Aggregation States and Proton Conductivity of Nafion in Thin Films

Transcription

1 2 nd Oct Fluoropolymer 2016 New Orleans, USA Aggregation States and Proton Conductivity of Nafion in Thin Films Department of Applied Chemistry and International Institute for Carbon-neutral Energy Research (WPI-I2CNER), Kyushu University Keiji Tanaka

2 Aggregation States and Proton Conductivity of Nafion in Thin Films Introduction Swelling Kinetics Proton Conductivity Conclusions

3 Polymer Electrolyte Fuel Cells separator H 2 polyelectrolyte ex. Nafion separator fuel electrode air electrode O 2 The device performance depends on water sorption in polyelectrolyte. To improve the cell performance, it is crucial to understand the swelling behavior of the polyelectrolyte film. Downsizing PEFC is one of the interesting developments due to portability.

4 T g / K Segmental Dynamics in Supported Thin Polystyrene (PS) Films h = 1.6 nm 2. h = 290 nm h PS SiO x layer Si substrate Film thickness / nm ACS Macro Lett. 1(11), (2012). d interfacial potential:f(d) C Φd Φd 8 vdw d long-range interaction ASiO ASiO A x x Si Φd vdw 2 12πd 12π d h 2 A: Hamaker constant

5 Aggregation States and Proton Conductivity of Nafion in Thin Films Introduction Swelling Kinetics Proton Conductivity Conclusions

6 Sample & Film Preparation Nafion Sigma Aldrich Inc. x = 5 ~ 13.5 y = 1000 SiO x Film thickness (h 0 ): 20, 47, 80 nm Nafion Silver Silver Glass (BK7) SiOx Silver Glass (BK7) SiOx Silver Glass (BK7) Glass (BK7) Deposition Spin-coating 3000 rpm, 60 s 313 K, 20 h under vacuum Deposition Nafion Silver Silver Glass (BK7) Glass (BK7)

7 Surface Morphology on silver on quartz in air 1.0 mm 1.0 mm in water 1.0 mm 1.0 mm R q : Root-mean-square roughness Nafion films were flat enough to be characterized both in air and water.

8 Surface Plasmon Resonance (SPR) p-polarized light Critical angle evanescent wave SPR Resonance angle Soft Matter 7(21), (2011).

9 SPR Curve for a Thin Nafion Film in Water Model used to fit the data Water (n = 1.33) Nafion (n = 1.35) silver BK7 parameters d i / nm Nafion 47 silver 34 d i : thickness of i th layer c 2 = h 0 : initial thickness

10 Reflectivity Time Dependence of SPR Curves s 1200 s 3600 s 6000 s s time Nafion / water h 0 = 47 nm Incident angle / deg. The resonance angle shifted to a higher angle with increasing time. The film became thicker in water with increasing time.

11 Swelling ratio Swelling Kinetics by SPR 1.5 Nafion films on silver (SPR) SiO x (SPR) RegimeⅢ 47 nm Time 10 3 / s RegimeⅡ RegimeⅠ A thin Nafion film thickened in the three steps. The swelling kinetics depended on the substrate.

12 Relation between Swelling Ratio and Hydrated Structures in Bulk bound water ionic cluster ionic channel sulfonic acid group free water water binding to sulfonic acid groups 1.05 Formation of sphere-like ionic clusters 1.26 bridge formation between clusters Yoshida et al. J. Membr. Sci. 68, 1 (1992). Laporta et al. PCCP. 1, 4619 (1999). Gebel et al. Macromolecules, 30, 7914 (1997). Gebel et al.. Polymer, 41, 5829 (2000). Schmidt-Rohr et al. Nat. Mater., 7, 75(2008). Hinatsu et al.. J. Electrochem. Soc., 141, 1493 (1994).

13 Reflectivity Reflectivity Neutron Reflectivity (NR) Soft Interface Analyzer (SOFIA) (BL-16, Japan Proton Accelerator Research Complex) neutron quartz q sample D 2 O conditions Elapsed time :15 min Wavelength (l n ) : 0.2 ~ 1.76 nm Incident angle(q) : 0.3 deg. 4 lsinq q π / q : incident angle q : scattering vector Film thickness 100 nm Film thickness 50 nm Roughness 0 nm 10 nm q / nm q / nm -1

14 NR Curves for Thin Nafion Films The shape of an NR curve strongly depended on the type of substrates.

15 Aggregation States of Nafion at Substrate Interface On silver silver Interfacial layer quartz On quartz Interfacial layers quartz D 2 O Nafion D 2 O Nafion (b/v) 10 4 / nm t = s Film thickness / nm single hydrated layer (b/v) 10 4 / nm t = s Film thickness / nm multi-lamellar hydrated layers

16 Swelling ratio Swelling Kinetics by NR 1.5 Nafion films on silver (SPR) silver (NR) 1.4 SiO x (SPR) SiO x (NR) 1.3 RegimeⅢ Time 10 3 / s RegimeⅡ RegimeⅠ

17 Plausible Swelling Model of a Thin Nafion Film in Water RegimeⅢ : : : bound water free water sulfonic acid group ionic cluster ionic channel RegimeⅡ RegimeⅠ RegimeⅠ RegimeⅡ RegimeⅢ / s dried Time swollen

18 Swelling ratio Water Diffusion Diffusion Coefficients based on Fickian model h h w h h 0, i 0, i 1 n0 8 2n 1 2 exp 2n 1 2 D i t t h 0, i 2 0, i D / cm 2 s cf. D in bulk : cm 2 s 1 RegimeⅠ(silver) RegimeⅡ(silver) RegimeⅢ(silver) 1.5 Nafion films on silver (SPR) silver (NR) 1.4 SiO x (SPR) SiO x (NR) RegimeⅢ RegimeⅡ 1.1 RegimeⅠ(SiO x ) RegimeⅡ(SiO x ) 1.0 RegimeⅢ(SiO x ) Time / s h 0 / nm : : : bound water free water sulfonic acid group RegimeⅠ RegimeⅠ RegimeⅡ RegimeⅢ dried ionic cluster Time swollen ionic channel The water sorption was restricted by the substrate interface.

19 Aggregation States and Proton Conductivity of Nafion in Thin Films Introduction Swelling Kinetics Proton Conductivity Conclusions

20 Conclusions Water sorption kinetics and proton conductivity in thin Nafion films were examined. Three-step swelling associated with structural change was observed. Hydrated layers were formed at the substrate interface. Water sorption was restricted by substrate interface. Out-of- and in-plane proton conductivity decreased and increased with decreasing thickness due to the interfacial hydrated layers, respectively.