Transport properties of composite membranes containing silicon dioxide and Nafion ®

Abstract

Silicon dioxide (SiO 2) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO 2 membranes were fabricated with SiO 2 content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO 2 content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO 2 content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO 2 content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO 2 nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.