Study On Low-cost Sulfonated Poly(ether Ether Ketone) Composite Membranes For Vanadium Redox Flow Battery

All vanadium redox flow batteries(VRFB) has become one of the most popular, the fastest growing and the most promising large-scale energy storage technologies due to its features such as: easy scale, long cycle life, free location.Perfluorinated sulfonic acid Nafion membranes are the mainly used ion exchange membranes(IEMs) in VRFB.However, Nafion membranes limitthe development of VRFB, due to the high cost and high vanadium ion permeationissues. Developing low-cost and high-selectivity IEMs is the important prerequisiteto promote the large scale application of VRFB,it is promisingfor SPEEKmembrane to replace Nafion membrane due to its lower cost, higher selectivity and easier scale preparation. In this work, we choose low-cost SPEEKmembrane as our study object, and conduct various modifications including: incorporating inorganic filler materials(graphene and graphene oxide), poly(tetrafluoroethylene)(PTFE) pore-filling and polydopamine(PDA) coating.To evaluate their integrated performanceand VRFB system applicability, the resultant composite membranes are investigated by various characterization tests.Firstly, we take the unique two-dimensional graphene and graphene oxidenanosheets as inorganic fillers to prepare SPEEK/Graphene(S/G) and SPEEK/Graphene oxide(S/GO) composite membranes. The 2D layered nanosheets are uniformly dispersed in SPEEK matrix, andcan serve as effective barriers to prevent vanadiumions from migrating through the membrane, leading to a decrease in vanadium ion permeability. Moreover, the interfacial interaction between the nanosheets and SPEEK matrixcontributes to improving the composite membranes’ mechanical properties and stability. Consequently, the VRFB performances are improved. At 80mAcm-2, the CEs of VRFBs assembled with Nafion 117、S/G and S/GO 2 membranes are 92.8% 、 96.4%and96.9%, respectively; and the capacity retention of VRFBs assembled with Nafion 117、S/GandS/GO 2 membranes after 300 cycles are 16.2%、56.3%and50.1%, respectively. Secondly, we fabricate a pore-filling composite membrane using highly stable PTFE polymer as the porous substrate to further improve the cycling stability of S/GOcomposite membranes for applications in long-life VRFBsystems.The S/GO 2@PTFE-VRFB has been normally run for 1200 charge-discharge cycles at a current density of 80 mA cm-2 in 2 mol L-1vanadiumelectrolyte with a stable CE(99%) and a very low capacity decay.PDA coating is adopted for the surface modification of SPEEK membrane to prepare PDA/SPEEK composite membrane. The thin PDA layer can serve as a blocking layer toprevent the crossover of vanadium ions and also asa protective film to prevent thesubstrate SPEEK membrane from being corroded by the highlyoxidative vanadium electrolyte,which is also helpful to improve the composite membranes’ mechanical property and thermal stability. Compared with Nafion 117 membrane, the optimal PDA/SPEEK composite membrane possesses higher CE(80mAcm-2, 92.8% vs 98.8%) and better cycling stability(after 150 cycles, capacity retention: 34% vs 70%).In a word, these attempts to prepare organic-inorganic, PTFE pore-filling, PDA coated composite membranes are all successful, since they can improve the pristine membrane’s properties and stability in different degrees, which possess great potential in low-cost, high-efficiency and long-life VRFB application.