1.   Introduction

The green and sustainable energy conversion and storage technology is the key to solve the problem of environmental pollution and energy shortage [1–3]. The technology of hydrogen production by water electrolysis and fuel cell is the optimal choice to realize the conversion between hydrogen energy and electric energy [4–5]. However, due to the complex proton–electron transfer process of oxygen evolution reaction (OER), OER has high overpotential and slow reaction kinetics, which seriously restricts the efficiency of hydrogen production and fuel cells by electrolytic water splitting [3,6–8]. Therefore, designing and synthesizing efficient OER catalysts to reduce OER reaction overpotential and accelerate the reaction rate is a focus of research. The best OER electrocatalysts reported so far are mainly noble metal-based catalysts, such as RuO₂ and IrO₂ [9–10]. However, low precious metal reserves in the earth’s crust, high cost, and short service life of precious metal catalysts seriously restrict the large-scale commercial application in the field of hydrogen production by water electrolysis and fuel cells [11–12].

Fortunately, more and more studies have reported that transition metal-based materials (e.g., MnO₂, LDH, etc.) abundant in the crust exhibited good electrocatalytic OER properties [13–17]. Specially, NiFe-based catalysts showed high efficiency OER electrocatalysis. Over the past decade, NiFe-based oxides, hydroxides, sulfides, and selenides have been explored for OER reactions, showing great promise for NiFe-based materials as low-cost OER catalysts [18–21]. Among various NiFe-based OER catalysts, NiFe-based oxides are effective OER catalysts and have been shown to have great potential for OER electrocatalysis [22–23]. Moreover, spinel compounds with AB₂O₄ general form have attracted special attention due to their stable structure and abundant catalytic active sites, and become a very competitive OER electrocatalyst [24]. In addition, the abundant element valence and structural flexibility in its unique spinel configuration can provide more possibilities to improve its catalytic OER. For example, Chen et al. [25] synthesized a NiFe₂O₄ spinel electrocatalyst with 100% (111) face exposure directly on nickel foam (NF) by hydrothermal methods. Abundant vacancy defects were produced on the NiFe₂O₄ (111) surface by argon plasma etching. The experimental results showed that the introduction of multi-vacancy effectively regulated the electronic structure of the active center and optimized the adsorption energy of H₂O molecules and reaction intermediates on electrocatalysts, which was the key to reduce reaction kinetics and improve electrochemical OER activity. In addition to metal oxides, NiFe alloy materials also show excellent OER catalytic performance [26–29]. For example, Xu et al. [26] found that different Ni/Fe ratios lead to significant differences in surface composition and structure of NiFe alloys, which modulated their electrocatalytic performance in OER. In addition, the ultra-thin oxide layer on the surface of NiFe alloys is the key to the high catalytic activity of NiFe alloys. The results of experimental characterization and theoretical calculation exhibited that Ni₄Fe₁ alloy with well-customized oxide/metal interface contributed to the formation of active substances, and the number of oxygen vacancies in the surface oxides optimized the adsorption interaction between O^* intermediate and Ni₄Fe₁ alloy, thus greatly improving its electrocatalytic activity.

Due to large surface area, abundant and uniformly dispersed active sites, adjustable chemical composition, and frame structure, metal-organic framework (MOF) materials are expected to replace noble metal-based catalysts as efficient OER pre-catalysts [30–33]. Li et al. [34] significantly improved the OER catalytic activity of MOF by introducing other metal cations into MIL-53(Fe). When FeNi_0.24Co_0.4-MIL-53 reached the current density of 20 mA·cm⁻², only 236 mV of overpotential was required, and the Tafel slope was 52.2 mV·dec⁻¹. In addition, Zhang et al. [35] prepared a series of bimetallic organic frameworks by a one-step method and obtained bimetallic oxides by pyrolysis of these bimetallic MOFs. These oxide catalysts showed good OER catalytic activity. The optimal catalyst needed only 335 mV overpotential to reach 10 mA·cm⁻² current density in 1.0 M KOH, and the Tafel slope was 55.6 mV·dec⁻¹.

Based on the above discussion, a series of MOF pyrolysis products in this work were prepared by high-temperature calcination using FeNi-MOF as precursor and their OER electrocatalytic performance was investigated. Compared with other samples, NiFe₂O₄/FeNi₃/C material formed at 450°C (FeNi₅-MOF-450) exhibited the optimal OER activity, and the current densities of 10 and 100 mA·cm⁻² can be reached at the overpotentials of 307 and 377 mV with the Tafel slope of 56.2 mV·dec⁻¹. Moreover, FeNi₅-MOF-450 displayed the superior stability during the electrochemical tests. This work provides a feasible way to develop new and efficient transition metal based OER catalysts.

2.   Experimental

2.1   Synthesis of FeNi_n-MOF (n = 0, 3, 5, and 7)

FeNi_n-MOF can be obtained by solvothermal reaction, where n represents the mole ratio of added Fe and Ni. In a typical synthesis of FeNi₅-MOF, 16.6 mg (0.1 mmol) of 1,4-terephthalic acid (H₂BDC), 145.4 mg (0.5 mmol) of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), and 27.0 mg (0.1 mmol) of ferric chloride hexahydrate (FeCl₃·6H₂O) were weighed and dissolved in 12 mL N,N-dimethylacetamide (DMA). The reaction solution was then transferred to a 20 mL high pressure reactor. The reactor was placed in the oven at 150°C for 6 h, and then cooled to room temperature. The product was washed three times each with water and ethanol, and dried in 60°C vacuum for 12 h. In addition, Fe-MOF, FeNi₃-MOF, and FeNi₇-MOF were synthesized as control groups. Among them, the synthesis steps of Fe-MOF, FeNi₃-MOF, and FeNi₇-MOF were exactly the same as those of FeNi₅-MOF (FeNi₃, FeNi₅, and FeNi₇ merely reflect the initial molar ratio of raw materials), except the addition amounts of Ni(NO₃)₂·6H₂O were different (0, 0.3, and 0.7 mmol of Ni(NO₃)₂·6H₂O were added, respectively).

2.2   Synthesis of FeNi_n-MOF-T

0.2 g FeNi₅-MOF sample prepared in the previous step was weighed into the porcelain boat and the porcelain boat was placed in the tubular furnace. The tubular furnace was heated to 450°C with the heating rate of 5°C·min⁻¹ and kept for 1 h in nitrogen atmosphere. After naturally cooled to room temperature, FeNi_n-MOF-450 was obtained. In addition, samples such as FeNi_n-MOF-200, FeNi_n-MOF-300, FeNi_n-MOF-400, FeNi_n-MOF-500, FeNi_n-MOF-550, and FeNi_n-MOF-600 were synthesized as control groups. Except for changing the calcination temperature (T), the other steps are exactly the same as the synthesis steps of FeNi_n-MOF-450.

2.3   Material characterization

The phase of electrocatalysts was characterized through X-ray diffraction (XRD, Bruker D2 PHASER). The morphologies were obtained using a scanning electron microscope (SEM, HITACHI S-4800) and a transmission electron microscope (TEM, China JEM-1400Plus). Fourier transform infrared (FT-IR) spectra were recorded by Bruker VERTEX70. Raman spectra were obtained by a LabRAM HR confocal Raman microscopy system. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Thermo Scientific ESCALAB 250Xi.

2.4   Electrochemical measurements

All OER tests in this article were performed with a typical three-electrode system (Gamry INTERFACE 1000 T) at room temperature. The synthetic samples, a graphite rod, and a calomel electrode (SCE) were separately used as the working electrode, the counter electrode, and the reference electrode. The test solution was 1.0 M KOH solution. All test results were corrected by the formula ($E_{\rm{RHE}}=E_{\rm{SCE}}+ 0.0592{\rm{pH}} +$$0.242$, where $E_{\text{RHE}}$ is the reversible hydrogen electrode potential after conversion, and $E_{\text{SCE}}$ is the actual measured potential using saturated calomel electrode.) and corrected for the iR-drop. Before the OER test, the working electrode was scanned with cyclic voltammetry (CV) until the sample was stable. Linear sweep voltammetry (LSV) were recorded at 5 mV·s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10⁻⁵ to 10⁻² Hz with the amplitude of 5 mV. To obtain the electrochemical double layer capacitance ($C_{\text{dl}}$) value, CV curves were collected at 1.38 to 1.48 V (vs. reversible hydrogen electrode (RHE)) with the scan rates of 20 to 100 mV·s⁻¹, respectively. The $C_{\text{dl}}$ values can be calculated by using the linear slopes of curves of Δj/2 vs. scan rate (Δj stands for the difference in current density). The electrochemical active area (ECSA) and the normalized current density were calculated using the double layer capacitance, according to the equations $\text{ECSA =} C_{\text{dl}}\text{/}C_{\text{s}}$ and $j_{\text{normalized}}=j\text{/ECSA}$, where $C_{\text{s}}$ is the specific capacitance of the material and j is current density. Tafel plots were calculated to understand the reaction mechanism of OER performance according to the Tafel equation η = b × lgj + a (η stands for overpotential, j stands for current density, and b stands for slope). The stability was evaluated by using CV at 100 mV·s⁻¹ for 2000 cycles. The long-term stability test was completed using chronopotentiometric measurement without iR drop compensation.

3.   Results and discussion

3.1   Material morphology and structure characterization

The preparation process of FeNi_n-MOF-T catalyst is shown in Fig. 1. Firstly, FeCl₃·6H₂O and Ni(NO₃)₂·6H₂O were used as nickel and iron sources, respectively. H₂BDC and DMA were severed as organic ligand and solvent. The product (FeNi_n-MOF) can be obtained by solvothermal reaction at 150°C for 6 h. Then, FeNi_n-MOF was used as the precursor and further calcined at different temperatures for 1 h, and the calcined products (FeNi_n-MOF-T) were obtained.

Figure  1.  Schematic illustration of the synthesis of FeNi_n-MOF-T.

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According to previous reports [36], FeNi_n-MOF was synthesized by solvothermal method in this work. In order to investigate the material structure change of FeNi₅-MOF during the pyrolysis process, the structural information of MOF and the products formed by the calcination of MOF at different temperatures (FeNi₅-MOF-T, T = 200, 300, 400, 450, 500, 550, and 600°C) were deteced by XRD and thermogravimetric analysis (TGA). As shown in Fig. 2(a), the diffraction peak of FeNi₅-MOF sample corresponded well with that of MIL-53 (Fe), indicating the successful preparation of the precursor of MOF. However, due to some lattice expansion caused by the incorporation of Ni in the process of MOF synthesis, the FeNi₅-MOF diffraction peak shifted to the left [37]. The diffraction peaks of FeNi₅-MOF-450 corresponded to NiFe₂O₄ (PDF#54-0964) and FeNi₃ (PDF#38-0419), indicating that NiFe₂O₄ and FeNi₃ nanoparticles were produced by MOF pyrolysis at 450°C. Yet, the XRD data of FeNi₅-MOF-500 and FeNi₅-MOF-600 showed that the diffraction peak corresponding to NiFe₂O₄ was weakened and the intensity corresponding to FeNi₃ was enhanced, which indicated that the metal oxides gradually disappeared and Fe and Ni formed alloy nanoparticles with the pyrolysis temperature rising. Furtherly, this was also verified by the TGA (Fig. S1).

Figure  2.  (a) XRD patterns of FeNi₅-MOF and products at different temperatures; (b) FT-IR spectra and (c) Raman spectra of FeNi₅-MOF products calcined at different temperatures.

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In addition, in order to analyze the structural changes of materials during heat treatment, the functional groups of products calcined at different temperatures were characterized by FT-IR spectra. As shown in Fig. 2(b) and Table 1, the absorption peak of FeNi₅-MOF at 3634 cm⁻¹ indicated the presence of physically adsorbed water molecules in the MOF channel. When the calcination temperature exceeded 300°C, the absorption peak here disappeared. The absorption peaks near 3420 and 1580 cm⁻¹ respectively represented the absorption peaks corresponding to different vibration modes of the –OH group. When the calcination temperature was higher than 450°C, the absorption peaks of –OH group disappeared, indicating that there were still associated –OH groups in FeNi₅-MOF-450. This may be the presence of –COOH groups without coordination in FeNi₅-MOF. The absorption peak at 1500 cm⁻¹ represented the absorption peak of benzene ring skeleton. This absorption peak did not disappear until the calcination temperature exceeded 500°C, which indicated that the MOF skeleton completely collapsed above 500°C. The absorption peaks near 750 and 820 cm⁻¹ represented the p-binary substitution of benzene ring, and these two absorption peaks also disappeared when the calcination temperature exceeded 500°C, which also proved that the MOF skeleton completely collapsed when calcined above 500°C. The absorption peak near 2183 cm⁻¹ was generated by the vibration of C–O bond. When the calcination temperature was higher than 300°C, the absorption peak near 2183 cm⁻¹ disappeared, representing C–O bonds were broken. Combined with XRD pattern analysis, NiFe₂O₄ was formed inside the material when the temperature exceeded 300°C. The formation of spinel oxide (AB₂O₄) was confirmed by the absorption peaks in the range of 400–600 cm⁻¹ in the infrared spectra of the materials calcined at 300–500°C.

Table  1.  FT-IR spectra peak positions of FeNi₅-MOF and FeNi₅-MOF-T (T = 200, 300, 400, 450, 500, 550, and 600°C; v stands for wavenumber)

Material	v(H2O) / cm−1	v(–OH) / cm−1	v(benzene) / cm−1	v(C–O) / cm−1	v(phenyl-binomial substitution) / cm−1
FeNi5-MOF	3634	3425, 1586	1500	2185	753, 820
FeNi5-MOF-200	3634	3418, 1571	1500	2181	748, 824
FeNi5-MOF-300	3640	3422, 1589	1500	2188	754, 827
FeNi5-MOF-400	—	3421, 1571	1504	—	752, 822
FeNi5-MOF-450	—	3411, 1570	1505	—	752, 822
FeNi5-MOF-500	—	—	1506	—	752, 822
FeNi5-MOF-550	—	—	—	—	—
FeNi5-MOF-600	—	—	—	—	—

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In order to further explore the evolution process of MOF skeleton during pyrolysis, Raman spectroscopy was performed on FeNi₅-MOF and FeNi₅-MOF-T products at different temperatures, and the results were shown in Fig. 2(c). When the heat treatment temperature was lower than 300°C, the characteristic peaks of Raman did not change, which indicated that the material still maintained a complete frame structure. When the pyrolysis temperature reached more than 400°C, the Raman characteristic peaks at 860, 1134, and 1426 cm⁻¹ disappeared, indicating the disintegration of the MOF skeleton. Moreover, D bands (1350 cm⁻¹) and G bands (1600 cm⁻¹) of carbon structure appeared in the Raman curves of FeNi₅-MOF-550 and FeNi₅-MOF-600, which demonstrated that the material was obviously carbonized when the heat treatment temperature exceeded 550°C.

Based on the above analysis, the morphologies of FeNi₅-MOF and FeNi₅-MOF-450 were investigated by SEM and TEM. As shown in Fig. 3(a), the morphology of FeNi₅-MOF showed a 1D rod-like appearance, which is similar to the classic MIL-53(Fe). This result was consistent with the XRD conclusion, further indicating that FeNi₅-MOF and MIL-53(Fe) have similar microscopic characteristics. The morphology of FeNi₅-MOF-450 (Fig. 3(b)) maintained the rod-like structure of the MOF, however after pyrolysis, the surface of the material was uneven and attached with a layer of carbon matrix, which is conducive to the charge transfer between the electrolyte and the active substances inside the catalyst, and improves the electrocatalytic activity of the sample [38]. At the same time, the outer carbon helped to protect active sites of the inner layer from the corrosion of the electrolyte. TEM images of FeNi₅-MOF-450 (Fig. 3(c)) showed that the diameter of nanoparticles inside the catalyst was about 80 nm. High-resolution transmission electron microscope (HR-TEM) images (Fig. 3(d)) showed that the crystal plane spacing was 0.208 and 0.280 nm, corresponding to the (111) facets of NiFe₂O₄ and the (200) facets of FeNi₃, and the particles were surrounded by carbon layer [39]. The elemental mapping images of FeNi₅-MOF-450 (Fig. 3(e)–(i)) showed that Ni, Fe, C, and O elements were evenly distributed in FeNi₅-MOF-450.

Figure  3.  (a) SEM image of FeNi₅-MOF; (b) SEM image of FeNi₅-MOF-450; (c) TEM and (d) HR-TEM images of FeNi₅-MOF-450; (e–i) elemental mapping images of FeNi₅-MOF-450.

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In order to investigate the chemical bonds and morphology of FeNi₅-MOF-450 surface atoms, XPS characterization analysis was performed on FeNi₅-MOF and FeNi₅-MOF-450. The XPS spectra of FeNi₅-MOF and FeNi₅-MOF-450 are shown in Fig. 4. Firstly, the characteristic peaks of Ni 2p, Fe 2p, C 1s, O 1s in Fig. 4(a) and (b) indicated the composition of elements in FeNi₅-MOF and FeNi₅-MOF-450, which was consistent with the result of elemental mapping. The Ni 2p spectrum of FeNi₅-MOF-450 can be fitted to six characteristic peaks, as shown in Fig. 4(c). Two characteristic peaks exist at 857.4 and 875.1 eV, corresponding to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively, which are attributed to the Ni–O bond. The peaks at 866.4 and 881.9 eV are satellite peaks. Compared with the Ni 2p diagram of FeNi₅-MOF, the Ni 2p characteristic peak of FeNi₅-MOF-450 moves 1.3 eV towards the direction of high binding energy, indicating that there is a strong electronic interaction between Ni and Fe after pyrolysis, which can regulate the electronic environment of the metal active center and thus improve the electrocatalytic performance. The Fe 2p spectrum of FeNi₅-MOF-450 can be fitted into 6 characteristic peaks, as shown in Fig. 4(d). The two characteristic peaks of 706.9 and 720.8 eV correspond to Fe 2p_3/2 and Fe 2p_1/2 of Fe⁰. The two characteristic peaks of 711.4 and 725.6 eV correspond to Fe 2p_3/2 and Fe 2p_1/2 of Fe³⁺, respectively, which attributed to the Fe–O bond. The peaks at 715.6 and 730.86 eV are two satellite peaks [40]. For the C 1s spectrum (Fig. 4(e)), 284.8, 286.7, and 288.4 eV are consistent with the binding energies of C=C, C=O, and O–C=O, respectively [41]. Compared with FeNi₅-MOF, the peak areas of the characteristic peaks C=O and O–C=O in FeNi₅-MOF-450 are obviously reduced, indicating the disintegration of the MOF skeleton. The O 1s spectrum (Fig. 4(f)) shows the presence of characteristic peaks of metal–oxygen (531.2 eV), C–O (532.9 eV), O–C=O (534.6 eV). Compared with FeNi₅-MOF, the M–O bond characteristic peaks in FeNi₅-MOF-450 are significantly decreased, which indicates the formation of alloy material in the material. These results further prove the formation of bimetallic oxides and bimetallic alloys.

Figure  4.  (a) XPS full spectrum of FeNi₅-MOF; (b) XPS full spectrum of FeNi₅-MOF-450; (c) Ni 2p, (d) Fe 2p, (e) C 1s, and (f) O 1s spectrum.

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3.2   Evaluation of OER electrocatalytic activity

OER electrocatalytic activity of all samples was tested in 1.0 M KOH. By comparing the OER activity of MOF precursors with Ni metal different amounts, it was proved that FeNi₅-MOF had the highest electrocatalytic activity (Fig. S2). In addition, RuO₂ was used as a comparison sample to conduct relevant OER activity tests to compare whether FeNi₅-MOF-450 electrocatalyst has higher activity under the same test conditions. As shown in Fig. 5(a), although RuO₂ requires a lower overpotential than FeNi₅-MOF-450 at low current density (less than 44 mA·cm⁻²), FeNi₅-MOF-450 requires a lower overpotential to achieve higher current density beyond 44 mA·cm⁻². As shown in Fig. 5(b), the overpotential of FeNi₅-MOF-450 at 10 mA·cm⁻² (η₁₀) and 100 mA·cm⁻² (η₁₀₀) are 307 mV and 377 mV, respectively, which are significantly lower than those of FeNi₅-MOF (325 mV and 401 mV), FeNi₅-MOF-400 (340 mV and 430 mV), and FeNi₅-MOF-600 (360 mV and 510 mV). As shown in Table 2, the electrocatalytic activity of FeNi₅-MOF-450 is also significantly better than those of other reported MOF-based or FeNi-based materials [35,42–52]. Therefore, FeNi₅-MOF-450 can be served as a promising electrocatalyst for OER. Fig. 5(c) shows the Tafel slope of FeNi₅-MOF and pyrolysis products at different temperatures. Similarly, The Tafel slope of FeNi₅-MOF-450 (56.2 mV·dec⁻¹) is also significantly lower than those of FeNi₅-MOF (77.9 mV·dec⁻¹), FeNi₅-MOF-400 (78.2 mV·dec⁻¹), and FeNi₅-MOF-600 (99.4 mV·dec⁻¹), which indicates that FeNi₅-MOF-450 has faster reaction kinetics than other catalysts in OER process.

Figure  5.  (a) Polarization curve of different catalysts; (b) comparison diagram of overpotentials of different catalysts at current densities of 10 and 100 mA·cm⁻²; (c) Tafel slope of different catalysts; (d) Δj–scan rate diagram of different catalysts; (e) Nyquist diagram of different catalysts; (f) long-term stability testing of FeNi₅-MOF-450.

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Table  2.  Comparison of OER catalytic performance of FeNi₅-MOF-500 and other reported MOF-based or NiFe-based catalyst in the 1.0 M KOH (η₁₀ and η₁₀₀ refer to the overpotentials of catalysts at current densities of 10 and 100 mA·cm⁻²)

Catalysts	Overpotential / mV	Reference
FeNi5-MOF-500	307 (η10), 377 (η100)	This work
CoNi1@C	355 (η10)	[35]
Co2P/CoP@NPGC	340 (η10)	[42]
Ni@Ni-NC	371 (η10)	[43]
Fe0.5Ni0.5Pc-CP	317 (η10)	[44]
CoMo-MI-600	316 (η10)	[45]
C@NiCo12	330 (η10)	[46]
Ni/NC-600	336 (η10)	[47]
Ni(OH)2@CoB	320 (η10)	[48]
Ni2MoN/NF	392.49 (η100)	[49]
Ni-OH/P	490 (η100)	[50]
Ni9S8	420 (η100)	[51]
Ni7Fe3	388 (η100)	[52]

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To analyze the activity of FeNi₅-MOF-450 electrocatalysts, the ECSA values of FeNi₅-MOF-400, FeNi₅-MOF-450, and FeNi₅-MOF-600 were evaluated and compared. Since the ECSA of a catalyst is proportional to its double-layer capacitance (C_dl), the ECSA of a catalyst can be evaluated by measuring the double layer capacitance of the catalyst. Fig. S3 shows the CV curves of FeNi₅-MOF-400, FeNi₅-MOF-450, and FeNi₅-MOF-600 at different sweep speeds. C_dl data are shown in Fig. 5(d). The C_dl value of FeNi₅-MOF-450 is 9.34 mF·cm⁻², which is significantly larger than those of FeNi₅-MOF-400 (6.53 mF·cm⁻²) and FeNi₅-MOF-600 (5.63 mF·cm⁻²). The moderate pyrolysis temperature (450°C) plays an important role in regulating the morphology and structure of catalyst, which increases the ECSA. Moreover, the suitable pyrolysis temperature adjusts the composition ratio between NiFe₂O₄ and FeNi₃, which is helpful for the formation of bimetallic alloys. The interface interaction between NiFe₂O₄ and FeNi₃ may improve the electronic structure of active sites in FeNi₅-MOF-450. Therefore, FeNi₅-MOF-450 exhibits the superior electrochemical activity than FeNi₅-MOF-400 and FeNi₅-MOF-600. In order to further investigate the catalytic kinetics of the catalyst, the EIS of FeNi₅-MOF-450 and other samples were tested. As shown in Fig. 5(e), the charge transfer resistance (R_ct) value of FeNi₅-MOF-450 is obviously lower than those of FeNi₅-MOF-400 and FeNi₅-MOF-600, indicating that the surface charge transfer rate of FeNi₅-MOF-450 electrocatalyst is faster. The stability of catalyst is also an important index to measure the performance of catalyst, which determines whether the catalyst can be used commercially on a large scale. Fig. 5(f) displayed that the stability of FeNi₅-MOF-450 was tested by constant voltage method. The FeNi₅-MOF-450 catalyst can work stably for more than 40 h at a voltage of 320 mV. TEM images and HR-TEM images of FeNi₅-MOF-450 after stability test further demonstrate that FeNi₅-MOF-450 can still maintain good structural stability after working more than 40 h (Fig. S4). Therefore, the above OER test results demonstrate that FeNi₅-MOF-450 is an OER electrocatalyst with great electrocatalytic performance.

4.   Conclusion

In this work, NiFe₂O₄/FeNi₃/C composites were prepared by solvothermal and high temperature pyrolysis. During the pyrolysis process, it is found that metal oxides are formed inside the material first, and when the temperature reaches a certain degree, FeNi alloy is formed inside the material and the metal oxides gradually disappear. Thanks to the synergistic effect between Ni and Fe metals and the interface interaction between NiFe₂O₄ and FeNi₃, the catalyst exhibits high efficiency OER electrocatalytic activity. The catalytic performance of the composite can be optimized by adjusting the metal molar ratio of Fe and Ni and the calcination temperature. Among them, the FeNi₅-MOF-450 electrocatalyst with the optimal performance showed better OER catalytic activity than commercial RuO₂ at high current density. Moreover, FeNi₅-MOF-450 owned a low overpotential of 377 mV at the current density of 100 mA·cm⁻² with Tafel slope of 56.2 mV·dec⁻¹, which was lower than that of commercial RuO₂ electrocatalyst. At the same time, the OER activity of FeNi₅-MOF-450 remained almost unchanged during the 40 h stability test. Therefore, FeNi₅-MOF-450 can perform as an efficient electrocatalyst for water splitting. This work provides a new way to prepare NiFe-based electrocatalysts with high catalytic performance of OER.