Percolation conductivity in Nafion membranes

This article deals with the method of determination of a threshold volume fraction of the conductive phase within perfluorosulfonic acid ionomer Nafion. Experiments have been performed with the commercial Nafion-120 and Nafion-427 membranes equilibrated with concentrated sodium chloride and sodium hydroxide solutions at 353 K. It has been stated that the insulator-to-conductor transition in membranes occurs at the critical volume fraction of the conductive phase (V_c) equal to 0.1. The same V_c has been estimated for a geometrical cluster-network model. Lower than the theoretical V_c for a classical dense-packed-hard-sphere model (V_c = 0.15), the volume fraction for the membranes is caused mainly by channels connecting the ionic clusters. The critical exponent t has been calculated for both membranes and found to be equal to 1.6 for Nafion-120 and 1.5 for Nafion-427. Both these constants correspond to those theoretically predicted for 3D systems. The ratios of sodium ion mobility in the internal membrane solution to its mobility in the equilibrating NaCl or NaOH solutions (u₊/u₊) are below unity, and they are dependent on the nature and concentration of the electrolyte.