Preparation And Investigation Of Nafion Composite Membranes For Vanadium Redox Flow Battery

All vanadium redox flow battery(VFB)is one of promising large-scale energy storage techniques for grid management(load leveling and peak shaving),backup power and utilization and integration of renewable energy sources.Nafion membrane,as a key component of VFB,is the most commonly selection in VFB due to its high proton conductivity and excellent physicochemical stability.However the high cost,serious vanadium ion permeability and water transmission of Nafion also results in lower battery efficiency and fast capacity fading,limiting the commercial progress of Nafion.Thus,further modification of Nafion membrane is crucial.In this thesis,zirconium phosphate(ZrP),graphene oxide(GO)and polybenzimidazole(PBI)are introduced to do modification of Nafion membrane to hire excellent cell perfotrmance.Preparation of coating layer to decrease vanadium ion permeasion.The protons conduction of membrane is retained by introducing materials that carring proton conduction groups,fabricating ultra-thin coating layer and reducing thickness of membrane.Firstly,ZrP/Nafion 212 composite membranes(ZrP/N212)are prepared by impregnating method.Addtion of ZrP to enhence vanadium ions barrier effect.The protons conduction of membrane can be well retained due to the phosphate groups of ZrP.In the results of performances,vanadium ion permeability of ZrP/N212-1 is redused to 56.6 % of N212 by sacrificing 20 % of conductivity.Moreover,the coulombic efficiency(CE 80.4-96.4 %)and energy efficiency(EE 75.7-80.9 %)of ZrP/N212-1 are higher than N212(CE 66.9-91.7 %;EE 63.1-75.6 %)at the whole current density.Then,GO/Nafion composite membranes,with orientated GO nanosheets in parallel to the surface of the ultra-thin coating layer(400 ~ 440 nm),are prepared by spin coating method.Orientation of GO maximizes the vanadium ions barrier effect of GO.Spin coating with water suspensor leads to uniform dispersion of GO and good binding between GO/Nafion coating layer and substrate Nafion membrane.The GO/Nafion membrane(M-2)achieves lower vanadium ion permeability,only 2.64 % of the pristine Nafion(P-N),and higher CE and EE(98.8 % and 84.5 %,respectively)comparing with the P-N(90.1 % and 75.7 %,respectively)at a current density of 80 mA cm-2.M-2 exhibits excellent battery performances over 200 charge-discharge cycles.The capacity decay rate is about 0.23 % per cycle,much lower than those assembled with Nafion 212(0.40 % per cycle)and the P-N(0.44 % per cycle),while keeping complete structure.At last,bilayer PBI/Nafion composite membranes(~ 21 μm),with ultra-thin coating layer(120 ~ 414 nm),are prepared by spin coating method.Dense structure of PBI layer maximizes the vanadium ions barrier effect of composite membrane.Reducing thickness of Nafion membrane by pretreatment in N,N-dimethylformamide(DMF)and designing ultra-thin PBI coating layer,reducing cost and retaining protons conduction is achieved.In the results of performances,there is no vanadium ion permeation happened on PBI/Nafion composite membrane(P/N-0.05%).Moreover,the single cell performance of P/N-0.05%(CE 91.8-96.9 %;EE 70.6-85.6 %)is higher than Nafion serious membranes(N211: CE 59.7-90.4 %;EE 56.0-73.4 %.P-N: CE 57.8-90.9 %;EE 51.2-64.6 %)at the whole current density.P/N-0.05% exhibits excellent battery performances during charge-discharge cycles at a current density of 50 mA cm-2.The capacity decay rate is about 0.19 % per cycle,much lower than those assembled with N211(1.04 % per cycle)and P-N(0.92 % per cycle).And the stability of PBI/Nafion composite membrane also been certified by 200 charge-discharge cycles test.