Thickness Dependence Impact on Proton Transport Proton Properties of Nafion® Thin-Film on Platinum Model Electrode

Abstract

Introduction Improving oxygen reduction reaction (ORR) activity of cathode catalyst is required for wide spread of polymer electrolyte fuel cells. Because ORR occurs at the three-phase-boundary among the cathode catalyst, ionomer and oxygen gas, understanding the impact the ionomer on the cathode catalyst is important to achieve the high performance PEFCs. Especially, the structure of the ionomer on the cathode is strongly influenced by humidity, thickness, annealing treatment [1-3]. In this study, we examined the proton conductivity the Nafion® thin-film with various thickness from 5 to 200 nm. The correlation between structure and proton conductivity of the Nafion on Pt electrode was discussed. Experimental For proton conductivity measurements, Nafion thin-films were prepared on interdigitated array of gold electrodes on SiO2 substrate by soaking the electrode on Nafion dispersion. For GISAXS measurements, Nafion thin-films were prepared on Pt sputtered P doped-silicon substrates by spin casting of Nafion dispersion. Proton conductivity under humidity was measured by applying an alternating potential of amplitude 100 mV over a frequency 7 MHz to 0.01 Hz. GISAXS measurements were performed under humidity in beamline BL40B2 at SPring-8, Japan. Results and Discussion In the Nafion thin-films with thickness more than 50 nm, EIS results indicated that the hydrophilic domains grew gradually as the thickness increases. In contrast, the hydrophilic domains, which works as the protonic path, grows larger with increase of the thickness, leading to reduce the activation energy of proton conductivity. According to GISAXS, the hydrophilic domain disappears in the Nafion thin-film with thickness less than 50 nm. These results indicate that the structure of Nafion thin-film drastically changes with the thickness. It is anticipated that the proton conductivity of the in-plane in the Nafion film is larger than that of the out-of-plane because the hydrophilic domain of the in-plane grows larger than that of the out-of-plane. The anisotropy of the ordered structure in the Nafion thin-film is enhanced by the interaction between the ionomer and substrate. These results indicate that the proton conductivity in the Nafion is largely influenced by the amount of the hydrophobic domain. Acknowledgments This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference: [1] A. Kusoglu, D. Kushner, D. K. Paul, D. Karan, M. Hickner, A. Weber, Adv. Funct. Mater., 24, 4763 (2014) [2] D. K. Paul, H. K. Shim, J. B. Giorgi, K. Karan, J. Polym. Sci. B Polym. Phys., 54, 1267 (2016) [3] D. K. Paul, R. McCreery, K. Karan, J. Electrochem. Soc., 161, F1395 (2014)