Effect of Radical‐Mediated Cross‐Linking on Partially Fluorinated Aromatic Anion Exchange Membranes and their Applications in Alkaline Water Electrolysis Cells

To investigate the effect of cross‐linking on partially fluorinated anion exchange membranes tethered with trimethylpropyl side chains (QPAF‐C3), styrene‐based cross‐linker is introduced into the precursor copolymers and then cross‐linked via free radical reaction. The one‐pot cross‐linking and quaternization reactions are successful as confirmed through nuclear magnetic resonance spectra. By solution casting, the resulting polymers provide flexible membranes (xQPAF‐C3‐VB) with 9.1–36.0% degree of cross‐linking. The cross‐linking results in smaller hydrophilic/hydrophobic phase‐separated morphology as confirmed by transmission electron microscopy images. The cross‐linking effect on the membrane properties is observed in the suppressed water uptake and decreased hydroxide ion conductivity. Among the cross‐linked membranes, xQPAF‐C3‐VB membranes with 17.4% degree of cross‐linking and 1.16 meq g−1 of ion exchange capacity exhibit the highest hydroxide ion conductivity (56 mS cm−1 at 30 °C) that is comparable to that of the pristine membrane (54 mS cm−1). The cross‐linking contributes to improving the thermomechanical properties with higher glass transition temperature. The cross‐linked xQPAF‐C3‐VB is applied to alkaline water electrolyzer to achieve high efficiency (74%) and reasonable performance (1.67 V at 1.0 A cm−2).

To investigate the effect of cross-linking on partially fluorinated anion exchange membranes tethered with trimethylpropyl side chains (QPAF-C3), styrene-based cross-linker is introduced into the precursor copolymers and then cross-linked via free radical reaction.The one-pot cross-linking and quaternization reactions are successful as confirmed through nuclear magnetic resonance spectra.By solution casting, the resulting polymers provide flexible membranes (xQPAF-C3-VB) with 9.1-36.0%degree of cross-linking.The cross-linking results in smaller hydrophilic/ hydrophobic phase-separated morphology as confirmed by transmission electron microscopy images.The cross-linking effect on the membrane properties is observed in the suppressed water uptake and decreased hydroxide ion conductivity.Among the cross-linked membranes, xQPAF-C3-VB membranes with 17.4% degree of cross-linking and 1.16 meq g À1 of ion exchange capacity exhibit the highest hydroxide ion conductivity (56 mS cm À1 at 30 °C) that is comparable to that of the pristine membrane (54 mS cm À1 ).The cross-linking contributes to improving the thermomechanical properties with higher glass transition temperature.The cross-linked xQPAF-C3-VB is applied to alkaline water electrolyzer to achieve high efficiency (74%) and reasonable performance (1.67 V at 1.0 A cm À2 ).
conditions.In the last decade, a number of alkaline stable and highly conductive AEMs have been developed via different synthetic methods. [10]For example, we have reported highly alkaline stable AEMs containing hexyltrimethylammonium cations that exhibited high ion conductivity (115 mS cm À1 at 80 °C) and survived for 1000 h in 8 M KOH at 80 °C without change in the conductivity. [11]Jannasch et al. synthesized AEMs containing piperidinium groups which exhibited high ion conductivity (146 mS cm À1 at 80 °C) and durability for 2900 h in 2 M KOH at 90 °C. [12]Peng et al. developed highly conductive and stable poly(terphenyl piperidinium)-based AEMs which achieved ion conductivity of 137 mS cm À1 at 80 °C and durability for 5000 h in 1 M NaOH at 80 °C. [13]Most of those AEMs contained high ion exchange capacity (IEC) for the sake of the conductivity and exhibited high water absorbability, high dimensional swelling, and/or low mechanical strength.These countervailing properties remain issues for AEMs, in particular, for AEMWE applications, where AEMs are always in contact with liquid water.
][20][21] Hickner et al. developed poly(olefin)-based cross-linked AEMs using triallyl methyl ammonium iodide as cross-linkers, revealing excellent thermal stability, low WU (from 50 to 13 wt% at 20 °C), low dimensional swelling (from 34% to 15% at 20 °C), high ion conductivity (from 30.1 to 51.3 mS cm À1 at 80 °C), and reasonable alkaline stability (96% remaining conductivity after 240 h in 2 M KOH at 80 °C) as well as high tensile strength (from 1.9 to 20.4 MPa) but lower elongation at break (from 80% to 10%). [22][17][18]22] In this work, cross-linking via free radical reaction of olefinic compounds was applied in our partially fluorinated AEMs containing pendent ammonium head groups.The WU, dimensional stability, hydroxide ion conductivity, alkaline stability, and mechanical strength of the resulting cross-linked AEMs were thoroughly investigated to optimize the degree of cross-linking.The optimized AEM was tested in water electrolysis cell using our in-house NiCoO anode catalyst which we have been developed recently. [23] Results and Discussion 2.1.Preparation of Cross-Linked xQPAF-C3-VB Copolymer PAF-C3 copolymers with pendent N,N-dimethyl-aminopropyl groups were prepared according to the literature.[24] To introduce the styrene-based cross-linker, PAF-C3 copolymers with different composition, n/m = 0.5 and 0.9 (determined by the proton nuclear magnetic resonance ( 1 H NMR) integral ratios), were partially quaternized with 4-vinylbenzyl chloride (VBC) using various amount (10, 20 and 50 mol% relative to the tertiary amino groups) (Table S1, Supporting Information).Subsequently, the remaining amino groups were quaternized with dimethyl sulfate to obtain the quaternized QPAF-C3-VB copolymers in high yields (90-99%) (Scheme 1).
The quaternized QPAF-C3-VB was further treated with conc.HCl to exchange the counter ions to chlorides.The chemical structure of the quaternized QPAF-C3-VB was confirmed by the 1 H NMR spectra (Figure 1) in which the olefinic protons associated with the styrene moieties were detected at 5.2, 5.7, and 6.6 ppm (peaks 14, 15, and 16).In addition, the methylene protons of benzyl side chain (N þ -CH 2 -Ar, peak 11) were detected at 4.5 ppm.Two aromatic peaks of styrene groups were detected at 7.35 and 7.46 ppm (peaks 12, 13) suggesting the successful quaternization with VBC.The intensity of VBC peaks (11, 12, 14, 15,  and 16) increased as increasing the VBC feed ratio, suggesting that the reaction was controllable.The degree of quaternization with VBC [n/(n þ o)] calculated from the 1 H NMR integral ratios was in fair agreement with the feed ratios (Table S1, Supporting Information).The disappearance of the peak at 2.2 ppm of -N-(CH 3 ) 2 groups indicated that the remaining amino groups of PAF-C3 were successfully quaternized.Furthermore, the peak at 3.0-3.2ppm (peak 10) was ascribed to methylene protons adjacent to the quaternary ammonium (N þ -CH 3 ).
Then, the quaternized QPAF-C3-VB copolymers were subjected to the cross-linking reactions via free radical reaction of the styrene groups using 2,2'-azobis(isobutyronitrile) (AIBN) as initiator (Scheme 1).xQPAF-C3-VB copolymers were obtained as the product in reasonably high yields (98%).The cross-linked structure was analyzed by the 1 H NMR spectra (Figure 2) in which the olefinic protons (peaks 14, 15, 16) were smaller than those of the precursor QPAF-C3-VB copolymers.The unreacted olefinic proton peaks were larger for larger VBC composition (50 mol%).New aliphatic peaks appeared at 1.5 ppm (peak 19) assignable to the attached initiator (X) ((CH 3 ) 2 -C-CN)), at 2.1 ppm (peak 17) assignable to -CH 2 groups, and at 2.7 ppm (peak 18) assignable to cross-linked -CHgroups, suggesting that the cross-linking reaction was successful.The cross-linking degree was calculated from the 1 H NMR integral ratios relative to the attached initiator and terminal styrene groups that ranged from 9.1 to 40.5 mol% (Table 1).
All xQPAF-C3-VB copolymers were soluble in DMSO but insoluble in CHCl 3 regardless of the copolymer composition and the degree of cross-linking (Table 1).xQPAF-C3-VB copolymers were cast from DMSO solution, however, the membrane-forming capability was strongly dependent on the amount of VBC as well as the IEC value.From the low-IEC precursor PAF-C3 (1.19 meq g À1 ), all three cross-linked copolymers provided   flexible membranes regardless of the degree of cross-linking.From higher-IEC precursor PAF-C3 (1.50 meq g À1 ), flexible membranes were obtained at low and moderate degree of cross-linking (9.1 and 17.4 mol%) but not obtained at higher degree of cross-linking (40.5 mol%).Even higher IEC membranes were unsuccessful due to either high water absorbability or lack of membrane formability.The IEC of xQPAF-C3-VB membranes was measured by the Mohr titration method.At low IEC values (IEC calc = 0.96-0.99meq g À1 ), the titrated IECs were in fair agreement with the calculated IECs (IEC tit = 1.02-1.06).However, at higher IECs (IEC calc = 1.34 À 1.41 meq g À1 ), the titrated IEC values were lower than the expected (IEC tit = 1.00-1.17meq g À1 ), in particular, at higher degree of crosslinking.The ion exchange reaction may not be efficient for the membranes with higher IEC and higher degree of cross-linking.

Morphology
The morphology of the cross-linked xQPAF-C3-VB membranes was investigated by transmission electron microscopy (TEM) images of the stained samples (with PtCl 4 2À ) (Figure 3).The membranes exhibited distinct phase-separated morphology composed of well-interconnected hydrophilic and hydrophobic clusters.At low degree of cross-linking (9.1% and 17.4%), the domain sizes for the hydrophilic clusters (0.5 AE 0.08 nm) and the hydrophobic clusters (0.8 AE 0.2 nm) were similar.The domain sizes were slightly larger at higher degree of cross-linking (40.5%), 0.6 AE 0.12 nm for the hydrophilic clusters and 1.0 AE 0.24 nm for the hydrophobic clusters.The uncross-linked QPAF-C3 exhibited larger domain sizes (1.4 AE 0.23 nm for the hydrophilic clusters and 2.5 AE 0.5 nm for the hydrophobic clusters, Figure S1, Supporting Information), suggesting that the cross-linking suppressed the development of ionic and nonionic clusters, presumably due to the restriction of the polymer main chain with the cross-linked structure.

Water Uptake and Hydroxide Ion Conductivity
The WU of the cross-linked xQPAF-C3-VB membranes at 30 °C is plotted as a function of the degree of cross-linking (Figure 4a), in which the red symbols represent higher IEC and black symbols represent lower IEC.As expected, the cross-linked xQPAF-C3-VB exhibited lower WU compared to the uncross-linked QPAF-C3 membranes.However, the effect of swelling depression was not further pronounced as increasing the cross-linking degree.The number of water molecules per ammonium groups (λ) was 27.2 and 28.9 for the uncross-linked QPAF-C3 membranes and 18.3-21.8for the cross-linked xQPAF-C3-VB membranes (Figure 4b).The results suggest that the increased cross-linking degree did not affect the water molecules located closely to the ammonium groups, which was in agreement with the minor differences in the hydrophilic cluster sizes as discussed above.
The hydroxide ion conductivity of the cross-linked xQPAF-C3-VB membranes at 30 °C is plotted as a function of the degree of cross-linking (Figure 4c).In general, the conductivity decreases as increasing the degree of cross-linking due to suppressed WU as well as less developed ionic channels.Among them, xQPAF-C3-VB membranes with 17.4% degree of cross-linking and high IEC of 1.16 meq g À1 exhibited high ion conductivity (56 mS cm À1 ), comparable to that of the uncross-linked QPAF-C3 membrane (54 mS cm À1 ).The conductivity showed significant increase at 15.8% and 17.4% degree of cross-linking (for low and high IECs, respectively) accompanied by subsequent increase in the WU and λ values, suggesting that the xQPAF-C3-VB membranes with appropriate degree of cross-linking utilized the water molecules.
The swelling ratio (SW) of xQPAF-C3-VB membranes was measured at 30 and 80 °Cas a function of the degree of crosslinking (Figure 5).In general, the cross-linked membranes exhibited smaller swelling (SW = 10-19% at 30 °C and SW = 18-32% at 80 °C) compared to the corresponding uncross-linked membranes (SW = 32% at 30 °C and 52% at 80 °C) due to mitigated WU in the former membranes.Among the cross-linked membranes, xQPAF-C3-VB (1.16 meq g À1 ) with 17.4% degree of crosslinking exhibited the largest swelling (SW = 32%).The swelling ratio was not much affected by the degree of cross-linking similar to the WU behavior as discussed above.
Temperature dependence of the hydroxide ion conductivity of xQPAF-C3-VB membranes was evaluated in water, as shown in Figure 6.The conductivity increased as increasing the temperature from 35 to 80 °C for uncross-linked and cross-linked membranes.Among them, xQPAF-C3-VB (1.16 meq g À1 ) with 17.4% degree of cross-linking exhibited the highest conductivity (56 mS cm À1 at 30 °C and 80 mS cm À1 at 80 °C) which was slightly lower than that of the uncross-linked QPAF-C3 (54 mS cm À1 at 30 °C and 99 mS cm À1 at 80 °C). [24]The apparent activation Determined from the 1 H NMR integral ratios; b) Calculated theoretically from the xQPAF-C3-VB assuming complete cross-linking of the styrene groups; c) Determined from Mohr titration method.
energies (E a ) for the hydroxide ion conduction were estimated from the slopes, which ranged from 8.7 to 9.8 kJ mol À1 for xQPAF-C3-VB membranes compared to 10.5 kJ mol À1 for the uncross-linked QPAF-C3, [24] suggesting that the ion conduction mechanism was not changed by the cross-linking.

Alkaline Stability
Alkaline stability of the xQPAF-C3-VB membranes was investigated in 4 M KOH at 80 °C for 1,036 h (Figure 7), where the ion conductivity of the cross-linked membranes (measured at 40 °C)   decreased gradually with the testing time.Among them, xQPAF-C3-VB (IEC = 1.16 meq g À1 ) with 17.4% degree of crosslinking demonstrated the highest alkaline stability maintaining 19 mS cm À1 and 47.5% of initial conductivity which was somewhat less stable than the uncross-linked QPAF-C3 membrane (remaining conductivity was 24 mS cm À1 % and 61%). [24]The other cross-linked membranes suffered from lower conductivities and/or faster degradation rate.For example, xQPAF-C3-VB (IEC = 1.17 meq g À1 ) with 9.1% degree of cross-linking showed significant drop in the conductivity from 39 to 2.0 mS cm À1 (5% remaining).
To elucidate the alkaline degradation mechanism of xQPAF-C3-VB membranes, the post-test samples after 1036 h were subjected to nuclear magnetic resonance (NMR) analyses (Figure 8), in which the decrease in intensity of the main peak (10) assigned to (N þ -CH 3 ) and the appearance of broad peaks at 2.0 and 2.7 ppm suggest that the ammonium groups degraded via nucleophilic substitution mechanism.The remaining peak (10)  1.24 meq g -1 1.17 meq g -1 1.06 meq g -1 1.03 meq g -1 1.16 meq g -1 1.02 meq g -1    larger and the broad peaks at 2.0-2.7 ppm were smaller in the post-test xQPAF-C3-VB (1.16 meq g À1 ) with 17.4% degree of cross-linking compared to those of the other cross-linked membranes, suggesting higher stability.The peaks (17, 18, and 19) associated with the cross-linker were either slightly decreased in intensity or overlapped with the new broad peaks (2.0-2.7 ppm).
A small peak was detected in the enlarged spectra around 5.3 ppm (-CH = CH 2 groups) suggesting minor degradation via the Hoffman elimination mechanism.While the remaining conductivity of the post-test xQPAF-C3-VB membranes was not dependent on the initial IEC and the degree of cross-linking, it was dependent more on the initial WU (Figure S2, Supporting Information).With larger amount of the absorbed water, the ammonium groups would be less sterically hindered and have more chances of the nucleophilic attack by hydroxide ions.

Mechanical Properties
The viscoelastic properties of the cross-linked xQPAF-C3-VB were investigated through dynamic mechanical analysis (DMA) in which the storage moduli (E'), loss moduli (E''), and tan δ were measured as a function of the temperature, as shown in Figure 9.
The cross-linked xQPAF-C3-VB membranes exhibited higher E' and E'' at 25 °C in most cases and maintained higher values at elevated temperature compared to those of the uncross-linked QPAF-C3 membrane.The glass transition temperature (T g ) of the membranes was detected in tan δ curves and is plotted as a function of the degree of cross-linking (Figure S3, Supporting Information) in which the T g of xQPAF-C3-VB membranes was 70-75 °C and ≈10 °C higher than that of the uncross-linked QPAF-C3 membrane.The T g seemed less dependent on the degree of cross-linking.Among them, xQPAF-C3-VB with 1.16 meq g À1 and 17.4% degree of cross-linking showed the highest T g and enhanced viscoelasticity.The results reveal that the cross-linking effectively improved the thermomechanical properties of the QPAF-C3 membranes.

Alkaline Water Electrolysis Cell Performance
For the alkaline water electrolysis cell, xQPAF-C3-VB (IEC = 1.16 meq g À1 , 17% degree of cross-linking, and 57 μm thick) membrane was selected due to its highest ion conductivity, alkaline stability, and mechanical robustness.The cell was assembled using Ni 0.8 Co 0.2 O and Pt/C for the anode and cathode catalysts, respectively.In Figure 10, the onset voltage was detected at 1.42 V that was in agreement with our previous work on different anion exchange membrane (1.44V) [25] and that (≈1.45 V) obtained by rotating disk electrode for the similar NiCoO catalyst. [23]The cell achieved 1.67 V at 1.0 A cm À2 and 1.82 V at 2.0 A cm À2 .The current efficiency was 74% at 1.0 A cm À2 which was comparable to that of our previous partially fluorinated and uncross-linked anion exchange membrane (76%). [25]It seemed that the smaller water absorbability in the cross-linked xQPAF-C3-VB membrane did not affect the efficiency of the water electrolysis.The ohmic resistance at 1.0 A cm À2 of the current density was 103 mΩ cm 2 , which was higher than that (83 mΩ cm 2 ) estimated from the hydroxide ion conductivity (80 mS cm À1 ) at 80 °C and the thickness (57 μm) of x-QPAF-C3-VB membrane, presumably due to the interfacial resistance between the membrane and the catalyst layers.The AEMWE performance of the xQPAF-C3-VB membrane using 1 M KOH at 80 °C was in fact better than those of the recently reported AEMs (considering the differences in cell components such as catalyst and electrode binder). [26,27]

Conclusion
Anion exchange membranes containing pendent trimethyl propyl ammonium head groups and perfluorohexylene groups were successfully cross-linked with styrene via free radical reaction.The cross-linked and quaternized xQPAF-C3-VB copolymers   exhibited good solvent solubility and membrane-forming capability of at least up to 36.0%degree of cross-linking.The restricted polymer chains by the styrene cross-linker resulted in less developed phase-separated morphology with smaller ionic and nonionic clusters compared to those of the uncross-linked membranes.Although the cross-linking did not affect the water molecules around the ammonium cations, it contributed to lower WU.The cross-linking did not impact much the ion conductivity despite of suppressed water absorbability and less developed ionic clusters.The ion conduction mechanism did not change as confirmed through comparable activation energy values with the uncross-linked membrane.More significant cross-linking effect was obtained in the thermomechanical properties, in which the cross-linked membranes exhibited 10 °C higher glass transition temperature.Among them, xQPAF-C3-VB with 17.4% degree of cross-linking showed glass transition at 75 °C compared to 62 °C for the uncross-linked membrane.A selected cross-linked membrane with 17.4% degree of cross-linking and IEC = 1.16 meq g À1 exhibited high performance in alkaline water electrolysis cell (1.67 V at 1.0 A cm À2 ).
Synthesis of Styrene-Tethered Copolymers (QPAF-C3-VB): A typical procedure is as follows.In a glass vial filled with N 2 , PAF-C3 copolymer (0.12 g) dissolved in 5 mL DMAc and 0.02-0.13mmol of VBC were added.The mixture was stirred at r.t for 24 h.Then, dimethyl sulfate (5.0 equiv.) was added and the quaternization reaction was conducted for 48 h.The mixture was poured into excess of water to precipitate a white solid of QPAF-C3-VB.The product was collected and stirred in conc.hydrochloric acid for 3 h to exchange the counter ions for chloride ions.The solid was filtered, washed with pure water several times, and dried under vacuum to obtain QPAF-C3-VB in high yields (90-99%).
Preparation of Cross-Linked xQPAF-C3-VB Copolymers: A typical procedure is as follows.In a round flask, QPAF-C3-VB (0.5 g) was dissolved in 10 mL of NMP under nitrogen atmosphere.A catalytic amount of AIBN (1.0 mol%) was added to the solution and the mixture was heated at 80 °C for 3 h.The viscous solution was poured into excess of water to precipitate a yellowish-brown solid.The solid was filtered, washed with pure water several times, and dried under vacuum to obtain xQPAF-C3-VB copolymer (0.55 g, 98%).
Membrane Preparation: 10 wt% xQPAF-C3-VB solution in NMP was cast onto a flat glass plate and dried at 65 °C for 24 h.A transparent and flexible membrane (in Cl À ion form) was obtained (≈50-60 μm thick).
Ion Exchange Reaction: The xQPAF-C3-VB (in Cl À ion form) membrane was immersed in 1 M KOH at 80 °C for 24 h, washed with degassed ultrapure water (DUPW) several times, and stored in DUPW.
[30] Preparation of Membrane Electrode Assembly: A modified procedure to that in the literature was used. [25]The anode catalyst ink was prepared from homemade catalyst Ni-Co oxide (Ni 0.8 Co 0.2 O) [23] and in-house binder (QPAF-4, IEC = 1.5 meq g À1 ) [31] using methanol and pure water as solvents.First, the catalyst was suspended in methanol and stirred using a planetary ball mill for 30 min.To the mixture, 5 wt% of QPAF-4 in methanol was added.The mixture was homogenized in planetary ball mill for further 30 min.The ratio of dry binder to the catalyst was adjusted to be 0.15 by weight.The catalyst ink was mechanically stirred in a closed vessel for 8 h.The anode catalyst ink was sprayed onto one side of xQPAF-C3-VB membrane (IEC = 1.16 meq g À1 , 17.4% degree of crosslinking, and 57 μm thick) using a pulse-swirl-spray apparatus (PSS, Nordson Co., Ltd.) to form a catalyst-coated membrane (CCM) with catalyst loading of 2.0 mg cat cm À2 and active geometric area of 1.0 cm 2 .Porous transport layer (PTL) for the anode side was Ni PTL (Bekaert Co., Ltd.).For the cathode catalyst, Pt-C (TEC10E50E, Tanaka Kikinzoku Kogyo, K. K.) and in-house QPAF-4 (IEC = 1.5 meq g À1 ) were used.The ratio of the binder to carbon was adjusted to be 0.6 by weight.The catalyst ink was sprayed onto a GDL (TGP-H-120, Toray Co., Ltd.) to form gas diffusion electrode (GDE) using a PSS apparatus in which the catalyst loading was adjusted to be 1.0 mg Pt cm À2 .Membrane electrode assembly (MEA) was fabricated from the CCM, PTL, and GDE.On the both sides of the MEA, a gasket (EPDM, 300 μm thick) and a Ni separator with a straight flow channel for the generated gas were fixed.A current collector was gold-plated copper.The electrolysis cell was sealed by applying pressure of 8.5 kgf cm À2 .
Evaluation of Water Electrolysis Cell: The water electrolysis cell was operated at 80 °C in which 1 M KOH was supplied as electrolyte for both electrodes at 10 mL min À1 .Prior to the evaluation, the cell was preconditioned by sweeping the current density from 0 to 1 A cm À2 twice.

Figure 3 .
Figure 3. TEM images of the cross-linked xQPAF-C3-VB membranes fitted with histograms of the diameters of hydrophilic and hydrophobic clusters.

Figure 4 .
Figure 4. a) WU, b) number of water molecules per ammonium group (λ), and c) hydroxide ion conductivity at 30 °C of xQPAF-C3-VB as a function of the degree of cross-linking.

Figure 5 .
Figure 5. Swelling ratio of xQPAF-C3-VB membranes at 30 and 80 °C as a function of the degree of cross-linking.

Figure 7 .
Figure 7. Alkaline stability of the cross-linked xQPAF-C3-VB membranes in 4 M KOH at 80 °C (the conductivity was measured at 40 °C).

Figure 9 .
Figure 9. DMA curves of the cross-linked xQPAF-C3-VB membranes as a function of the temperature.