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- Allushi, Andrit, et al.
(author)
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Hydroxide conducting BAB triblock copolymers tailored for durable high-performance anion exchange membranes
- 2023
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In: Materials Advances. - 2633-5409. ; 4:17, s. 3733-3745
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Journal article (peer-reviewed)abstract
- Well-designed block copolymers with a controlled co-continuous microphase morphology can be applied as efficient anion exchange membranes (AEMs) for fuel cells and water electrolyzers. In the present work, we have prepared and studied a series of BAB triblock copolymers consisting of a central cationic polyfluorene A block with flanking hydrophobic polystyrene B blocks, where the fluorene units of the A block carried double pairs of piperidinium cations via flexible hexyl spacer chains. First, a polyfluorene tethered with bromohexyl chains was prepared by superacid-mediated polyhydroxyalkylation, and then modified to produce a bi-directional macroinitiator for atom transfer radical polymerization (ATRP). Next, ATRP of styrene was carried out to form BAB triblock copolymers with different lengths of the B blocks. Finally, the polyfluorene block was densely functionalized with piperidinium cations by Menshutkin reactions. Small angle X-ray scattering of block copolymer AEMs indicated the presence of both block copolymer phase domains (d~15 nm) and ionic clusters (d~6 nm). Atomic force microscopy showed clearly phase-separated morphologies with seemingly well-connected hydrophilic nanophase domains for ion transport. The AEMs reached hydroxide conductivities up to 161 mS cm-1 at 80 ºC. Moreover, the AEMs decomposed only above 250 °C and possessed excellent alkaline stability with no degradation detected by 1H NMR analysis after storage in 2 M aq. NaOH, at 90 °C during 672 h. Notably, the current block copolymer AEMs showed higher alkaline stability and hydroxide conductivity compared to AEMs based on corresponding statistical copolymers.
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| 3. |
- Allushi, Andrit, et al.
(author)
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Polyfluorenes Bearing N,N-Dimethylpiperidinium Cations on Short Spacers for Durable Anion Exchange Membranes
- 2023
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In: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 56:3, s. 1165-1176
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Journal article (peer-reviewed)abstract
- Alicyclic quaternary ammonium cations having all the β-protons in a strain-free ring structure are in general highly base resistant, and are thus very attractive to employ for anion exchange membrane (AEM) applications. However, tethering cations such as N,N-dimethylpiperidinium (DMP) to polymer backbones without introducing any weak links is quite challenging. In the present study, we have attached pairs of piperidine rings in their 4-position to fluorene and 2,7-diphenylfluorene via methylene bridges using straightforward SN2 reactions. These fluorenes were subsequently utilized as monomers in polyhydroxyalkylations to prepare poly(fluorene alkylene)s with different contents of the piperidine groups. AEMs were cast after quaternizing the piperidine groups to introduce DMP and spirocyclic 6-azonia-spiro-[5,5]undecane-6-ium (ASU) cations, respectively. The AEMs reached very high hydroxide ion conductivities, 100-156 mS cm-1 at 80 ºC, in the ion exchange capacity (IEC) range 1.8-2.4 mequiv. g-1. X-ray scattering showed ionomer peaks indicating ionic clustering with a characteristic distance d = 2.0-2.9 nm depending on IEC. The AEMs displayed high thermal stability, up to ~250 ºC, and 1H NMR data indicated no degradation after storage in 5 M aq. NaOH during 168 h at 90 ºC. However, degradation started under very severe conditions (10 M, 90 °C) with ~75% of the total ionic loss in all the AEMs assigned to Hofmann β-elimination. The overall results show that fluorene-based AEMs carrying DMP and ASU cations via methylene bridges display an attractive combination of ionic phase separation, thermal and chemical stability, and hydroxide conductivity.
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| 6. |
- Mansouri Bakvand, Pegah, et al.
(author)
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Poly(arylene alkylene)s with pendent benzyl-tethered ammonium cations for anion exchange membranes
- 2023
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In: Journal of Membrane Science. - : Elsevier BV. - 0376-7388. ; 668
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Journal article (peer-reviewed)abstract
- Polymeric anion-exchange membranes (AEMs) with alkali-stable quaternary ammonium (QA) cations are essential components for the development of alkaline membrane fuel cells and water electrolyzers. Ionic loss by β-elimination reactions typically accelerates at low water contents, i.e., high alkali concentrations, which makes QA cations attached to polymer backbones via benzylic sites interesting alternatives. In the present study, we synthesized and studied a series of ether-free poly(biphenyl alkylene)s (PB) and poly(p-terphenyl alkylene)s (PT) functionalized with different mono- or di-QA groups placed in benzylic positions. By employing different synthetic strategies, we systematically varied both the polymer backbone and the cationic structure to investigate the effect on morphology, alkaline stability and hydroxide (OH-) conductivity. Two precursor polymers were first synthesized via superacid-mediated polyhydroxyalkylations involving 4´-methyl-2,2,2-trifluoroacetophenone, and biphenyl and p-terphenyl, respectively. Next, these polymers were benzylbrominated to allow the introduction of trimetylammonium (TMA), quinuclidinium (Qui), piperidinium (Pip), and bis-piperidinium (bisPip) cations, respectively, through Menshutkin reactions. The ionic content was conveniently controlled by adjusting the degree of bromination through the efficient Wohl-Ziegler reaction. AEMs functionalized with bisPip groups efficiently formed ionic clusters to reach high hydroxide ion conductivities, up to 78 and 133 mS cm−1 at 20 and 80 °C, respectively, and only decomposed above 262 °C. After storage in 1 M aq. NaOH at 80 °C, AEMs functionalized with Qui, and bisPip cations showed less ionic losses in comparison to those carrying Pip and TMA cations, which may be due to the bulky structure of the cage-like Qui cation. Careful 1H NMR analysis indicated that at low alkaline concentration, loss by nucleophilic substitution at benzylic positions dominated, while ring opening by Hofmann β-elimination of the alicyclic QAs accelerated at higher alkali concentrations. The findings of the present study provided valuable insights into the influence of structure and position of QAs on the stability and degradation mechanisms of benzylic QA cations at different alkali concentrations.
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| 10. |
- Pan, Dong, et al.
(author)
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Improving poly(arylene piperidinium) anion exchange membranes by monomer design
- 2022
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In: Journal of Materials Chemistry A. - 2050-7488. ; 10:31, s. 16478-16489
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Journal article (peer-reviewed)abstract
- Energy conversion devices such as alkaline membrane fuel cells and water electrolyzers rely critically on durable anion exchange membranes (AEMs) with high hydroxide conductivity. In this context, poly(arylene piperidinium)s have emerged as one of the top candidate materials. Here, we report on the preparation and properties of poly(arylene alkylene piperidinium)s (PAAPs) with significantly higher alkaline stability than current state-of-the-art poly(arylene piperidinium)s derived from piperidone. A new piperidine trifluoromethyl ketone monomer (TFPip) was designed, synthesized and employed in superacid-mediated polyhydroxyalkylations with p- and m-terphenyl, biphenyl and fluorene, respectively. The pendant piperidine rings of the resulting polymers were then quaternized and cycloquaternized to form N,N-dimethylpiperidinium (DMP) and 6-azonia-spiro[5.5]undecane (ASU) cations, respectively. Polymers based on p- and m-terphenyl were cast into mechanically strong AEMs which reached OH- conductivities close to 80 and 180 mS cm-1 at 20 and 80 °C, respectively. The AEMs also displayed an excellent resistance against OH- attack. For example, AEMs carrying DMP cations showed a mere 14% ionic loss after storage in 5 M aq. NaOH at 90 °C during 20 days. In comparison, a corresponding benchmark poly(arylene piperidinium) AEM lost three times as many DMP cations (42%) under the same conditions. The results of the study demonstrate that the overall properties and alkaline stability of AEMs can be considerably improved by rational monomer design.
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