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## Creator

[Fang Xian](https://orcid.org/0000-0003-3005-4352), [Kenji Miyatake](https://orcid.org/0000-0001-5713-2635), Fanghua Liu, [Lin Guo](https://orcid.org/0000-0002-2880-1753), Ahmed Mohamed Ahmed Mahmoud, Vikrant Yadav, Toshio Iwataki, [Makoto Uchida](https://orcid.org/0000-0002-7847-3727), [Renzhi Ma](https://orcid.org/0000-0001-7126-2006)

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[Monolayer LDH nanosheet / quaternized polymer composite membranes with enhanced anion conductivity and alkaline durability for water electrolysis](https://mdr.nims.go.jp/datasets/664450d4-9a66-41bc-8bed-3fab748a3654)

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Monolayer LDH nanosheet / quaternized polymer composite membranes with enhanced anion conductivity and alkaline durability for water electrolysisFang Xian,a Kenji Miyatake,a,b,c* Fanghua Liu,a Lin Guo,a Ahmed Mohamed Ahmed Mahmoud,a,d Vikrant Yadav,a Toshio Iwataki,b Makoto Uchida,b Renzhi MaeaClean Energy Research Center, University of Yamanashi, Kofu, Yamanashi 400-8510, Japan.bHydrogen and Fuel Cell Nanomaterials Center, University of Yamanashi, Kofu, Yamanashi 400-8510, Japan.cDepartment of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan.dOffice of Institutional Advancement and Communications, Kyoto University, Kyoto 606851, Japan.eResearch Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan.AbstractAnion exchange membranes (AEMs) with high ion conductivity and alkaline resistance are of importance for achieving high performance and long-term durability of anion exchange membrane water electrolyzers (AEMWEs) and anion exchange membrane fuel cells (AEMFCs). In the present work, we have developed a series of composite membranes composed of inorganic two-dimensional (2D) layered double hydroxide nanosheets (LDH NS), with intrinsic alkaline stability and high hydroxide ion conductivity (approaching 10–1 S cm–1), and our original partially fluorinated, quaternized polymer-based copolymers (QPAF-4) via a facile solution-casting process. The obtained organic/inorganic composite membranes (QPAF-4/LDH NS) exhibited excellent mechanical properties, characterized with a maximum elongation over 300%. A high hydroxide ion conductivity (102.0 mS cm–1) was achieved on an optimized percentage of LDH NS at 3 wt.%. The hydroxyl groups on the surface of the LDH NS provided strong hydrogen bonds and highly dense Grotthuss conduction sites, which were responsible for good compatibility with the QPAF-4 matrix, contributing to high mechanical strength and high ion conductivity. Furthermore, the LDH nanosheets mitigated the chemical degradation of the ammonium groups of QPAF-4. As a result, a high retention of 80% conductivity was realized for the composite AEMs under harsh alkaline conditions (8 M KOH at 80 oC for 1,000 h) compared to only 43% for the pristine QPAF-4 membranes under the same conditions. Furthermore, an alkaline water electrolysis cell was constructed by combining the optimized QPAF-4/LDH NS composite membrane with an Ni2/3Fe1/3 LDH-rGO anode catalyst, which delivered a current density of 1 A cm-2 at 1.61 V (76.4% voltage efficiency), outperforming state-of-the-art AEM electrolysis systems.1. IntroductionHydrogen is considered an ideal renewable and clean fuel due to its high energy density (140 MJ kg-1), zero-emission, and high combustion calorific value.1-5 Currently, hydrogen is primarily produced by steam methane reforming from fossil fuel-based feeds, a method that emits CO2 and thus runs counter to the goal of carbon neutrality.4,5 Water electrolyzers (WEs), powered by renewable energy to produce green hydrogen, have thus attracted extensive interest.2,6,7 Among the three main types of low-temperature (˂ 100 oC) WEs, anion exchange membrane water electrolyzers (AEMWEs)8,9 not only offer advantages of high energy conversion efficiency and high current density operability,10 similar to proton exchange membrane water electrolyzers (PEMWEs),11 but also allow the use of earth-abundant transition metal catalysts, which can significantly reduce the cost. As a key component of AEMWEs, AEMs with high OH- conductivity (> 100 mS cm–1) are needed. The conductivity of AEMs generally can be increased by increasing the ion exchange capacity (IEC) of the membranes.12 However, increase in the IEC is often accompanied by excessive swelling in water, which lowers the mechanical strength (tensile strength and dimensional stability) of the membranes, as well as partially offsetting the gain in conductivity. Another crucial property of AEMs is the alkaline stability of AEMs for long-term operation of AEMWEs. It has been recognized that heteroatom linkages such as aryl ether groups are susceptible to attack by hydroxide ions at high temperatures.13,14 Subsequently, ether-free polyarylenes14,15 combined with functional cationic groups such as imidazolium16, quaternary ammonium17, metallocenes18, phosphonium19, and so on have been explored for achieving better chemical stability. More recently, Sun et al. reported that 1-methyl-3,3-diphenyl quinuclidinium monomers demonstrated an ionic loss of only 0.57% in 5 M NaOD for 1,440 h, as confirmed by 1H nuclear magnetic resonance (NMR) spectra, due to the structural stabilization of the quinuclidinium rings and that AEMs bearing quinuclidinium maintained 180 mS cm–1 (97% remaining of the initial conductivity) in 1 M KOH at 80 oC for 2,500 h.20 However, its elongation properties (15%-28%) were moderate. Compositing with inorganic materials has attracted extensive attention due to their great advantages in improving membrane conductivity and alkaline stability of AEMs, as well as their straightforward preparation process. Most of the previous works have focused on hydrophilic inorganic particles such as TiO221 and zirconia22 to utilize water absorption onto the inorganic particles to enhance the ionic conductivity of the composite membranes. The intrinsic properties of the particles and their dispersion within the polymer matrix are of great importance and can often significantly change other characteristics of the membranes. For example, it was found that Nafion-SnO2 composite membranes containing small-sized (~5 nm in diameter) nanoparticles exhibited a higher current density (~1.7 times) at 0.6 V in a fuel cell compared to that containing submicrometer particles (~110 nm) at 120 oC and 31% relative humidity (RH), which was attributed to their higher specific surface area, increasing the ion adsorption sites on the surface of the SnO2 nanoparticles.23 In addition to the oxide nanoparticles, two-dimensional (2D) inorganic materials with accessible high surface area have also been recently explored in hybrid membranes, such as graphene24, layered silicate25 and clay materials26. In particular, layered double hydroxides (LDHs), which are inorganic anionic conductors with low swelling and good alkaline stability, are expected to be ideal candidates as additives for hybrid AEMs.27-29 Zeng et al. used MgAl LDH particles as integrated inorganic MEAs for WEs by cold pressing. However, the WE performance was unsatisfactory, yielding a current density of 208 mA cm-2 at 2.2 V in 0.1 M NaOH solution due to its low anion conductivity (7.8 mS cm-1 at 60 oC, 98% RH).30 Wan et al. prepared an NiFe LDH coated PTFE-based AEM, which exhibited stable area resistance in 1 M NaOH at 60 oC.31 However, the hydroxide ion conductivities before and after the alkaline stability test were not mentioned. Those LDHs were bulk materials or multilayered platelets, which had low intrinsic anion conductivity (generally up to ca. 1 mS cm–1). Recently, an ultrahigh hydroxide ion conductivity (10–1 S cm–1 at 60 oC, 80% RH) has been reported on intrinsic monolayer LDH nanosheets.27 Self-standing membranes can be readily prepared by restacked LDH nanosheets. Nevertheless, their mechanical properties were not sufficient for practical applications. The anisotropy of 2D LDH nanosheets, i.e., the in-plane ion conductivity of the stacked membrane is significantly higher than its through-plane conductivity, is also a troublesome issue. To tackle this issue, LDH nanoparticles were incorporated with nanosheets to construct inorganic AEMs, which reduced the restacking order of the LDH nanosheets to some extent.29 In this aspect, it is attractive to maximize the advantages of LDH monolayer nanosheets by compositing suitable polymers with 2D LDHs. However, such a strategy has not been well-explored.Herein, we report composite AEMs using our original anion exchange polymer, QPAF-432, as a matrix for 2D LDHs. As shown in Fig. 1, QPAF-4, consisting of a partially fluorinated aromatic main chain and fluorenyl moiety with pendant ammonium head groups, displayed good mechanical properties but suffered from insufficient hydroxide ion conductivity and alkaline stability at high temperatures under harsh alkaline conditions. For LDH nanosheets, abundant hydroxyl groups covalently bonded in the 2D host layer can act as hydrogen-bond acceptors and donors, which would facilitate their good dispersion in, and compatibility with, the QPAF-4 matrix, thus forming fast ion-transport channels in the composite membranes. Other relevant properties of the prepared composite membranes, such as alkaline stability, dimensional stability, gas permeability, and water electrolysis performance were also evaluated. These findings have demonstrated that LDH monolayer nanosheets are highly promising as additives for AEMs for the applications in AEMWEs. Fig. 1. (a) Structure of the MgAl LDH nanosheets. (b) Chemical structure of QPAF-4. (c) Design principles of the organic/inorganic (QPAF-4/LDH NS) composite system. (d) Possible transport pathways in the QPAF-4/LDH NS composite membranes.2. Experimental2.1. Synthesis of QPAF-4 polymerPAF-4 polymer was synthesized by bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2, 17.49 mmol) promoted polycondensation of 3,3’-(2,7-dichloro-9H-fluorene-9,9-diyl)bis(N,N-dimethylpropan-1-amine) (AF, 2.03 mmol) and bis(3-chlorophenyl)perfluorohexane (PAF, 3.8 mmol) in the presence of 2,2’-bipyridine (BPY, 34.98 mmol) in N,N-dimethylacetamide (DMAc, 18 mL).32 After the polymerization at 80 oC for 3 h, the obtained mixture was poured into a mixed solution of methanol and hydrochloric acid to yield a black solid product, which was then washed with concentrated hydrochloric acid, potassium carbonate aqueous solution and deionized water, respectively. Subsequently, the pale-yellow product was dried at 60 oC in a vacuum oven.QPAF-4 was prepared by dissolving the above PAF-4 (2.51 g) in DMAc (14.4 ml). Dimethyl sulfate (MeSO4, 2 ml) was added dropwise into the mixture with stirring at room temperature (RT) for 60 h. The resulting mixture was poured into deionized water to obtain the QPAF-4 product, which was then washed with deionized water and dried in a vacuum oven at 60 oC.2.2. Preparation of MgAl LDH nanosheetsThe MgAl-CO3 LDH platelets were synthesized via hydrothermal reaction of Mg(NO3)2·6H2O and Al(NO3)3·9H2O (Mg/Al molar ratio of 2) mixed aqueous solution using hexamethylenetetramine (HMT, C6H12N4) as a hydrolysis agent.27,29 MgAl-CO3 LDH platelets were exchanged to MgAl-NO3 LDH by a salt-acid (NaNO3-HNO3) treatment. MgAl-NO3 LDH platelets were dispersed in formamide (1 g/L), followed by mechanical shaking at 170 rpm for 48 h to yield micrometer-sized monolayer nanosheets. The obtained large LDH nanosheets were subsequently sonicated in a water-bath ultrasonic cleaner for 4 h at RT to reduce the lateral dimensions to the nanoscale (~100–300 nm), yielding the nanosized LDH used in this study.2.3. Preparation of organic/inorganic (QPAF-4/LDH NS) composite membranesComposite membranes were prepared by a solution-casting method. Specifically, QPAF-4 was dissolved in DMAc solvent to form a homogenous solution. Then, MgAl LDH nanosheets in varied weight percentages (1-5 wt.%) relative to QPAF were added to the solution under vigorous stirring. The mixture was stirred overnight to ensure homogeneous dispersion and then sonicated for 5 min prior to membrane preparation. Finally, the mixture was cast onto a flat glass plate at 50 oC to obtain a composite membrane, abbreviated as QPAF-4/LDH NS-x%. Similarly, composite membranes were also prepared by mixing QPAF-4 with nanosized LDH nanosheets, abbreviated as QPAF-4/Nano-LDH NS-x%. The membranes in the form of MeSO4– were immersed in 1 M KOH at 80 oC and 3 M NaCl at RT for 48 h to prepare membranes in OH– and Cl– forms, respectively.2.4. Measurement of ion exchange capacity, water uptake, swelling ratio and OH– conductivity.Ion exchange capacities (IEC) of the membranes were determined by Mohr titration. Dry membranes in Cl– form (~20 mg) were immersed in ca. 12.5 mL of 0.2 M NaNO3 aqueous solution for 24 h, followed by titration with 0.01 M AgNO3 aqueous solution using K2CrO4 and NaHCO3 as an indicator and a pH adjuster, respectively.  The membranes in Cl– form were used to evaluate the water uptake and swelling at room temperature. The equations used for the calculation of these values are as follows: water uptake (%) = mwet-mdry x 100/mdry, swelling ratio (%) = Swet-Sdry x 100/Sdry, where, mwet and mdry represent the weights of membranes in the wet and dry states, respectively; and Swet and Sdry refer to  the membrane dimensions  (either in-plane or through-plane ) in the wet and dry states, respectively. Ion conductivities were determined using a four-probe cell equipped with a Solartron 1255B impedance analyzer over the 1 MHz to 1 Hz frequency range. All membranes were immersed in degassed ultrapure water at various temperatures, 30, 40, 60 and 80 oC. The resistances (R) were estimated from the intersection of the impedance spectra with the x axis. The OH– conductivity (σ) was calculated using the equation:  σ = l / (RS), where l (cm) is the electrode distance, and R (Ω) and S (cm2) represent the measured resistance and the conducting area of the sample, respectively. The activation energy (Ea) of the ion conduction was estimated as follows: σT ­= A exp(–Ea/kBT), where σ (S cm–1) is the ionic conductivity, T (K) and kB are the temperature and Boltzmann constant, respectively. Ea was estimated from the linear fitted slope of the ln(σT) vs. 1/T plot.2.5. Mechanical propertiesDynamic mechanical analyses (DMA) for the membranes in Cl– form were conducted on an ITK DVA-225 dynamic viscoelastic analyzer, where the temperature/humidity dependent storage modulus (E', Pa), loss modulus (E'', Pa), and tan δ (E''/E’) were recorded with varying temperature at 60% RH and with varying humidity at 80 oC, respectively.Tensile strengths of the membranes in Cl– form were measured on a Shimadzu universal testing instrument (Autograph AGS-J500N) equipped with a constant-temperature-humidity chamber. All membranes were measured after equilibration at 80 oC and 60% RH for at least two hours.2.6. Alkaline stability Alkaline stabilities of the membrane samples were evaluated in 8 M KOH at 80 °C. The OH– conductivities of the samples were monitored in degassed ultrapure water at 80 °C. Post-test membranes were analyzed by tensile strength testing and scanning electron microscope (SEM) and NMR spectra.2.7. Membrane electrode assembly (MEA) preparationThe anode catalyst ink containing in-house-prepared Ni2/3Fe1/3 LDH-rGO catalyst33 and 5 wt.% QPAF-4-1.5 (IEC = 1.5 meq g–1) ionomer dissolved in methanol and water was prepared, where the weight ratio of QPAF-4 binder to the catalyst was 0.15. The resulting anode catalyst ink was uniformly sprayed onto one side of a QPAF-4/LDH NS-3% composite membrane using a Nordson pulse-swirl-spray (PSS) apparatus to obtain a catalyst-coated membrane (CCM). The loading amount of the anode catalyst was 2.0 mg cm–2. Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo, K.K) and QPAF-4-2.0 (IEC = 2.0 meq g–1) binder were mixed in ethanol and water to form the cathode catalyst ink. The weight ratio of QPAF-4 binder to carbon was 0.6. The cathode ink was sprayed onto carbon paper (Toray TGP H-120) as a gas diffusion layer (GDL) using the PSS apparatus. The resulting gas diffusion electrode (GDE) had a catalyst loading amount of 1.0 mg cm–2. A nickel porous transport layer (PTL) was used as a support. The MEA containing CCM and GDE with nickel PTL was fabricated. Finally, the MEA was mounted in an electrolysis cell with a 300-μm thick gasket (EPDM) and a nickel separator with straight flow channels, where gold-plated copper was used as a current collector. 2.8. Anion exchange membrane water electrolysis (AEMWE) performanceThe AEMWE cell performance was assessed using an electrochemical workstation (Netsuden Ind. Co., Ltd., Japan) paired with a standard direct current power supply (PWR 401L, Kikusui Electronics Corp., Japan). The 1 M KOH electrolyte was supplied to both sides of the electrodes at a flow rate of 10 mL min–1 at 80 oC. After the electrolyte was supplied, the MEA was equilibrated at 1.0 V for over 48 h to ensure complete ion exchange. Polarization tests were performed by sweeping the current density from 0 to 2.0 A cm⁻2.3. Results and discussion3.1. Morphology of composite membranesThe QPAF-4 copolymer was synthesized and characterized according to our previous work (Scheme S1, Fig. S1).32 Well-crystallized micrometer-sized lamellar MgAl LDH platelets in carbonate form were synthesized by HMT hydrolysis under hydrothermal treatment.27 After exchanging the interlayer carbonate anion to nitrate, the MgAl LDH platelets were exfoliated in formamide to yield monolayer nanosheets. Detailed SEM and X-ray diffraction (XRD) characterizations are provided in Figs. S2 and S3. The obtained LDH nanosheets were redispersed in DMAc solvent. As shown in Fig. 2a, atomic force microscopic (AFM) image revealed monolayer MgAl LDH nanosheets with a lateral dimension of several micrometers with a thickness of ~ 0.8 nm. This result demonstrated that monolayer LDH nanosheets were successfully prepared and were uniformly dispersed in the DMAc solvent, which is beneficial for the subsequent composite membrane preparation. For comparison, small-sized LDH nanosheets ranging from 100 to 300 nm were also prepared by simple ultrasonic treatment (Fig. S4). Fig. S5 illustrates the preparation of QPAF-4/LDH NS composite membranes via solution casting from the homogeneous mixture of QPAF-4 polymer and LDH nanosheets in DMAc. Composite membranes with different proportions of LDH nanosheets were prepared. Homogeneous, transparent composite membranes were successfully obtained, most likely due to the high-quality monolayer structure of the LDH nanosheets, which ensured uniform dispersion and a hydrogen-bond network between QPAF-4 and the high-density hydroxyl groups on the surface of the LDH nanosheets. Fig. 2b displays the XRD patterns of the composite membranes with different proportions of LDH nanosheets. For the pristine QPAF-4 membrane, a broad peak was observed at ca. 19o, suggesting the amorphous structure of the polymer. By incorporating LDH nanosheets, small new peaks appeared at around 35o and 62o, which can be assigned to the (012) and (113) reflections of MgAl LDH, indicating the successful incorporation of the LDH nanosheets in the composite membranes. In addition, no distinct peaks assignable to the (003) basal spacing of LDH were observed, indicating that the LDH nanosheets did not form an ordered lamellar structure but were randomly well-dispersed in the QPAF-4 polymer-based composite membranes. The distribution of nanosheets was further investigated by cross-sectional SEM images (Fig. 2c-f). Compared to the pristine QPAF-4 membrane with dense and homogeneous morphology, LDH nanosheets tightly embedded in the QPAF-4 matrix could be visualized in the composite membranes containing 1 wt.% and 3 wt.% LDH nanosheets (Fig. 2d, e). When the proportion of nanosheets increased to 5 wt.%, the composite membranes seemed rather inhomogeneous due to the partial aggregation of LDH nanosheets (indicated by yellow arrows in Fig. 2f). It is noteworthy that similar morphology and alignment of nanosheets were also observed in the composite membranes containing small-sized nanosheets (Fig. S6).Fig. 2. (a) AFM image of MgAl LDH nanosheets dispersed in DMAc solvent. (b) XRD patterns of pristine QPAF-4 and QPAF-4/LDH NS composite membranes. SEM images of (c) pristine QPAF-4, (d) QPAF-4/LDH NS-1%, (e) QPAF-4/LDH NS-3% and (f) QPAF-4/LDH NS-5% composite membranes (a small peak observed around ~18° was attributed to local short-range ordering in the polymer rather than LDH reflections).Transmission electron microscopic (TEM) images were obtained to investigate the effect of LDH nanosheets on the hydrophilic/hydrophobic phase-separated morphology of the composite membranes with various nanosheet compositions (Fig. S7). The dark regions represent hydrophilic domains containing the ammonium groups and their aggregates, while the bright regions represent hydrophobic clusters composed of the polymer backbone. The domain sizes of all membranes were averaged at over 100 spots. All membranes showed nearly identical hydrophobic domain size (1.24 ± 0.25 nm). Although the LDH nanosheets were hydrophilic, the hydrophilic domain sizes were similar for the pristine QPAF-4 membrane (1.25 ± 0.21 nm), QPAF-4/LDH NS-1% (1.23 ± 0.27 nm), QPAF-4/LDH NS-3% (1.28 ± 0.23 nm) and QPAF-4/LDH NS-5% (1.28 ± 0.24 nm). The results suggest that the LDH nanosheets did not affect the phase-separated morphology of the polymer membranes and that the nanosheets were not stained with the PtCl42- ions used to stain the ionomer.The composite membranes were titrated to determine the IECs (Table S1). As an inorganic anion conductor, the theoretical IEC of MgAl-NO3 LDH was as high as 3.83 mmol g–1 (see the Supplementary Information). As a result, the composite membranes exhibited higher IEC values than that of the pristine QPAF-4 membrane. It is noted that the QPAF-4/LDH NS-5% composite membrane showed lower titrated IEC than that of the other composite membranes, because some of the aggregated LDH nanosheets did not participate in the ion exchange reaction.3.2. Water uptake, swelling behavior, and hydroxide ion conductivityThe water uptake of QPAF-4/LDH NS composite membranes in Cl– form was investigated at RT. As shown in Fig. 3a, the water uptake of the composite membrane increased slightly with increasing LDH nanosheet content due to the hydrophilic nature of the LDH. The dimensional change of the composite membranes displayed a decreasing trend, indicating that water molecules adsorbed on the LDH nanosheets did not contribute to the swelling of the QPAF-4 membranes. The OH– conductivities of the composite membranes were measured in nitrogen-sparged water (to avoid the effect of CO2 from the environment) at various temperatures from 30 to 80 oC (Figs. 3b and S8a). For the pristine QPAF-4 membranes, the OH– conductivity increased with temperature, from 46.7 mS cm-1 (at 30 oC) to 83.7 mS cm–1 (at 80 oC). The conductivities of the composite membranes increased and reached a maximum at 3 wt.% (59.6 mS cm–1 at 30 oC and 102.0 mS cm–1 at 80 oC), which would be attributable to the intrinsic high anion-conducting properties of the LDH nanosheets.27,29 In addition, the good dispersion and compatibility of LDH nanosheets in QPAF-4 polymer (as suggested by SEM images) maximizes the utilization of monolayer LDH nanosheets. With further increase of the LDH composition, the conductivity decreased because of the partial aggregation of nanosheets, which agreed with the lower titrated IEC value. The conductivity is replotted as a function of the number of absorbed water molecules per mobile anion (λ) in the composite membranes (Fig. S9). The conductivity increased with increasing λ from 24 to 26 due to the development of hydrophilic domains as ionic pathways but decreased at λ = 29 because of the practically lowered IEC and possible dilution of the ion concentration with excess water.34Fig. 3. (a) Water uptake and swelling ratio of QPAF-4/LDH NS composite membranes at room temperature. (b) OH– conductivity of QPAF-4/LDH NS composite membranes in water at 30 oC as a function of LDH nanosheet composition. (c) Temperature dependence of OH– conductivity of composite membranes in water. (d) Representative stress versus strain curves for QPAF-4/LDH NS composite membranes in chloride ion form at 60% RH and 80 oC. Minor fluctuations occasionally observed in some samples were likely due to slight misalignment of the membrane in the clamps or small displacement during sample mounting. Additional tensile curves were provided in the Supplementary Information (Fig. S10a) to confirm reproducibility.The effect of the lateral dimensions of the nanosheets on the anion-conducting properties of the composite membranes was also investigated. Notably, the OH– conductivity of the QPAF-4/Nano-LDH NS-1% was 110.8 mS cm–1 at 80 oC, higher than that of QPAF-4/ LDH NS-1% (97.5 mS cm-1) (Fig. S8b). The submicrometer-sized nano-LDH sheets would have created more intersections and thus more highly interconnected ionic pathways for higher ion mobility and conductivity. A similar phenomenon was observed in lamellar membranes composed of stacked LDH and GO nanosheets.27 However, QPAF-4/Nano-LDH NS-3% showed a lower conductivity of 88.4 mS cm–1 at 80 oC. This might be due to the intercalation of carbonate ions in the LDH NS from atmospheric carbon dioxide, which was consistent with the lower titrated IEC values. All membranes displayed Arrhenius-type temperature dependence of the conductivity (Fig. 3c). The activation energy (Ea) for OH– conduction was calculated from the slopes to be 11.7-13.3 kJ mol–1 (Table S2), suggesting a similar hydroxide ion conduction mechanism involving the hydrated hydroxide ions. 3.3. Mechanical propertiesThe tensile properties of QPAF-4/LDH NS composite membranes in Cl– form were measured at 80 oC and 60 % RH (Figs. 3d and S10). Compared to the pristine QPAF-4 membrane, the composite membranes exhibited comparable and ever higher elongation (> 300 %) and stress (> 19 MPa) values, indicating a positive effect of the LDH nanosheets on the mechanical properties of the composite membranes. The viscoelastic properties of the composite membranes (in Cl– form) were evaluated by dynamic mechanical analysis (DMA) under variable temperature (at 60% RH) and variable relative humidity (at 80 oC) (Fig. S11). In the relative humidity range from 0% to 90% RH (Fig. S11a-c), the E’ and E’’ values of all membranes decreased with increasing humidity, which was already shown to be intrinsic to the QPAF-4 polymer.32 There was a trend in increasing E’ and E’’ with increasing LDH nanosheet composition. Under variable temperature conditions (from RT to 100 oC) (Fig. S11d-f), a broad, distinct peak was observed in E’’, possibly associated with the glass transition, which was shifted to higher temperature with increasing LDH nanosheet content, possibly because the hydroxyl groups on the surface of the LDH nanosheets functioned as cross-linkers via hydrogen-bonding, suppressing the micro-Brownian motion of the segmented chains.3.4. Alkaline stability of composite membranesThe alkaline stabilities of the pristine QPAF-4, QPAF-4/Nano-LDH NS-1%, and QPAF-4/LDH NS-3% membranes were investigated under harsh conditions in 8 M KOH aqueous solution at 80 oC (Figs. 4a and S12). After 1,000 h, the QPAF-4 membrane lost 57% of the conductivity, decreasing from 83.7 to 35.8 mS cm–1. The QPAF-4/Nano-LDH NS-1% composite membrane retained higher OH– conductivity (46.5 mS cm–1 after 1,000 h) with comparable conductivity loss (58%). In contrast, the QPAF-4/LDH NS-3% composite membrane exhibited the highest OH– conductivity retention rate of 80% (81.1 mS cm–1 after 1,002 h, suggesting that the LDH nanosheets are helpful to chemically stabilize the composite membrane and to maintain high conductivity under the harsh alkaline conditions.    Fig. 4. (a) OH– conductivity of various membranes in 8 M KOH at 80 ℃ during different times. (b) 1H NMR spectra of the pristine QPAF-4 and composite membranes after the alkaline stability test. (c) Degree of alkaline decomposition via two possible degradation pathways. (d) SEM image and (e) stress versus strain curves of QPAF-4/LDH NS-3% composite membranes in Cl– form after the alkaline stability test. (f) H2 and O2 permeability of pristine QPAF-4 and QPAF-4/LDH NS-3% composite membranes at 80 oC and different humidities (30%, 60% and 90% RH). To investigate the effect of LDH nanosheets on the alkaline stability of the QPAF-polymers, NMR analyses on post measurement samples were conducted (Fig. 4b). The integral ratio of aromatic protons to aliphatic pendants remained unchanged for all membranes, suggesting that the QPAF-4 backbone and side chains were intact under alkaline conditions. However, four new peaks appeared at around 1.8 (Ha), 1.9 (Hb), 4.7 (Hc), and 5.5 (Hd) ppm, assignable to the degradation products of the trimethyl alkylammonium groups via nucleophilic substitution (Ha, Hb) and Hofmann elimination (Hc, Hd), respectively. The specific degradation pathways and products are described in Fig. S13. The degradation ratios of all membranes were calculated from the integral ratios in the 1H NMR spectra (Fig. 4c). The pristine QPAF-4 membranes showed 26.1% nucleophilic substitution and 25.3% Hofmann elimination. Notably, the composite membranes displayed slightly smaller degradation degrees of the ammonium head groups with 24.1% nucleophilic substitution and 25.3% Hofmann elimination for the QPAF-4/Nano-LDH NS-1%. The ammonium degradation occurred to a much smaller degree for the QPAF-4/LDH NS-3% composite membrane, with 10.6% nucleophilic substitution and 20.2% Hofmann elimination, accounting for the much higher percentage of remaining ammonium groups of 69.2%. These results were in line with the above-mentioned conductivity retention. The suppression of the ammonium degradation by LDH nanosheets most likely stems from the strong hydrogen bonds between the hydroxy groups on the surface of LDH nanosheets and the methyl groups of the quaternary ammonium groups of the polymers, lowering the production of free ammonium ions. Similar results were found for graphene-based composite membranes.35 In addition, the attack derived from nucleophilic substitution on the methyl groups might be mitigated by the steric effect of the LDH nanosheets, with large micrometer-scale lateral dimension, thereby improving the alkaline stability of the composite membranes.36,12 Fig. 4d shows the cross-sectional SEM image of the post-test QPAF-4/LDH NS-3% composite membrane, where LDH nanosheets were uniformly and tightly embedded in the QPAF-4 matrix, demonstrating the superior stability of LDH nanosheets under harsh alkaline conditions. The post-test QPAF-4/LDH NS-3% composite membrane was subjected to stress/strain measurement. As shown in Fig. 4e, the post-test QPAF-4/LDH NS-3% composite membrane exhibited a comparable maximum strain (345%) and a slightly lower stress (21 MPa) with an acceptable error, suggesting excellent robustness of the composite membranes during the severe alkaline stability test. 3.5. Gas permeabilityGas impermeability is a crucial property for anion exchange membranes for practical applications in electrochemical devices such as fuel cells and water electrolysis, to avoid direct reaction of cross-diffusing hydrogen and oxygen. The effect of 2D LDH nanosheets on the gas permeability of the composite membranes was investigated. As shown in Fig. 4f, the H2 and O2 permeabilities of both membranes increased with increasing humidity from 30% RH to 90% RH at a constant temperature of 80 oC, which can be attributed to the plasticization of the membranes with adsorbed water. Notably, the QPAF-4/LDH NS-3% composite membranes showed lower gas permeabilities at all humidities investigated (e.g., 5.99 × 10–10 cm3 (STP) cm cm–2 s–1 cm Hg–1 (O2) and 2.62 × 10–9 cm3 (STP) cm cm–2 s–1 cm Hg–1 (H2) at 90% RH) compared to those of the pristine QPAF-4 membranes (9.13 × 10–10 cm3 (STP) cm cm–2 s–1 cm Hg–1 (O2) and 3.33 × 10–9 cm3 (STP) cm cm–2 s–1 cm Hg–1 (H2) under 90% RH). The results may be reasonably explained by considering that the LDH nanosheets are less gas-permeable than the organic QPAF-4 polymers. This result demonstrates the effectiveness of employing these 2D inorganic materials as an inorganic filler for organic-inorganic functional composite membranes.3.6. Alkaline water electrolysis cell performanceFig. 5. (a) I-V curves and ohmic resistances for water electrolysis cells using QPAF-4/LDH NS-3% composite membrane and NiFe LDH-rGO catalyst. The results for pristine a QPAF-4 cell with NiFeO catalyst are shown for comparison.49 (b) I-V comparison of state-of-the-art AEMWEs (Refer to Table S3).The optimized QPAF-4/LDH NS-3% composite membrane was evaluated in a water electrolysis cell. An in-house-prepared Ni2/3Fe1/3 LDH-rGO catalyst with QPAF-4-1.5 (IEC= 1.5 meq g–1) as the ionomer binder was employed as the anode catalyst layer, whereas commercial Pt/C with QPAF-4-2.0 (IEC = 2.0 meq g–1) as the binder was used as the cathode. Fig. 5 presents the current-voltage (I-V) curves of the water electrolysis cell operated at 80 oC with 1 M KOH. The onset voltage of the QPAF-4/LDH NS-based cell was 1.41 V, slightly lower than the 1.46 V observed for the pristine QPAF-4 membrane-based cell. These values were close to the previously reported onset potential for oxygen evolution reaction (OER) on NiFe LDH-rGO and NiFeO catalysts measured in 1 M KOH using the rotating disk electrode technique50,33, suggesting that the NiFe LDH-rGO catalysts functioned well as the anode in the water electrolysis cell. The QPAF-4/LDH NS-based cell exhibited a lower ohmic resistance of 74 mΩ cm–2 at 2.0 A cm⁻2 than the 89 mΩ cm⁻2 of the pristine QPAF-4 cell, reflecting its higher ion conductivity. The results are indicative of the interfacial compatibility of the membrane and the electrodes. At 1.0 and 2.0 A cm⁻2 current density, the cell voltages of the QPAF-4/LDH NS-based cell were 1.61 V and 1.73 V with voltage efficiencies of 76.4% and 71.1%, respectively. This performance not only exceeded those of the pristine QPAF-4-based cell (72.8% at 1.0 A cm–2 and 66.8% at 2.0 A cm–2) but also outperformed many state-of-the-art AEMWEs reported in the literature (Fig. 5b, Table S3). For example, very recent studies reported current density of 2.1 and 3.8 A cm⁻2 at 1.8 V and 2 V, respectively, using QP(TP70-co-DBT30) and P(O-F50%-C50%)-GTA membranes with non-precious metal catalysts.47,48 Although a reference electrode was not employed, a lower Tafel slope of the cell voltage of the QPAF-4/LDH NS-based cell (79.6 mV decade–1, Fig. S14) than that of the pristine QPAF-4 cell (99.1 mV decade–1)49 indicate enhanced OER kinetics (assuming that the overpotential at the cathode with Pt/C catalyst was much smaller). Overall, the integration of LDH nanosheets, both in the membrane and anode, contributed to lower cell voltage and ohmic resistance and thus better voltage efficiency in practical AEMWEs. In-situ durability test revealed a slight increase in cell voltage (Fig. S15), which might be attributed to nanosheet catalyst aggregation and membrane–catalyst interfacial effects. Notably, the voltage consistently remained below 1.8 V throughout the 1000 h operation, with only minor voltage rise.4. ConclusionsIn summary, organic/inorganic (QPAF-4/LDH NS) composite anion conductive membranes with various contents of LDH nanosheets were prepared via the solution-casting method. It was revealed that the monolayer LDH nanosheets were compatible with the polymer and well-dispersed in the matrix, thus providing additional fast ion transport channels along the surface of the LDH nanosheets. The obtained QPAF-4/LDH NS-3% achieved improved dimensional stability (25 %), enhanced mechanical robustness (320%, 23.7 MPa) and high ion conductivity (102.0 mS cm–1 at 80 oC). Reducing the lateral size of the LDH nanosheets from a few micrometers to hundreds of nanometers slightly increased the conductivity of the composite membranes to 110.8 mS cm–1 by creating more interconnected ionic pathways for higher ion mobility. In addition, LDH nanosheets were not only alkali-resistant but also mitigated the degradation of the ammonium groups in the polymers through the formation of hydrogen bonds with the polymer matrix and the steric effect of the LDH nanosheets with large micrometer-scale lateral dimensions. As a result, QPAF-4/LDH NS-3% displayed much higher conductivity retention (80%, 81.1 mS cm–1 in 8 M KOH at 80 oC for 1,002 h) than that of the pristine QPAF-4 membrane (43%, 35.8 mS cm–1 in 8 M KOH at 80 oC for 1,000 h). Furthermore, the composite membrane exhibited lower gas permeability, presumably attributed to an inorganic filler effect of LDH NS. A water electrolysis cell employing the optimized QPAF-4/LDH NS composite membrane demonstrated superior performance, achieving 1.0 A cm–2 at 1.61 V, surpassing the performance of the pristine QPAF-4 membrane (1.69 V at 1.0 A cm-2).Conflicts of interestThere are no conflicts to declare.Date availabilityThe data supporting this article have been included as part of the ESI.AcknowledgementsThis work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO), the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through Grants-in-Aid for Scientific Research (23H02058) and MEXT Program: Data Creation and Utilization Type Material Research and Development Project (JPMXP1122712807), JST (GteX JPMJGX23H2) , Toshiaki Ogasawara Memorial Foundation,  and JKA promotion funds from KEIRIN RACE.Note and References1. M. Younas, S. Shafique, A. Hafeez, F. Javed and F. Rehman, Fuel, 2022, 316, 123317.2. T. Terlouw, C. Bauer, R. 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