The critical importance of catalyst layer ionomer in numerous transport phenomena (proton conduction, oxygen and water transport) and also in catalyst utilization at multiple length scales is well acknowledged [1]. Thickness-dependent properties such as proton conductivity, water uptake, swelling and water diffusivity have now confirmed that ionomer thin films possess properties significantly different than those of the bulk ionomer materials [2]. Previous work from our group, both independent and collaborative, confirmed suppression of proton conductivity in thin films [3], wettability changes with film thickness and thermal annealing [4], influence of substrate on internal structure [5] and water uptake [6], similarity of T2-relaxations in proton NMR of thin and thick films [7] and recent discovery of temperature-dependent vibrational mode of Nafion ionomer [8].

Tremendous progress made on the topic of ionomer thin films over the past decade has been summarily presented in a recent review [2]. However, many aspects of ionomer thin film structure and properties still remain unknown. For example, despite the confirmation of strong thickness-dependence of ionomer properties for film below 50 nm thickness, one question continues to elude us: what is the critical thickness below which ionic domains cannot be sustained? In a quest to answer this question, we have employed high-resolution electron microscopy to deduce the presence/absence of ionic domains in 4-30nm thin films. There is indication of a critical thickness below which domains are evidently not present.

Recent neutron reflectometry measurements from our group have focussed on the quantification of interfacial water (Pt/ionomer interface) and water in the bulk phase of the ionomer thin films on platinum substrate. The results acquired to date indicate that both interfacial and bulk water are dependent on the ionomer molecular structure. Complementing these measurements with GISAXS, ellipsometry, QCM and impedance spectroscopy – a clearer picture of the ionomer structure-property is emerging.

The talk will share these new results and discuss the implications of the findings on fuel cell behaviour.

Reference:

1. Karan, Current Opin. Electrochem. 5 (1), (2017) 27-35.
2. Kusoglu, A.Z. Weber Chem Rev, 117 (3) (2017), 987-1104
3. K. Paul, R. McCreery, K. Karan, J Electrochem Soc, 161 (14) (2014), F1395-F1402
4. K. Paul, H.K.K. Shim, J.B. Giorgi, K. Karan, J Polym Sci Part B: Polym Phys, 54 (13) (2016), 1267-1277
5. Kusoglu, D. Kushner, D.K. Paul, K. Karan, M.A. Hickner, A.Z. Weber, Adv Funct Mater, 24 (2014), 4763-4774
6. K. Shim, D.K. Paul, K. Karan, Macromolecules, 48 (22) (2015), 8394-8397
7. NE De Almeida, DK Paul, K Karan, GR Goward, J Phys Chem C 119 (3), (2015) 1280-1285
8. VO Kollath, K Karan, Physical Chemistry Chemical Physics 18 (37), (2016) 26144-26150