(Invited) In Situ Structural Analysis of PtCo Alloy Catalysts with Stabilized Platinum-Skin Layers and Durability/Activity Towards Oxygen Reduction Reaction
Authors: Inukai Junji , Iiyama Akihiro , Uchida Hiroyuki 
Published: February 2020

For polymer electrolyte fuel cells, Pt-based alloy catalysts have been widely studied. We have developed the nanocapsule method1,2 for the synthesis of PtCo alloy nanoparticles (n-PtCo) with well-controlled size and composition. On n-PtCo subsequently treated in hydrogen gas at high temperature (n-PtCoH2-HT), additional Pt-skin layers were formed (PtxAL-PtCo).3

Figure 1 shows the area-specific activities j
k for the oxygen reduction reaction (ORR) at carbon-supported n-PtCoH2-HT and PtxAL-PtCo (n-PtCoH2-HT/C and PtxAL-PtCo/C, respectively) and a commercial carbon-supported Pt catalyst (c-Pt/C) coated with Nafion in O2-saturated 0.1 M HClO4 solution at 65 oC and 0.85 V vs. RHE measured by a multi-channel flow double-electrode cell technique.2 Potential-step cycles were applied to the catalyst electrodes as the accelerated durability tests (ADTs) simulating load-change cycles of fuel cell vehicles4 in N2-saturated solution with no solution flow. Both the initial j
k and the durability were the highest at Pt2AL-PtCo/C.3

Figure 2 shows changes in the electrochemically active areas (ECAs) of n-PtCoH2-HT/C and PtxAL-PtCo/C during the ADTs in a constant solution flow at 65 °C so that the re-deposition of Pt became negligible. The ECA values of the catalysts decreased as the number of potential-step cycles increased, and the losses of the ECA of n-PtCo/C and PtxAL-PtCo/C after 5000 cycles were 42% and 26%, respectively. Therefore, the ECA loss was much mitigated by the existence of Pt skins.

Figure 3 shows the in-situ X-ray diffraction (XRD) pattern of the (111) peak of Pt-Co alloy of n-PtCo/C and PtxAL-PtCo/C measured during the ADT in a solution flow. For n-PtCo/C, the diffraction peak at ca. 27.6° was close to that for Pt (27.5°), suggesting that Co easily dissolved from n-PtCo into the hot acid electrolyte solution. The peak was small and broad even before the ADT, probably because of a low order of atoms in crystallite. In the XRD pattern of PtxAL-PtCo/C, a significantly large, distinct peak was observed before the potential cycles; the ordering of atoms in PtxAL-PtCo was very high. The peak was almost unchanged after the potential-step cycles, indicating that the crystallite structures of PtxAL-PtCo remained the same. The parameters of the Pt-Co alloy catalysts from the data obtained by the electrochemical measurements, transmission electron microscopy, and in-situ XRD are listed in Table 1. For n-PtCo/C, the crystallite size was approximately 2.3 nm during 5000 cycles. The number of crystallites per area and the metal retention ratio gradually decreased from 14 × 103 to 10 × 103 mm-2 and from 100% to 50%, respectively. Not only the dissolution but the migration/coalescence also played a role. For PtxAL-PtCo/C, the crystallite size was approximately 2.9 nm during 5000 cycles. No change was indicated in the shape and the number of the crystallites per area, as well as the loss of the metals. Interestingly, the ECA of PtxAL-PtCo/C gradually decreased during 5000 cycles by 26%. Therefore, it is assumed that the PtxAL-PtCo crystallites were aggregated into a larger particle consisting of multi-crystallites, while keeping the shape of each crystallite unchanged. The loss of ECA is explained by the loss of surface areas by a contact between crystallites. As shown in Fig. 1, the j
k value at PtxAL-PtCo/C remained the same level during the potential-step cycles. The coalescence of alloy crystallites without any dissolution of Pt and Co clearly explains this constant value of j
k. Analyzing an EXAFS of Pt in PtxAL-PtCo/C, the Debye-Waller factor for Pt–Pt in PtxAL-PtCo/C was even smaller than that of Pt nanoparticles with a similar size, revealing that the crystallinity of the outer Pt skin was extremely high.6 Thus, the existence of a rigid Pt skin realized the extensively high durability of PtxAL-PtCo/C. Further structural analyses of the Pt-Co catalyst at the atomic level are now in progress.

Acknowledgements

This work was carried out under the “SPer-FC” project, NEDO, Japan. The synchrotron radiation experiments were performed at BL14B2, BL16B2, BL19B2, and BL46XU of SPring-8 (No. 2015B3388, 2016A1760, 2016A1805, 2016A5390, 2016B1550, and 2016B1856, 2016B5390).

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Figure 1