Morphology and Transport in Ionic Membranes

Open Access
Author:
Disabb-miller, Melanie Lisa
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 12, 2013
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Michael Anthony Hickner, Committee Chair
  • Ralph H Colby, Committee Member
  • James Patrick Runt, Committee Member
  • Enrique Daniel Gomez, Committee Member
Keywords:
  • proton exchange membrane
  • anion exchange membrane
  • fuel cell
  • block copolymer
  • PEM
  • AEM
  • ion transport
  • morphology
  • superacid
Abstract:
Ion-containing polymers for fuel cell membranes have been studied to determine the chemical structure and ion content relationship to membrane water uptake, conductivity, and morphology. Random and block copolymer proton exchange membranes (PEMs) and anion exchange membranes (AEMs) with unique properties, such as diblock and triblock copolymers, superacidic moieties, and charge-delocalized polymer-tethered Ru-complex based cations, were investigated, and new metrics were developed to analyze fundamental ion transport behavior in these polymers. The morphology of the polymer systems was examined using small angle x-ray scattering (SAXS), small angle neutron scattering (SANS), and transmission electron microscopy (TEM). By studying a number of different ion-conducting systems using multiple techniques and deep analysis of structure-property relationships, a more complete picture of the property landscape of these materials was developed. Model diblock and unique triblock copolymer systems with center-functionalized blocks based on poly(styrene), PS, and poly(hexyl methacrylate), PHMA, were synthesized via atom transfer radical polymerization (ATRP). The PS block was functionalized for backbone-independent comparisons of PEM and AEM water uptake and conductivity to provide insight in how the properties of PEMs and AEMs compare and aid in further AEM development. The ratio of the mobile ion diffusion coefficients and dilute solution ion diffusivity (D/D0) was developed as a new metric, allowing for accurate comparison of polymer systems with different ion moieties and contents. Subsequently, it was determined that block copolymer PEMs and AEMs demonstrate the same barriers to ion transport if the mobility of the charge carrier is considered. Solution and membrane morphology was correlated for the PS-PHMA membrane systems using SAXS, SANS and TEM techniques. Two additional polymer systems incorporating unique Ru-complex-based and superacid ionic groups were investigated, as well. The effect of cross-link density on water uptake and conductivity was studied for bis(terpyridine) ruthenium-based AEMs with the new metrics to compare the conductivity of various AEM counterions. Finally, the conductivity, water uptake, and morphology of superacid random and block copolymer PEMs were explored. The perfluorosulfonic acid groups in these polymers led to enhanced conductivity over the alkyl and aryl sulfonic acid groups. Through this research, new insight was gained into the fundamental associations between water and ions in polymer membranes. These methods were applied to membranes with a wide variety of ionic groups and random and block copolymer PEM and AEM systems, with the goal to aid in the development and design of ion-conductive materials for a wide variety of applications with enhanced performance.