(Invited) Highly Active and Robust Pt-Skin/Pt Alloy Two-Way Catalysts for Oxygen Reduction and Hydrogen Oxidation in PEFCs
Authors: Uchida Hiroyuki , Shi Guoyu , Kobayashi Shun , Yano Hiroshi , Inukai Junji , Tryk Donald A. , Iiyama Akihiro 
Published: February 2020

Cathode catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) have been investigated toward increasing their activity and durability, which can promote the large-scale commercialization of fuel cell vehicles (FCVs) and residential co-generation systems. In contrast, anode catalysis R&D for the hydrogen oxidation reaction (HOR) has not been so intensive recently. One of the reasons is that the HOR rate is sufficiently fast at Pt/C anode catalysts with a low Pt loading (ca. 0.05 mgPt cm−2) for FCVs with pure H2 as the fuel. For residential PEFCs operated with reformate (H2-rich fuel gas), Pt-Ru anode catalysts have been used predominantly to mitigate the poisoning by CO (below 50 ppm). However, Ru is not only costly (mass activity for the HOR at Pt-Ru/C is low) but is also unstable at high potentials (E > 0.8 V).
We demonstrated that both bulk Pt-Co and commercial c-Pt3Co/C exhibited high CO-tolerance in 0.1 M HClO4 solution at temperatures below 70 °C.1 It has been clarified that the Co component leached out into the acidic solution, resulting in the formation of a thin Pt-skin layer on the alloy surface. The excellent CO-tolerance on the Pt-skin layer was ascribed to weaker CO adsorption, which can maintain active sites for the HOR, due to its electronic structure being modified by the underlying alloy.1 However, the CO-tolerance was lost after being heated ≥ 80 °C due to increased dealloying of the Co component, leading to a thick Pt layer. Such dealloying of Co from c-Pt3Co/C catalysts during load-change cycles is also a serious issue.
Very recently, we have succeeded in suppressing the dealloying of Co by forming a thin, uniform, stabilized Pt-skin (one to two atomic layers: xAL) on Pt-Co alloy nanoparticles (PtxAL–PtCo), which act as a two-way catalyst for the ORR cathode with high activity and durability2,3 and the HOR anode with high activity, CO tolerance, and robustness at 70 and 90 °C.4

This presentation focuses on the following topics concerning the unique characteristics of the stabilized Pt-skin catalysts, including the analyses of microstructure and electronic structure.
1) Important role of Pt-skin layer and underlying alloy for enhancement of the ORR activity5, 6

The j
k value for the ORR at 0.9 V on Pt-skin/Pt100−xCox(111) single crystals reached a maximum at x = 27 atom% that was ca. 27 times higher than that on a pure Pt(111) electrode.5 By the use of in situ STM, in situ SXS, ex situ angle-resolved XPS, and DFT calculations, we have succeeded in correlating this high j
k with the specific surface structure and electronic structure.6

2) Potential-cycle induced change in crystal structure of PtxAL–PtCo/C
We have found that the crystal structure of PtxAL‒PtCo nanoparticles [face-centered tetragonal (fct) + face-centered cubic (fcc)] was converted to single-phase fcc, which exhibited high ORR activity and durability, surprisingly, only by means of potential cycling from 0.05 V to 1.0 V in 0.1 M HClO4 solution at room temperature.
3) Multilateral analyses of the enhancement of HOR activity and CO-tolerance for the PtxAL‒PtM/C7, 8

It was found by in situ FTIR,7
in situ XAS,8 and DFT calculations8 that the adsorption of CO on terrace sites on the PtxAL skin covering the PtM alloy (M= Fe, Co) was weakened by the modified electronic structure.
This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from NEDO of Japan.

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