For residential polymer electrolyte fuel cells (PEFCs) operated with H₂-rich (reformate) fuel gas, the hydrogen oxidation reaction (HOR) at Pt catalysts is severely poisoned even by trace amounts of CO contained in the reformate due to a strong blocking of active sites. So far, Pt-Ru alloy catalysts have been used to mitigate such CO poisoning. However, the HOR mass activity at Pt-Ru alloys is not sufficiently high, so that a large amount of precious metals (Pt and Ru) must be used. In addition, since Ru is not stable at high potentials (≥ 0.8 V), leaching into the acidic electrolyte membrane, the anode must be protected so as not to be oxidized by air.

Recently, we have developed a Ru-free hydrogen anode catalyst by forming a stabilized Pt-skin (one to two atomic layers: xAL) on PtM-alloy (M = Fe, Co, and Ni) nanoparticles supported on carbon black (Pt_xAL‒PtM/C).^1,2 These new catalysts exhibited a high HOR activity, in both the presence and absence of CO, as well as a robustness.^3,4 The apparent values of mass activity (MA_app) for the HOR on Pt_xAL‒PtFe/C at 20 mV vs. RHE in H₂-saturated 0.1 M HClO₄ solution were 2‒3 times larger than those for a commercial Pt₂Ru₃/C (c-Pt₂Ru₃/C) catalyst in the absence and presence of adsorbed CO (CO_ad) at 70 °C as shown in Fig. 1. The retention of MA_app on these Pt_xAL-PtM/C catalysts after an accelerated durability test (ADT) simulating daily start-stop cycles repeated exposure to the reformate gas and air (2,500 potential cycles between 0.02 V and 0.95 V) in N₂ purged-0.1 M HClO₄ solution at 70 ^oC was as high as ca. 80 % as shown in Fig. 2. The apparent area-specific activity values at the Pt_xAL‒PtM/C remained unchanged during ADT. This suggests that the dealloying of the M component in the Pt_xAL‒PtM/C was suppressed by the stabilized Pt-skin layer.

To clarify the mechanism of such an enhancement of HOR activity and CO-tolerance for the Pt_xAL‒PtM/C, we performed multilateral-analyses, by in-situ attenuated total reflection Fourier transform infrared reflection-adsorption spectroscopy (ATR-FTIRAS), density functional theory (DFT) calculations, and in-situ X-ray absorption spectroscopy (XAS) techniques. It was found that the adsorption energy of CO on terrace sites on the Pt_xAL skin covering the PtM alloy was weakened by the modified electronic structure. The HOR activity of Pt_xAL‒PtFe/C with pre-adsorbed CO was found to recover appreciably after bubbling with CO-free pure H₂. Such a recovery can be ascribed to an increase in the number of active sites by a transfer of linearly adsorbed CO_ad at terrace sites (CO_{L, terrace}) to step/edge sites (CO_{L, step/edge}) on the surface of the Pt-skin layer, without removal of CO_ad from the surface.^4,5 We will also discuss possible mechanisms for the CO-tolerant HOR.

This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The synchrotron radiation experiments were performed at BL14B2 (Proposal Nos. 2015B3388 and 2016A1805) and BL16B2 (2016A5390 and 2016B5390) of SPring-8 in cooperation with the Device-Functional Analysis Department of NISSAN ARC Ltd.