Thermally cross-linked sulfonated poly(ether ether ketone) membranes containing a basic polymer-grafted graphene oxide for vanadium redox flow battery application

Abstract

Sulfonated poly(ether ether ketone) (SPEEK) with a very high degree of sulfonation (DS) and poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) are prepared and used as a polymer matrix and a filler, respectively, to prepare thermally cross-linked membranes for vanadium redox flow battery (VRFB) application. The thermally cross-linked SPEEK membranes containing ABPBI-GO exhibit significantly enhanced physico-chemical stabilities due to the formation of the well-developed cross-linked structures by the chemical bonds formed through the solvothermal process and the acid-base interactions between the benzimidazole groups in filler and sulfonic acid groups in polymer chain. In addition, ABPBI-GO in the polymer matrix is found to be effective in decreasing the vanadium ion permeability while retaining high proton conductivity producing a high-performance VRFB membrane. The ionic selectivity of the thermally cross-linked SPEEK membrane containing 0.5 wt% of ABPBI-GO (TC-SPEEK/0.5) is 13.15 x 10(7) mS min cm(-3), which is approximately 8 times larger than that of Nafion 212 (1.57 x 10(7) mS min cm(-3)). Accordingly, the VRFB cell fabricated using TCSPEEK/0.5 membrane exhibits higher coulombic efficiencies (CE: 98.1-99.4%), voltage efficiencies (VE: 92.5-85.2%), and energy efficiencies (EE: 90.7-84.7%) than that using Nafion 212 (CE: 89.2-93.3%, VE: 90.7-81.2%, EE: 80.9-75.7%) at current density of 40-100 mA cm(-2). Furthermore, the long-term stability of the VRFB cell using TC-SPEEK/0.5 membrane evaluated by the cycle test under realistic VRFB operation conditions is also found to be much better than that using Nafion 212.