Transport Properties of Perfluorosulfonate Membranes Ion Exchanged with Cations

Abstract

In this work, the properties of univalent, that is, Li+, Na+, NH4 +, and TEA+ form perfluorosulfonate (PFSA) membranes are studied and compared to the properties of H+ form materials. Properties of these polymer membranes including water uptake, density and conductivity, were investigated for membranes exposed to various water activity levels. The water uptake by the membranes decreased in the order H+ > Li+ > Na+ > NH4 + > TEA+, the same order as the hydration enthalpy (absolute values) of cations. Conductivity values did not strictly follow this order, indicating the importance of different factors besides the hydration level. The partial molar volume of water is derived from the density data as a function of water content for the various membrane forms. This provides further insight into the water, cation, and polymer interactions. Factors that contribute to the conductivity of these membranes include the size of cations, the electrostatic attraction between cations and sulfonate group, and the ion-dipole and hydrogen bonding interactions between cations and water. NH4 + transport is surprisingly high given the low water uptake in NH4 + form membranes. We attribute this to the ability of this ion to develop hydrogen bonded structures that helps to overcome electrostatic interactions with sulfonates. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was used to measure the diffusion coefficient of water in the membranes. FT-IR spectroscopy is employed to probe cation interactions with water and sulfonate sites in the polymer. Overall, the results reflect a competition between the strong electrostatic interaction between cation and sulfonate versus hydration and hydrogen bonding which vary with cation type.

Ionic Mobility in Ion-Exchange Membranes

“…The energy of interactions of large cesium cations with water molecules is insufficient to destroy the network of hydrogen bonds of water molecules, and the dependence of the 133 Cs chemical shift values on hydration degree is not pronounced [96,98]. This conclusion correlates well with dependences of 1 H chemical shifts on hydration degree [99,100].…”

“…The proton transfer is also facilitated by the change in the location of the short O-H…O bonds through the proton hops, followed by the formation of other short bonds due to the vibrations of the H-bond network. The Grotthuss mechanism explains the higher rate of proton transfer in ion-exchange membranes than that of other cations (Figure 2) [100,[116][117][118][119]. This is also the reason for the violation of the regularity of the change in water diffusion coefficients with a radius of monovalent ion (Figure 1).…”

“…Using 1 H pulsed field gradient NMR, it was shown that at the same hydration degrees of membranes, the water diffusion coefficients decreased in the sequence H + > Ba 2+ > Cs + > Na + > Li + (Figure 1) [99][100][101]. In NMR studies, it is usually called the water self-diffusion coefficient.…”

“…At the same relative humidity, a lower barium content in the membranes results in less water absorption by the membranes. In addition, the activation energy of ion transfer increases sharply when increasing the cation charge, which determines the higher strength of its bonds with the environment [82,100,102].…”

Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices

“…71 There have been several explanations postulated for the observed behavior with Nafion 212 in alkaline electrolytes: the partitioning of ions in the electrolyte does not track the bulk electrolyte for high KOH concentrations; the increased ionic strength more effectively screens the repulsive interactions between the perfluoroalkylsulfonates, constricting the membrane's pores; the dynamics of water and ions in the pores are comparably slow for concentrated liquid electrolytes; or the low degree of phase separation in the KOH, resulting in fewer regions of high hydrophilicity to allow for ion shuttling. 4,[71][72][73][74] Contrasting this behavior, AquaPIM 1 membranes exhibit their highest membrane ionic conductivity for electrolytes whose pH is greater than the pK a of the amidoxime (Figures 5B and 5C). Thus, while AquaPIM 1's membrane ionic conductivity is 0.18 mS cm À1 for the 1.0 M NaCl electrolyte and 0.70 mS cm À1 for the 1.0 M NH 4 Cl electrolyte, its conductivity jumps orders of magnitude to 7.9 mS cm À1 for the 1.0 M KOH electrolyte and advances further to 21.5 mS cm À1 at 5.0 M KOH and eventually backtracks to 16.9 mS cm À1 in 40% aqueous KOH (w/w) due to its higher viscosity.…”