Hydration and Diffusion of H+, Li+, Na+, Cs+ Ions in Cation-Exchange Membranes Based on Polyethylene- and Sulfonated-Grafted Polystyrene Studied by NMR Technique and Ionic Conductivity Measurements

Abstract

The main particularities of sulfonate groups hydration, water molecule, and alkaline metal cation translation mobility were revealed by nuclear magnetic resonance (NMR) and ionic conductivity measurements techniques in cation-exchange membranes MSC based on cross-linked sulfonated polystyrene (PS) grafted on polyethylene with ion-exchange capacity of 2.5 mg-eq/g. Alkaline metal cation hydration numbers (h) calculated from temperature dependences of 1H chemical shift of water molecule for membranes equilibrated with water vapor at RH = 95% are 5, 6, and 4 for Li+, Na+, and Cs+ ions, respectively. These values are close to h for equimolar aqueous salt solutions. Water molecules and counter ions Li+, Na+, and Cs+ diffusion coefficients were measured by pulsed field gradient NMR on the 1H, 7Li, 23Na, and 133Cs nuclei. For membranes as well as for aqueous chloride solutions, cation diffusion coefficients increased in the following sequence: Li+ < Na+ < Cs+. Cation and water molecule diffusion activation energies in temperature range from 20 °C to 80 °C were close to each other (about 20 kJ/mol). The cation conductivity of MSC membranes is in the same sequence, Li+ < Na+ < Cs+ << H+. The conductivity values calculated from the NMR diffusion coefficients with the use of the Nernst–Einstein equation are essentially higher than experimentally determined coefficients. The reason for this discrepancy is the heterogeneity of membrane pore and channel system. Ionic conductivity is limited by cation transfer in narrow channels, whereas the diffusion coefficient characterizes ion mobility in wide pores first of all.

Ionic Mobility in Ion-Exchange Membranes

“…Among them, the most stable are H 5 O 2 + and H 9 O 4 + ions, which are formed stepwise during membrane hydration. Their formation is confirmed by 1 H NMR data [ 72 , 107 , 108 , 109 , 110 ]. The authors of [ 111 ] reported a significant change in the properties of Nafion membranes with an increase in their hydration degree to above three water molecules per functional group.…”

“…The correctness of these conclusions is proved by the fact that the diffusion coefficients for aqueous solutions of alkali metal chlorides, determined using both methods, are equal and close to the values found by NMR for membranes samples [ 107 ]. It should also be mentioned that the type of ion-exchange membrane significantly affects the ratio of the diffusion coefficients measured using NMR spectroscopy and conductivity [ 133 ].…”

“…The self-diffusion coefficients of lithium, sodium, and cesium cations in grafted membranes with high water uptakes, determined using PFG-NMR spectroscopy data, approach those of aqueous solutions [ 107 ]. The character of their change correlates well with that for the diffusion coefficients calculated from the conductivity using the Nernst–Einstein equation [ 107 , 129 ]. However, their values are more than an order of magnitude higher than those calculated from conductivity [ 107 ].…”

“…Thus, the self-diffusion coefficients obtained from NMR data characterize ions in large pores. The lower diffusion coefficients calculated from the conductivity obviously characterize the mobility of cations moving in narrower channels connecting these pores [ 107 ]. These data unambiguously indicate that the ionic conductivity of membranes is limited by the transfer of ions in narrower channels connecting the pores.…”

“…This model was developed based on the combination of NMR data and ionic conductivity of hybrid membranes [ 172 ]—e.g., conductivity of perfluorinated sulfonic acid MF-4SK membranes doped with a small amount of different oxides increases, while diffusion coefficients of proton-containing groups, determined by the PFG-NMR technique, increase in some cases, while others decrease compared to the pristine MF-4SK membrane. This emphasizes that the increase in conductivity cannot be attributed to the properties of the intraporeal solution, but to the expansion of the channels connecting the pores [ 107 , 172 ]. With an increase in the nanoparticle size, the elastic forces of the pore walls, caused by their deformation during expansion, increase according to the Hooke law, and the osmotic pressure becomes insufficient for their further expansion.…”