Low Ion Exchange Capacity Research Articles

Internal and interfacial structure analysis of graft‐type fluorinated polymer electrolyte membranes by small‐angle X‐ray scattering in the high‐q range

AbstractThe hierarchical structures of poly(styrenesulfonic acid)‐grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) with different scale ranges including lamellar spacing, interfacial thickness, and intra‐structure of conducting layers were evaluated by small‐angle X‐ray scattering in terms of background scattering (I B(q)). First, I B(q) was roughly estimated by modifying Ruland's method and then optimized to avoid overestimation using a “contribution factor,” which is defined as the contribution of I B(q) to the observed scattering intensities over the entire q range. Then, I B(q) was optimized again by using the model for the deviation from Porod's law also proposed by Ruland to select proper q‐range for interfacial thickness evaluation. The lamellar spacing, which is observed in the low‐q range, was not altered by background correction. In contrast, in the high‐q range, the interfacial thickness and the internal structures can be estimated only after correction for the background scattering data. The interfacial thickness of the final membranes (ETFE‐PEMs) is affected by both the graft‐polymerization and sulfonation processes and because the change in membrane structures is observed during propagation steps at lower ion exchange capacity (IEC) range (IECs < 2.4 mmol/g), which should affect the mechanical strength of graft‐type PEMs.

Hydrophilic Flexible Ether Containing, Cross-Linked Anion-Exchange Membrane Quaternized with DABCO.

Anion-exchange membranes (AEM) with high ion content usually suffer from excessive water absorption and dilution effects that impair conductivity and mechanical properties. We herein report a novel ether containing a cross-linking strategy without adopting high ion-exchange capacity (IEC). The ether-containing cross-links and the quaternized structure are created simultaneously by introducing an ether-containing flexible hydrophilic spacer between two 1,4-diazabicyclo[2,2,2,2]octane or DABCO molecules; the resultant bi-DABCO structure was further employed to react with chloromethylated polysulfone. The long spacer with the ether moiety may benefit the hydroxide ion transport, and the cross-links will control the swelling and water absorption of the AEM. The two ether groups in the long spacer of the cross-links will also shield the DABCO cation from OH- attack due to an electron-donating effect. The prepared membranes exhibited an improved conductivity of 31 mS/cm (at 25 °C) at a comparatively low IEC (1.08 mmol/g) with a rational water absorption and low swelling ratio (95.0 and 27.1%, respectively); they also displayed an enhanced alkaline stability in 1 M NaOH aqueous solution at 80 °C for 150 h. The density functional theory study and physical characterization after the alkaline treatment further confirm the better chemical stability of the cross-linked membrane over its counterpart. Our work presents an effective strategy to balance AEM conductivity and robustness.

Covalent bonding-triggered pore-filled membranes for alkaline fuel cells

Highly conductive and physico-chemically stable ion transport channels (ITCs) are desirable for numerous membrane-integrated sustainable technologies such as fuel cells, redox flow batteries, reverse electrodialysis, etc. However, a “trade-off” exists among the ion exchange capacity, conductivity, and stability of the ITCs. In this context, we hereby propose a facile “covalent bonding-triggered (CBT)” strategy for filling of pre-formed membrane pores and formation of CBT-adopted ITCs (CITCs) and high-performance anion exchange membranes (AEMs). The resulting CITCs-integrated AEMs offer high hydroxide conductivity (39.8 mS cm−1 at 30 °C) at low ion-exchange capacity (0.97 mmol g−1) and excellent chemical stability (<6% conductivity loss after being treated in 2 M NaOH aqueous solution at 60 °C for 7 days) due to aligned interfacial reactive sites within the CITCs. Therefore, the fabricated AEMs further exhibit high fuel cell performance, reaching a peak power density of 315 mW cm−2 at 60 °C. Thus, the proposed CBT pore-filling strategy guides the design for the formation of high-performance and durable ITCs, required for fabrication of efficient AEMs.

Quaternized graphene oxide modified PVA-QPEI membranes with excellent selectivity for alkali recovery through electrodialysis

Composite anion exchanges membranes are prepared by incorporating quaternized graphene oxide (QGO) within poly(vinyl alcohol) (PVA) or the mixture of PVA and quaternized polyethyleneimine (QPEI), followed by cross-linking with formaldehyde. The PVA-QGO series of membranes have low ion exchange capacities (IECs) of 0.10–0.37mmol/g and low water uptakes (WR) of 16.1%–22.0%, leading to high area resistances (>60Ωcm2). The PVA-QPEI-QGO series of membranes show much lower area resistances of 2.47–6.04Ωcm2, which is ascribed to the improved IECs (0.86–1.37mmol/g) and WR (46.8%–71.4%). Both series of membranes show higher alkali resistance than commercial membrane AMV. Their weight losses are in the range of 0.5%–8.9% after immersion in 60°C 2mol/L NaOH alkaline solution for 32 days, while the value of AMV membrane is 19.2%. When utilized for electrodialysis (ED) application to recover NaOH from NaOH/Na2WO4 mixture, the prepared membranes exhibit high selectivity, as evidenced by the low leakage ratio of Na2WO4. The leakage ratio of the optimized composite membrane to WO42− is only 5.1%, while the leakage ratio of AMV membrane is up to 61.4%. To explore this reason, the composite membranes are selected for the ED separation of monovalent ions. The separation efficiency (S) of the optimized membrane is 83.9%, which is similar to the value of commercial anion selective membrane NEOSEPTA® ACS (88.7%), confirming further the selectivity of the composite membranes.

Improved performance of quaternized poly(arylene ether ketone)s/graphitic carbon nitride nanosheets composite anion exchange membrane for fuel cell applications

A series of novel nanocomposite anion exchange membranes (QPAEK) were prepared by incorporating graphitic carbon nitride (g-C3N4) nanosheets into chloromethylated poly(arylene ether ketone)s and the following Menshutkin reaction. Fundamental properties of the nanocomposite membranes with various g-C3N4 nanosheets content were investigated systematically. The prepared QPAEK-CN-0.5 membrane with a low ion exchange capacity (IEC) of 1.31 mmol g−1 exhibited an improved ionic conductivity of 34.5 mS cm−1 at 80 °C, because of the hydrogen bonding interactions between alkaline quaternary ammonium groups and amine/imine groups of g-C3N4 nanosheets. It also displayed a low methanol permeability because the triazine units in g-C3N4 nanosheets can block the methanol crossover but allow water molecules and hydroxide ions to transport. In addition, the QPAEK-CN-0.5 membrane exhibited a power density of 49 mW cm−2 at 80 °C under 100% relative humidity. Furthermore, the molecular dynamics simulation method has been carried out to provide additional insightful structure-performance relationships of the nanocomposite membrane. Overall, this study demonstrated that the introduction of g-C3N4 nanosheets is an effective way to enhance the properties of the anion exchange membrane, especially in ionic conductivity, alkaline resistance and methanol permeability due to their special composition and lattice structure.

Blend anion exchange membranes containing polymer of intrinsic microporosity for fuel cell application

High hydroxide conductivity and alkali resistance of anion exchange membranes (AEMs) is the basic requirement for realizing the practical application of anion exchange membrane fuel cells. In this work, we develop a novel blend AEM incorporating polymer of intrinsic microporosity (PIM). PIM features highly rigid backbone and loose packing so that the resultant blend AEM containing PIM possesses increased free volume so that the ion clusters formed by microphase separation may get easily percolated and form pathways for hydroxide ion transport. The blend AEM containing 30% PIM shows a low ion exchange capacity (IEC) of 0.77 mmol g−1, but a relatively high conductivity of 16.9 mS cm−1 at 30 °C, and yields a H2/O2 fuel cell power density of 163 mW cm−2 at 60 °C. The membrane also shows a high mechanical robustness (tensile strength 43 MPa) and alkaline stability as compared with that without PIM incorporation. To the best of our knowledge, this work is the first report of PIM incorporation in blend AEM, and provides a new and effective strategy to promote hydroxide ion conduction in AEMs at relatively low IEC, which may pave the way to achieve better balanced conductivity and stability.

Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electro-electrodialysis

Comb-shaped polysulfone anion exchange membranes (AEMs) containing various pendant alkyl side chains were synthesized and characterized. Ionic clusters aggregation is observed in transmission electron microscopy (TEM) image and small-angle X-ray scattering (SAXS). The comb-shaped quaternized ammonium polysulfone (Cx-QAPSU) membranes show enhanced dimensional stability and ultra-low membrane area resistance (Rm). Notably, the C16-QAPSU with 16 carbon atoms has the best hydrophilic-hydrophobic phase separation structure and lowest Rm of 2.5 Ω cm2 despite low ion exchange capacity (IEC) of 1.55 mmol/g and swelling ratio of 5.3%. Moreover, comb-shaped AEMs show much higher tensile strength (TS) compared to QAPSU membrane under the same IEC level. The C16-QAPSU with lowest Rm of 2.5 Ω cm2 exhibits the highest TS of 16.7 MPa. These results indicate that the long alkyl side chains especially the length of the pendant side chain of Cx-QAPSU up to C16 achieve an electrochemical-mechanical balance of AEMs. In electro-electrodialysis systems, the comb-shaped AEMs exhibit higher purification efficiency of wet phosphoric acid than non-comb-shaped AEMs.

Conductive Channels in Anion Conducting Multiblock Copolymers: Different Number and Type of Head-Groups

The conductivity and physical properties of the ion conductive channels in a multiblock copoly(arylene ether) polymer was quantitatively measured as a function of (i) number of tethered head-groups per polymer repeat unit and (ii) type of cation head-groups. Multiblock copoly(arylene ether)s with 1, 2, 3 and 4 alkyl chain tethered quaternary ammonium head-groups on each hydrophilic repeat unit were synthesized via polycondesation, Friedel-Crafts, and reduction reactions, Figure 1. The effect of the number of ionic groups on the ion exchange capacity (IEC), morphology and properties of the anion exchange membranes was investigated. As the ionic group density increased, both the ionic conductivity and water uptake of the membranes increased. The 3-tether membrane formed the most efficient ionic channels as determined by the highest ionic conductivity/IEC. The 2, 3, and 4-tether (per hydrophilic repeat unit) membranes had 9 to 10 bound waters of hydration per ionic group pair, however, the amount of unbound, freezable waters increased with the number of ion pairs per polymer repeat unit. The unbound, freezable water was unproductive and led to lower ionic conductivity/IEC ratio, such as in the 4-tether membrane. The optimum membrane had three head-groups per hydrophilic repeat unit (X3Y5-3): 130.6 mS/cm ionic conductivity at 80oC, 58.3% water uptake, IEC of 1.83 meq/g. All the membranes showed acceptable thermal stability and alkaline stability in 1 M KOH at 60oC for 1000 h. As the number of head-groups per hydrophilic repeat unit increased, the mechanical strength of the membranes decreased. The two-tether (per hydrophilic repeat unit) block copolymer was used to compare three previously reported head-groups: quaternary trimethylammonium (TMA), quinuclidium (ABCO), and tris(2,4,6-trimethoxyphenyl)phosphonium (TTMPP) cations in terms of size of the conducting channels, ionic conductivity of the mobile hydroxide ion, mechanical properties, quantity of productive and unproductive water, and chemical stability of the membrane in base. The interdomain spacing acquired from small angle X-ray scattering showed that multiblock copolymers with larger cations formed larger ion conduction channels in the membrane. However, the larger cations resulted in lower ion exchange capacity (IEC) even though the polymer backbone and tether arrangements were identical. TMA was the most stable cation after exposure to 1 M NaOH at 60oC for 20 days. ABCO had a lower number of bound water molecules and a 22% loss in ion conductivity after treatment in 1 M NaOH at 60oC for 20 days due to the higher hydroxide ion concentration in the ion conductive blocks. Membranes with TMA head-groups also had the best mechanical properties, while the TTMPP head-group was the poorest. Thus, the alkyl trimethylammonium head-group is the best suited cation head-group among those studied with this backbone. In addition, two membrane preparation methods were compared. Quaternization before membrane casting had better morphology and membrane properties than quaternization after membrane casting. The presence of the cation head-groups assists in phase segregation. Figure 1

Mid-block quaternized polystyrene-b-polybutadiene-b-polystyrene triblock copolymers as anion exchange membranes

A novel mid-block, side-chain-type quaternized triblock copolymer was prepared from commercial polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymers by Markovnikov addition reaction with HBr, azidation, and Cu(I)-catalyzed click reaction for high-performance anion exchange membranes (AEM). After the optimization of reaction conditions, the bromination, azidation and subsequently Cu(I)-catalyzed click reactions have been confirmed to be quantitative according to the NMR results. The anion-conductive, mid-block triblock copolymer (SBS-c-QA) membrane was obtained from casting its DMF solution. As a control experiment, the mid-block quaternized SBS (SBS-QA) membrane was also synthesized by direct quaternization of brominated SBS with trimethylamine. The SBS-c-QA membrane with pendent quaternary ammounium groups showed much lower water uptake than that of SBS-QA membrane. However, comparable bicarbonate conductivities of 5.6 mS/cm at 20 °C and 20.5 mS/cm at 80 °C were obtained for SBS-c-QA membrane in spite of its low ion exchange capacity (IEC) (1.10 meq./g) and low water uptake (18 wt%). Thus, the SBS-c-QA membrane displayed high IEC-normalized conductivity which can be attributed to the side-chain architecture of the triblock copolymer. The alkaline stability testing indicated that SBS-c-QA membrane showed a higher alkaline stability than SBS-QA membrane. A high retention of bicarbonate conductivity (70%) was observed after degradation testing in 1 M NaOH at 80 °C for 216 h. These results suggested that incorporating the side-chain-type QA cations into the mid-block of the triblock copolymer was an effective method to fabricate AEMs with both high ionic conductivity and excellent alkaline stability.

Thermally triggered polyrotaxane translational motion helps proton transfer

Synthetic polyelectrolytes, capable of fast transporting protons, represent a challenging target for membrane engineering in so many fields, for example, fuel cells, redox flow batteries, etc. Inspired by the fast advance in molecular machines, here we report a rotaxane based polymer entity assembled via host–guest interaction and prove that by exploiting the thermally triggered translational motion (although not in a controlled manner) of mechanically bonded rotaxane, exceptionally fast proton transfer can be fulfilled at an external thermal input. The relative motion of the sulfonated axle to the ring in rotaxane happens at ~60 °C in our cases and because of that a proton conductivity (indicating proton transfer rate) of 260.2 mS cm−1, which is much higher than that in the state-of-the-art Nafion, is obtained at a relatively low ion-exchange capacity (representing the amount of proton transfer groups) of 0.73 mmol g−1.

Fuel Cell Catalyst Layers and Membrane-Electrode Assemblies Containing Multiblock Poly(arylene ether sulfones) Bearing Perfluorosulfonic Acid Side Chains

A sulfonated poly(arylene ether sulfone) multiblock copolymer bearing perfluorosulfonic acid side chains has been studied as both the membrane and cathode catalyst layer ionomer in fuel cell membrane-electrode assemblies. The multi-block, hydrocarbon-backbone polymer was designed to possess good conductivity with low ion exchange capacity and low water sorption to mimic perfluorosulfonic acid ionomers, but without the synthetic difficulty. A H2/O2-fuel cell power density of 1080 mW·cm−2 was achieved at 80°C, 100% RH using membrane-electrode assemblies incorporating the sulfonated poly(arylene ether sulfone) membrane and traditional PFSA ionomer in the catalyst layer. An accelerated stress test using 30% RH, 90°C, H2/Air demonstrated durability to open circuit potentials of more than 400 h, which is 4 times longer than for Nafion 211. No membrane thinning was observed over the same period, which is attributed to low gas crossover and inhibition of membrane degradation reactions, despite the rate of water transport across the membrane remaining high. Cathode catalyst layers fabricated using 20 wt% sulfonated poly(arylene ether sulfone) as the ionomer exhibited comparable power densities to benchmark PFSA MEAs, under high current densities, over a wide range of operational relative humidities, demonstrating that hydrocarbon-backbone, solid polymer electrolytes hold technological promise as both the membrane and catalyst layer ionomer in fuel cell membrane-electrode-assemblies.

Bent-twisted block copolymer anion exchange membrane with improved conductivity

Inadequate hydroxide conductivity of anion exchange membrane (AEM) is one of the obstacles restricting practical application of anion exchange membrane fuel cell. In this work, we report the design, fabrication and properties of AEMs based on bent-twisted non-coplanar block copolymer. The bent-twisted polymer backbone creates high free volume size so that hydroxide ion transport is facilitated. The free-volume effect can also facilitate microphase separation due to enhanced chain segment motion. With the free-volume holes working synergistically with microphase-separation enabled ion channels, the membrane showed a high conductivity (35mScm−1 at room temperature) at a relatively low ion exchange capacity (IEC, 1.25meqg−1); it also exhibited a good dimension stability, the swelling ratio being 4.7% at room temperature and kept virtually unchanged with temperature because of relatively low IEC. A H2/O2 fuel cell employing the fabricated AEM yielded a high power density (262mWcm−2 at 50°C). Our work opens up a new route to fabrication of highly conductive AEM with relatively low IEC.

Highly proton conducting proton‐exchange membranes based on fluorinated poly(arylene ether ketone)s with octasulfonated segments

ABSTRACTA new bisphenol monomer containing a pair of electron‐rich tetra‐arylmethane units was designed and synthesized. Based on this monomer, along with commercial 4,4′‐(hexafluoroisopropylidene)diphenol A and 4,4′‐difluorobenzophenone, a series of novel poly(arylene ether ketone)s containing octasulfonated segments of varying molar percentage (x) (6F‐SPAEK‐x) were successfully synthesized by polycondensation reactions, followed by sulfonation. Tough, flexible, and transparent membranes, exhibiting excellent thermal stabilities and mechanical properties were obtained by casting. 6F‐SPAEK‐x samples exhibited appropriate water uptake and swelling ratios at moderate ion exchange capacities (IECs) and excellent proton conductivities. The highest proton conductivity (215 mS cm−1) is observed for hydrated 6F‐SPAEK‐15 (IEC = 1.68 meq g−1) at 100 °C, which is more than 1.5 times that of Nafion 117. Furthermore, the 6F‐SPAEK‐10 membrane exhibited comparable proton conductivity (102 mS cm−1) to that of Nafion 117 at 80 °C, with a relatively low IEC value (1.26 meq g−1). Even under 30% relative humidity, the 6F‐SPAEK‐20 membrane (2.06 meq g−1) showed adequate conductivity (2.1 mS cm−1) compared with Nafion 117 (3.4 mS cm−1). The excellent comprehensive properties of these membranes are attributed to well‐defined nanophase‐separated structures promoted by strong polarity differences between highly ionized and fluorinated hydrophobic segments. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 25–37

Synthesis and properties of quaternized polyolefins with bulky poly(4-phenyl-1-butene) moieties as anion exchange membranes

Polyolefin-based anion exchange membranes (AEMs) with well-defined ionic domains for anion transport were synthesized by introducing bulky poly(4-phenyl-1-butene) (P4PB) moieties into quaternized polyolefinic copolymers via heterogeneous catalytic copolymerization of 4-phenyl-1-butene with 11-bromo-1-undecene over a Ziegler–Natta catalyst and subsequent quaternization. Well-defined hydrophobic-hydrophilic separation was observed for the as-obtained membranes, as confirmed by atomic force microscopy (AFM). The membranes showed high hydroxide conductivity (10.4mScm−1) despite their low ion exchange capacity (IEC, 0.98meqg−1). When compared with membranes containing poly(4-methyl-1-pentene) (PMP) or polypropylene (PP) moieties, the membranes obtained herein showed lower conductivities under similar conditions at IEC values higher than 1.2meqg−1. Excessive swelling of the P4PB membranes resulting from a high water uptake (WU) was assumed to decrease the ion concentration in the membrane. Thus, lower volumetric IEC values were observed for AEMs with IEC higher than 1.2meqg−1. A maximum hydroxide conductivity of 18.9mScm−1 at room temperature was obtained for the P4PB-TMA-43 membrane (IEC = 2.17meqg−1, WU = 262.4wt%). Similar to our previous report on polyolefin-based AEMs with PMP or PP moieties, side chains with nine methylene units between the polymer backbone and the quaternary ammonium (QA) cations were very stable and showed no apparent degradation upon storage in a 10M NaOH aqueous solution at 80°C for 20d. We demonstrated that a polyolefin-based molecular architecture combined with poly-QA side chains attached via flexible spacer units with bulky side chain moieties achieved AEMs with efficient phase separation and high alkaline stability characteristics.

Multiblock poly(Phenylene ether nitrile)s with pendant sulfoalkoxyl side chain for H2/air fuel cells at low humidity condition

ABSTRACTA series of multiblock poly(phenylene ether nitrile)s with pendant sulfoalkoxyl side chains have been developed as proton exchange membranes for fuel cells. The membranes were obtained by a solution casting method and exhibited good thermal stability, flexibility, and mechanical strength. The membranes displayed well‐developed microphase separation, which largely contributed to their excellent ion conduction ability. One of the new membranes with a low ion exchange capacity of 1.57 mequiv g−1 showed higher proton conductivity than Nafion 212 over the entire RH range (30–95%). The maximum power output generated in a single cell test reached up to 0.754, 0.640, and 0.414 W cm−2 at 70 °C under 80%, 50%, and 30% RH conditions, respectively. The current density of the membrane obtained at 0.6 V (I0.6) was as high as 640 mA cm−2, which was much higher than that of Nafion 212 (375 mA cm−2 at 30% RH), suggesting its superiority for a more rapid system start‐up. Furthermore, the in situ durability test at 50% RH was performed at a constant current loading, and the membrane did not show any significant voltage reduction over the 400 h testing period. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1940–1948

Ion transport properties of mechanically stable symmetric ABCBA pentablock copolymers with quaternary ammonium functionalized midblock

ABSTRACTAnion exchange membranes (AEMs) are a promising class of materials for applications that require selective ion transport, such as fuel cells, water purification, and electrolysis devices. Studies of structure–morphology–property relationships of ion‐exchange membranes revealed that block copolymers exhibit improved ion conductivity and mechanical properties due to their microphase‐separated morphologies with well‐defined ionic domains. While most studies focused on symmetric diblock or triblock copolymers, here, the first example of a midblock quaternized pentablock AEM is presented. A symmetric ABCBA pentablock copolymer was functionalized to obtain a midblock brominated polymer. Solution cast films were then quaternized to obtain AEMs with resulting ion exchange capacities (IEC) ranging from 0.4 to 0.9 mmol/g. Despite the relatively low IEC, the polymers were highly conductive (up to 60 mS/cm Br− at 90 °C and 95%RH) with low water absorption (&lt;25 wt %) and maintained adequate mechanical properties in both dry and hydrated conditions. X‐ray scattering and transmission electron microscopy (TEM) revealed formation of cylindrical non‐ionic domains in a connected ionic phase. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 612–622

Comb-Like Poly(Ether-Sulfone) Membranes Derived from Planar 6,12-Diaryl-5,11-Dihydroindolo[3,2-b]Carbazole Monomer for Alkaline Fuel Cells.

Highly conductive anion exchange membranes (AEMs), along with the ability to suppress swelling, are critical but challenging requirements for alkaline fuel cell applications. To achieve this criterion, a series of poly(ether sulfone)s (PESFs) with flexible alkyl imidazolium pendants attached directly on large planar 6,12-bis(4-hydroxyphenyl)-5,11-dihydroindolo[3,2-b]carbazole (DCP) units is reported. The planar DCP units stabilize the hydrophobic phase through strong π-π interactions and also facilitate the formation of ionic conducting channels through self assembly of hydrophilic pendants. The AEM prepared here, based on rational design, has a relatively low ion exchange capacity (IEC) of 1.86 × 10-3 mol g-1 and exhibits high hydroxide ion (OH- ) conductivity of 101 × 10-3 S cm-1 , a low swelling ratio of 9.3% and a water uptake of 39.6%. Furthermore, the AEMs reported in this paper have excellent stability in 1 m NaOH solution at 80 °C over 500 h. Therefore, the synthesized polymers offer a new insight into the design of high performance materials for AEMs.

Highly Conductive Anion-Exchange Membranes from Microporous Tröger's Base Polymers.

The development of polymeric anion-exchange membranes (AEMs) combining high ion conductivity and long-term stability is a major challenge for materials chemistry. AEMs with regularly distributed fixed cationic groups, based on the formation of microporous polymers containing the V-shape rigid Tröger's base units, are reported for the first time. Despite their simple preparation, which involves only two synthetic steps using commercially available precursors, the polymers provide AEMs with exceptional hydroxide conductivity at relatively low ion-exchange capacity, as well as a high swelling resistance and chemical stability. An unprecedented hydroxide conductivity of 164.4 mS cm(-1) is obtained at a relatively a low ion-exchange capacity of 0.82 mmol g(-1) under optimal operating conditions. The exceptional anion conductivity appears related to the intrinsic microporosity of the charged polymer matrix, which facilitates rapid anion transport.

Wittig reaction constructed an alkaline stable anion exchange membrane

Pendant type anion exchange membranes are currently believed to the state of the art membrane due to enhanced stability. Yet, challenge lies in the limited strategy to construct such structure. Herein, pendant type anion exchange membranes with flexible quaternary ammonium groups attached to poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbone via length-tunable spacers were successfully prepared by mild and one-pot Wittig reaction. The as-prepared membranes show restricted water swelling, high hydroxide conductivity and low methanol permeability at low ion exchange capacity due to spacer induced microphase separation. Among them, QPPO-SC6 exhibits highest hydroxide conductivity (from 37.6 mS/cm at 20°C to 99.5mS/cm at 80°C) and good chemical stability in alkaline environment at 80°C. The mechanical and thermal properties of these membranes were also investigated. Results prove that our strategy in preparing anion exchange membranes with flexible spacers has great application potential in anion exchange membrane fuel cells.

Synthesis and properties of bonding layer containing flexible and fluorinated moieties for hydrocarbon-based membrane electrode assemblies

For membrane electrode assemblies (MEAs) consisting of hydrocarbon proton exchange membranes (PEMs) and Nafion-coated catalysts, the interfacial incompatibility between the membrane and electrodes has been a constant issue because it can seriously deteriorate the performance and long term stability of the MEAs. In this study, new bonding materials based on sulfonated aromatic copolymers with flexible and partially fluorinated structures (SPE/F) were prepared and tested as a proton conductive bonding layer on the surface of the hydrocarbon PEMs to increase the adhesive force to the catalyst layer. The SPE/F copolymers were synthesized by polycondensation of sulfonated and non-sulfonated dihalide monomers with two dihydroxy monomers, 4,4′-dihydroxy diphenyl ether (DHDPE) and hexafluoro bisphenol A (HFB). The molar feed ratio of DHDPE to HFB was varied (10:0, 7:3, and 4:6) in order to evaluate the effect of the polymer structure on the physical, electrochemical, and adhesive properties. With an increase in the HFB ratio, the proton conductivity and dimensional change of the resulting polymers decreased due to the enhanced chain rigidity and lower ion exchange capacity. The maximum adhesive strength of the bonding layer was obtained using a DHDPE:HFB ratio of 7:3 (SPE/F-7/3). Finally, the MEA utilizing SPE/F-7/3 as the bonding layer exhibited higher performance in the initial unit cell test, as well as a reduced increase in resistance after the durability cycling test compared with pristine MEA without the bonding layer.