Graduation Semester and Year

2015

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Fuqiang Liu

Abstract

Low temperature traditional PEM Fuel Cells and also other fuel cells in general suffer from the acute yet long standing issues of cost and performance. This has severely hindered the commercialization of fuel cells but Alkaline Anion Exchange Membrane Fuel Cells (AAEMFCs) pose as a breakthrough technology by offering improved conversion efficiency (a virtue of fast kinetics in the alkaline media) and the lower costs (from the prospect of using non-noble metal catalysts). However, before systematic testing of non-noble metal catalysis is tested on this system, it is imperative to develop a reliable anion exchange membrane which lies at the heart of the fuel cell, yet in the past has suffered from problems of low conductivity and fast degradation. In this work, a series of novel AEMs are developed based on the guanidinium functional group. A bottom-up approach is taken starting with the synthesis of the prepolymer. The guanidine polymer is synthesized through a polycondensation reaction between a guanidinium salt and two different diamines. The guanidinium functional group attached directly to the polymer backbone to enhance both ionic conductivity and durability. As a result of this configuration and the resonance stabilized structure of guanidinium, it exhibited superior stability compared to commercial quaternary ammonium AEMs after being exposed in extreme conditions of 5 M KOH solution at 55°C for 50 h. This prepolymer is then subject to minor post modification such as crosslinking or tethering a lipophilic element to its main chain for suitable physio-chemical and mechanical properties. In addition, to achieve these optimum properties along with the required electrochemical performance, the membranes are eventually fabricated using two different approaches. The composite membrane is fabricated by incorporating guanidinium based polymer solution into a porous polytetrafluoroethylene (PTFE) film. Polymer crosslinking helped reinforce the mechanical strength of the membranes and interlock the guanidinium moieties to the porous PTFE. The hybrid blend membranes were obtained by blending the prepolymer with chitosan, another strengthening agent. Whereas the composite membrane displayed an outstanding ionic conductivity 80 mS cm-1 (at 20°C in deionized water), the hybrid blend membranes exhibited relatively lower values due to the effect from the blend components. However, the selectivity (ratio of ionic conductivity to methanol permeability) of the hybrid blend membranes is found to be superior even when compared to commercial membranes. Similarly, when used in a direct methanol alkaline fuel cells (DMAFCs) it fared even better than a commercial AEM reference reaching to an OCV of 0.69 V compared to the 0.47V of Tokuyama A201 at room temperature. Overall, the developed membranes demonstrate superior performance and therefore pose great promise for direct methanol anion exchange fuel cell (DMAFC) applications. Furthermore, through an experimental micro/nano phase analysis backed by simulation, the hydroxide transport process is highlighted which before now has not been well understood or experimentally probed in the past. The mechanism hypothesized may be transferrable to other guanidinium based membranes and extendible to other types of AEMs as well.

Keywords

Guanidinium, AEMFCs

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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