Synthesis and characterization of anion exchange blend membranes for vanadium redox flow battery applications

Abstract

In this dissertation, Anion Exchange Blend Membranes (AEBMs) were synthesized and applied in Vanadium Redox Flow Batteries (VRFBs). In the first paper, AEBMs were systematically optimized for VRFBs by varying the composition of polymers components. A bromomethylated poly (2, 6-dimethyl-1,4-phenylene oxide) (Br-PPO) was used as an anionic exchange precursor which was quaternized with 1,2,4,5-tetramethylmidazole (TMIm). A Polybenzimidazole-OO (PBI-OO, produced by Fuma-Tech) was used as a matrix polymer to provide mechanical strength. A minor amount of sulfonated polymer was used as an ionical cross-linker. Those AEBMs showed comparable Energy Efficiency (EE) with Nafion 212 membranes and one of the synthesized AEBMs (BM-TMIm 4) showed a superior Coulombic Efficiency (CE) of almost no decreasing after 300 charging-discharging cycles with a significant capacity retention of 77% of the initial value for after 300 charging-discharging cycles at a current density of 40 mA/cm2. Therefore, AEBMs are promising candidates for long-term operation in VRFBs if the proportion and type of the different components in the blend system is carefully adjusted. In the first paper, the composition of AEBMs were optimized for use in VRFBs. In the second paper, AEBMs were prepared with different polymer combinations. These AEBMs consisted of 3 polymer components. 1) F6-PBI (fluorinated PBI) or PBI-OO (non-fluorinated PBI): PBI was used as a matrix polymer, 2) Br-PPO: Br-PPO was used as an anion exchange polymer precursor by quaternizing with TMIm to provide anion exchange sites, 3) a partially fluorinated polyether or a non-fluorinated poly (ether sulfone): sulfonated polymer was used as an ionical cross-linker. The same weight ratios of three components were used in blend membranes, while different combinations of polymers were used. Similar properties of blend membranes such as ion exchange capacity, conductivity and swelling behavior showed since same amount of anion exchange polymer in each blend membrane was used. In VRFB test, all blend membranes showed better performances than the commercial membranes (Nafion: a cation exchange membrane and FAP 450: an anion exchange membrane) in terms of coulombic-, voltage- and energy efficiencies. One of the blend membranes (BM-TMIm4 FF), which is composed fluorinated polymers, exhibited excellent capacity retention showing no capacity decay over 550 charging-discharging cycles run at a current density of 40 mA/cm2. The outstanding performance of fluorinated polymers-based blend membranes probably is due to the highly stability of F6-PBI in an acidic condition. A pure F6-PBI membrane showed no structural changes in 30 % sulfuric acid solution for 9 days confirmed by Fourier-Transfrom Infrared Spectroscopy (FT-IR spectra), while a PBI-OO membrane was sulfonated after few days. Thus, it can be concluded that if proper matrix polymer chosen for blend membrane, the AEBMs in VRFBs are expected to exhibit superior performance. In the paper 3, AEBMs were synthesized by 3 steps based on Poly(pentafluorostyrene) (PPFSt) for use in VRFBs. Firstly, 1-(2-dimethylaminoethyl)-5-mercaptotetrazole was grafted onto PPFSt by nucleophilic substitution on the para-position. Secondly, the tertiary amino groups were quaternized with iodomethane to provide anion exchange sites. Thirdly, AEBMs were fabricated by blending of synthesized anion exchange polymer with F6-PBI. The blend membrane containing of 30% F6PBI showed better VRFBs performance that Nafion membrane in terms of energy efficiency, Open Circuit Voltage (OCV) and charging-discharging cycling. While the blend membrane containing of 40% F6-PBI displayed much longer OCV time and capacity retention by a charging-discharging test than that of Nafion membrane. It can be concluded that AEBMs are strong candidate for VRFB applications. The AEBMs tested in VRFBs have shown better performances than that of commercial reference membranes of a cation exchange membrane (Nafion) and an anion exchange membrane (FAP 450). By considering the battery test results of AEBMs in this study, therefore, it can be concluded that the AEBMs are very promising candidates as separators in VRFBs if the matrix polymer is chosen properly. One further potential application of those blend membranes can be used in phosphoric acid doped high temperature proton exchange membrane fuel cells (PA-doped HT-PEMFCs), since the AEBMs in this dissertation showed high thermal stability. In recent paper (Nat Energy 1, 16120 (2016)), anion exchange membrane showed very promising results in PA-doped PEMFCs displaying much better performances than that of a polybenzimidazole membrane (a standard membrane for PA-doped HT-PEMFCs) in the FC test. The results in our patent have shown also that those anion exchange membranes exhibited promising performances in the fuel cell test. Therefore, anion exchange blend membranes synthesized in this study are expected to excellent performances for phosphoric acid doped HT-PEMFCs.