A series of novel composite membranes, based on sulfonated poly(ether ether ketone) (SPEEK) with various graphene oxide (GO) loadings, were employed and investigated in vanadium redox flow battery (VRFB) for the first time. The scanning electron microscopy images of the composite membranes revealed the uniform dispersion of GO nanosheets in the polymer matrix due to the interaction between GO and SPEEK, as confirmed by Fourier transform infrared spectra. The mechanical and thermal parameters of the composite membranes increased, while the VO 2+ permeability decreased with increasing GO content.Random embedding of GO nanosheets in the membranes can serve as effective barriers to block the transport of vanadium ion, resulting in a significant decrease of vanadium ion permeability. The VRFB assembled with the composite membrane exhibited highly improved cell parameters and strikingly long cycling stability compared with commercial Nafion 117 membrane. With the protection of porous PTFE substrate, the pore-filling SPEEK/GO composite membrane based on VRFB ran for 1200 cycles with relatively low capacity decline.

Acid-base blend membrane prepared from sulfonated poly(ether ether ketone) (SPEEK) and polyacrylonitrile (PAN) was detailedly evaluated for vanadium redox flow battery (VRFB) application. SPEEK/PAN blend membrane exhibited dense and homogeneous cross-section morphology as scanning electron microscopy and energy-dispersive X-ray spectroscopy images show. The acid-base interaction of ionic cross-linking and hydrogen bonding between SPEEK and PAN could effectively reduce water uptake, swelling ratio, and vanadium ion permeability, and improve the performance and stability of blend membrane. Because of the good balance of proton conductivity and vanadium ion permeability, blend membrane with 20 wt % PAN (S/PAN-20%) showed higher Coulombic efficiency (96.2% vs 91.1%) and energy efficiency (83.5% vs 78.4%) than Nafion 117 membrane at current density of 80 mA cm(-2) when they were used in VRFB single cell. Besides, S/PAN-20% membrane kept a stable performance during 150 cycles at current density of 80 mA cm(-2) in the cycle life test. Hence the SPEEK/PAN acid-base blend membrane could be used as promising candidate for VRFB application.

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