Electrochemical Properties and Cell Performance of Pd Core-Pt Shell Structured Catalyst Synthesized By Direct Displacement Reaction Method

Abstract

1. Introduction Carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) was synthesized by a simple direct displacement reaction (DDR) method in which Pd core was spontaneously displaced with a Pt precursor upon stirring in N2 saturated H2SO4 aqueous solution at 70°C [1]. Compared with our previous modified Cu-UPD/Pt displacement method [2], DDR method is a simple and suitable synthetic method for mass production of the catalyst. In DDR method, Pt shell was more uniformly formed and the Pt shell coverage was increased. In this study, oxygen reduction reaction (ORR) activity and durability of the catalyst were examined and single cell performance was evaluated. 2. Experimental Pt/Pd/C catalyst was synthesized by DDR method [1]. 600 mg of Pd/C core (particle size: 5.0 nm, metal loading: 46 wt.%, ISHIFUKU Metal Industry) was dispersed in 800 mL of water and pH was adjusted to 1 by 4 M H2SO4 at 5°C under N2 gas atmosphere. K2PtCl4 corresponding to Pt monolayer shell was added and stirred at 5°C for 0.5 h, followed by stirring at 70°C for 3 h. ORR activity of the catalyst was evaluated by RDE technique performed in O2 saturated 0.1 M HClO4 at 25°C with a positive scan rate of 10 mV/s at 1,600 rpm. Pt/Pd/C catalyst was activated by a high activation protocol HAP performed in Ar saturated 0.1 M HClO4 at 80°C [3]. Durability of the catalyst was evaluated by an accelerated durability test ADT (rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80°C for 10,000 cycles). A membrane electrode assembly (MEA) with active area of 1 cm2 was fabricated by using 25.4 µm thick NafionTM NR211. Pt loading in Pt/Pd/C cathode was 0.11 mg/cm2. I-V performance of the MEA was evaluated at 80°C with H2-Air feeding of 418 and 988 NmL/min., respectively. 3. Results and Discussion Nonuniform Pt shell formation observed in Pt/Pd/C catalyst synthesized by modified Cu-UPD/Pt displacement method was suppressed in Pt/Pd/C catalyst synthesized by DDR method as demonstrated in Fig. 1. CVs of Pd/C core and Pt/Pd/C catalysts are summarized in Fig. 2. Large hydrogen desorption wave observed on Pd/C core in a potential range of 0.05-0.1 V, which is a character of Pd, was decreased in Pt/Pd/C catalyst synthesized by Cu-UPD method, and the wave was further reduced in Pt/Pd/C catalyst synthesized by DDR method, indicating that Pt shell coverage was increased by DDR method. Since potential difference between Pd and Pt2+ in DDR method could be decreased compared with the difference between Cu and Pt2+ in Cu-UPD method, Pt shell was more slowly and uniformly formed and the coverage was increased in DDR method. Furthermore, number of fine catalyst particles decreased and mean diameter increased in the catalyst synthesized by DDR method, suggesting that fine Pd core particles were preferentially dissolved and re-deposited by Cl- anions released from Pt precursor of K2PtCl4.Change in ORR mass activity of Pt/Pd/C catalysts with HAP and ADT is summarized in Fig. 3. Initial ORR mass activity of Pt/Pd/C catalyst synthesized by DDR method exhibited 1,419 A/g-Pt, which was 3.3 times higher than that of Pt/Pd/C catalyst synthesized by Cu-UPD method. The activity was enhanced to 1,688 A/g-Pt by HAP, corresponding to 5.3-fold of a reference Pt/C catalyst (320 A/g-Pt, TEC10E50E, TKK). After ADT, ORR mass activity of the catalyst decreased to 917 A/g-Pt. Thus, the catalyst exhibited higher durability than the catalyst synthesized by Cu-UPD method (after ADT: 389 A/g-Pt). I-V performance of Pt/Pd/C catalyst synthesized by DDR method overcame the performance of a reference Pt/C catalyst (TEC10E50E, Pt loading: 0.10 mg/cm2, TKK) as demonstrated in Fig. 4. The Pt/Pd/C catalyst exhibited current density of 0.061 mA/cm2 at cell voltage of 0.85 V (IR free), which was 3.7-fold of the Pt/C catalyst. We think that Pt/Pd/C catalyst synthesized by DDR method is a promising candidate for achieving highly active and durable catalyst for ORR. Acknowledgement We thank to Mrs. Daimaru and Mikami for MEA evaluation. This study was supported by NEDO, Japan.