Direct observation of hydration of a Nafion membrane using APXPS and AIMD simulation

The hydration of perfluorinated sulfonic-acid ionomers is the most important phenomenon that determines their transport and electrical properties. To bridge the gap between the macroscopic electrical properties and the microscopic water-uptake mechanism, we investigated the hydration process of a Nafion membrane using ambient-pressure x-ray photoelectron spectroscopy (APXPS) from vacuum up to ∼90% relative humidity at room temperature. The O 1s and S 1s spectra provided a quantitative analysis of the water content (λ) and the transformation of the sulfonic acid group (-SO₃H) to its deprotonated type (-SO₃ ^-) during the water-uptake process. Taking advantage of a specially designed two-electrode cell, the conductivity of the membrane was determined by electrochemical impedance spectroscopy before APXPS measurements with the same conditions, thereby establishing the connection between the electrical properties and the microscopic mechanism. By means of ab initio molecular dynamics simulations based on density functional theory, the core-level binding energies of O- and S-containing species in the Nafion + H₂O system were obtained.