Construction of self-supporting bimetallic organic framework electrocatalysts and investigation of their electrocatalytic oxygen evolution reaction performance

Abstract

The development of non-precious metal electrocatalysts with high activity and stability is pivotal for advancing water electrolysis technology. Although metal–organic frameworks (MOFs) offer advantages such as tunable structures and well-defined sites, conventional powdered electrocatalysts commonly suffer from issues including deactivation and detachment of active sites, as well as poor conductivity. To address this, this work employs an in situ synthesis strategy on a substrate. Using nickel–iron foam as both a substrate and a metal source and a one-step solvothermal method, arrays of nickel–iron bimetallic MOF nanosheets were successfully grown directly onto the framework surface, constructing a binder-free, self-supporting electrode (NiFe-MIL, MIL stands for Materials Institute Lavoisier). In situ growth ensures robust chemical bonding and efficient electron transfer between the MOF active layer and the conductive substrate, fundamentally eliminating active site shielding and detachment issues caused by insulating binders. The bimetallic synergistic effect optimises the material's electronic structure at the atomic level, effectively modulating the adsorption energy of oxygen-containing intermediates and thereby significantly enhancing the intrinsic activity. Benefiting from this unique structural design, the fabricated NiFe-MIL electrocatalyst exhibits an overpotential of merely 265 mV at 10 mA cm⁻² in 1 M KOH electrolyte, with a Tafel slope as low as 41.4 mV dec⁻¹. It also demonstrates impressive electrocatalytic durability. This work not only validates the feasibility of utilising substrates as in situ metal sources for MOF construction but also provides novel insights for designing highly efficient and stable bimetallic electrodes.

---

1. Introduction

Energy is the driving force and cornerstone of human civilisation. However, use of fossil fuels exacerbates the risk of global climate change.^1,2 To achieve “carbon peaking” and “carbon neutrality”, it is imperative to foster the advancement of alternative clean energy sources.^3,4 Hydrogen energy is widely regarded as the best energy carrier due to its pollution-free combustion products, high energy density, and high combustion calorific value.^5,6 As an efficient hydrogen production technology, water electrolysis involves two key half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).^7,8 Nonetheless, the OER poses limitations such as high reaction energy barriers and slow reaction kinetics, which directly affect the energy conversion efficiency of the entire water splitting process.⁹ The precious metals IrO₂ and RuO₂ are currently the benchmark materials in the field of OER catalysts.^10,11 Yet, their scarcity, high cost, and poorest stability constrain their practical applicability in water electrolysis.¹² Therefore, developing more efficient, cost-effective, and stable catalysts has emerged as a pivotal objective within the domain of water electrolysis technology.¹³

Metal–organic frameworks (MOFs) have garnered increasing attention from scholars in the energy sector due to their extraordinary properties, including exceptionally high specific surface areas, tunable pore structures, and the versatile coordination bonds that can be formed between metal ions and organic ligands.^14–16 However, powdered electrocatalytic materials cannot be directly employed as electrode materials, and practical electrode fabrication using them heavily relies on polymeric binders. This inevitably leads to the blockage of catalytic active sites and an increase in interface resistance, and the electrocatalytic coating often peels off from the electrode surface under the long-term erosion of gas bubbles generated during water electrolysis, seriously restricting the performance and long-term stability of the catalyst. To address these issues, researchers have developed self-supporting electrocatalyst systems. This strategy involves the direct growth of active materials on a conductive substrate. By avoiding the resistances and detachment caused by binders, and leveraging the strong interaction formed between the active materials and the substrate, it ensures efficient electron transfer and catalytic stability. Moreover, their porous structure and tunable surface properties significantly promote the exposure of active sites, electrolyte infiltration, and bubble desorption, thereby optimizing mass transport and reaction kinetics. On the other hand, from the perspective of intrinsic material properties, single-metal MOFs often exhibit narrow electronic band structures and a limited number of catalytic active sites due to their simple composition, structure, and morphology. This to some extent restricts their advanced performance and scope of application.¹⁷ To overcome these challenges, researchers developed a bimetallic MOF material system by introducing a second metallic element.^18,19 Compared to single-metal MOFs, bimetallic MOFs can significantly enhance the catalytic activity by optimising electronic structures, improving electrical conductivity, and lowering reaction energy barriers. Furthermore, this interaction contributes to enhancing the overall stability of the framework.^20–22 For example, Cheng et al. synthesized NiMn-based bimetallic organic framework nanosheets supported by porous channel carbon fibers through hydrothermal and ligand exchange strategies. The strong synergy between adjacent Ni and Mn nodes endows the active NiO₆ centers of NiMn-MOFs with rapid OER kinetics.²³ A team from Shandong University proposed to use highly crystalline Cu-BDC as a template and introduce Co²⁺ through ion exchange to successfully construct a Co/Cu-DBC highly crystalline material with Cu–O–Co asymmetric bimetallic sites. At a current density of 10 mA cm⁻², the overpotential was only 251 mV.²⁴ In recent years, among the many reported bimetallic-based electrocatalysts, nickel–iron-based materials have attracted considerable attention due to their strong electronic and atomic interactions, which can significantly improve catalytic performance, and their low cost.²⁵ It is widely believed that the interaction between Ni and Fe atoms is crucial for regulating the electronic structure around the active site and enhancing OER kinetics.²⁶ For example, Tan et al. improved the charge transfer efficiency by doping Fe into a metal–organic framework (Ni) and adding carbon black (CB), thus preparing Ni₉Fe₁-BDC-0.15CB. The MOF itself has an extremely high specific surface area, and the incorporated iron optimises the electronic structure of the Ni sites. Additionally, the high dispersion of metal species in MOFs exposes more active sites.²⁷ Zhang et al. prepared two-dimensional conjugated phenylenediamine frameworks (CPF-Fe/Ni) through ion exchange under microwave conditions. In the catalytic process, the bimetallic sites between adjacent layers synergistically form bridging catalytic centres, while the conjugated framework provides a highly favorable conductive framework for mass transfer, ensuring long-term stability. Under alkaline conditions, the overpotential for the OER at a current density of 10 mA cm⁻² is only 194 mV.²⁸ In view of this, the development of novel, highly efficient, self-supporting NiFe bimetallic MOF catalysts has become a hot topic of research.^29,30

Based on the above considerations, this study adopts a one-step hot solvent method to synthesize self-supporting bimetallic NiFe-MIL nanosheet electrocatalysts using both nickel and iron as the base and providing a metal source in situ. The effects on the structure and performance of the catalysts were studied through characterization methods such as XPS, FT-IR spectroscopy, and Raman spectroscopy. The experimental results show that the highly open and porous nature of the ultrathin nanosheet structure is one of the key factors in enhancing the OER activity. At the same time, doping Ni into the Fe-based catalyst can effectively regulate the local electronic environment of the active sites through the bimetallic cooperative effect, increase the conductivity and improve the OER kinetics, thereby overall enhancing the electrocatalytic performance. Impressively, the prepared catalyst can maintain stable operation for over 400 hours in the long-term stability test, offering broad application prospects.

2. Experimental section

2.1. Materials and chemicals

All chemicals and materials used in the experiment were purchased and can be used directly without further purification. 2-Aminoterephthalic acid (C₈H₇NO₄) was purchased from Aladdin Ltd. N,N-Dimethylformamide (DMF, 98%) and anhydrous ethanol (C₂H₆O) were purchased from Sinopharm Group Chemical reagent Co., Ltd. Ferronickel foam (NFF, Ni : Fe approximately 3 : 7) and foam iron were purchased from Suzhou Keshenghe Metal Materials Co., Ltd, with a thickness of 1.5 mm.

2.2. Sample preparation process

2.2.1. Synthesis of NiFe-MIL. The cleaning process of NFF: the foam nickel iron was cut into small pieces measuring 20 mm × 30 mm × 1.5 mm. First, it was ultrasonically cleaned in 2 M HCl for 5 minutes to remove surface oxides, then ultrasonically cleaned with ethanol and deionised water for 5 minutes each, and the cleaned NFF was dried and kept aside for later use.

The NiFe-MIL nanosheets were prepared according to the method described in the literature with minor modifications.³¹ 2.0 mmol of 2-aminoterephthalic acid (NH₂-BDC) was dissolved in a mixture of 25 mL of N,N-dimethylformamide (DMF), 2.5 mL of anhydrous ethanol and deionised water, and stirred for 20 minutes to form a uniform solution. The cleaned foam nickel–iron substrate together with the above solution was transferred to a 50 mL reactor. The oven was heated to 130 °C and maintained for 6 hours, then allowed to cool to room temperature naturally. After the reaction was complete, the prepared sample was washed several times with DMF, then vacuum dried at 60 °C for 12 hours, yielding the NiFe-MIL electrode material grown in situ on a foam nickel–iron substrate.

2.2.2. Synthesis of Fe-MIL. For comparative studies, Fe-MIL samples were prepared using the same synthesis method, with the only difference being that the foam nickel–iron substrate was replaced with a foam iron substrate.

2.3. Physical characterization

To analyse the structure and elemental composition of the samples, scanning electron microscopy (SEM) images were acquired utilizing a field emission scanning electron microscope (Hitachi, Regulus-8100), and transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analyses were performed on a transmission electron microscope (Hitachi, HT-7800). In addition, an X-ray diffractometer (XRD, PC2500 JEOL) was used to analyse the crystal phase structure of the samples. The functional groups of the sample were characterised using an FT-IR spectrometer (PerkinElmer Frontier). X-ray photoelectron spectroscopy (XPS, Shimadzu, AXIS ULTRA DLD) testing was carried out to evaluate various atomic valences and elemental information.

2.4. Electrochemical measurements

This study employed a three-electrode system and utilised the CHI760E electrochemical workstation to systematically evaluate the OER performance of the material. During the experiment, a 1 M KOH solution was used as the electrolyte. The working electrode (0.5 × 0.5 cm⁻²), reference electrode, and counter electrode were the prepared sample, Hg/HgO, and a carbon rod, respectively. The catalytic efficacy of various materials was evaluated through linear sweep voltammetry (LSV), employing a scan rate of 1 millivolt per second (mV s⁻¹). The electrochemical surface area (ECSA) was calculated by cyclic voltammetry (CV) measurements in the non-Faradaic potential region at different scan rates to determine the electrochemical double layer capacitance (C_dl). Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100 kHz to 0.1 Hz. Stability tests were conducted on the samples using the chronopotentiometric (C–P) method.

3. Results and discussion

3.1. Structural characterization

In the solvothermal process, nickel ions and iron ions released from the foam nickel–iron substrate act as metal centres and coordinate with 2-aminoterephthalic acid organic molecules through strong coordination bonds, ultimately generating NiFe-MIL nanosheets in situ. SEM images show that both the prepared NiFe-MIL and Fe-MIL exhibit distinct nanosheet morphologies, uniformly distributed on the substrate (Fig. 1a–c and Fig. S1), compared with Fe-MIL, NiFe-MIL shows a relatively thin nanosheet morphology. This ultra-thin nanosheet structure has highly open porous characteristics, which not only provide a large specific surface area and an abundance of catalytic active sites, but also promote rapid bubble desorption, thereby effectively improving the efficiency of the water electrolysis reaction.^32,33 The TEM image further confirmed the nanosheet morphology of the NiFe-MIL samples (Fig. 1d). Additionally, HRTEM delineated the crystal structure of NiFe-MIL, elucidating its microscopic architecture. As shown in Fig. 1e, the lattice stripe spacings are 0.257 and 0.284 nm, corresponding to its (312) and (303) crystal planes, respectively. However, no obvious lattice fringes were captured in the high-resolution transmission electron microscopy images of Fe-MIL (Fig. S2), which may be related to the extremely high sensitivity of the MOF structure to electron beams.^34,35 This phenomenon also indirectly indicates that the incorporation of nickel could enhance the structural stability of Fe-MIL. The crystal indices of NiFe-MIL can also be identified through SAED patterns (Fig. 1f). The corresponding element mapping diagram shows that Ni, Fe, C, N, and O elements are uniformly distributed throughout the nanoplate array, further proving the successful preparation of the sample (Fig. 1g).

Fig. 1 (a)–(c) SEM images of NiFe-MIL, (d) TEM image, (e) HRTEM image, (f) corresponding SAED pattern and (g) mapping images of NiFe-MIL.

The phase composition of the prepared catalyst was elucidated through XRD analysis. The characteristic peaks observed at approximately 9.1°, 16.7°, 18.2°, 20.7°, and 24.9° correspond to the (111), (103), (200), (201), and (211) planes of NiFe-MIL (CCDC No. 647646), indicating that NiFe-MIL was successfully grown on the foam nickel–iron substrate (Fig. 2a).³⁶ Raman and FT-IR spectroscopy analyses were carried out to elucidate the electronic structure and surface organic functional groups of NiFe-MIL and Fe-MIL. The peak shapes of the infrared characteristic peaks of the two samples are basically consistent and similar (Fig. 2b), indicating that they have similar crystal structures. The peaks at 1579 and 1389 cm⁻¹ can be assigned to the organic linkers ν_asym (–COO–) and ν_sym (–COO–); this further clarifies that Ni/Fe metal ions coordinate with carboxylate groups in H₂-BDC ligands to form metal–organic framework structures during the self-assembly synthesis of materials.³⁷ The peak of absorption at 3328 cm⁻¹ is associated with the asymmetric vibration of –NH₂.³⁸ The vibration band at 1244 cm⁻¹ is attributed to the C–N stretching mode of the benzene ring. The organic linker shows a C–H bending vibration band at 773 cm⁻¹.³⁹ The M–O vibration peak appearing at 574 cm⁻¹ indicates that Ni/Fe ions have successfully formed metal–oxygen bonds through direct coordination with carboxyl groups in the ligands.⁴⁰ In addition, the peak of M–O in the Raman spectrum is in the range of 575–698 cm⁻¹. Two vibration peaks at 1392 and 1582 cm⁻¹ are attributed to the inward and reverse stretching regions of the carboxylic acid group (Fig. S3).⁴¹ XPS clearly shows the presence of Ni, Fe, C, N, and O elements, which is consistent with the elemental distribution mapping results (Fig. 2c). As shown in Fig. S4, the XPS spectrum of C 1s exhibits three peaks at 284.78 eV, 286.05 eV, and 288.81 eV, which are attributed to C=C, C–O, and C=O, respectively.⁴² The high-resolution O 1s spectrum depicted in Fig. 2d shows three peaks at binding energies 531.52 eV, 532.19 eV, and 533.36 eV. These peaks belong to the M–O bond, the oxygen component in the carboxylate ion, and surface-adsorbed oxygen, respectively. Compared with the O 1s spectrum of Fe-MIL, in the O 1s spectrum of NiFe-MIL the binding energy peak shows a positive shift of 0.15 eV.⁴³ The Ni 2p spectrum of NiFe-MIL shows the characteristic peak of Ni²⁺ (Fig. 2e), and the peaks at 852.58 eV and 869.75 eV are attributed to the metal Ni in the substrate, while the two peaks at 856.02 eV and 873.94 eV correspond to Ni 2p_3/2 and Ni 2p_1/2, respectively. The two peaks at 861.57 eV and 879.97 eV are the satellite peaks of Ni.⁴⁴ The Fe 2p_3/2 peak of NiFe-MIL appears at a binding energy of 712.65 eV, while that of Fe-MIL appears at 713.79 eV (Fig. 2f). The negative shift observed in the Fe 2p spectrum indicates that the introduction of Ni can effectively regulate the electronic structure of Fe sites, prompting the redistribution of electron cloud density between Ni and Fe, increasing the nuclear electron cloud density of Fe, thereby promoting charge transfer and driving electrons to flow from Ni to Fe through the bridging oxygen (M–O–M) bonds. This generates a stronger bimetallic synergistic effect, which is the microscopic origin of the enhanced catalytic performance.^45,46

Fig. 2 (a) XRD patterns of NiFe-MIL and Fe-MIL, (b) and their corresponding FT-IR spectra, (c) full-scan XPS spectra of NiFe-MIL, and high-resolution XPS spectra of (d) O 1s, (e) Ni 2p of NiFe-MIL and Fe-MIL, and (f) Fe 2p of NiFe-MIL.

3.2. Electrochemical performance for the OER

The OER performance of NiFe-MIL was investigated under alkaline conditions using a typical three-electrode system. Fig. 3a shows the LSV curves of various catalysts. At a current density of 10 mA cm⁻², the overpotential of NiFe-MIL is only 265 mV, which is much lower than those of Fe-MIL (314 mV), commercial RuO₂ (286 mV) and NFF (333 mV). The OER kinetic process of the catalyst was evaluated using the Tafel curve. The smaller the Tafel slope, the more rapid the kinetic reaction process.⁴⁷ As expected, NiFe-MIL had the lowest Tafel slope of 41.4 mV dec⁻¹, indicating a faster charge transfer rate (Fig. 3b). The charge transfer resistance of the catalysts was evaluated using EIS (Fig. 3c).⁴⁸ NiFe-MIL exhibits the lowest charge transfer resistance, thanks to the bimetallic synergistic effect, which effectively reduces charge transfer resistance. This property enhances the intrinsic conductivity of electrocatalysts by promoting electron transfer efficiency at the electrode–electrolyte interface, thereby improving the catalytic performance. To further explain why NiFe-MIL can enhance the OER catalytic performance, CV tests were conducted on NiFe-MIL and Fe-MIL at different scan rates (Fig. S5). The ECSA is evaluated by characterising the C_dl of the catalyst.⁴⁹ The C_dl value of NiFe-MIL is 1.2 mF cm⁻², higher than that of Fe-MIL (1.0 mF cm⁻²) (Fig. 3d), indicating that the excellent catalytic performance of NiFe-MIL is due to its larger active surface area. As shown in Fig. 3e, the current densities normalised by the geometric area (j_geometry) and ECSA (j_specific) were compared, and the results indicate that NiFe-MIL not only has high epigenetic activity but also has high intrinsic activity. Stability is an important indicator for the commercialisation of catalysts. In chronopotentiostatic testing, NiFe-MIL can sustainably operate for 400 hours at a current density of 10 mA cm⁻² (Fig. 3f). There is no significant change in the LSV curve before and after 5000 cycles of the CV test, as shown in Fig. S6. Additionally, NiFe-MIL also exhibits long-term stability of 300 hours even at a high current density of 100 mA cm⁻² (Fig. 3g). In addition, the NiFe-MIL also demonstrates significant competitive advantage over previously reported NiFe metal electrocatalysts (Table S1). Given its outstanding performance in alkaline media for the OER, NiFe-MIL was employed directly as the anode catalyst to evaluate practical applications. A dual-electrode system was constructed by pairing it with commercial Pt/C as the cathode catalyst. As depicted in Fig. 3h, NiFe-MIL required only 1.73 V to drive a current density of 10 mA cm⁻², approaching the performance of the RuO₂‖Pt/C system. Stability testing revealed minimal voltage fluctuation after 100 hours of continuous operation at 10 mA cm⁻², as depicted in Fig. 3i, demonstrating outstanding water-splitting stability. Collectively, these results confirm NiFe-MIL's exceptional performance in both activity and stability, establishing it as a promising candidate material for efficient, low-cost electrochemical energy conversion technologies.

Fig. 3 OER performance in alkaline medium (1 M KOH). (a) OER LSV, (b) Tafel plots, (c) EIS, (d) C_dl values, (e) comparison of current density normalized by the geometric area (j_geometry) and ECSA (j_specific), (f) and (g) stability test of the NiFe-MIL electrode at 10 mA cm⁻² and 100 mA cm⁻² current density, (h) LSV curves of NiFe-MIL‖Pt/C and RuO₂‖Pt/C toward overall water splitting in a two-electrode configuration and (i) the chronopotentiometry measurement of NiFe-MIL‖Pt/C at 10 mA cm⁻².

3.3. Exploration of the catalytic mechanism

In order to further deepen our understanding of the catalytic mechanism, using the Arrhenius equation ln j = − (E_a/RT) + A, activation energy (E_a) was determined based on polarisation curves that vary with temperature. Fig. 4a and b depict the LSV curves of NiFe-MIL and Fe-MIL at different temperatures. As can be seen from Fig. 4c, the synthesized NiFe-MIL exhibits a lower E_a value (9.72 kJ mol⁻¹) compared to Fe-MIL (14.9 kJ mol⁻¹). The activation energy indicates the energy barrier in the OER reaction process.⁵⁰ The lower energy observed in NiFe-MIL indicates that it has more favourable thermodynamic properties. As a key technique for studying electrocatalytic reaction kinetics, EIS provides critical information on catalyst kinetics and elucidates the dynamic evolution pathway of adsorbed oxygen intermediates on the electrocatalyst surface throughout the OER process. The specific reaction pathway is shown in Fig. S7. R_ct is related to the charge transfer at the electrode interface. Therefore, the kinetics of the catalytic reaction were assessed based on the correlation between R_ct and the applied potential.⁵¹In situ EIS tests were conducted on the prepared samples within the potential range of 1.538–1.598 V (Fig. 4d and e), and the corresponding Nyquist plots were plotted (Fig. 4f). The R_ct value of NiFe-MIL is lower than that of Fe-MIL, indicating that NiFe-MIL has more efficient electron transfer and faster OER kinetics. In the OER process, it reduces the adsorption energy barrier of OH*, and its low R_ct and efficient OH* adsorption capacity make it a highly promising OER catalyst.⁵² The above study shows that the bimetallic synergistic effect of NiFe-MIL catalysts can promote both the thermodynamic and kinetic processes of chemical reactions.

Fig. 4 (a) and (b) LSV curves at different temperatures, (c) Arrhenius diagram at 1.5 V (vs. RHE), (d) and (e) Nyquist plots at different potentials, and (f) total charge transfer resistance at different potentials of NiFe-MIL and Fe-MIL.

3.4. Chemical and physical characterization after the OER

To gain further insight into the morphological structure and compositional stability of NiFe-MIL following OER testing, analyses including SEM, HRTEM, and XPS were conducted. In Fig. S8 and Fig. 5a, the SEM and HRTEM images reveal that NiFe-MIL exhibits slight surface collapse following OER testing, yet retains its original plate-like structure. Concurrently, NiFe-MIL catalysts before and after electrolysis display similar lattice fringes (Fig. 5b). Additionally, energy dispersive spectroscopy (EDS) patterns demonstrate the uniform distribution of Ni, Fe, C, N, and O elements (Fig. 5c). To probe the electronic structural changes and compositional stability, the high-resolution XPS spectra (O 1s, Ni 2p, Fe 2p) of the NiFe-MIL electrocatalyst were analyzed before and after the OER durability test. Post-test, a negative binding energy shift was observed in the O 1s spectrum (Fig. 5d). The Ni⁰ peak area in the Ni 2P spectrum exhibits a marked reduction (Fig. 5e), potentially attributable to the oxidation of metallic Ni within the nickel–iron foam substrate in solution. The overall negative binding energy shift across the spectrum indicates an increase in electron density surrounding Ni. This phenomenon may contribute to enhancing the electronic conductivity of NiFe-MIL, thereby optimising its electrocatalytic performance. The Fe 2p spectrum, however, remained virtually identical (Fig. 5f). The overall preservation of the elemental valence states in the catalyst provides direct evidence for the exceptional compositional stability of the nickel and iron species under prolonged OER conditions.

Fig. 5 Characterization of NiFe-MIL after OER tests: (a) and (b) HRTEM images, (c) EDS elemental mapping images, and high-resolution XPS spectra of (d) O 1s, (e) Ni 2p, and (f) Fe 2p.

4. Conclusion

In summary, this study used a solvothermal synthesis method to successfully obtain a self-supporting electrocatalyst made of NiFe-MIL bimetal in the form of nanosheets; the morphology of nanosheets exposes more active sites, the bimetallic synergistic effect optimises the electronic structure and the self-supporting structure improves the charge transport efficiency. The prepared catalyst at a current density of 10 mA cm⁻² requires only 265 mV overpotential for the OER, with a small Tafel slope (41.4 mV dec⁻¹). It is worth noting that NiFe-MIL has exceptional durability, lasting up to 400 hours. Experiments have demonstrated that Ni/Fe ions successfully form metal–oxygen bonds through direct coordination with carboxyl groups in ligands, facilitating electron transfer between Ni and Fe. The introduction of Ni modulates the electronic structure of Fe-MIL, promoting charge transfer and consequently augmenting the catalytic activity. Fe³⁺ itself acts as an intrinsic active centre, forming a synergistic effect with Ni sites, further enhancing the catalytic performance. In addition, the ultra-thin nanosheet structure of NiFe-MIL provides a high specific surface area, promoting mass transfer and adsorption of reactants (OH⁻). Although NiFe-MIL exhibits outstanding electrocatalytic performance, the catalyst currently faces challenges in achieving large-scale production to meet industrial demands. Future research must further optimise the synthesis methodology to bridge the gap between laboratory and industrial applications.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Data available on request from the authors.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5nj03372a.

Acknowledgements

This research is supported by the Natural Science Foundation Project of Jilin Province (YDZJ202401471ZYTS).

References

1. Giddaerappa, N. Kousar, S. Kumar, M. Hojamberdiev and L. K. Sannegowda, Electrochim. Acta, 2024, 475, 143575.
2. Z. Huang, L. Chen, H. Zhang, M. Humayun, J. Duan, Q. Zhu, M. Bououdina, Y. Cao, Y. A. Attia, G. Kardas and C. Wang, Chem. Eng. J., 2024, 501, 157628.
3. L. Wang, Z. Wang, L. Chu, Z. Huang, M. Yang and G. Wang, Int. J. Hydrogen Energy, 2024, 64, 830–841.
4. B. Sun, S. Wang, X. Li, W. Zhang, J. Li, Q. Pan, F. Liu and Z. Su, Int. J. Hydrogen Energy, 2023, 48, 9353–9361.
5. A. M. Sadeq, R. Z. Homod, A. K. Hussein, H. Togun, A. Mahmoodi, H. F. Isleem, A. R. Patil and A. H. Moghaddam, Sci. Total Environ., 2024, 939, 173622.
6. M.-M. Shi, D. Bao, J.-M. Yan, H.-X. Zhong and X.-B. Zhang, Acc. Mater. Res., 2024, 5, 160–172.
7. G. Patil, S. Daniel and L. Koodlur Sannegowda, Chemistry, 2024, 30, e202401759.
8. Y. Wang, L. Jing, W. Jiang, Y. Wu, B. Liu, Y. Sun, X. Chu and C. Liu, J. Colloid Interface Sci., 2024, 671, 283–293.
9. L. Xiao, X. Bai, J. Han, T. Tang, S. Chen, H. Qi, C. Hou, F. Bai, Z. Wang and J. Guan, Nano Res., 2024, 17, 2429–2437.
10. T. Mou, D. A. Bushiri, D. V. Esposito, J. G. Chen and P. Liu, Angew. Chem., Int. Ed., 2024, 63, e202409526.
11. H. Shooshtari Gugtapeh and M. Rezaei, ACS Appl. Mater. Interfaces, 2023, 15, 34682–34697.
12. Z. Wang, Y. Liang, T. Fang, X. Song, L. Yang, L. Wen, J. Wang, D. Zhao and S. Wang, Nanomaterials, 2025, 15, 294.
13. Y. Zhao, Y. Wang, Y. Dong, C. Carlos, J. Li, Z. Zhang, T. Li, Y. Shao, S. Yan, L. Gu, J. Wang and X. Wang, ACS Energy Lett., 2021, 6, 3367–3375.
14. H. Ji, T. Huang, G. Zhang, S. Cao, Y. Sun and H. Pang, Chin. J. Chem., 2025, 43, 1417–1441.
15. L. Jing, Y. N. Wang, W. Jiang, Y. Wu, B. Liu, C. Liu, X. Chu and G. Che, Int. J. Hydrogen Energy, 2024, 69, 195–202.
16. C. Wang, Y. Chen, M. Zhong, T. Feng, Y. Liu, S. Feng, N. Zhang, L. Shen, K. Zhang and B. Yang, J. Mater. Chem. A, 2021, 9, 22095–22101.
17. M. Han, X. Zhang, H. Gao, S. Chen, P. Cheng, P. Wang, Z. Zhao, R. Dang and G. Wang, Chem. Eng. J., 2021, 426, 131348.
18. S. He, Z. Li and J. Wang, J. Solid State Chem., 2022, 307, 122726.
19. C. Li, W. Zhang, Y. Cao, J.-Y. Ji, Z.-C. Li, X. Han, H. Gu, P. Braunstein and J.-P. Lang, Adv. Sci., 2024, 11, e2401780.
20. S. Liu, Z. Zhang, M. Liu, Z. Wang, Q. Zhang and L. Li, Surf. Interfaces, 2024, 44, 103734.
21. Y. Yang, J.-L. Zhang, W.-B. Liang, J.-L. Zhang, X.-L. Xu, Y.-J. Zhang, R. Yuan and D.-R. Xiao, Sens. Actuators, B, 2022, 362, 131802.
22. K. Liu, Y. Chen, X. Dong, Y. Hu and H. Huang, Electrochim. Acta, 2023, 456, 142441.
23. M. Shang, B. Zhou, H. Qiu, Y. Gong, L. Xin, W. Xiao, G. Xu, C. Dai, H. Zhang, Z. Wu and L. Wang, J. Colloid Interface Sci., 2024, 669, 856–863.
24. W. Cheng, X. F. Lu, D. Luan and X. W. D. Lou, Angew. Chem., Int. Ed., 2020, 59, 18234–18239.
25. C. Liang, H. Ai, L. Lin, X. Lu, L. Li, H. Zhang, P. Wang, Z. Zheng, Z. Wang, H. Cheng, Y. Dai, D. Xing, B. Huang and Y. Liu, Small, 2025, 21, e2500744.
26. J. Zhou, Z. Han, X. Wang, H. Gai, Z. Chen, T. Guo, X. Hou, L. Xu, X. Hu, M. Huang, S. V. Levchenko and H. Jiang, Adv. Funct. Mater., 2021, 31, 2102066.
27. X. Wang, H. Niu, X. Wan, Z. Zhang, F. R. Wang and Y. Guo, J. Colloid Interface Sci., 2022, 624, 160–167.
28. Y. Zang, D.-Q. Lu, K. Wang, B. Li, P. Peng, Y.-Q. Lan and S.-Q. Zang, Nat. Commun., 2023, 14, 1792.
29. C. H. Chiang, H. T. Chen, W. Y. Chen, W. T. Wang, S. P. Feng and C. G. Wu, Adv. Energy Mater., 2024, 14, 2400346.
30. J. Li, J. Zhang, J. Shen, H. Wu, H. Chen, C. Yuan, N. Wu, G. Liu, D. Guo and X. Liu, Mater. Chem. Front., 2023, 7, 567–606.
31. Y. Bao, H. Ru, Y. Wang, K. Zhang, R. Yu, Q. Wu, A. Yu, D.-S. Li, C. Sun, W. Li and J. Tu, Adv. Funct. Mater., 2024, 34, 2314611.
32. W. Peng, W. Zhang, Y. Lu, W. Li, J. He, D. Zhou, W. Hu and X. Zhong, J. Colloid Interface Sci., 2024, 664, 980–991.
33. W. Hou, K. Peng, S. Li, F. Huang, B. Wang, X. Yu, H. Yang and H. Zhang, J. Colloid Interface Sci., 2023, 646, 265–274.
34. H. Gao, Z. Yang, J. Yu, A. Kong, Y. Sun, S. Yang, B. Peng, G. Wang, F. Yu and Y. Li, Int. J. Hydrogen Energy, 2024, 81, 718–726.
35. J. Wu, Z. Yu, Y. Zhang, S. Niu, J. Zhao, S. Li and P. Xu, Small, 2021, 17, e2105150.
36. X. Feng, R. Long, C. Liu and X. Liu, Chem. Eng. J., 2023, 454, 139765.
37. C.-P. Wang, Y. Feng, H. Sun, Y. Wang, J. Yin, Z. Yao, X.-H. Bu and J. Zhu, ACS Catal., 2021, 11, 7132–7143.
38. L. Shao, Z. Yu, X. Li, X. Li, H. Zeng and X. Feng, Appl. Surf. Sci., 2020, 505, 144616.
39. Z. Ye, R. Oriol, C. Yang, I. Sirés and X.-Y. Li, Chem. Eng. J., 2022, 433, 133547.
40. R. Yuan, C. Yue, J. Qiu, F. Liu and A. Li, Appl. Catal., B, 2019, 251, 229–239.
41. X. Xu, H.-C. Chen, L. Li, M. Humayun, X. Zhang, H. Sun, J. Jia, C. Xu, M. Bououdina, L. Sun, X. Wang and C. Wang, Adv. Funct. Mater., 2024, 34, 2408823.
42. N. Liu, W. Huang, M. Tang, C. Yin, B. Gao, Z. Li, L. Tang, J. Lei, L. Cui and X. Zhang, Chem. Eng. J., 2019, 359, 254–264.
43. S. Li, H. Chai, L. Zhang, Y. Xu, Y. Jiao and J. Chen, J. Colloid Interface Sci., 2023, 642, 235–245.
44. L. Zhong, N. Wang, L. Sun, X. Xie, L. He, M. Xiang and W. Hu, J. Colloid Interface Sci., 2025, 683, 489–498.
45. M. Ying, R. Tang, W. Yang, W. Liang, G. Yang, H. Pan, X. Liao and J. Huang, ACS Appl. Nano Mater., 2021, 4, 1967–1975.
46. H. Zhang, H. Zhou, H. Wang, Y. Wang, X. Yang, D. Wu, P. Yuan, M. He, W. Wei and T. Yang, Adv. Energy Mater., 2025, 15, 2403464.
47. K. Yatsuzuka, K. Adachi, A. Li, S. Kong, S. Hamamoto, D. Hashizume, R. Nakamura and H. Ooka, J. Phys. Chem. C, 2023, 127, 22457–22463.
48. Ruqia, M. A. Asghar, S. Ibadat, S. Abbas, T. Nisar, V. Wagner, M. Zubair, I. Ullah, S. Ali and A. Haider, Molecules, 2022, 27, 6396.
49. Kiran Varsha, S. R. Ananda, L. K. Sannegowda and S. Aralekallu, Sustainable Energy Fuels, 2025, 9, 2287–2293.
50. H. He, P. Kou, Z. Zhang, D. Wang, R. Zheng, H. Sun, Y. Liu and Z. Wang, J. Colloid Interface Sci., 2024, 653, 179–188.
51. Z.-X. Huang, K. Li, J.-M. Cao, K.-Y. Zhang, H.-H. Liu, J.-Z. Guo, Y. Liu, T. Wang, D. Dai, X.-Y. Zhang, H. Geng and X.-L. Wu, Nano Lett., 2024, 24, 13615–13623.
52. P. Sabhapathy, P. Raghunath, A. Sabbah, I. Shown, K. S. Bayikadi, R.-K. Xie, V. Krishnamoorthy, M.-C. Lin, K.-H. Chen and L.-C. Chen, Small, 2023, 19, e2303598.

---