Preparation and properties of quaternized poly(phthalazinone ether ketone ketone) anion-exchange membrane for all-vanadium redox flow battery

Abstract

As one of stationary energy storage device, all-vanadium redox flow battery (VRB) could effectively balance the relationship of electricity generation and demand side, in which interests are based on having features of high efficiency, flexible design, low cost and so on. Ion exchange membrane is one of the key components of VRB, separating redox-active ions and allowing the transfer of non-reaction species. Nafion series from Dupont company could ideally satisfy the requirements of high conductivity and good stability but facing the disadvantages of poor vanadium blocking ability in VRB. Modified Nafion membranes partially conquered the problem of poor ion selectivity but still suffering from high cost. Cationic exchange membranes prepared from non-fluorinated polymers have lower vanadium ion permeability and cost while cationic exchange groups hinder the further improvement of vanadium ion blocking. On the contrary, anionic exchange membranes (AEMs) exhibit low vanadium ion diffusion coefficient because of Donnan exclusive effection. It is much of importance to explore new AEMs. And the lifetime of the VRB cell depends on the selectivity and stability of the membranes to some degree. However, most of ion exchange membranes for VRB application showed low selectivity or stability. In order to improve the selectivity of ion exchange membranes, poly(phthalazinone ether ketone ketone) containing bromomethyl moieties (BPPEKK) with varied content of bromomethyl were dissolved to prepare BPPEKK base membranes using solution casting method. Quaternized poly(phthalazinone ether ketone ketone) (QBPPEKK) anion-exchange membranes were prepared from the amination of BPPEKK membranes in trimethylamine aqueous solutions. The measured ion exchange capacity (IEC) values of QBPPEKK membranes were in the range of 1.15–1.51 mmol/g, which were lower than the theoretical IEC (IECT) values. The result indicated the incomplete amination reaction of bromomethyl groups with trimethylamine. Properties of QBPPEKK membranes including water uptake, swelling ratio, area resistance and vanadium ion permeability were investigated. Water uptake and swelling ratio of QBPPEKK membranes increased with the increase in content of bromomethyl in BPPEKK, while area resistance and vanadium ion permeability decreased. The vanadium ion mass transfer coefficient of QBPPEKK membranes was in the range of 2.3×10-5 –5.0×10-5 cm/min, less than that of Nafion 117 membrane (11.9×10-5 cm/min). QBPPEKK membranes showed much lower vanadium ion permeability than Nafion 117. VRB with each QBPPEKK membrane showed higher columbic efficiency (CE) than the battery with Nafion117 membrane. QBPPEKK membrane with higher IEC exhibited higher voltage efficiency and energy efficiency (EE). When the content of bromomethyl reached 0.99, the VRB with QBPPEKK90 membrane had EE of 87.7% (current density: 40 mA/cm2), higher than that of Nafion117 membrane (86.0%). When the charge-discharge current density increased from 20 to 80 mA/cm2, the CE of VRB with QBPPEKK90 membrane increased from 97.1% to 99.0%, and EE of VRB cell with QBPPEKK90 decreased from 91.1% to 80.2%. After QBPPEKK membranes were immersed in VO2+ solution for 60 d, the tensile strength of the membranes was more than 36 MPa, which was lower than that of the original QBPPEKK90 membrane. There were no significant changes on microstructure of QBPPEKK membrane after the membrane had been immersed into 1.5 mol/L VO2+ solutions for 60 d. Compared with the original QBPPEKK90 membrane, CE changed slightly and EE decreased 1.4% at the charge-discharge current density ranging from 20 to 60 mA/cm2 after the membrane was immersed in VO2+ solution for 60 d. The results suggested that QBPPEKK membranes showed good chemical stability.