In this work, a novel method of producing polyolefin-silica composite membrane with enhanced wettability and tailored pore sizes was presented. The supercritical CO2 was used as the medium for intercalation of the silica precursor into the polymer matrix, which allowed obtaining a uniform distribution of SiO2 nanoparticles in the material structure. The proposed method of impregnating Celgard matrix with tetraethoxysilane (TEOS) in supercritical carbon dioxide and following acid-catalyzed hydrolysis and condensation leads to significant changes in ion transport properties of Celgard membranes in aqueous electrolytes. This allows one to obtain separators suitable for aqueous electrochemical energy storage systems such as aqueous vanadium redox flow batteries. (C) 2019 Elsevier B.V. All rights reserved.