Local alkalinity enables high-performance pure water anion exchange membrane electrolysis

Domain: physics
Published: 2026-02-04
Editorial outcome: major_revision

Abstract
Anion exchange membrane water electrolyser is a highly promising electrolyser technology, but its performance in pure water is severely limited by the unsatisfactory OH− conductivity of the membrane. To overcome this critical challenge, we develop a local alkalinity engineering strategy that employs TiO2 nanoparticles in catalyst layers. These nanoparticles enrich OH− in the electric double layer at both electrodes, creating self-sustaining alkaline micro-environments (pH ~14), as confirmed by a scanning electrochemical microscopy technique integrating pH microelectrodes. As a result, the engineered electrolyser achieves a high current density of 3.0 A cm−2 at 2.08 V, approaching that of the precious-metal-based proton exchange membrane water electrolyser under identical conditions. In addition, the local alkalinity alleviates the degradation of non-noble metal catalysts and membrane, thus enabling the electrolyser to realise long-term stability of ~1400 h at 1.0 A cm−2. We also demonstrate that this local alkalinity strategy can be readily extended to different types of membranes and scaled up, providing a universal tactic to boost the performance of anion exchange membrane water electrolysers.

Introduction
Powered by renewable electricity, proton exchange membrane water electrolysers (PEMWEs) that can produce high-purity H2 directly from pure water are currently the leading technology for sustainable H2 production1–5. Nevertheless, PEMWEs rely on precious-metal-based catalysts (IrO2 and Pt), and their large-scale future deployment would considerably increase capital expenditures and the consumption of precious metals6,7. Assuming a global H2 supply of 400 GW per year, the annual demand for Ir would reach ~126 t—far exceeding its annual production (~1 t)5–7. Recently, anion-exchange membrane water electrolysers (AEMWEs)8–15 have attracted intensive attentions, as they eliminate the need for precious metals while retaining key advantages similar to PEMWEs.

Despite significant progress, the current performance of AEMWEs still lags far behind that of PEMWEs3,5. Specifically, the hydrogen production current density of AEMWEs supplied with pure water typically ranges 0.2–0.8 A cm−2 at 2.0 V16–22, lower than that of PEMWEs (2.0–3.0 A cm−2 at 2.0 V)23–25. The main challenge lies in the low ion conductivity of hydroxyl anions (OH−) in commercial anion membranes (10−3–10−2 S cm−1), compared to that of proton (H+) in PEMs (>10−1 S cm−1)10–12,26–28. Consequently, OH− generated at the cathode (2H2O + 2e− ! H2 + 2OH−) cannot be rapidly transported through the AEM to the anode for the oxygen evolution reaction (OER)20,21,26, slowing the reaction kinetics at both electrodes. More critically, insufficient OH− supply leads to anodic acidification, further degrading the long-term durability of AEMWEs (<200 h)17,18,22.

To tackle the challenges posed by inefficient OH− transport in AEMs, research efforts have focused on modifying the functional groups and/or backbone structures of AEM/ionomer to enhance OH− conduction10,29–34. While significant advancement has been achieved, improvements in OH− conductivity are usually accompanied by increased water uptake and membrane swelling9–11,35, which severely compromise the durability of AEMWEs. Therefore, the fundamental issue of inadequate OH⁻ transport remains unresolved, leading to sluggish reaction kinetics at both electrodes and high overpotentials in AEMWE. To mitigate these limitations, most reported AEMWE studies still employ alkaline electrolytes instead of pure water36–45. However, this approach introduces critical issues, including bipolar plate passivation by alkali and the need for circulation/recovery of corrosive electrolytes8,9. These issues not only raise maintenance costs but also generates substantial environmental concerns. Therefore, developing strategies to overcome the current AEMWE limitations caused by inadequate OH− transport remains imperative.

In this work, we overcome the performance and stability limitations of AEMWEs by introducing titanium dioxide (TiO2) to regulate the local microenvironment pH at both anode and cathode catalyst layers, establishing optimal alkaline conditions. Using a pH microelectrode coupled with scanning electrochemical microscopy (SECM), we precisely monitor in situ pH distributions in the catalyst layers. Our results show that TiO2 generates localised alkalinity within 5 µm from the catalyst surface, simultaneously enhancing reaction kinetics and OH− transport. Crucially, this alkaline microenvironment effectively stabilises non-precious metal catalysts and the AEM, leading to improved AEMWE durability.

Results
Identifying the performance bottleneck of AEMWE supplied with pure water
We assembled a typical AEMWE using NiFe oxide and NiMo alloy as the anode and cathode catalysts (Supplementary Figs. 1–3), respectively, and commercial Sustainion membrane as the AEM. As shown in Fig. 1a, the AEMWE demonstrates poor performance when supplied with deionized (DI) water (hereafter, AEMWE was supplied with DI water unless otherwise specified). Electrochemical impedance spectroscopy (EIS) reveals both high ohmic resistance (Rs) and charge transfer resistance (Rct) (Fig. 1b, inset), corresponding to inefficient OH− transport and sluggish reaction kinetics21,46, respectively. Notably, adding KOH to the feed water reduces both Rs and Rct values (Fig. 1b), consequently increasing the hydrogen production current density (Fig. 1a).

To investigate the reaction microenvironment of the AEMWE, we integrated a pH microelectrode (~10 µm tip) with SECM to monitor the in situ pH distributions in the cathode and anode catalyst layers (Fig. 1c, Supplementary Note 1 and Supplementary Fig. 4). This system operates with the microelectrode measuring local pH in the AEMWE through bipolar plate openings while SECM controls its precise positioning. As illustrated in Fig. 1d, at 1.9 V cell voltage, the local pH values in the anode (pHanode) and cathode catalyst layers (pHcathode) are 3.7–4.1 and 10.1–10.4, respectively. Similar results were also found in AEMWEs with different catalyst layer thicknesses (Supplementary Figs. 5–7), confirming weakly acidic (anode) and weakly alkaline (cathode) microenvironments—a deviation from the ideal strongly alkaline conditions required for efficient operation of AEMWE. This phenomenon arises from the low OH− conductivity of AEM—OH− produced at the cathode cannot be rapidly transported to the anode through the membrane20,21,26, which slows reaction kinetics and increasing the local pHanode. In this case, the anode proceeds OER via 2H2O ! O2 + 4H+ + 4e−, causing acidification. Notably, AEMWE supplied with 1.0 M KOH maintains pHanode and pHcathode values of ~14 (Supplementary Fig. 8). These collective results demonstrate that the poor performance of AEMWE operated in DI water (Fig. 1a, b) stems from non-ideal pH conditions. Moreover, the anodic acidification further compromises the stability of the AEMWE, which will be discussed in detail later.

Creating alkaline microenvironments for both electrodes
To overcome the fundamental performance limitations of AEMWEs caused by non-ideal pH conditions, we developed a microenvironment pH-regulating strategy for both electrodes—a previously unexplored approach in AEMWEs. According to our previous work47, the introduction of Cr2O3 (Cr3+, pK: ~2.0548) on a cobalt oxide catalyst can dynamically split natural seawater (pH ~7.9) and enrich OH− near the catalyst surface. Here, we employed TiO2 to regulate local pH in AEMWEs (Supplementary Figs. 9–11, and Supplementary Note 2), as it combines: (i) electrochemical stability across full HER and OER potential windows49,50, and (ii) optimal OH− affinity (Ti4+, pK: 2.2051). Based on these properties, we deposited ~ 5 nm TiO2 nanoparticles onto pre-synthesised anode NiFe oxide (anode) and NiMo alloy (cathode) catalysts (Fig. 2a, b and Supplementary Figs. 12–15).

We then assembled AEMWEs using TiO2-modified anode/cathode catalysts and commercial Sustainion membranes (Supplementary Tables 1, 2 and Supplementary Fig. 16). These electrolysers, designated as TiO-modified AEMWE-x (where a larger x represents an increasing TiO2 loading), were subjected to local pHanode and pHcathode monitoring. As illustrated in Fig. 2c, d, when no cell voltage was applied, the pHanode and pHcathode of these TiO2-modified AEMWEs are both ~7.0, the same as that of DI water. As the voltage increases, the pHanode and pHcathode values increase. Moreover, when the electrolysers were operating under the same voltage, the greater the coating amount of TiO2, the higher the pHanode and pHcathode values (Fig. 2c, d and Supplementary Fig. 16). Additionally, the pHanode and pHcathode of the TiO2-modified AEMWE-3 approach 14.0 when the cell voltage is >1.55 V (Fig. 2c, d; Supplementary Figs. 16 and 17), demonstrating that strong alkalinity was generated around both the anode and cathode catalyst layers. This important finding is further supported by the measured two-dimensional uniform distribution of pHanode and pHcathode (~14) in this TiO2-modified AEMWE-3 at 1.90 V (Fig. 2e).

Moreover, we quantified the regions where the local alkalinity acts on the surface of the anode and cathode catalyst layers. Specifically, we varied the distance between the pH microelectrode and the catalyst surface, and measured the local pHanode and pHcathode of TiO2-modified AEMWE-3. As shown in Fig. 2f, when the distance between the two is less than 5 µm, the measured pH ranges 13.8–12.9, and this value rapidly drops to ~7.0 when the distance exceeds 5 µm. For pHcathode, a similar trend was observed (Fig. 2g). These results indicate that the local strong alkalinity is in the range of 5 µm on the anode and cathode catalyst surface.

Mechanism of TiO2-mediated OH⁻ enrichment
To elucidate the formation mechanism of local alkalinity in AEMWEs, in situ surface-enhanced Raman spectroscopy was employed (Fig. 3). For the anode in a neutral electrolyte, TiO2 (Fig. 3a) and TiO2-modified NiFe oxide (Fig. 3c; Supplementary Figs. 18 and 19) exhibit potential-dependent enhancement of the H–O–H bending vibration (~1600 cm⁻1) from adsorbed water (H2O*)52, accompanied by emerging OH* peaks (~800 cm⁻1)52,53. This indicates active water dissociation on TiO2 (Supplementary Note 3 and Supplementary Fig. 20), in sharp contrast to pristine NiFe oxide under the same conditions (Fig. 3b), which shows negligible H2O* and OH* signals. Obviously, the potential-dependent evolution of these spectral features shows a clear correlation with the pH measurements (Fig. 2c), where TiO2-modified AEMWEs exhibit evident OH− accumulation (and thus increased pHanode), while the pristine AEMWE shows negligible change. This direct correspondence provides spectroscopic evidence that the local alkalinity originates from accelerated water dissociation on TiO2 (H2O → OH− + H⁺). Our density functional theory calculations support this point, revealing that TiO2 exhibits a low energy barrier for water dissociation under anode potential (Supplementary Fig. 21). This facilitated dissociation leads to the accumulation of OH− within the electrical double layer (EDL) due to electrostatic attraction to the positively charged electrode surface (Supplementary Fig. 22), while the generated H+ is rapidly extracted from the EDL under the applied potential gradient (Fig. 3e). This selective ion redistribution creates an alkaline microenvironment at the anode interface.

At the cathode, while the pristine TiO2 and NiMo alloy in neutral electrolyte (Fig. 3f, g) show weak H2O* signals, TiO2-modified NiMo alloy exhibits enhanced H2O* intensity and rapidly increasing H* signals (~1950 cm⁻1)24 with applied potential (Fig. 3h and Supplementary Fig. 23). These observations clearly demonstrate that the local alkalinity at the cathode (Fig. 2d) develops through a mechanism distinct from the anode’s direct water dissociation on TiO2. This should be a synergistic mechanism between TiO2 and NiMo alloy (Fig. 3e), where the NiMo alloy catalysers the Volmer step of HER (H2O + e− → OH− + H*)54, while oxophilic TiO2 50,55 adsorbs and enriches OH⁻. This OH− feedback accelerates the Volmer step on NiMo alloy, thereby establishing a self-sustaining alkaline microenvironment.

The above results demonstrate that at both electrodes, TiO2 enriches OH− in the EDL. The resulting alkaline environments enable the anode NiFe oxide and cathode NiMo alloy catalysts to proceed via alkaline OER and HER mechanisms, respectively. This is evidenced by the pronounced O–O bond53 (Supplementary Fig. 24) and H* intermediates observed on the TiO-modified NiFe oxide and NiMo in neutral electrolyte (Fig. 3c, h), similar to their behaviour in the alkaline electrolyte (Fig. 3d, i; Supplementary Note 4; Supplementary Figs. 25 and 26). Undoubtedly, this effect can enhance the reaction kinetics at both electrodes.

Performance of locally alkaline AEMWE
As discussed above, the performance of the pristine AEMWE is limited by unfavourable OH− transport and sluggish reaction kinetics (high Rs and Rct, Fig. 1b). EIS measurements show that the Rs and Rct values of the locally alkaline AEMWE (TiO2-modified AEMWE-3) are reduced compared to those of the pristine AEMWE (Fig. 4a). Moreover, it is found that the local alkalinity can also improve the OH− conductivity of AEM in DI water, with its value close to that of the membrane in 1.0 M KOH (Fig. 4b and Supplementary Fig. 27). This finding is reasonable that due to the boosted hydrogen evolution kinetics, a large amount of OH− is produced (2H2O + 2e− ! H2 + 2OH−), which increases the occupancy and transport of OH− in the membrane. Therefore, the local alkalinity can solve the critical issues of inefficient OH− transport and sluggish reaction kinetics in AEMWEs.

As a result, the locally alkaline AEMWE delivers a high hydrogen generation current density of 3.0 A cm−2 at 2.08 V, which is 8 times that of the pristine AEMWE (Fig. 4c; Supplementary Figs. 28 and 29). This performance is competitive with those reported for AEMWEs supplied with pure water16,17,19–21,56 and approaches that of the state-of-the-art PEMWE23–25 (Supplementary Table 3). It is noteworthy that the slight performance reduction of the locally alkaline AEMWE compared with that of the pristine AEMWE operated in 1.0 M KOH is due to the fact that TiO2 covers some active sites on the anode and cathode catalysts (Supplementary Fig. 30).

We further validated this microenvironment regulation strategy in electrolysers incorporating additional commercial AEMs (AEMIONTM and FAA), alongside the above investigated Sustainion membrane (Figs. 1 and 2). These membranes employ distinct OH− conduction mechanisms: Sustainion uses imidazole-functionalized polystyrene, AEMIONTM relies on methylated polybenzimidazole, and FAA depends on quaternary ammonium groups57. TiO2-modification consistently establishes local alkalinity across all membrane types (Supplementary Figs. 31 and 32), leading to performance enhancements. Specifically, at 1.9 V, the modified Sustainion-, AEMIONTM-, and FAA-based AEMWEs exhibit 9-, 8-, and 7-fold increases in hydrogen production current density, respectively, compared to their unmodified counterparts (Fig. 4d; Supplementary Figs. 33 and 34). This universal performance improvement across diverse membrane chemistries demonstrates the broad applicability of our microenvironment regulation strategy.

Long-term stability of locally alkaline AEMWE
Another key challenge of AEMWEs for practical applications is their poor long-term stability17,22,27. As discussed above, the local reaction environment of the anode catalyst in the pristine AEMWE is weakly acidic (pH ~4.0, Fig. 1d), which can obviously cause structural degradation of non-precious metal catalysts. To clarify this point, we quantified Fe and Ni dissolution from the anode NiFe oxide catalyst after 100 h operation at 1.9 V in both the pristine and locally alkaline AEMWEs. Note that the pHanode values of these AEMWEs are 3.7 (pristine), 8.7, 12.7 and 13.8 (locally alkaline) (Fig. 2c and Supplementary Fig. 35). As shown in Fig. 5a, the pristine AEMWE shows substantial metal dissolution (43 at% Fe, 59 at% Ni), confirming the destabilising effect of acidic conditions (Supplementary Fig. 36). Conversely, locally alkaline AEMWEs exhibit progressively reduced dissolution with increasing pHanode. Specifically, for the AEMWE with local pHanode of 13.8, the loss of Fe and Ni from the anode catalyst becomes negligible (Fig. 5a and Supplementary Fig. 36). Therefore, the in situ generated local alkalinity can effectively inhibit the detrimental dissolution of the anode catalyst.

Furthermore, according to previous works17,58, the anodic acidification can lead to degradation of membranes. Therefore, we pay close attention to the effect of pHanode on the structural changes of AEM after its assembly and operation in the electrolyser. Figure 5b shows the X-ray photoelectron spectroscopic (XPS) spectra of the original and used Sustainion membranes with different pHanode. As illustrated, the original membrane exhibits evident N+ (401.5 eV) and C−N+ (287.0 eV) peaks58 from its imidazole groups, which are crucial for OH⁻ conduction. Notably, as pHanode decreases, the intensities of the N+ and C−N+ signals decrease, suggesting H+-induced degradation of imidazole groups. Furthermore, when the membrane was operated at pHanode values of 8.7 and 3.7, a O−C=O (288.4 eV) appears, indicating oxidative degradation of the polymer chain58. In contrast, there is no noticeable change observed in the membrane of the locally alkaline AEMWE with pHanode of 13.8. These results demonstrates that in situ generated local alkalinity can effectively supress AEM degradation.

Afterwards, we evaluated the long-term stability of our locally alkaline AEMWE (TiO2-modified AEMWE-3). As shown in Fig. 5c, this electrolyser works stably for more than 1400 h at a high current density of 1.0 A cm−2, with a voltage drift as small as 60 µV h−1. Such stability is better than those reported for AEMWEs operating in pure water10,16–18,22,29 (in the range of 10–200 h and measured at lower current densities of 100–500 mA cm−2). This finding is further supported by our measured changes in the local pHanode and pHcathode of our locally alkaline AEMWE as a function of time (Fig. 5d, e and Supplementary Fig. 37), which demonstrated that the strong local alkalinity in the anode and cathode catalyst layers was well maintained during the operation of the electrolyser.

Scale up of locally alkaline AEMWE
We fabricated a 10-cell locally alkaline AEM stack connected in series (Fig. 6a), featuring a total electrode area of 250 cm² (25 cm² per single cell). As shown in Fig. 6b, individual cells perform similarly, obtaining a current density of 2.0 A cm−2 at 1.94 ± 0.02 V, while the overall stack achieves the same current density at 21.0 V. Moreover, our locally alkaline AEM stack demonstrates good durability with a degradation rate of 117 µV h−1 during 500 h of operation at 1.0 A cm−2 (Fig. 6c). Assuming maintenance of >80% initial performance at this decay rate, the projected operational lifetime of the stack exceeds 32,000 h—a duration that would satisfy industrial requirements. Importantly, the locally alkaline AEM stack supplied with DI water achieves an energy consumption of 49.5 kWh kg−1 H2 and an energy conversion efficiency of 74.5% (Fig. 6c), comparable to those of the state-of-the-art PEM stack based on precious metal catalysts (46.6 kWh kg−1 H2 and 79.2% conversion efficiency), also approaching the Department of Energy’s (DOE) 2050 target for AEMWE technology (75% conversion efficiency).

To assess economic viability, we conducted a levelized cost of hydrogen (LCOH) analysis (Supplementary Note 5 and Supplementary Figs. 38–41). The results demonstrate cost advantages for our locally alkaline AEM stack compared to precious-metal-dependent PEM systems, with a median LCOH reduction of 25.4% (Supplementary Fig. 38). When compared to KOH-enhanced AEM systems, our approach maintains competitive performance while effectively avoiding the operational challenges associated with alkaline corrosion and wastewater treatment, exhibiting economic stability (Supplementary Figs. 39–41). This combination of technical advantages and economic benefits positions our locally alkaline AEMWE technology as an ideal solution for commercial hydrogen production.

Methods
Materials synthesis
The NiFe oxide catalyst was fabricated by a solvothermal method. Specifically, a precursor solution was firstly prepared by dissolving 0.5 mM Ni(NO3)2·6H2O (≥99%, Aladdin), 0.5 mM Fe(NO3)3·9H2O (≥99%, Aladdin) and 10 mM urea (≥99%, Aladdin) in 35 mL of DI water. Then, a pre-cleaned Ni fibre felt was placed vertically in the precursor solution, transferred to a 50 mL Teflon autoclave and treated at 120 °C for 12 h. Afterwards, the Ni fibre felt was thoroughly washed with DI water, dried at room temperature (25 °C), and heated in N2 atmosphere at 600 °C for 2 h to obtain the NiFe oxide catalyst. Note that the loading mass of as-prepared NiFe oxide catalyst on Ni fibre felt was ~3 mg cm−2.

The NiMo alloy catalyst was synthesised according to the previous work54. Firstly, a precursor solution was prepared by dissolving 0.08 M Ni(NO3)2·6H2O and 0.02 M (NH4)6Mo7O24·4H2O (99%, Aladdin) in 30 mL of deionized water. Afterwards, a pre-cleaned Ni fibre felt was placed in the precursor solution, then transferred to a 50 mL Teflon autoclave, and treated at 150 °C for 6 h. Next, the Ni fibre felt was washed with DI water, dried naturally and reduced at 600 °C for 2 h under H2/Ar (v/v=15/85) atmosphere to obtain NiMo alloy catalyst. Note that the loading mass of NiMo alloy catalyst on Ni fibre felt was ~3.2 mg cm−2.

For the TiO2-modified NiFe oxide or NiMo alloy catalysts, a titanium precursor was prepared by dissolving titanium isopropyl alcohol (TiC9H19NO4) in ethanol. Then, the Ni fibre felt loaded with NiFe oxide or NiMo alloy catalyst was immersed in the precursor solution, dried naturally and heated at 400 °C for 3 h in N2 flow to obtain the TiO2-modified NiFe oxide or NiMo alloy catalyst. Note that for 4, 8, 12 and 20 at% TiO2-modified NiFe oxide, 0.05, 0.1, 0.2, 0.5 M TiC9H19NO4 in ethanol were used as precursor, respectively. For 2, 7, 10 and 16 at% TiO2-modified NiMo alloy, 0.02, 0.1, 0.2, 0.5 M TiC9H19NO4 in ethanol were used, respectively.

Materials characterisation
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were carried out on a Hitachi S-4800 SEM and a JOEL 2100 TEM, respectively. X-ray diffraction patterns (XRD) were obtained using a Bruker D8 Advance diffractometer with a Cu Kα radiation operating at 20 kV. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were performed on an Agilent Varian 700. XPS spectra were collected using a Kα Thermo Fisher spectrometer. Local pH measurements were carried out on a commercial CHI920D SECM system (CH Instruments, China), which was placed on a shock-proof platform to minimise vibration disturbance. The whole testing system was put in a shielding box. The performance of AEMWEs was measured using a programmable DC power supply (ITECH).

In situ surface-enhanced Raman spectroscopy
In situ surface-enhanced Raman spectra were collected using a confocal Raman microscope (Horiba LabRAM HR Evolution) equipped with an excitation wavelength of 532 nm and a 50× objective (Supplementary Fig. 18). Each Raman spectrum was acquired over a collection time of 30 s and was the average of three measurements. The working electrode was a rough Au electrode loaded with catalyst. The counter electrode was a Pt wire and the reference electrode was Ag/AgCl/KCl (3.0 M). The latter was calibrated in H₂-saturated electrolyte using Pt foils as both counter and working electrodes. All OER and HER potentials were converted to the RHE scale by averaging the two zero-current potentials from cyclic voltammetry scans at 1 mV s−1. The conversion used was VRHE = VAg/AgCl + 0.61 V in 0.5 M Na₂SO₄ and VRHE = VAg/AgCl + 1.02 V in 1.0 M KOH. In addition, laser irradiation was performed for 1 h before in situ test.

Measurement of local pH distributions in catalyst layers of AEMWE
The Pt/IrO2 microelectrode was fabricated based on electrochemical cyclic voltammetry59. First, the iridium precursor solution for electrodeposition was prepared. Specifically, 0.075 g of IrCl4·xH2O (99.99%, Alfa Aesar) was dissolved in 50 mL of pure water and magnetically stirred for 30 min. Subsequently, 0.5 mL of H2O2 (30 wt%, Meryer) was added, and the mixture was further stirred magnetically for an additional 10 min. Then, 0.25 g of oxalic acid (99.5%, Meryer) was introduced, followed by continued stirring for another 10 min. Anhydrous potassium carbonate was gradually added to adjust the pH to 10.5. The resulting solution was stirred for 2 days, during which it gradually turned dark blue, and then stored at 25 °C for subsequent use. The electrodeposition experiments were conducted in a three-electrode configuration using the prepared iridium solution, with a 10 μm diameter Pt microelectrode serving as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl/KCl (3.0 M) electrode as the reference electrode. Cyclic voltammetry was carried out over a potential range of 0.2 to 0.75 V (vs. Ag/AgCl) at a scan rate of 50 mV s−1 for 100 cycles in the previously prepared Ir iridium solution. After electrodeposition, the Pt/IrO2 microelectrode was removed, rinsed with pure water, dried, and then aged in an oven at 100 °C for 2 h before being stored for further use.

To measure the local pH within AEMWEs, a three-electrode configuration was established using a Pt/IrO2 microelectrode as the working electrode, a Pt wire and an Ag/AgCl electrode as the counter electrode and reference electrode, respectively. Furthermore, precise movement of the Pt/IrO2 microelectrode was realised by the controlling of SECM. For measuring the pH within the catalyst layers, an area of 1×1 cm2 was opened in the bipolar plate of the AEMWE (Fig. 1c). During the test, the Pt/IrO2 microelectrode approached the catalyst layer at a rate of 0.2 μm s−1, and the open circuit potential (Eoc) was monitored. Once the tip of the Pt/IrO2 microelectrode contacted the catalyst layer, a marked change in Eoc was observed (Supplementary Fig. 4), and the Pt/IrO2 microelectrode was subsequently lifted upward at a rate of 0.2 μm s−1. When the tip was 1.0 μm away from the catalyst layer, Eoc was recorded, and the corresponding local pH value in the catalyst layer was deduced based on its relationship with Eoc (Supplementary Fig. 4), as shown detailed in the previous works47,59. The two-dimensional pH distribution with dimensions of 100×100 μm2 was mapped at a scan rate of 40 μm s−1. All measurements were performed for at least three repetitions.

Membrane electrode assembly (MEA)
For the AEMWE, commercial anion-exchange membranes (Sustainion X37-50, AEMION™ AF1-HNN8-50-X, or FAA-3-50; thickness: 50 μm) were employed. Each membrane was pre-treated with 1.0 M KOH for 48 h, and then treated with fresh KOH for 24 h. Next, the membrane was washed with DI water until the pH of the water reached neutral. The catalyst integration comprised two components: pre-grown catalysts directly on nickel fibres and catalysts spray-coated onto the membrane. For the spray-coated catalysts, NiFe oxide and NiMo alloy were first exfoliated from nickel felt substrates via ultrasonication. Catalyst inks were prepared by homogeneously dispersing 100 mg of the exfoliated catalysts in 10 mL of isopropanol/water mixture (9:1, v/v) with 10 wt% Sustainion XB-7 ionomer (relative to catalyst mass) under 3 h sonication. These inks were then deposited onto both sides of the pretreated AEM using ultrasonic spray coating, achieving a loading of approximately 1 mg cm⁻2. Then, the spray-coated AEM, the Ni fibres loaded with NiFe oxide (~3.0 mg cm−2) and NiMo alloy (~3.2 mg cm−2) catalysts, and Teflon gaskets were assembled together with a torque wrench tighten the screws to 5.0 Nm−1 (Supplementary Fig. 1).

For each cell of the PEM stack, a Nafion 115 membrane was utilised and subjected to sequential 1-hour treatments at 80 °C in 5 wt% H2O2, 1.0 M H2SO4, and DI water, Then, the membrane was rinsed with DI water until neutral pH was achieved. Following this, commercial IrO2 (2.0 mg cm−2) and Pt/C (0.5 mg cm−2) catalysts were sprayed onto both sides of the prepared PEM. For catalyst inks (Pt/C and RuO2) ink preparation, 100 mg of catalyst (Pt/C or RuO2) was ultrasonically dispersed for 2 h at 25 °C in a 10 mL mixture of isopropanol and water (1:3 v/v) containing 5 wt% Nafion. The assembly was completed by stacking the catalyst-coated membrane with Ti fibre felts and Teflon gaskets, and tightening the screws to a torque of 5.0 Nm−1 with a torque wrench.

Electrolyser and stack tests
During tests, the feed water/electrolyte was continuously supplied to both electrodes of the AEM electrolyser (active area: 1×1 cm2) and 10-cell stack (single cell active area: 5×5 cm2) using a two-channel peristaltic pump, with flow rates maintained at 20 mL/min for the electrolyser and 200 mL/min for the stack (Supplementary Note 6). The operating temperature of the electrolyser was maintained at 60 °C using a temperature controller and thermocouples integrated into the endplates. Note that the electrolytes used were pure water (pH= 7.04±0.08) or 1.0 M KOH (pH=13.93±0.03), both solutions were freshly prepared before each test. Prior to testing, the electrolyser (or stack) underwent a pre-activation procedure via chronopotentiometry, with the current density stepped from 0.1 to 1.0 A cm−2 (increments of 0.1 A cm−2, 1 min per step)24. Polarisation curves without iR compensation were then recorded by stepping the voltage from 1.4 to 2.4 V (increments of 0.02 V, 20 s per step). EIS was subsequently conducted at 1.5 V, with a frequency range from 100 kHz to 0.01 Hz and an amplitude of 5 mV.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This article reposts some interesting results on the effect of titanium oxide nanoparticles in the catalyst layer to improve the performance of non-PGM based AEMWE with pure water operation. The local pH value was measured to verify the effect of titanium oxide to increase the pH for better HER and OER in the operating cell. I am supportive of this article in Nature Comm, however, there are technical concerns that have to be well-addressed before the final decision can be made.\nMajor concerns:\n1) Although the authors claim that the AEMWE exhibited comparable performance with PEMWE under identical conditions, it is not mentioned how pure water was supplied. I believe that pure water was supplied to the anode side only since in the PEMWE water is not supplied to the cathode in order to obtain pure (or less humid) hydrogen. In AEMWE, dry cathode operation is also desirable but much more challenging since water has to be permeated through the membrane to the cathode. Effect of titanium oxide nanoparticle in the dry cathode reaction should be discussed, comparing with that of wet cathode reaction (water is supplied to both electrodes).\n2) Effect of titanium oxide nanoparticle on the mass transport (in particular, gas diffusivity) is concerned. Since one of the major advantages of membrane-based water electrolyzers is high current density operability, run the WE at least up to 4 A cm-2 of the current density (or 2.4 V of the cell voltage) in Figure 4c, as were done in Figure 1a.\n3) On page 11, why did membranes affect the electrochemical performance of the electrodes? If the authors mean AEMION and FAA were used as ionomer (binder) in the catalyst layer, it should be mentioned so (but I do not believe that) and the structural differences of those ionomers should be taken into account for the discussion.\n4) In addition to the catalyst materials, post-stability test analysis should be done for titanium particles (size and distribution, etc.).\nMinor concerns:\n1) On page 2 last line, the authors mention that alkaline water is used in most AEMWEs to improve the alkaline stability, but this is not very true. The alkaline water is needed rather to facilitate the electrochemical reaction and lower the overpotential.\n2) On page 19 line 3, specify the ionomer material used in the catalyst ink. Was it the same anon exchange polymer as the AEMs? What was the ionomer/catalyst weight ratio for each electrode?", "reviewer_2": "This paper reports the deposition of TiO₂ on both anode and cathode catalysts establishes an interfacial alkaline environment, thus enhancing water dissociation and enabling efficient pure water electrolysis in an anion exchange membrane electrolyzer. Furthermore, this approach mitigates the degradation of the membrane and metal catalysts, leading to prolonged operational stability under industrial-scale current densities. Before this paper can be accepted, the following issues should be addressed:\n1. In Figure 1c, pH detection on the electrode surface is achieved by introducing microelectrodes and scanning electrochemical microscopy (SECM). What is the principle of this test? Can detailed descriptions and more intuitive models be supplemented in the Supporting Information (SI)? After introducing this three-electrode system into the electrolyzer, will it have an impact on the original circuit of the electrolyzer? The paper mentions that pH detection with micrometer-scale spatial resolution can be achieved (Page 4, Line 79). I have a question regarding this: why can OH⁻ generated in the catalyst layer (on the nanometer scale) be detected in the micrometer-scale solution region? In addition, can the specific quantity of OH⁻ produced by TiO₂-facilitated water dissociation be provided, and can the dynamic process of OH⁻ diffusion from the catalyst layer to the electrical double layer and then to the micrometer-scale solution phase be elaborated? At the same time, please explain: can the rate of OH⁻ generation via water dissociation catalyzed by TiO₂ match the consumption rate of OH⁻?\n2. As described in Page 7, Line 118, after depositing TiO₂ on the catalyst, the pH of the system is 7 when no voltage is applied; as voltage is applied, the pH increases with the rise of voltage, even reaching strong alkalinity (pH = 14). It is known that TiO₂ coating on the catalyst surface significantly reduces conductivity, and its electron conductivity is almost negligible. Why can a large amount of OH⁻ still be generated as the voltage increases? What is the surface state of TiO₂ under this condition?\n3. Page 7, Line 124 mentions that the pH values of both the cathode and anode increase to 14; Page 8, Line 143 explains that OH⁻ accumulates in the electrical double layer at the anode because the anode is positively charged. However, the cathode is negatively charged—why does no H⁺ enrichment occur (which would result in acidity) at the cathode? Furthermore, the thickness of the electrical double layer is on the nanometer scale, while the detection depth mentioned in the paper is on the micrometer scale. Is this description sufficiently accurate?\n4. Page 8, Line 158 describes that as the potential decreases, the content of adsorbed water molecules (H₂O*) increases, and the content of the intermediate species adsorbed hydrogen (H*) also increases accordingly. If it is assumed that TiO₂ promotes water dissociation at the catalytic interface to generate H*, the amount of H₂O* at the interface should decrease—so why does the experimental result show that both increase simultaneously? In addition, what is the basis for assigning a specific characteristic peak to H*? It seems that supplementary verification data using D₂O or H₂¹⁸O are not provided in the Supporting Information. Similarly, for the assignment of the intermediate species OOH* during the oxygen evolution reaction (OER) mentioned in Page 9, Line 168, no further verification has been conducted using D₂O and H₂¹⁸O. Raman data appears to be the key basis for the paper to confirm the rapid water dissociation capability of TiO₂, and all the above questions need to be answered.\n5. Page 19, Lines 357–373 mentions that the catalyst is loaded via ultrasonic spraying. However, the previous text states that the anode and cathode catalysts are synthesized on nickel felt via hydrothermal synthesis. When preparing the catalyst ink, is the catalyst first ultrasonically exfoliated from the nickel felt and then loaded onto the membrane via ultrasonic spraying? In addition, the thickness of the commercial membrane used in the paper is not specified—what is its specific value? Meanwhile, the type of ionomer added to the anode and cathode catalysts also needs to be clarified—specifically, which type of ionomer is used? All the above information should be clearly supplemented in the paper to avoid misunderstanding among readers.", "reviewer_3": "Guo et al. report a ‘solid base’ strategy that enriches local hydroxide (OH–) concentration within the catalyst layer for high-performance pure-water AEMWE. They introduce TiO2 nanoparticles on both the anode and cathode catalysts, and then employ pH ultramicroelectrode and surface-enhanced Raman spectroscopy to in situ probe the interfacial microenvironment at the catalyst–electrolyte interface. The results reveal a TiO2-dominated localized alkalinity, accelerating the Volmer step at the cathode and water dissociation at the anode. They also evaluate electrolyzer and stack performance and estimate the levelized cost of hydrogen; collectively, these cross-scale investigations underscore the concept’s device-level potential. In parallel, the manuscript presents several notable weaknesses, particularly concerning (i) the definition and adaptation of the ‘solid-base’ concept and (ii) the mechanistic evidence demonstrating its operation. Before the manuscript can be considered for publication in Nat. Commun., the authors should systematically address the following issues.\n1. The primary issue lies in using the core concept of ‘solid base’. In heterogeneous catalysis and solid acid–base chemistry, a solid base denotes a solid that features surface basic sites, namely Lewis (e.g., O2–/M–O⁻) and Brønsted (e.g., –OH) sites, with representative materials such as alkali and alkaline-earth oxides and layered double hydroxides (LDHs). Its mechanism involves Lewis-type electron-pair donation and Brønsted-type proton abstraction at surface basic sites, thereby facilitating adsorption and activation of acidic substrates, which in turn promotes deprotonation, transesterification, and condensation, and regulates the interfacial microenvironment.\nHowever, TiO2 is generally amphoteric and often weakly acidic; in surface chemistry, it is more commonly regarded as a solid acid rather than a solid base. In addition, based on the authors’ experimental results, TiO2 primarily acts by enriching OH⁻ and modulating the local interfacial environments, thereby assisting the reaction kinetics rather than serving as the principal catalytic active site. This differs, in terms of terminological scope, from classical solid acid–base chemistry, where surface basic centers activate substrates via Brønsted proton abstraction or Lewis electron-pair donation. Borrowing the concept here risks implying that intrinsic basic sites of the material cause alkalization, whereas a more faithful explanation is local pH regulation under applied bias, driven by OH⁻ generation and ionic redistribution in the electric double layer. I recommend that the authors reconsider using the ‘solid-base’ concept and adopt a more precise conceptual framing aligned with their experimental evidence.\n2. The mechanistic role of TiO2 nanoparticles has not yet been systematically established. Although extensive in situ characterizations were performed, the data indicate that introducing TiO2 correlates with accelerated OER at the anode and HER at the cathode, and with enhanced near-interfacial OH⁻ concentrations in the catalyst layer (i.e., an elevated local pH). However, the pathway by which TiO2 leads to OH⁻ enrichment remains insufficiently evidenced, and the molecular-level mechanism is still unclear. The XPS results show predominantly Ti4+ with no discernible Ti3+ features, suggesting a low concentration of oxygen vacancies. According to established surface chemistry, stoichiometric TiO2 surfaces typically adsorb water molecularly, whereas dissociative adsorption leads to surface –OH being more favorable when oxygen vacancies are present. I recommend defect-sensitive measurements (e.g., EPR) and first-principles calculations to contrast water adsorption/dissociation on defective versus stoichiometric TiO2, thereby clarifying the specific mechanism, explaining the performance gains, and avoiding overly speculative interpretations.\n3. In Figures 3h and 3i, the strong band at ~1900 cm–1 is assigned to *H. Given its high intensity, this assignment warrants careful verification. I recommend isotope substitution experiments; if the band arises from a surface metal–H vibration, the wavenumber should exhibit a characteristic red shift. This control would help determine whether the band corresponds to *H rather than other adsorbed species."}, "author_response": "Response to Reviewer #1\nGeneral Comments:\nThis article reposts some interesting results on the effect of titanium oxide nanoparticles in the catalyst\nlayer to improve the performance of non-PGM based AEMWE with pure water operation. The local\npH value was measured to verify the effect of titanium oxide to increase the pH for better HER and\nOER in the operating cell. I am supportive of this article in Nature Comm, however, there are technical\nconcerns that have to be well-addressed before the final decision can be made.\nResponse:\nWe sincerely thank the Reviewer for his/her valuable comments, which have helped us significantly\nimprove the quality of our manuscript. In response, we have conducted additional experiments and\nprovided more detailed explanations and discussions to address the points raised and to further support\nour conclusions.\nOriginal Comment 1-1: Although the authors claim that the AEMWE exhibited comparable\nperformance with PEMWE under identical conditions, it is not mentioned how pure water was supplied.\nI believe that pure water was supplied to the anode side only since in the PEMWE water is not supplied\nto the cathode in order to obtain pure (or less humid) hydrogen. In AEMWE, dry cathode operation is\nalso desirable but much more challenging since water has to be permeated through the membrane to\nthe cathode. Effect of titanium oxide nanoparticle in the dry cathode reaction should be discussed,\ncomparing with that of wet cathode reaction (water is supplied to both electrodes).\nResponse:\nThanks for this comment. According to the suggestion of the Reviewer, we have compared the\nperformance of AEMWE under anode-only water feeding (dry cathode) and dual-side water feeding\nconditions. As shown in Fig. R1a, under anode-only feeding with DI water, the TiO -modified\nwas supplied under both feeding configurations (Fig. R1b). This indicates that under dry cathode\nconditions, water can still permeate from the anode to the cathode. Under such circumstances, the TiO\n2\nlayer at the cathode can still effectively capture OH⁻ ions and help maintain an alkaline micro-\nenvironment at the catalyst layer, similar to the case under dual-side water feeding.\nWe also compared the stability under anode-only and dual-side water feeding with DI water, and\nfound a slight decrease in stability under the anode-only feeding condition (Fig. R1c). This observation\naligns with the expectation of the Reviewer: the dry cathode configuration still faces inherent\nchallenges, including insufficient hydration causing membrane drying, reduced ion conductivity, and\n1\npotential degradation risks. Therefore, we adopted a dual-side water feeding configuration in this work.\nFigure R1. (a, b) Polarization curves of the TiO₂-modified AEMWE with (a) DI water and (b)1.0 M\nKOH, comparing anode-only and dual-side water feeding configurations. (c) Chronopotentiometry\ncurves of the TiO -modified AEMWE operating at 1.0 A cm⁻2, comparing anode-only and dual-side\n2\nwater feeding modes.\nIn response to fully address this comment, we have revised our manuscript as follows:\ni. Supplementary Note 5 has been added in the revised supporting information to discuss the water\nfeed mode of locally alkaline AEMWE.\n“Supplementary Note 5. Water feed mode of locally alkaline AEMWE.\nWe have compared the performance under anode-only and dual-side water feeding conditions. As\nshown in Supplementary Fig. 27a, under anode-only feeding with DI water, the locally alkaline\nAEMWE showed a very slight performance decrease. A similar trend was observed when 1.0 M KOH\nwas supplied under both feeding configurations (Supplementary Fig. 27b). This indicates that under\ndry cathode conditions, water can still permeate from the anode to the cathode. Under such\ncircumstances, the TiO layer at the cathode can still effectively capture OH⁻ ions and help maintain\n2\nan alkaline micro-environment at the catalyst layer, similar to the case under dual-side water feeding.\nWe also compared the stability under anode-only and dual-side water feeding with DI water and found\na slight decrease in stability under the anode-only feeding condition (Supplementary Fig. 27c). This\nsuggests that although water permeation from the anode to the cathode is confirmed, the dry cathode\nconfiguration still presents some limitations. These includes insufficient hydration leading to\nmembrane drying, reduced ion conductivity, and potential degradation issues.”\nii. Fig. R1 has been added as Supplementary Fig. 27 in the revised supporting information.\nOriginal comment 1-2: Effect of titanium oxide nanoparticle on the mass transport (in particular, gas\ndiffusivity) is concerned. Since one of the major advantages of membrane-based water electrolysers is\nhigh current density operability, run the WE at least up to 4 A cm-2 of the current density (or 2.4 V of\nthe cell voltage) in Figure 4c, as were done in Figure 1a.\nFollowing the Reviewer’s suggestion, we have provided the polarization curve of the TiO -modified\ncm⁻2 without any discernible mass transport limitation. This result is reasonable because the TiO\n2\nnanoparticles (3–5 nm) are confined to the catalyst layer and are far smaller than typical gas bubbles\n(tens to hundreds of micrometres, ACS Nano 2023, 17, 23299). Thus, their localized presence does not\nblock the primary pathways for gas transport.\nFigure R2. Polarization curve of the TiO -modified AEMWE supplied with DI water.\nhave been provided in the new Fig. 4c in the revised manuscript.\nOriginal comment 1-3: On page 11, why did membranes affect the electrochemical performance of\nthe electrodes? If the authors mean AEMION and FAA were used as ionomer (binder) in the catalyst\nlayer, it should be mentioned so (but I do not believe that) and the structural differences of those\nionomers should be taken into account for the discussion.\nResponse:\nWe thank the Reviewer for this comment. To clarify, the membranes (AEMION and FAA-3-50) were\nused as the membrane and not as the ionomer in the catalyst layer. The ionomer in all AEMWE\ninvestigated was Sustainion XB-7. The observed performance differences among these membranes\noriginate from their distinct intrinsic properties, including OH⁻ conductivity, water uptake, swelling\nbehaviour, and mechanical strength. These characteristics collectively govern the OH⁻ transport\nefficiency and interfacial contact resistance in the MEA, thus influencing overall electrolysis\nperformance. We note that among the three commercial membranes, Sustainion demonstrates the most\nfavourable combination of these properties (J. Electrochem. Energy Convers. Storage 2021, 18,\n3\n024001).\nTo support this point, we performed electrochemical impedance spectroscopy (EIS) on AEMWEs\nassembled with each membrane. As shown in Fig. R3a, the cell with the Sustainion membrane\nexhibited the lowest ohmic (R) and charge transfer (R ) resistances. This result is consistent with the\ns ct\nsuperior intrinsic properties of Sustainion, which collectively contribute to its enhanced performance\n(Fig. R3b).\nFigure R3. (a) EIS curves of TiO -modified AEMWEs assembled with three commercial membranes,\n2\nmeasured at 1.5 V, with the inset showing their corresponding R and R values.\ns ct\n(b) Polarization curves\nof the TiO -modified AEMWEs assembled with three commercial membranes.\ninformation, along with the corresponding discussion in the figure caption.\n“As shown, the cell with the Sustainion membrane exhibited the lowest R and R ₜ resistances (a). This\ns c\ntrend correlates well with its superior electrolysis performance (b).”\nOriginal comment 1-4: In addition to the catalyst materials, post-stability test analysis should be done\nfor titanium particles (size and distribution, etc.).\nResponse:\nFollowing the Reviewer’s suggestion, we conducted a detailed analysis using HAADF-STEM to\ncompare the size and distribution of TiO particles before and after the stability test. As shown in Fig.\n2 1\nb ) results from the electron beam irradiation damage following immersion in the electrolyte.\n2\n2 1 2\n(b , b ) after testing the TiO -modified AEMWE at 1.0 A cm−2 in DI water.\n1 2 2\nIn response, we have added Fig. R4 as Supplementary Fig. 35 in the revised supporting\ninformation, along with the corresponding discussion in the figure caption.\n“As shown, the TiO nanoparticles maintained their original distribution without noticeable\n2\nagglomeration or size change after stability testing.”\nOriginal comment 1-5-1:\n1) On page 2 last line, the authors mention that alkaline water is used in most AEMWEs to improve\nthe alkaline stability, but this is not very true. The alkaline water is needed rather to facilitate the\nelectrochemical reaction and lower the overpotential.\nResponse:\nFollowing the suggestion of the Reviewer, we have revised the relevant description in the revised\nmanuscript (pages 2-3):\n“Therefore, the fundamental issue of inadequate OH⁻ transport remains unresolved, leading to sluggish\nreaction kinetics at both electrodes and high overpotentials in AEMWE. To mitigate these limitations,\nmost reported AEMWE studies still employ alkaline electrolytes instead of pure water36-45.”\nOriginal comment 1-5-2:\n2) On page 19 line 3, specify the ionomer material used in the catalyst ink. Was it the same anon\n5\nexchange polymer as the AEMs? What was the ionomer/catalyst weight ratio for each electrode?\nResponse:\nAs suggested, we have provided the detailed catalyst ink preparation procedure in the Methods section\nof the revised manuscript (page 18):\n“Catalyst inks were prepared by homogeneously dispersing 100 mg of the exfoliated catalysts in 10\nmL of isopropanol/water mixture (9:1, v/v) with 10 wt% Sustainion XB-7 ionomer (relative to catalyst\nmass) under 3 h sonication.”\nResponse to Reviewer #2\nGeneral Comments:\nThis paper reports the deposition of TiO₂ on both anode and cathode catalysts establishes an interfacial\nalkaline environment, thus enhancing water dissociation and enabling efficient pure water electrolysis\nin an anion exchange membrane electrolyser. Furthermore, this approach mitigates the degradation\nof the membrane and metal catalysts, leading to prolonged operational stability under industrial-scale\ncurrent densities. Before this paper can be accepted, the following issues should be addressed:\nResponse:\nWe sincerely thank the Reviewer for his/her valuable comments, which have helped us significantly\nimprove the quality of our manuscript. In response, we have conducted additional experiments and\nprovided more detailed explanations and discussions to address the points raised and to further support\nour conclusions.\nOriginal comment 2-1-1:\nIn Figure 1c, pH detection on the electrode surface is achieved by introducing microelectrodes and\nscanning electrochemical microscopy (SECM). What is the principle of this test? Can detailed\ndescriptions and more intuitive models be supplemented in the Supporting Information (SI)?\nResponse:\nWe thank the Reviewer for this insightful comment. Our method (Fig. R5a, b) is based on the Nernstian\nresponse of iridium oxide (IrO ) to hydrogen ion activity. Specifically, the IrO /IrO(OH) redox couple\nx x\nexhibits a linear open-circuit potential (E ) dependence on pH (E ∝ -0.059 pH), according to the\noc oc\nNernst equation (Fig. R5c, d).\nTo implement this principle for spatially resolved local pH under operating conditions, we\nestablished the following experimental configuration. An ultra-micro pH sensor was fabricated by\nelectrodepositing an IrO film onto a platinum microelectrode tip (10 μm diameter). A 1 × 1 cm2\nx\n6\nobservation window was machined into the bipolar plate of the AEMWE (Fig. R5a), and the pH\nmicroelectrode was positioned ~1 μm above the catalyst layer surface using the high-precision\npositioning module of the SECM. During AEMWE operation, changes in E were recorded and\noc\nconverted into local pH values using the pre-calibrated E –pH curve (Fig. R5c, d).\noc\nFigure R5. (a, b) Schematic of SECM-pH ultra-microelectrode detection system. (c) Time and (d) pH\ndependence of E of Pt/IrO ultra-microelectrode in electrolyte with varied pH values.\noc x\nIn response to fully address this comment, we have added Fig. R5 as Supplementary Fig. 4 in the\nrevised supporting information, along with the detailed descriptions in the caption.\n“Note that the measurement principle is grounded in the Nernst equation and the\nunique electrochemical properties of IrO films. Specifically, the IrO /IrO(OH) redox couple exhibits\nx x\na Nernstian response to hydrogen ion activity, with its open-circuit potential (E ) demonstrating a\noc\nlinear dependence on solution pH (E ∝ -0.059 pH). Specifically, the relationship between E and\nOC oc\nthe value of pH can be described by the following equation,\npH = a × (E + b) (1)\noc\nwhere a and b were obtained by linear fitting of E against the pH of the electrolyte. On this basis,\noc\ncorresponding local pH value in the catalyst layer of AEMWE was derived according to the\ndependence of pH on E .”\noc\nAfter introducing this three-electrode system into the electrolyser, will it have an impact on the original\ncircuit of the electrolyser?\nResponse:\nThe integration of the SECM-pH microelectrode system does not interfere with the operation of the\nelectrolyser. The SECM-pH microelectrode circuit is a high-impedance, near-zero-current system\ndedicated solely to monitoring the open-circuit potential. Therefore, it operates completely\nindependently from the AEMWE’s original circuit.\nOriginal comment 2-1-3:\nThe paper mentions that pH detection with micrometre-scale spatial resolution can be achieved (Page\n4, Line 79). I have a question regarding this: why can OH⁻ generated in the catalyst layer (on the\nnanometre scale) be detected in the micrometre-scale solution region?\nResponse:\nWe thank the Reviewer for this valuable comment. The process can be described as follows: the\ngenerated OH⁻ first becomes enriched within the nanoscale electrical double layer (EDL) and then\ndiffuses outward through the microscale diffusion layer, where it is detected by the pH microelectrode.\nWe note that in our method, the pH microelectrode probes not the interior of the nanoscale EDL, but\nrather the resulting steady-state OH⁻ concentration within the diffusion layer. This spatial resolution\nlimitation arises fundamentally from the physical dimensions of the microelectrode tip, which has a\ndiameter of approximately 10 μm. Nevertheless, as addressed in our response to the Comment 2-1-4,\nour method offers a reliable semi-quantitative assessment of the local microenvironment in AEMWEs.\nOriginal comment 2-1-4:\nIn addition, can the specific quantity of OH⁻ produced by TiO₂-facilitated water dissociation be\nprovided and can the dynamic process of OH⁻ diffusion from the catalyst layer to the electrical double\nlayer and then to the micrometer-scale solution phase be elaborated?\nResponse：\nThanks for this insightful comment. We fully acknowledge the challenge in directly quantifying the\nexact amount of OH⁻ produced by TiO . To address this, we evaluated the effective local OH⁻\n2\nconcentration within the EDL through an indirect comparative method, wherein the performance of\nthe TiO -modified AEMWE operating with DI water was benchmarked against baseline systems fed\n2\nwith KOH solutions (0.8–1.1 M). As shown in Fig. R6, the performance of TiO -modified AEMWE\n2\nsupplied with DI water closely matched that of the 1.0 M KOH baseline, indicating that the local OH⁻\nconcentration within the EDL produced by TiO -facilitated water dissociation approaches but does not\n2\n8\nexceed 1.0 M (pH 14, the OH⁻ concentration in the EDL cannot surpass that of the bulk solution due\nto OH⁻ diffusion from the bulk to the EDL).\nFigure R6. Polarization curves of the TiO -modified AEMWEs supplied with DI water and KOH\n2\nelectrolyte (0.8–1.1 M).\nMoreover, as elaborated in our response to Comment 2-1-3, the generated OH⁻ enriches within the\nnanoscale EDL and then diffuses outward through the microscale diffusion layer, where it is detected\nby the pH microelectrode, yielding a measured pH of 13.8 (Fig. 2f). By combining the electrochemical\nperformance data with this local pH measurement, we estimate the actual pH within the EDL to range\nbetween 13.8 and 14. We also note that the actual OH⁻ generated by TiO would be higher than this\n2\nrange due to the consumption by OER.\nBased on these results, we conclude that our methodology offers a reliable semi-quantitative\nassessment of the local microenvironment in AEMWEs, even though the measurement probes the OH⁻\nconcentration within the diffusion layer rather than directly accessing the interior of the nanoscale EDL.\nIn response to fully address this comment, we have revised our manuscript as follows:\ni. Supplementary Note 1 has been added in the revised supporting information to clarify the\nassessment of the local pH in AEMWE by SECM-pH ultramicroelectrode.\n“Supplementary Note 1. Assessment of the local pH in AEMWE by SECM-pH ultramicroelectrode.\nIt should be noted that the generated OH⁻ first becomes enriched within the nanoscale EDL and then\ndiffuses outward through the microscale diffusion layer. The spatial resolution of our current method\nis fundamentally limited by the physical dimensions of the sensing probe, which has a tip diameter of\napproximately 10 μm. Consequently, our SECM–pH microelectrode measurements were conducted at\na controlled distance of approximately 1 μm from the catalyst layer surface. At this position, the pH\n9\nmicroelectrode detects not the interior of the nanoscale EDL, but rather the steady-state OH⁻\nconcentration within the diffusion layer.\nTo estimate the pH value within the EDL, we compared the performance of the TiO -modified\nM). As shown in Supplementary Fig. 16, the performance closely matched that of the 1.0 M KOH\nbaseline, indicating that the local OH⁻ concentration within the EDL approaches but does not exceed\n1.0 M (OH⁻ diffusion from the bulk to the EDL).\nThis interpretation is further supported by SECM-pH microelectrode measurements (recorded 1\nµm above the catalyst surface), which showed a pH of 13.8 (Fig. 2f). By integrating the electrochemical\nand local pH data, we estimate the local pH within the EDL to be between 13.8 and 14. We also note\nthat the actual OH⁻ generated by TiO -facilitated water dissociation would be higher than this range\n2\ndue to consumption by the oxygen evolution reaction.\nBased on these results, we conclude that our methodology offers a reliable semi-quantitative\nassessment of the local microenvironment in AEMWEs.”\nii. Fig. R6 has been added as Supplementary Fig. 16 in the revised supporting information.\nOriginal comment 2-1-5：\nAt the same time, please explain: can the rate of OH⁻ generation via water dissociation catalyzed by\nTiO₂ match the consumption rate of OH⁻?\nResponse:\nYes. As shown in Fig. R7, the overall performance of the TiO -modified AEMWE in DI water remains\n2\nnearly identical to that achieved in 1.0 M KOH, even at a very high current density of 4 A cm⁻2.\nThis\nclose agreement indicates that the rate of OH⁻ generation at the catalyst interface matches its\nconsumption rate during operation. Furthermore, our local pH measurements confirm that the TiO -\n2\nmodified AEMWE sustains a highly alkaline microenvironment (pH ∼14) across the operating voltage\nrange of 1.5 to 1.9 V (Fig. 2c, e and Fig. 5d). These collective results demonstrate that the OH⁻\ngeneration rate via TiO -faciliated water dissociation can match the consumption rate of OH⁻.\n2\n2\nelectrolyte.\nIn response, we have added Fig. R7 as Supplementary Fig. 21 in the revised supporting\ninformation, along with the corresponding discussion in the figure caption.\n“As shown, the overall performance of the TiO -modified AEMWE in pure water remains nearly\n2\nidentical to that in 1.0 M KOH, even at a high current density of 4 A cm⁻2, indicating that the rate of\nOH⁻ generation matches its consumption at the anode. This is further corroborated by our in situ pH\nmeasurements (Fig. 2c, e and Fig. 5d), which confirm that the TiO -modified AEMWE sustains a highly\n2\nalkaline microenvironment (pH ~ 14) from 1.5 to 1.9 V during operation.”\nOriginal comment 2-2:\nAs described in Page 7, Line 118, after depositing TiO₂ on the catalyst, the pH of the system is 7 when\nno voltage is applied; as voltage is applied, the pH increases with the rise of voltage, even reaching\nstrong alkalinity (pH = 14). It is known that TiO₂ coating on the catalyst surface significantly reduces\nconductivity, and its electron conductivity is almost negligible. Why can a large amount of OH⁻ still be\ngenerated as the voltage increases? What is the surface state of TiO₂ under this condition?\nResponse:\nWe agree with the Reviewer that a dense TiO coating can hinder electronic conductivity. However, in\n2\nour case, the ultra-thin (3-5 nm), dispersed TiO nanoparticles do not form a continuous insulating\n2\nlayer. This is corroborated by our EIS measurements in 1.0 M KOH (Fig. R8), which show no\nsignificant increase in ohmic resistance (R) after TiO modification. We note that the slight increase\ns 2\nin charge transfer resistance (R ) is attributed to partial coverage of active sites by the TiO\nct 2\nnanoparticles, which lowers the overall reaction kinetics.\n2\nwith the inset showing their corresponding ohmic (R) values.\ns\nMoreover, our in-situ Raman results reveal that the potential-dependent increase in local pH should\noriginate from enhanced water dissociation on the TiO surface (Fig. 3). To elucidate this observation,\n2\nwe calculated the water dissociation energy barrier on TiO under anode potential. As shown in Fig.\nwhich\nRHE\nprovides a fundamental explanation for the observed voltage-dependent increase in OH⁻ generation.\nFigure R9. Density functional theory (DFT) calculated energy barrier of water dissociation on TiO\n2\n(110) surface at 1.6 and 2.0 V ,\nwith the inset showing the atomic configurations of water\nRHE\ndissociation on the TiO surface in the initial state (IS), transition state (TS) and final state (FS). Colour\n2\ncodes: cyan and orange spheres denote Ti and O atoms in TiO , and red and white spheres represent O\n2\nand H atoms in H O molecule, respectively.\n2\n2 1g 1g\nand E ) of the rutile phase show negligible shift or change in intensity as the potential increases.\ng\nTherefore, we conclude that TiO maintains its structural integrity and does not undergo a phase\n2\ntransition or chemical decomposition under the operating conditions.\nFigure R10. In situ surface-enhanced Raman spectra of TiO at different anode potentials in 0.5 M\n2 4\nIn response to fully address this comment, we have revised our manuscript as follows:\ni. Fig. R8 has been added as Supplementary Fig. 14 in the revised supporting information, along with\nthe related discussion in the figure caption.\n“As shown, there is no significant increase in R after TiO modification, indicating that the introduced\ns 2\nTiO nanoparticles do not create additional resistive barriers within the electrode structure. It is\n2\nreasonable that the ultra-thin (3-5 nm), dispersed TiO nanoparticles do not form a continuous\n2\ninsulating layer on the catalyst (Supplementary Fig. 13).”\nii. Fig. R9 has been added as Supplementary Fig. 20 in the revised supporting information, along\nwith the related discussion in the revised manuscript (page 8) and Supplementary Note 2.\n“Our density functional theory calculations support this point, revealing that TiO exhibits a low\n2\nenergy barrier for water dissociation under anode potential(Supplementary Fig. 20).”\n“To further elucidate the mechanism, we performed DFT calculations to examine water dissociation\non TiO under applied potential. As shown in Supplementary Fig. 20, the energy barrier is significantly\n2\nlowered as the potential decreases, providing a fundamental explanation for the voltage-dependent\nincrease in OH⁻ generation. Based on these combined results, we conclude that the applied anodic\npotential—which activates the intrinsic catalytic property of the stable TiO surface—serves as the key\n2\ndriver for efficient water dissociation.”\nPage 7, Line 124 mentions that the pH values of both the cathode and anode increase to 14; Page 8,\nLine 143 explains that OH⁻ accumulates in the electrical double layer at the anode because the anode\nis positively charged. However, the cathode is negatively charged—why does no H⁺ enrichment occur\n(which would result in acidity) at the cathode?\nResponse:\nWe agree with the Reviewer that if the cathode develops through a similar mechanism like the anode’s\ndirect water dissociation on TiO ,the cathode’s negative charge would attract cations like H⁺. However,\n2\nin our case, as discussed in the main text and shown in Fig. 3, the local alkalinity at the cathode exhibits\na distinct mechanism. Specifically, NiMo alloy at the cathode catalyzes the Volmer step of HER (H O\n2\n+ e⁻ → OH⁻ + H*). The H* is rapidly consumed to produce H , while oxophilic TiO adsorbs and\n2 2\nenriches the generated OH⁻. This OH⁻ feedback accelerates the Volmer step on NiMo alloy, ultimately\nestablishing a self-sustaining an alkaline microenvironment. Due to the rapid consumption of H* and\nthe stabilization of OH⁻, no significant local decrease in pH (i.e., acidification) is observed.\nOriginal comment 2-3-2:\nFurthermore, the thickness of the electrical double layer is on the nanometer scale, while the detection\ndepth mentioned in the paper is on the micrometer scale. Is this description sufficiently accurate?\nResponse:\nWe fully agree with the Reviewer’s valid point regarding the scale difference between the EDL\n(nanoscale) and the detection depth of our method (micrometer scale). While our technique does not\ndirectly probe the EDL, it provides a reliable assessment of the local microenvironment within the\ncatalyst layer during AEMWE operation (please see details in our Responses to Comment 2-1-3 and\n2-1-4).\nOriginal comment 2-4-1:\nPage 8, Line 158 describes that as the potential decreases, the content of adsorbed water molecules\n(H₂O*) increases, and the content of the intermediate species adsorbed hydrogen (H*) also increases\naccordingly. If it is assumed that TiO₂ promotes water dissociation at the catalytic interface to generate\nH*, the amount of H₂O* at the interface should decrease—so why does the experimental result show\nthat both increase simultaneously?\nResponse:\nWe thank the Reviewer for this insightful comment regarding the simultaneous increase of H O* and\nbulk electrolyte. As the anode potential increases, the positively charged electrode surface enhances\n14\nthe flux of polar water molecules to the interface. This ensures that the rate of H O* re-adsorption\n2\nsurpasses its consumption by dissociation, thereby allowing both H O* and H* concentrations to rise\n2\nsimultaneously.\nOriginal comment 2-4-2:\nIn addition, what is the basis for assigning a specific characteristic peak to H*? It seems that\nsupplementary verification data using D₂O or H₂¹⁸O are not provided in the Supporting Information.\nSimilarly, for the assignment of the intermediate species OOH* during the oxygen evolution reaction\n(OER) mentioned in Page 9, Line 168, no further verification has been conducted using D₂O and H₂¹⁸O.\nRaman data appears to be the key basis for the paper to confirm the rapid water dissociation capability\nof TiO₂, and all the above questions need to be answered.\nResponse:\nFollowing the Reviewer’s suggestion, we have performed additional experiments using isotopic\nlabelling to confirm the peak assignment. As shown in Fig. R11, the characteristic peak shifts from\n1953 cm⁻1 in H O (Fig. R11a) to 1537 cm⁻1 in D O (Fig. R11b), clearly indicating the involvement of\n2 2\nhydrogen atoms in the observed vibrational mode. This isotopic shift is quantitatively supported by\ntheoretical calculation. The expected shift ratio γ for metal-hydrogen (M-H) to metal-deuterium (M-D)\nbending vibrations can be approximated by the following equation (Nat. Commun. 2023, 14, 5289):\n(cid:3049)((cid:2897)(cid:2879)(cid:2888)) (cid:3040)((cid:2897))(cid:2878)(cid:3040)((cid:2888)) (cid:3040)((cid:2897))(cid:2878)(cid:3040)((cid:2892)) (cid:3040)((cid:2892))\nꝩ = = (cid:3495) /(cid:3495) ≈ (cid:3495) = 70.7% (1)\n(cid:3049)((cid:2897)(cid:2879)(cid:2892)) (cid:3040)((cid:2897))×(cid:3040)((cid:2888)) (cid:3040)((cid:2897))×(cid:3040)((cid:2892)) (cid:3040)((cid:2888))\nwhere 𝑣 is the bending vibration frequency, and m is the relative atomic mass. The experimentally\nobserved shift ratio is 78.7%, which aligns well with the theoretical value of 70.7%, further confirming\nthat the peak at ~1950 cm⁻1 in Fig. 3 indeed corresponds to the H* species.\nFigure R11. In situ surface-enhanced Raman spectra of TiO -modified NiMo alloy at different cathode\n2\npotentials in (a) H O and (b) D O.\n2 2\nOOH*, we compared the signals in H O16 and H O18. Note that the peaks shift from 1011 and 1055\n2 2\ncm⁻1 in H O16 (Fig. R12a) to 950 and 973 cm−1 in H O18 (Fig. R12b), indicating that this species\n2 2\ncontains O atoms. This is further supported by the theoretical shift ratio (γ=94.3%) for 18O-18O/16O-\n16O bending vibrations in the O labelling experiment (J. Am. Chem. Soc. 2024, 147, 1334), as\napproximated by the following formula,\n(cid:3049)((cid:2899)(cid:3117)(cid:3124)(cid:2879)(cid:2899)(cid:3117)(cid:3124)) (cid:3040)((cid:2899)(cid:3117)(cid:3124))(cid:2878)(cid:3040)((cid:2899)(cid:3117)(cid:3124)) (cid:3040)((cid:2899)(cid:3117)(cid:3122))(cid:2878)(cid:3040)((cid:2899)(cid:3117)(cid:3122))\nꝩ = = (cid:3495) /(cid:3495) = 94.3% (2)\n(cid:3049)((cid:2899)(cid:3117)(cid:3122)(cid:2879)(cid:2899)(cid:3117)(cid:3122)) (cid:3040)((cid:2899)(cid:3117)(cid:3124))×(cid:3040)((cid:2899)(cid:3117)(cid:3124)) (cid:3040)((cid:2899)(cid:3117)(cid:3122))×(cid:3040)((cid:2899)(cid:3117)(cid:3122))\nThe theoretical γ value agrees well with the experimental shifts (93.9 and 92.1%), confirming that the\nobserved peaks at ~1010 and 1055 cm⁻1 in Fig. 3 originate from OOH*.\nFigure R12. In situ surface-enhanced Raman spectra of TiO -modified NiFe oxide at different anode\n2\npotentials in (a) H O16 and (b) H O18.\n2 2\nIn response to fully address this comment, we have added Figs. R11 and R12 as Supplementary\nFigs. 22 and 23, respectively, in the revised supporting information, along with the related discussions\nin the figure captions.\nOriginal comment 2-5:\nPage 19, Lines 357–373 mentions that the catalyst is loaded via ultrasonic spraying. However, the\nprevious text states that the anode and cathode catalysts are synthesized on nickel felt via hydrothermal\nsynthesis. When preparing the catalyst ink, is the catalyst first ultrasonically exfoliated from the nickel\nfelt and then loaded onto the membrane via ultrasonic spraying? In addition, the thickness of the\ncommercial membrane used in the paper is not specified—what is its specific value? Meanwhile, the\ntype of ionomer added to the anode and cathode catalysts also needs to be clarified—specifically,\nwhich type of ionomer is used? All the above information should be clearly supplemented in the paper\nto avoid misunderstanding among readers.\nFollowing the suggestion of the Reviewer, the related preparation details of MEA have been specified\nin the Methods section in the revised manuscript (pages 17-18).\n“For the AEMWE, commercial anion-exchange membranes (Sustainion X37-50, AEMION™ AF1-\nHNN8-50-X, or FAA-3-50; thickness: 50 μm) were employed. Each membrane was pre-treated with\n1.0 M KOH for 48 h, and then treated with fresh KOH for 24 h. Next, the membrane was washed with\nDI water until the pH of the water reached neutral. The catalyst integration comprised two components:\npre-grown catalysts directly on nickel fibers and catalysts spray-coated onto the membrane. For the\nspray-coated catalysts, NiFe oxide and NiMo alloy were first exfoliated from nickel felt substrates via\nultrasonication. Catalyst inks were prepared by homogeneously dispersing 100 mg of the exfoliated\ncatalysts in 10 mL of isopropanol/water mixture (9:1, v/v) with 10 wt% Sustainion XB-7 ionomer\n(relative to catalyst mass) under 3 h sonication. These inks were then deposited onto both sides of the\npretreated AEM using ultrasonic spray coating, achieving a loading of approximately 1 mg cm⁻2. Then,\nthe spray-coated AEM, the Ni fibres loaded with NiFe oxide (~3.0 mg cm−2) and NiMo alloy (~3.2 mg\ncm−2) catalysts, and Teflon gaskets were assembled together with a torque wrench tighten the screws\nto 5.0 N m−1 (Supplementary Fig. 1).”\nResponse to Reviewer #3\nGeneral Comments:\nGuo et al. report a ‘solid base’ strategy that enriches local hydroxide (OH–) concentration within the\ncatalyst layer for high-performance pure-water AEMWE. They introduce TiO nanoparticles on both\n2\nthe anode and cathode catalysts, and then employ pH ultramicroelectrode and surface-enhanced\nRaman spectroscopy to in situ probe the interfacial microenvironment at the catalyst–electrolyte\ninterface. The results reveal a TiO -dominated localized alkalinity, accelerating the Volmer step at the\n2\ncathode and water dissociation at the anode. They also evaluate electrolyzer and stack performance\nand estimate the levelized cost of hydrogen; collectively, these cross-scale investigations underscore\nthe concept’s device-level potential. In parallel, the manuscript presents several notable weaknesses,\nparticularly concerning (i) the definition and adaptation of the ‘solid-base’ concept and (ii) the\nmechanistic evidence demonstrating its operation. Before the manuscript can be considered for\npublication in Nat. Commun., the authors should systematically address the following issues.\nResponse:\nWe sincerely thank the Reviewer for his/her valuable comments, which have helped us significantly\nimprove the quality of our manuscript. In response, we have conducted additional experiments and\nprovided more detailed explanations and discussions to address the points raised and to further support\n17\nour conclusions.\nOriginal comment 3-1:\nThe primary issue lies in using the core concept of ‘solid base’. In heterogeneous catalysis and solid\nacid–base chemistry, a solid base denotes a solid that features surface basic sites, namely Lewis (e.g.,\nO2–/M–O⁻) and Brønsted (e.g., –OH) sites, with representative materials such as alkali and alkaline-\nearth oxides and layered double hydroxides (LDHs). Its mechanism involves Lewis-type electron-pair\ndonation and Brønsted-type proton abstraction at surface basic sites, thereby facilitating adsorption\nand activation of acidic substrates, which in turn promotes deprotonation, transesterification, and\ncondensation, and regulates the interfacial microenvironment.\nHowever, TiO is generally amphoteric and often weakly acidic; in surface chemistry, it is more\n2\ncommonly regarded as a solid acid rather than a solid base. In addition, based on the authors’\nexperimental results, TiO primarily acts by enriching OH⁻ and modulating the local interfacial\n2\nenvironments, thereby assisting the reaction kinetics rather than serving as the principal catalytic\nactive site. This differs, in terms of terminological scope, from classical solid acid–base chemistry,\nwhere surface basic centres activate substrates via Brønsted proton abstraction or Lewis electron-pair\ndonation. Borrowing the concept here risks implying that intrinsic basic sites of the material cause\nalkalization, whereas a more faithful explanation is local pH regulation under applied bias, driven by\nOH⁻ generation and ionic redistribution in the electric double layer. I recommend that the authors\nreconsider using the ‘solid-base’ concept and adopt a more precise conceptual framing aligned with\ntheir experimental evidence.\nResponse:\nWe sincerely thank the Reviewer for this insightful and constructive comment regarding the use of the\n“solid base” concept. We agree that this terminology, as rigorously defined in classical solid acid-base\nchemistry, may not precisely describe the role of TiO in our system.\npure water anion exchange membrane electrolysis” and have systematically updated all relevant\ndescriptions throughout the revised manuscript and supporting information accordingly.\nOriginal comment 3-2:\nThe mechanistic role of TiO nanoparticles has not yet been systematically established. Although\n2\nextensive in situ characterizations were performed, the data indicate that introducing TiO correlates\n2\nwith accelerated OER at the anode and HER at the cathode, and with enhanced near-interfacial OH⁻\nconcentrations in the catalyst layer (i.e., an elevated local pH). However, the pathway by which TiO\n2\nleads to OH⁻ enrichment remains insufficiently evidenced, and the molecular-level mechanism is still\nunclear. The XPS results show predominantly Ti4+ with no discernible Ti3+ features, suggesting a low\n18\nconcentration of oxygen vacancies. According to established surface chemistry, stoichiometric TiO\n2\nsurfaces typically adsorb water molecularly, whereas dissociative adsorption leads to surface –OH\nbeing more favourable when oxygen vacancies are present. I recommend defect-sensitive\nmeasurements (e.g., EPR) and first-principles calculations to contrast water adsorption/dissociation\non defective versus stoichiometric TiO , thereby clarifying the specific mechanism, explaining the\n2\nperformance gains, and avoiding overly speculative interpretations.\nResponse:\nWe thank the Reviewer for this insightful comment and fully agree that oxygen vacancies (O ) are\nv\nusually recognized as active sites that enhance water dissociation on TiO surfaces. Following the\n2\nsuggestion, we synthesized oxygen vacancy (O )-rich TiO by annealing the pristine sample in an\nv 2\nH /Ar atmosphere at 400 °C. EPR, XPS, and ultraviolet-visible (UV-Vis) absorption (400–800 nm)\n2\ncollectively confirmed the successful introduction of oxygen vacancies (Fig. R13a , b , c ). However,\n1 1 1\npost-OER polarization tests revealed that the O signals were significantly diminished (Fig. R13a , b ,\nv 2 2\nc ), indicating that the generated O are refilled under anode potential.\n2 v\nFigure R13. (a ) EPR, (b ) XPS, and (c ) UV-visible (UV-Vis) absorption spectra of as-synthesized\n1 1 1\npristine and oxygen vacancy (O )-rich TiO samples. (a ) EPR, (b ) XPS, and (c ) UV-Vis absorption\nv 2 2 2 2\nspectra of pristine and O -rich TiO₂ samples after OER tests in 0.5 M Na SO (pH=7).\nv 2 4\nThis finding was further corroborated by in situ Raman spectroscopy. As depicted in Fig. R14a\nand b, the E peak intensity for the O -rich TiO was notably lower than that of the pristine TiO at\ng v 2 2\nopen-circuit potential (E ), due to the presence of O . However, when the anode potential was raised\noc v\n19\nto 1.2 V , the E peak intensity of the O -rich TiO increased and became nearly identical to that of\nRHE g v 2\nthe pristine TiO , again confirming the rapid filling of oxygen vacancies under operational conditions.\nv\nTiO (Fig.R14c)\nv 2\nfrom open-circuit potential (E ) to 1.8 V in 0.5 M Na SO (pH=7). (c) Measured local pH values\noc RHE 2 4\non TiO surface in 0.5 M Na SO . Note that this local pH measurement was performed by an IrO -\n2 2 4 x\nmodified rotating ring-disk electrode (RRDE) method as we previously established (Nat. Energy 2023,\n8, 264).\nTo further elucidate the mechanism, we performed DFT calculations to examine water dissociation\non TiO under applied potential. As shown in Fig. R15, the energy barrier is significantly lowered as\n2\nthe potential decreases. This interpretation is further supported by in situ Raman spectra (Fig. R14a,\nb), which show increasing H₂O* and OH* signal intensities with rising anode potential for both pristine\nand Ov-rich TiO . Based on this combined experimental and theoretical evidence, we conclude that\n2\nthe applied anode potential—which activates efficient water dissociation on TiO —serves as the key\n2\ndriver for creating the local alkaline environment.\nRHE 2\nthe initial state (IS), transition state (TS) and final state (FS). Colour codes: cyan and orange spheres\ndenote Ti and O atoms in TiO , and red and white spheres represent O and H atoms in H O molecule,\n2 2\nrespectively.\nIn response to fully address this comment, we have revised our manuscript as follows:\ni. Supplementary Note 2 has been added in the revised supporting information to clarify the\nmechanical role of TiO in water dissociation.\n2\n“Supplementary Note 2. Mechanical role of TiO in water dissociation.\npotential role of oxygen vacancies (O ), which are widely recognized as active sites for enhancing\nv\nwater dissociation on TiO₂ surfaces. We synthesized O -rich TiO by annealing the pristine sample in\nv 2\nan H /Ar atmosphere at 400 °C. EPR, XPS, and ultraviolet-visible (UV-Vis) absorption (400–800 nm)\n2\ncollectively confirmed the successful introduction of oxygen vacancies (Supplementary Fig. 19a , b ,\n1 1\nc ). However, post-OER polarization tests revealed that the O signals were significantly diminished\n1 v\n(Supplementary Fig. 19a , b , c ), indicating that the oxygen vacancies are filled under anodic\n2 2 2\npotential.\nThis finding was further corroborated by in-situ Raman spectroscopy. As depicted in\nSupplementary Fig. 19d, e, the E peak intensity for the defective TiO was notably lower than that of\ng 2\nthe pristine TiO at open-circuit potential (E ), due to the presence of O . However, when the anode\n2 oc v\npotential was raised to 1.2 V , the E peak intensity of the defective sample increased and became\nRHE g\nnearly identical to that of the pristine TiO , again confirming the rapid filling of oxygen vacancies\n2\nunder operational conditions. Therefore, water dissociation behaviour under anode potential was\nalmost identical for both the O -rich and pristine TiO .\nv 2\nTo further elucidate the mechanism, we performed DFT calculations to examine water\ndissociation on TiO under applied potential. As shown in Supplementary Fig. 20, the energy barrier\n2\nis significantly lowered as the potential decreases, providing a fundamental explanation for the\nvoltage-dependent increase in OH⁻ generation. Based on these combined results, we conclude that the\napplied anodic potential—which activates the intrinsic catalytic property of the stable TiO surface—\n2\nserves as the key driver for efficient water dissociation.”\nii. Figs. R13-R15 have been added as Supplementary Figs. 19 and 20 in the revised supporting\ninformation, respectively.\nOriginal comment 3-3:\nIn Figures 3h and 3i, the strong band at ~1900 cm–1 is assigned to *H. Given its high intensity, this\n21\nassignment warrants careful verification. I recommend isotope substitution experiments; if the band\narises from a surface metal–H vibration, the wavenumber should exhibit a characteristic red shift. This\ncontrol would help determine whether the band corresponds to *H rather than other adsorbed species.\nResponse:\nFollowing the suggestion of the Reviewer, we performed isotope substitution experiments by\ncomparing the spectra in H O and D O electrolytes. As anticipated for a hydrogen-containing species,\n2 2\nthe peak shifts from 1953 cm⁻1 in H O (Fig. R16a) to 1537 cm⁻1 in D O (Fig. R16b). This isotopic\n2 2\nshift is quantitatively consistent with the theoretical prediction for a metal-hydrogen (M–H) vibration.\nThe theoretical shift ratio (γ) for the vibrational frequency of M–H to M–D can be approximated by\nthe following equation (Nat. Commun. 2023, 14, 5289; Angew. Chem. Int. Ed. 2023, 62, e202300989):\n(cid:3049)((cid:2897)(cid:2879)(cid:2888)) (cid:3040)((cid:2897))(cid:2878)(cid:3040)((cid:2888)) (cid:3040)((cid:2897))(cid:2878)(cid:3040)((cid:2892)) (cid:3040)((cid:2892))\nꝩ = = (cid:3495) /(cid:3495) ≈ (cid:3495) = 70.7% (3)\n(cid:3049)((cid:2897)(cid:2879)(cid:2892)) (cid:3040)((cid:2897))×(cid:3040)((cid:2888)) (cid:3040)((cid:2897))×(cid:3040)((cid:2892)) (cid:3040)((cid:2888))\nwhere 𝑣 is the bending vibration frequency, and m is the relative atomic mass. The theoretical γ value\nagrees well with the experimental shift (78.7%), confirming that the observed peak at ~1950 cm⁻1 in\nFig. 3 indeed originates from H*.\nFigure R16. In situ surface-enhanced Raman spectra of TiO -modified NiMo alloy at different cathode\n2\npotentials in (a) H O and (b) D O.\n2 2\nIn response to fully address this comment, we have added Fig. R16 as Supplementary Fig. 22 in\nthe revised supporting information, along with the related discussions in the figure captions.\nEND OF RESPONSE\n22"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am mostly happy with the revision. One more concern I have noticed this time is that the flow rate of water/electrolyte was very high (200 mL/min) for a small electrode cell (1 cm2). Add the explanation why the authors used such high flow rate with the utilization of water at a specific current density (1 A/cm2, for example). It would be better to comment the effect of the flow rate on the performance/durability.", "reviewer_2": "The authors have made a commendable effort in revising the manuscript and have addressed most of the previous comments. However, a few minor issues still remain to be resolved:\n(1) The authors must clarify whether the reported local alkalinity effect is unique to TiO2 or can also occur on other metal oxides. Comparative experiments are essential to substantiate whether this phenomenon represents a general property of oxides or merely an artifact specific to TiO2.\n(2) As shown in Figure R5a, when measuring the pH value of the electrolytic cell, were changes in the pH values at multiple points taken into account?\n(3) In Figure R12, the peaks at 1010 and 1015 exhibit a shift in 18O, so the authors assign them to OOH. Whether these two peaks show a shift in heavy water (D2O) to further verify this assignment? In Figure 3a, does the OH group at ~800 cm-1 shift in heavy water (D2O) and 18O-labeled water?", "reviewer_3": "The authors have addressed my comments. I recommend the publication of this work."}, "author_response": "Response to Reviewer #1\nOriginal Comments:\nI am mostly happy with the revision. One more concern I have noticed this time is that the flow rate of\nwater/electrolyte was very high (200 mL/min) for a small electrode cell (1 cm2). Add the explanation\nwhy the authors used such high flow rate with the utilization of water at a specific current density (1\nA/cm2, for example). It would be better to comment the effect of the flow rate on the\nperformance/durability.\nResponse:\nThank you for allowing us to clarify this important detail regarding the flow rate. We sincerely\napologize for the error in our initial manuscript. The flow rates were mistakenly transposed: the\noptimal flow rate for the single electrolyser (4 cm2 electrode area) is 20 mL/min, while that for the 10-\ncell electrolysis stack (250 cm2) is 200 mL/min.\nFigure R1. (a, b) Polarization curves of the TiO₂-modified AEM electrolyser and stack under varied\nflow rates. (c) Chronopotentiometry curves of the TiO\n2\n-modified AEM electrolyser operating at 1.0 A\ncm⁻2 under varying flow rates.\nFollowing your suggestion, we systematically investigated the effect of flow rate on performance\nof the single electrolyser and the 10-cell stack. For the single electrolyser, as shown in Fig. R1a,\nincreasing the flow rate from 5 to 20 mL/min resulted in a slight performance improvement at high\ncurrent density, however, further increases in flow rate did not lead to additional performance gains.\nThis is reasonable that a flow rate of 20 mL/min is sufficient to provide optimal mass transport to\nsustain the high reaction rate while efficiently removing generated gas bubbles, respectively. A\nsimilar trend was observed for the stack (Fig. R1b). The final flow rate was selected as 200 mL/min by\ncomprehensively considering both performance benefits and overall system energy consumption.\nmL/min exhibited a marginal advantage (Fig. R1c). Therefore, flow rates of 20 mL/min for the AEM\nelectrolyser and 200 mL/min for the AEM stack were adopted in this work.\nIn response to fully address this comment, we have revised our manuscript as follows:\ni. The flow rates for the electrolyzer and stack were detailed in the Methods section of the revised\nmanuscript (Page 18):\n“During tests, the feed water/electrolyte was continuously supplied to both electrodes of the AEM\nelectrolyser and stack using a two-channel peristaltic pump, with flow rates maintained at 20\nmL/min for the electrolyser and 200 mL/min for the stack (Supplementary Note 6).”\nii. Fig. R1 has been added as Supplementary Fig. 29 and Supplementary Note 6 has been included\nin the revised Supporting Information to further discuss the effect of flow rate on the performance\nof both the single electrolyzer and the 10-cell stack.\n“In addition, we systematically investigated the effect of flow rate on performance of the single\nelectrolyser and the 10-cell stack. For the single electrolyser, as shown in Supplementary Fig. 29a,\nincreasing the flow rate from 5 to 20 mL/min resulted in a slight performance improvement at high\ncurrent density, however, further increases in flow rate did not lead to additional performance gains.\nThis is reasonable that a flow rate of 20 mL/min is sufficient to provide optimal mass transport to\nsustain the high reaction rate while efficiently removing generated gas bubbles, respectively. A\nsimilar trend was observed for the stack (Supplementary Fig. 29b). The final flow rate was selected\nas 200 mL/min by comprehensively considering both performance benefits and overall system\nenergy consumption. Furthermore, at 1 A cm⁻2, stability was largely comparable across the tested\nflow rates, though 20 mL/min exhibited a marginal advantage (Supplementary Fig. 29c). Therefore,\nflow rates of 20 mL/min for the AEM electrolyser and 200 mL/min for the AEM stack were adopted\nin this work.”\nResponse to Reviewer #2\nGeneral Comments:\nThe authors have made a commendable effort in revising the manuscript and have addressed most of\nthe previous comments. However, a few minor issues still remain to be resolved:\nResponse:\nWe sincerely thank the Reviewer for his/her valuable comments, which have helped us significantly\nimprove the quality of our manuscript. In response, we have conducted additional experiments and\nprovided more detailed explanations and discussions to address the points raised and to further support\n2\nour conclusions.\nOriginal comment 2-1:\nThe authors must clarify whether the reported local alkalinity effect is unique to TiO or can also occur\n2\non other metal oxides. Comparative experiments are essential to substantiate whether this phenomenon\nrepresents a general property of oxides or merely an artifact specific to TiO .\nWe thank the reviewer for raising this insightful comment. According to classical Lewis acid-base\ntheory, hard acids tend to strongly interact with hard bases (such as OH⁻), making materials with strong\nLewis acidity more conducive to creating a locally alkaline environment on the catalyst surface. To\nverify this, we selected another strong Lewis acid oxide, Cr O , whose Cr3⁺ ion has a pK value (~2.05)\n2 3 a\nsimilar to that of Ti4⁺ in TiO (~2.20). In situ local pH measurements in neutral electrolyte (0.5 M\n2 4 2 3\nup to 14 at 1.7 V (Fig. R2a), confirming the formation of an alkaline microenvironment. This\nRHE\nobservation is further supported by performance tests (Fig. R2b): Cr O modification significantly\n2 3\nenhances the OER activity of the NiFe anode, achieving performance comparable to that in 1.0 M\nKOH. Together, these results confirm that strong Lewis acid metal oxides can generally induce local\nalkalization. Notably, although Cr O can induce local alkalization, it faces the risk of oxidation to\n2 3\nsoluble CrO ²⁻ at high potentials. For this reason, we selected TiO that remains stable within the OER\n4 2\npotential range to construct a durable alkaline microenvironment in this work.\nFigure R2. (a) Local pH measured on the Cr O -modified and pure NiFe oxide electrodes surfaces\n2 3\nduring OER in 0.5 M Na SO (pH~7). Note that this local pH measurement was performed by an IrO -\n2 4 x\nmodified rotating ring-disk electrode (RRDE) method as we previously established (Nat. Energy 2023,\n8, 264). (b) Corresponding OER polarization curves of the Cr\n3\n-modified and pure NiFe oxide\n3\nelectrodes in 0.5 M Na SO , and pure NiFe oxide electrode in 1.0 M KOH.\n2 4\nIn response to fully address this comment, we have added Fig. R2 has been added as\nSupplementary Fig. 9 and Supplementary Note 2 in the revised supporting information to discuss\nthe general capability of strong Lewis acid oxides to induce local alkalinity.\n“Supplementary Note 2. General capability of strong Lewis acid oxides to induce local alkalinity.\nAccording to classical Lewis acid-base theory, hard acids tend to strongly interact with hard bases\n(such as OH⁻), making materials with strong Lewis acidity more conducive to creating a locally\nalkaline environment on the catalyst surface. To verify this, we selected another strong Lewis acid\noxide, Cr O , whose Cr3⁺ ion has a pK value (~2.05) similar to that of Ti4⁺ in TiO (~2.20). In situ\n2 3 a 2\nlocal pH measurements in neutral electrolyte (0.5 M Na SO ) show that the surface pH of the Cr O -\n2 4 2 3\nmodified electrode increases with potential, reaching up to 14 at 1.7 V (Supplementary Fig. 9a),\nRHE\nconfirming the formation of an alkaline microenvironment. This observation is further supported by\nperformance tests (Supplementary Fig. 9b): Cr O modification significantly enhances the OER\n2 3\nactivity of the NiFe anode, achieving performance comparable to that in 1.0 M KOH. Together, these\nresults confirm that strong Lewis acid metal oxides can generally induce local alkalization. Notably,\nalthough Cr O can induce local alkalization, it faces the risk of oxidation to soluble CrO ²⁻ at high\n2 3 4\npotentials. For this reason, we selected TiO that remains stable within the OER potential range to\n2\nconstruct a durable alkaline microenvironment in this work.”\nOriginal comment 2-2:\nAs shown in Figure R5a, when measuring the pH value of the electrolytic cell, were changes in the pH\nvalues at multiple points taken into account?\nResponse:\nYes, we considered spatial variations in the local pH measurements. Specifically, all local pH values\nin this work are averages obtained from three distinct points. Additionally, we performed local pH\nmapping with 3884 points (Fig. 1d and Fig. 2e), which confirmed a uniform distribution of local pH\nacross the measured area. These results confirm the reliability of our measurement approach.\nIn response, we have updated the descriptions related to the local pH measurements in the captions\nof Fig. 1 (page 4), Fig. 2 (page 6), and Fig. 5 (page 12) as follows:\nIn caption of Fig. 1: “d, Two-dimensional pH distribution measured across 3884 points within the\nanode and cathode catalyst layers of the AEMWE supplied with deionized water.”\nIn caption of Fig. 2: “Note: The local pH values shown in panels c, d, f, and g represent the average\nof measurements taken at three distinct points, with error bars indicating the corresponding standard\n4\ndeviations.”\nIn caption of Fig. 5: “Note: The local pH values in panels d and e are averaged from measurements at\nthree independent points; error bars denote the standard deviations.”\nOriginal comment 2-3-1:\nIn Figure R12, the peaks at 1010 and 1015 exhibit a shift in 18O, so the authors assign them to OOH.\nWhether these two peaks show a shift in heavy water (D O) to further verify this assignment?\nThank you for the valuable feedback, which prompted us to revisit and correct the peak assignment.\nFollowing your suggestion, we conducted comparative measurements in H O and D O. As shown in\n2 2\nFig. R3a, b, shifting from H O to D O resulted in negligible changes in peak position, confirming that\n2 2\nhydrogen atoms are not directly involved in the vibrational mode corresponding to these peaks.\nBased on the combined evidence from H/D and 16O/18O isotopic experiments, we have revised the\nassignment of the peaks near ~1010 and 1055 cm⁻1 to the O–O stretching vibration, rather than to an\nOOH-related species as initially proposed. It is well-documented in the literature (Chem. Rev. 2023,\n123, 6257) that O–O bond formation represents the rate-determining step in the OER mechanism and\nis generally favored under alkaline conditions. While this revision clarifies the vibrational assignment,\nit does not affect the conclusion of this work: local pH measurements confirm the presence of a strongly\nalkaline microenvironment, and the observed OER performance aligns closely with that achieved in\n1.0 M KOH. These results collectively support that the OER mechanism under our conditions follows\nan alkaline pathway.\nFigure R3. In situ surface-enhanced Raman spectra of TiO -modified NiFe oxide at different anode\n2\npotentials in (a) H O and (b) D O.\n2 2\n(page 9).\n“This is evidenced by the pronounced O-O bond53 (Supplementary Fig. 24)…”\nMoreover, Fig. R3 has been added as Supplementary Fig. 24 in the revised supporting information,\nalong with the related discussions in the figure caption.\nOriginal comment 2-3-2:\nIn Figure 3a, does the OH group at ~800 cm-1 shift in heavy water (D O) and 18O-labeled water?\nFollowing the reviewer’s suggestion, we performed isotopic experiments to verify the assignment of\nthe peak near 800 cm⁻1 to adsorbed OH*. First, comparison between the spectra collected in H₂¹⁶O and\nD₂¹⁶O shows a clear shift from 789 cm⁻1 (Fig. R4a) to 638 cm⁻¹ (Fig. R4b), confirming the involvement\nof hydrogen atoms in the vibrational mode. In addition, comparison between H₂¹⁶O and H₂¹⁸O reveals\na shift from 789 cm⁻¹ (Fig. R4a) to 752 cm⁻¹ (Fig. R4c), indicating that oxygen atoms are also part of\nthe observed species. Taken together, the consistent isotopic shifts observed in both H/D and ¹⁶O/¹⁸O\nexperiments provide strong evidence that the peak around 800 cm⁻¹ corresponds to the bending\nvibration of adsorbed OH*.\nFigure R4. In situ surface-enhanced Raman spectra of TiO -modified NiFe oxide at different anode\n2\npotentials in (a) H 16O, (b) D 16O and (c) H 18O.\n2 2 2\nIn response to fully address this comment, we have added Fig. R4 as Supplementary Fig. 19 in\nthe revised supporting information, along with the related discussions in the figure caption:\n6\n“We performed isotopic experiments to verify the assignment of the peak near 800 cm⁻1 to adsorbed\nOH*. First, comparison between the spectra collected in H₂¹⁶O and D₂¹⁶O shows a clear shift from\n789 cm⁻1 (a) to 638 cm⁻¹ (b), confirming the involvement of hydrogen atoms in the vibrational mode.\nFurthermore, the resulting experimental shift (80.8%) agrees well with the theoretical shift ratio (γ =\n72.8%) for O-H/O-D bending vibrations, calculated as follows16, 17,\n𝑣(𝑂−𝐷) 𝑚(𝑂)+𝑚(𝐷) 𝑚(𝑂)+𝑚(𝐻)\nꝩ = = √ /√ =\n𝑣(𝑂−𝐻) 𝑚(𝑂)×𝑚(𝐷) 𝑚(𝑂)×𝑚(𝐻)\n72.8% (2)\nwhere 𝑣 is the bending vibration frequency, and m is the relative atomic mass.\nIn addition, comparison between H₂¹⁶O and H₂¹⁸O reveals a shift from 789 cm⁻¹ (a) to 752 cm⁻¹\n(c), indicating that oxygen atoms are also part of the observed species. This is further supported by\nthe theoretical shift ratio (γ=99.8%) for 18O-H/16O-H bending vibration, calculated as follows16, 17,\n𝑣(𝑂18−𝐻) 𝑚(𝑂18)+𝑚(𝐻) 𝑚(𝑂16)+𝑚(𝐻)\nꝩ = = √ /√ = 99.8%\n𝑣(𝑂16−𝐻) 𝑚(𝑂18)×𝑚(𝐻) 𝑚(𝑂16)×𝑚(𝐻)\n(3)\nThe theoretical γ value closely matches with the experimental shift (95.3%). Taken together, the\nconsistent isotopic shifts observed in both H/D and ¹⁶O/¹⁸O experiments provide strong evidence that\nthe peak around 800 cm⁻¹ corresponds to the bending vibration of adsorbed OH*.”\nResponse to Reviewer #3\nGeneral Comments:\nThe authors have addressed my comments. I recommend the publication of this work.\nResponse:\nWe sincerely thank the Reviewer for recommending the publication of our work.\nEND OF RESPONSE\n7"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have addressed all the points I raised and revised the manuscript accordingly. I recommend acceptance of the manuscript in its current form."}, "author_response": ""}]

Decision letter