Activity Effects in Nafion: A Common Model to Characterize Numerous Physical Properties

Johna Leddy; Nadeesha P. W. Rathuwadu

doi:10.1149/MA2026-01492422mtgabs

Among its extraordinary properties, the ion exchange polymer Nafion (R) is superselective for cations and sustains proton flux of several A cm -2 . Nafion, a perfluorosulfonic acid polymer, forms a robust nanostructure as low dielectric constant (~ 2) fluorocarbon segregates from high dielectric (~80) water filled domains. As superacids, perfluorosulfonic acids are fully dissociated in water. For Nafion 1100 with density of 1.8 g cm -3 , the ionic strength in the ~ 5 nm water filled domains is > 5 M, where ions are separated by a few water molecules. [1] Nafion is widely used as membranes and cast films, and its physical properties are broadly measured. This includes Nafion density and equivalent weight as well as thermal, mechanical, electrical, and optical properties; swelling in organic solvents; gas solubilities; cation selectivity; and ionic conductivity and mobility. [1,2] Data are typically reported across a range of cations, or solvents. At high to extreme ionic strength, electrochemical activities of ion i , a i , differ substantially from the concentrations of i , [ i ]. Electrochemical potentials of thermodynamics use activity coefficient i to describe the relationship as a i = i [ i ]. For low ionic strength ( ≲ 0.1 M), extended Debye Huckel (DH) models for electrostatic interactions specify i . At > 0.1 M, DH equations are insufficient. Stokes and Robinson (SR) [3] envisioned the difference in concentration and activity as concentration [ i ] is moles of i per total mass of water and activity a i is moles of i per total mass of free water. Ions carry waters of hydration. Water molecules bound to ions are not free. Bound waters decrease free water and increase activity. At low , free and total waters are the same as the fraction bound is negligible and DH applies. As increases toward 5 M, only a small fraction of water is free and a i far exceed [ i ] as i > 1. The SR model is applied to Nafion measured properties of selectivity, solvent swelling, gas solubility, decomposition temperatures, and ion conductivity for 9 monocations to find the number of bound waters of hydration, n j . For all properties and 7 of 9 cations, n j is fixed within < 10 % relative error. n j ranges between 2.0 for Cs + and 4.3 for Ag + . Two distinct n j values are found for H + (4.6 and 1.9) and Li + (5.3 and 3.2). Lithium exchanged Nafion swells (ΔV/V) in nonaqueous solvents where ΔV/V varies linearly with DH / SR as ΔV/V (%) = (381 ± 7) D H / SR - (79 ± 4); n Li = 3.4 ± 0.7 for 29 solvents. Free and bound solvent impacts activity (SR). Activity well models numerous physical properties of Nafion across various cations and solvents. The SR model of Nafion activity allows control and design of specific Nafion properties. Acknowledgments: This work was supported by NSF and ARO. References Johna Leddy, Modification of Nafion Membranes: Tailoring Properties for Function, Chapter 6, in Nanomaterials for Sustainable Energy, ACS Symposium Series, Vol. 1213, (2015) Jingbo Louise Liu and Sajid Bashir, Ed. American Chemical Society, Washington, DC, pp 99–133, DOI: 10.1021/bk-2015-1213.ch006 H. Yeager, Transport Properties of Perfluorosulfonated Polymer Membranes. In Perfluorinated Ionomer Membranes; Yeager, H., Eisenberg, A., Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. R. H. Stokes and R. A. Robinson, Ionic Hydration and Activity in Electrolyte Solutions, Journal of the American Chemical Society 70 (5) (1948) 1870-1878.

Activity Effects in Nafion: A Common Model to Characterize Numerous Physical Properties

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