Solution casting Nafion/polytetrafluoroethylene membrane for vanadium redox flow battery application

Solution casting method was adopted using Nafion and polytetrafluoroethylene (PTFE) solution to prepare Nafion/PTFE blend membranes for vanadium redox flow battery application. The physicochemical properties of the membranes were characterized by using water uptake, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermal analysis (TA). The electrochemical properties of the membranes were examined by using electrochemical impedance spectroscopy (EIS) and single cell test. Despite the high miscibility of PTFE with Nafion, the addition of hydrophobic PTFE reduces the water uptake, ion exchange capacity (IEC) and conductivity of blend membranes. But PTFE can increase the crystallinity, thermal stability of Nafion/PTFE membranes and reduce the vanadium permeability. The blend membrane with PTFE (30 wt%, N0.7P0.3) was chosen and investigated for VRB single cell test. The energy efficiency of this VRB with N0.7P0.3 membrane was 85.1% at current density of 50 mA cm−2, which was superior to that of recast Nafion (r-Nafion) membrane (80.5%). Self-discharge test shows that the decay of open circuit potential of N0.7P0.3 membrane is much lower than that of r-Nafion membrane. More than 50 cycles charge–discharge test proved that the N0.7P0.3 membrane possesses high stability in long time running. Chemical stabilities of the chosen N0.7P0.3 membrane are further proved by soaking the membrane for 3 weeks in highly oxidative V(V) solution. All results suggest that the addition of PTFE is a simple and effective way to improve the performance of Nafion for VRB application.