Relationship between synthesis conditions and ORR activity of PtCo nanowire/C catalysts for PEFC cathodes

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Introduction Polymer electrolyte fuel cells (PEFCs) are used as power sources for automobiles because of their advantages such as the low environmental impact and high energy conversion. However, despite their convenience, they have not become widespread. One of the reasons is their high cost. In general, expensive Pt is used as the electrocatalyst for PEFCs, and high catalytic activity and durability are required with the reduced amount of Pt. Until now, Pt nanoparticles have been commonly used as the electrocatalysts, but there have been problems with Pt elution and agglomeration during PEFC operation. Recently, nanowire (NW) structure, which suppresses the aggregation of catalyst particles, has attracted attention. 1-3) In the formation of NWs, metal carbonyls, such as Co2(CO)8 and Mo(CO)6, act as strong adsorbents on Pt and the surface direction of the NWs. Oleylamine (OAm) also plays a specific role in nanostructure formation with capping ligands, using various organic or inorganic compounds as the precursors alone or in combination with other reactants. In this study, the cathodic PtCo NW/C catalysts were prepared with different amounts of OAm, and their effects on NW morphology, ORR activity, and durability were investigated. Experimental To prepare the PtCo NW, 100 mg of Pt(acac)2,75 mg of Co2 (CO)8, and 80 mg of Vulcan XC-72R were ultrasonically dispersed into OAm (25, 50, 100, and 150 mL), respectively. The mixture was then heated at 100 °C for 2 h while being stirred. Once the product was collected, it was washed multiple times with a mixture of EtOH and cyclohexane. The samples were naturally dried at room temperature and heat treated at 350 ℃ under H2/Ar flow. For removing the residual amount of Co, acid treatment was performed in acetic acid at 70 ℃ for 0.5 and 2 h. Finally, PtCo NW catalysts were prepared and noted as PtCoNW /C OAm25, PtCoNW/C OAm50, PtCoNW/C OAm100, and PtCoNW/C OAm150, respectively. The morphology and composition of PtCo NW/C were characterized by XRD, TEM, TGA, and XRF analysis. CV, CO stripping, and LSV were conducted for evaluation of ORR activity. MEA was fabricated using the PtCoNW/C catalyst as the cathode and evaluated for the I-V characteristic tests. ADTs were also performed to evaluate the durability of the PtCoNW/C catalysts. Results and Discussion XRD patterns showed that the peaks originating from the Pt (111) plane of the four PtCo NW/C catalysts were shifted to the higher angles, indicating that formation of the PtCo alloy. The XRD peaks shifted to the lower angle for the catalyst treated with acid for 2 h, compared to the catalyst treated with acid for 0.5 h. Figure 1 shows, the TEM image of PtCo NW was formed and uniformly dispersed on carbon support. The average diameter of the PtCo NW/C catalyst treated with acid for 2 h was smaller than that of the catalyst treated with acid for 0.5 h. The average diameters were changed with increasing amount of OAm25, OAm50, OAm100, OAm150 (3.6, 2.3, 2.0, and 2.1 nm), resperctively. Electrochemical active surface area (ECSA) was calculated from the CV and CO stripping curves. The ECSA of the PtCo NW/C OAm100 catalyst after 2 h of acid treatment was the largest among the four catalysts. The PtCo NW/C OAm100 catalyst had the highest mass activity (MA) calculated from the LSV. As shown in Figure 2, the cell voltage at high current density region (1.0 A cm－ 2) of the NW catalysts was lower degradation rate, compared to the commercial PtCo/C catalyst. Among all PtCo NW/catalysts, the PtCo NW/C OAm100 catalyst was showed excellent durability. This would be due to the NW morphology, which suppresses agglomeration.In conclusion, it was revealed that the addition of an appropriate amount of OAm would improve the ORR activity of the NW morphology of PtCo/C catalyst.This work was partly supported by NEDO, Japan. References (1) X. Wang, et.al., Nano Lett. 16,1467 (2016).(2) H. Zhang, et.al., Sci. Adv. 12, 2548 (2019).(3) T. Wang, et.al., Energy Mater. 9, 1803771 (2019).(4) K. Jiang, et al., Sci. Adv. 3,1601705 (2017). Figure 1