Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Brian C Benicewicz

Abstract

After approximately 20 years of development, polybenzimidazole (PBI) chemistries and the concomitant manufacturing processes have evolved into commercially produced membrane electrode assemblies (MEAs). Commerical PBI MEAs can operate reliably for over two years at elevated temperatures of 120-180°C due to the physical and chemical robustness of PBI membranes. Recently, the Department of Energy has issued a target of 40,000-80,000 hours for stationary (i.e. combined heat and power, back-up power) polymer electrolyte membrane / proton exchange membrane (PEM) fuel cells. It is known through observation that over time, PBI membranes at 180°C creep in a direction perpendicular to compressive forces, thus also changing the composition of the membrane. To reach this goal of 40k hours, enhancement of the mechanical properties of PBI membranes is of great importance to prevent membrane creep. From a manufacturing standpoint, new approaches of improving the long-term mechanical properties of PBI membranes are needed which are cost effective and compatible with current manufacturing processes that have been developed for these unique membrane materials. Herein, we review the history of PBI gel membranes (Chapter 1) and propose multiple novel approaches to enhancing the mechanical properties of PBI membranes and report the in-depth structure-to-property relationships of these modified membranes (Chapters 2-5).

Chapter 2 and Chapter 3 each focus on the synthesis and processing of three new series of polybenzimidazole copolymer membranes (3,5-pyridine-r-2OH-PBI, 3,5-pyridine-r-para-PBI, and 3,5-pyridine-r-meta-PBI; 2,5-pyridine-r-meta-PBI, 2,5-pyridine-r-para-PBI, and 2,5-pyridine-r-2OH-PBI, respectively) using the PolyPhosphoric Acid (PPA) Process. Monomer pairs with high and low solubility characteristics were used to define phase stability-processing windows for preparing membranes with high temperature membrane gel stability. Creep compliance of these membranes (measured in compression at 180°C) generally decreased with increasing polymer content. Membrane proton conductivities decreased in a relatively constant manner with increasing membrane polymer content. Fuel cell performances of some high-solids copolymer membranes (up to 0.66 V at 0.2 A cm-2 following break-in) were comparable to para-PBI (0.68 V at 0.2 A cm-2) despite lower phosphoric acid (PA) loadings in the high solids membranes. Long-term steady-state fuel cell studies showed these copolymer MEAs maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm-2 for over 9000 h. Phosphoric acid that was continuously collected from the long-term study demonstrated that acid loss is not a significant mode of degradation for these membranes. The PBI copolymer membranes' reduced high-temperature creep and long-term operational stability suggests that they are excellent candidates for use in extended lifetime electrochemical applications.

Chapter 4 offers an in-depth investigation of the structure-to-function properties of 2,5-py-r-para-PBI, 2,5-py-r-meta-PBI, 3,5-py-r-para-PBI, and 3,5-py-r-meta-PBI high polymer content membranes. Theoretical calculations of dipole strength and ground state geometries of model compounds (repeat units) were determined using Spartan'10 software. The complete protonation of each model compound was determined as energetically favorable. Protonation bond energy data indicates that steric hindrance partially impedes the ability of each lone electron pair of nitrogen in an sp2 orbital to bond. Data gathered from the PPA Process of these random copolymers indicate that greater proportions of flexible PBI moieties and stronger dipoles enhance the solubility of polymer chains, which consequently affects their abilities to form stable gels. High temperature creep compliance tests indicated that thermal gel stability decreased with increasing proportions of pyridines, more flexible PBI moieties, or PBIs with stronger dipoles. Higher dipole strengths of the fully protonated model compounds correlate with increased solubility and decreased gel thermal stability in phosphoric acid environments. Electrochemically, membranes composed of more soluble PBIs tend to demonstrate lower anhydrous proton conductivity and fuel cell voltage.

Chapter 5 reports the investigation of alternative approaches at modifying the structures of PBI membranes to enhance the mechanical and electrochemical properties. Research focused on the viability of a PBI polymer blending approach, the synthesis of novel polyetherbenzimidazoles (PEBIs) and polyphosphonobenzimidazoles (phos-PBI), and the inclusion of various small-molecule organic additives to PBI membranes. These efforts demonstrated mixed success and provided valuable insights into the structure-to-property relationships of PBI gel membranes.

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