Introduction

Proton exchange membrane water electrolysis (PEMWE) has attracted global attention and is one of the most viable technologies for producing green hydrogen using intermittent renewable energy and finally realizing carbon neutrality1,2,3. PEMWE is superior to its competitors (e.g., alkaline water electrolysis), due to its high current density, ultra-high-purity hydrogen, low ohmic losses, compact stack design, easy maintenance and quick startup/shutdown4,5,6. Up to now, iridium oxide (IrOx) has been the most pivotal electrocatalyst for the oxygen evolution reaction (OER)7,8,9,10. Although IrOx is the truly commercially available catalyst and exhibits the highest stability among other catalysts, its intrinsic catalytic activity is still insufficient under operational conditions2,3,11,12,13,14,15,16,17,18. To address the issue, substantial efforts have been devoted to exploring Ir-based OER electrocatalysts with high catalytic activity, such as morphology control, alloying, and interface engineering3,7,9,10,19,20,21. Unfortunately, most of the previously reported OER electrocatalysts have not yet met the activity requirement of industrial-level application, e.g., 1.80 V @ 3.0 A cm−2. For this purpose, the exploration and development of a feasible synthetic strategy to further improve their catalytic activities is urgent.

In a catalytic system, trace sulfonates (e.g., SO42−, − SO3−) adsorbed on the catalyst surface can enhance the OER process by forming hydrogen bonds with H atoms of OER intermediates, which facilitates deprotonation (Fig. 1a)22,23. The sulfonates converted/reconstructed by a traditional method (Fig. 1b) are usually adsorbed onto the catalyst surface. And the interactions between sulfonates and catalysts are weak; therefore, the sulfonates leach from the catalyst severely, which leads to an inevitable decline in the catalyst activity during prolonged operation1,22,24,25. Therefore, achieving the robust anchoring of sulfonates to the catalyst surface remains challenging. In response, we propose that sulfonates can be immobilized on the catalyst surface in the form of covalent bonds to enhance catalyst stability.

Fig. 1: Mechanism of SO42− in proton transfer and synthesis of anion-modified catalysts.
Fig. 1: Mechanism of SO42− in proton transfer and synthesis of anion-modified catalysts.
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a Schematic of SO42− adsorbed on the catalyst surface to accelerate proton transfer. Graphical illustrations for synthesis of b typical SO42−-modified catalysts and c our −SO3H-grafted catalysts.

Herein, we propose a strong bonding strategy where sulfonic groups ( − SO3H) are robustly grafted to the catalyst surface through covalent bonding. Taking commercial Ir/IrOx as an example, 1,3-propanesultone was selected as a monomer to graft −SO3H to the Ir/IrOx surface (denoted as Ir/IrOx~SO3H) (Fig. 1c). As evidenced by Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS), the −SO3H groups were stably anchored on the Ir/IrOx surface. Tafel analysis, deuterium kinetic isotope effects (KIEs), and the electrochemical in situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) verified the pivotal role that the grafted −SO3− anions played in enhancing the proton transfer process, promoting the generation of *OOH species on the catalyst active sites, and accelerating the OER kinetics. Moreover, the PEMWE with an Ir/IrOx ~ SO3H anode showed better stability at a current density of 1.0 A cm−2, with a demonstrated operational lifespan of over 1000 h. In terms of catalytic activity, the cell voltage at 3.0 A cm−2 was reduced from 1.88 V to 1.75 V on the commercial Ir/IrOx after a simple −SO3H modification, surpassing that activity of Ir/IrOx. Moreover, the strong bonding strategy enables the efficient, scalable synthesis of anion-modified catalysts with reduced energy and time consumption during the traditional catalyst synthesis processes. Particularly, compared with Ir/IrOx catalyst ink, the Ir/IrOx ~ SO3H catalyst ink was less easy to settle after standing for 24 h, which was beneficial for the fabrication of industrial membrane electrode assembly (MEA). This work strategically provides a direct and simple synthetic route for preparing stabilized anion-modified catalysts and it also offers a vital insight into developing high performance catalysts.

Results

Recent studies show that the sulfonates adsorbed on the catalyst surface can promote proton transfer, thus facilitating the OER process (Fig. 1a)10,12,13. For instance, Zhong’s group found that the surface sulfonic groups ( − SO3H) on Fe active sites can capture H+ from *OH or *OOH species, thereby promoting proton transfer and reducing the overpotential of OER26. Notably, in SO42−-modified Co3O4 (Co3O4-SO4), the upper O atoms of sulfate radicals (SO42−) formed hydrogen bonds with H atoms of *OOH, making it easier to deprotonate22. However, in sulfonate-modified catalysts synthesized via the traditional route (Fig. 1b), the interactions between sulfonates and catalysts are weak. And the severe leaching of the surface-reconstructed sulfonates happens during the OER process, thereby lowering the catalytic activity and breaking the stability1,25. In response, stabilizing the sulfonates on the catalyst surface has become essential. In this work, the enhanced binding between sulfonates and catalysts is expected to be achieved through covalent bonding, thereby simultaneously preventing the leaching and enhancing proton transport. The Ir/IrOx ~ SO3H catalyst was efficiently synthesized through a single step nucleophilic substitution reaction. Compared with conventional synthesis methods, this approach requires fewer reaction steps (reduced from 2–3 to 1), operates at significantly lower temperatures (110 °C vs. 300–500 °C), and enables precise and stable grafting of −SO3H groups. Specifically, hydroxyl groups on Ir/IrOx surface reacted with 1,3-propanesultone at 1 atm and 110 °C. The sulfonic groups (−SO3H) were covalently attached to the Ir/IrOx surface via propoxy linkers (−OCH2CH2CH2−) (Fig. 1c). Detailed procedures are provided in the supplementary information.

Fourier transform infrared (FTIR) was firstly conducted to analyze the surface structures of catalyst materials. As shown in Fig. 2a, both Ir/IrOx ~ SO3H and Ir/IrOx exhibited typical O−H stretching and bending vibration peaks at 3430 cm−1 and 1629 cm−1, respectively14,27,28. The Ir−O stretching vibration peak around 540 cm−1 remained consistent for both Ir/IrOx ~ SO3H and Ir/IrOx29,30. And the new characteristic bands in the range of 1213–1000 cm−1 corresponding to the stretching vibrations of −SO3H groups in Ir/IrOx ~ SO3H indicated the functionalization of Ir/IrOx surface with −SO3H31,32,33.

Fig. 2: Structural characterization of Ir/IrOx ~ SO3H catalyst.
Fig. 2: Structural characterization of Ir/IrOx ~ SO3H catalyst.
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a FTIR spectra and b S 2p XPS spectra of Ir/IrOx ~ SO3H and Ir/IrOx catalysts. c EDS elemental mapping of Ir/IrOx ~ SO3H catalyst. Source data are provided as a Source data file.

The element contents and electronic structures of Ir/IrOx ~ SO3H catalyst were studied by X-ray photoelectron spectroscopy (XPS). Compared with the XPS spectrum of commercial Ir/IrOx (Fig. S1, supplementary information), the S 2p signal at 168.1 eV in the XPS spectrum of Ir/IrOx ~ SO3H is also viewed, besides Ir, O, and C signals. The S content in Ir/IrOx ~ SO3H, as determined by XPS, was ~1.84 at.% (Table S1). As exhibited in Fig. 2b, the high-resolution S 2p spectrum of Ir/IrOx ~ SO3H was deconvoluted into two orbital splitting peaks assigned to the S 2p3/2 (167.8 eV) and S 2p1/2 (169.0 eV) in −SO3H, respectively33,34. With the help of XPS spectra, it is further proved that −SO3H was successfully grafted to Ir/IrOx.

The energy dispersive spectroscopy (EDS) elemental mapping (Fig. 2c) intuitively revealed the presence and an even distribution of S elements on the nanoscale of Ir/IrOx ~ SO3H catalyst, further verifying the successful synthesis of −SO3H-grafted Ir/IrOx catalyst. Furthermore, the high-resolution S 2p spectra of Ir/IrOx ~ SO3H (Fig. S2) also indicated uniform S distribution, with less than 5% variation in peak intensity across three randomly selected spots.

The thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) curves of Ir/IrOx ~ SO3H and commercial Ir/IrOx catalysts in N2 were presented in Fig. S3. From the initial mass loss (~3.6%) between 30 and 115 °C, it was apparent that Ir/IrOx ~ SO3H held more adsorption water than commercial Ir/IrOx (~1.8%) due to the introduction of −SO3H groups, indicating that the interaction between Ir/IrOx ~ SO3H and water molecules was enhanced. The second stage (116–228 °C) of mass loss was attributed to the destruction of lattice water. Beyond 229 °C, a significant mass loss (~12.1%) was assigned to the decomposition of −OCH2CH2CH2− linkers and −SO3H groups, along with dehydroxylation of Ir/IrOx ~ SO3H species29,35,36,37.

The X-ray diffraction (XRD) results (Fig. S4) confirmed that no phase transformation occurred in Ir/IrOx before and after −SO3H modification. Scanning electron microscopy (SEM, Fig. S5) revealed that both Ir/IrOx ~ SO3H and Ir/IrOx catalysts exhibited similar microstructure, consisting of irregular nanoparticles and aggregated clusters with coarse surfaces. Notably, the −SO3H grafting process did not alter the catalyst morphology.

The above studies prove that the −SO3H groups have been successfully grafted to commercial Ir/IrOx by using the strong bonding strategy proposed in this work.

The OER catalytic performances of Ir/IrOx ~ SO3H catalyst were evaluated by a series of electrochemical tests.

The OER performance of the Ir/IrOx ~ SO3H catalyst was first evaluated in a three-electrode system using 0.5 M H2SO4 as the electrolyte, with commercial Ir/IrOx as the reference. The iR-corrected linear sweep voltammetry (LSV) curves (Fig. S6a) demonstrated the better OER activity of Ir/IrOx ~ SO3H, which required an overpotential of 233 mV to achieve a current density of 10 mA cm−2, lower than that of commercial Ir/IrOx (255 mV). Without iR correction, the overpotentials required to achieve 10 mA cm−2 were 259 mV for I r/IrOx ~ SO3H and 281 mV for Ir/IrOx. The catalytic stability of Ir/IrOx ~ SO3H was investigated by the chronopotentiometry test. As presented in Fig. S6b, Ir/IrOx ~ SO3H displayed robust stability at 10 mA cm−2, delivering a decay rate of only 0.4 mV h−1 during more than 100 h test. The cyclic voltammetry (CV) curves of Ir/IrOx ~ SO3H and Ir/IrOx catalysts were also recorded. Subsequently, following the method described in previous literature38, we monitored the current density evolution of both catalysts during cycling at 1.67 V (the anodic vertex potential of our CV window, 0.95–1.67 V vs. RHE) to compare their activity. This potential was chosen because it represents an oxidative extreme that maximizes catalytic activity, amplifies degradation signatures, minimizes bubble-induced artifacts, and prevents anodic dissolution. As shown in Fig. S7, the OER current density of Ir/IrOx ~ SO3H at 1.67 V (vs. RHE) slightly increased with the cycle number and was consistently larger than that of Ir/IrOx at each cycle, suggesting the promotional effect of −SO3H on the catalytic activity.

Their LSV curves (Fig. 3a) were further performed using a membrane electrode assembly (MEA) method (Fig. S8). The Ir/IrOx ~ SO3H catalyst displayed better catalytic activity than that of Ir/IrOx control at high current densities.

Fig. 3: Electrocatalytic performance and stability study after OER.
Fig. 3: Electrocatalytic performance and stability study after OER.
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a The LSV curves of Ir/IrOx ~ SO3H and Ir/IrOx catalysts recorded in 0.5 M H2SO4 solution at 80 °C. b Polarization curves of PEMWEs assembled with Ir/IrOx ~ SO3H anode and Ir/IrOx anode, respectively. c Chronopotentiometry curves of a PEMWE assembled with either Ir/IrOx ~ SO3H or Ir/IrOx anodes at 1.0 A cm−2. The temperature of the input water was controlled at 80 °C. The measured ohmic resistance was 24.9 ± 0.1 mΩ for the Ir/IrOx ~ SO3H-PEMWE and 23.4 ± 0.1 mΩ for the Ir/IrOx-PEMWE. d FTIR spectra and e S 2p XPS spectra of Ir/IrOx ~ SO3H catalyst before and after OER. Source data are provided as a Source data file.

In order to explore the intrinsic catalytic activity of the Ir/IrOx ~ SO3H catalyst, its electrochemically active surface area (ECSA) was measured (the detailed computing method provided in the supplementary information), and commercial Ir/IrOx was also carried out as reference (Fig. S9). The ECSA value of Ir/IrOx ~ SO3H, which possessed a good OER activity, was very close to that of Ir/IrOx (24.9 m2 g−1 vs. 24.4 m2 g−1), illustrating that ECSA was not the primary factor to improve the catalytic activity in this catalyst system25,39. In view of the same test conditions, we speculated that the high catalytic activity of Ir/IrOx ~ SO3H profited from the grafted −SO3H groups themselves.

The electrochemical impedance spectroscopy (EIS) spectra and a Randles model simulation were depicted in Fig. S10, showing a significantly lower charge transfer resistance (Rct) of Ir/IrOx ~ SO3H (76.4 Ω vs. 167.2 Ω for commercial Ir/IrOx). The EIS data indicated that −SO3H grafting efficiently improved charge transport property, thereby boosting the favorable OER kinetics40,41. We further conducted Gibbs free energy calculations for the OER. In the corresponding OER free energy diagrams (Fig. S11), the theoretical overpotential (η) of Ir/IrOx ~ SO3H was 0.67 eV, substantially lower than that of Ir/IrOx (0.92 eV). This suggested that the −SO3H grafting facilitated the proton transfer process, thereby enhancing the OER performance.

The catalyst hydrophobicity/hydrophilicity can affect the OER activity8. The hydrophilicity of both Ir/IrOx ~ SO3H and commercial Ir/IrOx was characterized via the water contact angle (CA). The CA between water and Ir/IrOx ~ SO3H was 114.8°, which was lower than that of Ir/IrOx (142.2°) (Fig. S12). This observation manifested that the presence of −SO3H groups greatly boosted water adsorption on the Ir/IrOx ~ SO3H catalyst surface, consistent with the TGA results. A local hydrophilic environment established on the Ir/IrOx ~ SO3H surface was conducive to water diffusion, ion transport, and the fast desorption of gas bubbles, which accounted for a large proportion in promoting the Ir/IrOx ~ SO3H catalytic activity, especially under large current densities34,42,43.

The high OER catalytic activity of the Ir/IrOx ~ SO3H catalyst assembled into a proton exchange membrane water electrolysis (PEMWE) cell (Fig. S13) was further verified under actual working environments. For comparison, commercial Ir/IrOx was also assembled into a PEMWE cell. The polarization curves were obtained by recording the cell voltages at different current densities (Fig. 3b). The comparison of the cell activities of Ir/IrOx ~ SO3H and other reported Ir-based catalysts is summarized in Table S2, highlighting the advantage of Ir/IrOx ~ SO3H in PEMWE. Especially, at a current density of 3.0 A cm−2, the cell voltage of the Ir/IrOx ~ SO3H-PEMWE was 1.75 V, which was 125 mV less than that of the Ir/IrOx-PEMWE. The anodic and cathodic faradaic efficiencies of Ir/IrOx ~ SO3H were determined to be 97 ± 1% and 97 ± 1%, respectively (Fig. S14), confirming the absence of detectable side reactions during PEMWE operation. The polarization curve of Ir/IrOx ~ SO3H with low loading (0.4 mgIr cm−2 for the anode and 0.1 mgPt cm−2 for the cathode) was also measured, showing good performance (Fig. S15 and Table S3). Figure S16 further revealed that the Ir/IrOx ~ SO3H catalyst exhibited improved proton conductivity, resulting in lower cell voltages at high current densities (>2.0 A cm−2) in PEMWEs.

Long-term stability is a fundamental standard to measure the performance of catalysts and to validate the strong bonding strategy. Chronopotentiometry (CP) tests were performed in a PEMWE equipped with either Ir/IrOx ~ SO3H or Ir/IrOx anodes at a current density of 1.0 A cm−2 fed with pure water at 80 °C (Fig. 3c). The Ir/IrOx ~ SO3H-PEMWE demonstrated better performance, exhibiting a consistent lower cell voltage (70–100 mV reduction) and comparable stability compared to the Ir/IrOx-PEMWE at 1.0 A cm−2. The Ir/IrOx ~ SO3H-PEMWE operated stably for over 1000 h with only a minimal potential increase of 17 mV. This performance is competitive with that of most anion-modified catalysts reported previously (Table S4), highlighting the good durability and structural robustness of Ir/IrOx ~ SO3H.

After OER, the chemical structure of Ir/IrOx ~ SO3H was analyzed by FTIR and XPS. The −SO3H groups persisted (Fig. 3d) and sustained their original chemical states (Fig. 3e), and the S content in Ir/IrOx ~ SO3H, as determined by XPS, remained constant (Table S5) after OER, thus verifying the structural and chemical stability of Ir/IrOx ~ SO3H and the feasibility of the strong bonding strategy.

In summary, the results and characterizations above reveal that −SO3H groups are stably anchored to the Ir/IrOx surface and can effectively enhance the activities of catalysts.

The distinguished performances of the Ir/IrOx ~ SO3H catalyst motivate us to explore the effect of −SO3H on enhancing catalytic activity.

The OER kinetics are typically assessed through Tafel slope and kinetic isotope effect (KIE) measurements, which are the primary kinetic parameters for OER. As illustrated in Fig. 4a, Ir/IrOx ~ SO3H showed a relatively small Tafel slope of 45.1 mV dec−1, markedly lower than that of commercial Ir/IrOx (88.0 mV dec−1), implying a high reaction rate and accelerated reaction kinetics promoted by the grafted −SO3H groups during OER process24,44,45. −SO3H functionalization also altered the rate-determining step of Ir/IrOx, as detailed in the supplementary information regarding the Tafel slope analysis.

Fig. 4: OER-enhancing mechanism analysis.
Fig. 4: OER-enhancing mechanism analysis.
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a Tafel plots, b the KIE values vs. overpotential, and c the Ea values vs. overpotential of Ir/IrOx ~ SO3H and Ir/IrOx catalysts with iR correction, error bars represent the standard deviation (n = 3 independent experiments). All tests (Tafel slope, KIE, Ea) employed a three-electrode system. Electrolyte: 0.5 M H2SO4 for Tafel and Ea tests; 0.5 M H2SO4 and 0.5 M D2SO4 for KIE tests. d In situ ATR-SEIRAS spectral lines as a function of test time for Ir/IrOx ~ SO3H catalyst recorded in 0.1 M HClO4 at 1.55 V (vs. RHE). e Schematic illustration of the OER mechanism on the Ir/IrOx ~ SO3H surface. Source data are provided as a Source Data file.

Deuterium KIE can deliver the proton transfer kinetic messages of OER46,47, and a KIE value larger than 1.5 (primary KIE) is regarded as evidence that proton transfer participates in the rate-determining step or at least, in one of the steps influencing the reaction rate39,47,48. The KIE tests in 0.5 M D2SO4/D2O solution were carried out, and then the KIE values were computed according to the detailed calculated methods in supplementary information49. The curves of KIE value as a function of overpotential were depicted in Fig. 4b, both Ir/IrOx ~ SO3H and Ir/IrOx presented primary KIEs, suggesting that the cleavage of O − H bonds dominated their rate determining steps during the OER processes39,47. Notably, Ir/IrOx ~ SO3H showed the smaller KIE value than that of Ir/IrOx at each of the same overpotential, implying that certain functional groups (−SO3H groups in this catalyst system) existed near the catalytic centers to accelerate the OER kinetics47.

We employed the apparent activation energy (Ea) of both Ir/IrOx ~ SO3H and Ir/IrOx catalysts to analyse how the grafted −SO3H groups made a positive contribution to the catalytic activity. The Ea values at fixed overpotentials were calculated following the Arrhenius equation (the detailed computing method provided in the supplementary information)49. As shown in Fig. 4c, the Ea value of Ir/IrOx ~ SO3H was smaller than that of Ir/IrOx at each of the same overpotential, which was in line with the lower Rct and Tafel slope. The result reflected that the −SO3H groups introduced were conducive to reducing the Ea, changing the entropy of activation, and expediting the charge transfer process in OER39,41,49,50.

To further uncover the OER mechanism of the Ir/IrOx ~ SO3H catalyst, in situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) at 1.55 V (vs. RHE) in a 0.1 M HClO4 electrolyte was performed to monitor the generation of possible reaction intermediates during the catalytic process. The configuration of the custom-built electrochemical cell is shown in Fig. S17. The absorbance curves versus reaction time of Ir/IrOx ~ SO3H and Ir/IrOx were exhibited in Figs. 4d and S18, respectively. Excluding a sharp peak at ~1241 cm−1 classified as the Si-O-Si bonds in the ATR crystal and a broad peak at ~1060 cm−1 assigned to ClO4− vibrations, there was a visible absorption peak at ~1151 cm−1, marking the generation of *OOH intermediates (Fig. 4d)12,44,51,52. Conversely, no obvious *OOH peak was observed for Ir/IrOx (Fig. S18). The in situ ATR-SEIRAS results indicated that the grafted −SO3H groups facilitated both deprotonation and proton transfer processes during the OER via hydrogen bonding interactions, thereby generating more *OOH intermediates and accelerating the OER. Notably, as depicted in Fig. 4d, the stretching modes of −SO3H (1207, 1105, and 1011 cm−1) persisted at similar intensities throughout the test process for Ir/IrOx ~ SO3H, which implied the robust anchoring of −SO3H to the Ir/IrOx surface and the stable −SO3H structure in Ir/IrOx ~ SO3H under OER conditions, thereby offering a good PEMWE durability.

The above analysis defined that the −SO3H groups promote the OER process by enhancing the proton transfer process and boosting the generation of *OOH. Finally, we deduced a fundamental understanding of the OER mechanism on Ir/IrOx ~ SO3H (Fig. 4e): (1) initial water adsorption at the Ir/IrOx ~ SO3H surface, where H2O forms hydrogen bonding with −SO3−, facilitating the production of *OH species; (2) *OH deprotonation accelerated by hydrogen bonding between *OH and −SO3−, generating *O species; (3) *O hydration with H2O to produce *OOH intermediates; (4) *OOH deprotonation promoted by −SO3− interactions and O2 release.

Therefore, we attribute the accelerated OER kinetic and enhanced OER activity to the introduction of −SO3H groups.

When preparing the catalyst ink, we found an interesting phenomenon: the Ir/IrOx ~ SO3H catalyst ink was less easy to settle than Ir/IrOx catalyst ink after standing for 24 hours. The aggregation behavior of Ir/IrOx ~ SO3H catalyst was studied by zeta potential (ζ-potential) and dynamic light scattering (DLS) spectra, and Ir/IrOx catalyst was also carried out as reference. It was quite clear that the ζ-potential of Ir/IrOx catalyst shifted from −13 mV to −30 mV after grafting the −SO3H groups (insert text in Fig. 5a and 5d), further implying the successful synthesis of Ir/IrOx ~ SO3H and the enhanced dispersity of Ir/IrOx ~ SO3H catalyst solution.

Fig. 5: Characterization and fabrication of large-scale MEA.
Fig. 5: Characterization and fabrication of large-scale MEA.
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Scattering intensity vs. particle size distribution plots as a function of the standing time for a Ir/IrOx ~ SO3H and d Ir/IrOx catalyst inks. SEM images: top view of b Ir/IrOx ~ SO3H and e Ir/IrOx catalyst layers, cross-section of c Ir/IrOx ~ SO3H and f Ir/IrOx catalyst layers on Nafion 212 membrane prepared by the ultrasonic-spray method. g Picture of the slot-die coating equipment working on a practical note. h Picture of large-scale Ir/IrOx ~ SO3H MEA (54 cm × 45 cm). Source data are provided as a Source data file.

Figure 5a exhibited the DLS spectral lines from the fresh and aged Ir/IrOx ~ SO3H− catalyst inks, comparing hour 0, hour 12, and hour 24. No noticeable shift was observed in DLS spectral lines as a function of standing time, verifying that the Ir/IrOx ~ SO3H catalyst particles were less likely to settle and agglomerate during the aging process. Ir/IrOx catalyst inks were measured using the same methodology. As shown in Fig. 5d, the spectral lines of Ir/IrOx catalyst inks reflected a visible negative shift with increasing standing time. In addition, the average particle size of 142 nm in the fresh Ir/IrOx ~ SO3H catalyst ink was significantly lower than that in the fresh Ir/IrOx catalyst ink (210 nm). The above results implied that Ir/IrOx ~ SO3H catalyst ink possessed smaller average particle size and better storage stability than Ir/IrOx catalyst ink due to the introduction of −SO3H groups, thereby increasing the repulsive forces between negatively charged Ir/IrOx ~ SO3H catalyst particles and Nafion side chains, which could minimize the agglomeration of Ir/IrOx ~ SO3H catalyst particles and render them more dispersive53.

Next, the morphologies of Ir/IrOx ~ SO3H and the Ir/IrOx catalyst layers deposited on Nafion 212 membrane via the ultrasonic-spray method were investigated by SEM. The surface morphology of Ir/IrOx ~ SO3H catalyst layer was more uniform and compact compared to Ir/IrOx, as depicted in Fig. 5b, e. From the cross-section images of both (Figs. 5c, f), Ir/IrOx ~ SO3H revealed the dense and even packing of the tiny catalyst particles, which was conducive to increasing the accessibility and availability of active sites and establishing consecutive oxygen and water transfer channels, further enhancing OER activity54,55.

In actual operation, the efficient large-scale synthesis of Ir/IrOx ~ SO3H catalysts in quantities of tens of grams was realized via the strong bonding strategy in the lab, which was suited to the wholesale production of MEA. The high quality of the Ir/IrOx ~ SO3H catalyst ink, evidenced by its good dispersion stability, enabled the production of a large-area MEA using slot-die coating (refer to Fig. 5g). Furthermore, the large MEA (54 cm × 45 cm) highlighted the promising potential for industrial-scale application of Ir/IrOx ~ SO3H catalyst gained through the strong bonding strategy (Fig. 5h).

Discussion

In summary, we propose a strong bonding strategy to achieve the robust anchoring of sulfonates to the catalyst surface, thereby preventing their leaching. Taking Ir/IrOx as the study model, −SO3H groups were successfully grafted to Ir/IrOx catalysts, and the large-scale synthesis of Ir/IrOx ~ SO3H catalyst was performed smoothly. The resultant Ir/IrOx ~ SO3H catalyst exhibits high activity (1.75 V @ 3.0 A cm−2) while maintaining good stability (>1000 h). The enhanced performances benefit from the grafted −SO3H, which contributes to enhancing the proton transfer process and promoting the formation of *OOH intermediates, thereby accelerating the OER kinetics. Moreover, the stable modification of −SO3H groups ensures the durability of Ir/IrOx ~ SO3H during OER process. Furthermore, Ir/IrOx ~ SO3H catalyst particles are less likely to settle and agglomerate in their inks due to the enhanced repulsive forces between Ir/IrOx ~ SO3H and Nafion. Overall, this study puts forward a feasible way for the rational design and successful preparation of anion-modified catalysts for practical industrialization.

Methods

Chemicals

Iridium oxide (Ir/IrOx, SS-Ir-1001) was ordered from Anhui Contango New Energy Technology Co., Ltd. 1,3-Propanesultone (99.0%) was ordered from Adamas. Polyvinylidene fluoride (PVDF) was ordered from Shanghai Aladdin Biochemical Technology Co., Ltd. Toluene (99.5%), methanol (99.7%), sulfuric acid (H2SO4, 98.0%), isopropanol (IPA, 99.7%), perchloric acid (HClO4, 72.0%) were ordered from Sinopharm Chemical Reagent Co., Ltd. Deuteric sulfuric acid (0.5 M D2SO4 in D2O) solution was ordered from Wuhan Isotope Technology Co., Ltd. Titanium felt (2GDL10, 0.25 mm) was provided by Bekaert. Carbon paper (Sigracet 28 BC) was provided by Suzhou Sinero Technology Co., Ltd. Nafion ionomer (5 wt. %) and Nafion 212 membrane were ordered from DuPont Co. All the chemicals were at analytical grade and utilized as received without additional purification. Ultrapure water (18.2 MΩ cm) was used throughout this study. The 0.5 M H2SO4 (pH = 0.30 ± 0.05) and 0.1 M HClO4 (pH = 1.00 ± 0.03) electrolytes were prepared fresh for immediate use by diluting concentrated sulfuric acid and concentrated perchloric acid, respectively.

Synthesis of Ir/IrOx ~ SO3H

The sulfonic group (−SO3H) modified Ir/IrOx (Ir/IrOx ~ SO3H) was synthesized via the ring opening reaction of 1,3-propanesultone31. The reaction was carried out at 110 °C for 24 h with the mass ratio of Ir/IrOx and 1,3-propanesultone of 1:0.5. The Ir/IrOx ~ SO3H was separated by filtration, washed with deionized water, and dried in a vacuum oven. By extending the reaction time to 48 h and following the synthesis protocol, we had successfully achieved large-scale production (tens-of-gram batches) of the Ir/IrOx ~ SO3H catalyst.

Nafion membrane pretreatment

A commercially available Nafion 212 membrane (thickness: 50.8 µm) was used directly for electrochemical cell tests. For the proton exchange membrane water electrolysis and membrane electrode assembly tests, the Nafion 212 membranes had dimensions of 4 cm × 4.5 cm and 3 cm × 3 cm, with corresponding active areas of 2 cm × 2.5 cm and 1 cm × 1 cm, respectively.

Materials characterizations

Fourier transform infrared (FTIR) spectra were gained using a Bruker INVENIO infrared spectrophotometer equipped with a built-in mercury cadmium telluride (MCT) detector in a range of 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) spectra were measured by PHI GENESIS photoelectron spectrometer. Especially, the Ir/IrOx ~ SO3H catalyst was characterized after undergoing a 200-h stability test under potentiostatic conditions (1.60 V vs. RHE) in a three-electrode configuration (the Ir/IrOx ~ SO3H/Ti-felt electrode, PVDF binder, and 0.1 M HClO4 electrolyte). Post-OER characterization included FTIR and XPS analysis (denoted “After OER”). Field emission transmission electron microscopy (FE-TEM) and energy dispersive spectroscopy (EDS) elemental mapping were conducted using a JEOL JEM-2100F 200 kV FE-TEM equipped with an Oxford’s energy dispersive X-ray spectrometer. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were conducted using a Perkin Elmer-Pyris 1 TGA instrument at a heating rate of 10 °C min−1 from 35 °C to 800 °C under nitrogen flow. Powder X-ray diffraction (XRD) test was performed using a Rigaku Smatlab 9 KW diffractometer, equipped with a Cu-Kα X-ray source possessing a wavelength (λ) of 0.1542 nm. The scanning range was set from 20 to 80 degrees, with a scanning speed step set at 0.02 degrees and an integration time of 0.3 s per step. Scanning electron microscopy (SEM) images were taken by a Zeiss-Ultra 55 SEM detector. The water contact angle (CA) of the sample was measured using the sessile drop method. The catalyst was firstly dispersed in isopropanol and then applied to the surface of a Nafion 212 membrane using ultrasonic spraying. After hot pressing, the membrane was secured onto the sample stage of the contact angle analyzer. Finally, water was directly dropped onto the catalyst surface to measure the water contact angle. Dynamic light scattering (DLS) and zeta potential (ζ-potential) measurements were performed on the diluted catalyst ink using a Malvern-ZS 90 and repeated three and three times, respectively, to ensure the repeatability of the results. The catalyst loading on the membrane electrode assembly was measured by energy dispersive X-ray spectroscopy (EDX) using an EDX600PLUS device from Skyray Instrument Co. Ltd.

Electrochemical characterizations

Membrane electrode assembly (MEA) test

The homemade membrane electrode assembly (MEA) setup56 (Fig. S8) featured an active area of 1 cm2, with a 0.5 M H2SO4 solution maintained at 80 °C supplied to the anode at a consistent flow rate of 100 mL min−1. Titanium felt was used as the diffusion layer for the anode, while carbon paper served as the diffusion layer for the cathode. The reference electrode (saturated mercurous sulfate electrode) was positioned on the side near the anode. A Pt foil was used as the counter electrode. The catalyst loading was maintained at 1.0 mgIr cm−2 on the working electrode. To assess the oxygen evolution reaction (OER) activity of different catalysts, the catalyst on the working electrode was first subjected to 10 cycles of cyclic voltammetry (CV) at a scan rate of 50 mV s−1 for activation. Following this, the linear sweep voltammetry (LSV) testing was conducted from 0.5 to 1.8 V vs. reversible hydrogen electrode (RHE) at a scan rate of 10 mV s−1.

Three-electrode system electrochemical measurement

The LSV, CV, and electrochemical impedance spectroscopy (EIS) spectra of catalysts were obtained using a three-electrode system on an electrochemical workstation (Autolab PGSTAT204), which was equipped with a built-in EIS analyzer. The system was controlled, and data were acquired using the Nova 2.1.7 software package. Before the measurements, all 0.5 M H2SO4 electrolytes were purged with argon to eliminate dissolved oxygen. The LSV curves were obtained with a scan rate of 5 mV s−1 in the potential range of 0.95–1.67 V vs. RHE. The CV curves were conducted in the range of 0.3–1.0 V vs. RHE at a scan rate of 50 mV s−1. The EIS spectra were measured at 1.47 V (vs. RHE) with frequency ranging from 100 kHz to 0.05 Hz at an amplitude of 10 mV. To prepare the working electrode, 5 mg catalyst powder, 980 μL of a mixed solution (water:ethanol = 5:1, v/v), and 20 μL of 5 wt. % Nafion solution was mixed and sonicated to create a uniform ink. Then, a volume of 4.5 μL of this ink, corresponding to a catalyst loading of 0.32 mgIr cm−2, was drop-cast onto a clean glassy carbon rotating disk electrode (RDE, Autolab, 3 mm in diameter) and allowed to dry under ambient conditions. A saturated mercurous sulfate electrode (MSE, E0 = 0.652 V vs. RHE) was employed as the reference electrode, and a Pt foil was used as the counter electrode. The reference electrode was calibrated by performing a CV scan in a specific potential range near the open-circuit potential in a saturated K2SO4 solution. The scan rate should be as slow as possible (typically 1 mV s−1) in order to ensure the system reaches equilibrium. Identify the two electrode potentials where the current density is zero, calculate their average value, and use this as the calibrated reference electrode potential. Following calibration, the potential of the reference electrode was determined to be 0.652 V. The measured potential was then calibrated to the RHE scale using the following Eq. (1)57:

$${E}_{{RHE}}={E}_{{Hg}/{{Hg}}_{2}{{SO}}_{4}}+0.652+0.0592\times {pH}$$
(1)

The applied potential with iR compensation is denoted as potential (V vs. RHE).

Calculation of electrochemical active surface area (ECSA)

The electrochemical active surface area (ECSA) of Ir/IrOx ~ SO3H and Ir/IrOx was computed by the double-layer capacitance (Cdl) inferred from the CV curves at different scan rates. For the Ir/IrOx ~ SO3H catalyst, the potential was scanned from 0.30 to 0.40 V vs. MSE at elevated scan rates (20–150 mV s−1), while for Ir/IrOx, the potential was scanned from 0.25 to 0.35 V vs. MSE. The anodic and cathodic currents at 0.35 V vs. MSE for Ir/IrOx ~ SO3H and at 0.30 V vs. MSE for Ir/IrOx were plotted against the scan rates. Then, linear fitting was applied to these data points, and the average slope of both the anodic and cathodic plots represented the Cdl value. The ECSA was then obtained by dividing the Cdl by a constant factor of 0.035 mF cm−2, according to the previous report56,58,59. The calculation is performed according to the following Eq. (2):

$${ECSA}=\frac{{C}_{{dl}}}{0.035\,{mF}\,{{cm}}^{-2}}$$
(2)

Proton exchange membrane water electrolysis (PEMWE) test

Proton exchange membrane water electrolysis (PEMWE) test was performed using a DC power source (PWR401L, KIKUSUI Trading Co., Ltd.). A schematic diagram illustrating the configuration of the custom-built PEMWE cell is shown in Fig. S13. The MEA was fabricated using the catalyst-coated membrane (CCM) method. In this process, the Nafion 212 membrane was coated with anodic and cathodic catalysts on its respective sides. The anode was prepared at 80 °C by applying catalyst ink containing the catalysts, Nafion, and isopropanol in a mass ratio of 1:0.3:40. The Ir loading on the anode (Ir/IrOx ~ SO3H and Ir/IrOx) was measured to be 1.0 mgIr cm−2. Similarly, the cathodes with 40% Pt/C were prepared following the same procedures, achieving a Pt loading of 0.2 mgPt cm−2. Titanium felt was used as the diffusion layer for the anode, while carbon paper served as the diffusion layer for the cathode, and they were carefully assembled against titanium plates in a single-cell configuration with an active area of 5 cm−2. Deionized water at 80 °C was fed to the anode at a flow rate of 100 mL min−1. The electrochemical impedance spectrum of the PEMWE was measured using an electrochemical workstation (Autolab PGSTAT204). The high frequency resistance value, which corresponds to the ohmic resistance of the electrochemical cell, was found to be 24.9 ± 0.1 mΩ for the Ir/IrOx ~ SO3H-PEMWE and 23.4 ± 0.1 mΩ for the Ir/IrOx-PEMWE.

The faradaic efficiency (FE) tests were conducted using a drainage method at 25 °C60. The time courses of gas (oxygen and hydrogen) generation were collected during water electrolysis at a constant current density of 3.0 A cm−2, with the corresponding cell voltage stabilized at 1.75 V. The anodic FE was calculated by dividing the amount of evolved oxygen observed by the amount of oxygen expected from the charge passing through the anode electrode, according to the Eq. (3):

$${FE}=\frac{4F\times {n}_{{O}_{2}}}{Q}\times 100\%$$
(3)

where F is the faradaic constant (F = 96485 C mol−1), \({n}_{{O}_{2}}\) is the number of moles of O2 produced, which is estimated using the ideal gas law. Q is the total charge passed through the cell during the measurement period.

Similarly, the cathodic FE was determined from the measured and theoretical volumes of hydrogen. The gases produced by the PEMWE anode were analyzed by gas chromatography, and only hydrogen and oxygen were found.

Theoretical methods for OER

To estimate the reaction free energy of intermediates at the catalyst (*) surface in four-electrons OER procedure, the reaction free energy calculation2,24,25 followed these elementary steps:

$${H}_{2}O+* \rightleftarrows * {OH}+{H}^{+}+{e}^{-}$$
$$ * {OH}\rightleftarrows * O+{H}^{+}+{e}^{-}$$
$${H}_{2}O+* O\rightleftarrows * {OOH}+{H}^{+}+{e}^{-}$$
$$ * {OOH}\rightleftarrows * +{O}_{2}+{H}^{+}+{e}^{-}$$

The Gibbs free energy change (ΔG) of each intermediate is defined as:

$$\Delta G=\Delta E+\Delta {E}_{{ZPE}}-T\Delta S$$
(4)

where ΔEZPE is the difference in zero-point vibrational energy between the adsorbed and gas phases, T is the temperature (298.15 K), and ΔS represents the entropy change. The theoretical overpotential is determined by \(\eta=\max {G}_{i}-1.23\), where Gi denotes the free energy change for each OER step.

Proton conductivity measurement

Proton conductivities were conducted following the previous reported method56,61. The catalysts were sandwiched between two pieces of Nafion 212 membranes and loaded into a PEMWE cell. Cell voltages and current densities were then measured at 80 °C and used to derive the proton conductivity of the catalyst.

Tafel

The steady-state Tafel plots were obtained using chronoamperometry on rotation disk electrodes (1600 rpm) with iR compensation. The potential was held constant between 1.25 V and 1.75 V vs. RHE with a step of 20 mV. Each potential step was retained for 10 s to ensure stability, and the final current value at each step was recorded.

Moreover, the OER mechanism involves both a pre-equilibrium step (PES) and a rate determining step (RDS), which collectively determine the catalyst’s Tafel slope62. The Tafel slop (b) is inferred from the following Eq. (5)62,63:

$$b=\frac{d\eta }{d\log {{{\rm{i}}}}}=\frac{{k}_{B}T{{\mathrm{ln}}}10}{\left(\gamma+{r}_{{rds}}{\alpha }_{{rds}}\right)e}=\frac{59\,{{\mathrm{mV}}}/{{\mathrm{dec}}}}{\gamma+{r}_{{rds}}{\alpha }_{{rds}}}=\frac{59\,{{\mathrm{mV}}}/{{\mathrm{dec}}}}{\beta }$$
(5)

where η is the overpotential, i is the current density, γ is the number of electrons transferred to the reaction intermediate, rrds is 0 for a chemical step and 1 for an electrochemical reaction step as the RDS and αrds is the transfer coefficient of the RDS. The apparent transfer coefficient (β) is defined as β = γ + rrdsαrds. The term kBTln10/e is 59 mV dec−1 at 298 K. Here, kB is the Boltzmann constant, T is the absolute temperature (K), and e is the elementary charge.

According to the widely accepted conventional adsorbate evolution mechanism (AEM) in commercial IrO2, the OER involves multiple elementary steps, as shown below64:

$${M}^{ * }+{H}_{2}O\to M-{OH}+{H}^{+}+{e}^{-}$$
$$M-{OH}\to M-O+{H}^{+}+{e}^{-}$$
$$M-O+{H}_{2}O\to M-{OOH}+{H}^{+}+{e}^{-}$$
$$M-{OOH}\to {M}^{ * }+{O}_{2}+{H}^{+}+{e}^{-}$$

The significantly higher Tafel slope of 88.0 mV dec−1 for Ir/IrOx leads to an apparent transfer coefficient of β = 0.67; i.e., the first electrochemical (rrds = 1) reaction step in the electrocatalytic cycle of OER is the RDS, and no electron is transferred before the rds (γ = 0). Consequently, the transfer coefficient of the rds is α = 0.67. The Tafel slope is determined by a chemical PES followed by an electrochemical RDS. Therefore, the PES and RDS for Ir/IrOx can be described as follows:

$$\begin{array}{cc}M * +{H}_{2}O\to M-{H}_{2}O & ({{{\rm{PES}}}})\end{array}$$
$$\begin{array}{cc}M-{H}_{2}O\to M-{OH}+{H}^{+}+{e}^{-} & ({{{\rm{RDS}}}})\end{array}$$

In this mechanism, the catalyst active sites become saturated with adsorbed water (*H2O), and the reaction rate is limited by proton dissociation from *H2O.

The Tafel slope (b) of Ir/IrOx ~ SO3H is 45.1 mV dec−1 and translates to an apparent transfer coefficient of β = 1.31; i.e., one electron is transferred (γ = 1) before the rate determining electrochemical (rrds = 1) reaction step (rds) so that the transfer coefficient of the rds amounts to α = 0.31. The Tafel slope is determined by an electrochemical PES followed immediately by an electrochemical RDS. Therefore, the PES and RDS for Ir/IrOx ~ SO3H can be described as follows:

$$ M *+{-{SO}}_{3}^{-}+{H}_{2}O\to \left(M-{OH}+{-{SO}}_{3}H\right)\to M-{OH} \\ +{-{SO}}_{3}^{-}+{H}^{+}+{e}^{-} ({{{\rm{PES}}}})$$
$$ M-{OH}+{-{SO}}_{3}^{-}\to \left(M-O+{-{SO}}_{3}H\right)\to M-O+{-{SO}}_{3}^{-} \\ +{H}^{+}+{e}^{-} ({{{\rm{RDS}}}})$$

For the Ir/IrOx ~ SO3H system, the extremely rapid intermolecular proton transfer leads to apparent electrochemical dominance in both the RDS and the PES.

Relative to Ir/IrOx, Ir/IrOx ~ SO3H exhibited RDS-induced *OH coverage enhancement, promoting *OOH precursor formation and subsequent accumulation, as definitively verified by in situ ATR-SEIRAS (Fig. 4d). In conclusion, −SO3H functionalization alters the rate determining step of Ir/IrOx and thus accelerates the OER kinetics, as evidenced by Tafel slope measurements.

Kinetic isotope effect (KIE)

The electrochemical measurements of the catalysts were performed in both protonic (0.5 M H2SO4 in H2O) and deuteric (0.5 M D2SO4 in D2O) solutions to evaluate the KIE. The pD value of the deuteric solution was calculated by adding 0.41 to the pH value measured using a glass membrane pH electrode connected to a pH meter. Equation (6) was used to determine the potential on the reversible deuterium electrode (RDE) scale49.

$${E}_{{RDE}}={E}_{{Hg}/{{Hg}}_{2}{{SO}}_{4}}+0.0592\times {pD}+0.652+0.013$$
(6)

The term +0.013 arises from the difference in the standard equilibrium potentials between the deuterium couple (D2/D+) and the proton couple (H2/H+). This correction accounts for the distinct electrochemical properties of hydrogen and deuterium, which result in a small shift in their equilibrium potentials.

The overpotentials of the OER in both the protonic and deuteric solutions were determined using the following Eqs. (7) and (8), respectively65:

$$\eta={{{{\rm{E}}}}}_{{RHE}}-1.229{{{\rm{V}}}}$$
(7)
$$\eta={{{{\rm{E}}}}}_{{RDE}}-1.262{{{\rm{V}}}}$$
(8)

The KIE was calculated using Eq. (9)47:

$${{{\rm{KIE}}}}={\left|\frac{{j}_{{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}}}{{j}_{{{{{\rm{D}}}}}_{2}{{{\rm{O}}}}}}\right|}_{\eta }$$
(9)

where \({j}_{{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}}\) and \({j}_{{{{{\rm{D}}}}}_{2}{{{\rm{O}}}}}\) are the catalytic current density in the protonic and deuteric solution, respectively, at the same overpotential (η).

Apparent activation energy

The apparent activation energy (Ea) for the acidic OER was determined by performing LSV measurements of the catalysts in a 0.5 M H2SO4 solution at various temperatures. For a heterogeneous electrocatalytic reaction, Ea can be derived from the slope of a fitting Arrhenius plot (Eq. 10)49,66:

$${\left|\frac{\partial \left({\log }_{10}j\right)}{\partial \left(1/T\right)}\right|}_{\eta }=-\frac{{E}_{a}}{2.303{{{\rm{R}}}}}$$
(10)

where j represents the current density, R is the ideal gas constant (8.314 J K–1 mol–1), and T is the absolute temperature (K).

In situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS)

ATR-SEIRAS data were acquired using a Bruker INVENIO infrared spectrophotometer. The configuration of the custom-built electrochemical cell is shown in Fig. S17. The working electrode was prepared by coating either Ir/IrOx ~ SO3H or Ir/IrOx catalyst (both with PVDF binder) onto an Au/Si substrate. A saturated mercurous sulfate electrode (MSE, E0 = 0.652 V vs. RHE) served as the reference electrode and a graphite rod as the counter electrode. The Au/Si substrate was prepared through the following process67,68. The first step involved mechanical polishing of the Si prism’s reflecting plane with Al2O3 powders (0.3 µm followed by 0.05 µm). The Al2O3 residue was removed by ultrasonic cleaning in ultrapure water. The reflecting plane of the Si prism was soaked in piranha solution for over 3 h and then thoroughly rinsed with ultrapure water. The subsequent etching step involved immersing the reflecting plane in a 40 wt. % NH4F solution for 90 s. Finally, a gold film was deposited on the Si prism’s reflecting plane by immersing it in a plating solution at 60 °C for 3 min. The plating solution was formulated with the following composition: 0.015 M HAuCl4, 0.15 M Na2SO3, 0.05 M Na2S2O3, and 0.05 M NH4Cl.

ATR-SEIRAS spectra were acquired with the following parameters: unpolarized infrared radiation, ~70° incidence angle, 4 cm−1 spectral resolution, and a time resolution of 30 s per spectrum. The electrolyte was 0.1 M HClO4, which was saturated by bubbling with argon gas prior to use. After the background spectrum was recorded at the open circuit potential, the electrode potential was then held at 1.55 V vs. RHE to record the experimental spectra. All potentials are presented without iR correction. All spectra are plotted as absorbance (a.u.), which is computed from the reflected radiation intensities using the relation A = −log(I/I0). The reflected radiation intensity I is that of the sample, and I0 is that of the background spectrum.