Effect of Electrolyte Additives on the Water Transfer Behavior for Alkaline Zinc–Iron Flow Batteries

Alkaline zinc–iron flow batteries (AZIFBs) are a very promising candidate for electrochemical energy storage. The electrolyte plays an important role in determining the energy density and reliability of a battery. The substantial water migration through a membrane during cycling is one of the critical issues that affect the reliability and performance of an AZIFB. In this work, it has been proven that the reason for water migration in AZIFBs is the synergetic effect of concentration gradient, different ionic strengths of negolyte and posolyte, and the electric field. To address the issue of water migration in AZIFBs, a series of additives are employed and the effects of additives on the water transfer behavior and electrochemical performance of AZIFBs are investigated in detail. The results indicate that all investigated additives can suppress water migration through a polybenzimidazole membrane because of the shrunken gap of osmotic pressure and ionic strength between negolyte and posolyte. Moreover, organic additives such as glucose can decrease battery performance because of the increased polarizability of the electrode, whereas inorganic additives such as Na2SO4 demonstrate no distinct effect on battery performance. Specifically, an AZIFB that employs Na2SO4 as an additive in the negative electrolyte can afford a Coulombic efficiency of ∼99% and a voltage efficiency of ∼88% for 120 cycles at 80 mA cm–2, together with a good effect for inhibiting water migration behavior. This paper presents an effective way to suppress water migration and increase the reliability of AZIFBs.