Highly conductive hydroxide exchange membranes containing fluorene-units tethered with dual pairs of quaternary piperidinium cations

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T1 - Highly conductive hydroxide exchange membranes containing fluorene-units tethered with dual pairs of quaternary piperidinium cations

AU - Allushi, Andrit

AU - Pham, Thanh Huong

AU - Jannasch, Patric

N1 - Available online 24 April 2021

PY - 2021/8/15

Y1 - 2021/8/15

N2 - In the pursuit of anion exchange membranes (AEMs) with high alkaline stability and hydroxide conductivity, we have prepared and characterized a series of poly(fluorene alkylene)s in which each fluorene unit was functionalized with dual pairs of quaternary piperidinium cations on flexible alkyl spacer chains. First, ether-free precursor polymers were synthesized in superacid mediated polyhydroxyalkylations of 2,2,2-trifluoroacetophenone, 9,9-dimethyl-2,7-diphenyl-9H-fluorene, and different amounts of 2,7-dibromo-9,9-bis(6-bromohexyl)-fluorene to regulate the degree of bromoalkylation. Subsequently, the bromoalkyl side chains were utilized to introduce bis-piperidinium (bisPip) cations via Menshutkin reactions. These materials formed transparent and mechanically strong AEMs upon casting. At 80 °C, the hydroxide conductivity of bisPip AEMs reached 85 and 150 mS cm–1 at ion-exchange capacities (IECs) of 2.0 and 2.8 mequiv g–1, respectively. Moreover, the bisPip AEMs showed high alkaline stability with an ionic loss of merely 6% following immersion in 5 M aq. NaOH solution for a period of 168 h at IEC = 2.8 mequiv g–1. Under these conditions, 1H NMR data indicated that a β-hydrogen in an alkyl spacer chain was about 8 times more susceptible to attacks by hydroxide ions than a β-hydrogen in a piperidinium ring. In comparison, corresponding AEMs with fluorene units functionalized with monoPip cations (i.e., a single pair of piperidinium cations per fluorine unit) showed lower conductivity and alkaline stability under the same conditions, demonstrating the advantage of locally concentrating the cations in the polymer structure by employingbisPip side chains.

AB - In the pursuit of anion exchange membranes (AEMs) with high alkaline stability and hydroxide conductivity, we have prepared and characterized a series of poly(fluorene alkylene)s in which each fluorene unit was functionalized with dual pairs of quaternary piperidinium cations on flexible alkyl spacer chains. First, ether-free precursor polymers were synthesized in superacid mediated polyhydroxyalkylations of 2,2,2-trifluoroacetophenone, 9,9-dimethyl-2,7-diphenyl-9H-fluorene, and different amounts of 2,7-dibromo-9,9-bis(6-bromohexyl)-fluorene to regulate the degree of bromoalkylation. Subsequently, the bromoalkyl side chains were utilized to introduce bis-piperidinium (bisPip) cations via Menshutkin reactions. These materials formed transparent and mechanically strong AEMs upon casting. At 80 °C, the hydroxide conductivity of bisPip AEMs reached 85 and 150 mS cm–1 at ion-exchange capacities (IECs) of 2.0 and 2.8 mequiv g–1, respectively. Moreover, the bisPip AEMs showed high alkaline stability with an ionic loss of merely 6% following immersion in 5 M aq. NaOH solution for a period of 168 h at IEC = 2.8 mequiv g–1. Under these conditions, 1H NMR data indicated that a β-hydrogen in an alkyl spacer chain was about 8 times more susceptible to attacks by hydroxide ions than a β-hydrogen in a piperidinium ring. In comparison, corresponding AEMs with fluorene units functionalized with monoPip cations (i.e., a single pair of piperidinium cations per fluorine unit) showed lower conductivity and alkaline stability under the same conditions, demonstrating the advantage of locally concentrating the cations in the polymer structure by employingbisPip side chains.

KW - Anion exchange membrane

KW - Hydroxide conductivity

KW - Alkaline stability

KW - Anion exchange membrane fuel cell

KW - Polyhydroxyalkylation

U2 - 10.1016/j.memsci.2021.119376

DO - 10.1016/j.memsci.2021.119376

M3 - Article

VL - 632

JO - Journal of Membrane Science

JF - Journal of Membrane Science

SN - 0376-7388

M1 - 119376

ER -