Monovalent Cation Exchange Membranes with Extreme Selectivities and Permeance Enabled by Quaternized and Crosslinked Ultrathin Films

Wind and solar power generate energy intermittently. Low-cost, grid-scale energy storage can help ensure energy is available on demand. Redox flow batteries are promising grid-scale storage technologies, but their cost is too high for broad use. A major reason is the costly membrane that separates the two liquid electrolytes and allows electric charge to pass within the cell. The best membranes are expensive and can account for 30% of the system cost. Current membranes also cannot fully prevent the liquid electrolytes from mixing, which lowers efficiency and shortens battery life. This project will study a new class of thin, low-cost membranes that selectively permit transport of the desired ions while blocking others. The project will investigate how these improved properties translate to better performance in redox flow batteries. The research will support economic competitiveness by lowering the cost of grid-scale energy storage and by improving the reliability of an abundant renewable power supply. The project will contribute to a skilled workforce by training graduate and undergraduate students. Finally, the project will support outreach activities in the Rochester, NY region to teach middle and high school students about how polymers enable everyday applications and next-generation technologies. This project will use selective cation transport in monovalent-selective cation exchange membranes (MCEMs) to enable efficient, reversible electrochemical energy storage in vanadium redox flow batteries (VRFBs). The central hypothesis is that ultrathin, highly charged, crosslinked polyelectrolyte films coated onto conventional cation exchange membranes (e.g., Nafion) will disproportionately slow the diffusion of multivalent vanadium ions relative to protons through electrostatic repulsion, limited free volume, and controlled hydration. The project will first develop synthetic capabilities to establish the synthesis-structure-property relationships in these MCEMs and then show how the membrane properties translate to enhanced energy storage performance in vanadium-based redox flow batteries. The project will develop a two-step vapor-phase synthesis in which poly(4-vinylpyridine) films deposited by initiated chemical vapor deposition (iCVD) are simultaneously quaternized and crosslinked with mono- and dihaloalkanes, giving independent control over charge and crosslink densities. It will then characterize structure–property relationships across a broad compositional space using in situ spectroscopic ellipsometry on model samples to measure water uptake and transport measurements in thin film composite membranes to characterize ion conductivity and selectivity. The selective layers will be coated onto Nafion® and onto fluorine-free membranes such as sulfonated poly(ether ether ketone) for performance validation through galvanostatic cycling in single-cell VRFBs. The project will deliver a rational design framework linking membrane composition to selective transport, ultrathin membranes that combine high permeance with monovalent selectivity, and design strategies for lower-cost, sustainable, fluorine-free membranes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.