Change in ORR Activity of Pd Core-Pt Shell Structured Catalyst with Different Pt Shell Coverage

Abstract

Introduction Reduction of the amount of expensive Pt cathode catalyst in PEFCs is essential for their worldwide commercialization. Core-shell structured catalyst, in which Pt monolayer (PtML) shell is formed on non-Pt metal core surface, is one of the key technologies. Furthermore, it has been reported that ORR activity of the PtML shell is enhanced when Pd nanoparticles are used as core materials [1, 2]. Recently, we developed a new process for the Pt shell formation which needs no precise potential control and is suitable for scale-up synthesis of the core-shell structured catalyst (modified Cu-UPD/Pt replacement [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 new process is drastically enhanced with an accelerated durability test (ADT) performed at 80°C [3]. The ORR activity enhancement is considered to be due to a rearrangement of the surface Pt atoms and a decrease in number of low-coordinated Pt atoms associated with oxidative dissolution of Pd from the core nanoparticles. In this study, Pt/Pd/C catalysts with different Pt shell coverages are synthesized via the modified Cu-UPD/Pt replacement process and the influence of the Pt shell coverage on the ORR activity of the catalysts after ADT was investigated. Experimental Pt/Pd/C catalysts were synthesized with the modified Cu-UPD/Pt replacement process [3]. Carbon supported Pd core particles (Pd/C, Pd size: 4.2 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) were ultrasonically dispersed in 50 mM H2SO4 containing 10 mM CuSO4 and the solution was stirred with co-existence of a metallic Cu sheet at 5°C under Ar atmosphere. After stirred for 5 h, the Cu sheet was removed and K2PtCl4 with different concentrations was added to obtain the Pt/Pd/C catalysts with different Pt shell coverages. The Pt/Pd/C catalysts were characterized by TG, XRF, XRD, TEM, TEM-EDX, CV and XAFS. The ORR activity of the Pt/Pd/C catalysts was evaluated with the RDE technique in O2 saturated 0.1 M HClO4 at 25°C. The ADT was conducted 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 1,000 cycles. Results and discussion The compositions of the two Pt/Pd/C catalysts synthesized with different K2PtCl4 concentrations were Pt21Pd79 and Pt30Pd70 (atomic %) and their Pt shell thicknesses were calculated to be 0.6 and 1.0 ML, respectively (hereafter called Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C). CVs of the Pt/Pd/C catalysts are shown in Fig. 1. In the potential range of 0.05-0.1 V vs. RHE, hydrogen absorption/desorption peaks, which are characteristics of Pd, were observed and these peaks were stronger in the Pt0.6 ML/Pd/C catalyst, indicating that the Pt shell coverage is lower in the Pt0.6 ML/Pd/C catalyst. TEM images of the Pt0.6 ML/Pd/C catalysts are shown in Fig. 2. Small catalyst particles disappeared and the morphology changed into spherical shape after ADT. TEM-EDX analysis revealed that composition of the Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C catalysts changed to Pt48Pd52 and Pt50Pd50, respectively, after ADT. Suppose that the Pt shell does not dissolve during ADT, the Pd core dissolution in the Pt0.6 ML/Pd/C and Pt1.0 ML/Pd/C catalysts are estimated as 71% and 57%, respectively. Changes in ORR mass activity of the Pt/Pd/C catalysts after ADT are summarized in Fig. 3 together with a carbon supported Pt reference catalyst (Pt/C, Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK). The ORR mass activities of both Pt/Pd/C catalysts were enhanced after ADT and the enhancement was higher for the Pt0.6 ML/Pd/C catalyst (2.6-fold of the Pt/C catalyst). As described above, the Pd core dissolution by the ADT is larger in the Pt0.6 ML/Pd/C catalyst (71%), which is due to the lower Pt shell coverage in the catalyst (Fig. 1). Since the ORR activity enhancement of the Pt/Pd/C catalyst during ADT arises from rearrangement of the Pt shell associated with the Pd core dissolution, it is considered that the lower Pt shell coverage in the Pt0.6 ML/Pd/C catalyst is, the more the Pd core dissolution and the rearrangement of the Pt shell is enhanced. The micro-structural change of the Pt/Pd/C catalysts after ADT will be presented at the meeting. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.