Multi-interfacial nanosheet-intercalated structure with abundant oxygen vacancies promotes electrocatalytic oxygen evolution†

Di Wang^a, Zhe Sun^a, Wenguang Cui^b, Chaozhen He^c and Zhongkui Zhao*^a
aState Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, P. R. China. E-mail: zkzhao@dlut.edu.cn
^bCollege of Chemical Engineering Shijiazhuang University, 288 Zhufeng Street, Shijiazhuang 050035, Hebei, China
^cInstitute of Environmental and Energy Catalysis, School of Materials Science and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, China

First published on 18th July 2024

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1. Introduction

As a new type of renewable energy, hydrogen energy is widely regarded as the key to future energy transformation because of its high calorific value, low energy consumption and excellent combustion behavior.^1–3 Hydrogen can be produced by electrolysis of water, natural gas transformation, biomass conversion and other ways, among which hydrogen production by electrolysis of water is pollution-free, and is one of the most ideal solutions to replace traditional fossil energy. However, precious metal-based materials are currently used as industrial catalysts for the oxygen evolution reaction (OER) to overcome the high overpotential in the water-splitting process, but their high cost limits their large-scale commercial applications.^4–6 Therefore, the development of non-precious metal water-splitting electrocatalysts with low price, excellent performance and low energy consumption has become the top priority of current research.

In recent years, transition metal phosphide has been frequently used in the OER due to its unique hydrogenase-like catalytic mechanism.^7,8 With a growing understanding of the OER mechanism, studies have shown that spontaneously reconstructed metal (oxy)hydroxides after the OER are the true active species,^9,10 but the optimal conditions for the reconstruction process cannot be ensured. Therefore, controlled electrochemical reconstruction is an effective and feasible method to enhance OER activity, as demonstrated in previous work by our group.¹¹ Furthermore, interface engineering is considered an efficient strategy for improving the intrinsic activity of OER catalysts. Studies have reported that constructing crystalline–amorphous interfaces can more effectively promote OER activity, with the crystalline phase accelerating the electron transfer, and the irregular amorphous phase resulting in a random distribution of internal atoms, generating numerous unsaturated coordination atoms and providing abundant active adsorption sites.^12–14 Generally, low-cost transition metals, especially Fe, Mn, Co, and other 3d transition metals, are considered ideal additives for regulating interface construction.^15–17 It is now believed that oxygen vacancies (O_v), as the active sites in the OER, can regulate surface-oxygen interactions, thus accelerating the reaction kinetics and enhancing the catalytic activity.^18,19 In recent years, rare earth elements have also attracted attention due to their unique 4f sub electronic layer structure. Some studies have confirmed that introducing rare earth elements into transition metal catalysts can create corresponding oxygen vacancies. By utilizing the synergistic effect of the multi-interface catalyst and oxygen vacancies, the electronic structure can be optimized, the kinetic energy barrier can be lowered, and the performance of catalysts can be significantly improved.^20,21

Taking inspiration from the above, we envision a strategy based on electrochemical oxidation to obtain OER catalysts with higher activity by constructing interfaces and introducing oxygen vacancies. Therefore, we used the urea hydrothermal method to grow Ni₂P nanosheets on nickel foam (NF), and then introduced Fe and La in the process of Ni₂P reconstruction to obtain a multi-interfacial catalyst with a unique nanosheet-intercalated structure and rich oxygen vacancies, named (Fe,La)Ni₂P-r. Interestingly, in a 1.0 M KOH solution, the catalyst exhibited excellent OER catalytic activity, comparable to most reported catalysts. The unique structure exposes more active sites in the catalyst. Combined with DFT results, it was found that the strong electronic coupling at the interface and the synergistic effect of oxygen vacancies promote the breaking of the HO–O bond, leading to an increase in the content of intermediate species in *OOH and accelerating the release of oxygen. In addition, this study also provides a promising approach for designing multi-interfacial catalysts rich in oxygen vacancies.

2. Results and discussion

Based on the previous work of this group, the optimal conditions were first obtained by screening different metals and optimizing metal loading amounts and various parameters. Then, a unique nanosheet stacking structural (Fe,La)Ni₂P-r catalyst supported on a Ni foam (NF) substrate was prepared using a simple two-step method (Scheme 1). The atomic ratios were determined by ICP-AES and XPS tests, and the differences in the data were observed due to variation in the characterization methods (Table S1†). From the scanning electron microscopy images (Fig. 1A and S1†), it can be seen that the (Fe)Ni₂P-r and (Fe,Mn)Ni₂P-r catalysts exhibit a stacked nanosheet-intercalated morphology. However, after the introduction of Fe and La, a novel sheet stack structure with local bulges was unexpectedly discovered, as shown in Fig. 1B–D. The nanosheet stack structure of the catalyst is assembled with nanosheet units in a spiral layer-by-layer stacking manner, forming a unique structure of sheet assembly, which subsequently influences the electrochemically active surface area of the catalyst (Fig. S6 and Table S5†). The TEM image of the (Fe,La)Ni₂P-r catalyst (Fig. 1E) provides strong evidence that the catalyst is assembled from nanosheet units, consistent with the SEM results. The unique structure facilitates mass transfer in the oxygen evolution reaction.²² Further, the HAADF-STEM image (Fig. 1F) and EDX analysis (Fig. 1G) demonstrate the uniform distribution of Fe, La, Ni, and O elements on the (Fe,La)Ni₂P-r catalyst.

The XRD patterns in Fig. 1I reveal that the (Fe,La)Ni₂P-r catalyst differs from the reference samples by the appearance of new diffraction peaks at 31.59°, 40.93°, 42.99°, 44.30°, 44.65°, 47.78°, 49.15°, 49.76°, 50.28°, and 51.59°, matching perfectly with the (−110), (−201), (−202), (200), (003), (−113),(−212), (−203), (−210), and (120) crystal planes of LaOOH (PDF#19-0656), indicating the formation of the new species LaOOH. Clear lattice fringes of Ni₂P and LaOOH can be observed in the HRTEM image in Fig. 1H, confirming their high crystallinity, consistent with the XRD results. Meanwhile, the (Fe,La)Ni₂P-r catalyst exhibits distinct interfaces, with lattice spaces of 2.460 Å and 2.700 Å corresponding to the (201) plane of Ni₂P and the (−212) plane of LaOOH (Fig. S9B†). In addition, some regions have been observed without an obvious lattice fringe, confirming the formation of surface heterojunction interfaces.²³ Notably, the introduction of Fe and La induces a strong interface effect, effectively promoting the oxidation of Ni₂P, as evidenced by the disappearance of the diffraction peaks near 33°.^22,23 Overall, the formation of interfaces facilitates the generation of more active *OOH species, accelerating oxygen release. Additionally, the reconstruction layer thickness of 5.4 nm, significantly thicker than those of the (Fe)Ni₂P-r and (Fe,Mn)Ni₂P-r catalysts (as shown in Fig. S2†), further confirms this point.

X-ray photoelectron spectroscopy (XPS) was employed to analyse the surface chemical composition and electronic structure of the samples. Fig. S3A† shows the presence of Fe and La, confirming the successful introduction of Fe and La elements, consistent with the EDX results. All high-resolution spectra were calibrated based on the C 1s peak at 284.6 eV. As depicted in Fig. 2A, the Ni 2p spectrum of the (Fe,La)Ni₂P-r sample exhibits peaks at 853.8 eV attributed to metallic nickel and at 855.8 eV and 857.0 eV attributed to Ni²⁺ and Ni³⁺, indicating the coexistence of Ni²⁺ and Ni³⁺ in the (Fe,La)Ni₂P-r sample.^24–27 Comparison of the binding energies of Ni²⁺ in the different catalysts reveals a positive shift relative to the (Fe)Ni₂P-r sample for the (Fe,Mn)Ni₂P-r sample and the (Fe,La)Ni₂P-r sample by 0.1 eV and 0.3 eV, suggesting a stronger Fe–La interaction than Fe–Mn, facilitating more Ni oxidation to higher oxidation states, potentially beneficial for *OOH generation.^24,28 Furthermore, the ratio of Ni³⁺/Ni²⁺ in the (Fe,La)Ni₂P-r, (Fe,Mn)Ni₂P-r, and (Fe)Ni₂P-r samples confirms this, with values of 0.83, 0.75, and 0.72, respectively. In the Fe 2p XPS spectra (Fig. 2B), the (Fe,Mn)Ni₂P-r catalyst exhibits a slight negative shift (0.3 eV) compared to the (Fe)Ni₂P-r catalyst. Upon the transition of the metal from Mn to La, the (Fe,La)Ni₂P-r catalyst experiences an additional negative shift (0.2 eV) relative to the (Fe,Mn)Ni₂P-r catalyst (855.6 eV), attributed to the low electronegativity of metal La, facilitating the electron density transfer from La to Fe, resulting in more unsaturated Fe states, thereby altering the adsorption energy on the active sites and affecting its inherent OER activity.^29–31 The O 1s spectrum is deconvoluted into four characteristic peaks at 530.0 eV, 530.8 eV, 531.6 eV, and 532.8 eV (Fig. 2C), attributed to M–O (metal–oxygen in lattice oxygen), O–H (hydroxyl bond), O_v, and H₂O (adsorbed water molecules), respectively. Integration calculations of O under different states reveal that the O_v content of the (Fe,La)Ni₂P-r catalyst (23.1%) is higher than that of the (Fe,Mn)Ni₂P-r catalyst (17.6%). Electron paramagnetic resonance spectra further confirm the introduction of oxygen vacancies (O_v) at g = 2.000 for the (Fe,La)Ni₂P-r and (Fe,Mn)Ni₂P-r catalysts (Fig. S4†), with the signal intensity of the (Fe,La)Ni₂P-r catalyst being greater than that of the (Fe,Mn)Ni₂P-r catalyst, indicating that La introduction promotes the generation of oxygen vacancies (O_v), allowing *OOH adsorption and acquiring more active species, as confirmed.^32,33

More interestingly, under the influence of La³⁺, the adsorption energy of water on the surface of (Fe,La)Ni₂P-r sample is lower than that of (Fe,Mn)Ni₂P-r, demonstrating the excellent surface water adsorption ability of (Fe,La)Ni₂P-r. Relevant reports have shown that electron induced charge transfer can promote the adsorption ability of water, supporting the generation of M–OOH.^34,35 Similarly, in the XPS spectrum of La 3d, two peaks are observed corresponding to the satellite peaks (sat) of La 2d_5/2 and La³⁺ (Fig. 2D). For the (Fe)Ni₂P-r catalyst, the introduction of La is further verified, in accordance with the findings of EDX and ICP. For the P 2p spectrum (Fig. S3B†), an enhancement of the P–M peak is observed in the (Fe,La)Ni₂P-r catalyst, attributed to the stronger P–La bonding energy caused by La's lower electronegativity (1.1) compared to Fe (1.8) and Mn (1.5).²⁹ Therefore, the XPS results not only confirm the successful preparation of the (Fe,La)Ni₂P-r sheet–sphere heterostructure but also demonstrate that the introduction of Fe and La leads to a strong interface effect and enhancement of oxygen vacancies. This enhancement of oxygen vacancies and the shift in element binding energies may be attributed to the surface electron transfer induced by the interface charge redistribution among LaOOH, MOOH (M = Fe, Ni), and Ni₂P to maintain system charge neutrality.^36–40

The corresponding electrochemical tests on the prepared catalyst samples were performed in a 1.0 M KOH solution. The (Fe)Ni₂P-r catalyst and (Fe,Mn)Ni₂P-r catalyst are used as control samples. Fig. 3A shows the LSV curves of the different catalysts, and compared with the control catalyst samples and the pristine Ni₂P,¹¹ the (Fe,La)Ni₂P-r heterostructure exhibits a significantly higher current density at the same potential, demonstrating the strongest OER activity (Table S2†). The oxidation of metallic Ni before water oxidation causes the bulge in the curve, a phenomenon already documented.^41–43 To obtain the heterogeneous catalyst with the highest performance, the OER activity of different metals (Ce, Mn, In, Bi, Co, Zn, Cu, and Ca) is screened by an orthogonal test (Fig. S5A, Tables S7 and S8†), revealing that the (Fe,La)Ni₂P-r catalyst exhibits the highest current density at a constant potential. When the parameters of the (Fe,La)Ni₂P-r heterogeneous structure are modified, it is discovered that after 40 minutes of reconstruction at 1.60 V (vs. RHE), the best performance is obtained with a molar ratio of 10:1 (Fig. S5B–D†).

More importantly, the performance of (Fe,La)Ni₂P-r surpasses that of the commercial catalyst RuO₂ (overpotential of 246 mV at 10 mA cm⁻² current density), with (Fe,La)Ni₂P-r exhibiting an overpotential of 197 mV at 10 mA cm⁻² current density (Fig. 3B), comparable to most other reported catalysts (Table S9†). The Tafel slope reflects the intrinsic kinetics of the reaction. In Fig. 3C, the Tafel slope of the (Fe,La)Ni₂P-r catalyst introduced with La is 30 dec⁻¹, superior to those of the control samples (Table S3†). The electrochemical impedance spectroscopy results of the catalysts are displayed in Fig. 3D, where R_ct stands for the charge transfer resistance and R_s stands for the solution interface resistance. The R_ct of the (Fe,La)Ni₂P-r catalyst is the smallest (Table S4†), indicating that the introduction of Fe and La reduces the energy barrier encountered by the charge transfer at the electrode/electrolyte interface. Hence, the enhancement of the OER activity in the (Fe,La)Ni₂P-r catalyst may be attributed to the synergistic promotion of charge transfer by oxygen vacancies and interfaces, accelerating their intrinsic kinetics.³⁴ Exploring the specific activity and number of active sites of the catalysts are crucial for evaluating the OER activity. The CV curves of all catalysts at scanning rates of 20 mV s⁻¹ to 160 mV s⁻¹ were collected (Fig. 4A, S6C and D†), obtaining proportional curves of the scan rate and current density (Fig. 4B), and then calculating the ECSA of different catalysts based on the double-layer capacitance C_dl. The electrochemically active area of the (Fe,La)Ni₂P-r catalyst is 78.1 cm⁻², which is less than those of other catalysts, as indicated in Fig. S6A and Table S5.† Normalizing the ECSA to LSV polarization curves (Fig. 4C), it is evident that the specific activity of (Fe,La)Ni₂P-r is the highest, indicating the optimal intrinsic activity of the (Fe,La)Ni₂P-r sample. Fig. S6B† visually reflects the number of active sites of the catalysts. At 1.55 V (vs. RHE) potential, the TOF values of the (Fe,La)Ni₂P-r, (Fe,Mn)Ni₂P-r, and (Fe)Ni₂P-r catalysts are 0.52 s⁻¹, 0.34 s⁻¹, and 0.29 s⁻¹, respectively. Therefore, the higher OER activity of the (Fe,La)Ni₂P-r catalyst is partially attributed to the increase in the number of active sites of the catalyst. In addition, the catalyst has a Faraday efficiency of about 100% (Fig. S11 and Table S6†).

One of the most crucial criteria for evaluating the practical performance of a catalyst in OER applications is its long-term durability under strong alkaline operating conditions. The microsphere heterostructure material (Fe,La)Ni₂P-r maintains its activity for 100 hours at 20 mA cm⁻² without significant degradation, as evidenced by the nearly overlapping polarization curves before and after 100 hours, indicating the excellent stability of the catalyst (Fig. 4D and S7†). Characterization of the morphology and surface electronic structure of the catalyst after electrochemical testing and after 100 hours reveals, in Fig. S8,† that the catalyst largely retains its original microsphere structure, which is conducive to the exposure of active sites and facilitates the activation of O₂ due to its layered sheet–ball structure. Transmission electron microscopy images (Fig. S9†) further confirm that the morphology of the catalyst remains largely unchanged after 100 hours, with clear lattice stripes of LaOOH and Ni₂P and the presence of heterointerfaces. In the Fe 2p and Ni 2p spectra before and after the reaction (Fig. S10A and B†), a significant positive shift can be observed, attributed to the conversion of Fe–P and La–P bonds to Fe–O and La–O bonds, indicating surface restructuring of the catalyst during the OER.²⁸ The disappearance of the P–M bond in Fig. S10C† provides further evidence of surface restructuring to form FeOOH and LaOOH.⁴⁴ In Fig. S10D,† the presence of La³⁺ ions is still clearly observable after the reaction, confirming the precise introduction of La. Overall, all results demonstrate that the introduction of La generates a unique nanosheet stack structure with local bulges. The presence of interfaces between LaOOH, M–OOH (M = Fe and Ni), and Ni₂P, along with the enhancement of oxygen vacancies (O_v), induces charge redistribution, resulting in changes in the electronic structure, altering the adsorption energies of H and OH on the active sites, and promoting an increase in the *OOH content.

To investigate the influence of oxygen vacancies and heterointerfaces on the intrinsic mechanism of the catalyst reaction, density functional theory (DFT) calculations were employed. Firstly, models for all catalysts were established (Fig. 5A and S12†). In addition, a catalyst model without O_v (denoted as (Fe,La)Ni₂P-r(s)) was established on the surface of the (Fe,La)Ni₂P-r catalyst to visualize the effect of O_v (Fig. 5A). In order to better understand the influence of oxygen vacancies, the formation energies of oxygen vacancies in the different catalysts were calculated, and it was found that the energy barrier required for (Fe,La)Ni₂P-r was the lowest (Fig. 5B). At the same time, in order to make the calculation more accurate, as shown in Fig. S14,† we calculated the oxygen vacancies between the three different atoms of Fe–O_v–La, Fe–O_v–Ni, and La–O_v–Ni for (Fe,La)Ni₂P-r at different positions, and found that the formation energy between Fe–O_v2–La was the lowest (Fig. S14†). Fig. 5C demonstrates that the presence of oxygen vacancies reduces the Gibbs free energy from 1.88 eV to 1.49 eV, indicating that the oxygen vacancies between Fe and La significantly decrease the Gibbs free energy of the reaction process, facilitating the adsorption of the intermediate species and accelerating the oxygen release.⁴⁵

Furthermore, from Fig. 5C, it can be observed that the *OH to *O step in the (Fe,La)Ni₂P-r catalyst is the rate-limiting step for the whole reaction. The Gibbs free energy and overpotential (1.49 eV and 0.26 V) of the (Fe,La)Ni₂P-r catalyst are lower than those of (Fe,Mn)Ni₂P-r (1.76 eV and 0.53 V) and (Fe)Ni₂P-r (2.26 eV and 1.03 V), reflecting the superior OER performance of the (Fe,La)Ni₂P-r catalyst. Here, (Fe,Ni)OOH, (Fe,Mn,Ni)OOH, (Fe,La,Ni)OOH, and (Fe,La,Ni)OOH(s) represent the O_v-containing (Fe)Ni₂P-r, (Fe,Mn)Ni₂P-r, (Fe,La)Ni₂P-r, and O_v-free (Fe,La)Ni₂P-r catalysts, respectively. Combined with SEM, TEM, and XRD, it is evident that all catalysts retain a significant amount of Ni₂P after reconstruction, indicating that the doping of heteroatoms does not completely oxidize Ni₂P, while the generated MOOH (M = Fe, Mn, Ni, and La) exists on the surface of the catalyst. Unexpectedly, it was found that the doping of La generates a new species LaOOH, which exists in a crystalline form, indicating that amorphous FeNiOOH and crystalline LaOOH also formed a non-homogeneous interface at the catalyst surface.

Fig. 5D shows the differential charge distribution maps for all catalysts, where the red and blue regions represent electron accumulation and depletion, respectively. An intense electron density is observed around Ni in the (Fe,La)Ni₂P-r catalyst, indicating a strong coupling effect of electrons at the interface, which plays a crucial role in modulating the electronic structure, consistent with XPS results.²⁰ To determine the primary active species in the catalyst, adsorption energy calculations were performed for all possible sites. Benefiting from the interface effect and the weakening effect of oxygen vacancies on the limiting step barrier, Ni sites exhibit a lower Gibbs free energy and overpotential, with optimized intermediate models shown in Fig. S15.† It is noteworthy that different active sites have different rate-limiting steps for adsorption. For example, the overpotential of the Fe site in the *O to *OOH step is the lowest, with the lowest energy barrier (Fig. S16†). Thus, the experimental results are well confirmed by DFT calculations. The addition of Fe and La forms heterointerfaces and promotes the enhancement of oxygen vacancies, leading to an increase in the number of Ni primary active sites. The synergistic effect of the sharply increased Ni active sites and complementary multiple active sites is also one of the main reasons for the enhancement of OER activity.

3. Conclusions

In summary, based on electrochemical oxidation strategies, we introduced bimetallic Fe and La into the reconstruction process of a Ni₂P substrate and developed a nanosheet assembly structure with local protrusions. The electrocatalyst exhibits a low overpotential of 197 mV at a current density of 10 mA cm⁻², with a Faraday efficiency of about 100%. This excellent performance can be attributed to three aspects: (i) due to the unique structure, the synthesized catalyst has more active sites and excellent electrical conductivity, which promotes the mass transfer process, accelerates the intrinsic kinetics of the catalyst and promotes the release of oxygen. (ii) The redistribution of electrons at the interface and the synergistic effect between multiple active sites cause changes in the electronic structure, optimizing the adsorption and desorption strength of interface sites and optimizing the binding energy of different intermediates on the surface. (iii) The introduction of La induces the enhancement of O_v, thus changing the adsorption energy of H and OH at the active sites and increasing the content of *OOH, and the generated intermediate species *OOH was easily adsorbed on the surface of the (Fe,La)Ni₂P-r catalyst, improving the OER activity of the catalyst. In addition, the synergistic action of the primary active sites and complementary multiple active sites also promoted the OER. This study provides an effective strategy for constructing highly efficient multi-interface non-precious metal OER electrocatalysts rich in oxygen vacancies.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

D. Wang conceived and performed the experiments, collected and analyzed the data, and wrote the paper. Z. Sun and C. He performed the DFT calculation and participated in the discussion. W. Cui participated in the discussion. Z. Zhao conceived the idea, supervised the project work, led the data analysis and discussion, and prepared and finalized the paper. All the authors commented on the manuscript and have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21978030) and the Ministry of Education's New Century Excellent Talents Program for Universities (NCET-12-0079).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy00653d

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