Effect of Crystallographic Ordering in PtCo/C Alloy Catalyst on Durability

Abstract

Introduction Decrease of Pt usage in the PEFCs is essential for their worldwide commercialization. Pt-M alloy catalyst (M: 3d transition metals such as Fe, Co, Ni and Cu) is an attractive candidate for the decrease due to its high ORR activity [1, 2]. However, since the 3d transition metals M have lower redox potentials than the Pt, the metals M dissolve-out under the PEFC’s cathode condition, which decays the ORR activity of the Pt-M alloy catalysts. It has been reported that the decay of the ORR activity evaluated at room temperature (e.g., 25oC) can be suppressed with a crystallographic ordering of the catalyst [3]. Thus, in this study, we synthesized disordered and L10 ordered Pt-Co alloy catalysts and investigated their durability at PEFCs’ working temperature of 80oC using an accelerated durability test. Experimental Carbon supported PtCo alloy catalysts (PtCo/C) were synthesized by the following procedures. 0.5 mmol of Pt(acac)2 and 0.5 mmol of Co(acac)2 were dissolved in 50 ml of oleylamine (OAm) at room temperature and the solution was heated to 300oC under N2 atmosphere and kept for 1 h with stirring [4]. After the solution was cooled to room temperature, PtCo NPs were precipitated by adding 40 ml of ethanol and separated by centrifugation, followed by re-dispersion in n-hexane. These processes were repeated for 3 times to wash the PtCo NPs. Carbon support, Ketjen black EC-300J, was added to the n-hexane solution and stirred overnight to load the PtCo NPs, followed by filtration and drying. As-synthesized PtCo/C catalyst was heated at 400oC for 4 h to remove the OAm, followed by heating at 700oC for 1 h and 900oC for 1 h to promote crystallographic ordering under 15%-H2/Ar atmosphere. PtCo/C catalysts were characterized with TG-DTA, XRF, XRD, TEM, TEM-EDX and CV. ORR activity of the catalysts was evaluated by RDE technique in O2 saturated 0.1 M HClO4 at 25oC. Accelerated durability test (ADT) was performed at 25oC and 80oC using a rectangular wave potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE in Ar saturated 0.1 M HClO4 for 10,000 cycles. Results and Discussion XRF analysis revealed that bulk composition of the PtCo/C catalysts was Pt46Co54 (at.%) and TG analysis showed that metal loading was ca. 30 wt.%. XRD patterns of the PtCo/C catalysts are depicted in Fig. 1. The PtCo/C catalyst heated at 400oC showed a typical face-centered cubic (fcc) structure with disordered phase (hereafter, called as 400oC-Disorder). The broad peak at around 25o is attributed to the carbon support. The other four diffraction peaks are consistent with the fcc structure, corresponding to (111), (200), (220) and (311) planes. After heating at 700oC and 900oC, the XRD patterns transformed to an ordered intermetallic L10 Pt-Co phase (black triangles) (hereafter, called as 700oC-Order and 900oC-Order, respectively). TEM images of the PtCo/C catalysts are shown in Fig. 2. Mean diameters of 400oC-Disorder, 700oC-Order and 900oC-Order PtCo NPs were 5.4 nm, 5.8 nm and 6.0 nm, respectively, indicating that the mean diameter increased with increase of the heating temperature. It is well known that high ORR activity of the Pt-M alloy catalysts arises from ligand effect with the 3d transition metals M existing underneath of the topmost Pt shell in the catalysts [1, 2]. Therefore, durability of the Pt-M alloy catalysts strongly depends on dissolution of the M atoms under PEFC’s cathode condition. The Co dissolution from the PtCo/C catalysts with ADT performed at 25oC and 80oC is demonstrated in Fig. 3. The 400oC-Disorder PtCo/C catalyst lost all Co with the ADT. On the contrary, the 900oC-Order PtCo/C catalyst retained ca. 30% Co after the ADT performed at 80oC, indicating that the durability of the PtCo/C catalyst was enhanced with the crystallographic ordering from the disordered fcc phase to the ordered L10 phase. After the ADT, the 900oC-Order PtCo/C catalyst showed higher ORR mass activities than those of the 400oC-Disorder PtCo/C catalyst, also showing higher durability of the 900oC-Order PtCo/C catalyst (Fig. 4, black dotted line indicates a reference Pt/C catalyst; Pt mean diameter: 5.1 nm, Pt loading: 51 wt.%, TEC10E50E-HT, TKK). At the meeting, size effect of the L10 ordered PtCo/C catalyst on the durability will be also presented. Reference [1] T. Toda et al., J. Electrochem. Soc., 146, 3750 (1999). [2] V. R. Stamenkovic et al., Nat. Mater., 6, 241 (2007). [3] D. Wang et al., Nat. Mater., 12, 81 (2013). [4] Y. Yu et al., Nano Lett., 14, 2778 (2014). Figure 1