Bubble dynamics matters at high-rate water electrolysis

Domain: chemistry
Published: 2026-02-03
Editorial outcome: minor_revision

Abstract
Bubbles accumulation in the electrode limits anion exchange membrane water electrolyzer performance at industrial current densities (>1.0 A cm-2). Currently, conventional electrode designs prioritize the optimization of the electrochemically active surface area. However, this study reveals that bubble dynamics matters high-rate water electrolysis efficiency in anode-feeding mode in three ways: 1) cover active sites at the anode; 2) hinder water diffusion through the membrane; 3) cause water shortage at the cathode. Based on this mechanism, we propose an easy-to-prepare gradient stainless steel square hole mesh electrode. It not only offers a low cost ($8-150/m2), but also improves bubble dynamics. As a result, it reduces the cell voltage by 0.14 V at a current density of 5.0 A cm-2, even with a lower electrochemically active surface area compared to the stainless steel felt electrode. And it maintains a stable operation over 400 hours. This work redefines electrode engineering paradigms, shifting focus from electrochemically active surface area-centric approaches to two-phase flow management in water electrolyzers for industrial current densities-scale hydrogen production.

Introduction
The transition to a net-zero carbon economy hinges on the swift implementation of scalable and cost-efficient green hydrogen production technologies1,2. Water electrolysis, particularly when integrated with renewable energy, is the backbone of hydrogen production with minimal lifecycle carbon emissions3,4. Within the spectrum of water electrolysis technologies, anion exchange membrane water electrolyzer (AEMWE) has garnered attention due to its advantageous combination of features derived from both alkaline water electrolyzer and proton exchange membrane water electrolyzer (PEMWE). These advantages include compatibility with non-precious metal catalysts, lower material costs, and the capability to operate at high current densities, etc5,6. However, a major challenge lies in achieving industrial-scaled current densities while ensuring high efficiency and durability7. Addressing this challenge often requires enhancements in both oxygen evolution reaction (OER) activity and internal mass transport8–10.

As illustrated in Fig. 1, at lower current densities (<1.0 A cm−2)11, the electrochemical performance of AEMWE primarily relies on the intrinsic activity and electrochemically active surface area (ECSA) of the catalyst. Many studies have concentrated on optimizing the morphology and composition of catalysts12–14, and electrode structures15–17 to enhance ECSA and charge transport kinetics. However, as current densities increase to fulfill industrial specifications (>1.0 A cm−2)11, the effects of bubble dynamics on mass transport and interfacial processes become critically significant. The produced oxygen bubbles at the anode notably diminish the active sites of the electrodes18, disturb local pH and ion concentration gradients19, resulting in an elevated surface overpotential at a specific current density18. In addition, bubbles also hinder water diffusion through the membrane, and the diameter of the bubbles is related to the ohmic overpotential20. Especially in the anode-feeding mode, bubble restrictions on water diffusion across the membrane lead to water shortage at the cathode, resulting in insufficient supply of reactants21. Collectively, these factors diminish the efficiency and stability of the system. Consequently, effective bubble management at elevated current densities is important for improving the efficiency and stability of AEMWE.

Achieving effective bubble management can be accomplished at multiple scales. For example, some studies have focused on microstructural improvements22–24 or hydrophilic surface modifications25–27 at the microscopic level to achieve rapid bubble detaching from electrodes, and to mitigate mass transport overpotentials by renewing the electrolyte-electrode interface. Wan et al.28 proposed a direct membrane deposition method for fabricating three-dimensionally (3D) ordered catalyst layers (CLs) on AEM, significantly improving AEM water electrolysis performance to rival that of state-of-the-art PEMWE by increasing the exposure of active sites, enhancing the gas/liquid mass transport, and hydroxide ion (OH−) transport at the interface. The only drawback is the complexity of the processing methods and the need for careful handling. Ding et al.29 presented an effective one-step method for creating an optimized OER substrate, demonstrating that a double-layered nickel (Ni) mesh with NiFe-layered double hydroxide can enhance mass transport and bubble management, leading to improved performance in AEMWE. In addition, at the macroscopic scale, Wu et al.30 not only proposed a dual-layer channel flow field to improve the two-phase regimes in the flow channel and enabled the electrochemical performance of water electrolyzer to be enhanced by 0.15 V compared to that with the conventional serpentine flow field at a high current density of 5 A cm−2, and also incorporates machine learning methods to optimize key parameters of this design31. It is believed that these recent studies on bubble dynamics in water electrolyzers will inform simpler and more efficient bubble management strategies.

Stainless steel felt has emerged as a highly attractive integrated anode electrode in AEMWEs, serving a dual function as both an efficient OER catalysts under alkaline conditions and the porous transport layer (PTL)32–34. Its appeal lies in its relative ease of preparation and inherent structural robustness, offering a potentially cost-effective and durable alternative to more expensive catalyst-coated substrate electrodes32. Furthermore, in our previous research35, we found that changes in the surface morphology of stainless steel felt, such as reduced surface hydrophilicity, lead to a decrease in the number of bubbles generated and an increase in the diameter of bubble detachment, obviously increasing activation and mass transport and bubble overpotentials. Based on this, exploring multiple parameters such as pore size and hydrophilicity of stainless steel electrodes is a relatively simple and direct way to investigate which one of the electrode bubble dynamics and ECSA is the main factor affecting the electrochemical performance of AEMWE under high current conditions.

Herein, we employ a combination of techniques, including overpotential breakdown, distribution of relaxation times (DRT) analysis, potential measurements, and multiscale visualization methods, to explore the influence of bubble behaviors on mass transport and electrochemical performance in AEMWE with stainless steel electrodes that perform well under alkaline conditions. Firstly, it is demonstrated that enhancing bubble removal capabilities can substantially improve the electrochemical performance of electrodes with low ECSA. Therefore, we chose 316L gradient stainless steel mesh. This design not only surpasses the electrochemical performance of stainless steel felt electrodes in AEMWE but also simplifies the conventional electrode preparation process while simultaneously enhancing energy efficiency and reducing anode fabrication costs by over 99.86% compared with traditional IrO2 electrodes. This can also be seen as the systematic implementation of the “gradient structure+bubble control” strategy for AMEWE’s integrated stainless steel electrodes. The scientific positioning of this study is not on electrode design, but rather on using this design to reflect the more dominant role of bubble dynamics over electrode ECSA in AEMWE under high current conditions. Ultimately, this work highlights the need to balance ECSA optimization with two-phase flow management for high-rate operation. Addressing bubble-related mass transport issues could help improve the performance of future AEMWEs.

Results
Discovery of the dominant role of bubble dynamics at high current densities
Stainless steel electrodes have been shown to have strong OER activity under alkaline conditions, so we use an integrated (combined PTL and CL functions) stainless steel felt as the anode in this study32,35. First, the anode-feeding configuration is selected based on its demonstrated advantages over cathode- or both-feeding modes (Supplementary Fig. 4), consistent with prior studies36. Subsequently, the effect of the pore size of stainless steel felts on the electrochemical performance and bubble behaviors of AEMWE is investigated. Meanwhile, we confirm the optimal torque and thickness of the stainless steel felt according to Supplementary Fig. 5 and Fig. 6, and in the following experiments, only the pore size of the stainless steel felt is varied (5–50 μm), which is determined in Supplementary Table 1 and Supplementary Fig. 7. As shown in Fig. 2a, reducing the pore size of the stainless steel felts significantly decreases the cell voltage at high current densities, with the 5 μm pore size electrode achieving better overall performance compared with other pore sizes. The internal resistance measured by high-frequency resistance (HFR) varies slightly at different current densities, as shown in Supplementary Fig. 10e. After excluding ohmic overpotential (iR-free (E–iR) curves), this trend persists. Through utilizing the Tafel approximation method37, we obtain Tafel slopes in Fig. 2b and breakdown of all overpotentials in Fig. 2c. Specifically, the ohmic overpotential takes up the majority of all overpotentials. A smaller pore size improves interfacial contact between the rib and the electrode33, leading to reduced ohmic overpotential. For instance, a stainless steel felt with a 50 μm pore size results in over twice the ohmic overpotential compared to the 5 μm pore size felt. In addition, the activation and mass transport overpotentials in Fig. 2c are the sum of the cathode and anode, respectively, which was discussed in our previous study35. The activation overpotentials have optimal values at 10 μm and 30 μm pore sizes felt. The 5 μm pore size felt shows the minimum mass transport overpotentials.

We try to draw some firm conclusions about the effect of electrode pore size on internal mass transport of AEMWE. However, it is difficult to determine directly which portion of the overpotential at the cathode and anode is affected by the pore size of the anode electrode only by the Tafel approximation method35. Three-electrode configuration isolates the effect of anode pore size on OER performance (Supplementary Fig. 8a) and calculates the ECSA of all electrodes. For ECSA (Supplementary Fig. 8b), as the pore size increases, ECSA begins to decrease instead, which is consistent with performance differences at the low current density region in zero-gap membrane electrode assembly (MEA) electrolyzer (e.g., <0.5 A cm−2) (Supplementary Fig. 8c). A 5 μm pore size has the largest ECSA (Supplementary Fig. 8b and Fig. 9), it performs better than 30 and 50 μm but worse than 10 μm. The only reason for this is that the 10 μm pore size is more conducive to bubbles removal than the 5 μm pore size, and the rapid bubble discharge reduces the coverage of the active sites and compensates for the relatively low ECSA of the electrode itself, as shown in Fig. 2d (also see Supplementary Movie 1). Incidentally, the oversized pore size facilitates bubbles removal but cannot compensate for the relatively low ECSA of the electrode itself, e.g., 50 μm. Increasing pore size may not necessarily present smaller bubbles due to the thicker fibers and tortuous flow paths, increasing mass transport overpotentials38. The view comes to the practical zero-gap MEA electrolyzer, which differs in that there is no physical isolation of the cathode and membrane from the anode in this system. Moreover, it is hypothesized that the bubbles and pore size at the anode also affect the water diffusing through the membrane to the cathode, leading to a complex situation different from the three-electrode tests, depicted in Fig. 2e. DRT method is able to directly extract the activation and mass transport impedance of the cathode and anode, respectively, in AEMWE. From Fig. 2f, smaller pores (5 μm) increase the OER-related impedance (P2 peak) due to untimely detachment of bubbles. Larger pores reduce hydrogen evolution reaction (HER) impedance (P3 peak) by enhancing water cross-membrane but exacerbate mass transport impedance (P1 peak) and OH− transport (P4 peak) due to larger bubbles. The EIS curves at other current densities can be found in Supplementary Fig. 10, and stainless steel felt with different pore sizes also display trends similar to those shown in Fig. 2f. Notably, the ohmic, activation, and mass transport impedances all decrease for each stainless steel felt as the current density increases. This occurs because higher current densities promote rapid bubble formation and release, which helps to efficiently eliminate bubbles from previous cycles and thereby reduce each impedance component. Finally, it can be expected that at low current densities, the electrochemical performance depends on the ECSA value. However, at high current densities, the bubble dynamics matters high-rate water electrolysis efficiency of AEMWE more than that of ECSA. The initial electrode design principle at high current densities can be summarized as ensuring sufficient ECSA and enabling rapid detachment while maintaining smaller bubble size, so that the activation and mass transport overpotentials can be both reduced.

Reconfirmation of key processes in AEMWE affected by bubble dynamics
We investigate the influence of surface wettability on AEMWE performance and its key processes through controlled modification of the fiber surface morphology of stainless steel felt while maintaining consistent pore structure. This can be regarded as a special experimental design, using diluted nitric acid (HNO3) to reduce the ECSA of the stainless steel felts due to the oxidation of the fiber surface while enhancing its hydrophilicity by creating a roughened surface. This may better demonstrate whether bubble dynamics or ECSA plays the dominant role in AEMWE under high current densities conditions. Incidentally, compared to untreated samples, alcohol-washed stainless steel felts are cleaned with alcohol to remove grease and dirt. As bubbles detach from the electrodes, grease and dirt on untreated samples are gradually washed away by the bubbles. As a result, their electrochemical performance and ECSA do not differ significantly (Supplementary Fig. 11), and alcohol-washed stainless steel felts are used as the base case in this section. In a three-electrode system, Fig. 3a, b demonstrate that the acid-washed stainless steel felt presents better performance than the alcohol-washed stainless steel felt (base case) in both OER performance and impedance, and the performance gap becomes more pronounced with increasing current density. Notably, while alcohol-washed stainless steel felt exhibits 2.3-fold higher ECSA (3.81 mF cm−2 > 1.65 mF cm−2) than that of the acid-washed stainless steel felt (Fig. 3c and Supplementary Fig. 11), it fails to correlate with actual electrochemical performance. It suggests that evaluating electrode design solely based on ECSA may be insufficient under higher current densities.

Transitioning to a practical zero-gap MEA electrolyzer, acid-washed stainless steel felt achieves a 0.3 V voltage advantage over the alcohol-washed stainless steel felt at 5 A cm−2 (Fig. 3d). Using iR-free curves, the Tafel approximation method37 is applied to determine the Tafel slope. The internal resistance measured by HFR varies slightly at different current densities, as shown in Supplementary Fig. 12c. In particular, compared with alcohol-washing, acid-washing increases the Tafel slope (47.04 mV dec−1 to 49.39 mV dec−1) (Fig. 3e), which results in a significant increase in activation overpotentials at each current density. This reason may be that while the enhanced bubble dynamics of acid-washed felt improves bubble coverage on the electrode surface, it fails to sufficiently compensate for its inherently low ECSA. Nevertheless, this bubble dynamics enhancement yields more pronounced improvements in mass transport and ohmic overpotentials (Fig. 3f), ultimately resulting in a substantial overall performance enhancement through combined effects (Fig. 3d). The subsequent visualization results will help directly prove this hypothesis. The EIS and DRT analysis further resolves in detail that compared with the impedances of the alcohol-washed stainless steel felt, the anode activation impedance of the acid-washed stainless steel felt does not change significantly, the cathode activation impedance is lower, and the cathode and anode mass transport impedances are also both reduced (Fig. 3g), and this phenomenon becomes more obvious as the current density increases (Supplementary Fig. 12). The P4 peak shows the OH− transport inside the electrode and at the interface, related to water movement. Acid-washed stainless steel felt has a smaller peak 4 area than alcohol-washed stainless steel felt, indicating lower OH− transport impedance (Fig. 3g). This suggests that water transport from the anode to the cathode has been improved. Potential measurement in a customized electrolyzer (Supplementary Fig. 1) further validates this mechanism. Acid-washed stainless steel felt maintains a similar anode potential with alcohol-washed stainless steel felt, whereas alcohol-washed stainless steel felt exhibits significant cathode potential deviation at higher current densities (e.g., at 2.0 A cm−2), directly mirroring the accuracy of the DRT analysis (Fig. 3h).

The exact reason for the performance improvement of AEMWE caused by acid-washed stainless steel felt can be attributed to the improved bubble dynamics, which optimizes its inside key processes: (1) Reduced active site coverage mitigates anode activation and mass transport polarization; (2) Promoted water across the membrane improves ohmic polarization; (3) Increased transmembrane water alleviates cathode water shortage-induced activation. Specifically, microscale visualization characterization is employed to elucidate the relationship between electrode structure, electrochemical performance, and bubble behaviors. Scanning electron microscopy (SEM) imaging (Fig. 4a, b) reveals acid-washing-induced surface roughening, while contact angle measurements confirm 38% hydrophilicity improvement (wetting time reduction from 56.6 to 41.1 ms), promoting faster bubble evolution39. And in-situ bubble tracking35 (Fig. 4a and d) quantifies optimized bubble management (also see Supplementary Movie 2). Acid-washed stainless steel felt generates 20% smaller bubbles (56.4 to 44.9 μm diameter) with 33% higher number density (126 to 168 counts). This can reduce coverage and mass transport resistance on the electrode surface40. Crucially, the acid-washed stainless steel felt also consistently presents better OER performance under the constant-current test (Fig. 4c), proving the above key process (1). Direct monitoring of cathode-collected water (Fig. 4f) substantiates the above key processes (2) and (3). Finally, this improved bubble dynamics is ultimately identified as improving performance through the above three key processes (Fig. 4e). In addition, the duration of the performance gap between the acid-washed stainless steel felt and the alcohol-washed stainless steel felt is noteworthy. Initially, the acid-washed stainless steel felt presents better performance than the alcohol-washed stainless steel felt. However, their voltages become similar after approximately 30 h, and by around 85 h, the acid-washed stainless steel felt surpasses the alcohol-washed stainless steel felt (Supplementary Fig. 13a). After 120 h, both electrodes show significant degradation in electrochemical performance (Supplementary Fig. 13b) and ECSA (Supplementary Figs. 13c and 14), with the alcohol-washed stainless steel felt maintaining a higher ECSA throughout (Supplementary Fig. 13c). EIS data indicate that increased OER impedance is the main reason for the performance decline in the long term (Supplementary Fig. 13d). Specifically, SEM images (Supplementary Fig. 15) reveal that the fresh acid-washed stainless steel felt has a slightly rougher surface than the alcohol-washed stainless steel felt. XPS analysis (Supplementary Fig. 16) shows that diluted HNO3 etching forms a dense chromium-rich oxide layer. Although this layer reduces ECSA, it improves initial hydrophilicity (Supplementary Fig. 17) and bubble dynamics (Supplementary Fig. 18), explaining the early-stage performance advantage. Over time, the surface morphology of the two electrodes become similar (Supplementary Fig. 15). After 120 h, the decrease in oxyhydroxides and the significant loss of Cr, Fe, and Ni lead to reduced hydrophilicity and fewer active sites (Supplementary Fig. 16). As a result, the decreased ECSA and worsened bubble removal (Supplementary Fig. 19) become the primary reasons for the increased OER impedance and performance degradation of the acid-washed stainless steel felt after prolonged operation. Therefore, it is not recommended to use diluted HNO3 to treat stainless steel felt in industrial water electrolysis applications. This can be viewed as a special experimental design (reduced ECSA but improved bubble dynamics) to validate whether ECSA or bubble dynamics is the key factor to dominate the performance of AEMWE at high current densities. In addition, we also add hydrochloric acid (HCl) treatment samples to mitigate the surface oxidation, which not only increases ECSA but also presents better performance than alcohol-washed stainless steel felt (Supplementary Figs. 20–22).

In contrast, it is also worth mentioning that increasing ECSA to the same electrode substrate but deteriorating its bubble dynamics may degrade performance at high current densities, as its effect depends on the chosen method. For example, the morphology and surface composition of IrO2+ionomer-coated stainless steel felt are shown in Supplementary Figs. 23–25. When using piperION ionomer, an about 60% increase in ECSA, as the current density increases to 5.0 A cm−2, the iR-free cell voltage gap with uncoated stainless steel felt gradually narrows to a negligible level. When using Nafion ionomer, the AEMWE performance worsens by about 600 mV at 5.0 A cm−2, despite an about 123% increase in ECSA (Supplementary Figs. 26 and 27). The bubble behaviors detach from the above three electrodes help explain these differences (Supplementary Fig. 28).

Advantages of the gradient stainless steel mesh and operational stability
To enhance bubble removal and water transport at high current densities, we have designed a stainless steel mesh electrode with a gradient structure. Firstly, three-electrode configuration reveal that the non-gradient square hole mesh with sieves of 300 exhibit the worst OER performance, whereas gradient designs (both mat and square hole) demonstrated better performance at high current densities and even the gradient square hole mesh is better than all other designs (Supplementary Fig. 29). Critically, ECSA measurements (Fig. 5a and Supplementary Fig. 30) depict that the gradient square hole mesh is even lower than the non-gradient mat mesh with sieves of 200 or 1250, and is a quarter of the ECSA of the felt. Transitioning to a practical zero-gap MEA electrolyzer test, gradient stainless steel square hole mesh presents better performance compared to all other stainless steel electrode designs, achieving a cell voltage reduction of 0.14 V at 5 A cm−2 compared with stainless steel felt (Fig. 5b). In addition, mat mesh with sieves of 200 and square hole mesh with sieves of 60 exhibit cell voltages very close to those felt. Specifically, through the Tafel approximation (Supplementary Fig. 31), we obtain the breakdown for each overpotential (Fig. 5c). The internal resistance measured by HFR varies slightly at different current densities, as shown in Supplementary Fig. 33f. In terms of ohmic overpotential, mat mesh and square hole mesh have higher ohmic overpotentials due to their more pronounced localized surface heights, which do not ensure good interfacial contact with the rib of the flow field plate (Fig. 6a and Supplementary Fig. 32). By contrast, the gradient mat mesh and gradient square hole mesh may improve the bubbles removal from the anode to a large extent and significantly increase the water content of the membrane and cathode. Even with poor interfacial contact, they can also lower the ohmic overpotential. In terms of activation and mass transport overpotentials, the gradient square hole mesh is much lower than the other designs. DRT and EIS analyses (Fig. 5d and Supplementary Fig. 33) indicate that the anode impedance of gradient square hole mesh is close to that of the felt, but the cathode impedance is much lower than that of the felt. For the gradient mat mesh, its inferiority to the gradient square hole mesh can be explained by the higher cathode activation and mass transport impedance than that of the gradient square hole mesh. It can be seen that the layer of the gradient mat mesh close to the membrane has 1250 sieves. The sieve pore is too small for the diffusion of liquid water from the anode to the cathode, as shown in Fig. 6b. In the potential measurements, the gradient square hole mesh ensure that the anode potential is comparable to that of the felt but ensure that the overpotential of the cathode is much higher than that of the felt (Fig. 5e), which also doubly confirms that this design improves the overall electrochemical performance of AEMWE.

In the microscale visualization and quantitative statistics of the bubble behaviors (Fig. 6c, d) (also see Supplementary Movie 3), the number of bubbles removal from the gradient square hole mesh more than tripled (114 to 258 counts) and the diameter decreases from 39.4 to 29.9 μm compared to that of the felt, which is also responsible for the highest amount of cathode liquid water collected (Fig. 6b). In addition, the bubble behaviors of mat mesh, square hole mesh, and gradient mat mesh are also provided in Supplementary Fig. 34. The bubble behaviors of the electrodes also affect the millimeter-scale two-phase flow regimes in the flow field (also see Supplementary Movie 4). As shown in Fig. 7a, b, decreased bubbles clogging in the flow field caused by the gradient square hole mesh will cover less electrode area compared to the stainless steel felt. Specifically, Fig. 7c demonstrates that the difference in the proportion of plug, slug, and large bubble flow in the flow field caused by the two electrodes is small at low current densities. However, the ratio of effective area and dense tiny bubbles flow in the flow field caused by the gradient square hole mesh is significantly higher compared to the stainless steel felt, and this is especially evident at higher current densities, e.g., 3.0 A cm−2. Therefore, it can be hypothesized that the dense tiny bubbles flow does not cover the electrodes and affect the mass transport of water. Finally, we further investigate the durability of the gradient square hole mesh. The AEMWE fitted with this design was operated under anode-feeding (dry cathode) conditions and fed with 0.1 M KOH to the anode. Stable operation for 400 h (degradation rate: 0.035 mV h−1) at a current density of 1 A cm−2 (Fig. 5f) demonstrated its competitive application prospects compared with reported electrodes (Supplementary Table 2) in AEMWE.

For our laboratory-scale cell operating at high current densities (e.g., 5.0 A cm−2), low flow rates such as 1.0 and 3.0 mL min−1 fail to ensure adequate delivery of KOH solution to the anode electrode. Blindly increasing the KOH flow rate to 30 mL min−1 does not effectively enhance AEMWE performance as the gradient square hole mesh does and may even prove counterproductive (Supplementary Fig. 35a, b). Therefore, 5.0 mL min−1 represents the optimal flow rate at high current densities (Supplementary Fig. 35b). While higher flow rates promote bubble removal in flow channels (Supplementary Fig. 36), increased velocity toward the electrode in the Y-direction impedes bubble removal within the electrode (Supplementary Fig. 37), raising impedances for OER, mass transport, HER, and OH− transport (Supplementary Fig. 35c). Additionally, higher flow rates waste electrical energy and are not recommended. Therefore, optimizing electrode structures to overcome bubble clogging at high current densities may be a better solution. Finally, it is also worth emphasizing that the cost of a traditional IrO2 electrode is the highest, estimated at over $111,000/m2. In comparison, 316L stainless steel mesh costs about $8–150/m2, which is even lower than the over $7200/m2 for 316L stainless steel felt, over $4200/m2 for Ni felt, and over $350/m2 for Ni mesh. More details about the cost analysis of anode electrodes can be found in Supplementary Table 3. The gradient stainless steel mesh not only enhances electrolysis efficiency but also simplifies the preparation process, leading to a significant 99.86% reduction in anode manufacturing costs compared with the traditional IrO2 electrode. Thus, it presents a cost-effective and efficient alternative to traditional electrodes.

Methods
Materials
The experimental materials include 316L stainless steel felt and 316L stainless steel mesh from Fuershun Electronic Materials and carbon paper (TGP-H-090) from Toray Co., Japan; Pt/C (60%) catalyst from Johnson Matthey (UK); PiperION A membrane (60 µm) and Nafion ionomer (D520, 5 wt%) from Fuel Cell Store (USA); Potassium hydroxide (KOH) (Macklin, 95%) from Shandong Keyuan Biochemical Co., Ltd; Iridium oxide powder (IrO2, 99%) from Alfa Aesar. All chemicals are stored in dedicated chemical cabinets and are used without any further processing.

Preparation of membrane electrode assemblies
Stainless steel felts and meshes were cut into 4 cm2 (2.0 × 2.0 cm) and were cleaned in an ultrasonic bath with ethanol and DI water for 30 min, respectively. The catalyst loading was determined by the mass of catalyst in the ink divided by the coated geometric area (4 cm2). The catalyst loading of the cathode is 0.5 mg cm−2 and the catalyst ink was prepared by mixing 60 wt% Pt/C with ionomer (D520, 5 wt%) at a mass ratio of 4:1, and then ethanol was added into the solution as a solvent41. Subsequently, the ink was subjected to dispersion within an ultrasonic ice bath for 30 min42. Thereafter, the ink was sprayed onto the carbon paper and dried repeatedly, until the catalyst loading reached the desired value43. For the membrane, the square PiperION A (60 µm) were cut into 6.25 cm2 (2.5 × 2.5 cm) small pieces and were immersed in 1.0 M KOH solution at room temperature (25±2°C) for 24 h before being incorporated into the MEA32. Finally, MEA was created by combining an anode stainless steel electrode and cathode Pt-based carbon paper electrode with a PiperION A membrane in the middle.

For the anode porous transport electrodes used in AEMWE, the anode ink composed of IrO2 and ionomer was directly deposited onto 4 cm2 (2.0 × 2.0 cm) stainless steel felts via spray coating. The anode catalyst ink was prepared by bath sonication of a mixture containing the ionomer (either Nafion D520 or PiperION A), commercially available IrO2 catalyst with an Ionomer: Catalyst ratio of 0.4, isopropyl alcohol, and deionized water for 40 min. The resulting ink was then sprayed onto the electrode to achieve a catalyst loading of 1.6 mg cm−2.

Acid treatment
Firstly, stainless steel felts were cut into 4 cm2 (2.0 × 2.0 cm) and were cleaned in an ultrasonic bath with ethanol for 30 mins, respectively. After drying at 60°C, stainless steel felt is then immersed in the pickling solution to form a rough topography to obtain a hydrophilic surface, but cause fiber surface oxidation. The pickling solution was prepared by combining 200 mL of deionized (DI) water with 25 mL of 70% nitric acid and then heating it to a temperature of 60°C (±5°C). A stainless steel felt sample measuring 4 cm2 (2.0 × 2.0 cm) was weighed prior to the procedure. The sample was immersed in the solution for 10 min with vigorous stirring. Following this, the sample was thoroughly rinsed with DI water and dried in an oven at 100°C.

Hydrochloric acid (HCl) treatment can etch stainless steel fibers without causing fiber surface oxidation. The pickling solution was prepared by combining 150 mL of deionized (DI) water with 50 mL of 37% nitric acid and then heating it to a temperature of 60°C (±5°C). Two stainless steel mat mesh samples measuring 4 cm2 (2.0 × 2.0 cm). The samples were immersed in the solution for 30 and 60 min, respectively, with vigorous stirring. Following this, the samples were also thoroughly rinsed with DI water and dried in an oven at 100°C.

Characterization of anode stainless steel electrodes
The morphology and elemental composition of anode stainless steel felts and stainless steel meshes were characterized using SEM (Tescan MIRA LMS, Czech) with energy-dispersive X-ray spectroscopy mapping. The elements on the fiber surface were also determined by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha Nexsa). The liquid water contact angle test (Biolin Scientific Theta Lite) is employed to assess the rate at which liquid water penetrates the alcohol-washed and acid-washed stainless steel felts. A confocal laser scanning microscope (KEYENCE VK-X200) was used to characterize the structure of stainless steel felts and stainless steel meshes. The pore size distributions of the anode stainless steel felts with different pore sizes were measured by Mercury Intrusion Porosimetry (Micromeritics AutoPore IV 9510).

AEMWE fabrication and assembly
The AEMWE was constructed using a home-made fixture with serpentine flow fields. They are made of nickel (anode) and titanium (cathode), with a depth of 1.0 mm each. In order to prevent electrolyte leakage, fluorine rubber gaskets with a thickness of 0.4 mm were placed between the two half fixtures. In the middle of the two flow field plates is the prepared MEA. Before testing, the cathode and anode electrodes are soaked in fresh 1 M KOH for 1 h32. Before starting the experiment each day, the specific steps for preparing the electrolyte (1 M KOH) are as follows: dissolve the potassium hydroxide particles in deionized water in a fume hood. After preparing 1000 mL of 1 M KOH electrolyte each time, store it in a sealed glass bottle, and prepare the solution used everyday to ensure it is not contaminated. The cell was clamped by anode and cathode stainless steel end plates with four equally distributed bolts (3.0 N·m torque in each bolt).

Electrochemical measurements
During the experimental procedure, the AEMWE was heated and maintained at 80°C. A peristaltic pump (BT100-2J from Longer Pump) was employed to continuously deliver a 1 M KOH solution to the anode inlet of the AEMWE at a mass flow rate of 5 ml min−1. To ensure stable and reliable performance, the cell underwent a conditioning process (approximately 100 cycles of CV from 1.4 to 2.5 V)44.

For the electrochemical characterization of the AEMWE, an Autolab PGSTAT302N electrochemical workstation, equipped with an additional booster, was utilized. Initially, polarization curves were measured over a current density range of 0 A cm−2 to 5.0 A cm−2. The AEMWE was operated for 100 s at each specific point under low current densities (<1.0 A cm−2) and 300 s at each specific point high current densities (>1.0 A cm−2) to reach a steady state, and the cell voltage was recorded by averaging the voltage values over the last 10 s at each operating point. Subsequently, electrochemical impedance spectroscopy (EIS) measurements were conducted across a frequency range of 40 kHz to 1 Hz.

In the durability testing phase, a 0.1 M KOH solution replaced the previously used 1 M KOH solution for the anode feed. The electrolyte was replenished with water every 24 h to maintain a consistent KOH concentration throughout the experiment. A KUAIQU DC power supply was used to supply current, and the cell voltage was recorded every two hours by means of a paperless recorder.

Breakdown of overpotentials
Tafel approximation45–47, can be used to calculate the mass transport overpotential of AEMWE. The method involves the following steps. First, the total cell voltage (Ecell) was measured, and the HFR (Rs) was obtained from EIS tests. Next, the ohmic overpotential (ηohmic) was calculated by Eq. (1). For the kinetic analysis, the activation overpotential (ηactivation) was described by the Tafel equation (Eq. (2)). The parameters b and i0 were obtained by fitting the iR-free curves (Ecell − ηohmic). Finally, the mass transport overpotential (ηmasstransport) was obtained by Eqs. (3) and (4). The relative equations are as follows:

ηohmic = iRs (1)

ηactivation = b log (i/i0) (2)

Erev = 1.5184 − 1.5421 × 10−3 T + 9.523 × 10−5 T ln T + 9.84 × 10−8 T2 (3)

ηmasstransport = Ecell − Erev − ηactivation − ηohmic (4)

where T (K) is the temperature. i (A cm−2) is the current density.

Distribution of relaxation times (DRT) analysis
The DRT method directly derives electrochemical system relaxation characteristics from impedance spectra. It transforms frequency-domain impedance data into time-domain relaxation time distributions, enabling differentiation of primary and secondary electrochemical reaction processes48. Since electrochemical system polarization impedance is constant and the frequency in the EIS is usually set according to the logarithmic interval, the impedance reconstructed using DRT is typically described by Eq. (5).

ZDRT(f) = R0 + ∫−1+1 γ(ln τ) / (1+j2πfτ) dln τ (5)

Where ZDRT(f) represents the total impedance of the electrochemical system (in the complex plane), R0 represents the ohmic impedance, that is, the impedance value of the electrochemical system when the frequency is infinite, j is an imaginary unit, τ is the characteristic relaxation time, and f is the frequency. Building on the DRT tools framework developed by Ciucci et al.49,50, a genetic algorithm optimizes the frequency factor for a DRT-based EIS reconstruction method.

DRT peaks correspond to electrochemical reaction processes. The identification of the peaks was guided by characteristic frequencies summarized in literature for different electrochemical systems51, along with our previous research35. Specifically, the first peak appears in the range of 1–10 Hz, and is attributed to mass transport processes related to water/gas transfer. The second peak, observed in the range of 50–200 Hz, corresponds to the OER. The third peak, located at 300–800 Hz, is associated with the HER. Notably, the time constants for OER and HER differ by approximately one order of magnitude, allowing them to be clearly distinguished in the DRT plot. The fourth peak, found at frequencies above 1000 Hz, is typically linked to ionic transport processes occurring within the CL or at the interfaces.

Electrochemically active surface area (ECSA) calculation
The ECSA of the stainless steel electrodes was evaluated and compared based on their double-layer capacitance (Cdl). The measurements were conducted in a standard three-electrode configuration within a non-Faradaic potential window (0.774–0.974 V vs. RHE in 1.0 M KOH). CVs were recorded at scan rates from 40 to 160 mVs−1. The capacitive current (Δj = (ja–jc)/2) at the central potential of 0.874 V vs. RHE was plotted against the scan rate. The slope of this linear plot corresponded to Cdl. Since all electrodes are made from the same 316L stainless steel material, their specific capacitance (C) remains unchanged. Therefore, Cdl is directly proportional to the ECSA, and a direct comparison of Cdl values is valid for assessing their relative active surface areas.

High-speed visualization system
For observing micrometer-scale bubble behavior on the electrode surfaces (Supplementary Fig. 1), the Revealer M230M camera, coupled with a microscope, enabled clear visualizations. To record these bubble behaviors, an exposure of 5000 was used, again with a frame rate of 1000 fps. Two videos were captured, illustrating bubble behavior at the surfaces of stainless steel felt and stainless steel gradient square hole mesh, at a current density of 0.4 A cm−2. The experimental setup consisted of a three-electrode electrochemical measurement conducted in a square H-cell (Working electrode: stainless steel electrode, counter electrode: Pt sheet, reference electrode: Hg/HgO (1.0 M KOH), electrolyte: 1.0 M KOH, 200 mL per compartment). For quantitative analysis of bubble distribution, the bubble visualization image is first binarized and a hole-filling algorithm (BW) is applied. Then, the regionprops function is used to identify and extract parameters such as bubble size and count52.

The Revealer M230M high-speed camera, paired with microscale lenses, was utilized to visualize millimeter-scale two-phase flow within the flow field (Supplementary Fig. 1). In this investigation, a recording frame rate of 500 fps and an exposure of 500 were selected to capture the dynamics of the two-phase flow. Six videos were recorded, showcasing the two-phase flow in a configuration comprising stainless steel felt and stainless steel gradient square hole mesh, at current densities of 0.5, 1.0, and 3.0 A cm−2.

Three electrode measurements
Three electrode testing was conducted using 1.5 × 1.5 cm stainless steel felts or meshes as the working electrode, an Hg/HgO electrode as the reference, and a Pt sheet (1.5 × 1.5 cm) as the counter electrode in 1.0 M KOH (100 mL) at room temperature (25±2°C). For Hg/HgO reference electrode, it was calibrated against a reversible hydrogen electrode (RHE) in H2-saturated 1.0 M KOH at room temperature (25±2°C) using a Pt sheet as both working and counter electrode. The average potential was +0.926 V vs. RHE and was used for all potential conversions in three-electrode tests. The anode electrodes underwent conditioning by conducting 10 cyclic voltammetry sweeps within the voltage range of 0.3–2.0 V versus Hg/HgO. Subsequently, their performance was evaluated using linear sweep voltammetry from 0.3 to 2.0 V versus Hg/HgO at a scan rate of 10 mVs−1. To assess the electrolyte resistance and apply iR correction, potentiostatic EIS was performed at the open circuit voltage.

Anode potential and cathode potential test
The anode potential of this AEMWE was recorded in conjunction with the polarization curve test at room temperature (25±2°C). The cathode potential is calculated by the equation below:

Vc = Va − Ecell (6)

Vc (V) and Va (V) denote the anode and cathode potentials, respectively. The anode serves as the working electrode, while the cathode acts as the counter electrode. A 3 mm thick Hg/HgO electrode is utilized as the reference electrode. The potential of the anode is determined by using the open-circuit potential technique with the CHI650E electrochemical workstation. The anode flow field plate features a 4 mm diameter hole that connects to the anode channel inlet, allowing for the insertion of the Hg/HgO reference electrode, which is sealed using PEEK plastic, as shown in Supplementary Fig. 2. By minimizing the distance between the anode and the Hg/HgO reference electrode, the measurement of the anode potential is enhanced in terms of accuracy53.

Water diffusion measurements
We use anode-feeding mode, the cathode inlet is closed. At the anode there is diffusion of water to the cathode and some of the water undergoes HER at the cathode. The unreacted water is pushed to the cathode outlet by the hydrogen gas produced and is collected by silica gel particles, as shown in Supplementary Fig. 3. The weight of the collected water is recorded every hour.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript systematically demonstrates that bubble dynamics - rather than the conventionally emphasized electrochemical surface area - constitutes the performance-determining factor for anion exchange membrane water electrolyzers under industrially relevant current densities. Through modulation of pore architecture and wettability in stainless steel anodes, particularly via designed gradient square-pore electrodes, the authors present significantly enhanced bubble management capabilities and improved electrochemical performance. The work features clear logic, robust methodology, and provides a compelling challenge to existing AEMWE electrode design paradigms, demonstrating both academic significance and practical potential.\nHowever, the authors should address the following comments for improvement:\nMajor comments:\n1. The introduction fails to adequately highlight the reason for investigating how bubble behavior affects mass transport and electrochemical performance in stainless steel electrode. The literature review between Lines 72-92 requires further refinement to be more comprehensive. Furthermore, the authors should explicitly state whether this represents the first implementation of a systematic “gradient structure + bubble control” strategy in AEMWE technology. An expanded literature review or dedicated comparative discussion section should be added to clearly establish this work’s scientific positioning and breakthrough contributions.\n2. Regarding the conclusion that 10 μm pores facilitate bubble release more effectively than 5 μm pores in Figure 2d, a single snapshot may not provide sufficient evidence as it lacks statistical significance. To strengthen this claim, we recommend either: (a) Including supplementary video recordings at the same frame rate as supporting evidence, or (b) Performing quantitative analysis of bubble dynamics (e.g., average detachment size, residence time, and detachment frequency), following the methodology presented in Figure 3 of Article: ACS Appl. Mater. Interfaces 2025, 17, 7109−7118.\n3. In Figure 2f, it would be beneficial to: (a) Clearly label each peak position with arrows, and (b) Provide explicit explanation of the methodology used for identifying the four peaks.\n4. The manuscript currently lacks corresponding discussion of Fig. S9.\n5. Lines 193-194: How was surface oxidation of the stainless steel prevented? Additionally, how were potential impacts of this additional variable on the study’s conclusions mitigated?\n6. Lines 204-205: The statement “...which results in a little change in activation overpotentials...” requires revision. As clearly demonstrated in Fig. 3f, the acid-washed felt exhibits significantly higher activation overpotentials across all current densities compared to the alcohol-washed counterpart. This substantial difference should be properly acknowledged. Furthermore, pleasr add a brief discussion to explain the potential causes of these overpotential differences between the two treatment methods.\n7. Lines 216-217: “The exact reason for the above findings can be explained by the fact that the key processes of water electrolyzer are affected...” is too vague. Clarify: (1) what exact reason, and (2) which key processes. The statement should be made clear.\n8. Please mark the contact angle values in Figures 4a-b. Explain the statistical methods for bubble quantity and detachment size in Figure 4d in the METHODS section. Additionally, format “Daverage” as “Daverage” (with “average” as subscript) in Figs. 4d and 6d.\n9. Citations of current bubble dynamics studies would significantly strengthen the interpretation and impact of these results. Specifically, I recommend referencing: (a) Insights into bubble dynamics in Water Splitting in “INTRODUCTION Section”: Advances in Colloid and Interface Science 343 (2025) 103544; (b) Quantitative studies on hydrogen bubble evolution and mass transport mechanisms in “RESULTS Section”: Phys. Fluids 36, 073302 (2024); (c) Dynamic effects of bubble growth on electrode potential in “INTRODUCTION Section”: Chemical Engineering Journal 500 (2024) 156890; (d) Effect of bubble evolution on electrolytic performance in “RESULTS Section”: Journal of Power Sources 632 (2025) 236356.\n10. Lines 229-230: How to prove that enhanced transmembrane water transport is the cause of improved ohmic polarization?\n11. Figure S15: Since each curve is nearly identical, please provide enlarged partial views to demonstrate the differences between datasets. Additionally, verify all figures to ensure these differences are clearly represented.\nMinor comments:\n(1) The current text exhibits inconsistent tense usage and contains numerous grammatical errors. Please conduct a thorough review and revision. For example: (a) Lines 97-99: “Firstly, enhancing bubbles removal capabilities can ….” → “Firstly, it is demonstrated that enhancing bubble removal capabilities can substantially improve the electrochemical performance of electrodes with low ECSA.” (b) Lines 101-103: “but also simplifies the conventional….” → “but also simplifies the conventional electrode preparation process while simultaneously enhancing energy efficiency and reducing anode fabrication costs by 99.5% compared with traditional IrO2 electrodes. ” (c) Lines 123-125: “After that, how the ....” →“Subsequently, the effect of the pore size of stainless steel felts on the electrochemical performance and bubble behavior of AEMWE was investigated.”\n(2) In many instances, the sentences are excessively long and constitute comma splices. These should be divided into shorter, discrete sentences. For example: Lines 144-147: “The three-electrode test is utilized to consider only the effect of the pore size of the anode electrode on OER (Figure S7a), the 10 μm pore size shows the best OER performance, but the 5 μm pore size with the largest ECSA (Figures S7b and Figure S8) does not.”\n(3) In Fig. 2c caption, provide the full term “Mass transport (MT)” for the abbreviation. Similarly, when this term first appears in the main text, it should be presented in its complete form as “Mass transport (MT)”.\n(4) Line 150: The phrase “the lack of ECSA of the electrode itself” is inaccurate. “Lack” implies complete absence, which may not be the intended meaning. It is recommended to revise this to “the relatively low ECSA of the electrode itself” for better precision.", "reviewer_2": "In this manuscript, the authors experimentally demonstrated that bubble dynamics, which has been an overlooked factor in the performance degradation of AEMWE, plays a critical role under high current density conditions (> 1.0 A cm-2). Based on this insight, they proposed a new electrode design strategy. The study shows that even with a high ECSA, performance can degrade if bubble removal is inefficient, whereas effective bubble management can lead to superior performance even with a lower ECSA. The authors designed a gradient-structured stainless steel square hole mesh electrode to facilitate rapid bubble detachment and promote water diffusion.\nWhile a more quantitative analysis of catalytic activity and mass transport—such as the impact of ECSA reduction—is somewhat lacking, the study aligns well with the scope of Nature Communications by presenting a novel electrode structure and supporting it with diverse experimental evidence focused on bubble behavior.\nComments:\n1. The pore size of the stainless steel felt appears to be a critical variable in this study. However, the basis and method used to determine the pore size are not clearly stated. To enhance the reliability of the results, it would be beneficial to include a pore size characterization or provide additional explanation on how the values were obtained.\n2. The authors claim that smaller pore sizes lead to improved interfacial contact, but the experimental or theoretical basis for this is not clearly presented. It would be helpful to clarify whether this trend is supported by direct evidence in the current study or by previous literature that discusses the effect of pore size on interfacial contact.\n3. The ECSA values vary significantly depending on pore size, with the 5 μm sample showing the highest ECSA. However, as shown in Figure 2c, the activation overpotential of the 5 μm sample is higher than those of the 10 μm and 30 μm samples across the entire current density range. The authors attribute this to better bubble removal in the 10 μm sample, but it remains unclear whether this effect is strong enough to outweigh the benefits of higher ECSA. Additionally, if the influence of bubbles is minimal in the low current density region (e.g., < 0.5 A cm-2), it would be worth discussing whether the trends in this region reflect the ECSA-based expectations.\n4. To clearly demonstrate the effects of acid-washed and alcohol-washed treatments, it is essential to include a comparison with untreated samples. If data showing differences in electrochemical performance or ECSA before and after treatment are available, including them would help better convey the effectiveness of the surface treatments.\n5. Figure 5f presents an impressive durability result, showing stable operation over approximately 400 hours. If the voltage decay rate (mV h-1) during this period is quantified and compared with values reported in the literature, it could serve as a compelling metric to highlight the superior long-term performance demonstrated in this study.", "reviewer_3": "An et al. have implemented a multiscale visualization cell to explore how pore size and the hydrophilicity of a stainless-steel electrode impact the behavior of gas bubbles produced by water electrolysis. The authors find that bubbles cover anode active sites and hinder the transport of water to the membrane and cathode, such that maximizing the electrochemically active surface area does not necessarily lead to improved performance. These problems were then addressed in an engineered stainless-steel mesh leading to an improved anion exchange membrane water electrolyzer for operation at high (5 A cm-2) current densities.\nScientifically, the experiments and findings in this work are not particularly original and they do not differ much from recent papers from the same authors published in the last years (e.g. Wu & An et al. 2025 Adv. Science), except for a new “acid washing” treatment. Practically, the performance of the AEM electrolyzers in this work are not exceptional—many other works have been published (e.g. Yanagi & Peng et al. 2025 Joule) with improved performance at both low and high current densities, with more detailed characterization of transport layers and interfaces. I do not see the novelty or practical importance of this work that justifies its rapid communication in Nature Communications.\n1. There is very little data or discussion on the changes caused by the acid treatment. At early times, it would be helpful to know how the surface chemistry and < µm scale texture influences the gas release properties. For practical relevance, it would be helpful to know how long the difference between the two meshes persist, since the structure and composition of the anode will evolve during the break in period and much longer break in periods are used for industrial electrolysis systems.\n2. Despite the focus on transport properties, only a single anode flow rate appears to have been used (5 mL per min). How do these results change when the flow rate is increased/decreased. Obviously flow rate is an important design parameter for transport and some of the limitations addressed here might be equally addressed by increasing the flow rate of 1 M KOH. The authors should compare the sensitivity of their results at an increased flow rate (>20 mL min-1 as well as a reduced flow rate, e.g. 1 mL min-1 where the effects of gas evolution may be more evident).\n3. I doubt that the comparisons of IrO2, stainless steel, and Ni mesh—which are based on small scale suppliers for R&D research—are applicable to the costs of these materials at scale. Notably, the difference in the cost of Ni felt and IrO2 are not that much… How sensitive are the estimated costs to simple changes in catalyst loading or mesh thickness?"}, "author_response": "Response to Reviewer’s comments on Nature Communications\nmanuscript NCOMMS-25-39851-T\nWe really appreciate Editor and Reviewer for their valuable time and effort in reviewing our\nmanuscript and providing very constructive suggestions to improve the quality and clarity of our\nwork. In the following, we have taken great care to respond to the Editor’s and Reviewers’\ncomments as seen below and are confident that the manuscript has undergone substantial\nimprovements, making it suitable for Nature Communications:\nReviewer #1\nComment\nThis manuscript systematically demonstrates that bubble dynamics - rather than the\nconventionally emphasized electrochemical surface area - constitutes the performance-\ndetermining factor for anion exchange membrane water electrolyzers under industrially\nrelevant current densities. Through modulation of pore architecture and wettability in\nstainless steel anodes, particularly via designed gradient square-pore electrodes, the authors\npresent significantly enhanced bubble management capabilities and improved\nelectrochemical performance. The work features clear logic, robust methodology, and\nprovides a compelling challenge to existing AEMWE electrode design paradigms,\ndemonstrating both academic significance and practical potential.\nHowever, the authors should address the following comments for improvement:\nResponse:\nWe sincerely thank the Reviewer for the valuable comments.\nMajor Comment 1.1\nThe introduction fails to adequately highlight the reason for investigating how bubble\nbehavior affects mass transport and electrochemical performance in stainless steel electrode.\nThe literature review between Lines 72-92 requires further refinement to be more\ncomprehensive. Furthermore, the authors should explicitly state whether this represents the\nfirst implementation of a systematic “gradient structure + bubble control” strategy in\nAEMWE technology. An expanded literature review or dedicated comparative discussion\nsection should be added to clearly establish this work’s scientific positioning and\nbreakthrough contributions.\nResponse to 1.1:\nWe thank the Reviewer for the suggestions. We have already included a discussion of stainless steel\nelectrodes in the introduction, providing a detailed explanation of the reasons for investigating them\nand how bubble behaviors affect mass transport and electrochemical performance in stainless steel\nelectrodes based on our previous work.\nThe scientific focus of this study is not on designing electrodes. Instead, it centers on applying a\nmethod to directly reflect the more dominant role of bubble dynamics compared to ECSA in\nAEMWE under high current density conditions. This method combines a “gradient structure” with\n“bubble control” function, which is used on AEMWE stainless steel electrodes for the first time.\nRelevant discussion has been added into our revised manuscript:\nStainless steel felt has emerged as a highly attractive integrated anode electrode in AEMWEs,\nserving a dual function as both an excellent OER activity under alkaline conditions and the porous\ntransport layer (PTL)32–34. Its appeal lies in its relative ease of preparation and inherent structural\nrobustness, offering a potentially cost-effective and durable alternative to more expensive catalyst-\ncoated substrate (CCS) electrodes32. Furthermore, in our previous research35, we found that\nchanges in the surface morphology of stainless steel felt, such as reduced surface hydrophilicity,\nlead to a decrease in the number of bubbles generated and an increase in the diameter of bubble\ndetachment, significantly increasing activation and mass transport overpotentials. Based on this,\nexploring multiple parameters such as pore size and hydrophilicity of stainless steel electrodes is\na relatively simple and direct way to investigate which of the electrode bubble dynamics and ECSA\nis the main factor affecting the electrochemical performance of AEMWE under high current\nconditions.\nTherefore, we choose 316L gradient stainless steel mesh. This innovative design not only surpasses\nthe electrochemical performance of stainless steel felt electrodes in AEMWE but also simplifies\nthe conventional electrode preparation process while simultaneously enhancing energy efficiency\nand reducing anode fabrication costs by 99.86% compared with traditional IrO electrodes. This\n2\nalso represents the first systematic application of the 'gradient structure + bubble control' strategy\nfor integrated stainless steel electrodes in AMEWE. However, the scientific positioning of this\nstudy is not on electrode design, but rather on using this design to reflect the more dominant role\nof bubble dynamics over electrode ECSA in AEMWE under high current conditions.\nMajor Comment 1.2\nRegarding the conclusion that 10 μm pores facilitate bubble release more effectively than 5\nμm pores in Figure 2d, a single snapshot may not provide sufficient evidence as it lacks\nstatistical significance. To strengthen this claim, we recommend either: (a) Including\nsupplementary video recordings at the same frame rate as supporting evidence, or (b)\nPerforming quantitative analysis of bubble dynamics (e.g., average detachment size,\nresidence time, and detachment frequency), following the methodology presented in Figure\n3 of Article: ACS Appl. Mater. Interfaces 2025, 17, 7109−7118.\nResponse to 1.2:\nWe fully agree with the Reviewer and have added supplementary video recordings at the same\nframe rate as supporting evidence in the attached video files (Video S1). Screenshots of videos at\ndifferent times are as follows:\nMiddle\nEnd\nIn Figure 2f, it would be beneficial to: (a) Clearly label each peak position with arrows, and\n(b) Provide explicit explanation of the methodology used for identifying the four peaks.\nResponse to 1.3:\nThanks for these valuable suggestions, we have modified Figure 2f and added the relative\ndiscussion. The identification of the peaks was guided by characteristic frequencies summarized in\nliterature for different electrochemical systems51. Specifically, the first peak appears in the range of\n1-10 Hz, and is attributed to mass transport processes related to water/gas transfer. The second peak,\nobserved in the range of 50-200 Hz, corresponds to the OER. The third peak, located at 300-800\nHz, is associated with the HER. Notably, the time constants for OER and HER differ by\napproximately one order of magnitude, allowing them to be clearly distinguished in the DRT plot.\nThe fourth peak, found at frequencies above 1000 Hz, is typically linked to ionic transport processes\noccurring within the catalyst layer or at the interface.\nFigure 2. (f) EIS curves and DRT analysis at 1.0 A cm-2.\n[51] Ranz, M., Grabner, B., Schweighofer, B., Wegleiter, H. & Trattner, A. Dynamics of anion exchange membrane electrolysis:\nUnravelling loss mechanisms with electrochemical impedance spectroscopy, reference electrodes and distribution of relaxation times.\nJ. Power Sources 605, 234455 (2024).\nAction to 1.3\nRelevant modified Figure 2 and discussion have been added into our revised manuscript:\nThe identification of the peaks was guided by characteristic frequencies summarized in literature\nfor different electrochemical systems51, along with our previous research35. Specifically, the first\npeak appears in the range of 1-10 Hz, and is attributed to mass transport processes related to\nwater/gas transfer. The second peak, observed in the range of 50-200 Hz, corresponds to the OER.\nThe third peak, located at 300-800 Hz, is associated with the HER. Notably, the time constants for\nOER and HER differ by approximately one order of magnitude, allowing them to be clearly\ndistinguished in the DRT plot. The fourth peak, found at frequencies above 1000 Hz, is typically\nlinked to ionic transport processes occurring within the catalyst layer or at the interface.\nwith different pore sizes. (b) Tafel slope and iR-free cell voltage. (c) Breakdown of each\noverpotential at different current densities. (operating conditions: 80℃, PiperION A membrane\n(60 µm), the Pt/C cathode at a Pt loading of 0.5 mg cm-2, the anode is supplied with 1M KOH\nsolution at 5 ml min-1). (d) SEM image and bubble behaviors in stainless steel felts with different\npore sizes. (operating conditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane\ndirection parallel to the ground). (e) Schematic illustration depicting how varying pore sizes\ninfluence bubble behaviors at the anode and the diffusion of water through the membrane towards\nthe cathode. (f) EIS curves and DRT analysis at 1.0 A cm-2. (Mass transport (MT))\nThe manuscript currently lacks corresponding discussion of Fig. S9.\nResponse to 1.4:\nWe agree with the Reviewer and have added the relative discussion. As some figures were added,\nthe sequence of Figure S9 has been changed to Figure S10. The EIS curves at other current densities\ncan be found in Figure S10. Notably, the ohmic, activation, and mass transport impedance all\ndecrease for each stainless steel felt as the current density increases.\nAction to 1.4\nRelevant discussion has been added into our revised manuscript:\nThe EIS curves at other current densities can be found in Figure S10, and stainless steel felt with\ndifferent pore sizes also displays differences similar to those shown in Figure 2(f). Notably, the\nohmic, activation, and mass transport impedances all decrease for each stainless steel felt as the\ncurrent density increases. This occurs because higher current densities promote rapid bubble\nformation and release, which helps to efficiently eliminate bubbles from previous cycles and\nthereby reduce each impedance component.\nFigure S10. EIS curves of stainless steel felts with different pore sizes at different current densities\n(0.5-3 A cm-2): (a) 5 μm, (b) 10 μm, (c) 30 μm, (d) 50 μm.\nLines 193-194: How was surface oxidation of the stainless steel prevented? Additionally, how\nwere potential impacts of this additional variable on the study’s conclusions mitigated?\nResponse to 1.5:\nWe thank the Reviewer for the comment. In fact, nitric acid (HNO ) indeed has the oxidizing effect\n3\non the stainless steel fiber, which can reduce ECSA of electrode (Figure 3c) due to the accumulation\nof a dense chromium-rich oxide layer (Figure S16e). However, due to the roughness of the electrode\nsurface (Figures 4a-b) caused by HNO , the hydrophilicity of acid-washed felt is significantly\n3\nenhanced, which in turn improves its bubble dynamics (Figures 4a-b) and leads to a marked\nimprovement in performance (Figure 3d) of AEMWE at high current densities. This can be viewed\nas a special experimental design (reduced ECSA but improved bubble dynamics) to validate\nwhether ECSA or bubble dynamics is the key factor to dominate the performance of AEMWE at\nhigh current densities.\nThank you very much for the good question you raised. Regarding your suggestions, we also added\nhydrochloric acid (HCl) treatment samples to mitigate the surface oxidation, which not only\nincreases ECSA but also presents better performance than alcohol-washed electrode (Figure S20).\nCombined with HNO and HCl treatment samples, we are confident with our conclusions that\n3\nbubbles dominate the performance rather than ECSA at high current densities.\nFigure 3. Electrochemical performance of the alcohol-washed and acid-washed stainless steel felts\nin 1 M KOH through a standard three-electrode system: (c) Capacitive currents as a function of\nscan rate for ECSA. Electrochemical performance of the alcohol-washed and acid-washed\nstainless steel felts through a zero-gap MEA system: (d) Polarization and iR-free curves.\nFigure 4. SEM image, contact angle tests and bubble behaviors in stainless steel felts. (operating\nconditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane direction parallel to the\nground): (a) Alcohol-washed stainless steel felt. (b) acid-washed stainless steel felt.\nC, Cr, Fe, Ni and O.\nFigure S20. Comparison of electrochemical performance of HCl-washed stainless steel mat\nmeshes in 1 M KOH through a standard three-electrode system: (a) LSV curves and enlarged\npartial views, (b) capacitive currents as a function of scan rate for ECSA.\nAction to 1.5\nRelevant discussion has been added into our revised manuscript:\nThis can be viewed as a special experimental design (reduced ECSA but improved bubble\ndynamics) to validate whether ECSA or bubble dynamics is the key factor to dominate the\nperformance of AEMWE at high current densities. In addition, we also add hydrochloric acid (HCl)\ntreatment samples to mitigate the surface oxidation, which not only increases ECSA but also\npresents better performance than alcohol-washed electrode (Figures S20-S22).\nFigure S20. Comparison of electrochemical performance of the stainless steel felts and HCl-\nwashed stainless steel mat meshes in 1 M KOH through a standard three-electrode system: (a)\nLSV curves and enlarged partial views, (b) capacitive currents as a function of scan rate for\nECSA.\nvarious scan rates (40-160 mV s-1) for OER: (a) felt, (b) mat mesh etched with HCl for 0 h,\n(c) mat mesh etched with HCl for 0.5 h, (d) mat mesh etched with HCl for 1 h.\nFigure S22. SEM image of: (a) felt, (b) mat mesh etched with HCl for 0 h, (c) mat mesh etched\nwith HCl for 0.5 h, (d) mat mesh etched with HCl for 1 h.\nLines 204-205: The statement “...which results in a little change in activation\noverpotentials...” requires revision. As clearly demonstrated in Fig. 3f, the acid-washed felt\nexhibits significantly higher activation overpotentials across all current densities compared\nto the alcohol-washed counterpart. This substantial difference should be properly\nacknowledged. Furthermore, please add a brief discussion to explain the potential causes of\nthese overpotential differences between the two treatment methods.\nResponse to 1.6:\nWe agree with the Reviewer and we acknowledge that, compared to alcohol-washed felt, acid-\nwashed felt exhibits significantly higher activation overpotentials across all current densities (Tafel\nslope: 49.39 mV dec-1 vs. 47.04 mV dec-1). This reason may be that while the enhanced bubble\ndynamics of acid-washed felt improves bubble coverage on the electrode surface, they fail to\nsufficiently compensate for its inherently low ECSA. Nevertheless, this bubble dynamics\nenhancement yields more pronounced improvements in mass transport and ohmic overpotentials,\nultimately resulting in a substantial overall performance improvement through combined effects\n(Figures 3e-f).\nFigure 3. Electrochemical performance of the alcohol-washed and acid-washed stainless steel felts\nthrough a zero-gap MEA system: (d) Polarization and iR-free curves. (e) Tafel slope and iR-free\ncell voltage. (f) Breakdown of each overpotential at different current densities. (operating\nconditions: 80℃, PiperION A membrane (60 µm), the Pt/C cathode at a Pt loading of 0.5 mg cm-\n2, the anode is supplied with 1M KOH solution at 5 ml min-1).\nAction to 1.6\nRelevant discussion has been added into our revised manuscript:\nIn particular, compared with alcohol-washing, acid-washing increases the Tafel slope (47.04 mV\ndec-1 to 49.39 mV dec-1) (Figure 3e), which results in a significant increase in activation\noverpotentials at each current density. This reason may be that while the enhanced bubble\ndynamics of acid-washed felt improves bubble coverage on the electrode surface, they fail to\nsufficiently compensate for its inherently low ECSA. Nevertheless, this bubble dynamics\nenhancement yields more pronounced improvements in mass transport and ohmic overpotentials\n(Figure 3f), ultimately resulting in a substantial overall performance enhancement through\ncombined effects (Figure 3d). The subsequent visualization results will help directly prove this\nhypothesis.\nLines 216-217: “The exact reason for the above findings can be explained by the fact that the\nkey processes of water electrolyzer are affected...” is too vague. Clarify: (1) what exact\nreason, and (2) which key processes. The statement should be made clear.\nResponse to 1.7:\nWe agree with the Reviewer and have provided the clearer statement. The exact reason for the\nperformance improvement of AEMWE caused by acid-washed felt can be attributed to the\nimproved bubble dynamics, which optimizes its key processes (Figure 1): (1) Reduced active site\ncoverage mitigates anode activation and mass transport polarization; 2) Promoted water across the\nmembrane improves ohmic polarization; 3) Increased transmembrane water alleviates cathode\nwater shortage-induced activation.\nSpecifically, microscale visualization characterization is employed to elucidate the relationship\nbetween electrode structure, electrochemical performance, and bubble behaviors. SEM imaging\n(Figures 4a-b) reveals acid-washing-induced surface roughening, while contact angle\nmeasurements confirm 38% hydrophilicity improvement (wetting time reduction from 56.6 ms to\n41.1 ms), promoting faster bubble evolution. And in-situ bubble tracking (Figures 4a-d) quantifies\noptimized bubble management (also see video S2). Acid-washed felts generate 20% smaller\nbubbles (56.4 to 44.9 μm diameter) with 33% higher number density (126 to 168 counts).\nCrucially, the acid-washed stainless steel felt also consistently presents better OER performance\nunder the constant-current test (Figure 4c), proving that the above key process 1. Direct monitoring\nof cathode-collected water (Figure 4e) substantiates the above key processes 2 and 3.\nFinally, this improved bubble dynamics is ultimately identified as improving performance through\nabove three key processes (Figure 4f).\nFigure 1. Bubbles dynamics matters high-rate water electrolysis efficiency and have a major\nimpact in three areas: cover active sites at the anode, block water diffusion through the membrane,\nand water shortage at the cathode.\nconditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane direction parallel to the\nground): (a) Alcohol-washed stainless steel felt. (b) acid-washed stainless steel felt. (c) Time\ntransition of potential vs Hg/HgO at 0.4 A cm-2. (d) The distribution of bubble sizes counted in\nhigh-speed camera images. (e) Schematic illustration depicting how hydrophilicity influence\nbubble behaviors at the anode and the diffusion of water through the membrane towards the\ncathode. (f) The amount of water collected at the cathode across the membrane as a function of\ntime for AMEWEs with alcohol-washed and acid-washed stainless steel felts.\nAction to 1.7\nRelevant discussion has been modified into our revised manuscript:\nThe exact reason for the performance improvement of AEMWE caused by acid-washed felt can be\nattributed to the improved bubble dynamics, which optimizes its key processes: 1) Reduced active\nsite coverage mitigates anode activation and mass transport polarization; 2) Promoted water\nacross the membrane improves ohmic polarization; 3) Increased transmembrane water alleviates\ncathode water shortage-induced activation. Specifically, microscale visualization\n12\ncharacterization is employed to elucidate the relationship between electrode structure,\nelectrochemical performance, and bubble behaviors. SEM imaging (Figures 4a-b) reveals acid-\nwashing-induced surface roughening, while contact angle measurements confirm 38%\nhydrophilicity improvement (wetting time reduction from 56.6 ms to 41.1 ms), promoting faster\nbubble evolution38. And in-situ bubble tracking31 (Figures 4a-d) quantifies optimized bubble\nmanagement (also see video S2). Acid-washed felts generate 20% smaller bubbles (56.4 to 44.9\nμm diameter) with 33% higher number density (126 to 168 counts). Crucially, the acid-washed\nstainless steel felt also consistently presents better OER performance under the constant-current\ntest (Figure 4c), proving that the above key process (1). Direct monitoring of cathode-collected\nwater (Figure 4e) substantiates the above key processes (2) and (3). Finally, this improved bubble\ndynamics is ultimately identified as improving performance through above three key processes\n(Figure 4f).\nMajor Comment 1.8\nPlease mark the contact angle values in Figures 4a-b. Explain the statistical methods for\nbubble quantity and detachment size in Figure 4d in the METHODS section. Additionally,\nformat “Daverage” as “Daverage” (with “average” as subscript) in Figs. 4d and 6d.\nResponse to 1.8:\nThanks for these valuable suggestions. We have marked the contact angle values in Figures 4a-b\nand added the relative discussion about statistical methods for bubble quantity and detachment size.\nFor quantitative analysis of bubble distribution, the bubble visualization image is first binarized and\na hole-filling algorithm (BW) is applied. Then, the regionprops function is used to identify and\nextract parameters such as bubble size and count55. In addition, we have also modified Figures 4\nand 6 based on the suggestions.\n55. Yuan, S. et al. Bubble management in PEM water electrolysis via imprinting patterned grooves on catalyst layer. Int. J. Heat\nMass Transfer 212, 124249 (2023).\nconditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane direction parallel to the\nground): (a) Alcohol-washed stainless steel felt. (b) acid-washed stainless steel felt. (d) The\ndistribution of bubble sizes counted in high-speed camera images.\nFigure 6. (d) The distribution of bubble sizes counted in high-speed camera images.\nAction to 1.8\nRelevant modified figure and discussion have been added into our revised manuscript:\nFor quantitative analysis of bubble distribution, the bubble visualization image is first binarized\nand a hole-filling algorithm (BW) is applied. Then, the regionprops function is used to identify and\nextract parameters such as bubble size and count 55.\nconditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane direction parallel to the\nground): (a) Alcohol-washed stainless steel felt. (b) acid-washed stainless steel felt. (c) Time\ntransition of potential vs Hg/HgO at 0.4 A cm-2. (d) The distribution of bubble sizes counted in\nhigh-speed camera images. (e) Schematic illustration depicting how hydrophilicity influence\nbubble behaviors at the anode and the diffusion of water through the membrane towards the\ncathode. (f) The amount of water collected at the cathode across the membrane as a function of\ntime for AMEWEs with alcohol-washed and acid-washed stainless steel felts.\nof felt, mat mesh with sieves of 1250 and square hole mesh with sieves of 60. (b) The amount of\nwater collected at the cathode across the membrane as a function of time for AMEWEs with\ndifferent stainless steel electrode designs. (c) Bubble behaviors in different stainless steel electrode\ndesigns. (operating conditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane\ndirection parallel to the ground). (d) The distribution of bubble sizes counted in high-speed camera\nimages.\nCitations of current bubble dynamics studies would significantly strengthen the\ninterpretation and impact of these results. Specifically, I recommend referencing: (a)\nInsights into bubble dynamics in Water Splitting in “INTRODUCTION Section”:\nAdvances in Colloid and Interface Science 343 (2025) 103544; (b) Quantitative studies on\nhydrogen bubble evolution and mass transport mechanisms in “RESULTS Section”:\nPhys. Fluids 36, 073302 (2024); (c) Dynamic effects of bubble growth on electrode potential in\n“INTRODUCTION Section”: Chemical Engineering Journal 500 (2024) 156890; (d) Effect\nof bubble evolution on electrolytic performance in “RESULTS Section”: Journal of Power\nSources 632 (2025) 236356.\nResponse to 1.9:\nWe thank the Reviewer for the suggestions and have added the relevant citations.\nAction to 1.9\nRelevant citations have been added into our revised manuscript:\nThe produced oxygen bubbles at the anode notably diminish the active sites of the electrodes18,\ndisturb local pH and ion concentration gradients19, resulting in an elevated surface overpotential\nat a specific current density18.\nAnd in-situ bubble tracking35 (Figures 4a-d) quantifies optimized bubble management (also see\nvideo S2). Acid-washed felts generate 20% smaller bubbles (56.4 to 44.9 μm diameter) with 33%\nhigher number density (126 to 168 counts). This can reduce coverage and mass transport\nresistance on the electrode surface40.\nIn addition, bubbles also hinder water diffusion through the membrane, and the diameter of the\nbubbles is related to the ohmic overpotential20. Especially in the anode-feeding mode, bubble\nrestrictions on water diffusion across the membrane lead to water shortage at the cathode,\nresulting in insufficient supply of reactants21.\nSEM imaging (Figures 4a-b) reveals acid-washing-induced surface roughening, while contact\nangle measurements confirm 38% hydrophilicity improvement (wetting time reduction from 56.6\nms to 41.1 ms), promoting faster bubble evolution39.\n(a)→18. Unveiling micro- and nanoscale bubble dynamics for enhanced electrochemical water\nsplitting. Advances in Colloid and Interface Science 343, 103544 (2025).\n(b)→40. Hydrogen bubble evolution and gas transport mechanism on a microelectrode\ndetermined by cathodic potential and temperature. Physics of Fluids 36, (2024).\n(c)→20. Evolution of hydrogen bubbles on a microelectrode driven by constant currents and its\nimpact on potential response. Chemical Engineering Journal 500, 156890 (2024).\n(d)→39. How dual hydrogen bubble evolution inhibits electrolytic performance. Journal of\nPower Sources 632, 236356 (2025).\nLines 229-230: How to prove that enhanced transmembrane water transport is the cause of\nimproved ohmic polarization?\nResponse to 1.10:\nWe agree with the Reviewer and have added the relative discussion. Figure 4 illustrates the\nprinciples of how the acid-washed felt improves bubble dynamics at the anode compared to the\nalcohol-washed felt, which helps facilitate water across the membrane from the anode to the cathode.\nThis is demonstrated by the water collected at the cathode, i.e., water mass across the membrane,\nas shown in Figure 4f. Accordingly, in DRT results (Figure 3g), the fourth peak reveals the OH-\ntransport process inside the electrode and at the interface, which is closely linked to water transport.\nBased on this, when comparing alcohol-washed and acid-washed electrodes, we observe that the\narea under the fourth peak is smaller in acid-washed electrodes. This indicates a lower OH- transport\nimpedance, meaning that the enhanced transmembrane water transport is the cause of improved\nohmic polarization.\nFigure 4. (e) Schematic illustration depicting how hydrophilicity influence bubble behaviors at the\nanode and the diffusion of water through the membrane towards the cathode. (f) The amount of\nwater collected at the cathode across the membrane as a function of time for AMEWEs with\nalcohol-washed and acid-washed stainless steel felts.\nFigure 3. Electrochemical performance of the alcohol-washed and acid-washed stainless steel felts\nin 1 M KOH through a standard three-electrode system: (g) EIS curves and DRT analysis at 3.0 A\ncm-2.\nRelevant discussion has been added into our revised manuscript:\nThe P4 peak shows the OH- transport inside the electrode and at the interface, related to water\nmovement. Acid-washed electrodes have a smaller peak 4 area than alcohol-washed ones,\nindicating lower OH- transport impedance. This suggests that water transport from the anode to\nthe cathode has been improved (Figure 3g).\nMajor Comment 1.11\nFigure S15: Since each curve is nearly identical, please provide enlarged partial views to\ndemonstrate the differences between datasets. Additionally, verify all figures to ensure these\ndifferences are clearly represented.\nResponse to 1.11:\nWe fully agree with the Reviewer and have provided enlarged partial views. As some figures were\nadded, the sequence of Figure S11 and Figure S15 have been changed to Figure S20 and Figure\nS29, respectively.\nAction to 1.11\nRelevant figures have been modified into our revised manuscript:\nFigure S20. Comparison of electrochemical performance of the stainless steel felts and HCl-\nwashed stainless steel mat meshes in 1 M KOH through a standard three-electrode system: (a)\nLSV curves and enlarged partial views, (b) capacitive currents as a function of scan rate for\nECSA.\nFigure S29. LSV curves of the different stainless steel electrode designs.\nThe current text exhibits inconsistent tense usage and contains numerous grammatical\nerrors. Please conduct a thorough review and revision. For example: (a) Lines 97-99:\n“Firstly, enhancing bubbles removal capabilities can ….” → “Firstly, it is demonstrated\nthat enhancing bubble removal capabilities can substantially improve the electrochemical\nperformance of electrodes with low ECSA.” (b) Lines 101-103: “but also simplifies the\nconventional….” → “but also simplifies the conventional electrode preparation process\nwhile simultaneously enhancing energy efficiency and reducing anode fabrication costs by\n99.5% compared with traditional IrO2 electrodes. ” (c) Lines 123-125: “After that, how\nthe ....” →“Subsequently, the effect of the pore size of stainless steel felts on the\nelectrochemical performance and bubble behavior of AEMWE was investigated.”\nResponse to 1.1:\nThanks a lot for pointing it out, we have carefully checked throughout the manuscript and conducted\nthe revision based on the above kind suggestions.\nMinor Comment 1.2\nIn many instances, the sentences are excessively long and constitute comma splices. These\nshould be divided into shorter, discrete sentences. For example: Lines 144-147: “The three-\nelectrode test is utilized to consider only the effect of the pore size of the anode electrode on\nOER (Figure S7a), the 10 μm pore size shows the best OER performance, but the 5 μm\npore size with the largest ECSA (Figures S7b and Figure S8) does not.”\nResponse to 1.2:\nThanks for the kind suggestions, we agree with the Reviewer and have revised in the manuscript.\nAction to 1.2\nRelevant revisions have been made, for example:\nThree-electrode testing isolates the effect of anode pore size on OER performance (Figure S8a)\nand calculates the ECSA of all electrodes. For ECSA (Figures S8b), as the pore size increases,\nECSA begins to decrease instead, which consistent with performance differences at the low current\ndensity region in zero-gap MEA electrolyzer (e.g., < 0.5 A cm-2) (Figures S8c). A 5 μm pore size\nhas the largest ECSA (Figures S8b and S9), it performs better than 30 and 50 μm but worse than\n10 μm.\nMinor Comment 1.3\nIn Fig. 2c caption, provide the full term “Mass transport (MT)” for the abbreviation.\nSimilarly, when this term first appears in the main text, it should be presented in its complete\nform as “Mass transport (MT)”.\nResponse to 1.3:\nWe agree with the Reviewer and have provided the full term “Mass transport (MT)” for the\nabbreviation.\nFigure 2. (a) Polarization and iR-free curves for AEMWE with the stainless steel felt-based anode\nwith different pore sizes. (b) Tafel slope and iR-free cell voltage. (c) Breakdown of each\noverpotential at different current densities. (operating conditions: 80℃, PiperION A membrane\n(60 µm), the Pt/C cathode at a Pt loading of 0.5 mg cm-2, the anode is supplied with 1M KOH\nsolution at 5 ml min-1). (d) SEM image and bubble behaviors in stainless steel felts with different\npore sizes. (operating conditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane\ndirection parallel to the ground). (e) Schematic illustration depicting how varying pore sizes\ninfluence bubble behaviors at the anode and the diffusion of water through the membrane towards\nthe cathode. (f) EIS curves and DRT analysis at 1.0 A cm-2. (Mass transport (MT))\nMinor Comment 1.4\nLine 150: The phrase “the lack of ECSA of the electrode itself” is inaccurate. “Lack” implies\ncomplete absence, which may not be the intended meaning. It is recommended to revise this\nto “the relatively low ECSA of the electrode itself” for better precision.\nResponse to 1.4:\nWe fully agree with the Reviewer and have revised in the manuscript.\nComment\nIn this manuscript, the authors experimentally demonstrated that bubble dynamics, which\nhas been an overlooked factor in the performance degradation of AEMWE, plays a critical\nrole under high current density conditions (> 1.0 A cm-2). Based on this insight, they\nproposed a new electrode design strategy. The study shows that even with a high ECSA,\nperformance can degrade if bubble removal is inefficient, whereas effective bubble\nmanagement can lead to superior performance even with a lower ECSA. The authors\ndesigned a gradient-structured stainless steel square hole mesh electrode to facilitate rapid\nbubble detachment and promote water diffusion.\nWhile a more quantitative analysis of catalytic activity and mass transport—such as the\nimpact of ECSA reduction—is somewhat lacking, the study aligns well with the scope of\nNature Communications by presenting a novel electrode structure and supporting it with\ndiverse experimental evidence focused on bubble behavior.\nResponse:\nWe sincerely thank the Reviewer for the valuable comments.\nComment 2.1\nThe pore size of the stainless steel felt appears to be a critical variable in this study. However,\nthe basis and method used to determine the pore size are not clearly stated. To enhance the\nreliability of the results, it would be beneficial to include a pore size characterization or\nprovide additional explanation on how the values were obtained.\nResponse to 2.1:\nWe thank the Reviewer for the valuable suggestions. We procured stainless steel felt with different\npore sizes from the supplier and they provided the pore size specifications. Based on your\nsuggestion, it is important to confirm the actual pore size value. Firstly, we obtained Bubble Point\nTest data from the supplier (Table 1). Additionally, to verify the accuracy of the supplier’s data,\nwe performed further validation using mercury intrusion porosimetry (MIP) on samples with pore sizes\nof 5 μm, 10 μm, and 50 μm (Figure S7). These tests ultimately confirmed the reliability of the\nsupplier’s data provided.\nTable 1. Bubble point pressure and maximum pore size of stainless steel felt with different pore\nsizes provided by the manufacturer\nMaximum pore size Bubble point\n(μm) pressure (Pa)\n5 6800\n10 3700\n15 2450\n20 1900\n25 1550\n30 1200\n40 950\n50 750\n60 630\nAction to 2.1\nRelevant discussion, table 1 and MIP results have been added into our revised manuscript:\nMeanwhile, we confirm the optimal torque and thickness of the stainless steel felt according to\nFigures S5 and S6, and in the following experiments, only the pore size of the stainless steel felt is\nvaried (5–50 μm), which is determined in Table 1 and Figure S7.\nTable 1. Bubble point pressure and maximum pore size of stainless steel felt with different pore\nsizes provided by the manufacturer\nMaximum pore size Bubble point\n(μm) pressure (Pa)\n5 6800\n10 3700\n15 2450\n20 1900\n25 1550\n30 1200\n40 950\n50 750\n60 630\nComment 2.2\nThe authors claim that smaller pore sizes lead to improved interfacial contact, but the\nexperimental or theoretical basis for this is not clearly presented. It would be helpful to\nclarify whether this trend is supported by direct evidence in the current study or by previous\nliterature that discusses the effect of pore size on interfacial contact.\nResponse to 2.2:\nWe thank the Reviewer for the valuable suggestions. We confirmed that for all AEMWEs\nassembled with stainless steel felt of different pore sizes, the same fixture, other key components,\nand torque settings are used, with only the pore size of the anode stainless steel felt varied. As a\nresult, the high-frequency resistance (HFR) value accurately indicated the quality of interfacial\ncontact (HFR is the intersection point between the EIS spectrum in the high-frequency region and\nthe real axis (Z 'axis)). Figure 2f shows that stainless steel felts with smaller pore sizes have lower\nHFR values, indicating enhanced interfacial contact. Additionally, we have also cited the relevant\nprevious literature to directly support “smaller pore sizes lead to improved interfacial contact”.\nThe literature mentions that “The anode potentials show a decrease of potential loss going from\nSS100 to SS10. We hypothesize that this performance improvement is due to the larger interfacial\ncontact area and smaller pore size enabled by the smaller fibers in the SS10 anodes.”\n[FIGURE REDACTED]\n33. Xu, Q. et al. Integrated Reference Electrodes in Anion-Exchange-Membrane Electrolyzers: Impact of Stainless-Steel Gas-\nDiffusion Layers and Internal Mechanical Pressure. ACS Energy Lett. 6, 305–312 (2021).\nAction to 2.2\nRelevant discussion has been added into our revised manuscript:\nA smaller pore size improves interfacial contact between the rib and the electrode33, leading to\nreduced ohmic overpotential.\nThe ECSA values vary significantly depending on pore size, with the 5 μm sample showing\nthe highest ECSA. However, as shown in Figure 2c, the activation overpotential of the 5 μm\nsample is higher than those of the 10 μm and 30 μm samples across the entire current density\nrange. The authors attribute this to better bubble removal in the 10 μm sample, but it\nremains unclear whether this effect is strong enough to outweigh the benefits of higher\nECSA. Additionally, if the influence of bubbles is minimal in the low current density region\n(e.g., < 0.5 A cm-2), it would be worth discussing whether the trends in this region reflect the\nECSA-based expectations.\nResponse to 2.3:\nWe thank the Reviewer for the valuable suggestions.\nTo validate the impact of higher ECSA, we have added additional tests. Firstly, we ensured that\nstainless steel felts have consistent pore size (20μm). Subsequently, we ink-sprayed a mixture of\nthe noble IrO and the ionomer onto one side of the stainless steel felt to increase the ECSA of\n2\nelectrode (can be also called porous transport electrode (PTE)). For the ionomer, we provided the\nhydrophilic PiperIon and the hydrophobic Nafion. The surface morphology and composition\ndistribution of these three samples (uncoated stainless steel felt, Nafion+IrO coated stainless steel\n2\nfelt and PiperIon+IrO coated stainless steel felt) were characterized by SEM and energy-dispersive\nElectrochemical data (Figures S26-27) indicated that Nafion+IrO exhibits the most pronounced\n2\nincrease in ECSA. However, it significantly degrades AEMWE performance at elevated current\ndensities, with a voltage increase of approximately 600 mV at 3.0 A cm-2 compared to uncoated\nstainless steel felt. PiperIon+IrO exhibited a relatively smaller increase in ECSA, and its\n2\nperformance was slightly superior to that of the uncoated stainless steel felt. However, after\nexcluding ohmic overpotentials, the performance gap between PiperIon+IrO and the uncoated\n2\nstainless steel felt in AEMWE gradually narrowed to negligible levels as the current density\nincreased to 5.0 A cm-2. Contact angle measurements and bubble behaviors visualization (Figure\nS28) indicated that the uncoated stainless steel felt exhibits superhydrophilic surface wettability,\nwith a higher bubble count and smaller bubble diameters detaching from the electrode. For the other\ntwo samples, large bubbles covering the electrode surface prevent Nafion+IrO from fully\n2\nleveraging its advantage of higher ECSA. PiperIon+IrO exhibited hydrophilic surface wettability.\n2\nthose on the uncoated stainless steel felt. This was the primary reason for its performance\nimprovement, becoming negligible at high current densities. Therefore, we conclude that as current\ndensity increases, electrodes with weak bubble removal capability and high ECSA do not\noutperform and may even significantly underperform those with strong bubble removal capability.\nThis outcome depends on the method used to enhance ECSA. It is advisable to use electrodes that\nstrike a balance between enhancing ECSA and maintaining strong bubble removal capability.\nAdditionally, we provide a magnified view of the <0.5 A cm-2 region to demonstrate that trends in\nthis area align with ECSA-based expectations, indicating negligible bubble influence at low current\ndensities (Figure S8c).\nstainless steel felts with different pore sizes.\nAction to 2.3\nRelevant discussion has been added into our revised manuscript:\nThree-electrode testing isolates the effect of anode pore size on OER performance (Figure S8a)\nand calculates the ECSA of all electrodes. For ECSA (Figures S8b), as the pore size increases,\nECSA begins to decrease instead, which consistent with performance differences at the low current\ndensity region in zero-gap MEA electrolyzer (e.g., < 0.5 A cm-2) (Figures S8c).\nFinally, it can be expected that at low current densities, the electrochemical performance depends\non ECSA value. However, at high current densities, the bubble dynamics matters high-rate water\nelectrolysis efficiency of AEMWE more than that of ECSA. The initial electrode design principle\nat high current densities can be summarized as ensuring sufficient ECSA and enabling rapid\ndetachment while maintaining smaller bubble size, so that the activation and mass transport\noverpotentials can be both reduced.\nIn contrast, it is also worth mentioning that increasing ECSA to the same electrode substrate but\ndeteriorating its bubbles dynamics may degrade performance at high current densities, as its effect\ndepends on the chosen method. For example, the morphology and surface composition of IrO +\n2\nionomer-coated stainless steel felt are shown in Figures S23-25. When using piperIon ionomer, an\nabout 60% increase in ECSA, as the current density increases to 5.0 A cm⁻2, the iR-free cell voltage\ngap with uncoated stainless steel felt gradually narrows to a negligible level. When using Nafion\nionomer, the AEMWE performance worsens by about 600 mV at 5.0 A cm⁻2, despite an about 123%\nincrease in ECSA (Figures S26-27). The bubble behaviors detach from the above three electrodes\nhelp explain these differences (Figures S28).\n(b) surface SEM images. (c) The corresponding cross-sectional elemental mapping.\n2\nsectional and (b) surface SEM images. (c) The corresponding cross-sectional elemental mapping.\n2\nsectional and (b) surface SEM images. (c) The corresponding cross-sectional elemental mapping.\n2 2\ncoated stainless steel felts: (a) Polarization curves. (b) Enlarged local view of polarization curves\nwithin the range of 0–1.0 A cm-2. (c) iR-free curves. (d) Capacitive currents as a function of scan\nrate for ECSA.\nFigure S27. CV curves of stainless steel felts with different coatings at various scan rates (120-220\nmV s-1) for OER: (a) uncoated, (b) IrO +PiperIon coated, (c) IrO +Nafion coated.\n2 2\n(operating conditions: ambient temperature, three electrode tests, in-plane direction parallel to the\nground): (a) uncoated, (b) IrO +PiperIon coated, (c) IrO +Nafion coated.\n2 2\nKOH through a standard three-electrode system: (a) LSV curves, (b) capacitive currents as a\nfunction of scan rate for ECSA. Electrochemical performance of the stainless steel felts with\ndifferent pore sizes through a zero-gap MEA system (c) Enlarged local view of polarization\ncurves within the range of 0–0.6 A cm-2 for stainless steel felts with different pore sizes.\nComment 2.4\nTo clearly demonstrate the effects of acid-washed and alcohol-washed treatments, it is\nessential to include a comparison with untreated samples. If data showing differences in\nelectrochemical performance or ECSA before and after treatment are available, including\nthem would help better convey the effectiveness of the surface treatments.\nResponse to 2.4:\nWe thank the Reviewer for the valuable suggestions. According to the suggestion, we supplemented\nthe electrochemical data of the untreated samples. In Figure S11, it can be found that untreated\nsample exhibits performance and ECSA very close to those of alcohol-washed electrode. In fact,\nthe purpose of cleaning the stainless steel electrode with alcohol before testing is merely to remove\ngrease and dirt, and it is not a special treatment. As bubbles detach from electrode, the grease and\ndirt on the untreated samples are washed away by the bubbles over time. Therefore, their\nelectrochemical performance and ECSA differ little after a period of conditioning.\n(40-160 mV s-1) for OER: (a) untreated sample, (b) alcohol-washed, (c) acid-washed. (d)\nCapacitive currents as a function of scan rate for ECSA. (e) LSV curves.\nAction to 2.4\nRelevant discussion has been added into our revised manuscript and figure has been added into\nour revised supplemental:\nIncidentally, compared to untreated samples, alcohol-washed stainless steel felts are cleaned with\nalcohol to remove grease and dirt. Therefore, as bubbles detach from the electrodes, grease and\ndirt on untreated samples are gradually washed away by the bubbles. As a result, their\nelectrochemical performance and ECSA do not differ significantly (Figure S11), and alcohol-\nwashed stainless steel felts are used as the base case in this study.\nFigure S11. CV curves of stainless steel felts with different treatment methods at various scan rates\n(40-160 mV s-1) for OER: (a) untreated sample, (b) alcohol-washed, (c) acid-washed. (d)\nCapacitive currents as a function of scan rate for ECSA. (e) LSV curves.\nFigure 5f presents an impressive durability result, showing stable operation over\napproximately 400 hours. If the voltage decay rate (mV h-1) during this period is quantified\nand compared with values reported in the literature, it could serve as a compelling metric to\nhighlight the superior long-term performance demonstrated in this study.\nResponse to 2.5:\nWe thank the Reviewer for the suggestions. Stable operation for 400 h (average degradation rate:\n0.035 mV h-1) at a current density of 1 A cm-2 (Figure 5f) demonstrated its promising application\ncompared with reported electrodes (Table 2) in AEMWE. We also add the long-term performance\ncomparison in the revised manuscript.\nFigure 5. (f) AEMWE durability using gradient stainless steel square hole mesh at current density\nof 1.0 A cm-2.\nAction to 2.5\nRelevant table has been added into our revised supplemental:\nTable 2. Long-term performance comparison of state-of-the-art AEMWEs\nCurrent density\nOperating Stability Decay rate\nAnode ‖ Cathode (A cm−2) @ Ref.\nconditions (h) (mV h-1)\nCell Voltage (V)\nS.S. mesh ‖ Pt/C 0.1 M KOH, 80 ℃ 1.0 @ 1.95 400 0.035 This work\n3D AEI-CEI-NiFe/Nf ‖ Pt/C 1.0 M KOH, 80 ℃ 10.0 @ 2.3 800 0.18 1\nS.S. felt ‖ Pt/C 1.0 M KOH, 60 ℃ 1.0 @ 1.72 400 0.268 2\nVCoP/AF‖VCoP/AF 1.0 M KOH, 60 ℃ 1.0 @ 1.68 200 0.2 3\nRT-NiFe-LDH ‖ Pt/C 1.0 M KOH, 70 ℃ 1.0 @ 1.71 100 0.41 4\nNiFe ‖ Pt/C 1.0 M KOH, 80 ℃ 1.67 @ 1.8 1500 0.2 5\nFe Ni -P S ‖ Fe Ni -P S 1.0 M KOH, 60 ℃ 2.50 @ 2.00 300 0.08 6\n0.2 0.8 0.5 0.5 0.2 0.8 0.5 0.5\nCoCrO ‖ Pt/C 1.0 M KOH, 60 ℃ 1.50 @ 2.10 120 4.9 7\nx\nS.S. felt ‖ Pt/C 0.1 M KOH, 80 ℃ 2.0 @ 2.08 600 0.005 8\nCAPist-L1 ‖ Ni Mo/MoO 1.0 M KOH, 80 ℃ 1.0 @ 1.6 1500 0.027 9\n4 2\nNi-FeOOH ‖ Pt/C 1.0 M KOH, 80 ℃ 3.0 @ 1.6 500 0.2 10\nEtched CuCoO ‖ Pt/C 0.1 M KOH, 45 ℃ 0.5 @ 1.72 3000 0.0113 11\nNi P/Ni S ‖ Pt/C 1.0 M KOH, 75 ℃ 1.00 @ 1.88 140 1.9 12\n2 7 6\nCo O ‖ NiCo 0.1 M KOH, 80 ℃ 1.50 @ 2.2 515 0.136 13\n3 4\nNiFe ‖ MoO /A-Ni S 1.0 M KOH, 40 ℃ 2.0 @ 2.2 2500 0.03 14\nx 3 2\nIrO ‖ PtRu/C 1.0 M KOH, 60 ℃ 0.5 @ 1.57 300 0.29 15\nIrO ‖ Pt/C 1.0 M KOH, 80 ℃ 1.0 @ 1.75 100 1.2 17\n2\n1. Zheng, Y., Ma, W., Serban, A., Allushi, A. & Hu, X. Anion Exchange Membrane Water Electrolysis at 10 A·cm-2 Over 800\nHours. Angew. Chem. Int. Ed. 64, e202413698 (2025).\n2. Chen, B., Biancolli, A. L. G., Radford, C. L. & Holdcroft, S. Stainless Steel Felt as a Combined OER Electrocatalyst/Porous\nTransport Layer for Investigating Anion-Exchange Membranes in Water Electrolysis. ACS Energy Lett. 8, 2661–2667 (2023).\n3. Wan, L., Xu, Z., Xu, Q., Wang, P. & Wang, B. Overall design of novel 3D-ordered MEA with drastically enhanced mass\ntransport for alkaline electrolyzers. Energy Environ. Sci. 15, 1882–1892 (2022).\n4. Seijas-Da Silva, A. et al. Scalable synthesis of NiFe-layered double hydroxide for efficient anion exchange membrane\nelectrolysis. Nat Commun 16, 6138 (2025).\n5. Shaik, S. et al. Rapid Outgassing of Hydrophilic TiO2 Electrodes Achieves Long-Term Stability of Anion Exchange\nMembrane Water Electrolyzers. Nano-Micro Lett. 17, 186 (2025).\n6. Wan, L. et al. Dual regulation both intrinsic activity and mass transport for self-supported electrodes using in anion exchange\nmembrane water electrolysis. Chem. Eng. J. 431, 133942 (2022).\n7. Li, S. et al. Highly efficient anion exchange membrane water electrolyzers via chromium-doped amorphous electrocatalysts.\nNat Commun 15, (2024).\n8. Tricker, A. W. et al. Pathways Toward Efficient and Durable Anion Exchange Membrane Water Electrolyzers Enabled By\nElectro-Active Porous Transport Layers. Adv. Energy Mater. 2303629 (2024).\n9. Li, Z. et al. Seed-assisted formation of NiFe anode catalysts for anion exchange membrane water electrolysis at industrial-\nscale current density. Nat. Catal. 7, 944–952 (2024).\n10. Chen, N., Jiang, Q., Song, F. & Hu, X. Robust Piperidinium-Enriched Polystyrene Ionomers for Anion Exchange Membrane\nFuel Cells and Water Electrolyzers. ACS Energy Lett. 8, 4043–4051 (2023).\n11. Jang, M. J. et al. Efficient and Durable Anion Exchange Membrane Water Electrolysis for a Commercially Available\nElectrolyzer Stack using Alkaline Electrolyte. ACS Energy Lett. 7, 2576–2583 (2022).\n12. Wang, F.-L. et al. Porous heterojunction of Ni2P/Ni7S6 with high crystalline phase and superior conductivity for industrial\nanion exchange membrane water electrolysis. Appl. Catal. B Environ. 330, 122633 (2023).\n13. Tricker, A. W. et al. Design and operating principles for high-performing anion exchange membrane water electrolyzers. J.\nPower Sources 567, 232967 (2023).\n14. Lee, H. et al. Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water\nElectrolysis at Industrial-Scale Current Density. Adv. Sci. 12, 2502478 (2025).\n15. Hyun, J. et al. Anode Reinforcement by Polydopamine Glue in Anion Exchange Membrane Water Electrolysis. ACS Energy\nLett. 5240–5247 (2023) doi:10.1021/acsenergylett.3c02205.\n16. Kim, M.-G. et al. Realizing the potential of hydrophobic crystalline carbon as a support for oxygen evolution electrocatalysts.\nEnergy Environ. Sci. 16, 5019–5028 (2023).\n17. Cha, M. S. et al. Oligomeric chain extender-derived anion conducting membrane materials with poly(p-phenylene)-based\narchitecture for fuel cells and water electrolyzers. J. Mater. Chem. A 10, 9693–9706 (2022).\nComment\nAn et al. have implemented a multiscale visualization cell to explore how pore size and the\nhydrophilicity of a stainless-steel electrode impact the behavior of gas bubbles produced by\nwater electrolysis. The authors find that bubbles cover anode active sites and hinder the\ntransport of water to the membrane and cathode, such that maximizing the electrochemically\nactive surface area does not necessarily lead to improved performance. These problems were\nthen addressed in an engineered stainless-steel mesh leading to an improved anion exchange\nmembrane water electrolyzer for operation at high (5 A cm-2) current densities.\nResponse:\nWe sincerely thank the Reviewer for the valuable time and comments.\nComment\nScientifically, the experiments and findings in this work are not particularly original and\nthey do not differ much from recent papers from the same authors published in the last years\n(e.g. Wu & An et al. 2025 Adv. Science), except for a new “acid washing” treatment.\nPractically, the performance of the AEM electrolyzers in this work are not exceptional—\nmany other works have been published (e.g. Yanagi & Peng et al. 2025 Joule) with improved\nperformance at both low and high current densities, with more detailed characterization of\ntransport layers and interfaces. I do not see the novelty or practical importance of this work\nthat justifies its rapid communication in Nature Communications.\nResponse:\nWe thank the Reviewer for the comments. In fact, there are significant differences between this\nstudy and my previous work (Wu & An et al. 2025 Adv. Science), and we believe our performance\nis also excellent enough, and more detailed characterizations have been supplemented.\nWe can briefly introduce the differences between them. Most importantly, the novelty and focus of\nthis study do not lie in the electrode design, but in using special design to reflect the more dominant\nrole of bubble dynamics over electrode ECSA in AEMWE under high current conditions.\nUltimately, this work presents a concept to achieve a paradigm shift from ECSA-centric\noptimization of catalysts and electrode to bubble management for AEMWE, which is a\nlandmark shift. By addressing the “bubble bottleneck” problem, it is believed that future AEMWEs\nbased on this shift in concept can go beyond their current limitations and provide a sustainable and\neconomically viable pathway to achieve gigawatt-scale hydrogen production.\nFor our previous work (Wu & An et al. 2025 Adv. Science), it mainly discusses the significant\ndegradation of the stainless steel felt electrode in AEMWE after durability testing (Figure A),\ncharacterized by surface roughness caused by corrosion and oxide accumulation. We found that\nsurface changes cause the size of bubbles detached from the electrode to increase, thereby hindering\nwater flow towards the electrode, deteriorating performance, and exacerbating electrode\ndegradation. The key finding is the complex degradation mechanism (chemical/electrochemical\ndegradation and mechanical degradation associated with bubbles) of stainless steel electrodes in\nAEMWE.\nincluding durability test and bubble visualization at the μm- and mm- scales).\nDifferences between this work and our previous work (Wu & An et al. 2025 Adv. Science):\n(1) Regarding the methods used in this new study, we not only incorporate bubbles visualization at\nvarious scales and electrochemical characterization (such as overpotential breakdown and DRT\nanalysis) as in previous studies, but also include in-situ potential measurement and statistical\nevaluation of water transport capacity in this new study.\n(2) In terms of research focus, the relationship between electrode degradation and bubbles was\ninvestigated in previous study (Wu & An et al. 2025 Adv. Science), offering new insights for\ndesigning electrodes that ensure long-term stable operation. In the new study, we focus more on\nwhich factor (ECSA of electrode or bubble management) dominates water electrolysis\nperformance at high current densities. We find the three key processes: (1) bubbles cover active\nsites at the anode; (2) bubbles hinder water diffusion through the membrane, (3) it in turn causes\nwater shortage at the cathode. In conclusion, bubbles affect the internal mass transport of AEMWE,\nultimately controlling its performance at high current densities.\nFor the second work (Yanagi & Peng et al. 2025 Joule), it mainly explores water and oxygen\ndynamics in AEMWEs, highlighting that anode catalyst layers can promote water back-diffusion\nand reduce oxygen bubble overpotential, improving performance under dry-cathode conditions. In-\nsitu techniques reveal that effective removal of OER-generated water maintains higher pH and\nenhances anode kinetics. A novel coating strategy and optimized ink properties enable scalable\nfabrication of uniform anodes, achieving stable operation at 2 A cm-2 with minimal degradation\nover 1000 hours. Overall, the work presents a comprehensive approach to improve AEMWE\nperformance and durability for industrial applications.\nDifferences between our work and Peng’s Joule 2025:\n(1) It is somewhat unreasonable to compare our optimal performance with that of Peng's study\n(Yanagi & Peng et al. 2025 Joule). The optimized PTE in Peng's work involves mixing Co O\n3 4\ncatalyst particles with Nafion ionomer, modifying the ink properties by adding cellulose nanofibrils,\nand then coating the mixture onto stainless steel felt. In contrast, our work simply uses stainless\nsteel mesh with different mesh sizes, stacked to create a gradient structure. In addition, the\noperating conditions are different. The dry-cathode mode is adopted in our study, and the both-\nfeeding mode was used in Peng's study.\n38\n(2) The voltage of the water electrolyzer includes eversible voltage, ohmic overpotential,\nactivation\noverpotential, and mass transport overpotential. The absolute cell voltage of our study operates\nabove Peng’s study at high current densities. However, when excluding ohmic overpotential, our\niR-free performance is better than Peng’ work, which means our design presents the better\nimprovement in activation and mass transport overpotentials (Figure B&C). This also\ndemonstrates our superiority in regulating anode bubble dynamics. The ohmic overpotential\nmay be affected by the fixture, different batches of membranes, etc. Therefore, it is also more\nreasonable to compare the iR-free cell voltage between the two studies.\nFigure B. Performance comparison: (a) Yanagi & Peng et al. 2025 Joule using PTE and PTE with\nNCF, (b) AEMWE using stainless steel felt and gradient stainless steel square hole mesh (our\nstudy) (c) cell voltage, iR-free cell voltage at 2 and 4 A cm-2.\n(3) In fact, the absolute performance of our study is also sufficiently excellent. We have also\ncompared our work with other exemplary studies, demonstrating clear advantages in\nperformance (Figure C) and long-term stability (Table 2).\nFigure C. The performance comparison of this study with the reported literature.\n1. Wang, F.-L. et al. Porous heterojunction of Ni2P/Ni7S6 with high crystalline phase and superior conductivity for industrial\nanion exchange membrane water electrolysis. Appl. Catal. B Environ. 330, 122633 (2023).\n2. Zhang, J. et al. A High‐entropy Antiperovskite Nitride Enables Efficient Anion Exchange Membrane Water Electrolysis. Adv.\nMater. 2509042 (2025).\n3. Cha, M. S. et al. Oligomeric chain extender-derived anion conducting membrane materials with poly(p-phenylene)-based\narchitecture for fuel cells and water electrolyzers. J. Mater. Chem. A 10, 9693–9706 (2022).\n4. Chen, B., Biancolli, A. L. G., Radford, C. L. & Holdcroft, S. Stainless Steel Felt as a Combined OER Electrocatalyst/Porous\nTransport Layer for Investigating Anion-Exchange Membranes in Water Electrolysis. ACS Energy Lett. 8, 2661–2667 (2023).\n5. Li, S. et al. Highly efficient anion exchange membrane water electrolyzers via chromium-doped amorphous electrocatalysts.\nNat. Commun. 15 (2024).\n6. Zhu, Y. et al. Ru Single Atoms and Sulfur Anions Dual‐Doped NiFe Layered Double Hydroxides for High‐Current‐Density\nAlkaline Oxygen Evolution Reaction. Adv. Energy Mater. 2500554 (2025).\n7. Lee, H. et al. Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water\nElectrolysis at Industrial-Scale Current Density. Adv. Sci. 12, 2502478 (2025).\n8. Tricker, A. W. et al. Pathways Toward Efficient and Durable Anion Exchange Membrane Water Electrolyzers Enabled By\nElectro-Active Porous Transport Layers. Adv. Energy Mater. 2303629 (2024).\n9. Yu, P. C. et al. Nitrogen-mediated promotion of cobalt-based oxygen evolution catalyst for practical anion-exchange membrane\nelectrolysis. J. Am. Chem. Soc.146(29), 20379-20390 (2024).\n10. Xia, L. et al. Operando-informed precatalyst programming towards reliable high-current-density electrolysis. Nat. Mater. 1-9\n(2025).\n11. Wan, L. et al. Dual regulation both intrinsic activity and mass transport for self-supported electrodes using in anion exchange\nmembrane water electrolysis. Chem. Eng. J. 431, 133942 (2022).\n12. Zhao, C. et al. Synergistic Kinetics Acceleration and Mass Transport by Nanocone‐Nanoneedle Hierarchical Electrodes for\nEfficient Anion Exchange Membrane Water Electrolysis. Adv. Funct. Mater. e13731 (2025).\n13. Zhong, X. et al. Stabilization of layered lithium-rich manganese oxide for anion exchange membrane fuel cells and water\nelectrolysers. Nat. Catal. 7(5): 546-559 (2024).\n14. Seijas-Da, S. A. et al. Scalable synthesis of NiFe-layered double hydroxide for efficient anion exchange membrane electrolysis.\nNat. Commun. 16(1): 6138 (2025).\n15. Serban, A. et al. An oxide-promoted, self-supported Ni 4 Mo catalyst for high current density anion exchange membrane water\nelectrolysis. Energy Environ. Sci. 18(3): 1533-1543 (2025).\nTable 2. Long-term performance comparison of state-of-the-art AEMWEs\nCurrent density\nOperating Stability Decay rate\nAnode ‖ Cathode (A cm−2) @ Ref.\nconditions (h) (mV h-1)\nCell Voltage (V)\nS.S. mesh ‖ Pt/C 0.1 M KOH, 80 ℃ 1.0 @ 1.95 400 0.035 This work\n3D AEI-CEI-NiFe/Nf ‖ Pt/C 1.0 M KOH, 80 ℃ 10.0 @ 2.3 800 0.18 1\nS.S. felt ‖ Pt/C 1.0 M KOH, 60 ℃ 1.0 @ 1.72 400 0.268 2\nVCoP/AF‖VCoP/AF 1.0 M KOH, 60 ℃ 1.0 @ 1.68 200 0.2 3\nRT-NiFe-LDH ‖ Pt/C 1.0 M KOH, 70 ℃ 1.0 @ 1.71 100 0.41 4\nNiFe ‖ Pt/C 1.0 M KOH, 80 ℃ 1.67 @ 1.8 1500 0.2 5\nFe Ni -P S ‖ Fe Ni -P S 1.0 M KOH, 60 ℃ 2.50 @ 2.00 300 0.08 6\n0.2 0.8 0.5 0.5 0.2 0.8 0.5 0.5\nCoCrO ‖ Pt/C 1.0 M KOH, 60 ℃ 1.50 @ 2.10 120 4.9 7\nx\nS.S. felt ‖ Pt/C 0.1 M KOH, 80 ℃ 2.0 @ 2.08 600 0.005 8\nCAPist-L1 ‖ Ni Mo/MoO 1.0 M KOH, 80 ℃ 1.0 @ 1.6 1500 0.027 9\n4 2\nNi-FeOOH ‖ Pt/C 1.0 M KOH, 80 ℃ 3.0 @ 1.6 500 0.2 10\nEtched CuCoO ‖ Pt/C 0.1 M KOH, 45 ℃ 0.5 @ 1.72 3000 0.0113 11\nNi P/Ni S ‖ Pt/C 1.0 M KOH, 75 ℃ 1.00 @ 1.88 140 1.9 12\n2 7 6\nCo O ‖ NiCo 0.1 M KOH, 80 ℃ 1.50 @ 2.2 515 0.136 13\n3 4\nNiFe ‖ MoO /A-Ni S 1.0 M KOH, 40 ℃ 2.0 @ 2.2 2500 0.03 14\nx 3 2\nIrO ‖ PtRu/C 1.0 M KOH, 60 ℃ 0.5 @ 1.57 300 0.29 15\nIrO ‖ Pt/C 1.0 M KOH, 80 ℃ 1.0 @ 1.75 100 1.2 17\n1. Zheng, Y., Ma, W., Serban, A., Allushi, A. & Hu, X. Anion Exchange Membrane Water Electrolysis at 10 A·cm-2 Over 800\nHours. Angew. Chem. Int. Ed. 64, e202413698 (2025).\n40\n2. Chen, B., Biancolli, A. L. G., Radford, C. L. & Holdcroft, S. Stainless Steel Felt as a Combined OER Electrocatalyst/Porous\nTransport Layer for Investigating Anion-Exchange Membranes in Water Electrolysis. ACS Energy Lett. 8, 2661–2667 (2023).\n3. Wan, L., Xu, Z., Xu, Q., Wang, P. & Wang, B. Overall design of novel 3D-ordered MEA with drastically enhanced mass\ntransport for alkaline electrolyzers. Energy Environ. Sci. 15, 1882–1892 (2022).\n4. Seijas-Da Silva, A. et al. Scalable synthesis of NiFe-layered double hydroxide for efficient anion exchange membrane\nelectrolysis. Nat Commun 16, 6138 (2025).\n5. Shaik, S. et al. Rapid Outgassing of Hydrophilic TiO2 Electrodes Achieves Long-Term Stability of Anion Exchange\nMembrane Water Electrolyzers. Nano-Micro Lett. 17, 186 (2025).\n6. Wan, L. et al. Dual regulation both intrinsic activity and mass transport for self-supported electrodes using in anion exchange\nmembrane water electrolysis. Chem. Eng. J. 431, 133942 (2022).\n7. Li, S. et al. Highly efficient anion exchange membrane water electrolyzers via chromium-doped amorphous electrocatalysts.\nNat Commun 15, (2024).\n8. Tricker, A. W. et al. Pathways Toward Efficient and Durable Anion Exchange Membrane Water Electrolyzers Enabled By\nElectro-Active Porous Transport Layers. Adv. Energy Mater. 2303629 (2024).\n9. Li, Z. et al. Seed-assisted formation of NiFe anode catalysts for anion exchange membrane water electrolysis at industrial-\nscale current density. Nat. Catal. 7, 944–952 (2024).\n10. Chen, N., Jiang, Q., Song, F. & Hu, X. Robust Piperidinium-Enriched Polystyrene Ionomers for Anion Exchange Membrane\nFuel Cells and Water Electrolyzers. ACS Energy Lett. 8, 4043–4051 (2023).\n11. Jang, M. J. et al. Efficient and Durable Anion Exchange Membrane Water Electrolysis for a Commercially Available\nElectrolyzer Stack using Alkaline Electrolyte. ACS Energy Lett. 7, 2576–2583 (2022).\n12. Wang, F.-L. et al. Porous heterojunction of Ni2P/Ni7S6 with high crystalline phase and superior conductivity for industrial\nanion exchange membrane water electrolysis. Appl. Catal. B Environ. 330, 122633 (2023).\n13. Tricker, A. W. et al. Design and operating principles for high-performing anion exchange membrane water electrolyzers. J.\nPower Sources 567, 232967 (2023).\n14. Lee, H. et al. Suppressing Mo-Species Leaching in MoOx/A-Ni3S2 Cathode for Stable Anion Exchange Membrane Water\nElectrolysis at Industrial-Scale Current Density. Adv. Sci. 12, 2502478 (2025).\n15. Hyun, J. et al. Anode Reinforcement by Polydopamine Glue in Anion Exchange Membrane Water Electrolysis. ACS Energy\nLett. 5240–5247 (2023) doi:10.1021/acsenergylett.3c02205.\n16. Kim, M.-G. et al. Realizing the potential of hydrophobic crystalline carbon as a support for oxygen evolution electrocatalysts.\nEnergy Environ. Sci. 16, 5019–5028 (2023).\n17. Cha, M. S. et al. Oligomeric chain extender-derived anion conducting membrane materials with poly(p-phenylene)-based\narchitecture for fuel cells and water electrolyzers. J. Mater. Chem. A 10, 9693–9706 (2022).\n(4) From a research focus, our study does not pursue absolute performance at high current densities.\nWe focus more on which factor (ECSA of electrode or bubble management) dominates water\nelectrolysis performance at high current densities. We find the three key processes: (1) bubbles\ncover active sites at the anode; (2) bubbles hinder water diffusion through the membrane, (3) it in\nturn causes water shortage at the cathode. In conclusion, bubbles affect the internal mass transport\nof AEMWE, ultimately controlling its performance at high current densities. Based on this\nmechanism, we proposed a simple and easy-to-prepare gradient stainless steel square hole mesh\nelectrode. It not only has an extremely low cost ($8-150/m2 only) (Table 3 in response 3.3), but also\ngreatly improves bubble dynamics, ultimately enhancing the performance of AEMWE. Ultimately,\nthis work presents a new concept to achieve a paradigm shift from ECSA-centric\noptimization of catalysts and electrodes to effective bubble management for AEMWE.\n(5) In terms of characterization, for electrochemical characterization, we not only have polarization\ncurves, overpotential breakdown, in-situ potential measurement, but also DRT to help us further\nverify and analyze impedance. For electrode and interface characterization, this study achieved\nmulti-scale bubble observation from μm to mm scale by combining high-speed cameras with\nmicroscopes and microscale lenses, respectively. We have also supplemented characterization\ntechniques such as SEM, EDS mapping, XPS, etc. In addition, we also characterized the electrode\npore size using mercury intrusion porosimetry (MIP). The statistical analysis of transmembrane\nwater transport also provides evidence for the extent of improvement in water transport\nperformance.\nThere is very little data or discussion on the changes caused by the acid treatment. At early\ntimes, it would be helpful to know how the surface chemistry and < µm scale texture\ninfluences the gas release properties. For practical relevance, it would be helpful to know\nhow long the difference between the two meshes persist, since the structure and composition\nof the anode will evolve during the break in period and much longer break in periods are\nused for industrial electrolysis systems.\nResponse to 3.1:\nWe thank the Reviewer for the suggestions. For electrodes that have just been acid-washed and\nalcohol-washed, we have also supplemented XPS data. The diluted nitric acid (HNO ) has the\n3\noxidizing effect on the stainless steel fiber, which can reduce ECSA of electrode (Figure 3 (c)) due\nto the accumulation of a dense chromium-rich oxide layer (Figure S16 (e)). However, due to the\nroughness of the electrode surface (Figures 4 (a-b)) caused by HNO , the hydrophilicity of acid-\n3\nwashed felt is significantly enhanced, which in turn improves its bubble dynamics (Figures 4 (a-b))\nand leads to a marked improvement in performance (Figure 3 (d)) of AEMWE at high current\ndensities.\nFigure 3. Electrochemical performance of the alcohol-washed and acid-washed stainless steel felts\nin 1 M KOH through a standard three-electrode system: (c) Capacitive currents as a function of\nscan rate for ECSA. Electrochemical performance of the alcohol-washed and acid-washed\nstainless steel felts through a zero-gap MEA system: (d) Polarization and iR-free curves.\nFigure 4. SEM image, contact angle tests and bubble behaviors in stainless steel felts. (operating\nconditions: ambient temperature, 0.4 A cm-2, three electrode tests, in-plane direction parallel to the\nground): (a) Alcohol-washed stainless steel felt. (b) acid-washed stainless steel felt.\nCr, Fe, Ni and O.\nInitially, the acid-washed felt demonstrates significantly better performance than the alcohol-\nwashed felt for approximately 30 hours. However, by around 85 hours, the cell voltage of\nAEMWE with acid-washed felt begins to surpass the that with alcohol-washed felt (Figure S13\n(a)). After a 120-hour durability test, both electrodes experience significant degradation in\nperformance (Figure S13 (b)) and ECSA (Figure S13 (c)). Although this acid treatment only\nmaintained an advantage of 35 hours and a stalemate period of 85 hours, it was viewed as a special\nexperimental design (reduced ECSA but improved bubble dynamics) to validate whether\nECSA or bubble dynamics is the key factor to dominate the performance of AEMWE at high\ncurrent densities.\nFigure S13. Comparison of AEMWE using acid-washed felt and the alcohol-washed felt: (a)\nDurability at 2.0 A cm−2. (b) Polarization curves. (c) Capacitive currents as a function of scan rate\nfor ECSA. (d) EIS curves at 2.0 A cm−2.\nSpecifically, after 120 hours of operation, the surface morphology of both electrodes becomes\nalmost indistinguishable (Figures S15c and S15f). During the initial break in period (around 5\nhours), an increase in O content suggests the accumulation of oxyhydroxides on the electrode\nsurface (Figure S16), significantly enhancing hydrophilicity (Figure S17). By 120 hours, however,\n43\nthe decrease in oxyhydroxides (Figure S16) reduces the hydrophilicity of both electrodes, making\nthem more similar (Figures S17c and S17f). As a result, after prolonged operation, bubble behaviors\nshow no notable difference between the two electrodes (Figure S19). The performance degradation\nof the acid-washed felt is primarily due to the loss of active surface components. The diluted HNO\n3\ntreatment forms a protective oxide layer rich in Cr, which also contains oxides of Fe and Ni. After\n120 hours, the surface Cr content drops sharply from 3.82% to 0.43%, accompanied by increased\ndissolution of Fe and especially Ni, leading to a lower ECSA compared to the alcohol-washed felt\n(Figure S16). This reduced ECSA, along with diminished hydrophilicity, mainly contributes to the\nsignificantly increased OER impedance of the acid-washed felt after 120 hours (Figure S13 (d)) and\nwhy it cannot maintain its long-term advantages.\nFigure S15. SEM image of: (c) alcohol-washed felt_120 h, (f) acid-washed felt_120 h.\nFigure S16. XPS of acid-washed felt and the alcohol-washed felt after 0 h, 5 h, 120 h. XPS signals\nof (a) O 1s, (b) Fe 2p, (c)Ni 2p, (d) Cr 2p. (e) The atomic percentage of C, Cr, Fe, Ni and O.\nFigure S19. Bubble behaviors on the electrode surface at current densities of 0.01, 0.1 and 0.4A\ncm−2: (a) alcohol-washed felt_120 h and (b) acid-washed felt_120 h.\nAlthough the acid treatment electrode did not show improvement in long-term performance, but the\nfinding got from this sample is important, i.e., bubble dynamics is more critical than ECSA at high\ncurrent densities. Based on this finding, we further propose a new design, gradient stainless steel\nsquare mesh, which is simple and easy to prepare. It has demonstrated very good stability (400\nhours operation with a decay rate of 0.035 mV h-1 only), and its cost is only around $8-150/m2,\nmuch lower than traditional titanium fiber felt coated with IrO (> $111,000 /m2) (Table 3 is in\nAction to 3.1\nRelevant discussion has been added into our revised manuscript:\nIn addition, the duration of the performance gap between the acid-washed felt and the alcohol-\nwashed felt is noteworthy. Initially, the acid-washed felt outperforms the alcohol-washed felt.\nHowever, their voltages become similar after approximately 30 hours, and by around 85 hours,\nthe acid-washed felt surpasses the alcohol-washed felt (Figure S13a). After 120 hours, both\nelectrodes show significant degradation in (Figure S13b) and ECSA (Figure S13c and Figure S14),\nwith the alcohol-washed felt maintaining a higher ECSA throughout (Figure S13c). EIS data\nindicates that increased OER impedance was the main reason for the performance decline in the\nlong term (Figure S13d). Specifically, SEM images (Figure S15) reveal that the fresh acid-washed\nfelt has a slightly rougher surface than the alcohol-washed felt. XPS analysis (Figure S16) shows\nthat diluted HNO etching forms a dense chromium-rich oxide layer. Although this layer reduces\n3\nexplaining the early-stage performance advantage. Over time, the surface morphology of the two\nelectrodes becomes similar (Figure S15). After 120 hours, the decrease in oxyhydroxides and the\nsignificant loss of Cr, Fe, and Ni lead to reduced hydrophilicity and fewer active sites (Figure\nS16). As a result, the decreased ECSA and worsened bubble removal (Figure S19) become the\nprimary reasons for the increased OER impedance and performance degradation of the acid-\nwashed felt after prolonged operation. Therefore, it is not recommended to use diluted HNO to\n3\ntreat stainless steel felt in industrial water electrolysis applications.\nFigure S13. Comparison of AEMWE using acid-washed felt and the alcohol-washed felt: (a)\nDurability at 2.0 A cm−2. (b) Polarization curves. (c) Capacitive currents as a function of scan rate\nfor ECSA. (d) EIS curves at 2.0 A cm−2.\nh, (b) acid-washed felt_0 h, (c) alcohol-washed felt_120 h, (d) acid-washed felt_120 h.\nwashed felt_120 h, (d) acid-washed felt_0 h, (e) acid-washed felt_5 h, (f) acid-washed felt_120\nh.\nof (a) O 1s, (b) Fe 2p, (c)Ni 2p, (d) Cr 2p. (e) The atomic percentage of C, Cr, Fe, Ni and O.\nFigure S17. Contact angle tests: (a) alcohol-washed felt_0 h, (b) alcohol-washed felt_5 h, (c)\nalcohol-washed felt_120 h, (d) acid-washed felt_0 h, (e) acid-washed felt_5 h and (f) acid-washed\nfelt_120 h.\ncm−2: (a) alcohol-washed felt_0 h and (b) acid-washed felt_0 h.\ncm−2: (a) alcohol-washed felt_120 h and (b) acid-washed felt_120 h.\nComment 3.2\nDespite the focus on transport properties, only a single anode flow rate appears to have been\nused (5 mL per min). How do these results change when the flow rate is increased/decreased.\nObviously flow rate is an important design parameter for transport and some of the\nlimitations addressed here might be equally addressed by increasing the flow rate of 1 M\nKOH. The authors should compare the sensitivity of their results at an increased flow rate\n(>20 mL min-1 as well as a reduced flow rate, e.g. 1 mL min-1 where the effects of gas\nevolution may be more evident).\nResponse to 3.2:\nWe thank the Reviewer for the valuable suggestions. Following the suggestion, we conducted\nadditional tests with different flow rates.\nIn this study, our primary research focus is high-rate water electrolysis. We found that at high\ncurrent densities (e.g., 5 A cm-2), flow rates of 1.0 and 3.0 mL min-1 make the performance of\nAEMWE the worst (Figures S35a -b). It can be seen that the flow rates of 1.0 and 3.0 mL min-1\n51\nfail to remove the bubbles in the flow field in time, resulting in the channel being almost filled with\nbubbles (Figure S36). It may fail to ensure sufficient KOH solution supply to the electrodes. 5.0\nmL min-1 presents the better performance (Figures S35a -b). Further increasing the flow rate to 30\nmL min-1 may facilitate bubble removal in the flow channel (Figure S36), but the increased velocity\nof KOH solution moving toward the electrode in the Y-direction impedes bubbles removal within\nthe electrode (Figure S37). This increases impedances for OER, mass transport, HER and OH⁻\ntransport (Figures S35c), and results in a cell voltage increase of up to 83 mV in AEMWE. However,\noptimizing the electrode structure (gradient square hole mesh) improves performance by\napproximately 137 mV (Figures S35a). This also indicates that blindly increasing the flow rate of\nKOH cannot equally alleviate oxygen bubble transport limitations. Instead, it may restrict bubbles\ntransport. Moreover, increased flow rates represent wasted electricity and are not recommended.\nUltimately, we advocate optimizing key components within AEMWE such as electrodes to\novercome oxygen bubble transport limitations at high current densities.\nFigure S35. (a) Comparison of how 1 M KOH flow rate and electrode design affect cell voltage of\nAEMWE. (b) Cell voltage at 5.0 A cm-2. (c) EIS curves and DRT analysis at 5.0 A cm-2.\nFigure S37. Effect of flow rate on Y velocity of fluid in flow field. (Zoom the Z-axis by five times)\nRelevant discussion has been added into our revised manuscript:\nFor our laboratory-scale cell operating at high current densities (e.g. 5.0 A cm-2), low flow rates\nsuch as flow rates of 1.0 and 3.0 mL min-1 make the performance of AEMWE the worst (Figures\nS35a -b). It can be seen that the flow rates of 1.0 and 3.0 mL min-1 fail to remove the bubbles in the\nflow field in time, resulting in the channel being almost filled with bubbles (Figure S36). It may fail\nto ensure sufficient KOH solution supply to the electrodes. Blindly increasing the KOH flow rate\nto 30 mL min-1 does not effectively enhance AEMWE performance like the gradient square hole\nmesh does and may even prove counterproductive (Figures S35a-c). Therefore, 5.0 mL min-1\nrepresents the optimal flow rate at high current densities (Figures S35b-c). While higher flow\npromotes bubble removal in flow channels (Figure S36), increased velocity toward the electrode\nin the Y-direction impedes bubble removal within the electrode (Figure S37), raising impedances\nfor OER, mass transport, HER, and OH⁻ transport (Figures S35d-e). Additionally, higher flow\nrates waste electrical energy and are not recommended. Therefore, optimizing electrode structures\nto overcome bubble clogging at high current densities may be a better solution.\nFigure S35. (a) Comparison of how 1 M KOH flow rate and electrode design affect cell voltage of\nAEMWE. (b) Cell voltage at 5.0 A cm-2. (c) EIS curves and DRT analysis at 5.0 A cm-2.\nby five times)\n.\nI doubt that the comparisons of IrO2, stainless steel, and Ni mesh—which are based on small\nscale suppliers for R&D research—are applicable to the costs of these materials at scale.\nNotably, the difference in the cost of Ni felt and IrO2 are not that much… How sensitive are\nthe estimated costs to simple changes in catalyst loading or mesh thickness?\nResponse to 3.3:\nWe thank the Reviewer for the suggestions. We have conducted a more detailed statistical analysis\n(including different suppliers, catalyst loadings, mesh sizes, thicknesses, etc) of the costs of\ndifferent anode electrodes, as shown in Table 3. Among them, the traditional titanium fiber felt\ncoated with IrO electrodes has the highest cost, exceeding $111000/m2. In contrast, 316L stainless\n2\nsteel mesh is the most economical, priced at around $8-150/m2, reflecting a substantial cost\nreduction of at least 99.86%.\nAction to 3.3\nRelevant discussion and modified table 3 have been added into our revised supplemental:\nFinally, it is worth emphasizing that the cost of a traditional IrO electrode is the highest, estimated\n2\nat over $111000/m2. In comparison, 316L stainless steel mesh costs about $8-150/m2, which is\neven lower than the over $1200/m2 for 316L stainless steel felt, over $4200/m2 for Ni felt, and over\n$350/m2 for Ni mesh. More details about cost analysis of anode electrodes can be found in Table\n3. The gradient stainless steel mesh not only enhances electrolysis efficiency but also simplifies\nthe preparation process, leading to a remarkable 99.86% reduction in anode manufacturing costs\ncompared with traditional IrO electrode. Thus, it presents a cost-effective and efficient alternative\n2\nto traditional electrodes.\nTable 3 Cost comparison of different anode electrodes\nTypes Unit Standard Price USD/square Remark\nTitanium 1 mg/cm2 $111,000 /m2\nFrom Laborxing18\nfiber felt 2 mg/cm2 $159,000 /m2\ncoated with Loading 3 mg/cm2 $127,000 /m2 From Alibaba19\nIridium Oxide\n4 mg/cm2 $400,000 /m2 From Accurate Atom, Inc.20\nIrO\n2\n0.1 mm\n0.2 mm\nThickness\n0.3 mm From Amazon21,\n(pore size $9,725- $10,800 /m2\n0.4 mm and eBay22\n10 𝜇m)\n0.5 mm\n0.6 mm\nStainless steel 5 𝜇m $6,500 /m2\nfiber felt 10 𝜇m $7,200 /m2\nFrom Amazon23\nPore size 20 𝜇m $7,200 /m2\n(0.4mm 30 𝜇m $8,450 /m2\nthickness) 40 𝜇m $1,200 /m2\n60 𝜇m $1,200 /m2\nFrom Alibaba24\n80 𝜇m $1,200 /m2\n100 𝜇m $1,200 /m2\nStainless steel Sieve 10 Mc-master-Carr26,\n$8-150 /m2\nmesh counts 20 and Alibaba27\n30\n57\n60\n100\n300\n10\n20\n50\n80\n0.1mm $21,700 /m2\n0.25mm $24,650 /m2\nThickness\n0.4mm $24,650 /m2\n(pore size From Amazon28\n0.6mm $25,200 /m2\n10 𝜇m)\n0.8mm $30,750 /m2\nNickel fiber\n1.0mm $42,000 /m2\nfelt\n10 𝜇m\nPore size\n20 𝜇m\n(0.4mm\n30 𝜇m $4,200-5,500 /m2 From Alibaba29\nthickness)\n40 𝜇m\n40 𝜇m\n4 $1,400 /m2\n9 $1,400 /m2\nSieve\n64 $1,220 /m2 From Mc-master-Carr30\ncounts\n100 $870 /m2\n400 $560 /m2\n10 $7,200 /m2\n20 $7,200 /m2\n40 $7,200 /m2\nNickel mesh\n80 $7,800 /m2\nSieve\n100 $7,600 /m2 From Amazon31\ncounts\n150 $8,800 /m2\n200 $4,500 /m2\n300 $10,400 /m2\n400 $10,400 /m2\nSieve\n10-300 $350-625 /m2 From Alibaba32\ncounts\n0.3 mm $15,600 /m2\n0.5 mm $15,600 /m2\nThickness\n1 mm $16,700 /m2\n(110ppi)\n1.5 mm $17,200 /m2 From Amazon33\n(high\n2 mm $19,400 /m2\ndensity)\n3 mm $21,000 /m2\n5 mm $25,000 /m2\n0.5 mm $2,200 /m2\nThickness\nNickel foam 1 mm $2,200/m2\n(110ppi)\n1.5 mm $2,200 /m2 From Amazon34\n(low\n2 mm $2,200 /m2\ndensity)\n3 mm $2,500 /m2\nFiltration 40 PPI\naccuracy 80 PPI\n$3,250 /m2 From Amazon35\n(PPI) (2 mm\nthickness)\n58"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed all my previous comments in their responses, and this revised version is now acceptable for publication. Congratulations to the authors. However, it should be noted that there is an unnecessary figure label “(d)” in Figure 7 of the main text, and the figure labels in Supplementary Figures S18 and S19 have been incorrectly marked.", "reviewer_2": "All the comments from the reviewer have been considered and properly answered by the authors. The reviewer, therefore, recommends publication of this work.", "reviewer_3": "I appreciate the additional experiments that the authors have performed and the explanations provided about the ways this work differs from their previous study published in Advanced Science.\nI still do not feel this work has produced significant or noteworthy results sufficient for publication in Nature Communications. The key insight from this work (that commercially available meshes and simple acid treatments can improve AEM electrolysis in KOH and that gas transport affects the electrochemically active surface area) seem to be based on optimization and do not represent major insights which are not already known to electrolysis practioners. The difference between the performance and water transport with and without acid washing (compared to alcohol washing) or for different meshes is marginal (see figure 4f, 6b)."}, "author_response": "Response to Reviewer’s comments on Nature Communications\nmanuscript NCOMMS-25-39851A\nWe really appreciate Editor and Reviewer for their valuable time and effort in reviewing our\nmanuscript and providing very constructive suggestions to improve the quality and clarity of our\nwork. In the following, we have taken great care to respond to the Editor’s and Reviewers’\ncomments as seen below and are confident that the manuscript has undergone substantial\nimprovements, making it suitable for Nature Communications:\nReviewer #1\nComment\nThe authors have adequately addressed all my previous comments in their responses, and\nthis revised version is now acceptable for publication. Congratulations to the authors.\nHowever, it should be noted that there is an unnecessary figure label “(d)” in Figure 7 of the\nmain text, and the figure labels in Supplementary Figures S18 and S19 have been incorrectly\nmarked.\nResponse:\nWe sincerely thank the reviewer for the positive feedback and kind congratulations. We have\ncarefully corrected these errors in the revised manuscript and supplementary information.\n1\nComment\nAll the comments from the reviewer have been considered and properly answered by the\nauthors. The reviewer, therefore, recommends publication of this work.\nResponse:\nWe are very grateful to the reviewer for their time and effort in reviewing our manuscript and for\nthe positive recommendation.\nComment\nI appreciate the additional experiments that the authors have performed and the\nexplanations provided about the ways this work differs from their previous study published\nin Advanced Science.\nI still do not feel this work has produced significant or noteworthy results sufficient for\npublication in Nature Communications. The key insight from this work (that commercially\navailable meshes and simple acid treatments can improve AEM electrolysis in KOH and that\ngas transport affects the electrochemically active surface area) seem to be based on\noptimization and do not represent major insights which are not already known to electrolysis\npractioners. The difference between the performance and water transport with and without\nacid washing (compared to alcohol washing) or for different meshes is marginal (see figure\n4f, 6b).\nResponse:\nWe appreciate the reviewer’s further comments. However, we believe there may have been a\nmisunderstanding regarding the primary scientific focus and the core contribution of our work.\nWe would like to clarify that the central theme of this study is not the optimization of surface\ntreatments (acid vs. alcohol washing) or the use of commercial meshes.\nInstead, the major scientific insight we aim to communicate is the fundamental shift in performance-\nlimiting factors at high current densities. Specifically:\n1. Bubble dynamics vs. ECSA: We demonstrate that at industrially relevant current densities (e.g.,\n> 1.0 A cm-2), the bubble dynamics becomes the dominant factor governing performance,\noutweighing the influence of the electrochemically active surface area (ECSA). This challenges\nthe conventional emphasis on maximizing ECSA through complex catalyst nanostructuring.\n2. Decoupling of factors: While the reviewer noted \"marginal\" differences in Figures 4f and 6b,\nwe would like to highlight that this is precisely part of our key finding: despite similar ECSA\nor initial surface conditions, the performance diverges significantly under high-current\noperations due to mass transport limitations that are often overlooked in standard lab-scale\ncharacterizations.\n3. Significance for practitioners: Our work reveals that for large-scale AEM electrolysis,\nengineering the physical structure for efficient bubble removal is more critical than incremental\nimprovements in intrinsic catalytic activity. This provides a clear, actionable direction for the\nfield to bridge the gap between lab-scale discovery and industrial-scale application.\nWe have revised the Introduction and Conclusion sections to more explicitly emphasize that the\n\"bubble dynamics at high current density\" is our primary discovery, ensuring it is not overshadowed\nby the description of the experimental preparation methods.\n4"}]

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