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Article

Preparation of S-Doped Ni-Mn-Fe Layered Hydroxide for High-Performance of Oxygen Evolution Reaction

by
Jiefeng Wang
1,
Shilin Li
2,
Yifan Guo
2,
Jiaqi Ding
2 and
Zhi Lu
2,3,*
1
School of Mechanical and Aeronautical Manufacturing Engineering, Anyang Institute of Technology, Anyang 455000, China
2
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China
3
Luoyang Key Laboratory of High Purity Materials and Sputtering Targets, Henan University of Science and Technology, Luoyang 471003, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(7), 825; https://doi.org/10.3390/coatings15070825
Submission received: 19 May 2025 / Revised: 28 June 2025 / Accepted: 12 July 2025 / Published: 15 July 2025
(This article belongs to the Special Issue Advanced Coatings for Enhanced Electrochemical Catalysis and Energy Storage Technologies)
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Abstract

A novel catalyst with a metal sulfide/hydroxide heterostructure was prepared by introducing sulfur ions into NiMnFe layered hydroxide by a simple hydrothermal method, using a series of characterization methods and electrochemical tests to explore the optimal sulfur ion doping amount. The XPS results show that the introduction of sulfur ions leads to a change in metal electron delocalization, which is conducive to the OER procedure. The newly formed metal sulfide can not only improve the conductivity of NiMnFe LDH/NF electrode materials but also enhance the intrinsic catalytic activity of the materials. The electrochemical performance indicated that the S2-NiMnFe LDH/NF catalyst required only 205 mV overpotential to provide a current density of 10 mA−2, and the Tafel slope was only 45.79 mV dec−1. In addition, the large turnover frequency value (1.2614 S−1) reflects the excellent intrinsic activity of the novel catalytic material.

1. Introduction

The depletion of energy has become one of the problems faced by countries around the world. To solve this problem, scientists have undertaken a lot of research to gradually replace traditional energy with new energy [1,2]. As a new energy source, hydrogen energy provides high energy output without pollution, and has attracted wide attention from researchers [3,4]. Electrolysis of water by direct current is one of the methods that is used to produce hydrogen and has been used for industrial hydrogen production. However, it relies on an external supply of electricity [5]. Some researchers have proposed that solar panels can be combined with electrolytic water hydrogen production devices to save energy [6,7]. The hydrogen produced by such a device is more environmentally friendly and sustainable, which helps to move away from dependence on traditional energy sources, reducing carbon emissions and realizing the sustainable use of energy.
However, the rate of hydrogen production in water electrolysis faces a high energy barrier and needs catalysts for it to be lowered. The efficiency of hydrogen production by electrolysis is always constrained by the oxygen evolution reaction (OER). This is because its complex four-electron transfer process harshly limits overall reaction efficiency [8,9]. Therefore, there is a pressing need to explore OER catalysts with low overpotentials, high efficiency, and long-term stability to accelerate their electron transfer and improve reaction efficiency. Currently, precious metal catalysts show the most significant catalytic effect available on the market, such as IrO2 and RuO2, and are usually used as the benchmarks of OER catalysts [10,11]. Limited by the high cost and scarcity of precious metals, the development and utilization of transition metal catalysts with high catalytic effect have become the goal for candidate materials expected by industry. In recent years, transition metal layered hydroxide (LDH) and transition metal sulfide (TMS) nanomaterials have attracted the attention of many scholars due to their unique electronic structures, tunable active sites, and rich surface chemistries [12,13]. In general, they are devoted to boosting the electrochemical performance of catalysts by in situ doping, valence state regulation, and interface engineering regulation [14,15,16]. Recently, researchers have attempted to combine TMSs with LDHs and utilize the synergistic effect to design efficient catalysis for water electrolysis [17,18]. Liu et al. [19] synthesized the NiCo2S4@NiFe LDH heterostructure by a three-step hydrothermal method, which exhibited a low overpotential of 201 mV at a current density of 60 mA cm−2; this demonstrated the excellent electrochemical performance of the heterostructure. Yang et al. [20] prepared FeCoNi hydroxide/sulfide heterostructured catalysts grown on NF by hydrothermal method, and this heterostructure enabled the catalyst to show excellent catalytic performance. It was found by electrochemical test that when the current density reached 10 mA cm−2, FeCoNi hydroxide/sulfide heterostructure catalyst required only 195 mV overpotential. Based on the above research, the unique nanostructure arrays formed by the compound of transition metal layered hydroxides and metal sulfides are an effective strategy for designing catalysts with excellent OER performance.
In this paper, NiMnFe LDH nanosheets were first prepared in situ on NF by a simple one-step hydrothermal method, after which sulfur-doped NiMnFe hydroxide/sulfide (denoted as S-NiMnFe LDH/NF) was prepared by using high-temperature a hydrothermal decomposition of thiourea. The prepared catalyst showed excellent OER performance in the electrochemical test of alkaline electrolyte. When achieving 10 mA cm−2 current density, the S-NiMnFe LDH/NF catalyst exhibited a low overpotential of 205 mV. The performance of S-NiMnFe LDH/NF benefited from the heterostructure of the sulfide and the layered hydroxide. The newly formed metal sulfide can greatly increase the conductivity of the catalyst and stimulated more active sites. Therefore, the preparation of a metal sulfide/hydroxide heterostructure is an effective measure to improve the performance of OER.

2. Materials and Methods

2.1. Materials

Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 98% purity), manganese (II) chloride tetrahydrate (MnCl2·4H2O, 98% purity), and ferric nitrate (III) nonahydrate (Fe(NO3)3·9H2O, 98% purity) were purchased from Shanghai Aladdin Biochemistry Science and Technology Company (Fengxian District, Shanghai, China). Urea (CO(NH2)2), ammonium fluoride (NH4F), thiourea (CH4N2S), and potassium hydroxide (KOH) were purchased from Sinopharm Corp (Huangpu District, Shanghai, China). Nickel foam (NF, 99.9% purity) was purchased from Shenzhen Advanced Wei Metals Co. (Longgang Distict, Shenzhen, China). All chemical reagents were analytically pure. Deionized water was produced by a UPC series ultrapure water dispenser in the laboratory.

2.2. Preparation of NiMnFe LDH/NF Catalysts

Firstly, 4 mmol Ni(NO3)2·6H2O, 1 mmol MnCl2·4H2O, and 1 mmol Fe(NO3)3·9H2O were mixed in 30 mL deionized water, and the solution was stirred continuously until the solution was transparent. Then, 12 mmol urea and 5 mmol ammonium fluoride were added, and stirring was continued for 15 min until all reagents were dissolved. The NF was ultrasonically cleaned by deionized water and 1 M HCl solution in turns to clean the surface at room temperature. Next, a pre-treated NF and the stirred solution were placed in a polytetrafluoroethylene tank. The NiMnFe LDH/NF catalyst was hydrothermally synthesized at 120 °C for 6 h. Finally, the specimens were flushed with water and alcohol and placed in a vacuum oven at 60 °C overnight.

2.3. Preparation of S-NiMnFe LDH/NF Catalysts

Different molar amounts of thiourea (1, 1.5, 2, 2.5, and 3 mmol) were added in five beakers of containing 30 mL deionized water and continuously stirred until the thiourea was completely dissolved. Subsequently, the thiourea solution was placed in a polytetrafluoroethylene tank along with NiMnFe LDH/NF, implementing a hydrothermal reaction at 150 °C for 2 h. The sulfur-doped layered NiMnFe hydroxide catalyst was prepared and was named Sx-NiMnFe LDH/NF for different sulfur contents (x is different thiourea molar amounts).

2.4. Characterization

The phase structure of the NiMnFe LDH/NF and S2-NiMnFe LDH/NF was analyzed using an X-ray diffractometer (XRD, Advance, Bruker, Kyoto, Japan) under Cu Kα radiation at 40 kV. Field emission scanning electron microscopy (FESEM, JSM-7800F, JEOL Ltd., Tokyo, Japan) was used to study the morphology and elemental composition of nanosheets. Transmission electron microscopy (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan) was used to analyze the crystal structures of the samples. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA) was utilized to assess the composition and oxidation states of the elements in the samples.

2.5. Electrochemical Measurements

A CHI660E electrochemical workstation (CH Instruments, Shanghai, China) was used to detect the OER performance of the catalysts. A standard three-electrode system was used in the 1 M KOH solution (pH 13.7), with the working electrodes (NiMnFe LDH/NF and Sx-NiMnFe LDH/NF), reference electrode (Ag/AgCl electrode), and counter electrode (platinum sheet). The electrolysis temperature was room temperature. Linear scanning voltammetry (LSV) curves of the catalysts were tested at a scan rate of 2 mV s−1. Non-Faraday intervals were used to test cyclic voltammetry (CV) curves to obtain electrochemical double-layer capacitance (Cdl). The electric potentials were referenced to the reversible hydrogen electrode (RHE) using the formula
ERHE = EAg/AgCl + 0.1989 V + 0.059 V × PH.
Electrochemical impedance spectroscopy (EIS) was tested between 0.1 Hz and 100 kHz. A current density of 10 mA cm−2 was maintained to test the stability of S2-NiMnFe LDH/NF by chronopotentiometry, and the LSV curves before and after 1000 CV cycles were compared to verify the catalytic activity of S2-NiMnFe LDH/NF.

3. Results and Discussion

3.1. Morphological and Structural Analysis of S-NiMnFe LDH/NF

Figure 1 illustrates a flowchart of the in situ growth of S-NiMnFe LDH nanosheet arrays on an NF substrate using a simple hydrothermal method. Firstly, a layer of Ni-Mn-Fe LDH nanosheet arrays is grown on the NF substrate; the nanosheets are uniformly arranged on the 3D skeleton of NF. In the second step, thiourea is used as a sulfur source for secondary hydrothermal reaction. The sulfur ion (S2−) is released after hydrolysis of thiourea and reacts with metal hydroxide at high temperature to form a thin layer of Ni-Mn-Fe sulfide on the surface of the nanosheet array. After that, the yellowish precursor transforms into a brown-black Ni-Mn-Fe sulfide.
Figure 2 shows the XRD and morphological analysis of the electrode materials. The XRD detection of NiMnFe LDH/NF and S2-NiMnFe LDH/NF are shown in Figure 2a, where the three strong diffraction peaks at 44.6°, 51.9°, and 76.4° correspond to the characteristic peaks of NF [21]. By analyzing the XRD data of NiMnFe LDH/NF, the characteristic peaks of the hydrotalcite-like structure (JCPDF No.89-7111) can be seen. The characteristic peaks located at 11.5°, 23.1°, 31.7°, 34.4°, 38.9°, 46.2°, 59.8°, and 61.1° correspond to the (003), (006), (100), (012), (015), (018), (110), and (113) crystallographic planes of NiMnFe LDH [22,23]. Compared with the standard card, the angle of the NiMnFe LDH peak is shifted slightly, which is attributed to the introduction of Mn and Fe elements; thus, the lattice size of nickel hydroxide is changed to form a jamborite mixture [24]. The XRD detect of S2-NiMnFe LDH/NF reveals that the peaks of the composition phases of NiMnFe LDH are basically unchanged when S-doped, which suggests that the introduction of S does not change the crystal structure of NiMnFe LDH. Sulfur may combine with Ni, Mn, and Fe atoms at the surface of the material rather than entering the interior of the lattice.
Figure 2b shows the scanning electron microscope diagram of NF, which clearly shows that NF has a 3D mesh skeleton structure. This can provide a larger area for the growth of the catalyst and effectively prevent the agglomeration of the catalyst. The inset shows the magnification diagram of the NF skeleton; the surface is uneven, which ensures the catalyst grows firmly on the surface without any binder. Figure 2c shows the morphology of NiMnFe LDH/NF; the nanosheet array grows very densely on the surface of NF and without agglomeration, which gives it excellent electrochemical properties.
Figure 2d–f shows the SEM images of S1-NiMnFe LDH/NF, S2-NiMnFe LDH/NF, and S3-NiMnFe LDH/NF catalysts, where the inset is a high magnification image. The figures show that the nanosheets of S2-NiMnFe LDH/NF catalyst is denser than others, which is one of the reasons for its high electrochemical performance. As can be seen from Figure 2d, when the amount of thiourea is low, the nanosheet appears to thicken and crack. When the amount of thiourea was increased to 2 mmol, the edges of the nanosheets showed cross-linking with each other, which could irritate more active sites that are preferentially exposed at the edge of the nanosheet; thus, it is necessary to match more oxygen-containing intermediates. When the amount of thiourea is further increased, the cross-linked nanosheet morphology disappears and becomes relatively thick.
To further determine the successful introduction of S into the crystal structure of NiMnFe LDH, the energy-dispersive spectrometry (EDS) of the S2-NiMnFe LDH and NiMnFe LDH nanosheets was tested (Figure 3). It can be clearly seen that the elements Ni, Mn, Fe, O, and S are relatively uniformly distributed on the surface of the nanosheets, which ensures good OER performance as well as stability of the catalyst. The atomic ratio of Ni: Mn: Fe: O: S in the EDS data of S2-NiMnFe LDH/NF is shown in Figure S3 of the Supplementary Information. The uniform distribution of S elements confirms the successful doping of S into NiMnFe LDH.
TEM images of NiMnFe LDH/NF and S2-NiMnFe LDH/NF are demonstrated in Figure 4, and the HRTEM images and selected electron diffractograms (SAED) were used to further analyze the effect of S doping on NiMnFe LDH nanosheets. Figure 4a,c are the TEM images of NiMnFe LDH/NF and S2-NiMnFe LDH/NF. In the figure, the nanosheets that oscillated down by ultrasound can be seen stacked together. These nanosheets are transparent under electron beam irradiation, which reflects the ultra-thin characteristics of NiMnFe LDH and S2-NiMnFe LDH nanosheets, consistent with the result of SEM. Figure 4b shows the HRTEM image of NiMnFe LDH/NF, where three lattice fringes can be clearly seen, and the lattice fringes located at 0.161 nm, 0.272 nm, and 0.281 nm correspond to the (110), (101), and (100) crystallographic planes of NiMnFe LDH. Figure 4d shows the HRTEM image of S2-NiMnFe LDH/NF, in which two relatively obvious lattice fringes located at 0.160 nm and 0.230 nm correspond to the (110) and (015) crystal planes of NiMnFe LDH, whereas one lattice fringe located at 0.167 nm conforms to the (003) crystal plane of FeS [25]. The insets in Figure 4b–d show SAED images of NiMnFe LDH/NF and S2-NiMnFe LDH/NF, respectively, and it can be clearly seen that both have three distinct diffraction loops, which proves the polycrystalline nature of NiMnFe LDH and S2-NiMnFe LDH.
Table 1 demonstrates the average pore volume and average specific surface area of the samples, the N2 adsorption-desorption isotherms and pore size distribution of the samples are shown in Figure S1 of the Supplementary Information. As shown in Table 1, when increasing the content of sulfur, the specific surface area and porosity of the material rise first and then decrease; S2-NiMnFe LDH/NF catalyst material has the largest specific surface area (8.189 m2/g) and the highest porosity (0.050 cm3/g). Therefore, suitable sulfur doping can provide larger specific surface area and higher porosity for NiMnFe LDH/NF, which can supply more active sites to the catalysts as well as promote the rapid diffusion of ions.
To further determine the effect of S element on NiMnFe LDH, XPS tests were performed. Figure 5a illustrates the XPS spectra of NiMnFe LDH/NF and S2-NiMnFe LDH/NF, where the presence of Ni, Mn, Fe, O, and S can be detected. Compared with NiMnFe LDH/NF, the signal of S is detected in the S 2p spectrum of S2-NiMnFe LDH/NF (Figure 5b), which further indicates that S is successfully introduced into the catalyst. Analysis of the spectrum of S 2p shows that the peak at the binding energy of 161.2 eV indicates the presence of low-oxidation-state sulfur ions in the catalyst. The peak at 162.7 eV indicates the presence of metal sulfide, and the presence form of S is mostly expressed as S2-. The peak of binding energy at 169.4 eV is a characteristic signal of the oxidation state of nickel–iron sulfide appears, which indicates some connection of bonds between the Ni, Fe, and S atoms [20,26].
By observing the Ni 2p spectra of S2-NiMnFe LDH/NF in Figure 5c, the two main peaks at 857.5 eV and 875.6 eV correspond to Ni 2p3/2 and Ni 2p1/2, with two satellite peaks at 863.5 eV and 880.9 eV [27]. The two peaks at 856.5 eV and 873.9 eV correspond to Ni2+, while the two peaks at 858.7 eV and 875.9 eV are attributed to Ni3+. After doping sulfur, it can be found that the proportion of Ni2+ increases as shown in Table S1 of the Supplementary Information, which may be caused by the less electronegative S atom replacing a portion of O atom, thereby transferring electrons to the Ni atom [28]. When Ni2+/Ni3+ increases, according to 2Ni3+ → 2Ni2+ + OV, more oxygen vacancy (OV) will be generated in the catalyst, which will play a synergistic role with nearby metals to reduce the reaction energy barrier and improve the electrocatalytic performance of the catalyst [29]. The spectrogram of Mn 2p (Figure 5d) shows that the Mn elements are mainly Mn2+, Mn3+, and Mn4+ present in NiMnFe LDH/NF and S2-NiMnFe LDH/NF. It was found that the proportion of Mn3+/Mn4+ decreases in NiMnFe LDH when doped with S, which indicates that the Mn ion transitions to high valence. The increase in Mn4+ in the intercalation of LDH will affect the overall electronic structure of the material, thereby increasing the electrical conductivity of the catalyst [30].
Figure 5e shows the Fe 2p spectrum. In the Fe 2p spectrum of S2-NiMnFe LDH/NF, the peaks at 711.8 eV and 716.3 eV are attributed to Fe2+ and Fe3+. The peaks at 706.5 eV reveal the presence of Fe-S bonds in this structure, and some Fe atoms are in the low oxidation state Feδ+ [26,31]. After doping S, the peak of Fe 2p spectrum has no obvious change, indicating that S doping has no significant effect on the oxidation state of Fe elements. The O 1 s spectrum of S2-NiMnFe LDH/NF (Figure 5f) has three peaks located at a binding energy of 531.7 eV, 532.9 eV, and 534.2 eV, corresponding to metal–oxygen–metal bonds (M-O-M), metal–hydroxide bonds (M-OH), and interlayer water molecules, respectively [32]. Comparing with the O 1 s spectra of NiMnFe LDH/NF, it is obvious that doping with S leads to an increase in M-O-M bonds, which means more lattice oxygen is produced, which is conducive to the process of OER.

3.2. Oxygen Evolution Reaction Performance Measurement

Using a standard three-electrode system to test the electrocatalytic oxygen evolution performance of NiMnFe LDH/NF and Sx-NiMnFe LDH/NF catalysts in 1 M KOH solution, Figure 6 reveals the corresponding data of the electrode materials. Figure 6a shows the LSV polarization curves of the catalysts after having 95% IR compensation, in which the S2-NiMnFe LDH/NF has the best OER activity. The introduction of sulfur alters the local coordination environments of Ni, Mn, and Fe, lowers the metal d-band centers, and optimizes the adsorption energies of the intermediates (*OH, *O, and *OOH), thus enhancing the catalytic activity of the materials [33]. This demonstrates that proper sulfidation can indeed improve the OER performance of LDHs.
Figure 6b shows the overpotential required for the catalyst at a current density of 10 mA cm−2, from which the S2-NiMnFe LDH/NF possesses the lowest overpotential (205 mV). It is lower than that of NiMnFe LDH/NF (241 mV), S1-NiMnFe LDH/NF (223 mV), S1.5-NiMnFe LDH/NF (222 mV), S2.5-NiMnFe LDH/NF (225 mV), and S3-NiMnFe LDH/NF (230 mV). By comparison, it was found that appropriate vulcanization can decrease the overpotential of NiMnFe LDH/NF catalyst. Figure 6c shows the Tafel slope of the catalyst, which is obtained by transforming the LSV curve. The S2-NiMnFe LDH/NF catalyst shows a low Tafel slope of 45.79 mV dec−1. It is lower than that of NiMnFe LDH/NF (51.07 mV dec−1), S1-NiMnFe LDH/NF (48.71 mV dec−1), S1.5-NiMnFe LDH/NF (48.19 mV dec−1), S2.5-NiMnFe LDH/NF (53.25 mV dec−1), and S3-NiMnFe LDH/NF (46.28 mV dec−1), which suggests that the S2-NiMnFe LDH/NF catalyst has faster reaction kinetics.
During the OER process, the sulfur-doped NiMnFe LDH surface is seemed undergoes dynamic remodeling to form highly active hydroxyl oxides (NiMnFe-OOH) under the applied voltage and strong oxidizing environment [34,35]. Sulfur doping lowers the activation energy barrier for surface remodeling, accelerates the remodeling process, and increases the proportion of high valence metals (Ni3+/Ni4+, Mn3+/Mn4+, and Fe3+/Fe4+) in the remodeled hydroxyl oxides to enhance the catalytic activity of the catalysts [36,37]. The formation of Ni-Mn-Fe-S bonds modulates the electron density, raises the Fermi energy level of the material, and enhances the electron transfer capability as well as weakens the strong adsorption of *OOH (avoiding “poisoning”), thus accelerating O2 release [38].
Figure 6d shows the EIS of the catalysts, from which the S2-NiMnFe LDH/NF catalysts have the highest slopes in the low-frequency region, which indicate the faster ion transport rate at the interface between the S2-NiMnFe LDH/NF electrode and the electrolyte [39]. This is due to the doping of sulfur ions. On the one hand, sulfur acts as an “electron bridge” to accelerate the charge redistribution between Ni/Mn/Fe, regulating the electronic structure of Ni-Mn-Fe tri-metal, and the obtained metal sulfide improves the conductivity and ion transport rate of the material. Moreover, the substrate NF is subjected to in situ sulfidation, and some sulfides are formed on the NF surface to form a conductive network together with S-NiMnFe LDH, which enhances the electron transport efficiency and further improves the electrical conductivity.
The relationship between the electrochemically active surface area (ECSA) and the Cdl of the catalyst can be estimated by the formula ECSA = Cdl/Cs. From the previous report, the specific capacitance Cs in 1 M KOH solution is about 0.04 mF cm−2, which indicates that ECSA is positively proportional to Cdl [40]. The CV curves were obtained by testing in non-Faraday intervals (1.2072 V vs. RHE–1.3072 V vs. RHE) and tested at different scanning speeds (10, 20, 30, 40, and 50 mV s−1). The Cdl values of the catalysts are shown in Figure 6e in which the ECSA of the NiMnFe LDH/NF catalyst after vulcanization was significantly increased. This proves that the doping of sulfur ions excited more active sites; a layer of metal sulfide covering the NiMnFe LDH nanosheets contributes to increasing the active specific surface area of the material.
The good performance of the metal-based catalysts was attributed to their electronic structure [41,42]. The electronic structure of metal-based catalysts is mainly related to the valence state, spin state, eg orbital of metals compound, and the covalently bonds between the metals and O element [43,44]. During the catalytic reaction in solution, the electron transfer capability in the reaction is determined by the number of charge carriers, band structure at the Fermi level, and the charge distribution of the metal ions [45,46]. The d-band theory suggests that the shift in the d-band center to Fermi level can improve the binding between the catalyst and the adsorbates [47]. Some methods have been confirmed to regulate the electronic activity and improve the intrinsic activity of catalysts, such as phase transitions [48], heterostructures [49], and non-metallic atom doping [50]. In particular, the doping of S and P elements have showed a promising effect [41,49,50]. These methods help to narrow the bandgap of the catalysts and contribute to increasing their conductivity, which lead to improved reaction kinetics [50]. In this study, the microelectronic structure of the metal hydroxides is changed by the interaction between the doped metal (Mn) and other transition metal atoms (Ni, Fe), and the overpotential for water splitting can be reduced evidently [51]. The doping of S contributes to form heterostructures in the electrocatalyst, which can provide more diffusion channels for the OER, improving electron transfer and mass structure [52].
The turnover frequency (TOF) of an electrode reflects its intrinsic catalytic activity; it refers the conversion rate of reaction molecules at a single active site. A larger TOF value indicates better intrinsic catalytic activity of the electrode. To evaluate the intrinsic catalytic activity of NiMnFe LDH/NF and Sx-NiMnFe LDH/NF, the TOF values of the samples were compared. As can be seen in Figure 6f and Table 2, the TOF value of the S2-NiMnFe LDH/NF at an overpotential of 300 mV (1.2614 S−1) is greater than that of NiMnFe LDH/NF (0.5388 S−1), S1-NiMnFe LDH/NF (0.5203 S−1), S1.5-NiMnFe LDH/NF (0.9733 S−1), S2.5-NiMnFe LDH/NF (0.2943 S−1), and S3-NiMnFe LDH/NF (0.4459 S−1). Therefore, the S2-NiMn LDH/NF catalysts have better intrinsic catalytic activity compared to other catalysts. The TOF value results show that the introduction of suitable sulfur atoms to replace some of the oxygen sites or form vacancies in the material can enhance the intrinsic activity of the material.
As shown in Table 3, some recently reported metal sulfide catalysts are compared. The special nanosheets morphology of S2-NiMnFe LDH/NF provide considerable active sites for OER and contributes to the competitive overpotential under 10 mA cm−2 than most of the reported works. According to the overpotentials listed in the table, the S2-NiMnFe LDH/NF has excellent OER performance.
A qualified catalyst materials should not only have excellent electrochemical properties but also good stability. The stability of S2-NiMnFe LDH/NF catalyst materials was verified by two methods. Figure 7a illustrates the potential change in S2-NiMnFe LDH/NF catalysts tested in 1 M KOH solution at a current density of 10 mA cm−2. The results demonstrated that the potential change in S2-NiMnFe LDH/NF remained essentially constant after maintaining a constant current test for 100 h with an activity decay of <5%, which was attributed to the structural stabilizing effect of Fe/Mn as well as the mechanical support of NF.
The inset in the figure shows the comparison of the OER performance of the tested S2-NiMnFe LDH/NF catalysts before and after 1000 CV cycles. The results showed that the OER performance of S2-NiMnFe LDH/NF catalysts did not change much after CV cycling. XRD of S2-NiMnFe LDH/NF after 100-hour chronopotentiometry test is shown in Figure S2 of the Supplementary Information, whose composition phases are same as Figure 2a. Figure 7b demonstrates the surface morphology of the S2-NiMnFe LDH/NF after the stability test. It can be visualized that the nanosheets did not collapse or fall off after the stability test, which further validates the excellent stability of the material.

4. Conclusions

In summary, the S-NiMnFe LDH/NF are successfully prepared by hydrothermal vulcanization NiMnFe LDH/NF. It is found that appropriate vulcanization can optimize the active center of NiMnFe LDH and promote the adsorption and desorption of intermediates. Doping of non-metallic S atoms can adjust the electronic structure or spin states of Ni, Mn, and Fe—resulting in a new charge distribution—and promote the activation of the reactant molecules. This leads to a reduction in the Gibbs free energy of the reaction intermediates, more in favor of charge transfer and chemical bond breaking. The OER performance test shows that in 1 M KOH solution, to achieve a current density of 10 mA cm−2, only the overpotential of 205 mV needs to be overcome. The Tafel slope is only 45.79 mV dec−1. It is proved that proper sulfurization of layered hydroxides is a helpful strategy to enhance the performance of OER. The S-NiMnFe LDH/NF is expected to be a high-performance catalyst for OER.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15070825/s1, Figure S1. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of the samples. Figure S2. XRD of S2-NiMnFe LDH/NF after 100-hour chronopotentiometry test. Figure S3. The atomic ratio of Ni: Mn: Fe: O: S in the EDS data of S2-NiMnFe LDH/NF. Table S1. The ratios of metal ion in NiMnFe LDH/NF and S2-NiMnFe LDH/NF.

Author Contributions

Conceptualization: J.W.; methodology: J.W. and Z.L.; software: S.L. and Z.L.; validation: S.L.; formal analysis: S.L.; investigation: J.W., S.L., Y.G., J.D. and Z.L.; data curation: S.L.; writing—original draft preparation: J.W.; writing—review and editing: Z.L.; supervision: Z.L.; project administration: J.W.; funding acquisition: J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Graduate Education Reform Project of Henan Province (No. 2023SJGLX096Y).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of S-NiMnFe LDH growing in situ on nickel foam.
Figure 1. Schematic diagram of S-NiMnFe LDH growing in situ on nickel foam.
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Figure 2. (a) XRD patterns of NF, NiMnFe LDH/NF, and S2-NiMnFe LDH/NF. Morphology of the (b) NF, (c) NiMnFe LDH/NF, (d) S1-NiMnFe LDH/NF, (e) S2-NiMnFe LDH/NF, and (f) S3-NiMnFe LDH/NF.
Figure 2. (a) XRD patterns of NF, NiMnFe LDH/NF, and S2-NiMnFe LDH/NF. Morphology of the (b) NF, (c) NiMnFe LDH/NF, (d) S1-NiMnFe LDH/NF, (e) S2-NiMnFe LDH/NF, and (f) S3-NiMnFe LDH/NF.
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Figure 3. EDS of (a1–a6) S2-NiMnFe LDH/NF and (b1–b5) NiMnFe LDH/NF.
Figure 3. EDS of (a1–a6) S2-NiMnFe LDH/NF and (b1–b5) NiMnFe LDH/NF.
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Figure 4. TEM images of (a) NiMnFe LDH/NF and (c) S2-NiMnFe LDH/NF. High-resolution images of (b) NiMnFe LDH/NF and (d) S2-NiMnFe LDH/NF with selected electron diffraction images.
Figure 4. TEM images of (a) NiMnFe LDH/NF and (c) S2-NiMnFe LDH/NF. High-resolution images of (b) NiMnFe LDH/NF and (d) S2-NiMnFe LDH/NF with selected electron diffraction images.
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Figure 5. (a) XPS spectra of NiMnFe LDH/NF and S2-NiMnFe LDH/NF. High-resolution XPS (b) S 2p spectra, (c) Ni 2p spectra, (d) Mn 2p spectra, (e) Fe 2p spectra, and (f) O 1 s spectra.
Figure 5. (a) XPS spectra of NiMnFe LDH/NF and S2-NiMnFe LDH/NF. High-resolution XPS (b) S 2p spectra, (c) Ni 2p spectra, (d) Mn 2p spectra, (e) Fe 2p spectra, and (f) O 1 s spectra.
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Figure 6. (a) LSV polarization curves and (b) corresponding overpotential at a current density of 10 mA cm−2. (c) Tafel slope curve. (d) Electrochemical impedance spectroscopy. (e) Double-layer capacitances and (f) TOF value of the catalysts at 300 mV overpotential.
Figure 6. (a) LSV polarization curves and (b) corresponding overpotential at a current density of 10 mA cm−2. (c) Tafel slope curve. (d) Electrochemical impedance spectroscopy. (e) Double-layer capacitances and (f) TOF value of the catalysts at 300 mV overpotential.
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Figure 7. (a) Chronopotentiometry curve of S2-NiMnFe LDH/NF at a constant current density of 10 mA cm−2, and the inset compares the LSV curve before and after the 1000 CV cycle. (b) The SEM image of S2-NiMnFe LDH/NF after stability test.
Figure 7. (a) Chronopotentiometry curve of S2-NiMnFe LDH/NF at a constant current density of 10 mA cm−2, and the inset compares the LSV curve before and after the 1000 CV cycle. (b) The SEM image of S2-NiMnFe LDH/NF after stability test.
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Table 1. The average pore volume and specific surface area of samples.
Table 1. The average pore volume and specific surface area of samples.
SampleAverage Pores Volume (cm3/g)Average Specific Surface Area (m2/g)
NiMnFe LDH/NF0.0426.382
S1-NiMnFe LDH/NF0.0416.271
S2-NiMnFe LDH/NF0.0508.189
S3-NiMnFe LDH/NF0.0345.412
Table 2. The electrocatalysis activity of Sx-NiMnFe LDH/NF.
Table 2. The electrocatalysis activity of Sx-NiMnFe LDH/NF.
SampleOverpotential (mV)
at 10 mA cm−2		Tafel (mV dec−1)TOF (S−1)
NiMnFe LDH/NF24151.070.5388
S1-NiMnFe LDH/NF22348.710.5203
S1.5-NiMnFe LDH/NF22248.190.9733
S2-NiMnFe LDH/NF20545.791.2614
S2.5-NiMnFe LDH/NF22553.250.2943
S3-NiMnFe LDH/NF23046.280.4459
Table 3. The overpotential of catalysts at current density of 10 mA cm−2 has been reported recently.
Table 3. The overpotential of catalysts at current density of 10 mA cm−2 has been reported recently.
CatalystCurrent Density (mA cm−2)Overpotential (mV)Reference
Co3-Ni7Fe3 LDH/TGF10252[12]
S-FeCoNiCuAlV20362 [13]
ZnFe LDH@NiCoS/NF10285[17]
NM/NCS/NS/NF 30282 [18]
NiCo2S4@NiFe LDH60201[19]
(FeCoNi)OH-S/NF10195[20]
FeCoNiSx/NF10231[53]
CuO/CuS10270[54]
Ni2FeS@NSC10247[55]
CoxFe1-x-AO10267[56]
CNTs/N-Cu2S10280[57]
Mo-Fe-Ni-S/NF10230[58]
Fe-NiS2@NC10255[59]
CoNi2S4@CoS2/NF10259[60]
NiFe-S@CNT10246[61]
NiFe LDH@CoS10259[62]
FeCoS2/XC-7210230[63]
NiFe/Co9S8/CC10219[64]
V-Ni3S2@NiFe LDH10209[65]
S2-NiMnFe LDH/NF10205This work
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Wang, J.; Li, S.; Guo, Y.; Ding, J.; Lu, Z. Preparation of S-Doped Ni-Mn-Fe Layered Hydroxide for High-Performance of Oxygen Evolution Reaction. Coatings 2025, 15, 825. https://doi.org/10.3390/coatings15070825

AMA Style

Wang J, Li S, Guo Y, Ding J, Lu Z. Preparation of S-Doped Ni-Mn-Fe Layered Hydroxide for High-Performance of Oxygen Evolution Reaction. Coatings. 2025; 15(7):825. https://doi.org/10.3390/coatings15070825

Chicago/Turabian Style

Wang, Jiefeng, Shilin Li, Yifan Guo, Jiaqi Ding, and Zhi Lu. 2025. "Preparation of S-Doped Ni-Mn-Fe Layered Hydroxide for High-Performance of Oxygen Evolution Reaction" Coatings 15, no. 7: 825. https://doi.org/10.3390/coatings15070825

APA Style

Wang, J., Li, S., Guo, Y., Ding, J., & Lu, Z. (2025). Preparation of S-Doped Ni-Mn-Fe Layered Hydroxide for High-Performance of Oxygen Evolution Reaction. Coatings, 15(7), 825. https://doi.org/10.3390/coatings15070825

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