ORR Activity Enhancement of PtML/Pd/C Core-Shell Catalysts By Potential Cycling
Pt/Pd/C catalyst was obtained from Pd/C core (4.5 nm, 30 wt.%, Ishifuku Metal Industry) using the modified Cu-UPD method . The Pd/C core and Pt/Pd/C catalyst were characterized by TG, XRD, XRF, TEM, TEM-EDX and CV. The ORR activity of the catalyst was measured by the RDE technique at 1,600 rpm in O2 saturated 0.1 M HClO4 at 298 K. The ADT was performed using rectangular-wave potential cycling (FCCJ, 0.6 V (3 s)/1.0 V (3 s) vs. RHE for 10,000 cycles) in Ar saturated 0.1 M HClO4at 353 K.
The ORR activity of the Pt/Pd/C catalyst was enhanced after ADT, showing by 2-fold higher ORR mass activity (512 A/g-Pt) than that of a commercially available Pt/C catalyst (2.8 nm, 46 wt.%, TEC10E50E, TKK). Morphological changes of the Pt/Pd/C catalyst after ADT is shown in Figure 1. The average size of the catalyst decreased from 5.6 to 4.9 nm and the catalyst particles changed into spherical particles. TEM-EDX analysis revealed that the Pd core dissolved out by 73 % and the Pt shell was thickened (to approximately 2.3 ML) after ADT.
The microstructural change of the Pt/Pd/C catalyst after ADT is schematically illustrated in Figure 2. The Pd core dissolved out through defects on the Pt shell during ADT. As a result, the Pt atoms at the shell were rearranged to give a defect-free surface to the Pt shell, by which the number of lower coordinated Pt atoms decreased. EXAFS analysis revealed that the Pt-Pt bond length decreased after ADT. It was concluded that the enhanced ORR activity after ADT was caused by a moderately induced compressive strain at the Pt shell, in addition to a decreased number of the lower coordinated Pt atoms.
Though the ADT protocol gave an enhanced mass activity (~ x2) to the Pt/Pd/C catalyst, there remains one problem that the electrochemical surface area (ECSA) decreased from 140 initially to 40-50 m2/g-Pt after ADT. Hence we optimized the potential cycling protocol to minimize the decrease in ECSA during ADT. The optimized protocol (0.4 V (300 s)/1.0 V (300 s) for 60 cycles) gave a high ECSA (> 80 m2/g) and a very high ORR mass activity (1002 A/g-Pt, x4 of Pt/C) after potential cycling.
This study was supported by NEDO, Japan.
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