Self-Diffusion of Ions in Nafion-117 Membrane Having Mixed Ionic Composition

Abstract

The self-diffusion coefficients (SDCs) of Na(+), Cs(+), and Ba(2+) have been determined in Nafion-117 membrane having mixed cationic compositions. Membranes with different proportions of Na(+)-Cs(+), Cs(+)-Ba(2+), Na(+)-Ba(2+), and Ag(+)-Ba(2+) cations have been prepared by equilibrating with solutions containing different ratios of these cations. The SDCs of the cations (D(Na), D(Cs), D(Ba)) and the ionic compositions of the membrane have been determined using a radiotracer method. For the Na-Cs and Cs-Ba systems, the SDCs of the cations have been found to be independent of the ionic compositions of the membrane. In the case of the Na-Ba system, D(Na) does not change with ionic composition, while D(Ba) has been found to be strongly dependent on the ionic composition of the membrane and decreases continuously with increasing Na(+) content in the membrane. Similar results have also been obtained for D(Ba) in the case of the Ag-Ba system. The specific conductivities (κ(imp)) of the membrane in mixed cationic forms have also been obtained from ac impedance measurement and compared with that (κ(cal)) calculated from the SDC data. For the Na-Ba system, the increment of κ(imp) with increase in the Na(+) content of the membrane has been found to be parabolic, whereas for the Na-Cs system the increment is linear. The reason behind the different behaviors for different types of ionic systems has been qualitatively explained based on different transport pathways of the cations in the membrane.

Structure-Transport Relationship of Perfluorosulfonic-Acid Membranes in Different Cationic Forms

“…A consequence of this phenomenon is that Nafion-H + and Nafion-Cs + have the similar d dry at 10% RH, although the former membrane accommodates 3 more water molecules per SO 3 sites (including the residual water). The unique properties of Nafion-Cs + owing to its very low charge density and strong interaction with sulfonate (SO 3 -) sites was discussed previously, and Cs + was suggested to be transferred through the interfacial regions, not the ionic domains [26,29].…”

“…Overall, cations have been demonstrated to influence a PFSA membrane's conductivity, selfdiffusion, water-uptake capacity in liquid water, transference number, permeability, and selectivity [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. In addition, most of these properties have been shown to be interrelated as most of them are influenced by the cation's hydration energy and interaction between the ionic moieties and water molecules.…”

Role of ionic interactions in the deformation and fracture behavior of perfluorosulfonic-acid membranes

“…2, 12-15, 17, 19-23 Investigations on PFSAs in proton and other cation forms have provided insight into the ion transport mechanisms, such as water-mediated ion transport and role of hydrogen bonding. 4,10,15,17,19,21,[24][25][26][27] Impact of a cations associated with not only its interaction strength and mobility within the ionic region, but also the intermediate region occupied by the fluoroether side-chains, which affect the polymer-chain segmental motion and morphology. 2,10,19,23,28 To elucidate how these interactions influence ionomer behavior, many studies focused on measuring the properties of PFSA membranes exchanged with different cationic forms.…”

Influence of external electrolyte on ion exchange in Nafion membranes

“…[13][14][15][16] Most of these studies focus on the effect of cations, but a few reports on anion dependence on transport of cations are also available. 17,18 Goswami and co-workers, for example, have shown that mercury(II) ions in solution are absorbed in different oxidation states, depending on the electrolyte present in the solution from which they are taken up by Nafion. When loaded from Hg(NO 3 ) 2 , the charge of the species inside the membrane has been found to be +2, as expected.…”