Electrochemical Properties of Pt Shell-Pd Core Structured Catalyst Synthesized via Direct Displacement Method

Introduction Carbon supported Pt shell-Pd core structured catalyst (Pt/Pd/C) is a promising candidate for decreasing Pt usage owing to high Pt utilization and ORR activity [1]. We found that ORR specific activity of Pt/Pd/C catalyst was drastically enhanced with accelerated durability test (ADT) performed at 80°C although electrochemical surface area (ECSA) of the catalyst largely decreased, which moderately enhanced ORR mass activity [2]. Thus, we developed high activation protocol (HAP) to mitigate ECSA decay and largely enhance ORR mass activity [3]. Furthermore, we developed H 2 -O 2 chemical activation treatment which mimics HAP performed on glassy carbon (GC) electrode and is suitable for mass-production of highly active catalyst [3]. Besides ORR activity enhancement, synthetic method of Pt/Pd/C catalyst suitable for mass-production is highly important. Although we demonstrated that Pt/Pd/C catalyst could be mass-produced by modified Cu-UPD/Pt displacement method [2, 4], the Cu-UPD method was complicated. Therefore, in this study, we synthesized Pt/Pd/C catalyst via direct displacement (DD) method in which Pd core was directly displaced with Pt precursors, which is much simpler synthetic method compared with Cu-UPD one. Durability and electrochemical properties of Pt/Pd/C catalyst synthesized via DD method were investigated. Experimental 200 mL of H 2 SO 4 aqueous solution (pH1) was deaerated with N 2 gas and 300 mg of carbon supported Pd core (Pd/C, mean diameter: 4.7 nm, metal loading: 33 wt.%, ISHIFUKU Metal Industry) was dispersed with stirring, and the solution temperature was controlled in a range of 5-70°C. Then, Pt precursors (K 2 PtCl 4 or H 2 PtCl 6 ) corresponding to Pt monolayer shell were added at the controlled temperature and stirred for 3 h. The obtained Pt/Pd/C catalyst was activated by HAP in which rectangular potential cycling of 0.4 V (300 s)-1.0 V (300 s) was applied to GC electrode in Ar saturated 0.1 M HClO 4 at 80°C for 60 cycles [3]. Pt/Pt/C catalyst was also activated with H 2 -O 2 chemical treatment which mimics HAP performed on GC electrode and is suitable for mass-production. 300 mg of Pt/Pd/C catalyst was dispersed in 2 M H 2 SO 4 aqueous solution at 80°C and H 2 and O 2 gasses were alternately introduced for every 6 minutes in which equilibrium potentials of H 2 (ca. 0.0 V) and O 2 (ca. 1.0 V) were alternately applied to the catalyst [3]. ADT was carried out by using rectangular potential cycling of 0.6 V (3 s)-1.0 V (3