Covalent catalyst immobilization in cross-linked ionomers for durable pure water anion exchange membrane electrolysis

Domain: physics
Published: 2025-11-24
Editorial outcome: major_revision

Abstract
Anion exchange membrane water electrolysis technology, which employs alkaline electrolytes, has emerged as a highly promising alternative to the acidic counterparts. However, the development of pure water-fed anion exchange membrane water electrolysis remains in its nascent stage, hindered by suboptimal ionomer performance, along with an unstable catalyst-ionomer interface induced by the Marangoni effect during the fabrication of catalyst layers. In this study, we introduce a strategy to overcome these challenges by employing in-situ covalent anchoring of the catalyst within cross-linked ionomer networks. Through synchrotron X-ray three-dimensional computed tomography characterization, complemented by extensive electrochemical analysis and multiphysics simulations, we demonstrate that the interconnected ionomer network substantially improves mass transport properties. Additionally, the covalently locked interfacial bonding effectively addresses delamination issues. Under rigorous pure water-fed conditions, our crosslink-immobilized catalyst layer demonstrates competitive durability (>1800 hours with a decay rate of 0.03 mV h−1) and performance (2.55 A cm−2 at 1.9 V). This approach presents an alternative paradigm for fabricating mechanically robust catalyst layers with enhanced durability and performance.

Introduction
Water electrolysis powered by renewable sources provides a sustainable route for meeting the growing demand for green hydrogen production1–3. Emerging anion exchange membrane water electrolysis (AEMWE) has garnered significant attention due to its distinctive advantages, including the utilization of cost-effective non-precious metal catalysts, fast reaction kinetics, and excellent compatibility with intermittent renewable energy sources4–6. Over the past decades, significant research efforts have been devoted to the development of advanced catalysts and anion exchange membranes (AEMs), enabling KOH-fed AEMWE to achieve performance competitive with their proton exchange membranes counterparts7–12. However, the utilization of KOH electrolyte presents inherent limitations, including carbonation formation, materials corrosion, and environmental challenges associated with electrolyte disposal, all of which compromises system performance and sustainability13,14. To address these limitations, recent research has shifted focus toward developing pure water-fed AEMWE systems as a promising alternative.

The development of pure water-fed AEMWE systems faces two primary technical challenges: insufficient ionic conductivity and suboptimal catalyst-ionomer interface properties. Recent studies have demonstrated that anion exchange ionomers, when employed as binder materials, play a pivotal role in facilitating multiphase mass transport and reinforcing system stability. Therefore, the development of high-performance ionomer materials through molecular structure engineering presents a promising solution to this bottleneck challenge. For instance, Kim et al. reported polystyrene ionomers with increased ion content could achieve current densities up to 2.7 A cm−2 at 1.8 V. However, their operational stability under pure water-fed conditions remains to be improved. This limitation primarily stems from the system’s heavy reliance on the ionomer-catalyst interface for electrocatalytic reactions in the absence of supporting electrolytes. The excessive dimensional swelling caused by highly conductive ionomers significantly compromises the adhesive strength of catalyst layers (CLs). Under high current density operation, rapid and heterogeneous bubble evolution disrupts the structural integrity of the CL, consequently impairing mass transport properties and system stability. Therefore, developing innovative ionomer molecular designs to overcome the fundamental trade-off between ionic conductivity and swelling behavior has become crucial for advancing the technology.

Furthermore, it is noteworthy that during the CL fabrication process, cracks and interfacial delamination caused by non-uniform coating flow and defect formation due to surface tension gradients during solvent evaporation also serve as a significant factor contributing to the poor durability of the CL. This defect formation originates from the large and nonuniform agglomerates in the catalyst ink. During deposition, particles migrate from high-shear to low-shear regions, exacerbating heterogeneous distribution and aggregation15,16. Subsequently, in the drying phase, these poorly dispersed agglomerates create imbalanced capillary forces between particles, generating heterogeneous porous structures with large agglomerates and phase-separated cracks17,18. This mass transfer phenomenon-driven by surface tension gradient during solvent volatilization-results in uneven coating flow and defect formation, known as the Marangoni effect18,19. These CL defects compromise the ionomer-catalyst interface, leaving it vulnerable to severe mechanical degradation under operational stresses such as rapid bubble removal and continuous water flow scouring. Achieving uniform ink dispersion through aggregate reduction is therefore essential to mitigate Marangoni effect and establish a homogeneous, stable ionomer-catalyst interface. Most notably, these aforementioned challenges are particularly pronounced in AEMWE system employing non-PGM catalysts. This phenomenon can be attributed to the intrinsically low active site density characteristic of non-PGM catalysts, necessitating higher catalyst loading to obtain commercially viable current density. As a result, the CL thickness in non-PGM configurations exhibits nearly one fold increase compared to iridium-based (Ir/C or IrO2) electrodes20, presenting significant challenges for maintaining effective and stable electrochemical reactions. These critical challenges have motivated our research initiative to develop mechanically robust CL via precisely engineered ionomer molecular design, aiming to simultaneously mitigate the Marangoni effect and achieve effective catalyst immobilization.

Herein, we propose an ionomer-assisted stabilization strategy designed to effectively anchor non-PGM metal (NiFe) catalyst particles for AEMWE. We developed a thermally triggered cross-linkable ionomer derived from polyarylpiperidine, which enables uniform catalyst distribution and robust immobilization during in-situ cross-linking process. Simultaneously, the crosslinked structure effectively suppresses swelling behavior, thereby improving the trade-off between conductivity and dimensional stability. The conceptual framework and implementation strategy of this approach are illustrated in Fig. 1a. The well-dispersed catalyst ink forms a continuous catalytic network via localized cross-linking of terminal vinyl groups under ultrasonic thermal activation, significantly mitigating deformation risks during the spray coating process. Subsequent to more extensive cross-linking during the thermal spraying process, the catalyst particles become firmly anchored within the three-dimensional interconnected ionomer network, ensuring structural stability. The synergistic effects of uniform dispersion and timely fixation throughout the ink-to-CL manufacturing effectively mitigate the Marangoni effect. Notably, this study provides the 3D characterization of ionomer distribution surrounding non-noble metal catalysts. This well-established 3D ionomer network architecture not only extends ion conduction pathways but also significantly alleviates catalyst aggregation and interface delamination. The advantages of the cross-link-immobilization strategy in stabilizing NiFe catalysts were systematically evaluated under challenging operational conditions, including elevated temperature, high flow velocity, high current density, and pure water-fed AEMWE system.

Results
The design and synthesis of MEA-adopted materials
The crosslinkable poly(fluorene-co-aryl piperidinium) (CPFTP) ionomer was synthesized through two-step procedure. First, the polyfluorene backbone was prepared via polyhydroxyalkylation. Subsequently, the target ionomer was obtained by functionalizing the polyfluorene backbone through a Menshutkin reaction, which involved the chemical grafting of vinyl benzyl chloride (VBC) with terminal cross-linkable vinyl groups and iodomethane (Fig. 1a). For comparative analysis, a non-crosslinkable poly(fluorene-co-aryl piperidinium) (PFTP) ionomer was synthesized without the VBC group9. The detailed synthesis routes for both ionomers are described in Supplementary Fig. 1. Supplementary Figs. 2 and 3 provide the chemical structure confirmation of the synthesized ionomers. The NiFe catalyst was synthesized by in situ reduction of metal precursors. The structure and morphology were confirmed with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) (Supplementary Figs. 4–6 and Table 1)21.

The catalyst ink was fabricated using the synthesized ionomer and catalyst through a widely adopted ultrasonication probe method. During tip sonication, high-frequency sound waves propagate into the solution, generating numerous tiny bubbles and vortices. These phenomena generate localized high-pressure zones and transient high-temperature regions that exhibit dual functions: Primarily, it destroys the surface tension and viscous force within the catalyst, and effectively disperse catalyst agglomerations22. Simultaneously, the well-transient dispersion process of aggregates facilitates localized ionomer cross-linking through thermal trigger, thereby immobilizing catalyst particles and preventing their re-agglomeration. Through in-situ Fourier-transform infrared spectroscopy (FT-IR) analysis, we systematically investigated the temperature-dependent cross-linking evolution of the ionomers, monitoring the dynamic process across a precisely controlled temperature range from 20°C to 100°C (Fig. 1b–d). The characteristic vibrational peaks associated with C=C bonds exhibited significant intensity reduction upon heating, including the C=C stretching vibration at 1630 cm−1, C=C-H stretching vibration at 3026 cm−1, and C=C-H antisymmetric stretching vibration at 3083 cm−1. Furthermore, the thermal cross-linking process was corroborated by the emergence and subsequent intensity increase of a new absorption band at 1296 cm−1, corresponding to methylene alkane torsional vibration (Fig. 1e). Therefore, the in-situ variable-temperature IR analysis conclusively demonstrates that the CPFTP ionomer exhibits temperature-responsive behavior, enabling efficient cross-linking upon thermal activation. Specifically, during ink preparation, the FTIR spectrum of CPFTP@NiFe ink after tip ultrasonication alone revealed weakened absorption bands at 1640 cm−1 (Supplementary Fig. 7) corresponding to the double bonds, suggesting partial cross-linking occurred during this step. Subsequent deposition and drying eliminated this peak entirely, confirming complete crosslinking.

Impact of ionomer on structural characteristics of catalyst ink
To verify the effect of crosslinked CPFTP ionomer on the prepared inks, we conducted rheological tests. The steady-shear viscosity measurements revealed significantly enhanced shear viscosity in CPFTP-ink, which can be primarily attributed to the occurrence of a three-dimensional network via cross-linking interactions between ionomer-coated catalyst particles (Fig. 2a). This increased viscosity would reduce the Marangoni number, which mitigates the Marangoni flow. Rheological oscillation measurements revealed that the CPFTP@NiFe ink exhibits a higher platform modulus (G’0 = 25.8 Pa), demonstrating the formation of fractal-like agglomerates into a three-dimensional continuous NiFe network. In contrast, the PFTP@NiFe ink demonstrated a lower plateau modulus (G’0 = 6.4 Pa), corresponding to a less interconnected and sparsely distributed NiFe network23. Notably, the yield stress point of PFTP@NiFe (8.4%) is lower than CPFTP@NiFe (16.7%), indicating that CPFTP@NiFe exhibits a greater capability in preserving the integrity of the network structure against external stress-induced damage (Fig. 2b). The distinct variations in rheological parameters indicate that the cross-linking of ionomer promoted the formation of a more connected and non-deformable network. Utilizing the acquired rheological parameters, including both G’0 and particle volume fraction, we determined the stability-related scaling exponent A through theoretical modeling approaches23,24, thereby enabling a comprehensive evaluation of the fractal characteristics and structure stability of the fabricated ink. Through quantitative analysis, the scaling exponents for CPFTP@NiFe and PFTP@NiFe were determined to be 4.76 and 2.39, respectively, as shown in Fig. 2c. Within the framework of colloidal gel classification system, protein gels exhibiting a scaling exponent of 3.4 is classified as transition gels, serving as a critical threshold that distinguished strong-link gels (A > 3.4) and weak-link gels (A < 3.4)25. These results establish that the CPFTP@NiFe ink corresponds to a strong-link gel, while PFTP@NiFe is a weak-link gel15. The robust and continuous NiFe gel network endow the CPFTP@NiFe ink with a high yield stress, effectively preventing particle precipitate during storage and minimizing particle migration induced by variations in shear stress during spray coating. Additionally, the measured surface tension of the CPFTP@NiFe inks (36.7 mN m−1) is higher than PFTP-assembled ink (33.2 mN m−1) (Supplementary Fig. 8), which contributes to improving the coating uniformity and ink stability at high shear forces. Consequently, this dual improvement in storage and operational stability not only mitigates the Marangoni effect but also enables the fabricating of a more uniform CL.

Ionomer performs dual critical functions within CLs, serving as both adhesive binders for catalyst and conductive media that facilitate OH− transport to active sites. The spatial distribution of ionomers around catalyst is therefore of paramount importance to the performance, making the achievement of uniform ionomer dispersion with minimal agglomeration of long-standing challenge in the field. Herein, through the application of synchrotron X-ray nano-CT technology, we have detailed morphological characteristics of ionomer-catalyst encapsulates at the microscale level (Supplementary Figs. 9 and 10). As displayed by the 3D reconstruction in Fig. 2d, e, the crosslinked ionomer (blue) is uniformly dispersed across the catalyst surface (red) in the CPFTP@NiFe sample. In contrast, the PFTP@NiFe sample exhibits a nonuniform spatial distribution and a reduced density of PFTP ionomers (Fig. 2f, g). Given that the same amount of ionomer was used in both catalyst inks, this distinct morphological difference suggests that the PFTP ionomer may undergo agglomeration. Further supporting evidence emerges from nano-scale characterization, where energy-dispersive X-ray (EDX) spectroscopy images (Fig. 2h) and corresponding line-scanning intensity profile (Fig. 2i) demonstrate a homogeneous distribution of iodine element from CPFTP, showing spatial correlation with both Ni and Fe distributions within the catalyst matrix. Therefore, the morphology analysis demonstrates that the in-situ cross-linking strategy significantly promotes the homogeneous distribution of ionomers closer to the catalyst surface. This optimization is a critical factor in enhancing electrode performance, as it enhances reactant accessibility to active sites and boosts catalytic efficiency26.

Impact of ionomer on CL microstructure
The CPFTP ionomer significantly upgraded multiple critical ink characteristics, including dispersion quality, rheological behavior, and structural retention features, effectively suppressing the formation of large aggregates. As supported by the dynamic light scattering measurements in Supplementary Fig. 11, the PFTP assembled ink shows a wide distribution characteristic with average particle clusters of 1202 nm, while the CPFTP-coated system shows a narrowed distribution and reduced particle sizes of 817 nm. During the spray-deposition-dry process, the residual terminal vinyl groups, which remain available for further cross-linking, undergo thermally activated reactions that strengthen catalyst immobilization effect. This synergistic combination of optimized ink dispersion and solvent evaporation-induced crosslinking enables progressive catalyst fixation, transitioning from localized points to volumetric stabilization, thereby effectively mitigating Marangoni effect-induced particle migration. Surface analysis via SEM and EDX spectrometry (Supplementary Figs. 13–15 and Fig. 3a, b) shows defect-free distribution of Ni, Fe, and I across the CL surface, which reveals that the CPFTP-catalyst dispersions form uniform, defect-free coatings on the nickel foam substrate. In contrast, the PFTP-encapsulated CL exhibit substantial defects, including pronounced cracking and bulk ionomer agglomerate (Fig. 3c). This is due to the fact that these larger PFTP aggregates generate greater surface tension gradients during solvent evaporation, intensifying the Marangoni effect. Shrinkage pits evolve into open holes when the coating is thin. As the thickness increases, stress-laminated pits propagate into macroscopic cracks. This observed catalyst/ionomer aggregator in PFTP@NiFe CL exhibits strong consistency with the 3D nano-CT results, further confirming the inhomogeneous distribution feature of PFTP ionomer. Clearly, such undesired catalyst/ionomer aggregators could obstruct mass transfer pores within CL, significantly increasing Knudsen oxygen diffusion resistance27. N2 adsorption measurements demonstrated a markedly lower adsorption capacity for the PFTP@NiFe CL (Fig. 3d), suggesting a reduced network of interconnected micropores within its structure. To evaluate the adhesion characteristics of NiFe catalyst with different ionomers, we conducted mechanical tests including 180° peel adhesion measurements and adhesive tape test (Fig. 3e, Supplementary Fig. 16 and Fig. 3f). The CPFTP-based CL demonstrated adhesion strength of 29.4 gf mm−1 and 85% CL retention, significantly outperforming the PFTP CL (20.1 gf mm−1, 44% CL retention). This enhanced performance originates from the well-interconnected CPFTP binding network and uniform CL distribution. Collectively, our integrated morphological and structural characterization reveals that the crosslinked CPFTP ionomer architecture addresses two fundamental limitations in CL fabrication: (1) in preventing shear-induced particle redistribution during electrode deposition and (2) suppressing surface tension-mediated defect generation during solvent evaporation. This rationally designed framework substantially improves the mechanical integrity of CPFTP-based CL, endowing them with notable durability against multifaceted operational stresses—encompassing hydrodynamic shear, thermal fluctuations, bubble collapse events, and mechanical fatigue—under realistic industrial electrolysis conditions.

Electrochemical performance analysis of a half-cell
In a standardized three-electrode system containing 1 M NaOH as the electrolyte, alkaline ionomers deliver elevate electrocatalytic activity compared to commercial Nafion without OH− conduction capability (Supplementary Fig. 17). This comparative analysis underscores the pivotal role of OH− transport in optimizing the OER process28. Notably, electrochemical characterization reveals that CPFTP@NiFe CL exhibits marginally high activity relative to its PFTP@NiFe CL (Fig. 3g), highlighting the beneficial effects of the crosslinked ionomer network. Supplementary Figs. 18 and 19 demonstrate that CPFTP@NiFe achieved the highest electric double-layer capacitance (EDLC) in both NaOH and pure water-fed conditions. Notably, it also retained a higher EDLC in the pure water-fed condition compared to that measured in the 1.0 M KOH. Specifically, CPFTP@NiFe and PFTP@NiFe retained 86.0% and 76.3% of their EDLC, respectively. This result reveals that a more uniform distribution of the CPFTP@NiFe CL exposes more active sites of the catalyst. Meanwhile, CPFTP’s interconnected ionomer network enables efficient ionic conduction without supporting electrolytes. This is further supported by the electrochemical impedance spectroscopy (EIS) results. As shown in Supplementary Fig. 20, the CPFTP@NiFe CL exhibited a lower ohmic resistance (Rohm) (0.45 Ω cm2) and charge transfer resistance (Rct) (0.47 Ω cm2) than PFTP@NiFe CL (Rohm = 0.47 Ω cm2, Rct = 0.63 Ω cm2) in 1.0 M NaOH system. And this resistance value gap is widened further under pure water system. This improved OH− accessibility is conducive to reducing the conduction resistance of OH− and providing more sufficient OH− reactants for enhancing OER performance29.

We subsequently assessed the ex-situ stability of the CL through chronopotentiometry (CP) testing at a current density of 10 mA cm−2. Conventional long-term durability studies employ stirring to enhance ion transport rates and bubble removal efficiency. We intentionally avoided stirring in this work, this strategic approach enables a more precise evaluation of intrinsic material properties by isolating two critical factors: (1) hydroxide ion transport to catalytic active sites and (2) oxygen diffusion characteristics at the mesoscopic scale. Over a testing period of at least 90 h, the CPFTP@NiFe CL maintains stability, exhibiting only an approximate 1% increase in overpotential, while the PFTP@NiFe CL displays significant fluctuations and an overpotential rise exceeding 12% (Fig. 3i). These results highlight the efficient OH− transport facilitated by CPFTP to the catalyst surface and its enhanced oxygen diffusion at the mesoscopic scale30. Notably, unlike PFTP@NiFe CL, which exhibits initial performance degradation, CPFTP@NiFe CL maintains stable operation without any observed overpotential increases during the critical first 10 h. This enhanced stability can be attributed to the significantly reduced dimensional changes in the crosslinked ionomer network, as evidenced by the lower swelling ratio of CPFTP (8.4%) compared to PFTP (15%). The cross-linked architecture effectively restricts ionomer-catalyst separation, thereby maintaining structure integrity during prolonged operation6,31. Overall, the CPFTP not only enhances catalytic activity but also significantly improves the ex-situ stability of CL.

Performance and durability evaluation of AEMWE
Although the cross-linking immobilized CL exhibits high OER activity and durability in half-cell configuration, it is important to note that a significant performance gap exists between half-cells and membrane electrode assembly (MEA), primarily due to the differences in anion and O2 transport mechanisms32,33. To bridge this gap and evaluate practical performance, we conducted AEMWE measurements under anhydrous cathode conditions, feeding a 1 M KOH solution to the anode at both 25 °C and 80 °C (Fig. 4a). As shown in Fig. 4b, the current density of both CPFTP and PFTP-based AEMWE increases with rising temperature, attributed to enhanced OH− transport efficiency and accelerated OER kinetics. Remarkably, the CPFTP-based cell achieves a current density of 5.6 A cm−2 at 1.9 V and 80 °C, significantly outperforming the PFTP-based AEMWE (3.8 A cm−2 at 1.9 V). This performance ranks in the medium-to-upper tier among reported non-PGM-AEMWEs (Supplementary Table 3) and is three times higher than that of the AEMWE with IrO2-anode (Supplementary Fig. S23).

To elucidate the underlying mechanisms for performance enhancement, high-frequency resistance (HFR) measurements provided initial insights. The HFR of the CPFTP-based MEA is approximately 17 mΩ at 80 °C, significantly lower than that of the PFTP-based MEA (38 mΩ) (Supplementary Fig. 24). This difference in HFR is primarily attributed to the anode CL, as both cells utilized the same AEM and cathode CL. Through EIS (Supplementary Figs. 25–27), we decoupled the total impedance into three fundamental components: ohmic resistance (Rohm), charge transfer resistance (Rct), and mass transport (Rmt)34. The Rct values of the CPFTP- and PFTP-based MEAs are similar (∼27 mΩ), but significant differences emerge in both Rohm (27.6 vs 30.2 mΩ) and Rmt (7.5 vs 12.2 mΩ) (Fig. 4c). The reduction Rohm and Rmt of CPFTP-CL can be attributed to two synergistic factors: (1) the well-interconnected ionomer network that enables efficient transport of OH− to the catalyst surface, and (2) its abundant micropore structure, which promotes the rapid removal of gas from the catalyst surface.

To demonstrate the effectiveness of the ionomer cross-linking immobilization strategy in stabilizing in situ AEMWEs, we evaluated the short-term durability of the AEMWEs under harsh operating conditions, including high electrolyte flow rates (60–260 mL min−1, voltage recording for 11 h at each flow rate), elevated current density (5.0 A cm−2), and temperature (80 °C). As the flow velocity increased, both the CPFTP- and PFTP-based AEMWEs exhibited a performance improvement trend (Supplementary Fig. 28a, b and Fig. 4d). The accelerated performance improvement in the CPFTP-based AEMWE can be attributed to its large exposed ECSA and three-dimensional ionomer network, which enables efficient transport of reactants (OH−) to the electrode surface, while its high porosity facilitates to rapid removal of reaction gas products. Interestingly, the stability retention profile exhibited non-linear behavior with varying flow velocities, deviating from expected norms (Supplementary Fig. 28c, d and Fig. 4e). Specifically, we observed that both CPFTP- and PETP-based AEMWEs exhibit significant performance attenuation at lower flow velocities. As flow velocity increases, the stability retention rate of the PFTP-based AEMWE follows a rise-and-decline pattern. In contrast, the CPFTP-based AEMWE reaches a maximum value and then stabilizes in a platform region. To investigate the mechanism behind this stability attenuation, we employed COMSOL Multiphysics simulations to analyze the spatial variations in flow velocity, reaction product distribution, and current density distribution within the flow field. Simulation results demonstrate a correlation between flow velocity and gas accumulation, with higher gas volume fractions observed in lower flow rates and subsequent reduction at elevated velocity (Fig. 4f). This trend may be attributed to the enhanced capability of the liquid to carry bubbles away from the electrode surface. The CPFTP-based AEMWE exhibits a relatively lower gas volume fraction compared to the PFTP-based AEMWE. Analysis of gas distribution contours on the electrode (Fig. 4g) and within the flow channels (Fig. 4h) further revealed that in CPFTP-based AEMWE, gas accumulation appears more concentrated in the flow channels rather than within the electrode, differing from the PFTP case. These observations indicate that a serious bubble covering over the PFTP electrode could reduce the active catalytic area and increase the electrode voltage. In contrast, the high porosity of the CPFTP-assembled electrode may help mitigate the bubble-related blocking effects. Additionally, higher flow velocities could lead to increased shear forces, particularly near the inlet and outlet of the CL, potentially raising concerns about CL damage and materials loss (Fig. 4i). When flow velocities surpass a critical threshold, flow-induced shear might cause catalyst detachment in PFTP-based CL due to insufficient mechanical stability, resulting in electrochemical performance decay. The crosslinked structure of the CPFTP-assembled CL, however, appears to provide improved structural integrity, which may provide reliable operation under harsh hydrodynamic conditions by mitigating shear-induced catalyst loss and helping maintain ionomer network stability.

This robust design ultimately manifests as the characteristic performance plateau observed at elevated flow rates. This hypothesis is supported by the presence of significantly higher catalyst concentrations in the anode circulating electrolyte of PFTP-based AEMWE. Even under relatively mild operating conditions (25 °C and 300 mA cm−2), the cell voltage of PFTP-based MEA shows a degradation rate of 0.454 mV h−1 over 110 h, with HFR rising from 25 to 32 mΩ (Fig. 5a). In contrast, CPFTP-based AEMWE demonstrates notable operational stability under identical conditions (Fig. 5b). Post-test dissection of the MEA combined with SEM analysis offers detailed insights into the micromorphology characteristics. Significant CL loss is observed on the PFTP@NiFe substrate, while the CPFTP@NiFe maintains well-preserved structural integrity at the micromorphology (Fig. 5c). These results further confirm that catalyst loss is another primary contribution to the performance degradation of the PTFP-assembled AEMWE, primarily caused by electrolyte-induced erosion and interface damage resulting from bubble accumulation and rupture35–38. The significantly enhanced durability of the CPFTP@NiFe primarily stems from two fundamental mechanisms: (1) the ionomer-anchored catalyst immobilization strategy effectively mitigates particle agglomeration and loss during MEA fabrication, resulting in highly uniform and stable CL; (2) the robust 3D ionomer network significantly enhances mechanical stability, effectively resisting deformation and displacement induced by external stresses. More significantly, our analysis reveals an optimal balance between bubble removal efficiency and CL stability across varying fluid velocities. The mechanical robustness of CPFTP@NiFe CL enables stable operation over a wider range of fluid velocities while maintaining structural integrity.

Pure-water electrolysis represents a key advantage of AEMWE technology, enabling significant reductions in system operation and maintenance costs. However, in a pure-water system, the ionomer serves as the sole ionic conductor, and the electrocatalytic reaction highly depends on the ionomer-catalyst interface. As a result, achieving optimal performance and durability in a pure-water system is considerably more challenging compared to a KOH-fed system39. We performed pure water electrolysis experiments using membrane electrodes fabricated with two distinct ionomer materials. As displayed in Fig. 5d, the electrochemical measurements at 1.9 V and 80 °C reveal a significant enhancement in current density from 1.53 A cm−2 for conventional PFTP to 2.55 A cm−2 for CPFTP@NiFe, highlighting the improvement achieved through ionomer crosslinking in facilitating OH− transport and O2 removal. Moreover, when compared with state-of-the-art pure water-fed AEMWEs documented in the literature, the CPFTP@NiFe CL demonstrates competitive performance metrics (Supplementary Table 4).

To evaluate the stability of PFTP- and CPFTP-based AEMWE with pure water feed, initially short-term stability tests were conducted under a high current density of 2.0 A cm−2 and 80 °C. As shown in Fig. 5e, the CPFTP-based AEMWE remained stable, while the PFTP-based AEMWE exhibited a relatively large voltage decay rate of 0.71 mV h−1. To analyze the degradation causes, we collected the EIS data before and after the durability test of PTFP-based AEMWE (Fig. 5f). The results showed increases in the cell’s Rohm, Rct, and Rmt increase by 4.3%, 11.2% and 6.0%, respectively. XPS analysis of the CL was performed to understand the CL-degradation mechanism (Supplementary Fig. 33). After 120 h of operation at 2.0 A cm−2, a new binding energy peak emerged at 292 eV in the C 1s spectrum of PFTP-CL, consistent with the formation of carbonyl and/or ester groups, while the C 1s spectrum of CPFTP-CL remained unchanged. This indicates that the degradation of PFTP ionomers led to reduced ionic transport to the catalyst and decreased electrochemically active area due to catalyst particle detachment, consequently increasing both Rohm and Rct. Conversely, the crosslinking of CPFTP enhances ionomer stability via steric hindrance effect40. Additionally, the observed increase in Rct and Rmt is primarily attributed to the structural destruction of CL. Beyond visible catalyst detachment, we further employed TEM combined with ultraomics technology to study the morphological changes of CLs before (beginning-of-life, BOL) and after (end-of-test, EOT) the in-situ AEMWE stability test (Supplementary Fig. 34). TEM images revealed that CPFTP@NiFe maintained a homogeneous catalyst distribution and intact microstructure from BOL to EOT. In comparison, PFTP@NiFe exhibited a relatively poor micromorphology, which further deteriorated after the durability test. Therefore, under electrolyte-free water operation, CPFTP not only enhances the physical stability of the CL through its cross-linked network structure, but also improves the oxidation resistance of the ionomer.

Most importantly, the pure-water fed CPFTP@NiFe AEMWE maintaining consistent performance for 1813 h under demanding conditions of high current density (1.0 A cm−2) and elevated temperature (80 °C), with a minor voltage decay rate of 0.03 mV h−1 (Fig. 5g). This notable long-term durability not only underscores the mechanical robustness of the anode CL but also highlights its potential for practical industrial applications. Through comparison with state-of-the-art pure-water-fed AEMWEs across multiple performance metrics—including current density, durability, and voltage degradation rate6,13,14,41–45 (Supplementary Table 4), our CPFTP-based AEMWE possesses comparable performance and long-term stability. These achievements underscore the significant potential of the developed CL for practical application in AEMWEs.

Methods
Materials
P-Terphenyl (TP, 98%), biphenyl (BP, 98%), 9,9′-dimethylfluorene (DF, 98%), 1-methyl-4-piperidone (MP, 98%), trifluoromethanesulfonic (TFSA, 99%), trifluoroacetic acid (TFA, 99%), iodomethane, and chloromethyl styrene (VBC, 98%) were purchased from energy-chemical Co., Ltd (China). Dichloromethane (DCM), potassium carbonate (K2CO3), potassium hydroxide (KOH), acetone, and dimethyl sulfoxide (DMSO) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Preparation of alkaline-resistant polymers
The poly(fluorene-co-aryl piperidinium) polymers were synthesized following recently reported literature9. TP (8.28 g), DF (0.777 g), MP (4.98 g), and DCM (18 mL) were added to a 200 mL three-neck flask with a mechanical stirrer. After that, TFA (4.8 mL) and TFSA (36 mL) were slowly added into the above solution under −3.2 °C, and the reaction proceeded at −3.2 °C for ~12 h. Finally, the viscous solution was poured into 1000 mL 1 mol L−1 NaOH solution and continued stirring for 48 h. Then, it was washed with DI water several times until neutral pH. The polymer was dried in an oven at 80 °C for 24 h to obtain poly(fluorene-co-arylmethylpiperidine) (PFAM-x). Then PFAM (3.0 g) and iodomethane (3 mL) were dissolved in DMSO (30 mL), and the reaction proceeded at 25 °C under stirring for 24 h in a dark environment. The resulting solution was precipitated in 300 mL of acetone and washed with DI water thrice. Finally, the polymer was dried to obtain a light-yellow poly(fluorene-co-aryl piperidinium) polymer (PFTP). The synthesis route of poly(fluorene-co-aryl piperidinium) polymer (CPFTP) is similar to PFTP. The PFAM (1.0 g), K2CO3 (0.646 g), and VBC (0.238 g) were dissolved in DMSO (12 mL). The reaction proceeded at 25 °C under stirring for 24 h. Then, add 2 mL iodomethane and stir for another 24 h. The reaction solution was treated in the same way as PFTP. Similarly, PFBP follows the same synthesis method, only replacing the TP monomer with the BP monomer.

Synthesis of NiFe catalyst
The synthesis method was modified based on a previously published work6. In a typical process, 30.3 g NiCl2·6H2O and 8.11 g FeCl3 were dissolved in 50 mL and 25 mL DI water, respectively. Metal precursor solution (1 mL) was prepared by mixing 0.665 mL NiC12 solution and 0.335 mL FeCl3 solution. The 2 M NaBH4 solution was freshly prepared by dissolving 0.378 g NaBH4 powder in 5 mL deionized water. Subsequently, 1 mL metal precursor solution was quickly injected into the prepared NaHB4 solution by stirring it at 25 °C. A large amount of black NiFe catalyst is rapidly generated, and a large number of bubbles are released. The stirring was maintained for about 1 min. After freeze-drying, the NiFe catalyst was obtained46,47.

Characterizations
The chemical structures of AEIs were confirmed by 1H NMR (Bruker 510, 400 MHz). In-situ variable temperature FT-IR spectra were collected using a NICOLET iS10 (Thermo Fisher Scientific). The CPFTP polymer was fixed between two Cu plates and heated from 20 to 100 °C in seventeen stages with an increment of 5 °C (5 °C temperature rise 5 min followed by a thermal hold at each temperature). FT-IR absorbance were recorded with a measurement every 5 °C. SEM and EDS mapping analysis were carried out on a Genimi SEM 500 (Carl Zeiss Microscopy Ltd., Germany) at an accelerating voltage of 3 kV, working distance (WD) of 4.6 mm with an in-lens detector. TEM was performed on a JEM-2100F (JEOL, Japan) at an accelerating voltage of 200 kV. XPS was measured using a Kratos Axis supra+ XPS system from a monochromated aluminum anode X-ray source with Kα radiation (1486.6 eV). The test power was 150 W, and the X-ray beam spot was 300 × 700 μm. The energy analyzer has a transmittance of 160 eV for the full spectrum and 40 eV for the narrow spectrum. The pore size distribution of CL was measured by using a surface aperture analyzer (BK200C, JWGB SCI. & TECH.). Dynamic shear rheology at an oscillation frequency of 1 Hz, strain from 0.01% to 1000%, storage modulus and loss modulus were recorded with 10 points per decade. To ensure reproducible results, a preshear process was carried out to remove the shear history. The plateau storage modulus (G′0) was extrapolated from the data corresponding to 0.01–0.03% data. The critical strain (γc) was determined by the intersection of the linear region and the non-linear region. The 3D Nano-CT was carried out at the hard X-ray imaging beamline 4W1A. During data collection, the sample was enclosed within a vacuum chamber. A series of two-dimensional (2D) tomogram images was acquired, each with an exposure time of 15 s. The sample was rotated over an angular span of 140°, from −70° to +70°, with data captured at every 0.5° interval. The resultant projection dataset was then processed to reconstruct into 3D tomography images using Avizo Fire VSG software (Visualization Sciences Group, Bordeaux, France)48. The 3M double-sided tape was attached to the electrode samples with a dimension of 2.0 × 4.0 cm using a rubber roller (diameter: 40 mm, width: 24 mm) to ensure consistent pressure through the experiments. The adhesion strength was recorded upon peeling at a 180° angle.

Electrochemical characterizations
A standard three-electrode configuration workstation (CHI 760E) records the electrochemical measurements for the samples without iR compensation. The catalyst inks were loaded on carbon fiber papers serving as the working electrode, while an Hg/HgO electrode and a platinum plate electrode were used as the reference and the counter electrode, respectively. The potential was referred to RHE by calibrating the reference electrode with Pt foil (Aldrich) as both the working and the counter electrodes in a sealed standard three-electrode. The electrolyte was saturated with high-purity H2 gas for 30 min prior to measurement. Cyclic voltammetry (CV) was performed at a scan rate of 1 mV s−1, and the zero current potential during the cathodic scan was defined as 0 V vs. RHE. The calibrated potential was calculated using the equation: ERHE = EHg/HgO + 0.9265 V. All potentials in this work are calculated relative to the RHE potential. For the preparation of the catalyst ink, typically, 2 mg of catalyst was dispersed into a mixed solution with 0.2 mL deionized water, 0.8 mL isopropanol, and 4.5 μL 5 wt.% ionomer solution, followed by 30-min ultrasonication to make a homogeneous ink. After that, to prepare the working electrode, 102 μL of catalytic ink is loaded onto one end of a piece of carbon paper (1 × 1.2 cm) to get a catalyst loading of 0.204 mg cm–2, with a loading area of 1 × 1 cm, and then dried naturally at 25 °C. An activation process of chronoamperometric (a constant current density of 10 mA cm−2) measurement was conducted before the OER performance tests. LSV curves were measured in fresh alkaline (1.0 M NaOH, NaOH dissolved in deionized water) solutions by a forward scan from 1.2 to 1.8 V vs. RHE at a scan rate of 5 mV s−1. The overpotential (η) during OER was estimated based on the formula as follows: η = ERHE − 1.23 V. Tafel plots were analyzed to assess the reaction kinetics by plotting η vs. the logarithm to base 10 of the current density. The Tafel slope was extracted from the Tafel equation, η = b log j + a, where b is the Tafel slope and j denotes the current density. The evaluation of the electrochemically active surface area (ECSA) for the samples is based on the double-layer capacitance (Cdl) results measured in the potential range of 0.1–0.3 V. By plotting the current density against the scan rate, the value of the fitting slope is Cdl. The ECSA was calculated using the formula: ECSA = Cdl/Cs. Cs denotes the specific electrochemical double-layer capacitance of the smooth surface substrate (0.04 mF cm−2). EIS was carried out by applying an amplitude of 10 mV over the frequency window from 0.01 to 100 kHz at 1.5 V vs. RHE. The stability tests of the catalyst were performed by chronopotentiometry at j = 10 mA cm−2.

Multiphysics simulations
The simulation was performed using Comsol Multiphysics software49–51. Several coupled physics fields, including water electrolyzer physics, laminar flow, Darcy’s law, and phase transfer of free and porous media flow in the software, are used for joint modeling. The water electrolyzer was used to study the reaction kinetics of water electrolysis. The laminar flow was used to solve the fluid flow in the flow channel. Darcy’s law was used to solve the electrolyte permeability characteristics of the diffusion electrode (reaction electrode). And the phase transfer in the flow of free and porous media was used to solve the diffusion movement of gases in the electrolyte channel and porous reaction electrode.

Fabrication of MEA
The anode and cathode were manufactured using the catalyst-coated substrate method. For the preparation of the anode, NiFe or PtRu/C catalysts, solvent (IPA/DI = 4/1), and CPFTP/ PFTP or PFBP ionomer solution (5 wt% in DMSO) were initially sonicated in an ice-water bath for 1 h to obtain a well-dispersed catalyst ink, the ratio of NiFe to ionomers is 9:1 and PtRu/C to ionomers is 3:1, respectively. The catalyst ink was sprayed on 10 × 10 cm NiFe foam or carbon paper (Toray-H-060) to prepare anode GDE with a NiFe loading of 4.0 mg cm−2 and cathode GDE with a PtRu/C loading of 0.5 mg cm−2. All the GDEs in the spraying process were heated on the 60 °C hot plate and adsorbed on the vacuum adsorption table. After spraying, the anode electrode containing CPFTP ionomer was heated on the 60 °C hot plate for 2 h to convert the CPFTP@NiFe anode CL into cross-linked forms thoroughly.

AEMWE performance evaluation
The AEM (PFTP, 37 ± 2 μm, 3.0 × 3.0 cm) and GDEs were immersed in 1.0 M KOH solution for 24 h before testing. AEM, GDEs, gasket, bipolar plate, and end plate were assembled into a single cell with an effective area of 5 cm2. The polarization curves were recorded at different conditions (25 °C, 80 °C, 1 M KOH, pH = 13.8 ± 0.1 or pure water) by specified voltage steps (EU harmonized polarisation curve test method B) from 1.3 to 2.0 V with an increment of 0.05 V using battery testing system (LANHE, Wuhan, China). The HFR of the cells was recorded via a potential control module by applying an amplitude of 10 mV at 1 kHz frequency. EIS was taken using an electrochemical workstation (Zahner E, Zahner electrochemical company, Germany). The HFR of the cells under different currents was recorded at 1 kHz by stepping the current from 0.05 A to 2 A with an increment of 0.05 A. Each voltage was held for one minute, using Impedance vs. Parameter Measurement. The EIS measurements were carried out at different current densities and temperatures. The frequency range was 100 kHz – 0.1 Hz. The amplitude current was set to 8% of the test current. The cell durability at a constant current density of 1.0 A cm−2 was recorded at 80 °C and pure-water fed.

Data availability
The authors declare that the data supporting the findings of this study are available within the paper, Supplementary Information and Source Data files. Further data beyond the immediate results presented here are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors propose an “ionomer-assisted locking” strategy to construct high-performance, durable catalyst layers (CLs) for anion exchange membrane water electrolyzers (AEMWEs). Specifically, they design an in-situ crosslinking reaction, thermally triggered during catalyst ink preparation and spraying processes, to form a robust three-dimensional ionomer network with uniform connectivity. This approach effectively mitigates the Marangoni effect often encountered during electrode fabrication, while simultaneously reinforcing the interface between catalyst particles and the ionomer to enhance both mechanical stability and mass transport properties.\n\nThe reviewer acknowledges the novelty of this concept and the notable improvements in durability it achieves. Furthermore, the manuscript demonstrates a thorough analytical approach, covering material design, catalyst ink characterization, electrochemical analyses, and simulation studies, which the reviewer finds particularly commendable. However, there are several key issues that warrant additional attention (major revision). Should these concerns be adequately addressed, the reviewer believes the manuscript would be well suited for publication in Nature Communications.\n\nReviewer Comments:\n1. In the Introduction, the authors emphasize the importance of the catalyst–ionomer interface and explain that “ionomer molecular design” capable of resolving the fundamental trade-off between ionic conductivity and swelling behavior is crucial. They argue that in-situ crosslinkable ionomer design can (1) suppress excessive expansion (swelling) of the ionomer binder and (2) withstand the shear force that occurs during the transition from catalyst ink to the catalyst layer (CL). However, this reviewer finds the connection between the catalyst–ionomer interface and the Marangoni effect to be unclear. In the Introduction, it would be helpful to provide a clear explanation of what the Marangoni effect is and why it is critically important for maintaining the stability of the catalyst–ionomer interface.\n2. It is somewhat ambiguous at which specific stage(s) the ionomer crosslinking reaction occurs, making it difficult to fully grasp the authors’ strategy. According to the manuscript, CPFTP can undergo thermal crosslinking when heated, and the authors describe that crosslinking may take place during three steps: (1) dispersion through tip sonication, (2) electrode fabrication by spraying, and (3) the drying process after spraying. Does the crosslinking fully conclude by the final drying stage? And to what extent does crosslinking proceed during the tip sonication step? If quantified, this information would greatly aid in understanding the work.\n3. Figure 1 uses FT-IR analyses to show how the crosslinking reaction in CPFTP progresses with increasing temperature. This reviewer understands that CPFTP undergoes faster crosslinking at higher temperatures. The authors claim that, because tip sonication can momentarily create high-temperature regions, CPFTP may crosslink in the catalyst ink during the sonication process. However, this is not direct proof that tip sonication indeed induces crosslinking in CPFTP. Although the rheological changes in the catalyst inks shown in Figure 2 could reflect CPFTP crosslinking, they could also simply indicate inherent differences in physical properties between PFTP and CPFTP, independent of whether crosslinking took place. Might there be another experiment that more directly demonstrates whether and how CPFTP undergoes crosslinking during tip sonication?\n4. In Figure 3e, is it possible that the data for PFTP and CPFTP were reversed?\n5. The data and description for Figure 4c appear inconsistent. The authors mention Rohm, Rct, Rmt, yet the graph includes Rs, and although the manuscript states that Rmt is 7.5 mΩ, the figure shows values closer to 30 mΩ. This discrepancy significantly complicates the reader’s understanding of the work.\n6. It would be valuable to disclose more details about the parameters input into the COMSOL simulations. When obtaining separate simulation results for PFTP and CPFTP, which parameter values were changed? Without this information, it is difficult to interpret only the results described in the text. While it is not necessary to present the entire COMSOL methodology, at least the key information should be included so that readers can follow the logic. Specifically, which parameters were varied between PFTP and CPFTP, and on what basis were those values chosen? Some explanation as to why those chosen values are appropriate would also be helpful.\n7. It would be good to incorporate Supplementary Figures 23 and 24 into Figure 5. Although the data in Figure 5 are certainly impressive, this reviewer believes it is crucial to highlight how CPFTP exhibits much better durability than PFTP. Beyond Supplementary Figures 23 and 24, the reviewer suggests incorporating additional data in Figure 5 to further demonstrate CPFTP’s superior durability.", "reviewer_2": "This manuscript focuses on enhancing the ionomer–catalyst interface through an in-situ crosslinking strategy aimed at improving interfacial adhesion. The authors present a simple method that achieves high performance and durability in AEMWE systems, particularly under pure water operation. However, the following points should be addressed before the manuscript can be considered for publication:\n\n1. As the authors state, \"During the deposition process, particle migration and aggregation are significantly exacerbated by shear force.\" To convincingly demonstrate the suppression of aggregation and crack formation through crosslinking, it would be more appropriate to coat the catalyst slurry onto a smooth, non-porous substrate (e.g., Si wafer or polymer film) using bar-coating under controlled shear conditions, followed by thermal crosslinking. This approach would provide a clearer assessment of effect of the ionomer for crosslinking. Additionally, the coating should maintain a catalyst loading greater than 1 mg/cm², similar to the high loading (4 mg/cm²) used in this study. Current observations of cracks on the porous metal foam substrate offer limited insight into morphological suppression attributed specifically to crosslinking.\n2. The claim \"This performance surpassed that of other reported AEMWEs without PGM anodes\" requires revision to include more recent literature. Specifically, the following two studies report higher performance for PGM-free anodes:\nhttps://doi.org/10.1039/D4EE01003E\nhttps://doi.org/10.1021/acsenergylett.4c00207\nMoreover, beyond these two references, the authors are encouraged to incorporate additional recent state-of-the-art AEMWE studies to provide a comprehensive and updated comparative context.\n3. Under pure water operation, a local pH decrease can potentially induce structural changes in the catalyst or degradation of the ionomer. The authors attribute improved performance and durability primarily to enhanced ionomer adhesion; however, it remains uncertain whether this alone sufficiently explains the observed performance and durability improvements. A comparative analysis with previously reported pure-water AEMWE studies should be included to clarify the potential influence of local pH-driven degradation phenomena, such as ionomer oxidation or catalyst structural changes, on the observed results.\n4. The manuscript attributes improved adhesion and mass transport to the crosslinked ionomer strategy. However, performance improvements are likely closely related to the triple-phase boundary structure. Therefore, additional electrochemical analyses, such as measurements of electric double-layer capacitance and ohmic resistance under both 1 M electrolyte and pure water conditions (using CV or impedance spectroscopy), are needed to quantify the extent of interfacial improvements and provide clear evidence supporting the claimed enhancements.", "reviewer_3": "This study presents a novel in-situ covalent anchoring strategy for stabilizing NiFe catalysts in AEMWE, addressing a critical technological bottleneck with broad potential implications for electrochemical devices. The reported durability and performance of the catalyst layer are among the best in literature. The combination of X-ray tomography, rheological measurements, electrochemical analysis and COMSOL simulations provides compelling evidence for the structure and function of the 3D ionomer network. The manuscript is well-written and suitable for Nature Communications after addressing the following key points.\n\n(1) The discussion on how cross-linking suppresses swelling and enhances ionic conduction is insightful, but the physical basis for Marangoni effect mitigation remains unclear. Could the authors quantify the interfacial tension changes? And a theoretical model would help correlate cross-linking density with flow instability suppression.\n(2) The homogeneous ionomer distribution is attributed to Marangoni effect mitigation, but how can the authors be sure that alternative mechanisms play no role in the system? Could the authors provide direct evidence connecting cross-linking to flow stabilization?\n(3) What is the kinetic mechanism of the temperature-dependent cross-linking evolution of the ionomers?\n(4) The claim of “uniform distribution” in Fig. 2 d-g requires more rigorous quantifications. Could the authors clarify why large blue/red domains in Fig. 2(e) do not contradict the uniformity claims?\n\nSome minor points are listed as follows:\n(1) Figure 2 (d-g): Please clarify the distinction between panels (d)/(e) and (f)/(g). All these panels look like 3D renderings instead of 3D slices.\n(2) Typos: Page 13, HER → HFR, regio → region."}, "author_response": "Response to reviewers’ comments\nWe are grateful to all reviewers for their time and constructive comments. Below is the point-to-point response.\nReviewers’ comments (blue); Authors’ response (black); Revised text in manuscript (yellow highlight).\n\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this manuscript, the authors propose an “ionomer-assisted locking” strategy to construct high-performance, durable catalyst layers (CLs) for anion exchange membrane water electrolyzers (AEMWEs). Specifically, they design an in-situ crosslinking reaction, thermally triggered during catalyst ink preparation and spraying processes, to form a robust three-dimensional ionomer network with uniform connectivity. This approach effectively mitigates the Marangoni effect often encountered during electrode fabrication, while simultaneously reinforcing the interface between catalyst particles and the ionomer to enhance both mechanical stability and mass transport properties.\nThe reviewer acknowledges the novelty of this concept and the notable improvements in durability it achieves. Furthermore, the manuscript demonstrates a thorough analytical approach, covering material design, catalyst ink characterization, electrochemical analyses, and simulation studies, which the reviewer finds particularly commendable. However, there are several key issues that warrant additional attention (major revision). Should these concerns be adequately addressed, the reviewer believes the manuscript would be well suited for publication in Nature Communications.\nResponse: We thank the reviewer for positive feedback and insightful comments to further improve the quality of our manuscript. We have addressed these comments point by point in the following paragraphs.\n1. In the Introduction, the authors emphasize the importance of the catalyst–ionomer interface and explain that “ionomer molecular design” capable of resolving the fundamental trade-off between ionic conductivity and swelling behavior is crucial. They argue that in-situ crosslinkable ionomer design can (1) suppress excessive expansion (swelling) of the ionomer binder and (2) withstand the shear force that occurs during the transition from catalyst ink to the catalyst layer (CL). However, this reviewer finds the connection between the catalyst–ionomer interface and the Marangoni effect to be unclear. In the Introduction, it would be helpful to provide a clear explanation of what the Marangoni effect is and why it is critically important for maintaining the stability of the catalyst–ionomer interface.\nResponse: Thank you for this professional suggestion. We have expanded the explanation of the Marangoni effect and its significance in the catalyst-ionomer interface stability within the introduction (Please see Lines 70-82).\nLines 70-82: “Furthermore, it is noteworthy that during the CL fabrication process, cracks and interfacial delamination caused by non-uniform coating flow and defect formation due to surface tension gradients during solvent evaporation also serve as a significant factor contributing to the poor durability of the CL. This defect formation originates from the large and nonuniform agglomerates in the catalyst ink. During deposition, particles migrate from high-shear to low-shear regions, exacerbating heterogeneous distribution and aggregation15, 16. Subsequently, in the drying phase, these poorly dispersed agglomerates create imbalanced capillary forces between particles, generating heterogeneous porous structures with large agglomerates and phase-separated cracks17, 18. This mass transfer phenomenon-driven by surface tension gradient during solvent volatilization- results in uneven coating flow and defect formation, known as the Marangoni effect18, 19. These CL defects compromise the ionomer-catalyst interface, leaving it vulnerable to severe mechanical degradation under operational stresses such as rapid bubble removal and continuous water flow scouring. Achieving uniform ink dispersion through aggregate reduction is therefore essential to mitigate Marangoni effect and establish a homogeneous, stable ionomer-catalyst interface.”\n2. It is somewhat ambiguous at which specific stage(s) the ionomer crosslinking reaction occurs, making it difficult to fully grasp the authors’ strategy. According to the manuscript, CPFTP can undergo thermal crosslinking when heated, and the authors describe that crosslinking may take place during three steps: (1) dispersion through tip sonication, (2) electrode fabrication by spraying, and (3) the drying process after spraying. Does the crosslinking fully conclude by the final drying stage? And to what extent does crosslinking proceed during the tip sonication step? If quantified, this information would greatly aid in understanding the work.\nResponse: We sincerely appreciate the reviewer's insightful suggestion regarding the specific stages of ionomer crosslinking. To verify the thermal crosslinking of CPFTP progress, we conducted FTIR characterization of CPFTP at key fabrication stages. As shown in newly Supplementary Fig. 6, after tip sonication alone, the FTIR spectrum showed weakened absorption at 1640 cm-1 (characteristic of C=C bonds), indicating partial crosslinking occurred during ultrasonication. Following full spray deposition and drying, this C=C absorption band disappeared completely, confirming full crosslinking by the final drying stage. We acknowledge the reviewer's point on quantification. While FTIR clearly shows the progression (weakening then disappearance of C=C peak), precise quantification of the extent of crosslinking during sonication proved difficult.\nSupplementary Figure 6. FTIR spectra of the CPTTP ionomer after tip ultrasonic and deposition drying treatment.\nThe figure was added as Supplementary Figure 6 in the revised Supporting Information, and corresponding discussions have been incorporated into the revised manuscript. Please see Lines 154-159.\nLines 154-159: “Specifically, during ink preparation, the FTIR spectrum of CPFTP@NiFe ink after tip ultrasonication alone revealed weakened absorption bands at 1640 cm-1 (Supplementary Fig. 6) corresponding to the double bonds, suggesting partial cross-linking occurred during this step. Subsequent deposition and drying eliminated this peak entirely, confirming complete crosslinking.”\n3. Figure 1 uses FT-IR analyses to show how the crosslinking reaction in CPFTP progresses with increasing temperature. This reviewer understands that CPFTP undergoes faster crosslinking at higher temperatures. The authors claim that, because tip sonication can momentarily create high-temperature regions, CPFTP may crosslink in the catalyst ink during the sonication process. However, this is not direct proof that tip sonication indeed induces crosslinking in CPFTP. Although the rheological changes in the catalyst inks shown in Figure 2 could reflect CPFTP crosslinking, they could also simply indicate inherent differences in physical properties between PFTP and CPFTP, independent of whether crosslinking took place. Might there be another experiment that more directly demonstrates whether and how CPFTP undergoes crosslinking during tip sonication?\nResponse: As addressed in our response to question 2, supplemental FTIR analysis confirmed crosslinking initiation during tip ultrasonication, evidenced by characteristic attenuation of the C=C bond signal (1640 cm-1).\n4. In Figure 3e, is it possible that the data for PFTP and CPFTP were reversed?\nResponse: We sincerely apologize for this oversight and appreciate your identifying the error in Fig. 3e. The data labels for PFTP and CPFTP were indeed reversed and were corrected in the revised manuscript.\nFig. 3e | 180o adhesion test profiles of the anode with PFTP and CPFTP binder.\n5. The data and description for Figure 4c appear inconsistent. The authors mention Rohm, Rct, Rmt, yet the graph includes Rs, and although the manuscript states that Rmt is 7.5 mΩ, the figure shows values closer to 30 mΩ. This discrepancy significantly complicates the reader’s understanding of the work.\nResponse: We thank the reviewer for identifying this error in Fig. 4c. The stacked bar annotations were inadvertently reversed in the original figure. We confirm the textual description is correct: the R and R of CPFTP-based MEA are 27.6 and 7.5 mΩ, respectively. The annotations have been ohm mt corrected in Fig. 4c of the revised manuscript.\nFig. 4c | R , R , and R values of the PFTP and CPFTP-AEMWE fitted from the EIS spectra.\nohm ct mt\n6. It would be valuable to disclose more details about the parameters input into the COMSOL simulations. When obtaining separate simulation results for PFTP and CPFTP, which parameter values were changed? Without this information, it is difficult to interpret only the results described in the text. While it is not necessary to present the entire COMSOL methodology, at least the key information should be included so that readers can follow the logic. Specifically, which parameters were varied between PFTP and CPFTP, and on what basis were those values chosen? Some explanation as to why those chosen values are appropriate would also be helpful.\nResponse: We sincerely appreciate the valuable comments. The specific surface area and porosity parameters differ between PFTP and CPTTP systems, as determined by experimental characterization: the specific surface areas of PFTP and CPFTP are 1e4 m-1 and 1.3e4 m-1, respectively, and the porosity is 0.70 and 0.85, respectively. These values derive from electrochemically active surface area measurement in three-electrode configurations and N 2 adsorption isotherms analysis. These parameters have been added to the revised Supporting Information (please see Supplementary Table 2).\n7. It would be good to incorporate Supplementary Figures 23 and 24 into Figure 5. Although the data in Figure 5 are certainly impressive, this reviewer believes it is crucial to highlight how CPFTP exhibits much better durability than PFTP. Beyond Supplementary Figures 23 and 24, the reviewer suggests incorporating additional data in Figure 5 to further demonstrate CPFTP’s superior durability.\nResponse: We have implemented all suggestions as recommended. The Supplementary Figures 23 and 24 are incorporated into Figure 5. Additionally, the Figures 5d-f in the revised manuscript are the newly supplemented data. The corresponding figure descriptions have been expanded in Lines 451-476 of the revised manuscript to reflect these updates.\nLines 451-476: “To evaluate the stability of PFTP- and CPFTP-based AEMWE with pure water feed, initially short-term stability tests were conducted under a high current density of 2.0 A cm-2 and 80 oC. As shown in Fig. 5e, the CPFTP-based AEMWE remained stable, while the PFTP-based AEMWE exhibited a relatively large voltage decay rate of 0.71 mV h-1. To analyze the degradation causes, we collected the EIS data before and after the durability test of PTFP-based AEMWE (Fig. 5f). The results showed increases in the cell’s R , R , and R increase by 4.3%, 11.2% and 6.0%, respectively. X-ray photoelectron spectroscopy (XPS) analysis of the CL was performed to understand the CL-degradation mechanism (Supplementary Fig. 32). After 120 h of operation at 2.0 A cm-2, a new binding energy peak emerged at 292 eV in the C1s spectrum of PFTP-CL, consistent with the formation of carbonyl and/or ester groups, while the C1s spectrum of CPFTP-CL remained unchanged. This indicates that the degradation of PFTP ionomers led to reduced ionic transport to the catalyst and decreased electrochemical active area due to catalyst particles detachment, consequently increasing both R and R . Conversely, the crosslinking of CPFTP enhances ionomer stability via steric hindrance effect 40. Additionally, the observed increase in R and R is primarily attributed to structure destruction of CL. Beyond visible catalyst detachment, we further employed TEM combined with ultraomics technology to study the morphological changes of CLs before (beginning-of-life, BOL) and after (end-of-test, EOT) the in-situ AEMWE stability test (Supplementary Fig. 33). TEM images revealed that CPFTP@NiFe maintained a homogeneous catalyst distribution and intact microstructure from BOL to EOT. In comparison, PFTP@NiFe exhibited a relatively poor micromorphology, which further deteriorated after the durability test. Therefore, under electrolyte-free water operation, CPFTP not only enhances the physical stability of the CL through its cross-linked network structure, but also improves the oxidation resistance of the ionomer.”\nFig. 5 | Performance and durability of AEMWEs. a, The durability and HFR of PFTP@NiFe and b, CPFTP@NiFe-based AEMWE at 80 oC and 300 mA cm-2 with 1.0 M KOH feed. c, SEM images of PFTP@NiFe and CPFTP@NiFe CL before and after AEMWEs stability test. d, Polarization curves of AEMWEs fed with pure water at 80 oC. e, Durability of PFTP@NiFe and CPFTP@NiFe-based AEMWE fed with pure water at 80 oC and 2.0 A cm-2. f, EIS sperume of PFTP@NiFe and CPFTP@NiFe-based AEMWE at beginning-of-life and end-of-test of the durability stability test. g, Durability of CPFTP@NiFe-based AEMWE fed with pure water at 80 oC and 1.0 A cm-2. h, Comparison of key merits with reported AEMWE, including current density, durability and decay rate.\nReviewer #2 (Remarks to the Author):\nThis manuscript focuses on enhancing the ionomer–catalyst interface through an in-situ crosslinking strategy aimed at improving interfacial adhesion. The authors present a simple method that achieves high performance and durability in AEMWE systems, particularly under pure water operation.\nHowever, the following points should be addressed before the manuscript can be considered for publication:\nResponse: We thank the reviewer for offering valuable and profound comments.\n1. As the authors state, \"During the deposition process, particle migration and aggregation are significantly exacerbated by shear force.\" To convincingly demonstrate the suppression of aggregation and crack formation through crosslinking, it would be more appropriate to coat the catalyst slurry onto a smooth, non-porous substrate (e.g., Si wafer or polymer film) using bar-coating under controlled shear conditions, followed by thermal crosslinking. This approach would provide a clearer assessment of effect of the ionomer for crosslinking. Additionally, the coating should maintain a catalyst loading greater than 1 mg/cm², similar to the high loading (4 mg/cm²) used in this study. Current observations of cracks on the porous metal foam substrate offer limited insight into morphological suppression attributed specifically to crosslinking.\nResponse: We sincerely appreciate the reviewers’ insightful comments regarding substrate selection and coating methodology. We agree that bar-coating on smooth non-porous substrate (e.g., Si wafer or polymer film) under controlled shear conditions would provide valuable morphological insights into crosslinking effects. However, the viscosity of our catalyst inks (< 100 Pa·s) falls below the operational range for bar-coating (typical 100-1000 Pa·s), and our laboratory currently lacks bar-coating equipment. We therefore employed the established spray-coating methodology for this fundamental investigation. Crucially, under identical ink preparation protocols, the results consistently demonstrate crosslinking's significant role in governing both ink rheology and catalyst layer morphology.\nTo further validate substrate independence, we deposited catalyst layers on a smooth polymer membrane and characterized their morphology. As shown in the newly Supplementary Fig. 14, both PFTP@NiFe and CPFTP@NiFe coatings exhibited identical morphological trends to those observed on porous substrates. Multiple experiments in this study utilized smooth, non-porous substrates to demonstrate substrate-independent effects: the catalyst inks spray-coated onto glass plate, dried, and scraped for the N₂ adsorption isotherms analysis. Similarly, samples for the 180o peel adhesion tests were prepared by spraying catalyst inks onto the polymer membrane. These complementary methodologies collectively confirm that substrate characteristics minimally influence ionomer crosslinking efficacy.\nSupplementary Figure 14. The morphology of the catalytic layer coated with ink on the surface of the membrane. a, EDS elemental mapping of Ni (blue), Fe (green), C (yellow) and I (red) in the CPFTP@NiFe electrode. b, SEM images of c, CPFTP@NiFe and c, PFTP@NiFe.\nTo elucidate rack formation mechanisms, we conducted Dynamic light scattering (DLS) analysis of catalyst inks. As shown in the newly Supplementary Fig. 10, PFTP-based ink exhibited a broad particle size distribution (reaching 1202 nm). In contrast, the CPFTP-based ink showed narrower distribution and reduced particle size (average 817 nm). This confirms superior dispersion in CPFTP ink, whereas PFTP ink contains larger, non-uniform aggregates. Consequently, these larger PFTP aggregates generate greater surface tension gradients during solvent evaporation, intensifying the Marangoni effect. Shrinkage pits evolve into open holes when the coating is thin. As the thickness increases, stress-laminated pits propagate into macroscopic cracks.\nSupplementary Figure 10. DLS measured size distributions of catalyst particles in PFTP@NiFe and CPFTP@NiFe catalyst ink slurries.\nThe Supplementary Figures 6 and 10 were added to the revised Supporting Information, and the corresponding revisions can be seen in the revised manuscript.\nLines 231-235: “As supported by the dynamic light scattering (DLS) measurements in Supplementary Fig. 10, the PFTP assembled ink shows a wide distribution characteristic with average particle clusters of 1202 nm, while CPFTP coated system shows a narrowed distribution and reduced particle sizes of 817 nm.”\nLines 246-249: “This is due to the fact that these larger PFTP aggregates generate greater surface tension gradients during solvent evaporation, intensifying the Marangoni effect. Shrinkage pits evolve into open holes when the coating is thin. As the thickness increases, stress-laminated pits propagate into macroscopic cracks.”\n2. The claim \"This performance surpassed that of other reported AEMWEs without PGM anodes\" requires revision to include more recent literature. Specifically, the following two studies report higher performance for PGM-free anodes:\nhttps://doi.org/10.1039/D4EE01003E\nhttps://doi.org/10.1021/acsenergylett.4c00207\nMoreover, beyond these two references, the authors are encouraged to incorporate additional recent state-of-the-art AEMWE studies to provide a comprehensive and updated comparative context.\nResponse: We appreciate your careful evaluation and valuable suggestion. The statement \"This performance surpassed that of other reported AEMWEs without PGM anodes\" has been revised to “This performance ranks in the medium-to-upper tier among reported non-PGM AEMWEs”.\nAccording to the suggestions of the reviewer, we have incorporated recent state-of-the-art AEMWE studies in the revised Supplementary Table 3 of the Supporting Information, please see the cited references 25-34.\n3. Under pure water operation, a local pH decrease can potentially induce structural changes in the catalyst or degradation of the ionomer. The authors attribute improved performance and durability primarily to enhanced ionomer adhesion; however, it remains uncertain whether this alone sufficiently explains the observed performance and durability improvements. A comparative analysis with previously reported pure-water AEMWE studies should be included to clarify the potential influence of local pH-driven degradation phenomena, such as ionomer oxidation or catalyst structural changes, on the observed results.\nResponse: We sincerely appreciate your expert insights, which have substantially strengthened this work. To address potential local pH-driven degradation mechanisms during pure water operation, including ionomer oxidation or catalyst structural changes, we conducted comparative short-term stability tests on PFTP- and CPFTP-based AEMWEs at 2.0 A cm-2 and 80 oC. As shown in Fig. 5e, the CPFTP-based AEMWE remained stable, while the PFTP-based AEMWE exhibited a relatively large voltage decay rate of 0.71 mV h-1. To analyze the degradation causes, we collected the EIS data before and after the durability test of PTFP-based AEMWE (Fig. 5f). The results showed increases in the cell’s R , R , and R increase by 4.3%, 11.2% and 6.0%, respectively. To clarify the potential influence of local pH-driven degradation phenomena, such as ionomer oxidation or catalyst structural changes. The X-ray photoelectron spectroscopy (XPS) of the CL was first performed in Supplementary Fig. 32 to analyze the ionomer-degradation mechanism. After 120 h of operation at 2.0 A cm-2, a new binding energy peak emerged at 292 eV in the C1s spectrum of PFTP-CL, consistent with the formation of carbonyl and/or ester groups, while the C1s spectrum of CPFTP-CL remained unchanged. This indicates that the degradation of PFTP ionomers led to reduced ionic transport to the catalyst and decreased electrochemical active area due to catalyst particles detachment, consequently increasing both R and R . Conversely, the crosslinking of CPFTP enhances ionomer stability via steric hindrance effect (ACS Appl. Energy Mater. 2021, 4, 14476–14487). Additionally, the observed increase in R and R is primarily attributed to structure destruction of CL. Beyond visible catalyst detachment, we further employed TEM combined with ultraomics technology to study the morphological changes of CLs before (beginning-of-life, BOL) and after (end-of-test, EOT) the in-situ AEMWE stability test. TEM images revealed that CPFTP@NiFe maintained a homogeneous catalyst distribution and intact microstructure from BOL to EOT. In comparison, PFTP@NiFe exhibited a relatively poor micromorphology, which further deteriorated after the durability test (Supplementary Fig. 33). Therefore, under electrolyte-free water operation, CPFTP not only enhances the physical stability of the CL through its cross-linked network structure, but also improves the oxidation resistance of the ionomer.\nThese newly added experiments (Figures 5e and f, Supplementary Figures 32 and 33) and discussions (Lines 451-476) have been incorporated into the revised manuscript.\nFig. 5 | e, Durability of PFTP@NiFe and CPFTP@NiFe-based AEMWE fed with pure water at 80 oC and 2.0 A cm-2. f, EIS sperume of PFTP@NiFe and CPFTP@NiFe-based AEMWE at beginning-of-life and end-of-test of the durability stability test.\nSupplementary Figure 32. XPS analysis of PFTP and CPFTP ionomer inside the CLs at beginning-of-life and end-of-test of the durability test.\nSupplementary Figure 33. TEM images inside the CLs of PFTP and CPFTP@NiFe at beginning-of-life and end-of-test of the durability test.\n4. The manuscript attributes improved adhesion and mass transport to the crosslinked ionomer strategy. However, performance improvements are likely closely related to the triple-phase boundary structure. Therefore, additional electrochemical analyses, such as measurements of electric double-layer capacitance and ohmic resistance under both 1 M electrolyte and pure water conditions (using CV or impedance spectroscopy), are needed to quantify the extent of interfacial improvements and provide clear evidence supporting the claimed enhancements.\nResponse: We have implemented this suggestion by incorporating a new measurement of electric double-layer capacitance and ohmic resistance under both 1.0 M electrolyte and pure water conditions. These data appear in Lines 287-301 and Supplementary Figures 17-19.\nLines 287-301: “Supplementary Fig. 17 and 18 demonstrate CPFTP@NiFe achieved the highest electric double-layer capacitance (EDLC) in both NaOH and pure water-fed conditions. Notably, it also retained a higher EDLC in the pure water-fed condition compared to that measured in the 1.0 M KOH. Specifically, CPFTP@NiFe and PFTP@NiFe retained 86.0% and 76.3% of their EDLC, respectively. This result reveals that a more uniform distribution of the CPFTP@NiFe CL exposes more active sites of the catalyst. Meanwhile, CPFTP's interconnected ionomer network enables efficient ionic conduction without supporting electrolytes. This is further supported by the EIS results. As shown in Supplementary Fig. 19, the CPFTP@NiFe CL exhibited a lower ohmic resistance (R ) (0.45 Ω cm2) and charge transfer resistance (Rct) (0.47 Ω cm2) than PFTP@NiFe CL (R = 0.47 Ω cm2, R = 0.63 Ω cm2) in 1.0 M NaOH system. And this resistance value gap is widened further under pure water system. This improved OH- accessibility is conducive to reducing the conduction resistance of OH- and providing more sufficient OH- reactants for enhancing OER performance.”\nSupplementary Figure 17. Cyclic voltammetry curves of a, QAFLN and b, PyFLN at different rotation speeds and 1.0 M NaOH. Cyclic voltammetry curves of c, QAFLN and d, PyFLN at different rotation speeds and pure water.\nSupplementary Figure 18. The electric double-layer capacitance of QAFLN and PyFLN at a, 1.0 M NaOH and b, pure water, respectively.\nSupplementary Figure 19. The EIS of QAFLN and PyFLN at a, 1.0 M NaOH and b, pure water, respectively.\nReviewer #3 (Remarks to the Author):\nThis study presents a novel in-situ covalent anchoring strategy for stabilizing NiFe catalysts in AEMWE, addressing a critical technological bottleneck with broad potential implications for electrochemical devices. The reported durability and performance of the catalyst layer are among the best in literature. The combination of X-ray tomography, rheological measurements, electrochemical analysis and COMSOL simulations provides compelling evidence for the structure and function of the 3D ionomer network. The manuscript is well-written and suitable for Nature Communications after addressing the following key points.\nResponse: The high and positive comments are highly appreciated.\n(1) The discussion on how cross-linking suppresses swelling and enhances ionic conduction is insightful, but the physical basis for Marangoni effect mitigation remains unclear. Could the authors quantify the interfacial tension changes? And a theoretical model would help correlate cross-linking density with flow instability suppression.\nResponse: We thank the reviewers’ insightful suggestions. Surface tension measurements of the catalyst inks revealed a significant increase for CPFTP-assembled ink (36.7 mN m-1) compared to PFTP-assembled ink (33.2 mN m-1), as shown in newly Supplementary Fig. 7. This crosslinking-induced elevation in surface tension reduces the capillary number, thereby enhancing coating uniformity and ink stability under high shear forces (Int. J. Hydrog. Energy, 2023, 48, 31758-31766.).\nWe then measured the particle size of the catalyst ink using the DLS. In Supplementary Fig. 10, PFTP-based ink exhibited a broad particle size distribution (reaching 1202 nm). In contrast, the CPFTP-based ink showed narrower distribution and reduced particle size (average 817 nm). This confirms superior dispersion in CPFTP ink, whereas PFTP ink contains larger, non-uniform aggregates. Consequently, these larger PFTP aggregates generate greater surface tension gradients during solvent evaporation, intensifying the Marangoni effect. Shrinkage pits evolve into open holes. As the thickness increases, stress-laminated pits propagate into macroscopic cracks. This phenomenon was alleviated in CPFTP@NiFe ink, as evidenced by the corresponding SEM morphology.\nFurthermore, we analyzed the Marangoni convection intensity within the inks using the Marangoni number (Ma), as defined by the Equation:\ndγRΔT\nMa = --------\ndT μα\nWhere is the change rate of fluid surface tension with temperature, R denotes the contact radius between the droplet and substrate, ∆T is the temperature difference at the droplet surface, µ is the dynamic viscosity, and α signifies the thermal diffusivity of the fluid. Thus, when the other conditions were identical, a larger µ would reduce Ma, thereby decreasing the intensity of Marangoni convection during solvent evaporation. The three-dimensional ionomer network of CPFTP formed by spontaneous cross-linking thermally triggered enhances the viscosity (as evidenced by the rheological measurement), which mitigates the Marangoni flow and promotes the formation of a uniform catalytic layer (Adv. Mater., 2024, 36, 2310752).\nSupplementary Figure 7. The surface tension of PFTP and CPFTP@NiFe inks.\nThe relevant discussions have been supplemented to the revised manuscript.\nLines 165-166: “This increased viscosity would reduce the Marangoni number, which mitigates the Marangoni flow.\nLines 189-192: “Additionally, the measured surface tension of the CPFTP@NiFe inks(36.7 mN m-1) is higher than PFTP-assembled ink (33.2 mN m-1) (Supplementary Fig.7), which contributes to improving the coating uniformity and ink stability at high shear forces.”\nLines 231-235: “As supported by the dynamic light scattering (DLS) measurements in Supplementary Fig. 10, the PFTP assembled ink shows a wide distribution characteristic with average particle clusters of 1202 nm, while CPFTP coated system shows a narrowed distribution and reduced particle sizes of 817 nm.”\nLines 246-249: “This is due to the fact that these larger PFTP aggregates generate greater surface tension gradients during solvent evaporation, intensifying the Marangoni effect. Shrinkage pits evolve into open holes when the coating is thin. As the thickness increases, stress-laminated pits propagate into macroscopic cracks.”\n(2) The homogeneous ionomer distribution is attributed to Marangoni effect mitigation, but how can the authors be sure that alternative mechanisms play no role in the system? Could the authors provide direct evidence connecting cross-linking to flow stabilization?\nResponse: We thank you for raising this fundamental question. Our experimental and theoretical analysis reveals that the relationship between homogeneous ionomer distribution and Marangoni suppression operates through three key mechanisms: Cross-linking significantly increases the ink's viscoelasticity (rheologically confirmed), creating an interconnected network; Progressive crosslinking inhibits internal compensatory flows during processing. This stabilizes liquid-gas interface and suppresses solutal Marangoni convection induced by local evaporation gradients.\nUnder identical processing conditions, crosslinked samples achieve superior homogeneity-validated through comprehensive homogeneous morphology characterization.\nWhile secondary mechanisms in such a complex dynamic process cannot be entirely excluded, our integrated evidence demonstrates compelling self-consistency: Rheology-microstructure correspondence & Crosslinking-homogeneity correlation & Microstructure-performance linkage.\nThis evidence robustly supports our core argument: ionomer cross-linking suppresses surface tension gradient-driven flows (Marangoni effect) through viscoelastic network formation, enabling homogeneous ionomer distribution.\nFor the connection between cross-linking and flow stabilization, we conducted the FTIR to monitor the crosslinking behavior of ionomers after tip ultrasonic and spray deposition drying treatment. As shown in Supplementary Fig. 6, the signal peak corresponding to double bonds (1640 cm-1) in the CPFTP spectrum weakened after tip-ultrasonic treatment. This indicates partial cross-linking occurred, consistent with the observed increase in the platform modulus of cross-linked ionomer from rheological testing. Furthermore, the complete disappearance of the 1640 cm-1 peak after deposition drying indicates full crosslinked during this stage. This significantly enhances the physical stability of the catalytic layer, as demonstrated by the 180o adhesion test. Therefore, FTIR analysis during catalyst ink processing, combined with subsequent characterization of the rheological properties and morphological distribution of cross-linked inks, provides direct evidence linking cross-linking to flow stabilization.\n(3) What is the kinetic mechanism of the temperature-dependent cross-linking evolution of the ionomers?\nResponse: The temperature-dependent cross-linking evolution of chloromethyl styrene-based ionomers follows a radical chain reaction mechanism. At elevated temperatures, thermal energy induces homolytic cleavage of the vinyl double bonds (C=C), generating primary radical species (R·). These radicals subsequently attack the vinyl double bonds of adjacent chloromethyl styrene units, forming propagating chain radicals (P·). Two propagating radicals (P.) terminate via coupling or disproportionation to form stable cross-linking bonds.\n(4) The claim of “uniform distribution” in Fig. 2 d-g requires more rigorous quantifications. Could the authors clarify why large blue/red domains in Fig. 2(e) do not contradict the uniformity claims?\nResponse: We sincerely appreciate the reviewer's astute observation regarding the apparent heterogeneity in Fig. 2(e). The term \"uniform distribution\" in our manuscript specifically denotes the significantly improved dispersion of CPFTP ionomers at the catalyst layer scale compared to the highly aggregated, unoptimized PFTP control samples. This enhanced uniformity is reflected in the enhancement of interface performance, as evidenced by the higher double-layer capacitance (Supplementary Fig. 18) and lower ohmic impedance (Supplementary Fig.19) observed in the CPFTP-based CL relative to the PFTP-CL.\nSome minor points are listed as follows:\n(1) Figure 2 (d-g): Please clarify the distinction between panels (d)/(e) and (f)/(g). All these panels look like 3D renderings instead of 3D slices.\n(2) Typos: Page 13, HER → HFR, regio → region.\nResponse: Thank you for your detailed observation.\nThe YZ orthogonal slices shown in panels (e) and (g) were extracted directly from the 3D images in panels (d) and (f). To help reviewers more clearly identify the differences, we have also provided orthogonal slice images (XY and XZ planes) from other perspectives in Supplementary Fig. 9 of the revised Supplementary Information.\nSupplementary Figure 9. Orthogonal slices (YZ, XY and XZ planes) of 3D Nano-CT of CPFTP@NiFe and PFTP@NiFe samples.\nAdditionally, all typos identified in the manuscript have been corrected as suggested."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript satisfactorily addresses my previous comments. Provided that the other reviewers’ concerns are also adequately addressed, I consider it suitable for publication in Nature Communications", "reviewer_2": "The authors have fully addressed the questions raised by the reviewers; therefore, I agree to publish it without further revision.", "reviewer_3": "I thank the authors for their detailed responses to my previous comments. As far as I can see, they have also responded adequately to the comments of the other reviewers. The additional experiments and discussion they provided enhance the clarity and credibility of the analyses of the underlying mechanisms. As such, I can happily recommend this paper for publication."}, "author_response": ""}]

Decision letter