Development of Highly Active and Durable Pt-Pd System Catalysts for PEFCs

Abstract

Introduction Carbon supported Pd core-Pt shell catalyst (Pt/Pd/C) and Pt-Pd alloy catalyst (PtPd/C) are attractive alternatives to a carbon supported Pt catalyst (Pt/C) because their ORR specific activities are enhanced with an accelerated durability test (ADT) performed at 80°C [1, 2]. However, ECSA of the catalysts drastically decreased with the ADT, resulting in slight increase of ORR mass activities. We explored the ADT protocol and developed a high activation protocol (HAP) to mitigate the ECSA decay and enhance the ORR mass activities. Furthermore, we developed a Cu-O2 treatment which mimics the HAP carried out on GC electrode and is suitable for mass-production of highly activated catalysts. We also applied SiO2 coating technique [3, 4] to prevent agglomeration of the catalyst particles with the ADT and improve durability of the catalysts. Experimental Pt/Pd/C was synthesized with a modified Cu-UPD/Pt replacement process [1]. A carbon supported Pd core (Pd/C, Pd size: 4.6 nm, Pd loading: 30 wt.%, Ishifuku Metal Industry) was 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 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 Pt shell. PtPd/C alloy catalyst was synthesized with an impregnation method. Pt(acac)2 and Pd(acac)2 were impregnated onto a carbon support using t-butylamine as a solvent (Pt20Pd80 in at.%), followed by reduction under 15% H2/Ar atmosphere at 600°C for 4 h. The catalysts were characterized by TG, XRF, XRD, TEM, CV and XAFS techniques. ORR activity of the catalysts was evaluated with RDE technique in O2 saturated 0.1 M HClO4 at 25°C. ADT was conducted using rectangular wave potential cycling of 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 Changes in ECSA and ORR mass activity of the Pt/Pd/C and PtPd/C catalysts with ADT, HAP and Cu-O2 treatment are summarized in Fig. 1 (dotted black lines show Pt/C catalyst; Pt size: 2.8 nm, Pt loading: 46 wt.%, TEC10E50E, TKK). The ECSA of the catalysts drastically decreased with the ADT, by which the ORR mass activity showed slight increases. On the contrary, the ECSA decay was mitigated with the HAP (rectangular wave potential cycling of 0.4V (300 s)-1.0 V (300 s) vs. RHE in Ar saturated 0.1 M HClO4 at 80°C for 30-40 cycles) and the ORR mass activity was highly enhanced, showing ca. 3-fold values of the Pt/C catalyst. Since the HAP is performed on the GC electrode, treated amount of the catalysts is extremely small (ca. 30 µg), we developed a Cu-O2 treatment (Fig. 2) to scale-up the HAP. In the Cu-O2 treatment, the catalyst powders are stirred for 300 s in 2 M H2SO4 containing 0.1 M CuSO4 at 80°C under an inert atmosphere (N2) with co-existence of a metallic Cu sheet, where the equilibrium potential of Cu2+/Cu (ca. 0.3 V) is applied to the catalysts when they contact with the Cu sheet. Next, the Cu sheet is removed and O2 gas is introduced in the solution, where the equilibrium potential of the ORR (ca. 1.0 V) is applied to the catalysts. As shown in Fig. 1, the ECSA decay by the Cu-O2 treatment is equivalent to that by the HAP and the ORR mass activity of the catalysts were equivalently enhanced, 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 and PtPd/C catalysts. However, the highly activated Pd@Pt NPs were severely agglomerated with the ADT and the ORR mass activity was decayed. In order to prevent the NPs agglomeration with the ADT, we applied SiO2 coating onto the Pd@Pt NPs [3, 4]. The SiO2 coating suppressed the NPs agglomeration and the ECSA decay was mitigated (Fig. 3), which highly enhanced the ORR mass activity of the Pt/Pd/C catalyst even after the ADT (Fig. 4). Acknowledgement This work was supported by NEDO, Japan.