The development of proton exchange membrane fuel cells (PEMFCs) have received increasing attention for its potential to get rid of dependence on fossil fuels and curb carbon emissions. However, the application of PEMFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum resources. It is well-known that the oxygen species generated on the Pt surface hiders ORR reaction at high potential region. Therefore, it is very important to evaluate the interaction between the oxygen species and the catalyst based on the electronic structure. However, the differences in XAFS spectra due to the presence of oxygen species are relatively small, and conventional detection methods, such as transmittance and total fluorescence yield, have difficulty distinguishing differences in electronic structure because the fine structure of the spectra is filled in by the lifetime width of the Pt 2p 3/2 core hole. In contrast, the new high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) monochromatized fluorescence lines (e.g., Pt Lα1), which are broadened only by Pt 3d 5/2 (~2.4 eV) rather than by Pt 2p 3/2 (~5.2 eV) core hole lifetime, are used to monitor core High-resolution X-ray absorption spectra with extended hole lifetimes can now be extracted. The present study covers conventional platinum catalysts supported on Vulcan carbon and TOYOTA MIRAI Gen.1 catalysts. The HERFD-XAS method was applied to elucidate the cause of the superior electrochemical properties of the TOYOTA MIRAI Gen.1 catalyst. operando Pt L_III edge HERFD-XAS measurements were performed on Pt/C and TOYOTA MIRAI Gen.1 catalysts under oxygen reduction conditions, and the high energy resolution revealed the interaction of oxygen species with the Pt surface.

Pt/C (29.1 wt %; TEC10V30E, average particle size of 3 nm, observed by TEM) catalyst was purchased from TKK Co. Ltd., Japan. The Pt/C catalyst ink contained water and 2-propanol in a 3:2 ratio (99.99%, FUJIFILM Wako Pure Chemical Corporation, Japan), and the amount of Pt/C was adjusted to form a 20 μg carbon cm^–2 catalyst layer after dropping 10 μL on a glassy carbon RDE (GC RDE, HOKUTO DENKO, Japan; 0.196 cm² area). TOYOTA MIRAI Gen.1 catalyst was also prepared by the above-mentioned preparation method. operando XAS measurements for Pt L_ΙΙΙ-edge of the catalysts were carried out at BL39XU of SPring-8, Hyogo, Japan. Prior to the experiments, the samples were electrochemically cleaned by CV in the range 0.0–1.1 V (vs. RHE). The electrode potential was swept cathodically from 0.5 to 1.1 V (vs. RHE).

Figure 1 shows the results for in situ HERFD-XAS spectra of Pt and MIRAI Gen.1 catalyst. For both samples, the white line intensity changes depending on the adsorption conditions. The obtained HERFD-XAS spectra were decomposed into oxidized (Pt-OH) and reduced (Pt metal) components. The ratio of the peak area intensity of the two components is proportional to the surface coverage assuming similar charge transfer among the catalysts. It is noted that enhancement of oxidation is significantly suppressed on MIRAI Gen.1 catalysts.

Figure 1. operando HERFD-XAS spectra for (a) TOYOTA MIRAI Gen.1 catalyst. (b) Pt/C (TEC10V30E)

Acknowledgement

This work was supported by a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).