Improving oxygen reduction reaction (ORR) activity of cathode catalyst is required for wide spread of polymer electrolyte fuel cells. Because ORR occurs at the interface between cathode catalyst and ionomer (Nafion), understanding the structure of Nafion on the cathode catalyst is important to design reaction field with high activity. The morphology of the Nafion on the cathode is strongly influenced by humidity, thickness, annealing treatment and so on[1-3]. The proton conductivity of Nafion increases with humidification, and the change degree depends on the thickness of Nafion.[3] However, the correlation between morphology and proton conductivity of the Nafion on the cathode have not still been clearly understood. In this study, we examined the correlation between morphology and proton conductivity under humidity of Nafion thin-film with annealing treatment via grazing incidence small angle X-ray scattering (GISAXS) and electrochemical impedance spectroscopy.
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. Annealing treatment was conducted under 160oC under N2 atmosphere for 1 hour for each thin-film prepared by the different methods. 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
The proton conductivity of the Nafion thin-film without annealing treatment was larger than that of the thin-film with annealing treatment under 20-80% relative humidity. Figure 1 shows that 1 D line profiles parallel to the substrate obtained from 2D GISAXS patterns of Nafion thin-films with or without annealing treatment under various relative humidity (RH). For the non-annealed thin-film (Figure 1 (a)), a broad peak, which is attributed to water domain in Nafion [4], appeared around q = 1.5 nm-1. The peak shifted lower q and the intensity increased with humidification. In the annealed thin-film (Figure 1(b)), the peak appeared around higher q (2.0 nm-1) higher than non-annealed thin-film, and the degree of intensity change with humidification was smaller. These results mean that annealing treatment suppresses growth of water domain in Nafion with humidity, which decreases proton conductivity.
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)
[4] A. Kusgglu, T. J. Dursch, A. Z. Weber, Adv. Funct. Mater., 26, 4961 (2016)