It is of great importance to decrease Pt usage in PEFCs for cost reduction. Carbon supported Pt shell-Pd core structured catalyst (Pt/Pd/C) is a promising candidate for the decrease owing to high Pt utilization and ORR activity . We found that ORR specific activity of the catalyst drastically enhanced with an accelerated durability test (ADT), while electrochemical surface area (ECSA) largely decreased due to agglomeration of the catalyst nanoparticles (NPs), resulting in a moderate increase of ORR mass activity . Thus, it is essential to mitigate the ECSA decay for further enhancement of the ORR mass activity. In the present study, poly-dopamine coating was investigated to suppress the catalyst NPs’ agglomeration with the ADT and further enhance the ORR mass activity of the catalyst.
Pt/Pd/C catalyst was synthesized by a modified Cu-UPD/Pt displacement method . 200 mg of Pt/Pd/C catalyst was added in 200 mL of 10 mM tris-HCl buffer solution (pH8.5) containing 2 mg/mL of dopamine hydrochloride and stirred at 30°C for 6 h under O2 gas bubbling (500 mL/min.), followed by filtering, washing and drying at 60°C in air . ADT was performed at 80°C for 10,000 cycles in Ar saturated 0.1 M HClO4 by using a rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE. Characterization of the catalyst was carried out by using TG, XRD, XRF, TEM and CV techniques. ORR activity of the catalyst was measured by RDE technique performed at 1,600 rpm in O2 saturated 0.1 M HClO4with positive scan rate of 10 mV/s at 25°C.
3. Results and Discussion
It has been reported that polymerization of the dopamine was accelerated by O2and that poly-dopamine (PD) can be coated on various materials such as metals, metal oxides, organic films and plastics at low temperature (30°C) [3, 4]. For carbon coating, carbonization of the PD is carried out by heat treatment in non-oxidative atmosphere  and it has been demonstrated that size of the PD coated PtFe NPs (6.5 nm) did not change before and after the heat treatment at 700°C, indicating that the carbon coating using the PD precursor well suppressed coalescence and sintering of the NPs against the heat treatment .
In this study, we investigated effect of the PD coating on durability of the Pt/Pd/C catalyst without the heat treatment. Figure 1 depicts TEM images of the Pt/Pd/C catalysts before and after the PD coating and after the ADT performed at 80°C for 10,000 cycles. Little morphological changes were observed before and after the PD coating and agglomeration of the catalyst NPs was well suppressed after the ADT, which mitigated the ECSA decay of the catalyst with the ADT (Figure 2). Figure 3 summarizes change in ORR mass activity of the catalyst with the ADT. The ORR mass activity of the PD coated Pt/Pd/C catalyst was enhanced by 3.8-fold of a reference Pt/C catalyst (TEC10E50E, TKK), implying that the PD coating without the heat treatment, i.e., not a carbon coating, effectively mitigated the catalyst NPs’ agglomeration and improved the durability of the Pt/Pd/C core-shell structured catalyst. In the meeting, the durability of the heat treated catalyst will be also presented.
This study was supported by NEDO, Japan.
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