In a typical all-vanadium redox flow battery (VRFB), species crossover through membranes not only deteriorates cell performance, but also reduces battery’s lifetime. To address this issue of species crossover, a better understanding of the ion transport behaviors through an ion exchange membrane is essential. The fixed charge, a very important and typical membrane property, has a deep correlation with ion transport behaviors: Firstly, the fixed charge affects ion distributions in the membrane as a result of the membrane selective adsorption. Hence, the ion transport rate is affected by all the three of transport modes including diffusion, migration and convection. Moreover, the ion selective adsorption is crucial to obtain the accurate ion diffusivity. Secondly, counter ions in the membrane may also transport via the swing of the charged functional groups. The characteristic of ion transport behavior in this way depends on the relative amount of the fixed charges and movable ions, and reflects in the ion diffusivity. However, both numerical and experimental investigations rarely concern about the intrinsic influence of the fixed charge on ion transport as the membrane parameters cannot be precisely controlled separately and an integrated impact of membrane is reflected in measured ion diffusivity. Therefore, it is urgent to explore how the fixed charge affects the ion transport behaviors.

In this work, we conceive, design and fabricate an experimental setup to study the characteristics of ion selective adsorption and ion diffusivities in an ion exchange membrane for VRFBs. As these two properties related to the relative amount of the fixed charges and movable ions, we take the amount of movable ions to be variables while the fixed charges constant to avoid the difficulty of precise production of membrane. We first perform an adsorption equilibrium test to explore the interactions between the fixed charge and movable ions. Two groups of vanadyl sulfate solution are prepared with a large difference in H⁺ concentration so that the amounts of total movable ions differ greatly. Each group contains three cases of vanadyl sulfate solution with the concentrations of the vanadium ion different. We then study the permeability to explore the transport mechanism. The membrane is placed between vanadyl sulfate solution and a blank one. The diffusivity can be acquired after a simple derivation. With an analysis of the same cases in the first step, the mechanism of the fixed charge affecting the ion diffusivity comes out.