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

Abstract

The aim of the study is to increase the stability and selectivity of a polyamide (PA) thin film composite (TFC) membrane (MT) 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 (M0) and MT membranes. The PDA layer significantly improves both the selectivity and stability of the pristine MT membrane. After coating the MT 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 MT 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.