Sorption and diffusion behaviors of water in Nafion 117 membranes with different counter ions

The sorption kinetic and equilibrium data obtained at different water activities allowed us to determine the water sorption isotherms and mean diffusion coefficient in the Nafion® membrane. The water sorption isotherm is reminiscent of systems in which there is water clustering at high water activities, with a marked Langmuir sorption at low water activity for the acid form of the membrane. The variation of the diffusion coefficient with the water activity showed a maximum value whose coordinates depend on the counter-cation nature. The smaller the counter-cation, i.e. the larger the counter-cation hydration energy, the higher the diffusion coefficient and the lower the water activity at the maximum point.

The sorption kinetic and equilibrium data of the H+, Li+, and Cs+ forms of the Nafion membrane were obtained at different water activities. The sorption isotherms and the mean diffusion in the Nafion® membrane were studied. The sorbed water molecules were divided into 3 different types of sorption modes, which are the Langmuir and Henry’s modes and water clusters. The smaller the counter-cation, i.e. the larger the countercation hydration energy, the higher the diffusion coefficient and the lower the water activity at the maximum point. The maximum points of the D for the Li+ and Cs+ forms of the Nafion membrane were consistent with the growth of the water cluster in the membranes while the D values for the acid form of the Nafion membrane decreases abruptly with the water activity in the vicinity of 0.6, where there is the onset of the water clustering. The behavior of the average diffusion coefficients of water for different cation forms of the Nafion membrane can be consistently interpreted on the basis of the existence of different types of sorbed water molecules including clustered water.