In Situ ATR-FTIR Analysis of the Structure of Nafion-Pt/C Interface By Use of MEA-Type Cell

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, H₂O_ad, 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 H₂O_adoriginally 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).