Na₂PdCl₄ were dissolved into DI water and pH of the solution was adjusted at 3.5 by NaOH, followed by addition of a carbon support (Ketjen black EC 300J). The solution was stirred at 60°C in air and NaBH₄ was added in drop-wise, forming the PtPd/C alloy catalyst [5].

Catalysts were characterized with TG, XRD, XRF, TEM, TEM-EDX and CV. ORR activities of the catalysts were evaluated with RDE technique in O₂ saturated 0.1 M HClO₄ at 25°C. ADT was conducted using a rectangular wave potential cycling (0.6 V (3 s)-1.0 V (3 s) vs. RHE, 10,000 cycles) in Ar saturated 0.1 M HClO₄ at 80°C.

Results and Discussion

XRD patterns of the catalysts are shown in Fig. 1. All samples exhibited characteristic diffraction patterns of fcc structure. In PtPd/C catalyst, composition was Pt₅₁Pd₄₉ (in at.%) and (220) diffraction peak angle was close to that of the Vegard’s law (dotted blue arrow), indicating that Pt and Pd are well-mixed and form alloy particle. In Pt/Pd/C catalyst, (220) diffraction peak angle did not change after Pt shell formation and peak symmetry was retained. TG and XRF analyses showed that the Pt shell formed on the Pd core surface corresponds to 1.1 times of Pt monolayer.

TEM images of the catalysts are depicted in Fig. 2. In PtPd/C catalyst, small size PtPd NPs disappeared after ADT and mean diameter of the catalyst increased. On the contrary, morphology of Pt/Pd/C catalyst changed into a spherical shape with ADT and mean diameter decreased. XRF compositional analysis revealed that 41 % and 70 % Pd were dissolved out from the PtPd/C and the Pt/Pd/C catalyst, respectively, after the ADT. 

Figure 3 summarizes ORR mass activity of the catalysts in comparison with that of a commercial Pt/C catalyst (TEC10E50E, TKK). Initial PtPd/C catalyst showed the highest ORR mass activity due to its high specific activity.  After ADT at 80°C, ORR mass activity of Pt/Pd/C catalyst was largely enhanced (3.0-fold of Pt/C catalyst). Interestingly, TEM-EDX analysis indicated that core-shell structure was retained even after the ADT, which suggests that the core-shell structure is stable although more than 50 % of the Pd core dissolved out with ADT. ORR mass activity of the PtPd/C alloyed catalyst could be enhanced with suppression of large decrease in electrochemical surface area with ADT.

Acknowledgement

This work was supported by NEDO Japan.

References

[1] A. U. Nilekar et al., Top. Catal., 46, 276 (2007).

[2] K. Sasaki et al., Angew. Chem. Int. Ed., 49, 8602 (2010).

[3] Y. Ikehata et al., 224^th ECS Meeting, Abstract #1497, San Francisco, USA (2013).

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

[5] K. Okuno et al., 81^st Japanese Electrochemical Society Meeting, Abstract PFC05, Osaka, Japan (2014).