High Activation Protocol for Pd Core-Pt Shell Structured Catalyst
Authors: Daimon Hideo , Okuno Kosuke , Mizoue Kota , Matsui Yuki , Higuchi Shunya , Aoki Naoya , Inoue Hideo , Doi Takayuki , Inaba Minoru 
Published: February 2020

Introduction

Carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) is a promising alternative to the conventional Pt/C catalyst because of high Pt utilization and enhancement of ORR activity [1, 2]. However, the Pd core dissolves-out under PEFC cathode conditions due to its lower redox potential compared with that of the Pt [3]. Recently, we found that the ORR activity of the Pt/Pd/C catalyst was drastically enhanced by an accelerated durability test (ADT) conducted at 80°C [4]. Associated with 70-80% dissolution of the Pd core by the ADT, the Pt shell thickened and a compressive strain was induced in the Pt shell, which was considered to enhance the ORR activity [4]. Since the core-shell structure was retained even after the ADT, the structure is to be stable in the Pt-Pd bimetallic system. In this study, a new protocol for high activation of the Pt/Pd/C catalyst was explored. Furthermore, we developed a H2-O2treatment to scale-up the protocol for the mass-production of highly activated Pt/Pd/C catalyst.

Experimental

Pt/Pd/C catalyst was synthesized with a modified Cu-UPD/Pt replacement process [4]. A carbon supported Pd core (Pd/C, Pd size: 4.2 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) was 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 was added to replace the Cu shell deposited on the Pd core surface with the Pt shell, forming the Pt/Pd/C catalyst. The Pt/Pd/C catalyst was characterized by TG, XRF, XRD, TEM, TEM-EDX, CV and XAFS. ORR activity of the Pt/Pd/C catalysts was 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) in Ar saturated 0.1 M HClO4
at 80°C for 10,000 cycles.

Results and Discussion

Electro-chemical surface area (ECSA) and ORR activity of the Pt/Pd/C catalyst are summarized in Table 1 together with a carbon supported Pt reference catalyst (Pt/C, Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK). The ORR mass activity of the Pt/Pd/C catalyst was enhanced by the ADT, and the enhancement is due to the increased ORR specific activity. Therefore, much higher ORR mass activity is to be obtained if the ECSA decay is decreased. It was found that a new potential cycling protocol, i.e., a rectangular wave potential cycling of 0.4 V (300 s)-1.0 V (300 s) vs. RHE (High Activation Protocol: HAP) decreases the ECSA decay. Figure 1 shows TEM images of the Pt/Pd/C catalysts. Agglomeration of the catalyst particles was suppressed by the HAP compared with the conventional ADT, which is considered to decrease the ECSA decay.
In order to scale-up the HAP for the Pt/Pd/C catalyst, we developed a H2-O2 treatment where equilibrium potentials of H2 and O2 in an acidic medium are used instead of the potentiostat. 100 mg of the Pt/Pd/C catalyst was dispersed in 2 M H2SO4 and H2 and O2 gases were alternatively introduced (for 300 s each) with stirring. In the H2-O2 treatment, ca. 0.0 V vs. RHE is applied to the catalyst when the H2 gas is introduced and ca. 1.0 V vs. RHE is applied when the O2 gas is introduced. Table 2 summarizes change in the electrochemical properties of the Pt/Pd/C catalysts after the H2-O2 treatment and a rectangular wave potential cycling (0.05 V (300 s)-1.0 V (300 s) vs. RHE). All properties after the H2-O2 treatment are almost equivalent to those after the potential cycling, indicating that the H2-O2treatment mimics the potential cycling and is suitable for the mass-production of highly activated Pt/Pd/C catalyst.

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., To
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[3] K. Sasaki et al., Angew. Chem. Int. Ed., 49, 8602 (2010).
[4] M. Inaba and H. Daimon, J. Jpn. Petrol. Inst., 58(2), 55 (2015).

Figure 1