Enhancing Proton Conductivity of Nafion Membranes with Hollow, Nanorod SiO2 Particles

The most common membrane for PEMFC’s is Nafion, which consists of a hydrophobic polytetrafluoroethylene (PTFE) backbone and hydrophilic sulfonic acid side chains. In Nafion, protons are conducted through the hydrated sulfonic side chains, making water management in the membrane essential; proton conductivity decreases at temperatures above 80 °C as water begins to evaporate. Improving ionic conductivity at high temperatures is important for applications in transportation (e.g., heavy vehicles), which experience a wide range of operating temperatures, and because higher temperatures (120-130 °C) otherwise improve fuel cell performance regarding carbon monoxide tolerance and catalyst performance.

As is well-established in the literature, a composite membrane of Nafion with silica nanoparticles can improve fuel cell performance at higher temperatures as pores in the nanoparticles provide physical pathways for ion transfer and may also allow for additional water storage. Previous studies have focused on spherical nanoparticles; however, the use of anisotropic particles may introduce additional benefits. Anisotropic filler particles have a larger surface area to volume ratio and may provide the same benefits as spherical particles at lower loading amounts. Furthermore, Nafion cast in aqueous alcohol solutions will aggregate into rod-like structures due to its hydrophobic backbone, offering the possibility for the silica nanoparticles to align with the Nafion aggregates and support existing proton conduction channels. Finally, the hollow tubes at the center of the nanoparticles may provide an additional avenue for proton transport.

In this work, preliminary investigation is completed into the efficacy of anisotropic silica nanoparticles as a Nafion additive. Commercial dispersions of Nafion D2821 are cast as pure films and with the addition of spherical and rod-shaped nanoparticles. The films are characterized through dynamic mechanical analysis (DMA), measurement of ionic conductivity, and water uptake measurements to evaluate the physical durability, proton conductivity, and water retention. Planned future work includes the development of a reliable method for casting equivalent films using non-commercial Nafion dispersions and investigating the effect of nanoparticle concentration.