Polymer electrolyte membrane (PEM) based on sulfonated poly (phenylene oxide) (sPPO) and sulfonated silica (sSiO₂) is synthesized for the application of redox flow battery (RFB). Synthesis of PEM based on sPPO and sSiO₂ is confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Proton conductivity (0.077 S cm^-1) of PEM based on sPPO and sSiO₂ is comparable to Nafion (0.078 S cm^-1). This is due to the incorporation of sSiO₂ having –SO₃H functional group. Increased –SO₃H functional groups in sPPO and sSiO₂ can increase the ion exchange capacity (IEC) of the membrane. It can lead to high proton conductivity because of the well-organized cluster network structure. In addition, sSiO₂ can play a role of barrier for the suppression of vanadium cross-over through the membrane. Therefore, the permeability of PEM (4.76 x 10^-9 cm² min^-1) based on sPPO and sSiO₂ is much lower than that of Nafion membrane (2.22 x 10^-7cm² min^-1). It means that the selectivity of PEM based on sPPO and sSiO₂ is much better than that of Nafion membrane. As a result of our study, it is found that our ion-selective PEM based on sPPO and sSiO₂ can be a promising candidate for energy device and water treatment system.