Synthesis and Characterization of Polymer Electrolyte Membranes with Controlled Ion Transport Properties

Open Access
Author:
Xu, Kui
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 18, 2011
Committee Members:
  • Qing Wang, Dissertation Advisor
  • Qing Wang, Committee Chair
  • Ralph H Colby, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Enrique Daniel Gomez, Committee Member
Keywords:
  • Fuel Cells
  • Polymer Electrolyte Membranes
  • Proton Conductivity
  • Methanol Permeability
  • Electrochemical Selectivity.
Abstract:
Polymer electrolyte membrane (PEM) fuel cells offer the potential to generate highly efficient and clean energy and have been considered as the next generation power producing technology for automotive, stationary, and portable applications. However, an important barrier that hampers the widespread use and practical implementation of PEM fuel cells is the lack of feasible polymeric electrolyte materials. Nafion and other analogous perfluorosulfonic acid polymers are currently the leading materials in this business, but they still present certain limitations. For example, they are not capable to work at temperatures higher than 90 oC and low humidification conditions. Elevating the working temperature of PEM fuel cells to above 100 oC is associated with a family of attractive benefits, including improved tolerance of electrodes to carbon monoxide, simplification of the cooling system and possible use of co-generated heat, and improved electrode reaction kinetics, etc. Moreover, perfluorosulfonic acid polymers fail to perform well in fuel cells fed with methanol due to the fuel crossover issue. Methanol is an important alternative fuel of choice to hydrogen, because it is free of massive production and distribution problems as for hydrogen. These issues lead to two major research thrusts in the current development of PEM materials, i.e. materials that can work at high temperatures (>100 oC), low humidities and that have low methanol crossover. This dissertation presents my recent efforts to overcome these materials challenges. A number of novel PEM materials have been created by rational design and sophisticated synthesis. Ion-containing block copolymers hold promise as next-generation PEM materials due to their capability to self-assemble into ordered nanostructures facilitating proton transport over a wide range of conditions. Ion-containing block copolymers, sulfonated poly(styrene-b-vinylidene fluoride-b-styrene), with varied degrees of sulfonation were synthesized. The synthetic strategy involved a new approach to chain-end functionalized poly(vinylidene fluoride) as a macro-initiator followed by atom transfer polymerization of styrene and sulfonation. Characterization of the polymers were extensively carried out by 1H and 19F nuclear magnetic resonance and Fourier-transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetry analysis. Tapping mode atomic force microscopy and transmission electron microscopy were applied to study the phase separation and self-assembled morphology. Strong dependence of ion exchange capacity, water absorption, morphology and proton conductivity on the degree of sulfonation has been found. It has been observed that the conductivities of the block copolymers are considerably higher than the random copolymers of polystyrene and sulfonated polystyrene possessing similar ion exchange capacities. Copolymers of vinylidene fluoride and perfluoro(4-methyl-3,6-dioxane-7-ene) sulfonyl fluoride containing amino end-groups were synthesized for the first time. The prepared amino-terminated polymers underwent cross-linking reactions with 1,3,5-benzene triisocyanate to form proton conductive networks. The chain-end crosslinked fluoropolymer membranes exhibited excellent thermal, hydrolytic and oxidative stabilities. The ion exchange capacity, water uptake, the state of absorbed water, and transport properties of the membranes were found to be highly dependent upon the chemical composition of the copolymers. The cross-linked membranes showed extremely low methanol permeability, while maintaining high proton conductivity at the same order of magnitude as Nafion. This unique transport feature gave rise to exceedingly higher electrochemical selectivity in relation to Nafion. The selectivity characteristics have been rationalized based on the formation of restrained ionic domains and the state of the absorbed water within the membranes. A series of new Nafion-based composite membranes were prepared via an in situ sol-gel reaction of 3-(trihydroxylsilyl) propane-1-sulfonic acid and solution casting method. The morphological structure, ion-exchange capacity, water uptake, proton conductivity, and methanol permeability of the resulting composite membranes were extensively investigated as functions of the content of sulfopropylated polysilsesquioxane filler, temperature, and relative humidity. Unlike the conventional Nafion/silica composites, the prepared membranes exhibit an increased water uptake and associated enhancement in proton conductivity compared to unmodified Nafion. In particular, considerably high proton conductivities at 80 and 120 °C under 30% relative humidity were demonstrated in the composite membranes, which are over 2 times greater than that of Nafion. In addition to a remarkable improvement in proton conductivity, the composite membranes displayed lower methanol permeability and superior electrochemical selectivity in comparison to the pure Nafion membrane. This work opens new opportunities of tailoring the properties of Nafion, the benchmark fuel cell membranes to obviate its limitations and enhance the conductive properties at high temperature/low humidity and in direct methanol fuel cells. A versatile and facile synthetic approach was developed for the preparation of a family of new ionomers with rigid aromatic backbones and pendant perfluorinated sulfonic acid groups. Variation in the chemical composition and structure of the new aromatic ionomers were performed to optimize PEM properties and fuel cell performance. The ionomers prepared from condensation polymerization of Sodium 1,1,2,2-tetrafluoro-2-(2’,3’,5’,6’-tetrafluoro-phenoxy)- ethane sulfonate and bisphenol monomers, e.g. hydroquinone, 4,4’-biphenol, or their mixture with appropriate ratio, exhibited comparable or greater proton conductivity in relation to Nafion. New aromatic ionomers also showed other outstanding PEM properties, e.g. high Tg, low methanol permeability, excellent thermal and chemical stability and good mechanical properties. Initial fuel cell testing of these ionomer at elevated temperatures demonstrated superior performance to Nafion membrane, indicating great potential for use in high temperature fuel cells.