The surface of porous polyethylene (SB-PE) membrane is modified with nickel coating using a hydrothermal method (SB-PE-Ni-x membrane). To optimize effects of pore size and hydrophilicity on properties of SB-PE-Ni-x, the amount of included nickel is varied. According to evaluations, structure of SB-PE-Ni-x is well preserved even under strong alkaline due to the formation of nickel silicate shell. The nickel silicate shell induces the preservation of stability and hydrophilicity and increase in ionic conductivity, while porosity and pore size of SB-PE-Ni-x are reduced from 67.4 % and 0.819 µm (SB-PE) to 57.9% and 0.557 µm (SB-PE-Ni-100). Chemical characterizations show that when nickel is coated onto SB-PE, porous structure and chemical bonding of SB-PE-Ni-x remain unchanged with high water uptake (32.5%) even after alkali treatment. This is evidence of high ionic conductivity. The performances of aqueous organic redox flow batteries (AORFBs) using mixed naphthoquinone and potassium ferrocyanide dissolved in potassium hydroxide are then investigated with SB-PE and SB-PE-Ni-x membranes. As a result, AORFB using SB-PE-Ni-50 shows high energy efficiency (80 %) and low capacity loss rate (0.0235 Ah∙L^-1 per cycle). This is because SB-PE-Ni-50 has optimal pore size showing less cross-over of active materials and high ionic conductivity.