| Separator as the key component of vanadium redox flow battery(VRB), functions to stop the cross-contamination of vanadium species between the two electrolytes, but to allow the transfer of current carrier(e g. H+) to complete the circuit in VRB. Up to now, the mostly-used separator for VRB is perfluorosulfonate ion-exchange membrane(e. g. Dupont’s Nafion® series), which features good proton conductivity and long-term chemical stability. However, this kind of PEM is high-cost and suffering from a drawback of serious vanadium ion leakage, limiting the broader commercial application. To overcome the problems above-mentioned, alternatives of cost-competitive separators with high proton conductivity, low vanadium ion diffusion, good mechanical and chemical stability are actively being developed. In this work, we prepared some novel poly(arylene ether ketong)-based amphoteric ion-exchange membranes(AIEMs) for VRBs, and the main content is described as follows:(1) A series of benzimidazole-containing poly(arylene ether ketone)s were developed. Amorphous polymers named poly(arylene ether ketone-benzimidazole)s were synthesized. Evaluation of solubility reveals that the polymers could be soluble in commonly used organic solvents. Also, polymers containing content-tunable benzimidazole show high glass transition temperatures(Tg’s, 157-319 oC, by DSC and DMA) and excellent thermal stability(e.g. temperature of 5% weight loss, above 438 oC in air). Dielectric constants of the polymers at 25 oC are all less than 2.66 in the frequency range of 0.1-50 kHz.(2) A strategy to modify the highly sulfonated poly(ether ether ketone) was proposed. The blends with PAEK-alt-BI content up to 20 wt% via optical observation were transparent, indicating an excellent miscibility between the solfonated poly(ether ether ketone)(SPEEK) and poly(arylene ether ketone-alt-benzimidazole)(PAEK-alt-BI). This was further verified by a well-defined morphology and nanoscale domain size of the membranes according to TEM and SEM images. The compatibility between SPEEK and PAEK-alt-BI could be ascribed to the acid-base interactions between the sulfonic acid groups in SPEEK and the benzimidazole groups in PAEK-alt-BI as well as the similarity in the molecular structures of SPEEK and PAEK-alt-BI. It was found that, the addition of PAEK-alt-BI dramatically improved thermal stability and oxidative stabilities, dimensional stability, and mechanical properties of SPEEK membrane. Further, VRB with optimized blend shows the higher Coulombic efficiency(99.4% vs. 96.8%), voltage efficiency(75.9% vs. 3.3%) and energy efficiency(75.4% vs. 71.0%) than that of Nafion117.(3) An amphoteric ion-exchange membrane(AIEM) from fluoro-methyl sulfonated poly(arylene ether ketone) was fabricated. The AIEM and its covalently cross-linked membrane(AIEM-c) behave the highly-supressed vanadium-ion crossover and their tested VO2+ permeability(2.24×10-11cm2 min-1 & 1.28×10-11 cm2 min-1) are about 638 and 1117 times lower than that of Nafion117(1.43×10-8cm2 min-1), respectively. This is further typically verified by the lower VO2+ concentration inside AIEM that is less than half of that inside Nafion117 detected by EDX, in addition of the nearly 3 times longer battery self-discharge time. The ultra-low vanadium ion diffusion could be ascribed to the narrower ion transporting channel originated from the acid-base interactions and the rebelling effect between the positively-charged benzimidazole structure and VO2+ ions. It is found that, VRB assembled with AIEM exhibits the equal or higher Coulombic efficiency(99.0% vs. 96.4%), voltage efficiency(90.7% vs. 90.7%) and energy efficiency(89.8% vs. 87.4%) than that with Nafion117 and keeps continuous 220 charge-discharge cycles for over 25 days.(4) The relationship between structure and properties of AIEMs from fluoro-methyl sulfonated poly(arylene ether ketone) was revealed. A series of fluoro-methyl sulfonated poly(arylene ether ketone-x%benzimidazole)(60SPAEK-6F-co-x%BI) were fabricated and systematically evaluated. The investigations demonstrate that the compacted structure of 60SPAEK-6F-co-x%BI that originated from the self-assembly ionic cross-linking, and the positive charged benzimidazole moiety in AIEM matrix could synergistically reduce the vanadium species cross-contamination, of which the self-discharge time of AIEMs is 1.4-3.4 times longer than that of Nafion117. At 30, 50 and 70 mA cm-2, VRB single cells assembled with the optimized AIEM exhibit higher battery efficiencies and lower capacity loss than that with Nafion117. For example, at 70 mA cm-2, single cells assembled with AIEM show the compared Coulombic efficiency(98.8-99.7% vs. 96.9%), voltage efficiency(73.6-68.4% vs. 69.8%) and energy efficiency(68.1-67.7% vs. 67.7%) to that with Nafion117. In addition, the ex situ chemical stability test reveals that AIEM is able to tolerate the strong oxidizing VO2+ and remain intact by immersion in acidic 1.5 M(VO2+)2SO4 solution for over 370 days.(5) A series of novel acid-base blends was fabricated. Quaternized poly(ether sulfone)(QPES) was used to improve the sulfonated poly(arylene ether ketone) with hexafluoroisopropylidene groups(60SPAEK-6F), and the effect of QPES on the physical-chemical properties and VRB single cell performance. The investigations show that the ion-exchange capacity, thermal stability is slightly decreased while the area resistance is increased as the addition of QPES. VO2+ permeabilities of blends are 1.54-2.07×10-11cm2 min-1, which are higher than that of Nafion117(6.61×10-10cm2 min-1). The results demonstrate that VRBs assembled with the blends show the compared battery efficiency to that of Nafion117 during 560 charge-discharge cycles, indicating the good chemical stability of SPAEK-6F/QPES blends as well. However, the self-discharge time of VRBs assembled with the blends(>500h) are much longer than that of Nafion117(80h), indicating that the good battery performance and chemical stability of blends. |
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