A poly(naphthalene-co-biphenyl piperidinium)-based highly conductive and durable anion exchange membrane for electrochemical energy devices

Abstract

Anion exchange membranes (AEMs) play a pivotal role in diverse electrochemical energy systems, including water electrolyzers, fuel cells, and redox flow batteries. However, conventional AEMs are often hindered by inadequate ion conductivity and chemical instability, limiting their performance. In this study, we introduce a poly(naphthalene-co-biphenyl piperidinium) (QPNBM)-based AEM and ionomer for electrolyzer and flow battery applications. The QPNBM membrane demonstrates outstanding hydroxide ion conductivity (197 mS cm⁻¹) and chloride ion conductivity (112.4 mS cm⁻¹) at 100 °C, coupled with excellent ex situ durability of 1500 h in 1 M KOH at 80 °C. In a water electrolysis setup, using Pt/C and IrO₂ catalysts with a 1 M KOH solution, the QPNBM membrane incorporating a fluorinated poly(carbazole-co-biphenyl piperidone) ionomer achieved a peak current density of 1.82 A cm⁻² at 60 °C and 2.4 V. In aqueous organic redox flow batteries, the QPNBM membrane demonstrated outstanding performance, with an energy efficiency of ∼77% and exceptional capacity retention (>99.56% over 50 cycles) at a current density of 10 mA cm⁻². These findings underscore the potential of the QPNBM-based ionomer as a high-performance membrane material for AEM-based electrochemical energy systems.

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1. Introduction

The intensifying global energy crisis and the urgent need to achieve carbon neutrality by 2050 have highlighted the necessity of reducing reliance on fossil fuel-based electricity generation. To meet the growing energy demand, renewable energy sources such as wind, solar, and tidal energy must be significantly expanded.¹ However, the intermittent nature of these renewable sources poses a major challenge to their seamless integration into existing power grids.^2,3 To address this issue and ensure a reliable energy supply, efficient energy storage systems and conversion technologies are essential.⁴ Among the various storage solutions, hydrogen energy and redox flow batteries stand out as sustainable options for both short- and long-term storage.⁵ Hydrogen, with its extremely high gravimetric energy density of 142 MJ kg⁻¹,^6,7 is particularly crucial as an energy carrier. It plays a vital role in key industries such as steel production, fertilizers, and chemical manufacturing, and serves as a carbon-free fuel for electricity generation in fuel cells.⁸

Hydrogen can be sustainably produced from water, offering a promising pathway for its generation. However, the vast majority (∼100 million tonnes annually) is currently derived from fossil fuel-based processes, which are both energy-intensive and environmentally detrimental.⁹ Water electrolysis accounts for less than 0.1% of global hydrogen production due to the high costs of catalysts, membranes, electrolyzer accessories, and electricity.^9,10 Among the existing water electrolysis technologies, alkaline water electrolysis is the most mature but relies on highly corrosive electrolytes such as KOH, complicating long-term operation and scalability. Proton exchange membrane water electrolysis (PEMWE) offers superior energy conversion efficiency but depends on costly platinum group metal (PGM) catalysts and Nafion-based membranes, limiting its scalability.¹¹ In contrast, the AEM water electrolyzer has emerged as a cost-effective alternative, operating in alkaline media and enabling the use of non-PGM catalysts. This significantly reduces costs while maintaining high hydrogen purity and efficiency.^12,13 Recent advancements in AEMs have positioned water electrolyzers based on this technology as a viable solution for large-scale, low-temperature hydrogen production powered by renewable energy, contributing to the creation of sustainable hydrogen sources.¹⁴ In AEM water electrolyzers, hydrogen is produced at the cathode via water reduction, while hydroxide ions (OH⁻) migrate to the anode, where they are oxidized to generate oxygen and water.¹⁵

Despite their promise, the performance of AEM water electrolyzers is often hindered by the limited durability and ion conductivity of current membranes and ionomers. These issues stem from polymer backbone degradation and cationic functional group decomposition in alkaline environments, as well as low mechanical integrity caused by excessive water uptake.^16–19 To meet the performance demands of water electrolyzer cells, ideal AEMs must exhibit high ionic conductivity, robust chemical and mechanical stability under alkaline conditions, and long-term durability under operational stresses.^20–22 These attributes are equally critical for other electrochemical applications, including redox flow batteries and fuel cells.^23,24 Significant challenges remain, particularly in enhancing ion conductivity and ensuring long-term stability under elevated temperatures and harsh alkaline conditions.^25,26 Polymer degradation via nucleophilic substitution and Hofmann elimination processes, as well as high water uptake leading to swelling, are major hurdles.^27–29 The cost-effectiveness of monomers and the simplicity of the polymerization process are also key factors influencing the commercialization of AEMs. Various ion-conducting groups such as trimethyl ammonium, imidazolium, sulfonium, phosphonium, and organometallic groups have been explored³⁰ with polymers such as aryl ether polymers,^31,32 polysulfone (PSF),³³ poly(ether ether ketone) (PEEK),^34,35 polybenzimidazole (PBI),³⁶ poly(norbornene) (PNB),³⁷ and alkylammonium containing poly(carbazole).³⁸ However, most of these materials suffer from degradation and performance losses in alkaline media.^39,40

Arylene-based AEMs, particularly those containing biphenyl moieties, have shown promise but exhibit excessive swelling due to water absorption, compromising their mechanical stability at high temperatures.⁴¹ Attempts to mitigate these issues through chemical modifications, such as introducing hydroxyethyl groups or incorporating polyhedral oligomeric silsesquioxane, have improved dimensional stability but at the cost of reduced hydroxide conductivity.^42,43 Similarly, copolymerizing biphenyl molecules with fluorenyl has reduced water uptake but increased material costs, limiting commercial feasibility.⁴⁴ Polymer membranes incorporating robust aromatic components and specifically designed backbone structures exhibit enhanced strength, thermal stability, and operational efficiency. By strategically utilizing cost-effective and readily accessible hydrophobic building blocks, it is feasible to augment durability and functionality for demanding industrial applications without compromising ionic transport efficiency. Moreover, the biphenyl monomer is cost-effective and exhibits higher reactivity compared to other high-aryl monomers.⁴⁵ The polymer synthesized with the biphenyl monomer demonstrates excellent solubility in common organic solvents and enhanced alkaline stability, attributed to the more rigid triphenyl monomer, which induces greater internal stress.^45,46 The planar structure of naphthalene, characterized by its fused bicyclic conjugation, facilitates electrophilic substitution reactions effectively.⁴⁷ The integration of a hydrophobic, rigid naphthalene moiety into the polymer chain improves both mechanical and dimensional stability.^48,49 Additionally, the use of a low ring strain N-methyl piperidone moiety as a stable ion-conducting group enhances the alkaline stability of the synthesized polymer, achieved through a straightforward and metal-free polymerization process.^50–52

To address these challenges, we have developed a cost-effective, efficient, and durable poly(naphthalene-co-biphenyl piperidinium) (QPNBM) AEM. By copolymerizing piperidone with naphthalene and biphenyl, we avoided weak ether bonds and benzylic hydrogens, achieving enhanced chemical stability. The synthesized polymer features a nanophase-separated morphology with hydrophilic quaternary ammonium groups and hydrophobic naphthalene and biphenyl moieties, improving ion transport and mechanical properties. The resulting QPNBM membrane surpasses commercial and literature-reported AEMs in terms of water behavior, ionic conductivity, current density, and durability, making it a highly promising candidate for AEM water electrolyzer and redox flow battery applications.

2. Results and discussion

2.1 Polymer synthesis and characterization

The co-polymerization of electron-rich biphenyl and naphthalene monomers (in a molar ratio of 3 : 0.8) was carried out using the Friedel–Crafts polycondensation method, as shown in Fig. 1a. To introduce ion-exchangeable functionality, N-methyl piperidone was employed as a third monomer, with its reactivity enhanced via protonation at the carbonyl group using trifluoroacetic acid (TFA). The addition of trifluoromethanesulfonic acid (TFSA) as a superacid catalyst further improved the reactivity of the piperidone, producing a highly electrophilic piperidone dication.⁵³ Under these reaction conditions, the electron-rich biphenyl and naphthalene monomers underwent rapid condensation with the piperidone moiety. It was observed that the inclusion of a small amount of the ketonic monomer increased the polymerization rate and molecular weight of the resulting poly(naphthalene-co-biphenyl piperidinium) (PNBM) polymer.⁵⁴ Specifically, a piperidone monomer content exceeding 9.5% relative to the biphenyl and naphthalene monomers was found to enhance the polymerization rate. As polymerization progressed, solution's viscosity was increased, peaking after 8–9 hours of reaction. Prolonging the reaction beyond this point led to a decrease in viscosity, indicating that the propagation reaction was surpassed by chain termination processes, resulting in inactive polymer chains (¹H NMR in Fig. S1, ESI†). While the superacid-catalyzed polymerization offers energy-efficient and metal-free synthesis of high-performance polymers, we acknowledge and have addressed associated sustainability concerns through minimized catalyst use and appropriate waste management practices. After synthesizing the PNBM polymer, quaternization of the piperidone units was achieved by reacting with methyl iodide at room temperature, followed by ion exchange with KCl and KOH to yield the chloride and hydroxide forms of the membrane, respectively, referred to as QPNBM.

Fig. 1 (a) Synthetic route for the preparation of the PNBM polymer and its subsequent quaternization to form the QPNBM membrane. (b) ¹H NMR spectra of PNBM and QPNBM, showing the chemical shifts of methylene, methyl, biphenyl, and naphthalene protons, along with the emergence of quaternization-specific peaks.

The chemical structures of the synthesized polymer and its quaternized derivative were confirmed through ATR-FTIR and NMR spectroscopy. Fig. S2 (ESI)† shows the ATR-FTIR spectra of PNBM and QPNBM. Characteristic absorption bands at 2930 and 2956 cm⁻¹ correspond to the symmetric and asymmetric stretching vibrations of aliphatic C–H bonds, while peaks corresponding to aromatic C–H stretching vibrations appear at 3020 cm⁻¹ and 3049 cm⁻¹. A peak corresponding to C–N stretching vibration of the piperidone moiety was observed at 1141 cm⁻¹, and a peak corresponding to the quaternized C–N⁺ stretching vibration appeared at 980 cm⁻¹.⁵⁵ Additionally, a peak corresponding to the C–H bending vibration of naphthalene was detected at 779 cm⁻¹.⁵⁶ The ¹H NMR spectra of PNBM and QPNBM, shown in Fig. 1c, further confirm the polymer structures. Signals corresponding to the methylene protons of piperidone appear between 2.2 and 3.5 ppm, while the methyl protons give a distinct peak at 2.75 ppm. Signals from biphenyl and naphthalene protons are detected between 7.3 and 8.1 ppm. After quaternization, a new peak corresponding to the methyl protons of the quaternized piperidone emerged at 3.12 ppm, and shifts in the methylene protons were observed at 3.34 ppm and 2.8 ppm (Fig. S3, ESI†).

The inherent viscosities of PNBM and QPNBM copolymers in DMSO at RT were 5.96 dL g⁻¹ and 7.06 dL g⁻¹, respectively, indicating that both copolymers possess high molecular weights. Notably, there are only a few reports of polyaryl-based polymers with intrinsic viscosities exceeding 4.5 dL g⁻¹.⁵⁷ The high molecular weight of PNBM contributes to its excellent film-forming ability, superior mechanical strength, and low swelling ratio. In addition, the rheological properties of the polymer solution significantly influence membrane morphology and processability.^58,59 Rheological studies of the QPNBM polymer solution revealed Newtonian behavior at low angular frequencies (0.1 rad s⁻¹) at 25 °C. However, at higher angular frequencies, shear thinning behavior was observed due to polymer chain alignment along the shear stress direction (Fig. S4a, ESI†). At elevated temperatures, complex viscosity and storage modulus increased due to enhanced polymer chain interaction, as shown in Fig. S4a and b (ESI).† Furthermore, the polymer exhibited high storage and loss moduli across the entire frequency range, as shown in Fig. S4b and c (ESI).† The viscosity–temperature curve demonstrated Newtonian behavior up to 60 °C, beyond which non-Newtonian behavior emerged, attributed to strong polymer chain interactions (Fig. S4d, ESI†).

2.2 Membrane morphology

The QPBNM membrane, as shown in Fig. 2a and b, exhibits a transparent and flexible film structure. The FE-SEM images in Fig. 2c and d reveal a smooth surface and dense cross-section, with no visible holes or cracks on the surface. This uniformity ensures homogeneous hydration throughout the membrane, thereby preventing excessive swelling and the formation of dry spots on irregular and cracked surfaces. Additionally, EDX mapping (Fig. S5 and Table S1, ESI†) confirmed the homogeneous distribution of various elements across the polymer matrix. The phase-separated morphology of AEMs plays a critical role in the development of ionic channels, which facilitate water molecules to fill ion-transport pathways.⁶⁰ Enhanced phase separation and less tortuous ionic channels contribute to faster ion transport across the membrane. High-resolution TEM (HR-TEM) images (Fig. 2e) revealed the nanophase-separated structure of the QNPBM membrane, evident throughout the film interior. The darker regions correspond to the aggregation of hydrophilic piperidinium functional groups, while brighter regions represent the aromatic hydrophobic polymer backbone. This distinct separation between hydrophilic and hydrophobic phases underscores the membrane's well-developed nanophase morphology, essential for efficient ion transport.

Fig. 2 (a) Photograph of the transparent QPNBM membrane (21 × 15 cm) showcasing its clarity and uniformity, (b) demonstration of the flexibility of the QPNBM membrane, (c and d) FE-SEM images highlighting the smooth surface and dense cross-section morphology without visible defects, (e) HR-TEM image showing a nanophase-separated morphology, with hydrophilic phase width size in the inset figure, (f) two-dimensional AFM phase image in the iodide form, showing phase separation with hydrophilic and hydrophobic domains, and (g) SAXS profile of the QPNBM membrane indicating the maximum scattering vector of 2.6 nm⁻¹.

The phase-separated morphology of the air-dried QPNBM membrane in its iodide form is further illustrated in the AFM image (Fig. 2f). The darker regions in the AFM phase image indicate the aggregation of hydrophilic moieties with quaternary ammonium functional groups, while the light-yellow domains represent the aggregation of hydrophobic structures. The 3D-AFM image (Fig. S6, ESI†) provides additional confirmation of the phase-separated morphology, with prominent peaks corresponding to hydrophobic zones and troughs indicating hydrophilic regions. The phase separation morphology was further examined using SAXS analysis (Fig. 2g). The maximum scattering vector (q_max) was observed at 2.6 nm⁻¹, corresponding to a d-spacing (domain size) of approximately 2.4 nm. This finding aligns well with the hydrophilic domain sizes of around 2.4–3 nm observed in the HRTEM and AFM images, confirming the consistent nanophase-separated structure of the QPNBM membrane. In the analysis of the commercial FAA-3-50 membrane (50 ± 5 μm, Br⁻ form), a weak phase separation morphology was observed, as evidenced by the AFM image (Fig. S6c and d, ESI†). Additionally, no distinct peak was identified in the SAXS analysis, as depicted in Fig. S6f, (ESI).† In contrast, the QPNBM membrane exhibits a distinct and large phase morphology characterized by high hydrophilicity and superior ionic channels, which facilitate enhanced ion transportation compared to the FAA-3-50 membrane. Such well-defined phase separation is essential for effective ion transport in AEMs.

2.3 Water uptake, dimensional stability, and surface charge analysis

Effective water uptake is critical for forming water channels in AEMs to facilitate anion transport. However, excessive water uptake can lead to excessive swelling, dilution of charge concentration, and weakened mechanical characteristics. The water absorption characteristics of the QPNBM membrane in I⁻ and OH⁻ forms were evaluated at temperatures ranging from 30 to 80 °C, as shown in Fig. 3a. The water uptake and swelling ratio (Fig. 3b) increased with temperature due to an increase in water molecule diffusion, ionic mobility, and channel expansion in the membrane. The membrane in the I⁻ form exhibited stable water uptake (16–20%) across the temperature range, indicating minimal changes in water molecule diffusion and channel expansion. In contrast, the OH⁻ form demonstrated a significant increase in water uptake, from 67% at 30 °C to 161% at 80 °C. This dramatic increase highlights the high-water affinity of the OH⁻ form due to its smaller ionic size and higher charge density, suggesting that the absorbed water is localized within the hydrophilic channels. This behavior avoids the formation of long-range void spaces and maintains polymer chain packing, backbone polarity, and flexibility. Furthermore, the high-water uptake and swelling values of the QPNBM membrane in comparison to the FAA-3-50 membrane (Fig. S8a and b, ESI†) align with the result of high IEC, large hydrophilic phase width, and high conductivity values. Additionally, the dimensional swelling ratio of the QPNBM membrane, which remained consistent, further supports this observation, underscoring the membrane's ability to retain mechanical integrity alongside high water uptake.

Fig. 3 (a) Water uptake and (b) swelling ratio of the QPNBM membrane as a function of temperature. (c) DSC curve indicating the glass transition temperature of QPNBM, (d) TGA and DTA curves showing thermal stability and degradation steps, (e) DMA curve depicting the storage and loss moduli variation with temperature, and (f) stress–strain curve of the QPNBM membrane in the I⁻ form at room temperature.

The presence of positively charged functional groups on the QPNBM membrane generates an electrical double layer, facilitating selective counter-ion transport and influencing electro-osmotic flow across the membrane. In contrast, a higher zeta potential generates a stronger driving force for electroosmotic drag.^61,62 This phenomenon is critical for mitigating cathode dryness and anode flooding, common challenges in dry cathode AEM water electrolyzers and fuel cells.^63–65 The QPNBM membrane exhibited a low positive zeta potential in the pH range of 3–9, as compared to the commercial FAA-3-50 membrane (Fig. S7, ESI†). This low zeta potential reduces electro-osmotic drag, potentially enhancing the performance of QPNBM membranes in electrochemical devices.

2.4 Thermal and mechanical stability

Thermal stability is a critical attribute of AEMs that significantly influences their longevity and durability in the working temperature range. The aromatic backbone of the QPNBM membrane enhances its thermal robustness compared to aliphatic polymers. Differential scanning calorimetry (DSC) analysis revealed a high glass transition temperature (T_g) of ∼320–355 °C due to the rigid aromatic and naphthalene moieties in the polymer backbone, whereas FAA-3-50 showed low T_g's at around 110–140 °C and 230 °C, indicating a low softening temperature of the FAA-3-50 membrane (Fig. 3c & S8c, ESI†). Furthermore, thermogravimetric analysis (TGA) in an N₂ medium (Fig. 3d) showed the following degradation pattern: an initial weight loss at ∼130 °C attributed to water and residual solvent evaporation. Two subsequent degradation steps: the first at ∼380 °C (decomposition of piperidinium cations) and the second at ∼550 °C (breakdown of the aromatic polymer backbone). The high char yield (∼40 wt%) at 650 °C reflects the thermal stability imparted by the naphthalene and biphenyl backbone in the QPBNM membrane. Additionally, the FAA-3-50 membrane degrades at ∼230 °C (degradation of the pendant quaternary ammonium group) and ∼430 °C (decomposition of the aromatic backbone) (Fig. S8d, ESI†), reflecting low thermal stability compared to the QPNBM membrane. Mechanical properties were evaluated through storage modulus and stress–strain analysis. The storage modulus of the QPNBM membrane at 100 °C was 186.8 MPa (Fig. 3e), with a reduction at higher temperatures due to decreased chain interactions. The membrane exhibited two T_g values: ∼330 °C for the biphenyl segment and ∼350 °C for the naphthalene segment in the copolymer. Additionally, FAA-3-50 exhibited a low storage modulus of 2.5 MPa at 100 °C and demonstrated two distinct T_g's. The initial T_g occurred at approximately 140 °C, corresponding to the pendant quaternary ammonium segmental block, while the second T_g was observed at ∼230 °C, associated with the hydrophobic polysulfone block (Fig. S8e, ESI†).

The stress–strain curve at RT (Fig. 3f) revealed a tensile strength (TS) of 30.6 MPa and elongation at break (EB) of 22.9%. The stiff aromatic backbone contributes to tensile strength, while the piperidone linkage provides chain flexibility. In a hydrated state, the TS and EB were reduced to 21.05 MPa and 15.72% (Fig. S8, ESI†), attributed to water-induced plasticization of the polymer chains.⁶⁶ For the OH⁻ form of the QPNBM membrane, the TS and EB were 30 MPa and 58%, respectively. The notable change in mechanical properties compared to the I⁻ form highlights the disruption of polymer chain packing by the smaller OH⁻ ions, which create voids and enhanced elongation. The mechanical properties of dry QPNBM membranes align closely with those of commercial FAA-3-50 (TS: 33 MPa, EB: 60%, in OH⁻ form, Fig. S8f, ESI†) and FAA-3-20 (TS: 32 MPa, EB: 27%) membranes⁵⁷ and exhibit higher tensile strength than that of Nafion^®117 membrane (TS: 25.9 MPa and EB: 177%).^67,68 These attributes, combined with thermal stability, make QPNBM membranes suitable for use in electrolyzers, fuel cells, and flow batteries without requiring reinforcement.

2.5 Ion exchange capacity and ionic conductivity

The mobility of hydroxide ions in AEMs is inherently lower than proton mobility in cation exchange membranes due to their larger hydrated radii and stronger interaction with quaternary ammonium groups. To compensate, AEMs require a high density of charge carrier functional groups to maximize counterion mobility. This results in a high ion exchange capacity (IEC), a key parameter influencing ionic conductivity, water absorption, dimensional stability, and membrane morphology.^60,61 The IEC and hydration number data are summarized in Table S2 (ESI).† The IEC of the QPNBM and FAA-3-50 membranes in their Cl⁻ form, determined via titration, were ∼2.38 mmol g⁻¹ (and by NMR 2.59 mmol g⁻¹) and ∼2.2 mmol g⁻¹, which are optimal values for balancing water uptake, ion and water transport, and dimensional stability. The hydration numbers of the QPNBM and FAA-3-50 membranes in their hydroxide forms were 12.2 and 5.05, respectively. The high hydration number indicates a strong interaction and accessibility of water molecules with the functional groups in the QPNBM membrane, which directly contributes to its enhanced ionic conductivity compared to the FAA-3-50 membrane.

Given the high IEC and hydration numbers, the QPNBM membrane demonstrated enhanced ionic conductivity, a critical property for achieving high power densities in electrochemical systems at a moderate to high working temperature range. The ionic conductivity of the QPNBM membrane increased with temperature due to accelerated ionic migration across the membrane. As shown in Fig. 4a, the Cl⁻ ion conductivity of the QPNBM membrane increased from 41.5 mS cm⁻¹ at 30 °C to 112.4 mS cm⁻¹ at 100 °C, significantly outperforming the commercial FAA-3-50 membrane, which showed a conductivity increase from 8.62 mS cm⁻¹ to 29.4 mS cm⁻¹ over the same temperature range. Similarly, the OH⁻ ion conductivity of the QPNBM membrane increased from 69 mS cm⁻¹ at 30 °C to 197 mS cm⁻¹ at 100 °C, which is more than five times higher than the commercial FAA-3-50 membrane, whose conductivity increased from 14 mS cm⁻¹ to 44 mS cm⁻¹ over the same temperature range (Fig. 4b). These results underscore the superior ionic transport properties of the QPNBM membrane. A comparative analysis of water uptake, IEC, and OH⁻ ion conductivity between QPNBM and other reported membranes in the literature (Fig. 4c) highlights the QPNBM membrane's exceptional performance. Its high OH⁻ ion conductivity, moderate IEC, and balanced water absorption ensure dimensional stability and optimal performance in electrochemical systems. Furthermore, the low-temperature DSC curve revealed a bound water value of 9.32 for the ammonium functional groups in the QPNBM membrane, contributing to its high ionic conductivity (Fig. S9 and Table S2, ESI†). The OH⁻ ion transference number (t_m) of the QPNBM membrane was found to be 0.827, confirming that OH⁻ ions are the dominant charge carriers (Fig. S10(a) and Table S3, ESI†). Additionally, the hydroxide ion permeation rate of the QPNBM membrane exceeded that of FAA-3-50, as evidenced by the steeper concentration vs. time slope, further demonstrating its effective OH⁻ ion transport capability (Fig. S10(b) and Table S3, ESI†).

Fig. 4 (a and b) Cl⁻ and OH⁻ ion conductivity of the hydrated QPNBM membrane as a function of temperature, (c) comparison of water uptake, ion exchange capacities, and OH⁻ ion conductivity of QPNBM with literature-reported membranes at ambient temperature,^69–77 (d) retention of OH⁻ ion conductivity of QPNBM and FAA-3-50 membranes after storage in 1 M KOH at 80 °C over various time intervals, and (e) ¹H NMR spectra of the QPNBM membrane in DMSO d⁶/TFA, before and after storage in 1 M KOH solution at 80 °C for different durations.

2.6 Alkali stability analysis

A key challenge in designing stable and durable AEMs is ensuring the chemical stability of the polymer matrix under alkaline conditions, which often leads to cationic group degradation. The alkali stability of the QPNBM membrane was evaluated by immersing it in 1 M KOH at 80 °C. At regular intervals, the degradation of the cationic groups was examined using ¹H NMR spectroscopy and Electrochemical Impedance Spectroscopy (EIS).⁶² The QPNBM membrane exhibited remarkable stability (Fig. 4d), retaining 99.16% of its OH⁻ ion conductivity after 1008 hours of exposure to alkaline conditions. In contrast, the FAA-3-50 membrane exhibited a significantly reduced conductivity of 54.34% under the same conditions. Upon extending the experiment to 1512 hours, the QPNBM membrane experienced a 93.85% retention in conductivity, compared to 48.13% for the FAA-3-50 membrane. These results demonstrate the superior stability of the QPNBM membrane in alkaline environments. To investigate the degradation mechanism, ¹H NMR spectra of the QPNBM membrane materials were collected before and after 63 days (1512 hours) of exposure to 1 M KOH at 80 °C. The membrane samples, first ion exchanged to the Br⁻ form and dried, were dissolved in DMSO-d⁶ with ∼6% TFA for analysis. The ¹H NMR spectra revealed that Hofmann elimination, involving β-protons, and substitution reactions were the primary degradation pathways in this chemically harsh environment. Signals corresponding to vinyl protons appeared at 4.7–5.4 and 6.5 ppm, indicating degradation of cationic groups through Hofmann elimination (Fig. 4e and see Fig. S11 (ESI†) for the stability in 5 M KOH). Additionally, TFA induced a tertiary amine proton signal at ∼9.5 ppm due to the elimination of the piperidine ring. The absence of signals in the 8.2–9 ppm range suggests that nucleophilic ring-opening substitution or methyl substitution reactions did not occur. However, the broad peak above 10 ppm, arising from acid protons, obscured the clear identification of tertiary amine proton signals near 9.5 ppm. The weak intensity of the vinyl proton signal in the QPNBM polymer and the 6.15% conductivity reduction after prolonged exposure indicate that carbon dioxide may have contributed to the decline in conductivity. Nevertheless, the QPNBM membrane's stability under these conditions makes it a highly promising candidate for use in electrochemical systems requiring long-term alkaline durability.

2.7 Electrolyzer cell performance

AEMWEs represent an advanced low temperature solution for sustainable hydrogen production, where the membrane properties and durability play a critical role in determining system performance.⁷⁸ In AEMWEs, oxygen is generated at the anode through the oxidation of hydroxide ions, a process that relies heavily on the supply rate of OH⁻ ions. The efficiency of AEMWEs improves when an alkaline solution is introduced, as it enhances ionic conductivity. However, excess water from alkali feeding at the cathode can compromise hydrogen purity.⁷⁹ To achieve high-purity hydrogen, the alkaline solution was supplied exclusively to the anode, while the cathode side was maintained dry. In this anhydrous cathode AEMWE configuration, water molecules diffuse from the anode to the dry cathode. Consequently, high OH⁻ ion conductivity and efficient water diffusivity through the membrane electrode assembly (MEA) are essential for optimal dry cathode AEMWE performance.

The performance of the in-house built AEMWE setup (Fig. 5a) was assessed by recording polarization curves over a potential window of 1.2 to 2 V at a scan rate of 10 mV s⁻¹. The membrane electrode assembly was fabricated using the QPNBM membrane (thickness 90 ± 5 μm) and a 10 wt% QPNBM ionomer/catalyst dispersion. This membrane electrode assembly demonstrated a current density of 597 mA cm⁻² at 2 V. For comparison, a commercial FAA-3-50 membrane (thickness 50 ± 5 μm, Fuel Cell Store, USA) was utilized to prepare a membrane electrode assembly using a 10% FAA-3-50/DMSO ionomer solution and the catalyst dispersion. Both membrane electrode assemblies were prepared with the same catalyst loading for the anode and cathode and identical ionomer content, ensuring a fair performance comparison. Under identical testing conditions, the FAA-3-50-based membrane electrode assembly yielded a current density of 475 mA cm⁻² at 2 V, as depicted in Fig. 5b.

Fig. 5 (a) Image of the custom-built dry cathode water electrolyzer setup used for performance evaluation. (b) Polarization curves of QPNBM and FAA-3-50 AEMWEs, demonstrating the higher current density achieved by the QPNBM membrane. (c) Nyquist plot and the equivalent circuit of the QPNBM and FAA-3-50 membranes in AEMWEs, highlighting the lower ohmic resistance of the QPNBM membrane. (d) Breakdown of individual component resistances obtained from equivalent circuit fitting. (e) Polarization curves of the QPNBM membrane-electrode assembly at various anodic ionomer loadings. (f) Polarization curve of the QPNBM membrane-electrode assembly with two distinct ionomer loadings. (g) Membrane-electrode assembly of the QPNBM membrane operated under 500 mA cm⁻² at 60 °C in the presence of a PGM catalyst (Ir = 2 mg cm⁻² & Pt = 0.5 mg cm⁻²).

Following the polarization curve measurements, EIS was performed in potentiostatic mode at 1.6 V to evaluate resistances within the electrolyzer setup. In the Nyquist plot, the intercept on the real impedance axis (x-axis) in the high-frequency region represents the ohmic resistance (R_ohm), encompassing membrane resistance and interfacial contact resistances between components such as the bipolar plate, gas diffusion layer (GDL), and catalyst layer on the membrane.⁸⁰ The activation resistance (R_act) corresponds to the second intercept on the x-axis, determined from the semicircle radius. Equivalent circuit fitting was applied to analyze the contributions of individual components to the overall resistance. EIS data (Fig. 5c) reveal that the R_ohm of the QPNBM membrane was 0.262 Ω cm², significantly lower than 0.424 Ω cm² for the FAA-3-50 membrane. The lower R_ohm highlights the superior ionic conductivity of the QPNBM membrane. However, the R_act of the QPNBM membrane electrolyzer was higher, potentially due to suboptimal catalyst dispersion, catalyst agglomeration, or ionomer blockage of active sites. Further analysis of component-specific resistances, derived from equivalent circuit fitting, is shown in Fig. 5d. Despite the higher R_act, the low R_ohm enabled higher current density for QPNBM compared to the FAA-3-50-based electrolyzer. To optimize the QPNBM-based electrolyzer, efforts to reduce R_act through better solvent and ionomer selection or improved catalyst ink dispersion are necessary.

The influence of anodic ionomer content on electrolyzer performance was investigated by varying the ionomer loading from 10% to 30%. Higher ionomer content led to reduced performance, with the current density dropping from 597 to 319 mA cm⁻². This decline is attributed to reduced active site accessibility at higher ionomer loadings, despite improved catalyst (IrO₂) binding, as it simultaneously covers the active sites of the catalyst. Fig. 5e illustrates the performance variations with different ionomer loadings. Additionally, the performance of AEMWEs using QPNBM membranes with the Nafion117 ionomer was compared against that using the QPNBM ionomer. As shown in Fig. 5f, the QPNBM ionomer outperformed Nafion117, owing to its higher OH⁻ ion conductivity.

Durability testing of the QPNBM membrane-based AEMWE was performed in galvanostatic mode at 500 mA cm⁻², with 1 M KOH circulated at 60 °C. The QPNBM membrane retained stable performance for up to 470 hours, demonstrating excellent resilience at high current densities (Fig. 5g). Stability under corrosive conditions and elevated temperatures underscores its suitability for continuous hydrogen production. Performance comparison between QPNBM and FAA-3-50 membranes, with identical ionomer (fluorinated poly(carbazole-co-biphenyl piperidone); QPCBF) and catalyst loadings, further validated the higher efficiency of QPNBM. At 60 °C, the QPNBM membrane delivered a current density of 1.31 A cm⁻² at 2.4 V, more than twice the 0.497 A cm⁻² achieved by the FAA-3-50 membrane (Fig. 6a). This high current density is attributed to the strong ionic conductivity of the QPBNM membrane, implying low ohmic resistance, as observed in Fig. 6b. Increasing the cell temperature from 60 °C to 80 °C further enhanced the QPNBM membrane's performance, raising the current density from 1.31 A cm⁻² to 1.68 A cm⁻² (Fig. 6c). This improvement resulted from enhanced ionic conductivity and accelerated hydrogen and oxygen evolution rates. Nyquist plot analysis (Fig. 6d) confirmed a reduction in R_ohm from 0.25 ohm cm² at 60 °C to 0.237 ohm cm² 80 °C, while R_act also decreases with temperature due to faster reaction rates. The QPNBM membrane consistently achieved high current densities, demonstrating superior catalyst dispersion and reduced resistance, as detailed in the fitted Nyquist plot (Fig. S12, ESI†).

Fig. 6 (a) Polarization curves of QPNBM and FAA-3-50 membrane-based water electrolyzers prepared with the QPCBF ionomer, illustrating the superior performance of the QPNBM membrane at 60 °C. (b) Nyquist plot and equivalent circuit analysis of QPNBM and FAA-3-50 membranes in AEMWEs at 60 °C and 1.6 V potential, highlighting their lower ohmic resistance. (c) Polarization curve of the QPNBM membrane-electrode assembly showing the effect of temperature on performance, (d) Nyquist plots of the QPNBM membrane-electrode assembly at 60 °C and 80 °C, demonstrating the effect of temperature on ohmic and activation resistances, (e) stability study of QPNBM and FAA-3-50 membranes in a dry cathode water electrolyzer assembly under 500 mA cm⁻² at 60 °C, with PGM catalysts (Ir = 3.5 mg cm⁻² & Pt = 0.8 mg cm⁻²), showing superior durability of the QPNBM membrane.

Durability testing of the QPNBM and commercial FAA-3-50 membranes at 60 °C and 500 mA cm⁻² revealed stark differences. The FAA-3-50 membrane-based electrolyzer exhibited a degradation rate of 17.5 mV h⁻¹ over 24 hours, while the QPNBM membrane degraded at only 1.9 mV h⁻¹ after 95 hours, as shown in Fig. 6e. The rapid increase in potential for the FAA-3-50-based electrolyzer at high current densities is likely due to reduced membrane conductivity. In conclusion, the QPNBM membrane demonstrated superior performance and durability compared to the FAA-3-50 membrane, achieving better long-term stability than previously reported systems, as summarized in Table S4 (ESI).†

2.8 Flow battery performance

To expand the application of the prepared membrane, a flow battery test was conducted using the QPNBM membrane (thickness: 90 ± 5 μm; average pore diameter 3.168 nm; Fig. S13 and Table S5, ESI†). The flow battery was constructed using a 0.5 M MV anolyte (see permeability data Fig. S14a, ESI†), 0.5 M FcNCl catholyte, and 2 M aqueous NaCl as the supporting electrolyte. The cell test was performed in a nitrogen environment to prevent the degradation of active materials and side reactions in the presence of oxygen. To avoid the second reduction of MV, the charge–discharge potential window was set between 1.5 and 0.1 V.⁸¹ The battery was cycled at current densities ranging from 10 to 40 mA cm⁻², as shown in Fig. 7a.

Fig. 7 Redox flow battery performance using the QPNBM membrane under neutral pH condition, with anolyte (0.5 M MV in 2.0 M NaCl solution) and catholyte (0.5 M FcNCl in 2.0 M NaCl solution): (a) galvanostatic charge/discharge performance at current densities of 10, 15, 20, 25, 30, 35, and 40 mA cm⁻². (b) Average CE, VE, and EE under galvanostatic battery cycling at 10–40 mA cm⁻² current densities. (c) Voltage profile of charge/discharge cycles in the time range of 8 to 10 hours. (d) Capacity vs. cycle number for 50 cycles at 10 mA cm⁻², to evaluate the performance of CE, VE, EE, and charge/discharge capacity over 50 cycles.

The flow battery demonstrated a high coulombic efficiency (CE) of >95% across current densities from 10 to 40 mA cm⁻², resulting in closely overlapping energy efficiency (EE) and voltage efficiency (VE) values. At a current density of 10 mA cm⁻², galvanostatic testing provided a maximum capacity utilization of 72% and a VE of 79.7%, with both metrics decreasing as the current density increased (Fig. 7b). The decline in capacity utilization with increasing current density is attributed to polarization effects, which are closely linked to membrane resistance, charge and mass transfer resistance, and electrolyte resistance.⁸²

For fast charge and discharge cycles, the battery should operate at high current density while maintaining reasonable capacity utilization. The charge/discharge curve, with a potential hold at 10 mA cm⁻², is illustrated in Fig. 7c. At a current density of 30 mA cm⁻², the battery demonstrated excellent EE, reaching 60%. Additionally, this flow battery exhibited outstanding capacity retention and CE over 50 cycles at 10 mA cm⁻². The CE remained above 99.56% over the 50 cycles, while the capacity retention was 77.06%, with a per-cycle capacity retention rate of 99.54%, as shown in Fig. 7d. The capacity decay per cycle was 0.459, which may be attributed to active material decomposition or crossover.

The area-specific resistance (ASR) of the flow battery was measured at 1.92 Ω cm², determined by the EIS experiment in potentiostatic mode, as shown in Fig. S14b (ESI).† This ASR is lower than the Selemion® ASV and Selemion® AMV membrane ASRs of 4.54 and 2.8 Ω cm² in 2 M NaCl and 0.5 M MV/FcNCl systems.⁸³ In comparison, the FAA-3-50 membrane exhibited an ASR of 6.25 Ω cm²,⁸⁴ in the BTTMPB/FcNCl system at 2 M NaCl, three times higher than that of QPNBM. The lower ASR of the QPNBM membrane reflects its superior ionic conductivity, as demonstrated in ex situ conductivity investigations.

3. Conclusions

In this study, we successfully developed a cost-effective QPNBM AEM and ionomer with exceptional dimensional stability, durability, and ionic conductivity. The inclusion of the rigid naphthalene along with the right ratio of ionic moieties promoted a nanophase separated morphology, enhancing both the chemical and mechanical properties while significantly improving ionic transport. The unique backbone architecture of the QPNBM membrane enabled outstanding performance in the dry cathode AEMWE. In AEMWE applications, the QPNBM-based electrolyzer delivered a peak current density of 1.82 A cm⁻², which is more than triple that of the FAA-3-50 membrane (497 mA cm⁻²) with identical ionomer and catalyst loadings in the membrane-electrode assembly. Durability testing further highlighted the superior stability of the QPNBM membrane. It demonstrated negligible degradation during ex situ stability tests of up to 1500 hours and maintained in situ operational stability for 470 hours at 500 mA cm⁻², substantially exceeding the performance of FAA-3-50. In redox flow battery testing, the QPNBM membrane exhibited outstanding performance with a high CE (>99.56%) and strong capacity retention (77.06%) over 50 cycles, with minimal capacity decay per cycle. Additionally, the membrane's low ASR of 1.92 Ω cm² further underscored its excellent ionic conductivity. These findings establish the QPNBM membrane as a highly promising ionomer material for AEMs in both energy storage and conversion systems. Future improvements in catalyst dispersion and ionomer selection could further enhance its performance, paving the way for more efficient and cost-effective electrochemical technologies.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

AM acknowledges the Ministry of Education, Government of India, for the doctoral research fellowship. BPT gratefully acknowledges the Department of Science and Technology (DST), Government of India, for funding support under grant numbers DST/TMD/MES/2k18/02 and DST/TMD/EWO/WTI/2K19/EWFH/2019/73. The authors also express their gratitude to the Central Research Facility, IIT Delhi, for providing NMR, FESEM, HRTEM, and SAXS analysis. Anubhav Kumar is acknowledged for the help and discussion on flow battery characterization.

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Footnote

† Electronic supplementary information (ESI) available: The ESI includes details on materials, polymer and membrane synthesis, and characterization of the polymer and anion exchange membrane. Properties such as morphology, pore size, ion exchange capacity, conductivity, and stability of the membrane are provided. Structural and electrochemical analyses, including ATR-FTIR, NMR, AFM, rheology, and impedance studies, are described. Performance assessments in redox flow batteries and AEM water electrolyzers, along with a comparative analysis of AEM performance, are included. See DOI: https://doi.org/10.1039/d5ta02672e

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