Date of Award

Fall 2021

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Brian C. Benicewicz

Abstract

Polybenzimidazoles (PBIs) represent a class of performance polymers that display exceptional thermal and oxidative stability. For almost thirty years, PBI membranes have been investigated as promising candidates for next-generation alternative energy devices, including high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) and vanadium redox flow batteries (VRBs). Issues with the production of PBI membranes arise from their inherent low solubility in organic solvents. Thus, conventional approaches typically yield membranes that have limited molecular weight and generate significant waste in preparation, due to the large quantity of solvent required to dissolve PBIs for membrane casting.

Presented herein is the development of a new process, adapted from the sol-gel polyphosphoric acid (PPA) Process, to yield dense PBI films for the first time without the use of organic solvents. This significantly reduces the waste generated in making PBI films and allows much higher molecular weights to be achieved. Included in this work is the fabrication and characterization of dense PBI films produced by this method and applications in two electrochemical devices. In the case of HT-PEMFCs, the PBI membranes produced have demonstrated the ability to maintain high proton conductivities (>0.20 S cm-1 ) with lower levels of phosphoric acid. These new membranes also contain double the solids content and a five times enhancement in the creep resistance, compared to the previous membrane method. Thus, cell operation at high current densities was achieved with a very low degradation rate. This advanced membrane offers great potential in aviation and heavy-duty vehicle applications.

For VRBs, PBI membranes produced using the outlined technique result in two orders of magnitude reduction in vanadium permeability. This process also gives PBI membranes with higher ionic conductivity than those that are cast from an organic solvent and doped in acid, or the “conventionally imbibed” PBIs. Membranes produced according to this method thus have impressive cell cycling results and show great promise in grid-scale energy storage.

Rights

© 2021, Laura Ann Murdock

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