Effect of crosslinking on the properties of partially fluorinated anion exchange membranes

Abstract

A series of crosslinked, ammonium-functionalized, and partially fluorinated copolymers have been prepared and evaluated as anion exchange membranes. In order to investigate the effect of crosslinking on the membrane properties, precursor copolymers containing chloromethyl groups were crosslinked with various aliphatic diamines followed by quaternization with monoamines. Crosslinking was effective in lowering water absorbability at no expense of high hydroxide ion conductivity of the membranes. By tuning the degree of crosslinking (20 mol %) and crosslinker chain length (C6 and C8), the highest ion conductivity of 73 mS/cm (at 80 C in water) was achieved. Furthermore, alkaline stability of the membranes was also improved by the crosslinking; the remaining ion conductivity after the stability test (in 1 M potassium hydroxide at 80 C) was 8.2 mS/cm (after 1000 h) for the C6 crosslinked membrane and 1 mS/cm (after 500 h) for the uncrosslinked membrane, respectively. The ammonium groups attached with the crosslinkers seemed more alkaline stable than the uncrosslinked benzyltrimethylammonium groups, while the polymer main chain was intact under the harsh alkaline conditions.

Chemically Durable Poly(phenylene-co-arylene ether) Multiblock Copolymer-Based Anion Exchange Membranes with Different Hydrophobic Moieties for Application in Fuel Cells

“…QPP-6F has the lowest activation energy (11.5 kJ mol –1 ), followed by QPP-BPM, QPP-BPPH, and the Fumatech membrane with 12.2, 12.4, and 16.1 kJ mol –1 , respectively. These values correspond with reported values for AEMs, which indicates that the hydroxide ion is transported by a similar mechanism. , The lower activation energy of the QPPs, compared to that of the commercial Fumatech membrane, also implies a lower energy barrier to overcome for successful conductivity; this is due to the well-developed phase separation of the multiblock structure.…”

“…These values correspond with reported values for AEMs, which indicates that the hydroxide ion is transported by a similar mechanism. 42,43 The lower activation energy of the QPPs, compared to that of the commercial Fumatech membrane, also implies a lower energy barrier to overcome for successful conductivity; this is due to the well-developed phase separation of the multiblock structure. Further, the characteristic factor (Φ), defined as the ratio of the chloride conductivity to the water volume fraction, was determined to assess the overall performance of the membranes.…”

Series-Connected Tetracation Partially Cross-Linked Anion Exchange Membranes: Insight towards Consequences of Alkyl Spacer Length

“…The alkyl spacer between cationic head groups in a cross-link plays dual role: it render high water absorptions to the membrane by increasing free volume and it also suppress water absorption by the membrane sample if the length of spacer exceeds a critical limit because of increase in hydrophobicity. , At 30 °C, AEM-Q5-C X membranes with X = 3, 4, 6, 8, and 10 exhibited 27.89, 41.61, 35.54, 30.96, and 25.35% water uptake corresponding to the number of water molecules per QAC (referred as λ) of 7.48, 11.11, 9.67, 8.63, and 7.29. A study by Koronka et al also realized similar effect of alkyl spacer chain length on the number of water molecules per quaternary ammonium group . The lower water uptake by AEM-Q5-C3 membrane is more likely due to the close proximity of QACs at low spacer chain length, which hinders ion dissociation because of electrostatic repulsion; thus, the membrane matrix imparts low free ion concentration amenable to water uptake.…”

Partially fluorinated copolymers containing pendant piperidinium head groups as anion exchange membranes for alkaline fuel cells

“…These changes in tensile properties also suggest possible cross-linking to some extent. 45 In addition, loss of quaternary ammonium groups inevitably caused lower water absorbability which should result in increased Young's modulus by making the membrane stiffer and less elastic. 46 QPAF4-C3 and 6-pip membranes exhibited much smaller changes in SS curves after the alkaline stability test, indicating smaller degree of the cross-linking reaction.…”