Preparation of the Sulfonated Poly(ether ether ketone)/Functionalized Graphene Oxide Composite Membrane for Vanadium Redox Flow Battery

Abstract

The all-vanadium redox flow batteries (VRFBs), which utilize VO2+/VO2 + redox couples as the positive electrolyte and V2+/V3+ couples as the negative electrolyte, have been regarded as promising candidate for commercial energy storage system due to the high energy efficiency, flexible design and long life cycle1-3. The membrane is one of the key components in VRFBs because the performance of VRFBs depends on the properties of membrane. An ideal membrane for the VRFBs should have good proton conductivity, very low vanadium ion permeability, and low cost. Currently, Dupont’s Nafion membrane has been commonly used in VRFBs due to its high proton conductivity and excellent chemical stability. However, the Nafion membrane suffers from drawbacks, such as high vanadium permeability and high cost. Although considerable efforts have been devoted to address this challenge, they still remain a critical issue for wide commercialization of VRFBs. Herein, we propose the functionalized graphene oxide (fsGO) incorporated sulfonated poly(ether ether ketone) composite membrane. It has been demonstrated that the sulfonated GO (sGO) enhance the proton conductivity of membrane due to the improved the hydrophilicity of GO4-6. The sGO was further functionalized for more uniform and homogeneous dispersion in the composite membrane, and the effect of good dispersion on the membrane performance was investigated in this study. As expected, the uniformly dispersed and exfoliated fsGO was observed in the composite membrane compared to the sGO. In addition, the SPEEK/fsGO composite membrane exhibited very low vanadium ion permeability, which is about 8 times lower than that of Nafion 117 membrane. In practical VRFB single cell test, the SPEEK/fsGO composite membrane showed a higher coulombic and energy efficiency compared to the Nafion 117 membrane, and the SPEEK/fsGO exhibited very low discharge capacity decay (19.4 %) compared to the Nafion 117 membrane (72.3 %) after 200 cycles of charge-discharge process. Through the measuring the vanadium ion concentration of positive and negative electrolyte for VRFB cell after 200 cycles, it is obviously confirmed that the much lower vanadium ion crossover was observed at the VRFB cell using SPEEK/fsGO composite membrane, implying the high performance and long cycle life VRFB cell. This was attributed that uniformly dispersed and exfoliated fsGO could act as effective barrier to prevent the vanadium ion crossover through membrane. Based on these experimental results, we suggest that the SPEEK/fsGO composite membrane can be one of the promising candidates for VRFB application.