Electrostatic effect synergistically enabling the superior ion selectivity and ion conductivity of composite membrane for alkaline zinc-iron flow batteries

Alkaline zinc iron flow battery (AZIFB) is considered as an economical candidate for energy storage technologies. Ion conduction membranes as the key material of AZIFB directly affect the performance of efficiency and output power. However, the “Trade off” effect between ion selectivity and conductivity is a significant challenge in membrane development. Herein, we propose a method of simultaneously improving ion conductivity and ion selectivity by in-situ incorporating negative TiO 2 nanoparticles into the Nafion 212 membrane. The Nafion 212/TiO 2 composite membranes exhibit lower Zeta potential of −33.1 mV than the Nafion 212 membrane (−17.1 mV). This significant electronegativity of the composite membrane plays a key role in AZIFBs by inhibiting the crossover of the electronegative active species such as [Zn(OH) 4 ] 2 ⁻, [Fe(CN) 6 ] 4 ⁻, and [Fe(CN) 6 ] 3 ⁻. Additionally, since the charge carriers such as K + and Na + display electropositive properties, the electrostatic effect generated by the Nafion 212/TiO 2 composite membrane attracts these charge carriers, further influencing ion transport dynamics within the battery system. Therefore, the composite membrane synergistically enhances both ion selectivity and ion conductivity, resulting in impressive performance metrics for AZIFBs. Consequently, AZIFBs based on the composite membranes achieve a superior energy efficiency (EE) of 84.5 % and voltage efficiency (VE) of 84.9 % at a current density of 80 mA/cm 2 , which is much higher than that of the typical Nafion 212 membrane (EE: 68.2 %; VE: 68.9 %). This work offers significant insights for the enhancing the high-performance membranes for the practicality of AZIFBs.