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

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 (Pt_ML) 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 Pt_ML 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 Pt_{0.6 ML}/Pd/C and Pt_{1.0 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 Pt_{0.6 ML}/Pd/C catalyst, indicating that the Pt shell coverage is lower in the Pt_{0.6 ML}/Pd/C catalyst. TEM images of the Pt_{0.6 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 Pt_{0.6 ML}/Pd/C and Pt_{1.0 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 Pt_{0.6 ML}/Pd/C and Pt_{1.0 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 Pt_{0.6 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 Pt_{0.6 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 Pt_{0.6 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.

References

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

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

[3] M. Inaba and H. Daimon, J. Jpn. Petrol. Inst., 58(2), 55-63 (2015).