States of water in recast Nafion® films

• Published in: Journal of membrane science 2021-11, Vol.637, p.119645, Article 119645
• Main Authors: Ren, X., Gobrogge, E., Beyer, F.L.
• Format: Article
• Language: English
• Subjects: FTIR spectroscopy; Recast Nafion films; States of water; Water hydrogen-bonding network; Water-ionic clusters
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2021.119645

The amount and states of water in a polymer electrolyte significantly affect the material's morphology as well as its mechanical and transport properties, and thus determine performance in many important applications such as fuel cells, electrolyzers, and other electrochemical energy conversion devices. In this study, recast H+-Nafion® films were characterized using attenuated total internal reflection FTIR (ATR-FTIR) spectroscopy before and after thermal annealing and subsequent rehydration. The states of water in the hydrated films were determined by analyzing the O–H stretching band of water using principle component analysis and multivariate curve resolution. It was found that annealing at a temperature above the glass transition temperature caused the water hydrogen-bonding networks in the water-ionic clusters to become restricted, suggesting that annealing forms a robust polymer matrix confining the water-ionic clusters. The evolution of film morphology with increasing thermal-annealing temperature was confirmed with small-angle X-ray scattering (SAXS) and CO2 diffusion measurements. Fuel cell performance tests using annealed and unannealed Nafion® in the cathode catalyst layer found improved stability when the cathode catalyst was annealed, indicating that thermal history is an important design parameter in electrochemical energy conversion device fabrication. [Display omitted] •Three water states were identified in triflate acid solutions at a concentration