ORR Activity of Large-Scale Synthesized Pd Core-Pt Shell Catalysts

Abstract

Introduction It is of great importance to decrease Pt usage in the polymer electrolyte fuel cells (PEFCs) for their cost reduction. Pd core-Pt shell catalyst (Pt/Pd/C) is a promising candidate for the decrease due to its high Pt utilization efficiency and enhancement of oxygen reduction reaction (ORR) activity [1, 2]. Recently, we have reported that ORR specific activity of the Pt/Pd/C catalyst is drastically enhanced with an accelerated durability test (ADT) [3]. It is considered that the enhancement arises from rearrangement of the Pt shell associated with the Pd core dissolution, in which number of low-coordinated surface Pt atoms decreased and a compressive strain was induced in the Pt shell [3].In our previous study, the Pt/Pd/C catalysts were successfully synthesized with a modified Cu-UPD/Pt replacement process in a large-scale (50-100 g/batch) [4]. However, the degree of the ORR activity enhancement after the ADT had poor reproducibility although their initial physical and electrochemical properties were almost equivalent. In this study, structural change of the Pt/Pd/C catalysts with the ADT was analyzed using STEM-EDX and XAFS techniques to understand the poor reproducibility in ORR performance after the ADT. Experiment 80 g of carbon supported Pd core (Pd/C; particle size: 6.0 nm, Pd loading: 30 wt.%) was ultrasonically dispersed in 50 mM H2SO4 containing 20 mM CuSO4 and stirred at 5oC with coexistence of a metallic Cu sheet under Ar atmosphere. After pre-determined stirring time, the Cu sheet was removed and K2PtCl4 was added to replace under-potentially deposited Cu monolayer on the Pd core surface with Pt monolayer, forming Pt/Pd/C core-shell catalyst. The Pt/Pd/C catalyst was characterized with ICP, XRD and CO adsorption. ORR activity of the Pt/Pd/C catalyst was evaluated with RDE technique in O2 saturated 0.1 M HClO4 at 25oC. Accelerated durability test (ADT) was performed using a rectangular wave potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE at 60oC in Ar saturated 0.1 M HClO4for 10,000 cycles. Structure of the Pt/Pd/C catalyst was analyzed by STEM, STEM-EDX and XAFS (BL14B2 beam line at SPring-8). Results and Discussion Physical and electrochemical properties of two Pt/Pd/C catalysts, (a) and (b), which were synthesized in different batches (100 g/batch), are summarized in Table 1. Although initial properties are almost equivalent, ORR specific activity of (a) Pt/Pd/C catalyst is much enhanced by the ADT (434→966 µA/cm2, 2.2-fold) compared with that of (b) Pt/Pd/C catalyst (437→731 µA/cm2, 1.7-fold).HAADF-STEM images and STEM-EDX line analyses of the catalysts after the ADT are demonstrated in Fig. 1, revealing that there are two different structures in the catalysts. One retained Pd core-Pt shell structure with thickened Pt shell (Fig. 1 (A)) and the other transformed to a Pt enriched nanoparticle with large Pd dissolution (Fig. 1 (B)). The STEM-EDX analysis showed that the core-shell structure was much retained in the (a) Pt/Pd/C catalyst. Furthermore, XAFS analysis indicated that Pt-Pt bond distance of the Pt shell was more shortened in the (a) Pt/Pd/C catalyst after the ADT. Therefore, it can be concluded that the higher ORR specific activity of the (a) Pt/Pd/C catalyst after the ADT arises from retention of the core-shell structure and the shortened Pt-Pt bond distance (i.e., higher compressive strain in the Pt shell).Since it is considered that the inferior ORR specific activity of the (b) Pt/Pd/C catalyst after the ADT is mainly due to size distribution of the Pd core nanoparticles, synthesis of the Pd nanoparticles with narrower size distribution should be developed. At the meeting, cell performance using the Pt/Pd/C catalysts will be presented. Acknowledgement This work was supported by NEDO, Japan.