High Activation Protocol and Cu-O₂ Treatment for Highly Active Pd Core-Pt Shell Catalyst for PEFCs

Abstract

Introduction Carbon supported Pd core-Pt shell 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]. Recently, we found that ORR specific activity of the Pt/Pd/C catalyst was drastically enhanced with an accelerated durability test (ADT) conducted at 80°C [2]. During the ADT, the Pt shell rearranged associated with the Pd core dissolution and a compressive strain was induced in the Pt shell, which is considered to enhance the ORR specific activity [2]. However, ECSA of the catalyst decreased after the ADT. Thus, we developed a high activation protocol (HAP) using GC electrode to mitigate the ECSA decay and enhance ORR mass activity [3]. In this study, we explored influence of potential range in the HAP on ECSA decay and ORR activity enhancement of the Pt/Pd/C catalyst. Furthermore, we developed a Cu-O2 treatment to scale-up the HAP on the GC electrode for mass-production of highly activated Pt/Pd/C catalyst. Experimental Pt/Pd/C catalyst was synthesized with a modified Cu-UPD/Pt replacement process [2]. A carbon supported Pd core (Pd/C, Pd size: 4.6 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) was stirred in 50 mM H2SO4 containing 10 mM CuSO4 with co-existence of a metallic Cu sheet at 5°C under Ar atmosphere. After stirring for 5 h, the Cu sheet was removed and K2PtCl4 was added to replace under potentially deposited Cu shell on the Pd core surface with the Pt shell. ADT was carried out using rectangular wave potential cycling of 0.6 (3 s)-1.0 V (3 s) vs. RHE in Ar saturated 0.1 M HClO4 at 80℃ for 10,000 cycles. The Pt/Pd/C catalyst was characterized by TG, XRF, XRD, TEM and CV. ORR activity of the catalyst was evaluated with RDE technique in O2 saturated 0.1 M HClO4 at 25°C. Results and Discussion We explored influence of potential range in the HAP on electrochemical properties of the Pt/Pd/C catalyst using rectangular potential cycling of 0.05~0.8 V (300 s for low potential) to 1.0 V (300 s for high potential) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80℃ for 30 cycles. Changes in ECSA and ORR activity of the catalyst are demonstrated in Fig. 1. In comparison to ADT, ECSA decay of the Pt/Pd/C catalyst was mitigated with the HAP. Interestingly, ORR specific activity of the catalyst was largely enhanced when the low potential range was 0.2~0.6 V, which largely enhanced ORR mass activity of the catalyst. At the low potential range (0.2~0.6 V), it is considered that Pt shell rearrangement associated with Pd core dissolution was advanced due to sufficient oxidation/reduction of Pt and Pd. At low potential of 0.05 V, since hydrogen adsorbs on the Pt, it is considered that hydrogen adsorption hindered rearrangement of the Pt shell [4] and the ORR activity was not largely enhanced. We further developed a Cu-O2 treatment to scale-up the HAP performed on GC electrode. In the Cu-O2 treatment, the Pt/Pd/C catalyst powder (200 mg) is stirred at 80℃ for 300 s in 2 M H2SO4 containing 0.1 M CuSO4 with co-existence of a metallic Cu sheet under N2 atmosphere, where equilibrium potential of Cu2+/Cu (ca. 0.3 V) is applied to the catalyst powder when it contacts with the Cu sheet. Next, the Cu sheet is removed and O2 gas is introduced for 300 s, where equilibrium potential of ORR (ca. 1.0 V) is applied to the catalyst (Fig. 2). Figure 3 summarizes changes in ECSA and ORR mass activity of the Pt/Pd/C catalyst with the HAP and the Cu-O2 treatment (30 cycles). Compared with the HAP, the Cu-O2 treatment equivalently mitigated ECSA decay and enhanced ORR mass activity of the catalyst by ca. 3 times of reference carbon supported Pt catalyst (Pt/C, Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK), indicating that the Cu-O2 treatment mimics the HAP on the GC electrode and is suitable for mass-production of highly activated Pt/Pd/C catalyst. Acknowledgement This work was supported by NEDO, Japan. References [1] A. U. Nilekar et al., Top Catal., 46, 276 (2007). [2] M. Inaba and H. Daimon, J. Jpn. Petrol. Inst., 58(2), 55 (2015). [3] H. Daimon et al., The 228th Electrochemical Society Meeting, #1375, Phoenix, USA, October 2015. [4] Q. Xu et al., J. Electrochem. Soc., 155(3), B228 (2008). Figure 1