Inducing microstructural changes in Nafion by incorporating graphitic carbon nitride to enhance the vanadium-blocking effect

Two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets are introduced into a Nafion matrix to prepare a 'vanadium-blocking' recast Nafion membrane for vanadium redox flow battery (VRFB) applications. After 0.2 wt% g-C₃N₄ nanosheets are incorporated, the vanadium ion permeability of the composite membrane decreases from 3.3 × 10^-7 cm² min^-1 to 3.8 × 10^-9 cm² min^-1, which is a reduction of two orders of magnitude in comparison to the pristine recast Nafion membrane. This satisfactory result contributes to the physical blocking effect as well as the Donnan effect from the 2D morphology and functional amino groups of g-C₃N₄ nanosheets. Notably, this work reveals that the g-C₃N₄ nanosheets can help reinforce the vanadium-blocking effect by changing the microstructure of Nafion in addition to the well-known effects mentioned above. The g-C₃N₄ nanosheets induce shrinkage in the original spherical structure of the ion cluster and generate a new lamellar structure. Correspondingly, the amorphous phase of Nafion is interrupted, and the membrane crystallinity is reduced. The VRFB with an optimized composite membrane achieves a high coulombic efficiency of 97% and an energy efficiency of 83% at a current density of 160 mA cm^-2. Meanwhile, the battery exhibited excellent lifetime stability during a 100 charge-discharge cycling test.