Effect of casting solvent and annealing temperature on recast Nafion membranes for vanadium redox flow battery

The effect of casting solvent and annealing temperature on microstructure, proton conductivity, vanadium permeability, mechanical properties as well as single cell performances of recast Nafion membranes for vanadium redox flow battery (VRB) are conducted experimentally. Membranes are prepared from NMP (N-methly-2-pyrrolidone), DMF(dimethylformamide), DMSO (dimethyl sulfoxide) and EtOH (ethanol/water), annealed at 100 °C, 140 °C and 180 °C, respectively. The micromorphology of recast Nafion membranes is thoroughly examined by Fourier transform infrared (FT-IR), scanning electron microscope (SEM), ¹H NMR spectroscopy, X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). It is found that the formation of Nafion membranes is strongly influenced by both temperature and solvent. Results show that solvent is a primary factor affecting the microstructure of recast membranes at lower temperature, but its effect is no longer distinct at higher temperature. EtOH-140 membranes has the highest proton conductivity, NMP-180 has the lowest vanadium permeability, DMF-180 has the smallest ionic cluster, DMSO-180 has the best mechanical properties among the recast Nafion membranes. Single cell tests show that DMF-180 has the best comprehensive cell performances, its capacity decay rate is even smaller than that of commercial Nafion 212 membrane after charge-discharged for 100 cycles at current density of 80 mA cm⁻².