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Understanding Ion Conducting Polymeric Materials at Nanoscale for Renewable Energy Applications
Abstract
Ion conducting polymers (ionomers) play a critical role in the performance of fuel cells and other electrochemical devices. In fuel cells, ionomers are used as several tens of µm thick bulk, free-standing membranes and sub-µm thick layers as catalyst binders on electrodes. Interestingly, the material properties (ion conduction, hydration, nanostructure, etc.) of sub-µm thick ionomer layers are completely different from those of membranes. While the ionomer properties in bulk membranes have been investigated extensively, our understanding of ionomer thin-film behavior is still not up to the mark. This understanding is critical as ionomer chains experience confinement which slows down the proton diffusion and electrochemical reaction kinetics at the cathode leading to a low power density and performance of fuel cells. The poor proton conductivity within these thin ionomer layers is believed to be a complex outcome of confinement and interfacial interaction effects. As the film thickness approaches the radius of gyration of polymer chains, the interfacial effects become more prominent. The different interactions at different interfaces of ionomer films can impact the local glass transition temperature and mechanical properties which resulted from the water-polymer mobility and proton conductivity at different interfaces. However, mostly, ion conduction and other properties of polymer thin films are reported as average values for the entire samples. I, therefore, explored both the average- and depth-specific ion-conduction behavior of several fluorocarbon-based ionomers by exploring the nanoscale phase separations, morphologies, ionic domain characteristics, and hydration behaviors in thin films. I then leveraged this nanoscale understanding of conventional ionomers, and designed novel, plant-based, low-cost, environment-friendly, and efficient ionomers (LS) by strategically sulfonating lignin from different sources. Additionally, by making the composite film of sulfonated polysulfone (sPSf) and LS, I have demonstrated a strategy to improve the thin-film proton conductivity of sPSf compared to the thin-film proton conductors when used alone. These in-depth understandings of ionomers under thin-film confinement and new designs of ionomeric materials can pave pathways toward new designs of ionomer-catalyst interfaces for fuel cells and beyond.
Subject Area
Chemical engineering
Recommended Citation
Farzin, Seefat, "Understanding Ion Conducting Polymeric Materials at Nanoscale for Renewable Energy Applications" (2022). ETD collection for University of Nebraska-Lincoln. AAI29166640.
https://digitalcommons.unl.edu/dissertations/AAI29166640