Date of Completion

7-18-2016

Embargo Period

7-12-2018

Keywords

Ionomers, Sulfonated Poly(phenylene)s, Quaternized, Vanadium Redox Flow Battery, Permeability, Durability

Major Advisor

Christopher Cornelius

Associate Advisor

Yongku Cho

Associate Advisor

Radenka Maric

Associate Advisor

Mu-Ping Nieh

Associate Advisor

Steven Suib

Field of Study

Chemical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

In this study the physical, electrochemical, and transport properties, as well as performance in a working vanadium redox flow battery, was reported for a series of anionic and cationic Diels-Alder poly(phenylene)s. Parent methylated and non-methylated Diels-Alder poly(phenylene) homopolymers, and random and block copolymers, were synthesized. The parent materials displayed good solubility in various organic solvents, high glass transition temperatures (365-390 °C) and good thermal stabilities (>552 °C). Correlations were found between –mer length and inter-chain spacing, glass transition temperature, and occupied volume. Sulfonated materials were synthesized that displayed good mechanical and thermal properties. A sulfonated poly(phenylene) with an IEC of 2.0 meq/g had a conductivity of 88.4 mS/cm and water uptake of 61.1%. The material had comparable ion conductivity and free water content, however significantly lower methanol permeability, when compared to the commercial standard Nafion 117, suggesting smaller and less interconnected ionic regions. A series of quaternary ammonium homopolymers and random copolymers were synthesized that displayed moderate mechanical strength and thermal stability (>190 °C). An aminated homopolymer had an IEC of 2.2 meq/g, and possessed an ion conductivity of 26.9 mS/cm and a water content of 73.1%. The quaternary ammonium random copolymers displayed lower water content, as well as superior mechanical properties and alcohol rejection, suggesting larger and well-defined hydrophobic regions. A state of water analysis revealed decreased available free water, which resulted in lowered ion conductivities. Homogeneous amination preparation of the random and block copolymers resulted in adverse effects on mechanical, electrochemical, and transport properties, indicating a dilution effect.

The sulfonated poly(phenylene)s displayed high coulombic and moderate voltage efficiencies. The materials were optimized through control of ion content (IEC) and membrane thickness, resulting in superior performance over Nafion 117 at current densities of 10-50 mA/cm2. The quaternary ammonium materials displayed poor performance, predominantly due to electrochemical limitations resulting in poor voltage efficiency. Random copolymerization improved the coulombic efficiency at the cost of increased ohmic resistance. Membranes displayed no failure after 40 charge-discharge cycles. Rigorous ex-situ stability testing revealed discoloration and loss of flexibility in all materials. The effect appeared to be reduced in quaternary ammonium materials, suggesting electrostatic effects.

Available for download on Thursday, July 12, 2018

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