Abstract

Redox flow batteries (RFBs) are large-scale and long-duration electricity storage systems with decoupled energy and power units, granting them design flexibility. Despite their expanding applications, RFBs are still more expensive than competitive electricity storage systems due to the high cost of materials and relatively low power density. Hence, either the cost of materials must decrease through the development of cheaper materials with enhanced properties, or the power density must improve through efficient component design. This dissertation focuses on optimizing two main components of RFBs: ion-exchange membranes (IEMs) and flow field plates (FF plates). Anion-exchange membranes (AEMs) innately prevent cation cross-transfer. Therefore, AEMs are ideal to use as IEMs in RFB applications due to the cationic nature of the active species. However, AEMs lack chemical stability in the acidic environment of most RFBs. To increase the AEM durability, two durable polymers were blended with the lab-developed AEM, quaternized cardo poly (ether ketone) (QPEK-C): poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and polybenzimidazole (PBI). The blend of QPEK-C and PVDF-co-HFP resulted in an immiscible polymer-blended AEM, requiring morphology control via tuning the blend composition and casting temperature. The PVDF-co-HFP blending improved the durability of QPEK-C-based AEMs (21% conductivity loss vs. 63% over a week in 0.9M cerium (III/IV) in 2M methanesulfonic acid). However, the PVDF-co-HFP blending sacrificed ionic conductivity (11 vs. 13 mS.cm-1). In contrast, blending PBI with QPEK-C resulted in a miscible polymer-blended AEM. The PBI blending improved the durability of QPEK-C-based AEMs (100% ionic conductivity retention) while enhancing the ionic conductivity due to the synergic relationship between PBI and QPEK-C (>17 vs. 13 mS.cm-1). PBI/QPEK-C blended AEM outperformed commercial AEMs, showing that polymer blending is a useful strategy to improve the durability of membranes for RFBs and similar electrochemical systems. After improving the durability and performance of RFBs through AEM design, this dissertation focuses on investigating conductive polymer composites (CPCs) as a common set of materials for FF plates. A facile CPC fabrication method was introduced, and the effect of the shape and size of major and minor fillers were investigated in CPCs. As major fillers, Carbon nanofibers (CNFs) produced the most conductive and strongest CPCs. Elongated nanofillers of ~100 nm diameter were identified as the most effective minor fillers. As a result, thin CPCs (~0.9 mm) were developed with excellent chemical stability (Retaining conductivity over a week in 0.9M cerium (III/IV) in 2M methanesulfonic acid) and a lower area-specific resistance (ASR, 17.6-22.1 vs. 26.2 m.cm2) than common graphitic bipolar plates (BP, 3.175 mm). The lower ASR of CPCs promotes their use in FF plates of RFBs. Finally, this dissertation studies the effect of FF geometry and electrode properties on the performance of RFBs. Using dimensional analysis, nine dimensionless parameters were identified to comprehensively describe RFB systems. A semi-analytical model was developed, encompassing fluid dynamics, mass transport, kinetics, and thermodynamics. The model was validated with the literature data. The model provided a map of dimensionless mass transport and kinetics parameters and identified stagnant zones, especially at the end of the inlet and outlet channels of interdigitated FFs. Appropriate geometry modifications were proposed to inhibit the stagnant zones depending on the electrode thicknesses and channel sizes.

Degree

Doctor of Philosophy (PhD)

Author's Department

Energy, Environmental & Chemical Engineering

Author's School

McKelvey School of Engineering

Document Type

Dissertation

Date of Award

5-12-2025

Language

English (en)

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