(Invited) Pt-Alloy Anode and Cathode Catalysts for PEFCs
Authors: Uchida Hiroyuki 
Published: December 2024

The R&D of highly active and durable cathode catalysts for the oxygen reduction reaction (ORR: O2 + 4H+ + 4e−→2H2O) is very important for polymer electrolyte fuel cells (PEFCs). Bimetallic alloys of Pt such as Pt−Co exhibit higher ORR activities than that of pure Pt. It was found for polycrystalline Pt alloy electrodes that the dissolution of the surface was followed by a rearrangement of the Pt atoms, resulting in a Pt-skin layer, which can protect the underlying bulk alloy.1, 2 It was also shown that a modified electronic structure at the Pt-skin layer induced enhanced ORR activity.1, 2 Such Pt-skin/Pt alloy electrodes were found to maintain activities for the hydrogen oxidation reaction (HOR: H2 →2H+ + 2e−) in the presence of CO.3 However, in accelerated stress tests (ASTs) of Pt alloys in both states of bulk and nanoparticle (NP) in a half cell with a flow of hot acid solution (> 70°C), further dealloying certainly resulted in a deactivation of ORR and HOR by the formation of thicker Pt layer.1
−3 Thus, an essential point is the formation of a stable and uniformly thin Pt-skin layer, which can protect the underlying alloy from corrosion, as well as multilateral analyses of the electrocatalysis (for the ORR and HOR) and the degradation mechanism, towards designing new potential catalysts.
This presentation consists of the following topics for the cathode and anode catalysts.

Effect of Underlying Co Content on ORR Activity at Pt-Skin/Pt100−x
Co
x
(111) Single Crystal Electrodes4, 5

Because the surfaces of Pt alloy NPs usually consist of low-index facets such as (111), (100), and (110), well-defined alloy single crystals are essential to clarify the effects of the atomic arrangement on the activity. The ORR activities at a series of (111), (100), and (110) facets of Pt100−x
Co
x
single crystals were examined by rotating disk electrodes in 0.1 M HClO4. The Pt-skin/Pt73Co27(111) electrode exhibited the highest value of kinetically-controlled area-specific activity, jk
at 0.9 V, which was about 27 times higher than that on pure Pt(111).4 We clarified the layer-by-layer composition of Pt and Co by in-situ surface X-ray scattering (SXS), together with the modified electronic structure by ex-situ angle-resolved, grazing-incidence XPS (ARGI-XPS).5

Enhancement in the ORR Activity and Durability at Stabilized Pt-Skin–PtCo Catalysts6
−
8

We have successfully prepared PtCo alloy NPs, having one to two atomic layers of stabilized Pt-skin (Pt
x

AL), supported on carbon (PtxAL–PtCo/C). These catalysts exhibited high mass activity for the ORR, together with high durability.6, 7 A dramatic change in the crystal structure of Pt
x

AL–PtCo/C by potential cycles was analyzed.8

Suppression of H2O2 Formation at Pt-Skin–PtCo Anode Catalysts for Mitigation of PEM Degradation9

Polymer electrolyte membranes (PEMs) are degraded by ·OH radicals, generated from H2O2, which is produced by a reaction of hydrogen adsorbed on the Pt anode with O2 diffusing through the PEM. The H2O2 formation at Pt
x

AL–PtCo/C anode was found to be suppressed remarkably with maintaining superlative activity for the HOR.9, 10 We demonstrated a noticeably prolonged lifetime of a Nafion-PEM in a single cell test by using Pt-skin–PtCo/C anode, which was designed to be nearly identical with the in-house catalyst and prepared at 10 gram-scale by TKK.9 The mechanism for the suppression of H2O2 was analyzed by in-situ XAS.
This work was supported by funds from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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