The Structure–Activity Relationship in Membranes for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) are regarded as one of the most promising electrochemical technologies for grid-connected renewable energy storage systems. The performance of VRFBs, however, strongly depends on the membrane, one of the key components of VRFBs with critical dual functions of promotion of diffusion of active species (H⁺, H₃O⁺, SO₄ ²⁻, or SO₄H⁻) and inhibition of crossover of vanadium ions. This is intrinsically related to the microstructure of membranes. For example, large and connected ionic clusters or pores in membranes are favorable for the ion transfer, but detrimental to the ion selectivity. While small and isolated hydrophilic ion clusters or pores suppress the water uptake and ion transfer, the decreased swelling ratio would enhance chemical stability of membrane. Thus, comprehensive strategies are required to realize the optimal balance between the ion selectivity, proton conductivity, and chemical stability. This review focuses on the effects of microstructure of membranes on the ion transfer and the chemical stability, including introduction of the rigid groups, electron-withdrawing groups, and hydrophobic backbones, are reviewed. The prospect of the development of membranes with high ion selectivity and high-performance VRFBs is discussed.