Pt/Ionomer Interface Analyses with the Combination of Voltammetry and Spectroscopy

In catalyst layers of polymer electrolyte fuel cells, a solid polymer electrolyte, called “ionomer” for distinguishing it from the electrolyte membrane, is generally used to form good electrochemical interfaces on catalyst surfaces. However, model experiments using Pt single crystal electrodes covered with ionomer films suggested that sulfonate anion in perfluorinated sulfonic acid polymer (e.g. Nafion, with the side chain of OCF₂CF(CF₃)O(CF₂)₂SO₃H) blocks active sites on Pt catalyst and therefore, slows down the kinetics of oxygen reduction reaction (ORR) [1]. Recently, the catalyst poisoning was found to be weakened by using an ionomer with different anionic structure (NBC4, having two sulfonimide acid groups in its side chain ended with a perfluorobutane, instead of a sulfonic acid group), as shown by broader and smaller anion-related peaks (0.4 – 0.6 V) in the cyclic voltammogram (CV) of Pt (111) coated with NBC4 (black dashed curve in Fig. 1a) than that with Nafion (gray solid curve in Fig. 1a) [2]. In the present study, we report that the anion adsorptivity can be also controlled even by changing the structures of perfluorinated side chain of sulfonic acid ionomer.

                   In Fig. 1a, the black solid curve is the CV for a sulfonic acid ionomer with short side chains (Aquivion, with the side chain of O(CF₂)₂SO₃H). Although the peaks due to the adsorption and desorption of sulfonate anion are sharp, the peak potentials are higher and the peak intensities are weaker than those for the Nafion-coated Pt. Thus ionomer with the shorter side chain is less adsorptive on Pt surface.

                   Fig. 1b shows the CVs for a bare Pt (111) single crystal electrode in low-molecular-weight perfluoro-sulfonic acids of C₄F₉SO₃H (nonafluorobutane sulfonic acid, NFBSA) and C₂F₅OC₂F₄SO₃H (Perfluoro (2-ethoxyethane) sulfonic acid, PES). The CVs show reduction current due to residual oxygen and ill-defined features probably due to impurities. The Pt (111)-like H_upd plateaus are, however, seen between 0.07 – 0.4 V and anion-related peaks are observed (dashed circles). The potentials of the anion adsorption/desorption peaks are lower and the peak areas are larger for PES than for NFBSA.  In addition, the surface enhanced infrared absorption (SEIRA) spectra for a Pt polycrystalline film in the electrolyte of NFBSA showed the band for adsorbed sulfate anion on Pt at 0.4 – 0.7 V (Fig. 2a, the shaded region at ca. 1100 cm^-1) and therefore, the anion adsorption peak in the CV of Pt (111) may be not due to the adsorption of NFBSA anion itself, but due to the adsorption of sulfate anion as impurity, which was found to be contained with non-negligible concentration, <3000 ppm (corresponding to <1 mM for the 0.1 M electrolytes) in these types of chemical. (In this case, the other positive bands at 1066, 1140, 1250 and 1350 cm^-1in the SEIRA spectra can be ascribed to the orientations of NFBSA molecules to more IR-active states, not to their adsorptions on the Pt surface.) In contrast, because the SEIRA spectra for PES do not show the clear band due to adsorbed sulfate anion (Fig. 2b), the anion adsorption peak in the CV can be ascribed to the adsorption of PES anion itself. Therefore, presumably, the PES anion is much more adsorptive on Pt than NFBSA anion is, and the presence or absence of the ether group in the perfluoro-sulfonate anions affects the adsorptivity of the anions.

References

[1]R. Subbaraman et al., ChemPhysChem, 11(2010) 2825.

[2]K. Kodama et al., J. Electrochem. Soc. 161(2014) F649.