Humidity dependent structure of water at the interfaces between perfluorosulfonated ionomer thin film and Pt and HOPG studied by sum frequency generation spectroscopy

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Abstract

Humidity-dependent water structures at the interfaces between perfluorosulfonate ionomer (PFSI = Nafion) thin film and Pt and HOPG were determined at room temperature by using sum frequency generation (SFG) spectroscopy. At low relative humidity (RH) condition, a peak centered around 3600 cm 1 corresponding to water molecules interacting with sulfonic acid group in proton channel at PFSI surface was observed at both interfaces. However, while this peak increased with RH at PFSI/Pt interface, that at PFSI/HOPG did not increase. These results suggest that the structure of water molecules, especially the water molecules at the proton channel of PFSI surface, is strongly affected by the interaction of PFSI with Pt and HOPG.

Graphical abstract

Highlights

► Water at Pt/PFSI and HOPG/PFSI interfaces were studied by SFG spectroscopy. ► Water mainly exists at sulfonic acid site on the surface of proton channel at low RH. ► Water starts to adsorb even on hydrophobic fluorocarbon carbon site at high RH. ► Ordered water structure was observed at Pt/HOPG interface. ► Disordered water structure was found at HOPG/PFSI interface at high RH.

Introduction

Nafion is a perfluorosulfonate ionomer (PFSI), which contains a fluorocarbon backbone with pendant side chains that terminated with a sulfonate groups and the most commonly used in proton exchange membrane fuel cell (PEMFC) as membrane and ionomer [1], [2]. When the PFSI membrane is hydrated, the water clusters around sulfonate sites while the fluorocarbon domains tend to segregate from the water domains [3], [4]. The hydrophobic properties of the fluorocarbon backbone and the hydrophilic properties of the highly ionic sulfonate site, provides such unique structures. The hydrophilic domain of the PFSI facilitates the uptake and transport of water and proton [5], [6], [7] which strongly affect the performance of a PEMFC. PFSI is used not only as a membrane but also as an ionomer to ensure for the electrode reactions to proceed smoothly at three phase (gas, catalyst, and ionomer) boundary and, therefore, the molecular level understanding of water structure not only within the PFSI membrane but also at the interface between PFSI and gas, Pt (catalyst), and carbon (support and electron collector) is essential to clarify how water affect the cell performances. Behavior of water at gas–catalyst–ionomer interfaces on and within PFSI is strongly dependent on humidity. Thus, understanding of the hydration behavior of PFSI membrane/thin film under various conditions is a subject not only of scientific interest but also of great importance for the development of PEMFC.

Sum frequency generation (SFG) spectroscopy has been proved to be a powerful spectroscopic method to investigate the structure of water molecules at various interfaces [8], [9], [10], [11] because the second order nonlinear processes including SFG take place only at the interface where the inversion symmetry is necessarily broken.

Recently, we have reported the relative humidity (RH) dependent water structure at PFSI thin film surface by SFG spectroscopy [11]. At low RH environment, two main peaks were observed. One is a peak at 3720 cm 1 due to the “dangling OH” of water molecules adsorbed on PFSI surface. Since the nature of PFSI surface is hydrophilic due to Csingle bondF bond in the main chain, water molecules are not able to form hydrogen bond network with PFSI. Other is a peak at 3600 cm 1, which was assigned to water molecules interacting with sulfonic acid groups in proton channel at the PFSI surface. Intensities of these peaks increased with RH and a broad peak centered around 3200–3300 cm 1 due to water adsorbed on hydrophobic, i.e., fluorocarbon, sites of PFSI was observed when the PFSI thin film was exposed to a high RH environment (RH > 60%).

Since the electrode reactions take place at the interfaces between PFSI and gas, Pt, and carbon, humidity dependent water structure at the interfaces between PFSI and both Pt and HOPG were examined by SFG spectroscopy in this communication. Effect of the interaction of PFSI with Pt and HOPG on to the structure of interfacial water is discussed.

Section snippets

Experimental

After the quartz prism was cleaned by “piranha” solution (3:1 concentrated H2SO4 and 30% H2O2) for 30 min, and then rinsed thoroughly with ultrapure water (Millipore 18.2  cm resistivity), PFSI thin film was formed on the flat face of an IR grade hemicylindrical-shaped quartz prism for the SFG measurements by spraying PFSI solution (Dupon, DE1020; ~ 11 wt.% perfluorosulfonic acid aqueous solution) on the prism, which was being rotated at 2000 rpm. The thickness of the PFSI film was determined by

Results and discussion

Fig. 2(A) shows the SFG spectra in OH stretching region at the PFSI thin film/Pt interface measured at different RH conditions. At RH = 0% (Fig. 2(A)-(a)), no SFG signal due to water molecules on PFSI surface was detected suggesting that no water molecules exist at PFSI/Pt interface. When RH was increased to 37% (Fig. 2(A)-(b)), SFG peak centered around 3600 cm 1 (peak A) and a broad peak centered around 3400 cm 1 (peak B) were observed. The intensity of these peaks increased and peak B showed a

Conclusions

In summary, humidity-dependent water structure at the PFSI/Pt and PFSI/HOPG interfaces were studied at room temperature by using SFG spectroscopy and schematic models for the RH-dependent structures of these interfaces are proposed based on the experimental results. At low RH condition, a peak centered around 3600 cm 1 corresponding to water molecules interacting with sulfuric acid group in proton channel at PFSI surface was observed at both interfaces. When RH was increased, the amount of water

Acknowledgments

This work was partially supported by a Grant-in-Aid for Scientific Research (C) (No. 23550033), Japan Science and Technology Agency (JST), PRESTO, and World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, and a program for Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

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