Influence of temperature on the electrokinetic properties and power generation efficiency of Nafion® 117 membranes

In the present study we investigate the transport properties of Nafion® 117 membranes in temperatures ranging from ambient temperature up to 70 °C. The hydraulic permeability, streaming potential and ion conductivity have been measured as function of temperature in 0.03 M LiCl solutions in purposely designed, non-conductive set-ups. In particular, the apparent activation energies of the processes have been retrieved: 29.4 kJ mol−1, 9.3 kJ mol−1 and 22.9 kJ mol−1 for the hydraulic permeability, streaming potential coefficient and ion conductivity respectively. Based on the knowledge of the temperature dependence of these three independent properties the figure-of-merit of the electrokinetic energy conversion process has been calculated obtaining a monotonous increase of the efficiency with temperature. At 70 °C the electrokinetic efficiency is rather high about 26.6%:50% higher with respect to the one found at room temperature. The electrokinetic transport properties were also used to esteem the average pore size of the water channels in the polymer matrix resulting in pore diameters ranging approximately from 2.0 (25 °C) to 2.8 nm (70 °C).