Novel graphitic carbon nitride nanosheets/sulfonated poly(ether ether ketone) acid-base hybrid membrane for vanadium redox flow battery

Hybrid membranes (SPEEK/g-C3N4) composed of sulfonated poly(ether ether ketone) (SPEEK) and graphitic carbon nitride (g-C3N4) are fabricated via a solution-casting method for vanadium redox flow battery (VRB). The homogeneously dispersed g-C3N4 into SPEEK matrix is demonstrated by FT-IR and SEM. The physicochemical properties such as swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability, etc. change with the content of g-C3N4 nanosheets. SPEEK/g-C3N4-1.5 hybrid membrane exhibits a higher coulombic efficiency (CE: 97%) and energy efficiency (EE: 83.6%) at 30 mA cm-2, as compared with Nafion 117 (CE: 90% and EE: 73.8%) and SPEEK membrane (CE: 89.9% and EE: 76.1%). The self-discharge time of VRB with SPEEK/g-C3N4-1.5 (68 h) is longer than that of Nafion 117 membrane (26 h). A good structure stability against strong oxidizing and acid condition is proved under 300 charge-discharge cycles. The formed acid-base pairs between -NH2 groups of g-C3N4 and sulfonic acid groups of SPEEK contribute the comprehensive properties of vanadium ion permeability and its selectivity, accompanied with an enhanced proton transport channel. These SPEEK/g-C3N4 hybrid membranes provide the acceptable candidate for VRB application because of their high structure stability and ion selectivity.