Hydration and Mobility of Alkaline Metal Cations in Sulfonic Cation Exchange Membranes

Abstract

The interconnection of ionogenic channel structure, cation hydration, water and ionic translational mobility was revealed in Nafion and MSC membranes based on polyethylene and grafted sulfonated polystyrene. A local mobility of Li+, Na+ and Cs+ cations and water molecules was estimated via the 1H, 7Li, 23Na and 133Cs spin relaxation technique. The calculated cation and water molecule self-diffusion coefficients were compared with experimental values measured using pulsed field gradient NMR. It was shown that macroscopic mass transfer is controlled by molecule and ion motion near sulfonate groups. Lithium and sodium cations whose hydrated energy is higher than water hydrogen bond energy move together with water molecules. Cesium cations in possession of low hydrated energy are directly jumping between neighboring sulfonate groups. Cation Li+, Na+ and Cs+ hydration numbers (h) in membranes were calculated from 1H chemical shift water molecule temperature dependences. The values calculated from the Nernst–Einstein equation and the experimental conductivity values were close to each other in Nafion membranes. In MSC membranes, calculated conductivities were one order of magnitude more compared to the experimental ones, which is explained by the heterogeneity of the membrane pore and channel system.

Understanding Monovalent Cation Diffusion in Negatively Charged Membranes and the Role of Membrane Water Content

“…Several studies assert that counter-ion dehydration during the partitioning step plays a critical role in the ion transport mechanism in charged polymer membranes. , There is no evidence of such dehydration for the membranes in the present study, although it is possible that dehydration could occur in membranes that have even lower water content than SPM-GDMA(31). These results agree with recent measurements of hydration numbers in IEMs via NMR, , which demonstrate that counter-ion hydration numbers in several water-equilibrated IEMs are similar to those in solution. Notably, dehydration of Li + ions in a Nafion membrane was only observed at λ values well below 10 .…”

“…These two length scales can be quite different, as the diameter of a water molecule is approximately 2.8 Å, , while the distance between charged groups in typical IEMs is on the order of 10 Å. Assuming a mean jump distance for the ions equivalent to the diameter of a water molecule is more plausible than the alternative choice. ,,, Consequently, we investigated the ability of Mafé’s model to explain the experimental results in Figure A.…”

Water Molecules’ and Lithium Cations’ Mobility in Sulfonated Polystyrene Studied by Nuclear Magnetic Resonance

“…In the last decade, a number of works have appeared regarding the ionic conductivity and self-diffusion of water molecules, hydrated H + cations, alkali metal ions Li + , Na + , Cs + in Nafion membranes [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ], membranes on the basis of polyethylene with grafted sulfonated polystyrene (MSC) [ 8 , 15 ], poly(vinylidene fluoride)-graft-polystyrene sulfonic acid polymer electrolyte membranes [ 16 ], poly(vinyl alcohol)–poly(styrene sulfonic acid) blend membranes [ 17 ], ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid membranes [ 18 ], chlorosulfonated polyethylene, sulfonated polysulfone membranes [ 19 ], radiation-grafted fluoropolymers with similar poly(styrene sulfonic acid) [ 20 ], and block copolymer membranes containing sulfonated polystyrene blocks [ 21 ].…”

“…The activation energies of cations’ and water molecules’ self-diffusion in the temperature range from 20 °C to 80 °C, and are close to each other (16–18 kJ/mol). The self-diffusion coefficients in Nafion membranes are arranged in the sequence Li + ≥ Na + > Cs + [ 8 , 15 ]. This change in the translational mobility of cesium cations is explained by the fact that, in contrast to MSC membranes, in the Nafion membrane, the Cs + cation and sulfo groups form contact ion pairs, which in particular are evidenced by the lower number of hydrated cations and the higher self-diffusion activation energy in Nafions.…”

“…The structure of membranes based on sulfonated polystyrene is inhomogeneous, as usual. These membranes’ transport pathway contains hydrated pores connected through narrow channels, and macroscopic transfer is limited by particles passing through these channels [ 15 ]. Li + and Na + cations have high hydration energy.…”