A polydopamine-coated polyamide thin film composite membrane with enhanced selectivity and stability for vanadium redox flow battery

The aim of the study is to increase the stability and selectivity of a polyamide (PA) thin film composite (TFC) membrane (M-T) used in a vanadium redox flow battery (VRB). After immersion for different periods, different concentrations of polydopamine (PDA) are successfully self-polymerized on the surface of the PA TFC membrane to prepare an optimized MDx-y (x and y denote the dopamine concentration and reaction time, respectively) membrane that is used in a VRB. The structure and physicochemical properties of MDx-y membranes are thoroughly evaluated using SEM (scanning electronic microscope), AFM (atomic force microscope), FT-IR (Fourier transform infrared spectroscopy), XPS (X-ray photoelectron spectroscopy), and TG (thermogravimetry) and compared to a pristine substrate (M-0) and M-T membranes. The PDA layer significantly improves both the selectivity and stability of the pristine M-T membrane. After coating the M-T membrane with PDA, the average pore radius of the MD membrane is estimated to be 0.24-0.25 nm, which is ideal for separate protons (<0.24 nm) from hydrated vanadium ions (>0.6 nm) in terms of ion exclusion effect. As a result, the PDA coated M-T membranes show very low vanadium ion permeability and high proton to vanadium selectivity. The coulombic efficiency of the optimized PDA-coated TFC membrane (MD2.0-10) reaches 99.3% at 80 mA cm(-2), which is higher than both pristine TFC and commercial Nafion 115 (N115) membranes. Even after 158 charge-discharge tests, the energy efficiency of the MD2.0-10 membrane remains stable at greater than 80% at 80 mA cm(-2), a value that is superior to the pure TFC membrane. In addition, the MD2.0-10 membrane also exhibits excellent chemical stability, increased mechanical properties and high discharge capacity retention rate, suggesting that it has great prospects in VRB applications.