Preparation And Properties Of Polybenzimidazole-Based Anion Exchange Membranes For All Vanadium Redox Flow Battery

All vanadium redox flow battery ?VRB? has been identified to be one of the most promising technologies for large-scale energy storage due to its high safety, high cycle life,easily designed capacity, fast response and short construction time. Membrane separator is one of the key materials of a VRB, which directly influences the cycle life and energy conversion efficiency of the VRB. In this thesis, a series of low cost anion exchange membranes with high ionic conductivity, low vanadium permeability, high energy conversion efficiency, low self-discharge rate and long cycle life have been developed.In Chapter 2, a series of novel cross-linked blend membranes have been successfully prepared from poly?2,2'-?p-oxydiphenylene?-5,5'-bibenzimidazole? ?OPBI? and partially quaternized poly?1-vinyl imidazole? ?QPVI? at varied polymer weight ratios using 1,6-dibromohexane as the cross-linker. The effect of the QPVI content on membrane properties such as mechanical properties, thermal stability, ion exchange capacity ?IEC?,water uptake, swelling ratio and anion conductivity are investigated. The OPBI/QPVI-CL cross-linked blend membranes exhibit a lower VO2+ permeability ?9.92×10-9 - 4.52×10-10 cm2·min-1? than Nafion117 ?4.23×10-6 cm2.min-1? . The VRB single cells assembled with the OPBI/QPVI-CL cross-linked blend membranes show higher coulombic efficiency ?94.9 - 97.5%vs. 72.7%? and energy efficiency ?88.7 - 93.8% vs. 71.4%? at 10 mA·cm-2 than the one assembled with Nafion117. Furthermore, the VRB single cell assembled with the cross-linked blend membrane OPBI/QPVI?3:7, weight ratio?-CL shows little decay in energy efficiency after 300 charge-discharge cycles test at 80 mA cm-2 indicating a good cycle stability of this membrane.In the Chapter 3, a novel diepoxy groups-terminated quaternary ammonium salt N has been synthesized using N, N, N', N'-tetramethyl-1, 6-hexanediamine and 6-bromohexene as the raw materials. The quaternary ammonium salt N and bisphenol A diglycidyl ether ?A? are used as cross-linkers to react with a polybenzimidazole with pendant amino groups ?H₂N-PBI?yielding a series of novel cross-linked anion exchange membranes with good mechanical properties. The effects of the molar ratio of the amino groups of the H₂N-PBI to the epoxy groups of the quaternary ammonium N and bisphenol A diglycidyl ether on mechanical properties, thermal stability, ionic conductivity, water uptake and swelling ratio are investigated. In comparison with the VRB single cell assembled with Nafion212, the one assembled with the H₂N-PBI-N ?1:1, by molar?-A film shows higher voltage efficiency ?77.1%vs. 73.8%?, higher energy efficiency ?74.9% vs. 72.2%? and higher charge and discharge capacity ?275 m · Ah vs. 159 m · Ah? at a high current density of 80 mA · cm-2. The VRB single cell assembled with the H₂N-PBI-N?1:1?-A membrane showed no significant decrease in energy efficiency after 100 charge-discharge cycles at a current density of 80 mA · cm-2 indicating good chemical stability of this membrane.In Chapter 4, a monoepoxy group-terminated quaternary ammonium salt D has been synthesized from N, N, N', N'-tetramethyl-1, 6-hexanediamine and 6-bromohexene. A series of anion exchange membranes have been prepared by grafting the quaternary ammonium salt D onto H₂N-PBI followed by cross-linking with bisphenol A diglycidyl ether. The effects of graft yield and cross-linking density on the mechanical properties, thermal stability, water uptake, swelling ratio and ionic conductivity are investigated. The VRB single cell assembled with the H₂N-PBI-D?1:0.8, molar ratio?-CL membrane exhibits higher coulombic efficiency,higher voltage efficiency, higher energy efficiency, lower self-discharge rate, higher charge and discharge capacity at current densities of 10 - 80 mA · cm-2 than the one assembled with Nafion212. The VRB single cell assembled with the H₂N-PBI-D?1:0.8?-CL membrane shows little decay in coulombic efficiency, voltage efficiency and energy efficiency after 100charge-discharge cycles test at a high current density of 80 mA · cm-2 and 72.4% of the initial discharge capacity value is maintained indicating that the H₂N-PBI-D?1:0.8?-CL membrane is fairly stable.