English

• Hydrogen features high energy density, high fuel efficiency, and being free of pollution, which is regarded as an ideal substitute for fossil fuels. Electrochemical water splitting is a promising method for producing clean hydrogen fuel by using renewable energy [1–3]. The oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) are the main half-reactions that occur at the anode and cathode in electrochemical water splitting respectively. Among them, the OER is an important basis for supporting a variety of cathode reactions such as the HER, carbon dioxide reduction, and nitrogen reduction reaction to achieve the conversion and storage of renewable energy [4–7]. However, the OER involves multiple electron-protons coupling steps, which limits the efficiency of the overall water electrolysis [8–11]. Therefore, developing OER catalysts with sufficient catalytic activity and stability is crucial to the overall efficiency of the water splitting reaction.

  It is reported that the OER catalytic activity of some non-precious metal materials is close to or sometimes even better than that of noble metal IrO₂ or RuO₂, which include crystalline/amorphous metal oxides [12,13], layered double hydroxides [14,15], spinel-type oxides [16,17], perovskite-type oxides [18,19], transition metal sulfide (TMS) [20–22] and so on. Among them, nickel-iron-based catalysts stand out due to their great balanced oxygen adsorption/desorption abilities and are long considered as transition metals with relatively high activity [23–25]. Yet the evaluations of nickel-iron-based catalysts have always been carried out under low current densities. In practical applications, the catalytic current densities that catalysts are forced to provide often exceed 500 mA/cm² [26,27]. However, in the OER process at high current densities, it is often accompanied by various unfavorable microstructure evolutions of the catalytically active species (such as oxidative decomposition, structural changes, metal leaching and irregular aggregation) and the detachment of the catalytically active species from the working electrode, which is very likely to lead to the deactivation of the catalyst. High current density means rapid charge/mass transfer [28–30]. Therefore, a large active surface area, high electrical conductivity and excellent hydrophilicity/hydrophobicity must be integrated into electrocatalysts to provide high-speed kinetics. Although it is crucial to develop nickel-iron catalysts that can operate stably at high current densities, it is still challenging.

  It is also worth noting that iron, usually associated with negative emotions and considered undesirable, is a positive and necessary metal-based catalyst for the production of OER electrodes [31,32]. Since nickel-iron catalysts exist in the form of flake-like structures, researchers have long paid more attention to using the strategy of electronic regulation to enhance the intrinsic activity of the catalysts, while ignoring the impact on the morphological structure of nickel-iron hydroxides during the process of regulating the intrinsic activity and this point has not been taken seriously for a long time [33,34]. If the morphology of nickel-iron catalytic materials is optimized simultaneously with the regulation of their intrinsic activity, by reducing the nanoscale size of the catalysts to increase their electrochemically active area, and consequently increasing the number of surface-active sites, then the OER performance of the catalysts will be further improved. However, reports on the synchronous regulation of the electronic structure and morphology of nickel-iron catalysts are relatively lacking at present.

  In light of the above analysis, herein, we have synthesized a nickel-iron hydroxide co-regulated by non-metal sulfur and metal ruthenium (S_ARu_T-FeNiOH_x-5h) through a simple multi-step room-temperature impregnation strategy. Physical characterizations and theoretical analyses show that the regulation by ruthenium and sulfur can not only modulate the electronic structure of the nickel-iron active centers and optimize the OER reaction pathway, but also adjust the growth process of the nickel-iron hydroxide catalyst, enabling it a larger specific surface area (enhanced 1.25 times). This effectively promotes the substantial contact between the catalyst and the electrolyte solution, thus enabling S_ARu_T-FeNiOH_x-5h to require a low overpotential of 253 mV when reaching a high current density of 1000 mA/cm² with long-term stability (500 h), and these features endow it with a large prospect in practical industrial-level applications.

  The S_ARu_T-NiFeOH_x-5h was synthesized via a stepwise room temperature soaking strategy as described in Fig. 1a. First, Ru modified NiFeOH_x supported on iron foam (Ru_T-NiFeOH_x-5h) was achieved through a 5-h impregnation process, which was then used as template to prepare the S_ARu_T-NiFeOH_x-5h by introducing S species in a sodium sulfide solution. The mechanism of the above synthetic process was shown in Fig. 1b. Specifically, the surface of the IF first underwent an oxidation–reduction reaction with oxygen, generating a small amount of Fe³⁺ and OH⁻. When they encountered the added Ru³⁺ and Ni²⁺, the four species precipitated and grew attached to the surface of the IF, forming Ru_T-NiFeOH_x-5h sample. During this process, CO₂ from the air spontaneously dissolved in the solution to form CO₃^2- species, which acted as a charge-balancing anionic regulator. Additionally, when the Ru_T-NiFeOH_x-5h was soaked in a solution containing sodium sulfide, S^2- would combine with Fe³⁺, Ru³⁺, and Ni²⁺ on the surface of Ru_T-NiFeOH_x-5h to form local sulfides, thereby achieving the regulation of the catalytic performance of Ru_T-NiFeOH_x-5h. Throughout the process of synthesizing S_ARu_T-NiFeOH_x-5h, the following chemical reactions (Eqs. 1–5) may be involved.

  $ \mathrm{Fe} \rightarrow \mathrm{Fe}^{2+}+2 \mathrm{e}^{-} $

  (1)

  $ \mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-} $

  (2)

  $ \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O}+4 \mathrm{e}^{-} \rightarrow 4 \mathrm{OH}^{-} $

  (3)

  $ \mathrm{Fe}^{3+}+\mathrm{Ni}^{2+}+\mathrm{Ru}^{3+}+\mathrm{OH}^{-} \rightarrow \mathrm{Ru}_{\mathrm{T}}-\mathrm{FeNiOH}_{\mathrm{x}} $

  (4)

  $ \mathrm{Ru}_{\mathrm{T}}-\mathrm{FeNiOH}_{\mathrm{x}}+\mathrm{S}^{2-} \rightarrow \mathrm{S}_{\mathrm{A}} \mathrm{Ru}_{\mathrm{T}}-\mathrm{FeNiOH}_{\mathrm{x}} $

  (5)

  Figure 1

  

  Figure 1.  (a) Schematic illustration of the synthesis process of S_ARu_T-NiFeOH_x-5h catalyst. (b) Synthesis mechanism of S_ARu_T-NiFeOH_x-5h. (c) XRD patterns of the prepared NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. Comparison of the SEM images of (d) NiFeOH_x-5h, (e) Ru_T-NiFeOH_x-5h, and (f) S_ARu_T-NiFeOH_x-5h. (g) TEM and (h) HRTEM images of S_ARu_T-NiFeOH_x-5h.

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  Fig. S1 (Supporting information) was the optical photograph of IF, NiFeOH_x-5h, Ru_T-NiFeOH_x-5h and S_ARu_T-NiFeOH_x-5h. At each stage, the corresponding color has changed significantly. Fig. 1c showed the X-ray diffraction (XRD) of three substances including NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. It was found that among the three samples, only three obvious characteristic peaks of the IF appeared at 45.4°, 65.2°, and 82.4°, which corresponded to the (110), (200), and (220) crystal faces of the iron (PDF No. 00–001–1267).

  The morphology and structure of the prepared samples were further characterized by scanning electron microscopy (SEM). As shown in Fig. S2 (Supporting information), the initial IF had a relatively smooth and flat surface, which provided an idea foundation for the subsequent growth of the nickel-iron hydroxide precursor. After a 5-h room temperature soaking, the NiFeOH_x-5h grew on the surface of the IF in the form of interwoven nanosheets and the size of the nanosheets being approximately 4–6 µm in length (Fig. 1d). When ruthenium ions were involved in the growth of NiFeOH_x-5h, it was observed that the size of the nickel-iron hydroxide nanosheets noticeably decreased (about 500 nm), which might be related to the rapid nucleation of Ru(OH)₃ to promote catalyst growth (Fig. 1e). After further S^2- regulation, the smaller size of the nickel-iron hydroxide nanosheets was remained, indicating that the sulfur modification has not destroyed the structure of the Ru_T-NiFeOH_x-5h (Fig. 1f). Figs. 1g and h were scanning transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of S_ARu_T-NiFeOH_x-5h. As revealed, S_ARu_T-NiFeOH_x-5h showed a relatively thin petal-like nanosheet structure, and the size of the nanosheets was about 200 nm, which was corresponding to the SEM results in Fig. 1f. It is noteworthy that in the HRTEM images of S_ARu_T-NiFeOH_x-5h hardly any long-range ordered lattice fringes were found, confirming the amorphous state of the catalyst. SEM mapping and energy dispersive X-ray spectroscopy (EDS) revealed, as expected, the presence of Fe, Ni, O, S, and Ru (Fig. S3 in Supporting information), with the atomic proportions of Fe, Ni, O, S, and Ru being 51.23%, 8.44%, 35.92%, 4.13%, and 0.28%, respectively (Table S1 in Supporting information). Combined with the above XRD, SEM, SEM Mapping, and HRTEM analyses, it was believed that the nanosheets born on the iron foam were mainly amorphous nickel-iron hydroxide.

  In the comparative experiments mentioned above, it was found that the regulation of Ru³⁺ could effectively reduce the size of nickel-iron hydroxide nanosheets. Therefore, by comparing the morphology of NiFeOH_x-5h under different Ru³⁺ additions (0, 0.00001, 0.001, 0.005, and 0.01 mol/L), the regulatory mechanism of Ru³⁺ on the growth and performance of nickel-iron hydroxide was further explored. As shown in Figs. 2a-e, the size of nickel-iron hydroxide nanosheets regulated by ruthenium gradually decreased with the increase of Ru³⁺ concentration. It was also found that when the concentration of Ru³⁺ in the soaking solution was 0.005 mol/L, the Ru_T-NiFeOH_x-5h-0.005 exhibited the best OER performance, indicating that Ru³⁺ had the best regulatory effect on nickel-iron hydroxide under this condition (Fig. 2f). Based on SEM and LSV results, it was speculated that the regulation of ruthenium on NiFeOH_x was related to the solubility product constants (K_SP) of Ru(OH)₃, Ni(OH)₂, and Fe(OH)₃. Specifically, the K_SP value of Ru(OH)₃ was 1.0 × 10^–36, which was much smaller than that of Ni(OH)₂ (2.0 × 10^–15). Considering the trace amounts of Fe³⁺ in the solution, Ru³⁺ would rapidly combine with the small amount of OH^- to form a large number of fine nanoparticles, which acted as crystal nuclei to promote the attachment and growth of nickel-iron hydroxide on their surfaces, thereby forming a large number of fine nanosheet structures (Fig. 2g). Compared with the pure NiFeOHx-5h, this abundance of small-sized nanosheets could effectively promote full contact between the catalyst and the electrolyte, thereby enhancing the overall catalytic performance of the NiFeOH_x-5h.

  Figure 2

  

  Figure 2.  The influence of the concentration of Ru³⁺ in the soaking system on the growth process of the Ru_T-NiFeOH_x-5h. SEM images of the Ru_T-NiFeOH_x-5h at different Ru³⁺ addition levels: (a) 0 mol/L, (b) 0.0001 mol/L, (c) 0.001 mol/L, (d) 0.005 mol/L, and (e) 0.01 mol/L. (f) The polarization curves of Ru_T-NiFeOH_x-5h at different Ru³⁺ concentrations. (g) Schematic diagram of the synthesis mechanism of Ru_T-NiFeOH_x-5h catalyst.

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  Further investigations of the chemical composition and valence state of the surface elements were probed by the X-ray photoelectron spectroscopy (XPS) technique. As shown in Fig. S4a (Supporting information), the S and Ru signal, together with the Ni, Fe, and O element peaks, were clearly present in the full spectrum of S_ARu_T-NiFeOH_x-5h, in agreement with the EDS results above. In the core-level, the Fe 2p spectra of NiFeOH_x-5h exhibited two main peaks attributed to Fe 2p_1/2 and Fe 2p_3/2 respectively. The characteristic peaks with binding energies of 725.0 eV and 711.3 eV were assigned to Fe³⁺, another couple of characteristic peaks with binding energy of 733.0 and 715.5 eV were assigned to satellite peaks [35–37]. Compared with pure NiFeOH_x-5h, the peaks of Fe³⁺ in Ru_T-NiFeOH_x-5h have shifted towards lower binding energy by approximately 0.3 eV. This change indicated an increase in the valence state of the Fe element in Ru_T-NiFeOH_x-5h. On the contrary, the characteristic peaks of Fe³⁺ in S_ARu_T-NiFeOH_x-5h were located at lower binding energies of 724.0 and 710.6 eV (compared to Ru_T-NiFeOH_x-5h), which might be attributed to the weak electronegativity of coordination sulfur relative to oxygen (Fig. S4b in Supporting information). For Ni 2p in Fig. S4c (Supporting information), the peaks at 873.62 and 855.7 eV, together with satellite peaks at 873.6 and 861.9 eV (Sat.), are indicative of the Ni 2p_3/2 and Ni 2p_1/2 orbitals of the Ni²⁺ species [38–40]. The binding energy of Ni²⁺ in Ru_T-NiFeOH_x-5h exhibited a positive shift of 0.5 eV compared to NiFeOH_x-5h. This might originate from the interfacial electron transfer from Ni to Ru, thereby raising the average oxidation state of nickel. Conversely, the positions of the Ni²⁺ peaks in S_ARu_T-NiFeOH_x-5h were very similar to those in Ru_T-NiFeOH_x-5h, indicating that sulfur primarily enhanced the OER performance of the catalyst by affecting the electronic structure of iron. In addition to the XPS spectra of the metal species, O 1s XPS spectra of three samples were shown in Fig. S4d (Supporting information). As shown, the O 1s spectra of catalysts all contained M-O bond, M-OH bond, and adsorbed water located at 529.8, 531.3, and 532.8 eV, respectively [41–43]. It is evident that the regulation by sulfur and ruthenium resulted in a positive shift in the binding energy of oxygen, corresponding to the increase in the valence state of nickel and iron. In Fig. S4e (Supporting information), the Ru 3p core level spectrum of Ru_T-NiFeOH_x-5h showed two distinct characteristic peaks at 486.7 and 463.3 eV, corresponding to the Ru 3p_1/2 and Ru 3p_3/2 orbitals, respectively. These peaks unequivocally indicated the presence of Ru³⁺ [44–46]. Compared to Ru_T-NiFeOH_x-5h, the Ru characteristic peaks of S_ARu_T-NiFeOH_x-5h exhibited a negative shift of approximately 0.4 eV, which implied a decrease in the oxidation state of Ru atoms in S_ARu_T-NiFeOH_x-5h. Regarding the S 2p spectrum depicted in Fig. S4f (Supporting information), S_ARu_T-NiFeOH_x-5h exhibited two peaks at 168.5 and 162.8 eV, corresponding to the S 2p_1/2 and S 2p_3/2 orbitals, respectively, unequivocally indicating the presence of S^2- [47,48].

  The alkaline OER performances of S_ARu_T-NiFeOH_x-5h and reference catalysts were evaluated in a 1.0 mol/L KOH electrolyte. As illustrated in Fig. 3a, a comparison was made of the linear sweep voltammetry (LSV) curves for NiFeOH_x under different soaking synthesis time. At a reaction time of 5-h, the NiFeOH_x-5h exhibited optimal activity, corresponding to an overpotential of 320 mV at a current density of 1000 mA/cm². To further investigate the effect of Ru³⁺ regulation mode on the OER performance of the catalyst, LSV test of Ru_T-NiFeOH_x-5h and Ru_A-NiFeOH_x-5h was conducted. As shown in Fig. 3b, Ru_T-NiFeOH_x-5h required a smaller potential at the same current density. This observation signifies that Ru³⁺ contributed significantly to the NiFeOH_x-5h growth process. Similarly, it was found that S_A-NiFeOH_x-5h showed better OER performance when modified by S^2- after synthesis of NiFeOH_x-5h (Fig. 3c). These results suggested that sulfur was more likely to enhance the performance of catalyst by modifying the surface electronic structure of NiFeOH_x-5h. Based on the above analysis, the OER properties of different catalysts were further compared under the optimal conditions (5-h soaking synthesis time, adding Ru³⁺ together with growth of nickel-iron hydroxide, adding S^2- after the growth of nickel-iron hydroxide) to reveal the regulatory results of Ru³⁺ and S^2- on nickel-iron hydroxide. As expected, S_ARu_T-NiFeOH_x-5h, in Figs. 3d and e, exhibited the optimal OER activity, requiring overpotentials of only 253, 296, and 321 mV to attain the current densities of 100, 500, and 1000 mA/cm², respectively. While the IF catalyst always afforded the largest η values and needed of 366 and 462 mV to reach the current densities of 10 and 100 mA/cm², which was worse than those of NiFeOH_x-5h (η₁₀₀ = 275 mV, η₅₀₀ = 321 mV, and η₁₀₀₀ = 341 mV), S_A-NiFeOH_x-5h (η₁₀₀ = 268 mV, η₅₀₀ = 315 mV, and η₁₀₀₀ = 340 mV), Ru_T-NiFeOH_x-5h (η₁₀₀ = 262 mV, η₅₀₀ = 310 mV, and η₁₀₀₀ = 330 mV), and S_ARu_T-NiFeOH_x-5h (η₁₀₀ = 253 mV, η₅₀₀ = 296 mV, and η₁₀₀₀ = 321 mV). The large differences in overpotential between catalysts demonstrated that the presence of Ru atom and S atom could effectively regulate the intrinsic OER activity of NiFeOH_x-5h. Even if the electrolyte was seawater (1.0 mol/L KOH containing 0.5 mol/L NaCl), the above rule remains unchanged (Fig. S7 in Supporting information). Meanwhile, due to its ultra-low overpotential, S_ARu_T-NiFeOH_x-5h demonstrated superior OER activity compared to most reported transition-metal-based catalysts and commercial iridium or ruthenium oxides (Table S2 in Supporting information). Next, the LSV polarization curves of the prepared samples were processed according to the Tafel equation to obtain the Tafel slopes for each sample (Fig. 3f). Specifically, the Tafel slopes for IF, NiFeOH_x-5h, S_A-NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h were 88, 68, 67, 63, and 61 mV/dec, respectively. Notably, S_ARu_T-NiFeOH_x-5h exhibited the smallest Tafel slope, indicating its superior catalytic activity and faster reaction kinetics among the samples. In addition, Nyquist plots were applied to further analyze the electronic and mass transport of catalyst (Fig. 3g). It was evident that S_ARu_T-NiFeOH_x-5h retained the smallest semicircle, indicative of the lowest electron transfer impedance. This phenomenon could be attributed to the redistribution of the electron configuration of the entire catalytic material, resulting from the modification of Ru³⁺ and S^2- on NiFeOHx-5h. Meanwhile, to reflect the morphology regulation of ruthenium, the NiFeOH_x-5h and Ru_T-NiFeOH_x-5h were evaluated by electrochemically active surface area (ECSA). It was obvious that the presence of ruthenium in Fig. S5 (Supporting information) and Table S3 (Supporting information) increased the ECSA value of NiFeOH_x-5h by 1.25 times, which could effectively promote full contact between the catalyst and the electrolyte. Finally, the stability of the S_ARu_T-NiFeOH_x-5h was also evaluated at a current density of 1000 mA/cm². As shown in Fig. 3h and Fig. S6 (Supporting information), during the long-term stability test, the potential and structure change of S_ARu_T-NiFeOH_x-5h could be negligible, indicating that S_ARu_T-NiFeOH_x-5h could maintain idea stability when running under industrial current density.

  Figure 3

  

  Figure 3.  (a) Polarization curves of NiFeOH_x with different immersion time with iR compensation. (b) Comparison of polarization curves of NiFeOH_x-5h with diverse Ru³⁺ doping methods with iR compensation. (c) Comparison of polarization curves tests of NiFeOH_x-5h with diverse S^2- doping strategies with iR compensation. Comparison of (d) polarization curves and (e) overpotentials of IF, NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, S_A-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h catalysts in 1.0 mol/L KOH with iR. (f) Tafel and (g) Nyquist plots of different samples with iR compensation. (h) Chronopotentiometry curve of S_ARu_T-NiFeOH_x-5h at a current density of 1000 mA/cm² for 500h without iR compensation.

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  The Gibbs free energy changes of reaction intermediates on the surface of nickel-iron hydroxide were investigated using density functional theory (DFT) calculations. The structures of NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h were shown in Fig. 4a. According to literature report, the Ni site in (001) crystal plane of nickel-iron hydroxide was selected to simulate the effect of sulfur ruthenium atom regulation on the OER performance of the catalyst. Meanwhile, the proposed adsorption evolution mechanism (AEM) of OER on the surface of catalyst was applied in Fig. 4b. Fig. 4c was the density of states (DOS) diagram of the three catalysts. As displayed, the center of the d band of S_ARu_T-NiFeOH_x-5h was closest to the Fermi level, indicating that the adsorption effect of reaction intermediates on the catalyst surface was significantly enhanced. Fig. 4d showed the calculated Gibbs free energy and the corresponding values. It was obvious that the rate determining step RDS of the NiFeOH_x-5h was *O → *OOH (3.94 eV). On the contrary, for Ru_T-NiFeOH_x-5h and S_ARu_T-NiFeOH_x-5h, the above RDS changed to *OOH→O₂ and the reaction energy barrier decreased to 3.08 and 2.96 eV, respectively. Therefore, the calculated results of DFT revealed that the synergistic regulation of S and Ru could effectively improve the OER activity nickel-iron hydroxide, which was consistent with the experimental analysis.

  Figure 4

  

  Figure 4.  (a) The optimized structural model of NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. (b) AEM mechanism of OER on the surface of catalyst. (c) DOS diagram of NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. (d) The calculated Gibbs free energy and the corresponding values.

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  In this work, a nickel-iron hydroxide catalyst supported on iron foam modified with nonmetallic sulfur and metallic ruthenium (S_ARu_T-NiFeOH_x-5h) was synthesized by a two-step room temperature impregnation strategy. It was found that the presence of ruthenium can effectively regulate the growth process of nickel-iron hydroxide, which significantly reduced the size of nanosheets and increased the contact area between catalyst and electrolyte (1.25 times). The subsequent modification of nickel-iron hydroxide by sulfur could promote the increase of metal valence in the active center of nickel, and thus enhanced the intrinsic activity of the catalyst. Resultantly, the optimal S_ARu_T-NiFeOH_x-5h exhibited an excellent OER activity with a lower overpotential 321 mV at 1000 mA/cm². Meanwhile, the stability test of up to 500 h at such a high current density endowed S_ARu_T-NiFeOH_x-5h a large prospect for practical application.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Shengxia Yang: Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. Yukang Pan: Writing – review & editing, Writing – original draft, Formal analysis, Conceptualization. Tianyu Kong: Writing – review & editing, Formal analysis. Chaoran Jia: Conceptualization. Yueyang Cui: Writing – original draft, Formal analysis. Xuehua Li: Writing – original draft, Formal analysis. Yannan Zhou: Writing – original draft, Formal analysis. Haijun Liu: Writing – review & editing, Funding acquisition, Formal analysis. Xinyu Zhang: Writing – review & editing, Resources, Project administration, Investigation, Funding acquisition, Formal analysis. Bin Dong: Writing – review & editing, Supervision, Investigation, Formal analysis, Conceptualization. Qunwei Tang: Writing – review & editing, Writing – original draft, Resources, Project administration, Investigation, Funding acquisition, Formal analysis.

  Acknowledgments

  This work is financially supported by Shandong Provincial Natural Science Foundation (No. ZR2024QB021) and Qingdao Natural Science Foundation (No. 24–4-4-zrjj-21-jch) and National Natural Science Foundation of China (Nos. 62204098, 62304124, 22309107).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111540.

  
  
• 1. [1]

     M. Chen, N. Kitiphatpiboon, C. Feng, et al., eScience 3 (2023) 100111. doi: 10.1016/j.esci.2023.100111

  2. [2]

     G.A. Gebreslase, M.V. Martínez-Huerta, M.J. Lázaro, J. Energy Chem. 67 (2022) 101–137. doi: 10.1016/j.jechem.2021.10.009

  3. [3]

     K. Ayers, N. Danilovic, R. Ouimet, et al., Annu. Rev. Chem. Biomol. Eng. 10 (2019) 219–239. doi: 10.1146/annurev-chembioeng-060718-030241

  4. [4]

     R. Ram, L. Xia, H. Benzidi, et al., Science 384 (2024) 1373–1380. doi: 10.1126/science.adk9849

  5. [5]

     J. Zhang, X. Fu, S. Kwon, et al., Science 387 (2025) 48–55. doi: 10.1126/science.ado9938

  6. [6]

     Y. Chen, Q. Li, Y. Lin, et al., Nat. Commun. 15 (2024) 7278. doi: 10.1038/s41467-024-51521-4

  7. [7]

     S. Lu, Z. Zhang, C. Cheng, et al., Angew. Chem. Int. Ed. 64 (2025) e202413308. doi: 10.1002/anie.202413308

  8. [8]

     J. Song, C. Wei, Z.F. Huang, et al., Chem. Soc. Rev. 49 (2020) 2196–2214. doi: 10.1039/c9cs00607a

  9. [9]

     J. Zhao, Y. Guo, Z. Zhang, et al., Nat. Nanotechnol. 20 (2024) 57–66.

  10. [10]

      S. Yu, J. Li, Y. Du, et al., Coord. Chem. Rev. 520 (2024) 216144. doi: 10.1016/j.ccr.2024.216144

  11. [11]

      J. Xu, M. Li, B. Dong, L. Feng, Chin. Chem. Lett. 35 (2024) 108798. doi: 10.1016/j.cclet.2023.108798

  12. [12]

      J. Zhang, L. Xu, X. Yang, et al., Angew. Chem. Int. Ed. 63 (2024) e202405641. doi: 10.1002/anie.202405641

  13. [13]

      C.H. Park, H. Lee, J.S. Choi, et al., Adv. Mater. 36 (2024) 2403392. doi: 10.1002/adma.202403392

  14. [14]

      R. Fan, C. Liu, Z. Li, et al., Nat. Sustain. 7 (2024) 158–167. doi: 10.1038/s41893-023-01263-w

  15. [15]

      P. Roy Chowdhury, H. Medhi, K.G. Bhattacharyya, C. Mustansar Hussain, Coord. Chem. Rev. 483 (2023) 215083. doi: 10.1016/j.ccr.2023.215083

  16. [16]

      L. Chong, G. Gao, J. Wen, et al., Science 380 (2023) 609–616. doi: 10.1126/science.ade1499

  17. [17]

      M.G. Ahmed, Y.F. Tay, X. Chi, et al., Angew. Chem. Int. Ed. 64 (2025) e202416757. doi: 10.1002/anie.202416757

  18. [18]

      Y. Shan, J. Pan, Y. Song, et al., Chem. Phys. 582 (2024) 112292. doi: 10.1016/j.chemphys.2024.112292

  19. [19]

      P. Wang, C. Zhang, J. Ding, et al., Small 20 (2024) 2305662. doi: 10.1002/smll.202305662

  20. [20]

      S. Yu, J. Li, J. Yin, et al., Chin. Chem. Lett. 35 (2024) 110068. doi: 10.1016/j.cclet.2024.110068

  21. [21]

      S. Yu, D. Liu, C. Wang, et al., J. Colloid Interface Sci. 653 (2024) 1464–1477. doi: 10.1016/j.jcis.2023.10.006

  22. [22]

      S. Yu, N. Zhang, J. Li, et al., ACS Sustain. Chem. Eng. 12 (2024) 4551–4564. doi: 10.1021/acssuschemeng.3c07704

  23. [23]

      G.L. Li, F. Deng, T.G. Ma, et al., Chem. Eng. J. 505 (2025) 159290. doi: 10.1016/j.cej.2025.159290

  24. [24]

      Z. Li, G. Lin, L. Wang, et al., Nat. Catal. 7 (2024) 944–952. doi: 10.1038/s41929-024-01209-1

  25. [25]

      Y. Liu, Q. Song, T. Xu, et al., Appl. Catal. B Environ. Energy 363 (2025) 124811. doi: 10.1016/j.apcatb.2024.124811

  26. [26]

      D. Li, R. Xiang, F. Yu, et al., Adv. Mater. 36 (2024) 2305685. doi: 10.1002/adma.202305685

  27. [27]

      J. Liu, W. Du, S. Guo, et al., Adv. Sci. 10 (2023) 2300717. doi: 10.1002/advs.202300717

  28. [28]

      N. Yu, F.L. Wang, X.Y. Jiang, et al., J. Energy Chem. 102 (2025) 208–217. doi: 10.1016/j.jechem.2024.10.043

  29. [29]

      R. Wan, T. Yuan, L. Wang, et al., Nat. Catal. 7 (2024) 1288–1304. doi: 10.1038/s41929-024-01266-6

  30. [30]

      Y. Park, H.Y. Jang, T.K. Lee, et al., Nat. Commun. 16 (2025) 579. doi: 10.1038/s41467-025-55910-1

  31. [31]

      X. -Y. Zhang, Y. -R. Zhu, Y. Chen, et al., Chem. Eng. J. 399 (2020) 125831. doi: 10.1016/j.cej.2020.125831

  32. [32]

      X. Zhou, T. Yang, T. Li, et al., Nano Res. Energy 2 (2023) e9120086. doi: 10.26599/nre.2023.9120086

  33. [33]

      C. Yin, J. Li, S. Wang, et al., Carbon Energy 6 (2024) e553. doi: 10.1002/cey2.553

  34. [34]

      J. Wu, Z. Ma, L. Yu, et al., Sci. China Chem. 67 (2024) 2755–2766. doi: 10.1007/s11426-024-2077-2

  35. [35]

      Y. Gao, X. Sun, G.A.M. Mersal, et al., Adv. Compos. Hybrid Mater. 8 (2025) 117. doi: 10.1109/icosse65712.2025.00028

  36. [36]

      H. Liu, X. Lu, P. Shen, et al., ACS Appl. Nano Mater. 7 (2024) 7836–7844. doi: 10.1021/acsanm.4c00444

  37. [37]

      X. Tian, Y. Wang, F. Sun, et al., J. Alloy. Compd. 997 (2024) 174942. doi: 10.1016/j.jallcom.2024.174942

  38. [38]

      L. Fan, P. Zhang, B. Zhang, et al., ACS Energy Lett. 3 (2018) 2865–2874. doi: 10.1021/acsenergylett.8b01897

  39. [39]

      A.A. Abdelraouf, A.M. Abdelrahim, M.G. Abd El-Moghny, M.S. El-Deab, Int. J. Hydrog. Energy 101 (2025) 556–567. doi: 10.1016/j.ijhydene.2024.12.395

  40. [40]

      K. Shen, Y. Tang, Q. Zhou, et al., Chem. Eng. J. 471 (2023) 144827. doi: 10.1016/j.cej.2023.144827

  41. [41]

      Q. Zhou, C. Xu, J. Hou, et al., Nano-Micro Lett. 15 (2023) 95. doi: 10.1007/s40820-023-01066-w

  42. [42]

      Y. Jiang, Z. Song, M. Qu, et al., Small 20 (2024) 2400859. doi: 10.1002/smll.202400859

  43. [43]

      L. Kong, L. Pan, H. Guo, et al., J. Colloid Interface Sci. 664 (2024) 178–185. doi: 10.1016/j.jcis.2024.03.002

  44. [44]

      W.X. Zheng, X.X. Cheng, P.P. Chen, et al., Nat. Commun. 16 (2025) 337. doi: 10.70114/acmsr.2025.2.1.p337

  45. [45]

      L. Zhou, Y. Shao, F. Yin, et al., Nat. Commun. 14 (2023) 7644. doi: 10.1038/s41467-023-43466-x

  46. [46]

      R. Yao, K. Sun, K. Zhang, et al., Nat. Commun. 15 (2024) 2218. doi: 10.1038/s41467-024-46553-9

  47. [47]

      Y. Huang, L.W. Jiang, H. Liu, J.J. Wang, Chem. Eng. J. 441 (2022) 136121. doi: 10.1016/j.cej.2022.136121

  48. [48]

      M. Li, H. Li, H. Fan, et al., Nat. Commun. 15 (2024) 6154. doi: 10.1038/s41467-024-50535-2

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  Figure 1  (a) Schematic illustration of the synthesis process of S_ARu_T-NiFeOH_x-5h catalyst. (b) Synthesis mechanism of S_ARu_T-NiFeOH_x-5h. (c) XRD patterns of the prepared NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. Comparison of the SEM images of (d) NiFeOH_x-5h, (e) Ru_T-NiFeOH_x-5h, and (f) S_ARu_T-NiFeOH_x-5h. (g) TEM and (h) HRTEM images of S_ARu_T-NiFeOH_x-5h.

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  Figure 2  The influence of the concentration of Ru³⁺ in the soaking system on the growth process of the Ru_T-NiFeOH_x-5h. SEM images of the Ru_T-NiFeOH_x-5h at different Ru³⁺ addition levels: (a) 0 mol/L, (b) 0.0001 mol/L, (c) 0.001 mol/L, (d) 0.005 mol/L, and (e) 0.01 mol/L. (f) The polarization curves of Ru_T-NiFeOH_x-5h at different Ru³⁺ concentrations. (g) Schematic diagram of the synthesis mechanism of Ru_T-NiFeOH_x-5h catalyst.

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  Figure 3  (a) Polarization curves of NiFeOH_x with different immersion time with iR compensation. (b) Comparison of polarization curves of NiFeOH_x-5h with diverse Ru³⁺ doping methods with iR compensation. (c) Comparison of polarization curves tests of NiFeOH_x-5h with diverse S^2- doping strategies with iR compensation. Comparison of (d) polarization curves and (e) overpotentials of IF, NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, S_A-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h catalysts in 1.0 mol/L KOH with iR. (f) Tafel and (g) Nyquist plots of different samples with iR compensation. (h) Chronopotentiometry curve of S_ARu_T-NiFeOH_x-5h at a current density of 1000 mA/cm² for 500h without iR compensation.

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  Figure 4  (a) The optimized structural model of NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. (b) AEM mechanism of OER on the surface of catalyst. (c) DOS diagram of NiFeOH_x-5h, Ru_T-NiFeOH_x-5h, and S_ARu_T-NiFeOH_x-5h. (d) The calculated Gibbs free energy and the corresponding values.

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