Tungsten oxide embedded graphene oxide doped with SPEEK composite membrane for zinc–bromine redox flow batteries

Zinc-bromine redox flow batteries (Zn/Br₂ RFBs) are fingerprint candidates for large-scale energy storage applications owing to their low cost, flexibility, high energy density, and astonishing round-trip efficiency. However, during the charging and discharging process, the diffusion of bromine through the porous membrane creates significant capacity decay and reduces the membrane efficiency of Zn/Br₂ RFBs. In this respect, we developed various amounts of tungsten trioxide (WO₃) nanoparticle-decorated graphene oxide (WO₃@GO)-loaded sulfonated poly (ether ether ketone) (SPEEK) membranes and investigated their Zn/Br₂ redox flow battery performance. The optimum amount of WO₃@GO-loaded WO₃@GO/SPEEK-3 membrane exhibits higher mechanical stability and ionic conductivity, significantly restricting the bromine crossover through the membrane. Due to their higher hydrophilic ability and uniform dispersion, WO₃-loaded GO nanosheets provide strong interaction between the sulfonic acid group of the SPEEK membrane, which acts as an effective barrier for bromine crossover and significantly alters the Zn/Br₂ battery performance. As a result, at a current density of 40 mA cm⁻², the Zn/Br₂ single cell of the WO₃@GO/SPEEK-3 demonstrated higher coulombic efficiency (96.6 %), voltage efficiency (89.6 %), and energy efficiency (86.6 %), better than the WO₃@GO/SPEEK-5 and pristine SPEEK membranes. These performances indicate that the proposed low-cost WO₃@GO/SPEEK membrane can be highly applicable to other energy-related fields, including fuel cells and water treatment.