Construction of Ni₃S₄-CeO₂ nanocomposites for enhanced electrocatalytic alkaline oxygen evolution reactions

Abstract

Water electrolysis is an important way to achieve sustainable hydrogen production. However, the sluggish kinetics of the anodic oxygen evolution reaction (OER) brings about high energy consumption and low efficiency of the electrolysis process, which greatly limits its application. In this work, Ni₃S₄-CeO₂/NF electrocatalysts were prepared on nickel foam by secondary calcination and simple hydrothermal synthesis. The Ni₃S₄-CeO₂/NF catalysts exhibit outstanding electrochemical alkaline OER performance and durability. Among them, the optimized Ni₃S₄-CeO₂/NF catalyst requires only 110 mV overpotential to reach a current density of 10 mA cm⁻², with a Tafel slope of 42.3 mV dec⁻¹ in 1 M KOH. Furthermore, the current density of the Ni₃S₄-CeO₂/NF catalyst is stable over 20 hours, showing good catalytic stability. Studies have shown that the improved OER performance is due to CeO₂ accelerating charge transfer and adjusting the electronic structure of Ni₃S₄. This work provided a new strategy for constructing high activity alkaline OER electrocatalysts.

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1. Introduction

In recent years, Fe-, Co-, and Ni-based transition metals have gained recognition as the top contenders for replacing precious metals, thanks to their plentiful sources, cost-effectiveness, and high catalytic activity.^1–3 However, the following problems still exist: (1) the concentration of high valence metal ions in the sample is low. The research on Ni³⁺ is a hot spot now, but most of the research only focuses on Ni²⁺.⁴ (2) The surface area of the catalyst is small, causing inefficient usage of the catalyst's active centers.⁵ (3) The conductivity of transition metal oxides and hydroxides studied is relatively low.⁶

Based on the above problems, researchers have conducted a series of studies and found that nickel sulfide (Ni_xS_y) has high electronic conductivity.^7,8 It has attracted much attention due to its inherent metal properties (continuous nickel–nickel bonds) and good conductivity.⁹ The covalent nickel–nickel bond in Ni_xS_y has good electrochemical corrosion resistance. Therefore, the design and synthesis of nickel–sulfur compounds with different structures and different components are expected to yield a new type of oxygen evolution electrocatalytic material.^10,11 The self-supporting nickel foam is a three-dimensional network structure, and its pores are connected to the metal skeleton, which can provide active surfaces for various compounds, so it is widely used in the field of electrocatalysis. In addition, the construction of heterojunctions can precisely modulate the electronic configuration of materials, which is the key to achieving high-efficiency electrocatalytic activity.¹²

Studies have shown that fluorite-type cerium–cerium dioxide (CeO₂) can generate oxygen vacancies and create heterojunctions with alternative structures by virtue of the flexible multivalent properties and excellent conductivity between Ce³⁺ and Ce⁴⁺,¹³ ensuring that CeO₂ can form strong interactions with active components, thereby improving catalytic performance.^14–17 Therefore, CeO₂ is widely used as a “performance promoter” for various electrocatalysts in different reactions.¹⁸ For example, Guo et al. successfully developed a CeO₂@CoSe₂/CC heterostructure catalyst,¹⁹ which displayed superior catalytic behavior, requiring only 245 mV overpotential to reach 10 mA cm⁻² current density in alkaline media. Within this heterostructure, CeO₂, enriched with oxygen vacancies, enhances OH⁻ adsorption and accelerates the transformation of CoSe₂ into CoOOH at reduced potentials. Li et al. proposed a scalable strategy to achieve electron dynamic reflow by introducing a rare earth oxide Ce^δ+O_x with an adjustable valence state onto iron-doped cobalt–iron hydroxide (Fe–Co(OH)₂) during the oxygen evolution reaction (OER) process.²⁰ This design resulted in significantly improved intrinsic activity and enduring stability.

In this study, considering the unique advantages of CeO₂, Ni₃S₄-CeO₂/NF electrocatalysts were prepared by a simple hydrothermal method. The 0.2 mmol Ni₃S₄-CeO₂/NF electrocatalyst can achieve a current density of 10 mA cm⁻² when the overpotential is only 110 mV, and the respective Tafel slope is 42.3 mV dec⁻¹. In addition, the stability test of the chronoamperometric current for 20 h showed that the current density fluctuation was not obvious, and the material demonstrated outstanding stability performance. The introduction of CeO₂ accelerates the charge transfer rate²¹ and adjusts the electronic structure of Ni₃S₄,²² which greatly improves the oxygen evolution ability of the material.

2. Experimental

2.1 Synthesis of the carbon sphere template

6.3 g Glucose was added to 70 mL of deionized water and stirred until completely dissolved. The obtained glucose solution was sealed in a high-pressure sterilizer and reacted at 180 °C for 5 h. After reaching room temperature, the brown material was collected via centrifugation, purified by water washing, and finally dried at 60 °C. Following this, the sample underwent annealing for 5 hours at 500 °C in a tube furnace under a flowing nitrogen atmosphere.²³ After natural cooling, the carbon sphere template was obtained.

2.2 Synthesis of CeO₂

0.048 g of the carbon sphere template along with 2 mmol Ce(NO₃)₃·6H₂O were dispersed in 40 mL deionized water using ultrasonication for 10 min.²⁴ Subsequently, 5 mmol urea was added and subjected to ultrasonic dissolution for 20 min. After sealing in a high-pressure sterilizer, the solution was subjected to 12-hour processing at 120 °C. Following cooling, the CeO₂/C precursor was separated via centrifugation, purified through deionized water washing, and finally dehydrated in a 60 °C oven.²⁵ The CeO₂/C precursor was subsequently placed in a tube furnace and heat-treated at 400 °C for 2 hours under N₂ and air atmospheres,²⁶ respectively. After cooling at room temperature, CeO₂ was finally obtained.

2.3 Synthesis of Ni₃S₄/NF nanocomposites and Ni₃S₄–CeO₂/NF nanocomposites

0.291 g Ni(NO₃)₃·6H₂O, 0.3 g TAA and CeO₂ powders were combined with 30 mL deionized water and subjected to intense stirring for 2 h to obtain a uniform solution. The above mixture was poured into a 50 mL autoclave, and at the same time a pre-treated 2 × 3 cm² nickel foam (NF) was placed in it, followed by heating at 180 °C for 18 h. According to the addition amounts of CeO₂ (0 mmol, 0.15 mmol, 0.2 mmol, and 0.25 mmol), they can be named Ni₃S₄/NF, Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2, and Ni₃S₄-CeO₂/NF-3, respectively.

2.4 Characterization

The instruments and equipment used in the experiment are shown in the SI.

2.5 Electrochemical measurements

The preparation of electrodes and electrochemical measurements used in the experiment are shown in the SI.

3. Results and discussion

Firstly, a carbon sphere template was prepared by a high-temperature calcination method, and then CeO₂ was prepared by a hydrothermal method followed by direct calcination. Finally, the Ni₃S₄-CeO₂/NF electrocatalytic material was prepared by combining CeO₂ and Ni₃S₄ by a simple hydrothermal method. The detailed synthesis process of the Ni₃S₄-CeO₂/NF hybrid material is presented in Fig. 1.

Fig. 1 The synthesis process of Ni₃S₄-CeO₂/NF.

XRD analysis was performed to study the crystal phase and composition of CeO₂, Ni₃S₄, and Ni₃S₄-CeO₂/NF materials (Fig. 2a). For pure Ni₃S₄ and CeO₂, the characteristic peaks correspond to their respective standard cards (PDF#47-1739) and (PDF#34-0394). For Ni₃S₄–CeO₂/NF samples, obvious diffraction peaks emerged at 44.5°, 51.8° and 76.4°, assigned to the (111), (200) and (220) crystal planes of Ni foam (PDF#87-0712).²⁷ The characteristic peaks at 28.5°, 33.1°, 47.3°, 56.2° and 58.9° can be indexed to the (111), (200), (220), (311) and (222) planes of CeO₂, respectively (PDF#34-0394).²⁸ The XRD peaks at 31.2°, 37.9°, 49.9° and 54.8° point to the (311), (400), (511) and (440) planes of the Ni₃S₄ phase (PDF#47-1739), respectively,²⁹ which proves the successful composite of Ni₃S₄–CeO₂/NF.³⁰ These characteristic peaks are well-aligned with the distinctive peaks of Ni₃S₄ and CeO₂, indicating that the Ni₃S₄–CeO₂/NF sample is composed of the Ni₃S₄ phase and CeO₂ phase, and the successful synthesis of Ni₃S₄-CeO₂/NF is confirmed, and Raman spectroscopy further proves this point. With higher CeO₂ loading in the composites, the peak value of CeO₂ gradually rises, showing that the crystallinity of the composites is improved.

Fig. 2 (a) XRD patterns of Ni₃S₄, CeO₂ and Ni₃S₄-CeO₂/NF samples, (b) Raman spectra of Ni₃S₄/NF and Ni₃S₄-CeO₂/NF-2.

The structural properties of Ni₃S₄/NF and Ni₃S₄–CeO₂/NF materials were analyzed by Raman spectroscopy (Fig. 2b), and small characteristic peaks related to Ni₃S₄ can be detected at 136 cm⁻¹,³¹ 224 cm⁻¹, 337 cm⁻¹, and 379 cm⁻¹. On the spectrum of the composite, the F_2g band is located at 461 cm⁻¹, related to the triple degenerate vibration state of CeO₂, which further indicates the occurrence of the O–Ce–O band.^18,32,33 It is worth noting that the D band at 605 cm⁻¹ is associated with the ratio of Ce^IV to Ce^III,³⁴ pointing to the presence of oxygen vacancies.³⁵

X-ray photoelectron spectroscopy (XPS) was employed to examine the surface properties and elemental valence states in Ni₃S₄/NF and Ni₃S₄-CeO₂/NF electrocatalytic materials. The XPS total spectral image of Ni₃S₄-CeO₂/NF (Fig. 3a) proves the detection of Ce, O, Ni and S elements. The Ni 2p spectrum of the Ni₃S₄-CeO₂/NF catalyst (Fig. 3b) is decomposed into one satellite peak and four main peaks. The peaks of Ni 2p at 856.21 eV and 874.02 eV are consistent with those of Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2,³⁶ respectively, while the other two peaks concentrated at 860.42 eV and 877.94 eV are consistent with those of Ni³⁺ 2p_3/2 and 2p_1/2, respectively. The peak area of Ni³⁺ is larger than that of Ni²⁺, which is mainly because Ni²⁺ is oxidized by air on the surface.³⁰ The XPS spectrum of Ni₃S₄ in Fig. 3b shows a new peak at 852 eV. Judging from the binding energy position, the peak at 852 eV is most likely attributed to metal nickel.³⁷ This may be due to the reductive environment generated by the decomposition of thioacetamide during the hydrothermal process, resulting in the local production of a small amount of metal nickel.³⁸ However, the XPS spectrum of Ni₃S₄-CeO₂/NF-2 did not show a peak at 852 eV, because the introduction of CeO₂ stabilized the high valence state of nickel and inhibited the formation of metallic nickel,³⁹ which was crucial for regulating its catalytic performance. Compared with the original Ni₃S₄/NF, the Ni 2p peak in the Ni₃S₄-CeO₂/NF sample has a positive shift, which indicates that the local electron density around the Ni atom decreases and its oxidation state increases.^40–42 Moreover, the high valence of Ni species is conducive to the formation of key OER intermediates, such as Ni³⁺–OO, which is crucial for improving the electrocatalytic activity of the OER.⁴³ In addition, as evidenced by the S 2p spectral features in Fig. 3c, the peaks at 164.05 eV and 162.75 eV of Ni₃S₄-CeO₂/NF are attributed to S 2p_1/2 and S 2p_3/2, and the peak at 169.07 eV is attributed to the sulfate group, which confirms the successful preparation of Ni₃S₄. In addition, the Ce 3d region in Ni₃S₄-CeO₂/NF (Fig. 3d) can be divided into six peaks, of which the peaks at 878.84 eV and 874.87 eV point to the Ce 3d_5/2 energy level of Ce⁴⁺, and the three peaks at 881.00 eV, 884.28 eV and 898.37 eV point to the Ce 3d_3/2 energy level of Ce⁴⁺.⁴⁴ Compared to CeO₂, the Ce 3d peak of Ni₃S₄-CeO₂/NF shows a negative shift, indicating that some electrons are transferred from Ni₃S₄ to CeO₂. The O 1s region of Ni₃S₄-CeO₂/NF in Fig. 3e is asymmetric, indicating the occurrence of distinct oxygen forms. The peaks at 531.16 eV and 532.68 eV are related to OH⁻ and O²⁻. The peak of Ni₃S₄-CeO₂/NF at 531.16 eV corresponds to a high binding energy, indicating that the presence of CeO₂ boosts the water adsorption capacity of Ni₃S₄-CeO₂/NF. Compared with CeO₂ and Ni₃S₄, the increase of the binding energy of S 2p and Ni 2p and the decrease of the binding energy of Ce 3d indicate that there is electron transfer from Ni and S to Ce at the Ni₃S₄–CeO₂ interface.^45,46 The electron transfer from Ni₃S₄ to CeO₂ promoted the formation of active sites with high catalytic performance. Based on the analysis of the electron transfer direction, combined with the intrinsic electrical properties of N₃S₄ (a typical p-type semiconductor)⁴⁷ and CeO₂ (a typical n-type semiconductor),^48,49 a p–n heterojunction is formed between Ni₃S₄-CeO₂/NF composites. There is a strong electronic interaction between Ni₃S₄ and CeO₂, indicating that CeO₂ can adjust the electronic structure of Ni₃S₄.^50,51 The analytical results confirm that a heterogeneous interface leads to electron redistribution, which is beneficial for improving the intrinsic activity of electrocatalysts.

Fig. 3 (a) The full spectrum and (b) XPS spectra of Ni 2p, (c) S 2p, (d) Ce 3d, and (e) O 1s for Ni₃S₄-CeO₂/NF-2.

Scanning electron microscopy (SEM) was employed to examine the morphological features of the catalytic materials.⁵² From the SEM images of Ni₃S₄ (Fig. 4a), it can be seen that the Ni₃S₄ synthesized by the hydrothermal method presents the morphology of rod cactus. The spherical morphology of carbon spheres was observed (Fig. 4b), and the size distribution was uniform and the arrangement was tight. As illustrated in Fig. 4c, CeO₂ accumulates on the face of the carbon spheres in a rod-like manner. Carbon-based materials have attracted extensive attention due to their high specific surface area, excellent stability and high conductivity. In the process of preparing CeO₂ on the carbon sphere template, the carbon sphere provides a large number of active sites due to the spatial characteristics of its three-dimensional structure,⁵³ which enhances the mass activity and stability of CeO₂. It also plays a role in supporting and fixing cerium dioxide,⁵⁴ thereby facilitating better compounding with Ni₃S₄ materials. After CeO₂ and Ni₃S₄ nanoparticles were compounded by the hydrothermal method, they still retained the initial morphology, which indicated that their morphology was almost unchanged before and after compounding, showing that the morphology of the catalytic materials compounded by this method had certain stability (Fig. 4d and e). Different from the polished exterior of CeO₂, the face of the composite material is rougher, and it is evident that CeO₂ particles are uniformly distributed on the outer layer of Ni₃S₄. This morphology provides more favorable conditions for the charge transfer of the composite catalyst, thereby improving the water splitting performance of the composite catalyst. The elemental mapping diagram shows the homogeneous dispersion of the corresponding Ni, S, Ce and O components in the Ni₃S₄-CeO₂/NF electrocatalytic material (Fig. 4f–i), which proves that CeO₂ and Ni₃S₄ have been compounded together through experiments.

Fig. 4 (a) SEM images of Ni₃S₄, (b) carbon spheres, (c) CeO₂, (d and e) Ni₃S₄-CeO₂/NF-2, and (f–i) the corresponding EDS mappings of Ni₃S₄-CeO₂/NF-2.

The microstructure and crystal plane of Ni₃S₄-CeO₂/NF hybrid materials were characterized by transmission electron microscopy (TEM) (Fig. 5a and b). The morphology of the hybrid material observed in the TEM image is consistent with the results observed by SEM. High-resolution transmission electron microscopy (HRTEM) revealed the crystal structure and formation process of Ni₃S₄-CeO₂/NF.⁵⁵ From the micrograph of Ni₃S₄-CeO₂/NF (Fig. 5d), the interplanar distance of 0.237 nm points to the (400) plane of Ni₃S₄ (Fig. 5c). The lattice spacing of CeO₂, as determined from the (200) plane in Fig. 5e, measures 0.269 nm.⁵⁶ In general, SEM and TEM confirmed the successful preparation of Ni₃S₄-CeO₂/NF electrocatalytic materials.

Fig. 5 (a and b) TEM and (c–e) HRTEM images of Ni₃S₄-CeO₂/NF-2.

The OER performance of the synthesized electrocatalyst was assessed through linear sweep voltammetry (LSV) measurements in a 1.0 M KOH aqueous solution.⁵⁷ To examine how CeO₂ loading affects the oxygen evolution reaction performance of Ni₃S₄-CeO₂/NF composite materials, the LSV polarization curve of the Ni₃S₄-CeO₂/NF catalyst is shown in Fig. 6a. The overpotentials of Ni₃S₄, Ni₃S₄ NF, Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2 and Ni₃S₄-CeO₂/NF-3 are 922 mV, 309 mV, 127 mV, 110 mV and 363 mV, respectively, which can reach a current density of 10 mA cm⁻². The data demonstrated that the electrocatalytic performance of Ni₃S₄-CeO₂/NF was enhanced in proportion to the rising CeO₂ content, and the preferred content of Ni₃S₄-CeO₂/NF in the hybrid catalyst was 0.2 mmol. The Tafel slope value provided insights into the reaction kinetics. Fig. 6b shows the Tafel plots of Ni₃S₄-CeO₂/NF catalysts with different CeO₂ contents. Compared with Ni₃S₄ (560.69 mV dec⁻¹), CeO₂/NF (226.81 mV dec⁻¹), Ni₃S₄/NF (190.30 mV dec⁻¹), Ni₃S₄-CeO₂/NF-1 (180.24 mV dec⁻¹) and Ni₃S₄-CeO₂/NF-3 (185.07 mV dec⁻¹), the Tafel slope of Ni₃S₄-CeO₂/NF-2 (42.3 mV dec⁻¹) is significantly reduced, demonstrating that Ni₃S₄-CeO₂/NF-2 has better OER kinetics and catalytic efficiency.

Fig. 6 (a) LSV curves in 1 M KOH, (b) Tafel slopes, (c) Nyquist diagrams of Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2, and Ni₃S₄-CeO₂/NF-3, (d) C_dl curves of CeO₂/NF, Ni₃S₄/NF, Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2, and Ni₃S₄-CeO₂/NF-3, (e) stability test of Ni₃S₄-CeO₂/NF-2 under 20 h chronopotentiometry and (f) LSV curves of Ni₃S₄-CeO₂/NF-2 before and after 1000 CV cycles.

The charge transfer kinetics of the catalytic materials were analyzed using electrochemical impedance spectroscopy (EIS). The interfacial charge transfer resistance (R_ct) values of Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2 and Ni₃S₄-CeO₂/NF-3 are 71.81 Ω, 24.69 Ω and 28.75 Ω (Fig. 6c), respectively. Ni₃S₄ and CeO₂/NF cannot be displayed on the graph due to their large resistance. It shows that the catalytic reaction of Ni₃S₄-CeO₂/NF-2 transfers electrons quickly and has high conductivity. The interfacial capacitance (C_dl) is obtained from the CV diagram at various sweep rates (Fig. S1), which is positively correlated with the effective ECSA of the catalytic material. The C_dl value of Ni₃S₄-CeO₂/NF-2 (25.16 mF cm⁻²) is higher than those of Ni₃S₄ (0.02 mF cm⁻²), CeO₂/NF (1.82 mF cm⁻²), Ni₃S₄/NF (12.59 mF cm⁻²), Ni₃S₄-CeO₂/NF-1 (0.135 mF cm⁻²) and Ni₃S₄-CeO₂/NF-3 (6.47 mF cm⁻²) (Fig. 6d). The C_dl results show that Ni₃S₄-CeO₂/NF-2 has the largest ECSA, which can further expose the active sites of the catalytic material and promote the catalytic activity of the OER. Therefore, based on the performance comparison, due to the addition of CeO₂, an interfacial synergistic effect generated by the heterojunction between CeO₂ and Ni₃S₄ optimizes the electronic structure and accelerates the reaction kinetics, so that the performance of Ni₃S₄-CeO₂/NF composites is significantly improved. More importantly, Ni₃S₄-CeO₂/NF-2 was tested by chronoamperometry to evaluate its durability. Current density versus time is plotted in Fig. 6e. The catalyst can be stabilized at 10 mA cm⁻² for a long time at a constant overpotential, and the total change of overpotential after 20 hours is only 16 mV, which clearly shows the good OER durability of Ni₃S₄-CeO₂/NF-2. Moreover, the overpotential of 10 mA cm⁻² driven by Ni₃S₄-CeO₂/NF-2 is better than that of the reported OER catalytic materials (Table 1).

Table 1 Comparison of OER overpotentials (η) of Ni₃S₄-CeO₂/NF at 10 mA cm⁻² with the published data

Electrocatalysts	Overpotential η (mV)	Ref.
Ni3S4/NF	266	58
NM50-Ni3S4	257	59
Ce0.2-Ni3S4	230	60
Ni3S4/N,P-HPC	370	61
MnS/Ni3S4	350	62
Ni3S4/FeS@FNF	196	63
Ni3S4-CeO2/NF	110	This work

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In order to further evaluate the stability of Ni₃S₄-CeO₂/NF-2 as an anode, we also compared the morphology, composition and atomic percentage of each element of the composites before and after 20 h of reaction.

It can be clearly seen from the SEM images and the corresponding EDS mappings before and after 20 hours of reaction (Fig. 7a–j) that although the electrode material has some defects, the overall morphology remains intact and no obvious structural collapse is observed. This shows that our composite material design has excellent mechanical and structural stability in long-term electrochemical reactions, which can maintain the integrity of the electrode structure.

Fig. 7 (a) SEM image of Ni₃S₄-CeO₂/NF-2 and (b–e) the corresponding EDS mappings; (f) SEM image of Ni₃S₄-CeO₂/NF-2 after 20 h of reaction and (g–j) the corresponding EDS mappings.

We also use a semi-quantitative method to characterize the retention of sulfur inside the electrode, which is an effective indirect means to evaluate the loss of active substances. From the SEM and EDS spectra, it can be seen that Ni₃S₄-CeO₂/NF-2 before and after the reaction contains the same S, Ni, O and Ce elements. In Fig. S2 a and b, the percentage of sulfur atoms in the catalyst decreased from 60.05% to 45.01%, a decrease of 15.04%, while the percentage of oxygen atoms increased from 3.38% to 26.74%. This result shows that during the 20-hour reaction process, although sulfur is partially leaked, most of the sulfur is effectively limited inside the electrode structure, which proves that the sulfide composite anode we designed has good structural stability and chemical stability.

In order to explore the chemical state and composition of the electrode surface, we also carried out XPS tests on the composite material after 20 h of reaction. The full spectrum (Fig. 8a) shows that after the reaction, Ni₃S₄/CeO₂-NF still contains Ni, S, O and Ce elements. In Fig. 8b, the peaks at 163.69 eV and 162.39 eV are still attributed to S 2p_1/2 and S 2p_3/2, while the peak at 168.49 eV is attributed to the sulfate group. Compared with the composite before the reaction, in Fig. 8c, Ni 2p shifted to the right, indicating that Ni and S were oxidized to the main electrochemical active center (losing electrons) during the 20 h electrochemical reaction. At the same time, the spectral peak of Ce 3d (Fig. 8d) also moved to the high binding energy direction, and its valence state change (Ce³⁺ → Ce⁴⁺) effectively regulated the local electronic structure. At the same time, O 1s (Fig. 8e) moves to the left, indicating that Ce and oxygen form a synergistic effect.

Fig. 8 (a) The full spectrum and (b) S 2p, (c) Ni 2p, (d) Ce 3d, and (e) O 1s spectra of Ni₃S₄-CeO₂/NF-2 after 20 h of reaction.

4. Conclusion

In summary, CeO₂ was synthesized by a hydrothermal method followed by heat treatment, and then Ni₃S₄ was combined with different proportions of the as-prepared CeO₂ by a simple hydrothermal method to form the Ni₃S₄-CeO₂/NF hybrid electrocatalyst for the alkaline electrocatalytic oxygen evolution reaction. The Ni₃S₄-CeO₂/NF-2 electrocatalyst required only 110 mV overpotential to reach a current density of 10 mA cm⁻², with a Tafel slope of 42.3 mV dec⁻¹. At the same time, the current density of Ni₃S₄-CeO₂/NF-2 was only slightly attenuated after electrolysis at a constant potential for 20 h, showing good durability. This is mainly due to the electronic configuration alteration of Ni₃S₄ promoted by CeO₂ and the complementary effect caused by the interaction between Ni₃S₄ and CeO₂, which promotes the overall electrochemical oxygen evolution activity.

Author contributions

Shuqian Sun: methodology, investigation, writing – original draft, investigation, and validation. Nan Li: methodology and writing – review and editing. Xinding Lv: validation and investigation. Yi Wu: investigation and validation. Yue Li: investigation and validation. Can Zhao: validation and investigation. Jiangquan Ma: methodology and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI).

Supplementary information: the SI include: chemicals; characterization; preparation of electrodes; electrochemical measurements CV curves of Ni₃S₄, Ni₃S₄/NF, Ni₃S₄-CeO₂/NF-1, Ni₃S₄-CeO₂/NF-2, Ni₃S₄-CeO₂/NF-3; EDS spectra of Ni₃S₄-CeO₂/NF-2. See DOI: https://doi.org/10.1039/d5cy01006c.

Acknowledgements

The authors gratefully acknowledge support from the Changzhou University, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Natural Science Foundation of Jiangsu Province (BK20220630), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX24_1600). The authors would like to thank eceshi (https://www.eceshi.com) for assisting in XPS analysis and TEM analysis.

References

1. P. Zhai, C. Wang, Y. Zhao, Y. Zhang, J. Gao, L. Sun and J. Hou, Regulating electronic states of nitride/hydroxide to accelerate kinetics for oxygen evolution at large current density, Nat. Commun., 2023, 14, 1873.
2. M.-A. Ha, S. M. Alia, A. G. Norman and E. M. Miller, Fe-Doped Ni-Based Catalysts Surpass Ir-Baselines for Oxygen Evolution Due to Optimal Charge-Transfer Characteristics, ACS Catal., 2024, 14, 17347–17359.
3. L. Wang, J. Li, Q. Meng, M. Xiao, C. Liu, W. Xing and J. Zhu, Facilitating active NiOOH formation via Mo doping towards high-efficiency oxygen evolution, Catal. Sci. Technol., 2024, 14, 4166–4173.
4. N. Li, L. Ai, J. Jiang and S. Liu, Spinel-type oxygen-incorporated Ni³⁺ self-doped Ni₃S₄ ultrathin nanosheets for highly efficient and stable oxygen evolution electrocatalysis, J. Colloid Interface Sci., 2020, 564, 418.
5. J. W. Park, G. Park, M. Kim, M. Han, J. Jang, Y. Yamauchi, B. Yuliarto, P. Krüger, J. Kim, N. Park and H. Lim, Ni-single atom decorated mesoporous carbon electrocatalysts for hydrogen evolution reaction, Chem. Eng. J., 2023, 468, 143733.
6. W. Fang, H. Hu, T. Jiang, G. Li and M. Wu, N- and S-doped porous carbon decorated with in-situ synthesized Co–Ni bimetallic sulfides particles: A cathode catalyst of rechargeable Zn-air batteries, Carbon, 2019, 146, 476–485.
7. C. Gao, L. Pan, H. Wang, H. Guo, S. Melhi, M. A. Amin and J. Lin, Cerium doping-induced enrichment of Ni₃S₄ phase for boosting oxygen evolution reaction, ChemSusChem, 2024, 17, e202400056.
8. C. Zhang, X. Du, Y. Wang, X. Han and X. Zhang, NiSe₂@Ni_xS_y nanorod on nickel foam as efficient bifunctional electrocatalyst for overall water splitting, Int. J. Hydrogen Energy, 2021, 46, 34713–34726.
9. F. Zhong, A. Xu, Q. Zeng, Y. Wang, G. Li, Z. Xu, Y. Yan and S. Wu, Confining MoSe₂ Nanosheets into N-Doped Hollow Porous Carbon Microspheres for Fast-Charged and Long-Life Potassium-Ion Storage, ACS Appl. Mater. Interfaces, 2021, 13, 59882–59891.
10. A. Yi, L. Zhou, J. Xie, X. Shi, H. Liang, F. Wang, D. Zheng and X. Lu, Local spin-state tuning enables high-efficiency nickel sulfide cathode for stable alkaline Zn batteries, Chem. Eng. J., 2024, 491, 151958.
11. Y. Sugawara, T. Uchiyama, M. Shishkin and T. Yamaguchi, Electrochemical oxygen evolution catalyzing of metal sulfides: a systematic study of electronic effects, Catal. Sci. Technol., 2025, 15, 7761–7770.
12. Z. Li, M. Hu, P. Wang, J. Liu, J. Yao and C. Li, Heterojunction catalyst in electrocatalytic water splitting, Coord. Chem. Rev., 2021, 439, 213953.
13. T. Munawar, A. Bashir, M. U. Nisa, R. A. Alshgari, F. Mukhtar, S. Mohammad, M. N. Ashiq, M. F. Ehsan, F. Iqbal and S. I. Allakhverdiev, Unravelling the operando structural and chemical stability of rare earth metals co-doped CeO₂-based electrocatalysts for oxygen evolution reaction, Int. J. Hydrogen Energy, 2025, 137, 1097–1106.
14. N. Yao, R. Meng, F. Wu, Z. Fan, G. Cheng and W. Luo, Oxygen-Vacancy-Induced CeO₂/Co₄N heterostructures toward enhanced pH-Universal hydrogen evolution reactions, Appl. Catal., B, 2020, 277, 119282.
15. J. Liu, G. Xu, H. Zhen, H. Zhai, C. Li and J. Bai, Directed self-assembled pathways of 3D rose-shaped PtNi@CeO₂ electrocatalyst for enhanced hydrogen evolution reaction, J. Alloys Compd., 2023, 931, 167379.
16. X. Ding, R. Jiang, J. Wu, M. Xing, Z. Qiao, X. Zeng, S. Wang and D. Cao, Ni₃N–CeO₂ Heterostructure Bifunctional Catalysts for Electrochemical Water Splitting, Adv. Funct. Mater., 2023, 33, 2306786.
17. C. Lyu, J. Cheng, H. Wang, Y. Yang, K. Wu, P. Song, W.-m. Lau, J. Zheng, X. Zhu and H. Y. Yang, Construction of interface-engineered coral-like nickel phosphide@cerium oxide hybrid nanoarrays to boost electrocatalytic hydrogen evolution performance in alkaline water/seawater electrolytes, Adv. Compos. Hybrid Mater., 2023, 6, 175.
18. H. Sun, C. Tian, G. Fan, J. Qi, Z. Liu, Z. Yan, F. Cheng, J. Chen, C.-P. Li and M. Du, Boosting Activity on Co₄N Porous Nanosheet by Coupling CeO₂ for Efficient Electrochemical Overall Water Splitting at High Current Densities, Adv. Funct. Mater., 2020, 30, 1910596.
19. Q. Guo, Y. Li, Z. Xu and R. Liu, CeO₂-Accelerated Surface Reconstruction of CoSe₂ Nanoneedle Forms Active CeO₂@CoOOH Interface to Boost Oxygen Evolution Reaction for Water Splitting, Adv. Energy Mater., 2025, 15, 2403744.
20. M. Li, X. Wang, D. Zhang, Y. Huang, Y. Shen, F. Pan, J. Lin, W. Yan, D. Sun, K. Huang, Y. Tang, J.-M. Lee, H. Li and G. Fu, Atomic rare earths activate direct O-O coupling in manganese oxide towards electrocatalytic oxygen evolution, Nano Energy, 2024, 128, 109868.
21. P. Zhou, H. Pan, G. Hai, X. Liu, X. Huang and G. Wang, Expediting *OH accumulation kinetics on metal-organic frameworks-derived CoOOH with CeO₂ “accelerator” for electrocatalytic 5-hydroxymethylfurfural oxidation valorization, J. Energy Chem., 2024, 98, 721–732.
22. J.-C. Ni, D.-K. Liu, X.-Z. Song, S. Yu, X.-B. Wang, Y.-X. Luan, X.-M. Zhao, Z. Tan, C. Lv and X.-F. Wang, Regulation of d-band center in hollow CeO₂/CoFeP heterojunctions for boosting bifunctional oxygen/hydrogen evolution electrocatalysis, J. Alloys Compd., 2025, 1017, 179056.
23. K. Zhang, J. Du, D. Luo, H. Shi, J. Wang, J. Zhang, X. Liu, M. Liu, K. Mei, D. Liu, Y. Zhang and S. Li, Structurally ordered FeCo@FeCoO_x@NC dual-shell nanoparticles synthesized under micro-oxygen conditions: An efficient cocatalyst for BiVO₄ photoelectrochemical water oxidation, Appl. Catal., B, 2025, 363, 124779.
24. V. Sumalatha, C. Anujya, V. Balchander, B. Dhanalaxmi, M. Pradeep Kumar and D. Ayodhya, Hydrothermal fabrication of n-CeO₂/p-CuS heterojunction nanocomposite for enhanced photodegradation of pharmaceutical drugs in wastewater under visible-light and fluorometric sensor for detection of uric acid, Inorg. Chem. Commun., 2023, 155, 110962.
25. Y. Shen, L.-e. Liu, T. Li, R. Liu, J. Zhang, J. Chen, X. Zhang, X. Li, N. Jian and D. Wu, An ingenious integrated metal-organic frameworks-based ratiometric sensing platform for efficient, sensitive and real-time detection of tetracyclines, Food Chem., 2025, 472, 142892.
26. S. Jiang, Q. Li, Q. Zhao, L. Cheng and T. Jiang, A hierarchical core-shell CuO@FeCoP/CF heterostructure for efficient overall water splitting in alkaline media, J. Alloys Compd., 2025, 1010, 177291.
27. J. D. Gojgić, A. M. Petričević, T. Rauscher, C. I. Bernäcker, T. Weißgärber, L. Pavko, R. Vasilić, M. N. Krstajić Pajić and V. D. Jović, Hydrogen evolution at Ni foam electrodes and Ni-Sn coated Ni foam electrodes, Appl. Catal., A, 2023, 663, 119312.
28. H. Chen, M. Hu, P. Jing, B. Liu, R. Gao and J. Zhang, Constructing heterostructure of CeO₂/WS₂ to enhance catalytic activity and stability toward hydrogen generation, J. Power Sources, 2022, 521, 230948.
29. Z. Gao, C. Chen, J. Chang, L. Chen, P. Wang, D. Wu, F. Xu and K. Jiang, Porous Co₃S₄@Ni₃S₄ heterostructure arrays electrode with vertical electrons and ions channels for efficient hybrid supercapacitor, Chem. Eng. J., 2018, 343, 572–582.
30. D. Wu, X. Xie, J. Zhang, Y. Ma, C. Hou, X. Sun, X. Yang, Y. Zhang, H. Kimura and W. Du, Embedding NiS nanoflakes in electrospun carbon fibers containing NiS nanoparticles for hybrid supercapacitors, Chem. Eng. J., 2022, 446, 137262.
31. Y. Ren, S. Zhu, Y. Liang, Z. Li, S. Wu, C. Chang, S. Luo and Z. Cui, Hierarchical Ni₃S₄@MoS₂ nanocomposites as efficient electrocatalysts for hydrogen evolution reaction, J. Mater. Sci. Technol., 2021, 95, 70–77.
32. X. Guo, M. Li, L. Qiu, F. Tian, L. He, S. Geng, Y. Liu, Y. Song, W. Yang and Y. Yu, Engineering electron redistribution of bimetallic phosphates with CeO₂ enables high-performance overall water splitting, Chem. Eng. J., 2023, 453, 139796.
33. G. S. de Luna, P. Zeller, E. Öztuna, F. Maluta, A. Canciani, F. Ospitali, P. H. Ho, A. Paglianti, A. Knop-Gericke, G. Fornasari, J. J. Velasco-Vélez and P. Benito, In Situ Development of a 3D Cu-CeO₂ Catalyst Selective in the Electrocatalytic Hydrogenation of Biomass Furanic Compounds, ACS Catal., 2023, 13, 12737–12745.
34. N. Li, L. Huo, Q. Dong, B. Zhu, L. Huang and J. Ma, RuSe₂/CeO₂ heterostructure as a novel electrocatalyst for highly efficient alkaline hydrogen evolution, Nanotechnology, 2024, 35, 115602.
35. Y. Liu, L. Xiao, H. Tan, J. Zhang, C. Dong, H. Liu, X. Du and J. Yang, Amorphous/Crystalline Phases Mixed Nanosheets Array Rich in Oxygen Vacancies Boost Oxygen Evolution Reaction of Spinel Oxides in Alkaline Media, Small, 2024, 20, 2401504.
36. M. Zhang, Y. Zhang, L. Tang, Y. Zhu, J. Wang, C. Feng, S. Fu, L. Qiao and Y. Zhang, Synergetic utilization of 3D materials merits and unidirectional electrons transfer of Schottky junction for optimizing optical absorption and charge kinetics, Appl. Catal., B, 2021, 295, 120278.
37. X. Wang, Y. Shan, L. Wang, K. Chen and X. Yu, Self-supporting 3D cross-linked NiFe-Co/NC@NiMoO₄ electrode for efficient overall water splitting and rechargeable Zn-air batteries, Appl. Catal., B, 2025, 375, 125406.
38. A.-y. Lee, J. Kim, M. Pyeon, Y. Son, J. Yoo and K. Lee, Bifunctional Ni₃S₂ nanoflake/NiMoO₄ nanoneedle composite electrocatalysts for efficient urea oxidation and hydrogen evolution in sustainable water electrolysis, Chem. Eng. J., 2025, 512, 162044.
39. P. Zhou, D. Liu, Y. Xiong, Q. Chen, L. Wu, W. Yao, X. Yang and D. Wang, Construction of Mott-Schottky heterojunction triggering d orbital electron engineering in Ni₃S₂ to optimize electrocatalytic oxygen evolution, J. Power Sources, 2025, 653, 237698.
40. Y. Gao, C. Yang, F. Sun, D. He, X. Wang, J. Chen, X. Zheng, R. Liu, H. Pan and D. Wang, Ligand-Tuning Metallic Sites in Molecular Complexes for Efficient Water Oxidation, Angew. Chem., Int. Ed., 2025, 64, e202415755.
41. M. Liu, X. Zhong, X. Chen, D. Wu, C. Yang, S. Li, C. Ni, Y. Chen, Q. Liu and H. Su, Unraveling Compressive Strain and Oxygen Vacancy Effect of Iridium Oxide for Proton-Exchange Membrane Water Electrolyzers, Adv. Mater., 2025, 37, 2501179.
42. B. Jiang, C. Zhao, Y. Zhang, S. Gu and N. Zhang, Atomic-Scale Interface Engineering to Construct Highly Efficient Electrocatalysts for Advanced Lithium–Sulfur Batteries, ACS Nano, 2025, 19, 18332–18346.
43. S. Chen, Z. Zheng, Q. Li, H. Wan, G. Chen, N. Zhang, X. Liu and R. Ma, Boosting electrocatalytic water oxidation of NiFe layered double hydroxide via the synergy of 3d–4f electron interaction and citrate intercalation, J. Mater. Chem. A, 2023, 11, 1944–1953.
44. Z.-H. Wang, X.-F. Wang, Y.-S. Gong, X. Chen, W.-Y. Zhu, T. Zhang, Y.-H. Zhao, J.-C. Ni, Z. Tan and X.-Z. Song, CeO₂-modulated CoP derived from prussian blue analogue boosting hydrogen evolution reaction electrocatalysis, J. Alloys Compd., 2022, 913, 165334.
45. S. Zhang, C. Wang, W. Peng, J. Huang, S. Han, S. Li, R. Liu, S. Ma and H. Yao, Dual-phased nickel sulfide heterojunction of Ni₉S₈/Ni₃S₂ doped with Co assembled on Ni foam as a highly efficient electrocatalyst for alkaline oxygen evolution reaction, Chem. Eng. J., 2025, 519, 165471.
46. F. Sun, X. Tian, J. Zang, R. Zhu, Z. Hou, Y. Zheng, Y. Wang and L. Dong, Constructing composite active centers optimized with Cr-doped Ni_S/NiS₂ heterostructure for efficiently catalyzing alkaline hydrogen evolution reaction, Fuel, 2024, 363, 130999.
47. T. D. Vu, P. Khac Duy, H. T. Bui, S.-H. Han and H. Chung, Reduced graphene oxide–Nickel sulfide (NiS) composited on mechanical pencil lead as a versatile and cost-effective sensor for electrochemical measurements of bisphenol A and mercury (II), Sens. Actuators, B, 2019, 281, 320–325.
48. S. Guo, J. Wang, Y. Sun, L. Peng and C. Li, Interface engineering of Co₃O₄/CeO₂ heterostructure in-situ embedded in Co/N-doped carbon nanofibers integrating oxygen vacancies as effective oxygen cathode catalyst for Li-O₂ battery, Chem. Eng. J., 2023, 452, 139317.
49. P. Zhou, W. Yao, D. Liu, Q. Chen, G. Lv, J. Li, L. Wu and D. Wang, Rational electronic structure modulation in the space-charge region of α-Co(OH)₂/CeO₂ p-n heterojunction to boost hydrogen evolution reaction, J. Alloys Compd., 2025, 1012, 178489.
50. S. Iqbal, Spatial Charge Separation and Transfer in L-Cysteine Capped NiCoP/CdS Nano-Heterojunction Activated with Intimate Covalent Bonding for High-Quantum-Yield Photocatalytic Hydrogen Evolution, Appl. Catal., B, 2020, 274, 119097.
51. Y. Liu, L. Cheng, Y. Huang, Y. Yang, X. Rao, S. Zhou, T. Taylor Isimjan and X. Yang, Electronic Modulation and Mechanistic Study of Ru-Decorated Porous Cu-Rich Cuprous Oxide for Robust Alkaline Hydrogen Oxidation and Evolution Reactions, ChemSusChem, 2023, 16, e202202113.
52. S. Madadkhani, S. Nandy, K. H. Chae, P. Aleshkevych and M. M. Najafpour, Advances in Understanding Tungsten Disulfide Dynamics during the Hydrogen-Evolution Reaction: An Initial Step in Elucidating the Mechanism, J. Phys. Chem. Lett., 2024, 15, 5112–5119.
53. Y. Wu, R. Yao, K. Zhang, Q. Zhao, J. Li and G. Liu, RuO₂/CeO₂ heterostructure anchored on carbon spheres as a bifunctional electrocatalyst for efficient water splitting in acidic media, Chem. Eng. J., 2024, 479, 147939.
54. M. Chen, X. Xiao, X. Wang, Y. Lu, M. Zhang, J. Zheng and L. Chen, Self-templated carbon enhancing catalytic effect of ZrO₂ nanoparticles on the excellent dehydrogenation kinetics of MgH₂, Carbon, 2020, 166, 46–55.
55. R. K. Giri, S. H. Chaki, A. J. Khimani and M. P. Deshpande, Mechanistic insights into transport properties of chemical vapour transport grown CuInS2 single crystal, J. Alloys Compd., 2023, 959, 170487.
56. S. You, S. Xing and C. Jiang, Synergistic optimization of microstructures and properties of electrodeposited Ni–CeO₂ composite coatings with CeO₂ microparticles and CeO₂ nanoparticles, J. Mater. Res. Technol., 2024, 29, 181–195.
57. M. Nisa, K. M. Younes, B. Huwaimel, W. M. A. Khojali, W. Farouk Soliman, M. Abdullah and A. M. A. Henaish, Novel hydrothermally synthesized NiS nanomaterial decorated over g-CN nanosheet for OER activity, Fuel, 2024, 368, 131627.
58. N. Li, L. Ai, J. Jiang and S. Liu, Spinel-type oxygen-incorporated Ni³⁺ self-doped Ni₃S₄ ultrathin nanosheets for highly efficient and stable oxygen evolution electrocatalysis, J. Colloid Interface Sci., 2020, 564, 418–427.
59. K. Wan, J. Luo, C. Zhou, T. Zhang, J. Arbiol, X. Lu, B.-W. Mao, X. Zhang and J. Fransaer, Hierarchical Porous Ni3S4 with Enriched High-Valence Ni Sites as a Robust Electrocatalyst for Efficient Oxygen Evolution Reaction, Adv. Funct. Mater., 2019, 29, 1900315.
60. C. Gao, L. Pan, H. Wang, H. Guo, S. Melhi, M. A. Amin and J. Lin, Cerium Doping-Induced Enrichment of Ni3S4 Phase for Boosting Oxygen Evolution Reaction, ChemSusChem, 2024, 17, e202400056.
61. X. Hu, T. Li, Y. Tang, Y. Wang, A. Wang, G. Fu, X. Li and Y. Tang, Hydrogel-Derived Honeycomb Ni3S4/N,P-C as an Efficient Oxygen Evolution Catalyst, J. Alloys Compd., 2019, 25, 7561–7568.
62. W. Fan and G. Li, Waxberry-Like MnS/Ni₃S₄ as High-Efficiency Bi-Functional Catalyst for Zn-Air Batteries, Chem. – Eur. J., 2023, 29, e202300206.
63. P. Tan, Y. Wu, Y. Tan, Y. Xiang, L. Zhou, N. Han, Y. Jiang, S.-J. Bao and X. Zhang, In Situ Fast Construction of Ni₃S₄/FeS Catalysts on 3D Foam Structure Achieving Stable Large-Current-Density Water Oxidation, Small, 2024, 20, 2308371.

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