Sulfonated Poly(Ether Ether Ketone)/Graphene composite membrane for vanadium redox flow battery

Graphical abstract: Graphene sheets act as a physical barrier of vanadium ion diffusion in SPEEK/Graphene (S/G) composite membrane resulting in good selectivity, therefore higher coulombic and energy efficiencies and better cycle performance are achieved. - Highlights: • SPEEK/Graphene (S/G) composite membrane is first used in vanadium redox flow battery. • S/G composite membrane is homogeneous accompanied with high ion selectivity. • High VRB single cell efficiencies are obtained by using S/G composite membrane. • VRB with S/G composite membrane shows stable cycling performance and slow capacity decline during 500 cycles. - Abstract: A novel composite membrane of sulfonated poly(ether ether ketone) (SPEEK) and graphene was prepared and investigated in this paper. The scanning electron microscopy images of the composite membrane revealed the presence and the uniformity dispersion of graphene in the SPEEK matrix. The properties of the obtained composite membrane, including the water uptake, swelling ratio, ion exchange capacity (IEC), area resistance, proton conductivity, VO2+ permeability, ion selectivity, single cell performance, chemical stability and mechanical properties are evaluated in detail, compared to the pristine SPEEK membrane and Nafion 117 membrane. Self-discharge test proved that the composite membrane possessed the longest discharge time (61 h above 0.8 V). At 80 mA cm−2, a higher coulombic and energy efficiency for SPEEK/Graphene membrane was obtained, compared with Nafion 117 (96.4% vs 92.8% and 83.8% vs 79.5% respectively). Furthermore, the SPEEK/Graphene composite membrane exhibited high stability in 500 cycle's long time running