Study of electro-osmotic drag coefficients in Nafion membrane in acid, sodium and potassium forms by electrophoresis NMR

The water electro-osmotic drag coefficient of a cation _ Kdrag(cation)_ is defined as the number of water molecules which is dragged by a cation when it goes through a membrane at equilibrium state (i.e. in the absence of water concentration gradient within membrane). This coefficient is one of the key elements for modeling the water balance through Nafion membrane in a proton Exchange membrane Fuel cell (PEMFC). In this work, we have applied the method of electrophoresis NMR (ENMR) to determine the coefficient of electro-osmotic drag (Kdrag) within Nafion prepared in different cationic forms (H⁺, Na⁺ and K⁺). Applying, in all cases, the same method for determining Kdrag of Nafion membrane in many cationic forms should be appropriate in order to apprehend the mechanisms of water transport which occur in this membrane. This method consists in measuring the phase of the water NMR signal under the application of an electric current in a diffusion-like experiment, this phase being proportional to the flux of water within Nafion. The results show that the electro-osmotic coefficients of water within Nafion depend on the type of counter-cation and on the water content of Nafion. Due to proton lability, the electro-osmotic drag coefficient of water within Nafion in the acid form (Kdrag(H⁺)) is the lowest among the three studied cationic forms. In a dehydrated state of Nafion, the Kdrag(H⁺) is lower than 1, suggesting here that the hopping mechanism is favored. In the case of Nafion in sodium and potassium forms, Kdrag of these cations is always higher than 1. This can be rationalized by the vehicular mechanism and also by a strong interaction between the water solvation of the cation and the cation itself.