Preparation of Sulfonated Poly(arylene ether)/SiO2 Composite Membranes with Enhanced Proton Selectivity for Vanadium Redox Flow Batteries

Proton exchange membranes (PEMs) are an important type of vanadium redox flow battery (VRFB) separator that play the key role of separating positive and negative electrolytes while transporting protons. In order to lower the vanadium ion permeability and improve the proton selectivity of PEMs for enhancing the Coulombic efficiency of VRFBs, herein, various amounts of nano-sized SiO₂ particles were introduced into a previously optimized sulfonated poly(arylene ether) (SPAE) PEMs through the acid-catalyzed sol-gel reaction of tetraethyl orthosilicate (TEOS). The successful incorporation of SiO₂ was confirmed by FT-IR spectra. The scanning electron microscopy (SEM) images revealed that the SiO₂ particles were well distributed in the SPAE membrane. The ion exchange capacity, water uptake, and swelling ratio of the PEMs were decreased with the increasing amount of SiO₂, while the mechanical properties and thermal stability were improved significantly. The proton conductivity was reduced gradually from 93.4 to 76.9 mS cm^-1 at room temperature as the loading amount of SiO₂ was increased from 0 to 16 wt.%; however, the VO²⁺ permeability was decreased dramatically after the incorporation of SiO₂ and reached a minimum value of 2.57 × 10^-12 m² s^-1 at 12 wt.% of SiO_2. As a result, the H⁺/VO²⁺ selectivity achieved a maximum value of 51.82 S min cm^-3 for the composite PEM containing 12 wt.% of SiO₂. This study demonstrates that the properties of PEMs can be largely tuned by the introduction of SiO₂ with low cost for VRFB applications.