For polymer electrolyte fuel cells, Pt-based alloy catalysts have been widely studied. We have developed a novel nanocapsule method^1,2 for the synthesis of controlled PtCo alloy nanoparticles (PtCo/GCB, graphitized carbon black support to mitigate carbon corrosion), and modified the method³for covering hydrogen-treated PtCo/GCB (H-PtCo/GCB) with Pt-skin layers.

Figure 1 shows images obtained with a transmission electron microscope (TEM) of PtCo/GCB, H-PtCo/GCB, and Pt-skin coated PtCo catalyst (Pt_xAL-PtCo/GCB) with the histograms of the particle-size distributions. Particle sizes were very uniform with the diameters of 2.5±0.2 nm for PtCo/GCB and H-PtCo/GCB, and of 3.0±0.3 nm for Pt_xAL-PtCo/GCB. By the electron diffraction (Table 1), the lattice of PtCo/GCB was determined as fcc. After the hydrogen treatment at 400 ^oC,³ the lattice became fct for H-PtCo/GCB. After the addition of a Pt-skin on H-PtCo/GCB, the lattice was mostly (ca. 97%) fct and partially (ca. 3%) fcc, presumably due to the addition of a Pt-skin. By X-ray absorption fine structure (XAFS) measurements, Pt and Co atoms were found to be very uniformly distributed in PtCo/GCB. For H-PtCo/GCB, neither the atomic distances nor the coordination numbers showed a large change, whereas for Pt_xAL-PtCo/GCB, the Pt-Pt coordination number increased showing the formation of a Pt-skin layer.

The effect of the Pt-skin layers was examined in a 0.1-M HClO₄ solution at 65 ^oC with a multi-channel flow double-electrode cell² on the activity towards the oxygen reduction reaction and the durability examined by the accelerated durability test according to the protocol for load-change cycles⁴ recommended by the Fuel Cell Commercialization Conference of Japan (FCCJ) (Fig. 2). Both the durability and the mass activity evaluated at 0.85 V vs. the reversible hydrogen electrode were the highest at Pt_xAL-PtCo/GCB followed by H-PtCo/GCB. By in situ XAFS measurements, the layers of Pt skins were indicated to exist at the surface of the alloy particles in 0.1 M HClO₄. After 30,000 cycles, the initial high activity at PtCo/GCB was lost and became almost identical to that at a commercial Pt catalyst supported on carbon black (c-Pt/CB) because of Co leaching out.^5,6

The efficiency/usefulness of the atomically-controlled Pt-skin layers was clearly demonstrated. Detailed structural analysis during the durability test is now in progress.

Acknowledgements

This work was carried out under the “HiPer-FC” and “SPer-FC” projects, NEDO, Japan. The synchrotron radiation experiments were performed at BL14B2 and BL16B2 of SPring-8 (Proposal No. 2015B1851, 2015B1852, 2015B3388, 2015B5390, 2016A5390).

References

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2) H. Yano, J. M. Song, H. Uchida, M. Watanabe, J. Phys. Chem. C, 112, 8372 (2008).

3) M. Watanabe, H. Yano, D. A. Tryk, H. Uchida, J. Electrochem. Soc., 163, F455 (2016).

4) http://fccj.jp/pdf/23_01_kt.pdf in Japanese.

5) M. Watanabe, D. A. Tryk, M. Wakisaka, H. Yano, H. Uchida, Electrochim. Acta, 84, 187 (2012).

6) K. Okaya, H. Yano, K. Kakinuma, H. Uchida M. Watanabe, ACS Appl. Mater. Interfaces, 4, 6982 (2012).