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

Vitaliy I Volkov; Alexander V Chernyak; Daniil V Golubenko; Vladimir A Tverskoy; Georgiy A Lochin; Ervena S Odjigaeva; Andrey B Yaroslavtsev

doi:10.3390/membranes10100272

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

Membranes (Basel). 2020 Oct 1;10(10):E272. doi: 10.3390/membranes10100272.

Authors

Vitaliy I Volkov^{1

2}, Alexander V Chernyak^{1

2}, Daniil V Golubenko³, Vladimir A Tverskoy⁴, Georgiy A Lochin^{1

5}, Ervena S Odjigaeva^{1

5}, Andrey B Yaroslavtsev³

Affiliations

¹ Institute of Problems of Chemical Physics RAS, Chernogolovka 142432, Russia.
² Scientific Center in Chernogolovka RAS, Chernogolovka 142432, Russia.
³ Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow 119991, Russia.
⁴ Lomonosov Institute of Fine Chemical Technologies, MIREA-Russian Technological University, Moscow 119571, Russia.
⁵ Faculty of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia.

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.

Keywords: diffusion coefficient; hydration number; ionic conductivity; pulsed field gradient NMR; sulfonic cation-exchange membrane.