(403i) Influence of Water Content on Ion Transport Properties of Highly Charged Ion Exchange Membranes for Vanadium Redox Flow Batteries

With rising energy usage around the world and increasing pressure to reduce fossil fuel consumption in electricity generation, energy storage systems that help reduce volatility in electricity supply are increasingly important for the development next-generation, renewable power grids. Vanadium redox flow batteries (VRFBs) provide efficient long-term energy storage and cycling capability without coupling power output and storage capacity, thus meeting many of the requirements for bulk electrochemical energy storage. However, industry standard ion-exchange membranes (IEMs) for VRFBs exhibit low selectivity (due to high water content) and high cost of synthesis, thus limiting the lifetime and economic viability of VRFBs. Recently invented membrane materials fail to surpass the industry standard due to inadequate transport properties, low chemical stability in highly acidic and oxidative electrolyte solutions, and/or high synthetic cost. The lack of understanding of the relationship between membrane structure (e.g., water content) and IEM performance limits the rational design of new membrane materials, making fundamental studies in this area highly impactful. In this study, we systematically investigated the role of IEM water content to develop materials design parameters for next generation VRFB IEMs.

Membrane materials design parameters require a theoretical understanding of the individual effects on performance from water content and charge density. Previous research has failed to isolate the effects of water content and charge density on IEM performance to due synthetic coupling of the two parameters – highly charged membranes have a larger thermodynamic driving force for water uptake, thus increasing the equilibrium water content. Decoupling the water content and charge density was accomplished by synthesizing highly cross-linked IEMs via a simple and inexpensive free radical solution polymerization process. By varying the weight fraction of the crosslinker in the reaction, the internal resistance to swelling of the polymer network can be used to control the water content, leaving the charge density of the material (based on the amount of water in the membrane) constant. This process was used successfully to synthesize membranes with water volume fractions ranging from approximately 0.25 to 0.50. The performance of these membranes was then evaluated by measuring the membrane ionic conductivity and vanadium permeability, as well as ion partitioning into the membrane at conditions relevant for VRFBs. The changes in ion throughput and selectivity were correlated to changes in membrane water content. Nafion 117 membranes were used as a comparison to the industry standard. Nafion 117 was almost 10x less efficient at rejecting vanadium species crossover than the least selective membrane synthesized in this study due to the decreased water volume fraction and higher charge density of the synthesized membranes.