Preparation of a Sulfonated Poly(ether ether ketone)-Based Composite Membrane with Phenyl Isocyanate Treated Sulfonated Graphene Oxide for a Vanadium Redox Flow Battery

A sulfonated poly(ether ether ketone) (SPEEK)-based composite membrane was developed using functionalized graphene oxide (GO) as the carbon filler for a vanadium redox flow battery. Functionalization of sulfonated GO through phenyl isocyanate treatment (isGO) enabled GO to be dispersed uniformly in the composite membrane, which led to lower vanadium ion permeation. The isGO incorporated SPEEK (SPEEK/isGO) composite membrane exhibited very low vanadium ion permeability (1.0 × 10 −7 cm 2 min −1 ), which was approximately eight times lower than that of a Nafion 117 membrane (7.96 × 10 −7 cm 2 min −1 ). As a result, the SPEEK/isGO composite membrane showed low discharge capacity decay (19.4%) compared to the Nafion 117 membrane (72.3%) after 200 charge-discharge cycles. This was due to the uniformly dispersed and exfoliated isGO, which acted as a good barrier to prevent vanadium ion crossover through the membrane.