Abstract
The percolation threshold governs transport in perfluorosulfonic acid (PFSA) membranes by marking the for mation of an interconnected ionic network that enables efficient transport of solvent molecules and ions. Determining this threshold is critical for designing efficient membrane-based separation and electrochemical processes. In this work, ammonia is used as the solvent, with Nafion 117 and Aquivion as representative PFSA membranes. Both membranes demonstrated stability in ammonia atmosphere. An electrochemical pressure-
decay apparatus was developed to measure ammonia solubility, apparent diffusion coefficient, and through-plane ionic resistivity under pure ammonia atmospheres at 20, 35, and 50
◦C. When expressed as a function of the ammonia-to-sulfonic acid group molar ratio (λ), the diffusivity and ionic resistivity data obtained at the three temperatures mostly collapsed onto master curves. A simplified resistivity model applied to the ionic resistance master curve yielded an experimental upper bound for the percolation threshold of λ ∼ 4 4.5.
Molecular dynamics (MD) simulations of the ammonia/Nafion and ammonia/Aquivion systems were performed over λ = 2 7 to analyse ammonia clustering, network formation, and pore-size distributions. The simulations revealed a clear sharp transition in the interconnected ammonia network around λ = 4 5, in good agreement with the experimentally determined threshold. The combined experimental and computational approach provides a robust framework for identifying percolation thresholds in PFSA membranes exposed to swelling solvents.