Oxygen defect engineering on low-crystalline iron(III) oxyhydroxide as a highly efficient electrocatalyst for water oxidation

Abstract

Improving the water oxidation performance of non-precious nanoelectrocatalysts is the key to developing green hydrogen energy. Herein, we developed a simple method to synthesize FeOOH nanocatalysts with low crystallinity and oxygen vacancies (V_O). These catalysts demonstrate excellent electrocatalytic performance for water oxidation. The V_O-FeOOH catalyst exhibits an overpotential of 255 mV at 10 mA cm⁻² and maintains stability for more than 120 hours at a high current output (50 mA cm⁻²). DFT calculations show that the rate-determining step (RDS) of V_O-FeOOH and FeOOH is O* to OOH* (the Gibbs free energy (ΔG) of the RDS is 1.65 eV and 1.91 eV, respectively). This result indicates that V_O can effectively reduce the energy barrier from *O to *OOH of the OER process, thus improving the activity of the V_O-FeOOH nanocatalysts. Our focus was on utilizing one of the abundant metallic elements to fabricate defect-rich OER electrocatalysts with improved performance through a convenient one-step synthesis approach. This methodology shows great promise for the development of high-performance catalysts.

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1. Introduction

With the continuous economic development and increase in population, the continuously depleting fossil fuels and related environmental issues have attracted increasing attention from the public. As a result, there is an escalating demand among humanity for clean, renewable energy and environmental friendly energy conversion technologies.^1,2 In light of this, people began to develop energy conversion technologies, such as electrolyzers, zinc-air batteries, and fuel cells.³ The advancement of renewable hydrogen energy plays a crucial role in addressing future energy crises and achieving objectives, such as carbon neutrality.⁴

The electrolysis of water, a hydrogen production process characterized by mild reaction conditions and minimum equipment requirements, has been continuously evolving.⁵ Water electrolysis can be mainly divided into the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), where the OER plays a crucial role throughout the reaction, but its inherent slow kinetic process hinders its development.⁶ The R&D of high-performance catalysts can effectively reduce the overpotential required for the OER process, and noble metal-based catalysts are currently the most efficient OER electrocatalysts.⁷ However, the scarcity of raw materials severely limits the large-scale application and commercial production.^8,9 Therefore, the preparation of low-cost, efficient, and stable non-precious metal-based nanocatalysts is a key strategy to improve the OER and promote water electrolysis.^10,11

In recent years, transition metal-based phosphides,¹² borides,¹³ nitrides,¹⁴ hydroxides,¹⁵ and sulfides¹⁶ have attracted much interest as OER catalysts. Hydroxides exhibit superior electrocatalytic activity compared to noble metals IrO₂ and RuO₂.¹⁷ However, due to the low conductivity and the cumbersome preparation process, the utilization of FeOOH catalyst is in the initial stage.¹⁸ Three main factors influence the intrinsic property of iron(III) oxyhydroxide. First, FeOOH can be classified into four crystal structures: α (orthorhombic), β (tetragonal), δ (hexagonal), and γ (orthorhombic),¹⁹ of which β-FeOOH exhibits the highest OER activity.^20,21 Furthermore, recent studies have reported that the performance of mixed-phase FeOOH is superior to that of single-phase crystals, mainly due to its higher V_O.²² The second factor affecting its performance is crystallinity. Amorphous FeOOH provides abundant random orientations and unsaturated bonds for water molecules and intermediate reactions.²³ The results indicate that the performance of mixed amorphous-crystalline structures surpasses that of single-phase amorphous or highly crystalline species.²⁴ The last factor is the formation of cation or anion vacancies. Their formation leads to electron modulation and the formation of unsaturated bonds,²⁵ thus improving the conductivity of species and OER electrocatalytic performance. At present, the main methods of generating oxygen vacancies are thermal treatment, anion/cation doping, plasma treatment, and other technical treatments (laser, flame, stripping, and template strategy).²⁶ The reduction of NaBH₄ can effectively introduce V_O on the surface of the catalyst material. So far, NaBH₄ reduction has been widely used to produce V_O in Co₃O₄, CoFe₂O₄, NiMn₂O₄, Fe–Co oxides, etc.^27–29

Herein, we proposed a one-step reaction method to prepare FeOOH nanosheets with low crystallinity and abundant defects. This nanocatalyst was attached to a nickel foam (NF) surface using an organic binder, and its electrochemical results demonstrated excellent catalytic properties (activity and stability). As an OER catalyst, V_O-FeOOH exhibits good catalytic activity, characterized by low overpotential, reaction dynamics, and stability. At a current output of 10 milliamperes per cm², the required overpotential was 255 mV. It exhibited stability exceeding 120 h at a current output of 50 milliamperes per cm², surpassing most similar catalysts in terms of efficiency and stability. The low crystalline structure of the FeOOH nanosheet and the presence of V_O resulted in a high specific surface area with open spaces, abundant transfer channels,¹⁸ and exposure to more active sites, as well as the formation of unsaturated coordination bonds, which accelerated charge transfer. Through detailed theoretical calculation and analysis, it is shown that the energy barrier of key rate-limiting steps is reduced by the introduction of oxygen defects. This is of certain guiding significance for the development of OER nanocatalysts.

2. Experimental section

Preparation of the V_O-FeOOH sample: the V_O-FeOOH sample materials were prepared by the oxidation conversion method at room temperature.³⁰ First, 4.5 mmol Fe(NO₃)₃·9H₂O and 15 mmol NaBH₄ were dissolved in 75 ml deionized water, respectively. After fully dissolved, the NaBH₄ solution was slowly added to Fe(NO₃)₃·9H₂O solution under stirring, and a dark green liquid was obtained by stirring for 1.5 h under a N₂ atmosphere. The liquid was centrifuged and washed with deionized water and anhydrous ethanol until the solid became an orange-red solid. Finally, the solid obtained by centrifugation was dried overnight at 60 °C, obtaining the final red-brown solid powder (Fig. S2†), which was recorded as the V_O-FeOOH sample.

3. Results and discussion

3.1 Materials synthesis and characterization

The production of low-cost catalysts for electrocatalytic hydrogen production is of great significance. The iron element is abundant in the earth's crust, and the iron-based catalyst has high catalytic activity, which is very suitable for large-scale production of electrolytic water catalysts. Low-crystalline V_O-FeOOH reddish-brown powders containing V_O were successfully synthesized using a straightforward oxidation conversion method at ambient temperature.³⁰ Detailed synthesis and characterization are presented in the ESI (Fig. S1–S3†). The phase structure of this catalyst was analyzed by X-ray diffraction (XRD), as depicted in Fig. 1a. The diffraction peaks at 35.30°, 46.64°, and 64.72° correspond to the crystal planes (211), (411) and (541) of the V_O-FeOOH sample (PDF#75-1594), respectively. The remaining portion exhibited an amorphous nature, indicating the successful synthesis of low-crystallinity FeOOH in this study. Simultaneously, the surface of the V_O-FeOOH material was subjected to analysis using X-ray photoelectron spectroscopy (XPS). The full spectrum of XPS, as shown in Fig. S4,† reveals the coexistence of Fe and O in the compound V_O-FeOOH.³¹

Fig. 1 (a) XRD patterns, (b) Fe 2p and (c) O 1s high-resolution XPS spectra, (d) N₂ adsorption–desorption isotherms, (e) SEM image, (f) TEM image, (g) HRTEM image, (h) fast Fourier transform (FFT) of different regions and (i) corresponding intensity profiles along with the position, (j) SAED pattern of the V_O-FeOOH sample, and (k) HAADF-STEM image and elemental mappings of Fe and O.

Furthermore, the high-resolution Fe 2p spectrum of V_O-FeOOH, as depicted in Fig. 1b, exhibits two prominent peaks centered at 711.1 eV and 724.4 eV, which correspond to Fe 2p_1/2 and Fe 2p_3/2.³² It shows that Fe (II) is the main component of the V_O-FeOOH sample, and it also shows that unsaturated coordination bonds are formed in the sample, respectively. Additionally, two satellite peaks centered at 718.1 eV and 732.8 eV are attributed to these respective Fe 2p peaks.³³ According to the data presented in Fig. 1c, the spectrum of the nuclear energy level of the O 1s exhibits three distinct peaks. These peaks, located at 529.8 eV, 531 eV, and 531.8 eV, can be attributed to O²⁻ species, OH⁻ groups, and adsorbed oxygen species, respectively.³⁴

In addition, scanning electron microscopy (SEM) analysis revealed that the V_O-FeOOH sample exhibited a nanosheet morphology (Fig. 1e). This nanosheet structure provided an increased number of exposed active sites on the surface, thereby facilitating adsorption and dissociation reactions within the reaction medium. Furthermore, the corresponding Brunauer Emmett Teller (BET) analysis also indicated that the V_O-FeOOH sample had a significant specific surface area and a mesoporous structure concentrated in the range of 2–50 nm (Fig. 1d and S5†). According to the image presented in Fig. 1f, the nanosheet structure was further characterized using transmission electron microscopy (TEM), revealing distinct morphological features with a thin lamellar structure. To determine the thickness of V_O-FeOOH nanosheets, atomic force microscopy (AFM) measurements were performed, yielding a measured thickness of approximately 150 nm, which is consistent with the results observed by the SEM image (Fig. S6†). As the OER catalytic layer, the thickness of the nanosheets not only facilitates the exposure of more active sites but also resists the impact of bubble release, thus stabilizing the catalytic process.³⁵ Additionally, HRTEM investigations indicated that the lattice structure of V_O-FeOOH exhibited a low-crystalline configuration. In particular, lattice fringes corresponding to the crystal plane of V_O-FeOOH (211) were discernible in selected regions (Fig. 1g), providing evidence of the inherent low crystallinity of the V_O-FeOOH sample. To directly study the phase structure of V_O-FeOOH, the fast Fourier transform (FFT) of HRTEM images was used for analysis. Fig. 2g (I), (II), and (III) show three different FFT modes extracted from the red, orange, and green squares of Fig. 2g, which exhibit two wide and blurred halo diffraction rings (Fig. 2h) without any identifiable diffraction points, further revealing the characteristics of long-range disorder but short-term order.³⁶ The lattice fringe width of V_O-FeOOH was measured to be 0.26 nm from Fig. 1i. Compared with the (211) crystal plane of standard FeOOH, the lattice of the (211) crystal plane of low crystalline V_O-FeOOH was increased by about 2.36%. The formation of low crystalline species likely causes the lattice of FeOOH to produce tensile strain.³⁷

Fig. 2 (a) The linear sweep voltammetry (LSV) curve, (b) overpotential of fixed currents, (c) Tafel slope, (d) electrochemical impedance spectroscopy, (e) the C_dl obtained at specific potential, and (f) ECSA normalized linear sweep voltammetry curve for V_O-FeOOH, IrO₂, and NF in 1.0 M potassium hydroxide electrolyte. (g) Operando Nyquist and (h) Bode phase plots of the V_O-FeOOH sample at various overpotentials in a 1.0 M potassium hydroxide electrolyte. (i) Comparison of overpotentials of different catalysts.

The tensile strain of the V_O-FeOOH lattice was further studied by lattice images obtained from HRTEM images by inverse fast Fourier transform (IFFT) mode (Fig. S7†). The corresponding IFFT image shows that the lattice fringes on the (211) crystal plane have obvious tensile strain. The selected area electron diffraction (SAED) mode of V_O-FeOOH shows that the diffraction ring is widened, which belongs to the typical low crystalline state (Fig. 1j).³⁸ The energy dispersive X-ray (EDS) element mapping image of V_O-FeOOH demonstrated an even distribution of Fe and O within the V_O-FeOOH sample (Fig. 1k and S8†). Therefore, the coordination of unsaturated bonds and micro-pores and nanosheet structures of low-crystalline species have significant electrolyte permeability advantages, which are conducive to the rapid migration of electrons and electrolyte ions, thereby improving the catalytic performance.

3.2 Electrocatalytic OER performance

A comprehensive performance analysis of the prepared catalyst was carried out. To determine the ideal quantity of the V_O-FeOOH coating per unit area of NF, the following volumes were applied: 25, 50, 75, 100, and 125 μL, respectively. Fig. S9† illustrates that the OER catalytic activity was optimized using a coating amount of 100 μL. The electrocatalytic activity was tested by LSV in 1.0 M potassium hydroxide after iR compensation. As illustrated in Fig. 2a, V_O-FeOOH exhibited catalytic activity with an overpotential of 255, 295, and 335 mV at 10, 50, and 100 mA cm⁻², respectively, better than IrO₂ (295, 334, and 391 mV) and NF (335, 391, and 432 mV) (Fig. 2b). Furthermore, the overpotential of V_O-FeOOH prepared in this work at 10 mA cm⁻² surpasses that of some FeOOH catalysts and other types of nanocatalysts, as depicted in Fig. 2i and Table S1.†

The kinetics of the V_O-FeOOH samples were further evaluated by electrochemical impedance spectroscopy (EIS) and Tafel slopes. The results revealed that V_O-FeOOH (63.96 mV dec⁻¹) has a lower Tafel slope than the commercial catalysts IrO₂ (81.94 mV dec⁻¹) and NF (104.55 mV dec⁻¹), elucidating that the catalyst has faster OER dynamics (Fig. 2c). The measured EIS data were fitted using an equivalent circuit diagram (Fig. S10†), and the results revealed that V_O-FeOOH has a smaller charge transfer resistance and faster electrochemical OER catalytic characteristics (Fig. 2d). To further reveal the activity of V_O-FeOOH, IrO₂, and NF, the electrochemical active surface area (ECSA) (Fig. S11†) was determined by electric double layer capacitance (C_dl). The C_dl value of V_O-FeOOH is 4.96 mF cm⁻², which is higher than that of other contrast samples (Fig. 2e), indicating that there are more active sites exposed on the surface of V_O-FeOOH and the OER activity is higher. The ECSA of V_O-FeOOH was calculated to be 124 cm², and its large active area should be due to its nanosheet morphology and porous structure (Fig. 2a and S5†). When the LSV curve is normalized to ECSA, V_O-FeOOH exhibits higher catalytic activity than that of IrO₂ and NF (Fig. 2f), demonstrating that the large specific surface area and the structure of the oxygen vacancy improve its intrinsic activity.

The intrinsic reaction kinetics of the electrode/electrolyte interface was studied by in situ electrochemical impedance spectroscopy (EIS).³⁹Fig. 2g and S12† depict Nyquist plots for various catalysts, revealing that the resistance decreases with increasing external voltage. When compared to IrO₂ and NF catalysts, the V_O-FeOOH catalyst exhibits the lowest resistance across all potentials, indicating that the presence of oxygen vacancies significantly enhances the adsorption kinetics of oxygen-containing reactive species during the alkaline water oxidation process. The Bode diagram illustrates the phase relaxation process of the catalyst sample, as demonstrated in Fig. 2h and S13† for the V_O-FeOOH, IrO₂, and NF samples. Peaks in the low and high-frequency regions correspond to the charge transfer reaction occurring at the electrolyte and catalyst interface, as well as electron conduction within the catalyst's inner layer at high frequencies.⁴⁰ As the potential increases, the electron transfer resistance in the reaction process decreases notably. In the case of V_O-FeOOH, IrO₂, and NF electrodes, as the potential increases, the phase angle shifts towards the high-frequency region. Compared with that of the carbon cloth, the first phase angle of NF appears at 0.55 V, while that of the carbon cloth appears at 0.6 V (Fig. S14†),⁴¹ indicating that the substrate itself has high intrinsic activity. The catalyst supported on the NF surface effectively promotes the electron transfer between the electrolyte and the catalyst, thereby promoting electron conduction. These results indicate that V_O-FeOOH, IrO₂, and NF share similar interfacial reaction characteristics.^35,42,43

For electrocatalysis, good stability is an important application index.⁴⁴ The LSV curve and EIS of the V_O-FeOOH sample after 500 cycles in Fig. 3a and b show that the activity of V_O-FeOOH after CV cycles is slightly improved, and the charge transfer resistance is slightly reduced. This may be due to the incomplete activation of the catalyst in the early stage, demonstrating that the V_O-FeOOH catalyst has good stability under dynamic current. Fig. 3c and d show that the V_O-FeOOH catalyst can remain stable at different current potentials and densities. The chronopotentiometry curve in Fig. 3e and Fig. S15† shows that the V_O-FeOOH catalyst has good durability after the 120 h and 45 h OER test at 50 mA cm⁻² and 100 mA cm⁻², respectively, and the potential change can be ignored. However, the commercial catalyst IrO₂ has a significant performance decline in less than 20 hours of operation. The V_O-FeOOH sample has a higher catalytic stability than the IrO₂ catalyst. It can also be explained that the V_O-FeOOH coated on the surface of NF does not fall off, which proves that it is a feasible method to coat the nanocatalyst on the surface of NF.

Fig. 3 (a) The linear sweep voltammetry curves, and (b) electrochemical impedance spectroscopy of V_O-FeOOH before and after 500 CV cycles. (c) The multi-current steps and (d) multi-potential steps of V_O-FeOOH. (e) The long-time E–t curves of V_O-FeOOH and IrO₂ for the OER (j@50 mA cm⁻²).

To further evaluate the overall water splitting performance of the Vo-FeOOH catalyst, we used Pt/C and V_O-FeOOH to assemble an electrolytic cell (Fig. S16a and b†). The results show that, under the same conditions, the V_O-FeOOH||Pt/C electrolytic cell has better performance than the RuO₂||Pt/C commercial catalyst electrolytic cell (Fig. S16c†). It is worth noting that the performance of the catalyst did not show a downward trend in the long-term stability at 10 mA cm⁻², which is consistent with the results of the three-electrode system, indicating that it has excellent stability (Fig. S16d†). To explore the effect of catalyst morphology on stability, we characterized the catalyst supported on NF by SEM and TEM. The corresponding results showed that the morphology of the catalyst did not change significantly during the long-term stability test (Fig. S17a–c†). At the same time, HRTEM also proved that the phase of the catalyst did not change before and after the OER operation (Fig. S17d and e†).

To compare the electrocatalytic performance and mechanism of highly crystalline FeOOH and low crystalline V_O-FeOOH, we prepared highly crystalline FeOOH samples using the previously reported wet chemical method (Fig. S18†).²¹ The HRTEM image of FeOOH clearly shows that it has neatly arranged lattice fringes with a width of 0.151 nm, which belongs to the (002) crystal plane of FeOOH, indicating that it has high crystallinity (Fig. S19†). We have experimentally studied the defects of catalysts and demonstrated that the defects enhance the electron transfer and the activity of individual sites in the catalytic process. Raman spectroscopy is known to be sensitive to defects and lattice disorder.⁴⁵ As depicted in Fig. 4a, the conspicuous Raman peaks at 215, 279, and 391 cm⁻¹ correspond to the asymmetric stretching vibrations between the metal and hydroxide ions in the highly crystalline FeOOH catalyst.⁴⁶ These observations align with the characteristic crystalline features of FeOOH, providing further confirmation of its high crystallinity. The corresponding peak positions indicate that FeOOH exhibits Raman activity.⁴⁷ Meanwhile, the Raman spectrum of V_O-FeOOH showed a reduction in peak intensities corresponding to the characteristic peaks and the formation of a broad band, which aligns with the spectroscopic features of low crystallinity.^48,49 The coexistence of Fe and O in the FeOOH sample was observed through the full spectrum of XPS (Fig. S20†). By fitting the high-resolution O 1s spectra of V_O-FeOOH and FeOOH in Fig. S21,† it was observed that the V_O peak area of the V_O-FeOOH sample was distinctly larger than that of FeOOH, pointing that V_O-FeOOH has more V_O than FeOOH.

Fig. 4 (a) The Raman spectra, (b) EPR spectra, (c) LSV curve, (d) EIS spectrum, (e) Tafel slope of V_O-FeOOH and FeOOH, and (f) calculated TOF values of V_O-FeOOH and FeOOH at 1.55 V and 1.60 V.

Electron paramagnetic resonance (EPR) spectroscopy analysis also confirmed this conclusion (Fig. 4b). At g = 2.003, V_O-FeOOH exhibited a stronger symmetric EPR signal compared to FeOOH, confirming the formation of V_O in V_O-FeOOH. At the same time, by comparing the peak area of Fe²⁺ in V_O-FeOOH and FeOOH (Fig. S22†), it can be found that the area of Fe²⁺ in V_O-FeOOH is larger than that in FeOOH, indicating that the formation of low valence Fe²⁺ is caused by V_O species.⁵⁰ Subsequent electrochemical results revealed that low-crystallinity V_O-FeOOH exhibited higher electrochemical catalytic activity (Fig. 4c) and low-overpotential (Fig. S23†), faster electron transfer rate (Fig. 4d), and smaller Tafel slope (Fig. 4e) compared to high-crystallinity FeOOH. Further C_dl results also revealed that V_O-FeOOH had a larger effective electrochemical specific surface area (ECSA) compared to FeOOH (Fig. S24†).

The intrinsic property of nanocatalysts is closely related to the turnover frequency (TOF),⁴⁴ as shown in Fig. 4f. The TOF values of V_O-FeOOH and FeOOH at potentials of 1.55 V and 1.60 V were 0.04 and 0.021 s⁻¹, and 0.104 and 0.04 s⁻¹, respectively, indicating that the presence of low crystallinity and V_O species enhanced the property of the V_O-FeOOH electrode. The corresponding BET results also indicated that the low-crystallinity V_O-FeOOH had a larger specific surface area (156.40 m² g⁻¹) compared to the high-crystallinity FeOOH (123.58 m² g⁻¹) (Fig. 1d and S25a†), together with a more concentrated mesoporous distribution (Fig. S5 and S25b†). In conclusion, the research results suggest that low-crystallinity V_O-FeOOH facilitates the formation of V_O, thereby enhancing the formation of unsaturated sites and conductivity and thus improving the electrocatalytic activity of the OER process.

3.3 DFT simulation and catalytic mechanism

To further study the specific role of defects in the catalytic process, we carried out theoretical calculations on the catalytic process. Here, based on the experimental results depicted in Fig. 4c, the theoretical models of V_O-FeOOH and FeOOH structures were constructed for different active intermediates using the AEM mechanism under alkaline conditions (Fig. 5a and b). The density of states (DOS) for V_O-FeOOH and FeOOH is shown in Fig. 5c. V_O has a significant impact on the DOS of FeOOH. The d-band of the V_O-containing structure is closer to the Fermi level compared to FeOOH, indicating lower adsorption energy for OER active intermediates as compared to the FeOOH counterpart. V_O-containing FeOOH exhibits significantly higher DOS at the Fermi level compared to FeOOH, suggesting that higher DOS enables more charge carriers to directly participate in catalytic reactions, which can significantly enhance the electrode's OER property. Furthermore, the intensity of the Fermi level aligns well with electron transfer resistance (Fig. 4d), and an analysis of different elements reveals that the presence of V_O can effectively enhance the DOS of different atoms (Fig. S26†), introducing more active catalytic sites, thereby facilitating the progress of the OER process.^51,52

Fig. 5 Optimized configurations of (a) V_O-FeOOH and (b) FeOOH, the Fe-3d calculated DOS of (c) V_O-FeOOH and FeOOH, the Gibbs free energy of (d) V_O-FeOOH and FeOOH for each elementary step of the OER process, and (e) a schematic representation of the OER course of V_O-FeOOH in an alkaline environment.

Then we continue to study the OER catalytic activity of V_O-FeOOH, and further calculate the different models (Fig. S27 and S28†) based on the ΔG change of the 4e⁻ pathway to elucidate the water oxidation energy barrier of V_O-FeOOH and FeOOH counterpart.⁵³ The OER process of the widely accepted AEM mechanism can be divided into four stages, including three intermediate states (OH*, O*, and OOH*) (Fig. 5d). All possible reaction paths on V_O-FeOOH are shown in Fig. 5d and e, and the reaction path with the highest energy required is O* → OOH*, indicating that this step is the RDS in V_O-FeOOH and FeOOH counterpart. The ΔG_max in V_O-FeOOH is 1.65 eV, which is less than the 1.91 eV in FeOOH, indicating that V_O contributes to the formation of more strong active sites in V_O-FeOOH, thereby enhancing the OER performance of V_O-FeOOH samples. The DFT calculation showed that the defects optimized the adsorption capacity of the intermediates during the corresponding catalytic process, effectively reduced the energy barrier, and finally improved the catalytic efficiency and stability. On the whole, we improved the oxygen defect level of the FeOOH nanocatalyst through a simple strategy, and used the defects to improve the activation ability of the rate-limiting step of the water oxidation process, and finally improved the activity and stability of cheap iron(III) oxyhydroxide.

4. Conclusions

In summary, the V_O-FeOOH nanosheets with low crystallinity and abundant V_O were synthesized by a simple oxidation conversion method under an inert N₂ atmosphere. The results showed that V_O-FeOOH nanosheets provided a larger reaction area for the OER process and provided a fast channel for charge transfer, exposing more potential active sites. Defect-enriched V_O-FeOOH exhibits excellent catalytic performance in the water oxidation process. The overpotential at 10 mA cm⁻² is 255 mV, and it can operate stably for 120 h (j@50 mA cm⁻²), and the reaction kinetics are better than those of the FeOOH counterpart. DFT calculations show that V_O plays an important role in the OER process. V_O can effectively promote the d-band to approach the Fermi level and improve the conductivity of the V_O-FeOOH sample. At the same time, the ΔG of the RDS in the OER process was reduced from 1.65 eV to 1.91 eV, which improved the OER catalytic activity of V_O-FeOOH. The study provides an effective path for the synthesis of low-cost, high-performance energy conversion electrodes with a defective structure and lays a valuable theoretical foundation for the structure and performance of OER catalysts.

Author contributions

Yaning Fan: Data acquisition, writing-review & editing. Junjun Zhang: Funding acquisition, writing-review & editing. Kongliang Luo: Data acquisition, writing-review & editing. Xuanyu Zhou: Visualization. Weiwei Bao: Data acquisition, writing-review & editing. Hui Su: Writing & editing. Jiahua Zhao and Nailiang Wang: Writing & editing. Pengfei Zhang: Visualization and acquisition of funds. Zhenghong Luo: Supervision.

Conflicts of interest

The authors declare that they have no conflict of interests.

Acknowledgements

This work was supported by Ningxia Key Research and Development Program (No. 2021BEE03007), Inner Mongolia R & D Program Plan (2021ZD0042, 2021EEDSCXSFQZD006) and National Natural Science Foundation of China (Grant No. 21902123). Junjun Zhang thanks the Chinese Academy of Sciences Western Young Scholar Program for the scholarships and the Scientific Research Start-Up Project Program of the Ningxia University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi02043f

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