In Situ ATR-FTIR Analysis of the Structure of Nafion-Pt/C Interface By Use of MEA-Type Cell
Authors: Hara Masanori , Kunimatsu Keiji , Watanabe Masahiro , Uchida Hiroyuki 
Published: February 2020

To improve the performance of polymer electrolyte fuel cell (PEFC), fundamental understanding of the catalyst/electrolyte interface under various operating conditions is essential. In particular, the investigation of the adsorption behavior of Nafion electrolyte (or binder) and water on the Pt cathode catalyst surfaces during the cell operation is one of the most important topics for the increase in the mass activity for the oxygen reduction reaction (ORR) of the catalysts and optimizing the operating conditions. Recently, a new MEA-type attenuated total reflectance-Fourier transform infrared (ATR-FTIR) cell was applied to observe a structure of Nafion/polycrystalline Pt, Pt/C or Pt3Co/C interface at room temperature in our laboratory.1,2 We succeeded in analyzing the adsorption of Nafion and water on Pt, as well as an intermediate in the ORR, as a function of potential. In this study, we have examined interactions between Nafion, water, and Pt/C catalysts in MEA-type cell by subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) under a humidified N2 and O2atmosphere at 60 °C.
The experimental setup was described in our previous report in detail.2 The MEA-type ATR-FTIR cell consisted of a Nafion-coated Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo) working electrode prepared on Au thin film/Si plate/ZnSe-ATR prism, a gas diffusion electrode (GDE) as a counter electrode placed on a carbon separator with a gas flow field, and a Nafion NRE211 electrolyte membrane. The Pt loading of working and counter electrodes were 10 μg cm−
2 and 0.5 ± 0.1 mg cm−
2, respectively. A GDE-type reversible hydrogen electrode (RHE) was used as the reference electrode. The SNIFTIRS measurements were conducted with the combination of a potentiostat (HZ-5000, Hokuto Denko), a trigger interface (Varian), and an FTIR spectrometer (FTS6000, Bio-Rad) equipped with a MCT detector. The spectral resolution and angle of incidence were set at 4 cm−
1 and 70°, respectively. A set of SNIFTIR spectra of the Nafion–Pt/C interface was obtained at 60 °C and 100% RH under N2 and O2atmosphere. The single beam spectrum at 0.1 V was chosen as the reference state.
The SNIFTIR spectra obtained on the Nafion coated Pt/C catalysts from 0.2 to 1.0 V in fully humidified N2 atmosphere were shown in Figure 1. Two downward bands around 3500 and 1600 cm−
1 were assigned to the OH stretching mode, ν(OH), and HOH bending mode, δ(HOH), of water molecules, respectively. The δ(HOH) band consists of the broad band of hydrated protons, H+(H2O)n, centered around 1710 cm−
1 and adsorbed water on the catalyst surface, H2Oad, around 1610 cm−
1. Several bands observed between 1500 and 900 cm−
1 were assigned to vibration modes of Nafion. Three upward bands around 1050, 990, and 960 cm−
1 were assigned to the SO3
− symmetric stretching mode, ν(SO3
−), and two C−O−C stretching modes, ether moiety close to the polymer backbone, ν(COC)1, and ether stretching band mechanically coupled to -SO3
−, ν(COC)2, of the Nafion side chain, respectively.3 The bands around 1250 and 1100 cm−
1 were assigned to the CF2 assymmetric and symmetric stretching modes, ν
as(CF) and ν
s(CF), respectively, related to the perfluorocarbon backbone in the main chain and side chain.
Figure 2 shows changes in the band intensities of these species as a function of potential. The intensities of both δ(HOH) and ν(OH) decreased with increasing potential. In contrast, the band intensities for ν(SO3
−) and ν(COC) as well as ν
s(CF) increased with potential. This suggests that the sulfonate group in the Nafion side chain adsorbed specifically on the Pt catalyst surface while expelling H+(H2O)n or H2Oadoriginally adsorbed at less positive potentials.
We have also examined changes in the band intensities for water and Nafion under O2 atmosphere. The intensities for bands of the water molecules during the ORR were larger than those measured in N2. It was found that the ν(SO3
−) band in O2 commenced to increase at less positive potential (ca. 0.3 V) than that observed under N2atmosphere. Our results reveal that the ORR affects the adsorption of Nafion on the Pt catalysts surface.
This work was supported by funds for the HiPer-FC project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

1) K. Kunimatsu, T. Yoda, D. A. Tryk, H. Uchida, and M. Watanabe, Phys. Chem. Chem. Phys., 12, 621 (2010).
2) H. Hanawa, K. Kunimatsu, M. Watanabe, and H. Uchida, J. Phys. Chem. C, 116, 21401 (2012).
3) A. M. Gomez-Marin, A. Berna, and J. M. Feliu, J. Phys. Chem. C, 114, 20130 (2010).

Figure 1