An investigation of structure-property relationships in several categories of proton exchange membranes

The chemical and structural features of proton exchange membranes (PEMs) are related to their fuel cell relevant properties. The objective of this work is to understand structure-property relationships in PEMs through the fabrication and characterization of several classes of membranes. Incorporation of linear and angled monomers into the main chain of a polyimide permitted investigation of the effect of kinked versus linear polymers on membrane properties. The conductivity of angled sulfonated polyimide membranes is greater than those prepared from linear polymers, but water uptakes are lower. These differences are attributed to increased entanglements of angled polymers, which limit the degree of swelling and lead to increased proton concentration. Polyelectrolytes were incorporated into reinforcing materials to study the effect of incorporating and confining polyelectrolytes in the pores of reinforcing materials. The employment of reinforcing materials reduces conductivity, mobility, and permeance due to decreased ionomer content and connectivity of the ionomer. However, membranes are stronger and thinner, which compensates for these losses in terms of lower resistance and increased dimensional stability. Incorporating zirconium hydrogen phosphate (ZrP) and silicon dioxide (SiO2) into Nafion® membranes permitted investigation of their effect on membrane properties. Data for Nafion®/ZrP membranes support the theory that ZrP disrupts cohesive forces in Nafion®, causing it to absorb more water. The increased water content of the membranes does not result in increased conductivity because there is a concurrent decrease in proton concentration and mobility due to poorly conducting ZrP disrupting the conduction pathway and increased water content diluting protons and separating proton conduction sites. The decreasing density of the Nafion®/SiO2 composite membranes with increasing SiO2 content and the increased dimensional stability of the membranes increasing compared to unmodified Nafion® support the theory that a rigid scaffolding forms. Due to formation of void space that increases with increasing SiO2 content, water content increases, thus diluting the protons in the membrane, leading to lower conductivity. These structure-property relationships may be relevant to other membrane systems and should be considered when designing alternative systems for proton exchange membranes.