Abstract: Redox flow batteries (RFBs) have emerged as a promising stationary energy storage technology because of their capability to decouple power and energy, high safety, long-life cycling, and low cost. However, the high cost, low permeability & selectivity of redox species of commercially used separators hinder the practical applications. We apply atomic layer deposition (ALD) for the surface functionalization of the separator which can improve the permeability & selectivity of redox species. In this study, isoporous polycarbonate track-etched membranes (PCTEs) and thin-film were modified by sequential infiltration synthesis (SIS) and ALD to tune the pore size and pore/surface chemistry as a platform that can enable low-cost high-performance RFB membranes. We then used water permeance measurements to compare the water flux for a series of membranes to study the pore size, pore-wall/surface chemistry, and water contact angle. We employed in situ Fourier transform infrared spectroscopy (FTIR) to elucidate the chemical interactions and in situ spectroscopic ellipsometry (SE) to measure the physical and optical changes that occur during Al₂O₃ SIS in PC.