Study on poly(benzimidazole) based polymer electrolyte membranes for vanadium redox flow battery application

Abstract

The vanadium redox flow battery (VRFB) has been emerging as a promising candidate for large-scale energy storage system (ESS). Ion exchange membrane (IEM) is an essential component of VRFB, allowing proton conduction in addition to blocking crossover between positive and negative electrolytes. Achieving high proton conductivity, low vanadium ion permeability, and high chemical stability using a single material remains as a key challenge for hydrocarbon-based membranes in VRFB application. Among the hydrocarbon-based electrolyte membranes, polybenzimidazole (PBI) membranes achieve proton conduction when combined with strong acid, such as phosphoric acid (PA) or sulfuric acid. However, research on PBI membranes for VRFB application has been mainly focused on improving the performance of VRFB operation. This thesis presents the study on the correlation between the chemical structure of PBIs and the resulting physicochemical properties of the PBI membranes for VRFB application. The thesis is divided into 3chapters. Chapter 1 introduces VRFB, IEM, and their related research. Chapter 2 mainly focuses on the physicochemical properties and cell performance of N-substituted poly(2,5-benzimidazole) (ABPBI) membrane for VRFB application. The main objective is to suppress polymer chain packing through the introduction of an alkyl side group into the ABPBI. As a result, the increase in the free volume of the membrane and enhanced absorption capabilities were confirmed according to the introduction of the alkyl spacer into ABPBI. Furthermore, it is found that enhanced hydrophilic absorption capabilities directly affect the cell performance of VRFB. Also, the alkyl spacer grafted ABPBI membrane shows outstanding long-term stability for 500 cycles under actual VRFB operation. Chapter 3 discusses the preparation of ABPBI-derivative membrane by direct casting method and their physicochemical properties. The key objective is to expand the free volume of ABPBI-derivative membrane through the direct casting method. ABPBI-derivative membrane prepared by direct casting method shows different properties, such as the crystallinity and ion conduction, compared to ABPBI-derivative membrane prepared by solution casting method. As a result, the membrane prepared by direct casting method shows similar efficiencies under VRFB operation to the commercial membrane.