[Nafion/(WO₃)_x] hybrid membranes for vanadium redox flow batteries

Nafion‑tungsten oxide hybrid membranes, [Nafion/(WO₃)_x], with varying loading levels of WO₃ nanofiller (x = 0, 0.024, 0.329) are prepared and investigated as candidates for application as solid electrolytes in vanadium redox flow batteries (VRFBs). The thermal properties of [Nafion/(WO₃)_x] hybrid membranes are probed both by high-resolution thermogravimetric analysis (HR-TGA) and by modulated differential scanning calorimetry (MDSC). Vibrational spectroscopy studies are carried out by: (i) Attenuated Total Reflectance - Fourier Transform Infrared spectroscopy (ATR-FTIR); and (ii) Raman spectroscopy, to elucidate the secondary structure of [Nafion/(WO₃)_x] and study the interactions taking place between the nanofiller and the Nafion matrix. The electrical response of [Nafion/(WO₃)_x] is determined by Broadband Electrical Spectroscopy (BES) and the permeability towards VO²⁺ is measured by UV-VIS spectrometry. It is demonstrated that the [Nafion/(WO₃)_x] hybrid membranes exhibit a high ion selectivity (up to 10.6∙10³ S∙min∙cm⁻³ for [Nafion/(WO₃)_0.329]) that is much improved in comparison with that characterizing recast Nafion (6.5∙10³ S∙min∙cm⁻³). A structural model and a conductivity mechanism for the [Nafion/(WO₃)_x] hybrid membranes are proposed, in order to rationalize the experimental results and correlate the electrical response with the transport properties.