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Article

Improved HER/OER Performance of NiS2/MoS2 Composite Modified by CeO2 and LDH

by
Hao Li
1,†,
Feng Chen
2,†,
Xinyang Wu
3,
Dandan Wang
3,
Yongpeng Ren
3,4,* and
Yaru Li
3,4,*
1
Henan Key Laboratory of Green Building Materials Manufacturing and Intelligent Equipment, Luoyang Institute of Science and Technology, Luoyang 471023, China
2
School of Environmental and Biological Engineering, Henan University of Engineering, Zhengzhou 451191, China
3
Henan Key Laboratory of High-Temperature Metal Structural and Functional Materials, National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials, Henan University of Science and Technology, Luoyang 471000, China
4
Longmen Laboratory, Luoyang 471000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Materials 2024, 17(19), 4876; https://doi.org/10.3390/ma17194876
Submission received: 9 September 2024 / Revised: 25 September 2024 / Accepted: 30 September 2024 / Published: 4 October 2024
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Abstract

:
In recent years, there has been significant interest in transition-metal sulfides (TMSs) due to their economic affordability and excellent catalytic activity. Nevertheless, it is difficult for TMSs to achieve satisfactory performance due to problems such as low conductivity, limited catalytic activity, and inadequate stability. Therefore, a catalyst with a heterostructure constituted of a nickel–iron-layered double hydroxide, nickel sulfide, molybdenum disulfide, and cerium dioxide was designed. At the current density of 10 mA cm−2 in an alkaline solution, the catalyst exhibits a HER overpotential of 116 mV. In addition, an overpotential of 235 mV@150 mA cm−2 was displayed for OER. The catalyst showed a good retention rate (94.7% for HER, 98.6% for OER) after 160 h stability tests. The excellent electrochemical performance is attributed to the following points: 1. The self-supporting three-dimensional hierarchical structure provides abundant sites, fast ion diffusion channels, and electron transfer paths, and ensures structural stability. 2. The strong interfacial electron interaction between Ni3S2/MoS2 heterojunction and NiFe-LDH improves the OER reaction kinetics. 3. The Ce3+ and oxygen vacancies in CeO2 promote the dissociation of H2O and promote the HER reaction kinetics. This approach paves the way for developing highly efficient electrocatalysts for various electrochemical applications.

Graphical Abstract

1. Introduction

Nowadays, as industrial growth accelerates, the issues of energy scarcity and environmental pollution are escalating at an alarming rate. Due to its exceptional attributes such as high energy density, sustainability, cleanliness, and efficient heat release, hydrogen is widely regarded as a highly promising energy source [1,2]. Hydrogen production from water electrolysis is an effective method to produce hydrogen fuel, but the two half-reactions involved (hydrogen evolution reaction (HER), oxygen evolution reaction (OER)) face a huge reaction barrier, so Pt, Ir, and Ru-based precious metal catalysts are needed to accelerate the reaction kinetics [3]. The high cost and scarcity of noble metal-based materials limit the large-scale industrial application of electrolyzed water. Therefore, the design of non-precious metal catalysts with low cost, high activity, and strong stability has become the focus of research [4,5].
At present, the transition metal nickel sulfide/molybdenum disulfide composite (NiS2/MoS2) exhibits high activity for both HER and OER, and has become one of the most promising bifunctional electrocatalysts [6,7]. However, NiS2/MoS2 faces the problems of poor intrinsic activity, low conductivity, and insufficient stability of noble metals. Therefore, it is very important to develop efficient and stable transition metal matrix composite catalysts [8,9]. For example, Xu et al. synthesized a self-supporting heterostructure based on MoS2-CoFeLDH as an electrocatalyst for the complete decomposition of water in alkaline media. At 10 mA cm−2, the HER and OER of MoS2-CoFeLDH can reach 100 mV and 216 mV, respectively. The bifunctional catalyst requires a voltage of 1.55 V for total water decomposition and remains stable within 48 h. The strong electronegativity of LDH is beneficial to the adsorption of H, and the strong conductivity of MoS2 is beneficial to the desorption of H2 molecules [10,11,12]. Due to the strong adsorption capacity of LDHs for hydroxyl species, the addition of MoS2 promoted the migration of hydroxyl species, thus promoting the OER of the anode [13,14]. In their study, Li et al. synthesized a crystal-amorphous heterogeneous electrocatalyst, MoS2/NiVFe, by modifying MoS2 with a NiVFe-LDH using a two-step hydrothermal method. In alkaline and neutral media, at 10 mA cm−2, the HER and OER overpotentials are 78 and 141 mV, respectively, and the Tafel Slopes are 61.51 and 80.2 mV·dec−1, respectively. Crystal-amorphous heterostructures have been shown to provide a large electrochemically active surface area to enhance active sites and accelerate electron transfer. XPS and theoretical calculations reveal that the strong electron modulation effect of electron transfer from NiVFe-LDH to MoS2 leads to the adsorption of -OH at the high valence Ni site and the adsorption of -H at the electron-rich S site, which significantly reduces the potential barrier of water splitting [15,16,17,18].
In addition, doping rare earth elements is a viable approach to modify electron density and enhance the abundance of active sites. The unique 4f electronic structure of rare earth elements enables the coexistence of Ce3+ and Ce4+ oxidation states, facilitating surface oxygen ion exchange. The CeO2, known for its ample oxygen vacancies and strong affinity for oxygen-containing species, exhibits significant potential in electrocatalysis applications [19,20,21,22]. Tae-Hwan Oh developed a well-interconnected Ni3S2-MoS2-CeO2 nanostructure as a multifunctional electrocatalyst. Under alkaline conditions, the electrode exhibits significant oxygen evolution reaction (OER) performance, with a low overpotential (206.03 mV at 10 mA cm−2) and Tafel slope (40.19 mV·dec−1). The material also showed excellent stability, and the durability of the electrocatalyst was verified by chronopotentiometry. The inherent stability of CeO2 facilitates a robust and intimate electrochemical coupling with NiOOH. The combined presence of CeO2, MoS2, and NiOOH in the catalytic system leads to a synergistic interaction, resulting in improved catalytic performance [23,24,25]. Wu et al. developed and designed Ni3S2-CeO2 hybrid nanostructures as OER electrocatalysts by the electrodeposition method. Under 1.0 M KOH conditions, the Ni3S2-CeO2 electrode demonstrated a remarkable current density of 20 mA cm−2 with a low overpotential of 264 mV. This represents a decrease of 92 mV compared to the Ni3S2 alone. Additionally, the Tafel slope was measured at 146 mV·dec−1. The strong interfacial interaction between Ni3S2 and CeO2 greatly facilitates electron transfer within the Ni3S2 nanosheets, leading to enhanced water oxidation activity [26,27,28].
Based on the above considerations, LDH was introduced into NiS2/MoS2 transition metal sulfide to improve the OER performance of the composite material, and CeO2 modification was used to further promote the kinetics of hydrogen evolution and oxygen evolution reaction, so as to achieve low cost, high activity, and strong stability of the electrocatalyst to promote the performance of water electrolysis [29,30].
Therefore, the CeO2-modified self-supporting hierarchical structure (CeO2-LDH/Ni3S2/MoS2) was designed and prepared on a three-dimensional nickel foam substrate by a two-step hydrothermal method for efficient OER and HER, as illustrated in Scheme 1. The CeO2-LDH/Ni3S2/MoS2 catalyst exhibits an overpotential of 116 mV at a current density of 10 mA cm−2 for HER. For the OER, it achieves an overpotential of 235 mV at 150 mA cm−2. The i-t curve of HER and OER did not decay significantly after 160 h of operation, showing good electrocatalytic stability. Ce doping at the optimal level in LDH structures can enhance catalytic activity by adjusting the electronic structure and local chemical binding environment, resulting in enhanced catalytic activity. In addition, the heterogeneous interface constructed by CeO2 modification promotes the structural evolution and charge transfer of the catalyst. Furthermore, LDHs’ electronic structure can be modulated and the change transfer during catalysis can be accelerated via redox interactions between Ce3+ and Ce4+. The Ni3S2/MoS2-NiFe-LDH composites facilitate strong interfacial charge transfer, leading to enhanced charge redistribution and improved reaction kinetics. Ce3+ and oxygen vacancies in CeO2 can promote the dissociation of H2O. CeO2 becomes the active site of HER and OER, and greatly improves the activity of hydrogen and oxygen evolution.

2. Experiment

2.1. Nickel Foam Pretreatment

A certain size of nickel foam (NF, 2 × 2 cm2) was cut, and the NF was ultrasonically cleaned in acetone and 3 M HCl for 15 min, in order to remove the grease and oxide layer on its surface. The nickel foam was washed with deionized water and anhydrous ethanol many times and then dried at 60 °C for 12 h.

2.2. Preparation of Ni3S2/MoS2

Ni3S2/MoS2 was prepared by one-step hydrothermal sulfidation. An amount of 0.0968 g of Na2MoO4·2H2O (Aladdin, Shanghai, China) and 0.0601 g of TAA (Aladdin, Shanghai, China) were put in deionized water and magnetically stirred for 30 min to obtain a clear and transparent mixed solution. The pretreated nickel foam was placed in the above solution and transferred to a stainless steel autoclave (Aladdin, Shanghai, China). The hydrothermal temperature was set at 200 °C in a blast drying oven for 22 h. After the reactor was cooled to room temperature, the nickel foam was washed several times with deionized water and vacuum-dried overnight. The product was named Ni3S2/MoS2.

2.3. Preparation of CeO2-Ni3S2/MoS2

The synthesis procedure of CeO2-Ni3S2/MoS2 was similar to that of Ni3S2/MoS2. On this basis, 0.0174 g of Ce (NO3)3·6H2O2 (Aladdin, Shanghai, China) was added.

2.4. Synthesis of CeO2-LDH/Ni3S2/MoS2

CeO2-LDH/Ni3S2/MoS2 catalysts were prepared by a two-step hydrothermal method. Ni (NO3)2·6H2O (0.5452 g) (Aladdin, Shanghai, China), Fe(NO3)3·9H2O (0.2525 g) (Aladdin, Shanghai, China), NH4F (0.046 g) (Aladdin, Shanghai, China), and urea (0.375 g) (Aladdin, Shanghai, China) were dissolved in 25 mL deionized water and stirred by magnetic force for 30 min until the solution was clear and green. The CeO2-Ni3S2/MoS2 obtained in the third step was transferred together with the mixed solution to a 50 mL stainless steel autoclave with polytetrafluoroethylene lining, and reacted at 120 °C for 6 h in a blast drying oven. The product was named CeO2-LDH/Ni3S2/MoS2.
The preparation method of LDH/Ni3S2/MoS2 is the same as that of CeO2-LDH/Ni3S2/MoS2, except that Ce(NO3)3·6H2O was not added during the first hydrothermal step.

2.5. Material Characterization

The samples’ crystal structure was characterized using X-ray diffraction (XRD) with a Bruker X instrument (Bruker, Karlsruhe, Germany). The valence state and surface composition of the samples were simultaneously analyzed using X-ray photoelectron spectroscopy (XPS) with an ESCALAB 250 Xi instrument (Thermo Fisher, Waltham, MA, USA). The morphology of the samples was examined using field emission scanning electron microscopy (SEM) (Thermo Fisher, MA, USA) and transmission electron microscopy (TEM) with a JSM-IT800 and FEI Talos F200 s instrument (Thermo Fisher, MA, USA), respectively.

2.6. Electrode Preparation and Electrochemical Measurement

All electrochemical tests were performed on a Princeton electrochemical workstation with a three-electrode system (Aladdin, Shanghai, China) (reference electrode Ag/AgCl, graphite rod as counter electrode, working electrode). The catalyst supported or coated with nickel foam was used as the working electrode. The preparation of nickel foam-supported noble metal standard electrode is as follows: Pt/C or RuO2 powder samples were weighed and dispersed in 1 mL anhydrous ethanol, ultrasonicated for 10 min, and then 50 μL Nafion solution (5 wt%) was added and ultrasonicated for 25 min to obtain uniform black ink. These inks were evenly dropped on the NF and dried with a digital infrared baking lamp. All electrochemical tests were carried out in 1.0 M KOH (pH = 13.7) solution. All data in this paper are converted into data relative to the reversible hydrogen electrode according to the ERHE = EAg/AgCl0.197 + 0.0592 × pH–iR formula [31]. The OER and HER were characterized using linear sweep voltammetry (LSV) in a 1 M KOH solution at a scan rate of 5 mV s−1. The Tafel diagram is obtained by the equation ŋ = a + blog(j) [32]. The durability of the material was tested by chronoamperometry. In order to evaluate the electrochemical active surface area (ECSA) of the catalyst, the non-faradaic interval was selected, and the CV curves were drawn at the scanning rates of 60, 70, 80, 90, and 100 mV s−1, and the Cdl value was obtained by linear fitting. Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 100 kHz to 0.01 Hz, using an AC voltage amplitude of 10 mV. The stability of the catalyst was tested by chronoamperometry (i-t curve) at a constant potential.

3. Results and Discussion

3.1. Structure and Morphology Characterization

The synthesis process of CeO2-LDH/Ni3S2/MoS2 composites is shown in Scheme 1. In the first step, Ni3S2/MoS2 was prepared by one-step hydrothermal vulcanization using nickel foam as the nickel source, Na2MoO4·2H2O as the molybdenum source, and thioacetamide as the sulfur source. On this basis, CeO2-Ni3S2/MoS2 was prepared by the hydrothermal method by adding Ce(NO3)·6H2O. CeO2-LDH/Ni3S2/MoS2 was prepared by the hydrothermal method by adding Ni(NO3)2·6H2O, Fe(NO3)3·9H2O and urea.
The structure of the composites was analyzed by XRD. As shown in the XRD pattern of Figure 1, the strong diffraction peaks at 44.4°, 51.8°, and 76.3° on the nickel foam (NF) substrate correspond to the (111), (200), and (220) crystal planes of NF, respectively. In the first step of the hydrothermal process, NF was inevitably sulfurized. The diffraction peaks at 21.8°, 31.1°, 37.7°, 44.4°, 49.8°, and 55.2° corresponded to the (101), (110), (003), (202), (113), and (122) crystal planes of Ni3S2 (JCPDS#No.44-1418), respectively. After the second step of hydrothermal treatment, new diffraction peaks appeared at 11.4°, 23.0°, 34.4°, 39.0°, 46.0°, 59.9°, and 61.3°, corresponding to the (003), (006), (012), (015), (018), (110), and (113) crystal planes of NiFe-LDH (JCPDS#No.40-0215). In addition, no diffraction peaks of CeO2 and MoS2 were observed in the XRD pattern, which may be due to the low content or the aggregation of MoS2 naturally accumulated during the hydrothermal process to a certain extent, resulting in poor crystallinity. Figure 1b and Figure S1 show that NiFe-LDH, LDH/Ni3S2/MoS2, and CeO2-LDH/Ni3S2/MoS2 exhibit a flower-like shape composed of nanosheet structures. The nanosheets grown on the nickel foam are relatively uniform, smooth, and densely covered. Figure S1b,e are two different sheet morphologies of LDH/Ni3S2/MoS2, showing a more dispersed growth mode than NiFe-LDH (Figure S1a,d), forming a larger flower ball. In this transition, the vertically growing and spatially interconnected network is well maintained, suggesting that this structure helps to expose more catalytically active sites, provide open channels for electrolyte accessibility, and facilitate the rapid release of bubbles. In addition, Figure S1c,f show that CeO2-modified LDH/Ni3S2/MoS2 is uniformly grown on nickel foam, and the cross-linking of nanosheets in different growth directions leads to a more open porous structure. This not only makes the catalyst have a large surface area, which is conducive to the penetration of the electrolyte, but also makes the catalyst have strong permeability. The morphological characteristics of CeO2-LDH/Ni3S2/MoS2 materials were characterized by high-resolution transmission electron microscopy. The test results shown in Figure 1c,d are high-resolution transmission images. In the HRTEM images, the lattice fringes of 0.170 nm, 0.290 nm, 0.286 nm, and 0.21 nm are from the (122), (100), (110), and (202) crystal planes of Ni3S2, respectively. The lattice fringe of 0.249 nm corresponds to the (102) crystal plane of MoS2, and the lattice fringes of 0.298 nm and 0.19 nm are from the (111) and (220) crystal planes of CeO2. In addition, there are 0.23 nm lattice fringes corresponding to the (015) crystal plane of NiFe-LDH, which confirms the successful preparation of CeO2-LDH/Ni3S2/MoS2. Figure 1c confirms the lamellar structure. The surface scanning results of the elements show that Ni, Fe, Mo, Ce, O, and S are uniformly distributed in the CeO2-LDH/Ni3S2/MoS2 catalyst (Figure 1e). The results of XRD, SEM, and TEM prove that the CeO2-LDH/Ni3S2/MoS2 hierarchical structure material is successfully synthesized.
The surface composition and valence state of the samples are analyzed based on XPS. The XPS total spectrum of Figure S2 demonstrates the presence of Ni, Fe, O, Mo, S, and Ce elements in CeO2-LDH/Ni3S2/MoS2. As shown in Figure 2a, in the XPS spectrum of Ni 2p, the Ni 2p energy level is split into 2p1/2 and 2p3/2, corresponding to the binding energy of 873.42 and 855.81 eV, respectively, showing Ni2+. The satellite peaks are located at 880.65 and 862.62 eV, respectively. At this time, no metal Ni peak was observed, which is due to the uniform growth of NiFe LDH nanosheets on Ni3S2/MoS2, so it is difficult to detect Ni0 from nickel foam [33]. For the Fe 2p (Figure 2b) spectrum, two peaks at 724.7 and 714.09 eV could be attributed to Fe 2p1/2 and Fe 2p3/2, respectively, indicating the existence of the Fe3+ oxidation state. The other two satellite peaks correspond to the binding energies of 718 and 707.67 eV [34]. These results prove the formation of NiFe-LDH. The O 1s spectrum (Figure 2c) shows two peaks at 531.17 and 533.13 eV, which are related to M-OH and H-O-H in layered dioxygen, respectively, proving the presence of oxygen vacancies (Ov) and adsorbed oxygen (Oa) [35]. The oxygen vacancies in the material are conducive to the adsorption and dissociation of H2O and thus improve the electrocatalytic performance. Figure 2d displays the spectrum of Mo 3d, which has two characteristic peaks at 236.29 and 235 eV, corresponding to 3d3/2 and 3d5/2 of Mo6+, respectively. There are two characteristic peaks at 232.5 and 229.07 eV, corresponding to 3d3/2 and 3d5/2 of Mo4+, respectively. Another characteristic peak is 227.02 eV, which can be attributed to the formation of the Mo-S bond [36]. In Figure 2e, the XPS spectra of the S 2p region display binding energies of 164.98 and 162.13 eV, corresponding to S 2p1/2 and S 2p3/2, respectively, which are attributed to the metal sulfides corresponding to S2− and the strong S-O peak (167.32 eV) due to the oxidation of sulfides [37]. In the Ce 3d spectrum (Figure 2f), the peaks at 900.61 eV, 896.45 eV, and 892.63 eV are the characteristic peaks of Ce4+ 3d3/2, Ce4+ 3d3/2, and Ce3+ 3d3/2, respectively. The peaks at 882.74 eV, 879.74 eV, 875.97 eV, and 873.51 eV are the characteristic peaks of Ce4+ 3d5/2, Ce3+ 3d5/2, Ce4+ 3d5/2, and Ce3+ 3d5/2, respectively [38]. The CeO2 exhibits a flexible transition between the Ce3+ and Ce4+ oxidation states, which contributes to its excellent electronic and ionic conductivity, reversible oxygen ion exchange at the surface, and high oxygen storage capacity. These inherent properties make CeO2 a valuable co-catalyst, facilitating efficient charge transport and enhancing energy efficiency. Consequently, CeO2 plays a crucial role in enhancing the activity of water oxidation catalysts [39,40,41].

3.2. Electrochemical HER Performance

Based on the three-electrode system, the hydrogen evolution performance of the catalyst was studied in an alkaline medium (1.0 M KOH). The HER performance of different samples is shown in Figure 3a,b, and the single nickel foam has almost no HER performance. At a current density of 10 mA cm−2, the overpotential of CeO2-LDH/Ni3S2/MoS2 is 116 mV, lower than that of Ni3S2/MoS2 (156 mV), CeO2-Ni3S2/MoS2 (117 mV), LDH/Ni3S2/MoS2 (141 mV), and commercial precious metal Pt/C (60 mV). With the current density increased to 50 and 100 mA cm−2, the overpotentials of Ni3S2/MoS2, CeO2-Ni3S2/MoS2, LDH/Ni3S2/MoS2, CeO2-LDH/Ni3S2/MoS2, and commercial precious metal Pt/C were 313 and 380 mV, 191 and 230 mV, 231 and 270 mV, 176 and 205 mV, and 127 and 204 mV, respectively. It is confirmed that the combination of LDH and two-component transition metal sulfides can slightly improve the hydrogen evolution performance. CeO2 nanoparticles can effectively enhance the hydrogen evolution activity of transition metal sulfides. The experimental results show that the noble metal Pt/C has the best performance at low current density. With the increase in current density, the catalytic performance of CeO2-LDH/Ni3S2/MoS2 exhibits the best activity. It is proved that the CeO2-LDH/Ni3S2/MoS2 electrode is suitable for high current density.
In order to further understand the kinetics of the reaction, the Tafel slope curves of different samples were obtained according to the LSV curve. As shown in Figure 3c, the Tafel slope value of CeO2-LDH/Ni3S2/MoS2 is 85.53 mV dec−1, which is lower than that of LDH/Ni3S2/MoS2 (110.81 mV dec−1), CeO2-Ni3S2/MoS2 (107.53 mVdec−1), and Ni3S2/MoS2 (232.3 mVdec−1). This indicates that CeO2-LDH/Ni3S2/MoS2 has the optimal HER kinetics. In addition, in order to further understand the charge transfer resistance of the catalyst material, an EIS test was performed on the prepared material. The charge transfer resistance Rct value of CeO2-LDH/Ni3S2/MoS2 is 0.57 Ω, less than that of LDH/Ni3S2/MoS2 (1.06 Ω), CeO2-Ni3S2/MoS2 (0.79 Ω), Ni3S2/MoS2 (4.00 Ω), and pure NF(33.28 Ω), indicating that the introduction of Ce accelerates the electron transfer at the interface between the catalyst and the electrolyte [42,43,44].
Stability is also one of the important factors to evaluate the catalyst. Figure 3e shows the HER chronoamperometry curve for evaluating the electrochemical stability of CeO2-LDH/Ni3S2/MoS2. During the electrolysis process, the electrode can remain stable for 160 h. The curve showed no obvious attenuation. The current density was maintained at 10 mA cm−2 with a retention rate of 94.7%, which proved that the CeO2-LDH/Ni3S2/MoS2 electrode maintained excellent catalytic activity and good stability [45,46,47].

3.3. Electrochemical OER Performance

Based on the three-electrode system, the oxygen evolution performance of the catalyst was studied in an alkaline medium (1.0 M KOH). The results are shown in Figure 4a,b. There is almost no OER activity on the nickel foam substrate, but due to the three-dimensional porous structure and metal activity of nickel foam, there is an obvious oxidation peak at about 1.35 V, which proves the excellent properties of nickel foam as a skeleton material [48,49]. At a current density of 150 mA cm−2, the overpotential of CeO2-LDH/Ni3S2/MoS2 is 235 mV, lower than that of LDH/Ni3S2/MoS2 (246 mV), CeO2-Ni3S2/MoS2 (302 mV), and Ni3S2/MoS2 (377 mV), and even better than that of commercial noble metal RuO2 (423 mV). The overpotentials of Ni3S2/MoS2, CeO2-Ni3S2/MoS2, LDH/Ni3S2/MoS2, and CeO2-LDH/Ni3S2/MoS2 are 416, 484, 318, 346, 268, and 308 mV, respectively, as the current densities increased to 200 and 300 mA cm−2.
The intrinsic catalytic kinetics of the material was further evaluated by the Tafel slope curve. As shown in Figure 4c, the Tafel slopes of CeO2-LDH/Ni3S2/MoS2, LDH/Ni3S2/MoS2, CeO2-Ni3S2/MoS2, and Ni3S2/MoS2 are 131.56,192.29,135.37, and 269.05 mA cm−2, respectively. The CeO2-LDH/Ni3S2/MoS2 shows the lowest Tafel slope, which proves optimal reaction kinetics and further confirms excellent OER catalytic activity [50,51]. In addition, the charge transfer resistance of the interface between the electrocatalyst and the electrolyte was studied by testing the Nyquist curve in EIS. The semicircle diameter reflects the rate of electron transfer. The smaller the arc radius of the Nyquist curve, the smaller the charge transfer resistance Rct value. As shown in Figure 4d, the Rct values of LDH/Ni3S2/MoS2 (0.30 Ω), CeO2-Ni3S2/MoS2 (0.67 Ω), and Ni3S2/MoS2 (1.13 Ω) indicate that the CeO2-LDH/Ni3S2/MoS2 composite structure grown on NF has the smallest charge transfer resistance and the highest kinetic reaction rate, which is also consistent with the kinetic results of Tafel slope [52,53].
ECSA was used to evaluate the number of active sites of the catalyst. Since the ECSA is linearly proportional to the electrochemical double-layer capacitance (Cdl), the Cdl is commonly used to indirectly compare the size of the electrochemically active surface area. Figure S3 shows the variable speed CV curves of different catalysts. As can be seen in Figure 4e, the Cdl of the CeO2-LDH/Ni3S2/MoS2 electrode is 52.31 mF cm−2, which is higher than that of LDH/Ni3S2/MoS2 (38.72 mF cm−2), CeO2-Ni3S2/MoS2 (41.89 mF cm−2), and Ni3S2/MoS2 (21.09 mF cm−2), indicating that the CeO2-LDH/Ni3S2/MoS2 electrode combines LDH and CeO2 active components to expose more catalytic active sites, and the introduction of LDH increases the contact with water molecules. Thus, the OER catalytic activity is significantly improved [54].
Figure 4f shows the stability curve of the CeO2-LDH/Ni3S2/MoS2 composite electrode running for 160 h at 150 mA cm−2. It can be seen that the current of the catalyst is stable and smooth in the alkaline electrolyte, and the percentage of current density change is 98.6%, indicating good electrocatalytic stability. This is attributed to the corrosion resistance and good conductivity of the nickel foam substrate [55]. Additionally, the comparable reported catalysts of OER and HER are shown in Table S2 and Table S3, respectively.

3.4. Overall Water Splitting

In order to evaluate the electrocatalytic performance of overall water splitting, we constructed a two-electrode electrolyzer using CeO2-LDH/Ni3S2/MoS2 as both the anode and cathode in a 1.0 M KOH solution (Figure 5a). The results from Figure 5b,c demonstrate that the CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2 electrolyzer achieved a cell voltage of 1.126V at 10 mA cm−2, which is significantly lower than that of LDH/Ni3S2/MoS2 (1.286 V), CeO2-Ni3S2/MoS2 (1.356 V), and Ni3S2/MoS2 (1.574 V). Moreover, Figure 5d reveals that the CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2 electrode maintained 98.6% of its initial activity after operating for 160 h at 10 mA cm−2. These results validate the excellent durability of the CeO2-LDH/Ni3S2/MoS2 composite for overall water splitting in an alkaline electrolyte. Additionally, the comparisons of OER and HER activities between Ni3S2/MoS2-based electrocatalysts recently reported are displayed in Table S1 and Table S2, respectively. The CeO2-LDH/Ni3S2/MoS2 electrolyzer displayed a comparable cell voltage to that of the Ni3S2/MoS2-based electrocatalysts, as shown in Table S3.

4. Conclusions

In summary, we have successfully prepared a CeO2 and LDH co-modified hierarchical structure (CeO2-LDH/Ni3S2/MoS2) as a novel non-precious metal electrocatalyst by a two-step hydrothermal method. The CeO2-LDH/Ni3S2/MoS2 catalyst was tested for electrochemical hydrogen evolution and oxygen evolution under alkaline conditions. It was proved that the HER overpotential of CeO2-LDH/Ni3S2/MoS2 was 116 mV at a current density of 10 mA cm−2, and the OER overpotential was only 235 mV at 150 mA cm−2. In addition, the CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2 electrolyzer achieved a cell voltage of 1.126V at 10 mA cm−2. Moreover, the CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2 electrode maintained 98.6% of its initial activity after operating for 160 h at a current density of 10 mA cm−2.
The catalyst exhibits excellent HER/OER performance due to the LDH, which increases the oxygen evolution active site, promotes the contact between the electrode and the alkaline electrolyte, and accelerates the OER reaction kinetics. In addition, CeO2 modifies the local electronic structure, which is conducive to the interaction between electrons and accelerates the transfer of charges, thereby improving the electrocatalytic performance of OER/HER. CeO2 also promotes the change of morphology. The hierarchical structure composed of nickel foam, nanosheets, and nanoparticles increases the active sites, which is beneficial for the improvement of electrocatalytic performance. According to the activity of the catalyst, it can be used for water decomposition in the future to decompose water into hydrogen and oxygen, thereby achieving an environmentally friendly energy conversion process. Compared with traditional fossil fuel combustion, this technology can reduce a large number of greenhouse gas emissions and air pollutants. This work provides a new idea for the design of highly active and stable electrocatalytic materials.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma17194876/s1, Figure S1: SEM of (a,d) LDH, (b,e) LDH/Ni3S2/MoS2, (c,f) CeO2-LDH/Ni3S2/MoS2, Figure S2: XPS survey spectra of CeO2-LDH/Ni3S2/MoS2, Figure S3: Cyclic voltammograms (CVs) from 60 to 100 mV/s for (a) Ni3S2/MoS2, (b) CeO2-Ni3S2/MoS2, (c) LDH-Ni3S2/MoS2, (d) CeO2-LDH/Ni3S2/MoS2 at the different scan rates, Table S1: Comparison of OER catalytic activities between Ni3S2/MoS2 and other recently reported OER electrocatalysts in both 1 M KOH; Table S2: Comparison of HER catalytic activities between Ni3S2/MoS2 and other recently reported HER electrocatalysts in both 1 M KOH, Table S3: Comparison of water splitting performance between Ni3S2/MoS2 and other recently reported electrocatalysts in both 1 M KOH.

Author Contributions

Conceptualization and methodology, H.L.; investigation and resources, F.C.; data curation and writing—original draft preparation, X.W.; writing—review and editing, D.W.; supervision and project administration, Y.R.; funding acquisition and visualization, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data generated during and/or analyzed in this article are available from the corresponding author upon reasonable request.

Acknowledgments

This work was supported by the Longmen Laboratory Free Exploration Project (LMQYTSKT012), Major Scientific and Technological Innovation Project in Henan Province (231100220100), Youth Project of Henan Natural Science Foundation (222300420134), and the Key Research and Development Project of Henan Province (No. 241111231600).

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 17 04876 sch001
Scheme 1. Preparation of CeO2-LDH/Ni3S2/MoS2.
Scheme 1. Preparation of CeO2-LDH/Ni3S2/MoS2.
Materials 17 04876 sch001
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Figure 1. (a) XRD patterns of LDH/Ni3S2/MoS2 and CeO2-LDH/Ni3S2/MoS2, (b) SEM image of CeO2-LDH/Ni3S2/MoS2, (c,d) HRTEM images of CeO2-LDH/Ni3S2/MoS2. (e) Elemental mapping images of CeO2-LDH/Ni3S2/MoS2.
Figure 1. (a) XRD patterns of LDH/Ni3S2/MoS2 and CeO2-LDH/Ni3S2/MoS2, (b) SEM image of CeO2-LDH/Ni3S2/MoS2, (c,d) HRTEM images of CeO2-LDH/Ni3S2/MoS2. (e) Elemental mapping images of CeO2-LDH/Ni3S2/MoS2.
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Figure 2. High-resolution XPS spectrogram of CeO2-LDH/Ni3S2/MoS2: (a) Ni 2p, (b) Fe 2p, (c) O 1s, (d) Mo 3d, (e) S 2p, (f) Ce 3d.
Figure 2. High-resolution XPS spectrogram of CeO2-LDH/Ni3S2/MoS2: (a) Ni 2p, (b) Fe 2p, (c) O 1s, (d) Mo 3d, (e) S 2p, (f) Ce 3d.
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Figure 3. HER electrocatalytic performance in 1.0 M KOH: (a) LSV curve, (b) overpotential, (c) Tafel slope, (d) EIS spectrum, (e) durability of CeO2-LDH/Ni3S2/MoS2 determined by chronoamperometry at a current density of 10 mA cm−2.
Figure 3. HER electrocatalytic performance in 1.0 M KOH: (a) LSV curve, (b) overpotential, (c) Tafel slope, (d) EIS spectrum, (e) durability of CeO2-LDH/Ni3S2/MoS2 determined by chronoamperometry at a current density of 10 mA cm−2.
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Figure 4. OER electrocatalytic performance in 1.0 M KOH: (a) LSV curve, (b) overpotential, (c) Tafel slope, (d) EIS spectrum, (e) the Cdl, (f) durability of CeO2-LDH/Ni3S2/MoS2 determined by chronoamperometry at a current density of 150 mA cm−2.
Figure 4. OER electrocatalytic performance in 1.0 M KOH: (a) LSV curve, (b) overpotential, (c) Tafel slope, (d) EIS spectrum, (e) the Cdl, (f) durability of CeO2-LDH/Ni3S2/MoS2 determined by chronoamperometry at a current density of 150 mA cm−2.
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Figure 5. The two-electrode electrolyzers for overall water splitting of CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2, LDH/Ni3S2/MoS2||LDH/Ni3S2/MoS2, CeO2-Ni3S2/MoS2||CeO2-Ni3S2/MoS2, and Ni3S2/MoS2||Ni3S2/MoS2 in 1.0 M KOH. (a) The digital photograph of the O2 and H2 evolution from CeO2-LDH/Ni3S2/MoS2. (b) The polarization curves, (c) overpotentials at 10 mA cm−2, and (d) chronopotentiometry test at 10 mA cm−2 for CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2.
Figure 5. The two-electrode electrolyzers for overall water splitting of CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2, LDH/Ni3S2/MoS2||LDH/Ni3S2/MoS2, CeO2-Ni3S2/MoS2||CeO2-Ni3S2/MoS2, and Ni3S2/MoS2||Ni3S2/MoS2 in 1.0 M KOH. (a) The digital photograph of the O2 and H2 evolution from CeO2-LDH/Ni3S2/MoS2. (b) The polarization curves, (c) overpotentials at 10 mA cm−2, and (d) chronopotentiometry test at 10 mA cm−2 for CeO2-LDH/Ni3S2/MoS2||CeO2-LDH/Ni3S2/MoS2.
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Li, H.; Chen, F.; Wu, X.; Wang, D.; Ren, Y.; Li, Y. Improved HER/OER Performance of NiS2/MoS2 Composite Modified by CeO2 and LDH. Materials 2024, 17, 4876. https://doi.org/10.3390/ma17194876

AMA Style

Li H, Chen F, Wu X, Wang D, Ren Y, Li Y. Improved HER/OER Performance of NiS2/MoS2 Composite Modified by CeO2 and LDH. Materials. 2024; 17(19):4876. https://doi.org/10.3390/ma17194876

Chicago/Turabian Style

Li, Hao, Feng Chen, Xinyang Wu, Dandan Wang, Yongpeng Ren, and Yaru Li. 2024. "Improved HER/OER Performance of NiS2/MoS2 Composite Modified by CeO2 and LDH" Materials 17, no. 19: 4876. https://doi.org/10.3390/ma17194876

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