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Article

Sulfur-Doped Nickel–Iron LDH@Cu Core–Shell Nanoarrays on Copper Mesh as High-Performance Electrocatalysts for Oxygen Evolution Reaction

by
Zhichao Zhang
1,†,
Jiahao Guo
1,†,
Yuhan Sun
2,†,
Qianwei Wang
2,
Mengyang Li
2,
Feng Cao
2,* and
Shuang Han
1,*
1
School of Science, Shenyang University of Chemical Technology, Shenyang 110142, China
2
Key Lab for Anisotropy and Texture of Materials (MoE), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
J. Compos. Sci. 2023, 7(12), 486; https://doi.org/10.3390/jcs7120486
Submission received: 14 October 2023 / Revised: 9 November 2023 / Accepted: 21 November 2023 / Published: 23 November 2023
(This article belongs to the Special Issue Nanocomposite Materials for Energy, Environment and Sustainable Development)
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Versions Notes

Abstract

:
The oxygen evolution reaction (OER) is a slow step in electrocatalytic water splitting. NiFe layered double hydroxides (LDH) have shown promise as affordable OER electrocatalysts, but their performance is hindered by poor charge transfer and sluggish kinetics. To address this, we doped NiFe LDH with sulfur (S) using an in situ electrodeposition method. By growing S-doped NiFe LDH on Cu nanoarrays, we created core–shell structures that improved both the thermodynamics and kinetics of OER. The resulting S-NiFe LDH@Cu core–shell nanoarrays exhibited enhanced activity in water oxidation, with a low potential of 236 mV (at 50 mA cm−2) and a small Tafel slope of 50.64 mV dec−1. Moreover, our alkaline electrolyzer, based on these materials, demonstrated remarkable activity, with a low voltage of 1.56 V at 100 mA cm−2 and excellent durability. The core–shell nanoarray structures provided a larger electroactive surface area, facilitated fast electron transport, and allowed for effective gas release. These findings highlight the potential of S-NiFe LDH@Cu core–shell nanoarrays as efficient OER electrocatalysts.

1. Introduction

The depletion of non-renewable energy resources and their consequential environmental impacts necessitate the urgent exploration of alternative clean energy sources [1,2]. Hydrogen attracts much attention and is believed to be the ideal candidate due to its high energy density and lack of emission of greenhouse gases after its consumption [3,4]. Electrocatalytic water splitting, a highly promising method for hydrogen production, harnesses surplus electricity and water to yield exceptionally pure hydrogen [5]. However, the scarcity of cost-effective catalysts with remarkable activity and stability, particularly for the oxygen evolution reaction (OER) process, presents a significant challenge in the field of electrocatalytic water splitting [6,7]. The kinetics of the OER process are hindered by complex multi-proton coupled electron transfer processes, thereby limiting the efficiency of water electrolysis [8,9,10]. Precious metals and their oxides, including IrO2 and RuO2, are regarded as benchmark electrocatalysts due to their superior OER activity. Nonetheless, their limited availability and high cost impede their widespread implementation [11,12,13].
In recent investigations, the catalytic activity of oxides/hydroxides of first-row transition metals (TMs) such as nickel (Ni) and iron (Fe) towards the oxygen evolution reaction (OER) has been found to be remarkable [14,15]. Moreover, studies have revealed that binary compounds of these metals exhibit higher electrical conductivity and a greater abundance of unsaturated coordination cations compared to monovalent compounds [16,17,18]. These unsaturated coordination cations function as adsorbed surface-active sites, effectively enhancing OER activity by activating reaction intermediates. For instance, the group led by Wang successfully synthesized nanoscale NiFe-based compounds that exhibited outstanding OER performance [19,20]. Nevertheless, despite extensive efforts to improve the catalytic activity and stability of NiFe-based compounds for OER, their performance is still limited due to several factors. Primarily, in strongly alkaline electrolytes, NiFe-based compounds tend to display inherent instability, particularly under high overpotentials, resulting in erosion [21,22,23]. Additionally, during electrochemical testing, the aggregation of NiFe-based compound nanoparticles reduces the number of accessible active sites [24]. Furthermore, inefficient mass transport and electron transfer between the NiFe catalysts and the current collector further hinder OER performance [25]. Consequently, it is crucial to explore feasible approaches that can enhance the OER activities of NiFe-based compounds.
Indeed, element doping has been recognized as an effective strategy to modify the surface electronic structure of catalysts, thus achieving improved adsorption energy [26,27,28]. Various metal elements, such as Cr, Co, Mn, and others, have been introduced into NiFe hydroxides to enhance their catalytic performance [29,30,31,32,33]. Additionally, the incorporation of non-metallic elements like P, N, S, and others into NiFe LDH (layered double hydroxide) has been considered a promising approach to adjusting the metal sites or microstructure [34,35,36]. Unlike the doping of metal cations, non-metallic elements typically act as electron donors to modulate the electronic structure. Among these non-metallic elements, the incorporation of sulfur (S) has recently gained attention and has shown significant potential for enhancing the oxygen evolution reaction (OER) due to the localized disruption of lattice periodicity. For instance, Li et al. [37] prepared S-doped Ni4/5Fe1/5-LDH and observed that the interaction between surface S and Fe synergistically enhances OER activity. The presence of Fe-S substances regulates the bonding between Fe and O* intermediates. Moreover, the introduction of S atoms leads to the generation of new amorphous or defect-rich structures, which are favorable for material reconstruction in the electrolyte and promote the formation of high-valence metal active sites. However, further investigation is still required to fully understand the underlying mechanism behind the catalytic activity enhancement of NiFe hydroxides resulting from the introduction of S atoms.
Herein, a facile process involving the electrodeposition of chemical oxidation calcination precursors was employed to produce metal hydroxide. Specifically, an array of CuO nanowires (NWs) was fabricated on a copper mesh and subsequently reduced to Cu nanowires (NWs), establishing an effective electron pathway. The resulting nanocomposites were able to uniformly wrap around the surface of the Cu NWs, maximizing the specific surface area. To further optimize the performance, the S element was introduced to adjust the electronic structure. The S-NiFe LDH@Cu ternary compound exhibited excellent oxygen evolution reaction (OER) performance in an alkaline medium (1 M KOH). At current densities of 50 mA cm−2 and 100 mA cm−2, the overpotentials of the S-NiFe LDH@Cu catalyst were measured to be 236 mV and 261 mV, respectively, surpassing those of the original NiFe LDH@Cu catalyst. Additionally, an electrocatalytic cell assembled with S-NiFe LDH@Cu and Pt/C achieved a current density of 100 mA cm−2 at a voltage of only 1.56 V, which is significantly better than that of the RuO2‖Pt/C electrocatalytic cell (1.64 V). These findings demonstrate the enhanced performance of the S-NiFe LDH@Cu catalyst and its potential as an efficient electrocatalyst for the oxygen evolution reaction.

2. Materials and Methods

2.1. Materials

Nickel nitrate hexahydrate (Ni (NO3)2·6H2O), ferrous chloride (FeCl2·4H2O ), sodium hydroxide (NaOH), polyvinyl pyrrolidone (PVP, (C6H9NO)n), hydrochloric acid (HCL) and ammonium persulfate ((NH4)S2O8) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Anhydrous ethanol (C2H6O), potassium chloride (KCl), potassium hydroxide (KOH), and thiourea (CH4N2S) were purchased from Yongda Chemical Reagent Co., Ltd, Tianjin, China. The copper mesh was purchased from Anping Hangying Wire Mesh Products Co., Ltd., Hengshui, China. The Nafion solution was purchased from Minnesota Minerals and Manufacturing Company, Saint Paul, MN, USA. All chemical reagents were of analytical grade and no further purification was required.

2.1.1. The Preparation of Cu Nanowires (NWs) as the Substrate

The copper mesh (geometric surface area of 1 × 3 cm2 and thickness of 2.0 mm) was washed as a substrate and ultrasonicated in 1.0 M hydrochloric acid (HCl) for 10 min to remove oxides on the surface. Then, the copper mesh was treated with deionized water and ethanol, respectively, for 10 min. After drying, the copper mesh was placed in the mixed solution of NaOH and (NH4)S2O8 at room temperature for 10 min to obtain blue Cu(OH)2 NWs, which were washed and dried with ultra-pure water and ethanol for use. The Cu(OH)2 NWs were calcined in a box furnace (300 °C, 1 h) to obtain a black-brown CuO NWs substrate (with a heating rate of 1 °C/min), and then, the CuO NWs were electrochemically reduced to Cu NWs. Then, under an applied voltage of −1.25V (vs. Hg/HgO/1 M KOH), CuO NWs were employed as the working electrode, a platinum mesh served as the counter electrode, Hg/HgO was used as the reference electrode, and Cu NWs were obtained through the process of electrochemical reduction.

2.1.2. The Preparation of S-NiFe LDH@Cu Electrodes

Using a standard three-electrode system, the Cu NW substrate was employed as the working electrode, and the platinum mesh (Pt) and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Electrochemical deposition was carried out using a CHI760D potentiostat in galvanostatic mode at −1.0 V vs. SCE. The electrolyte solution consisted of 0.01 mol FeCl2·4H2O, 0.5 g of PVP, 0.5 g of KCl, 0.02 mol of Ni(NO3)2·6H2O, and an amount of thiourea of 0.002 mol in 100 mL of deionized water. After deposition, the electrodes were rinsed with Milli-Q water and prepared for further analysis. The same method was also used to prepare NiFe LDH@Cu.

2.2. Material Characterization

X-ray diffraction (XRD) is an effective means to determine the crystal structure of materials. The phase constitutions of the alloys were examined using a Smartlab X-ray diffractometer from the Japanese company Rigaku. The test working current was 200 mA and the working voltage was 40 kV, with Cu Kα Ray (λ = 1.54 Å) as the incident source, filtered using a graphite monochromator, using θ/2θ. Linkage mode was used for continuous scanning in the range of 10–90°, and the scanning speed was 5°/min.
Scanning electron microscopy (SEM) can interact with materials through incident electrons, and then, obtain the morphological information of materials. The field emission scanning electron microscope JSM-7001F produced by Japan Electronics Corporation (JEOL) was used in this experiment to obtain surface morphology.
Transmission electron microscopy (TEM) is a method used to observe more detailed crystal structure and phase composition. In this experiment, the JEM-2100F transmission electron microscope produced by Japan Electronics Corporation (JEOL) was used to obtain the information on material morphology and crystal structure.

2.3. Electrochemical Measurements

All electrochemical measurements were carried out using a three-electrode system with an electrochemical workstation (Zahner iM6e). A Hg/HgO electrode was used as the reference electrode and a graphite rod was used as the counter electrode. The working electrode was made by connecting the electrodeposited sample (1 × 1 cm2) and the wire with silver glue and two-component curing glue (AB glue). For Rubidium oxide, 5 mg of the sample and 5 μL 5% Nafion solution were added to an 800 μL mixture of water/ethanol with a volume ratio of 3:1 and dispersed via ultrasonication for 30 min to form a homogeneous ink. After that, 30 μL of this ink was carefully dropped onto a Cu mesh with an area of 1 × 1 cm2 5 times and dried in air. Line sweep voltammogram (LSV) polarization curves were recorded with a scan rate of 5 mV s−1 over a potential range of 1 to 2 V vs. RHE in 1 M KOH. A Tafel slope was calculated from the LSV polarization curve according to the following equation:
η = a + b log j
where j is the current density, b is the Tafel slope, and η is the overpotential.
Cyclic voltammograms (CV) at various scan rates (10, 20, 30, 40, 50 mV/s) were collected in the 0.8−0.95 V vs. RHE range. Double-layer capacitance (Cdl) can be estimated using CV measures. The electrochemical surface area (ECSA) was assessed from the electrochemical double-layered capacitance (Cdl). Electrochemical impedance spectroscopy (EIS) measurements were carried out at a potential of 1.555 V vs. RHE with frequencies of 0.1 Hz to 100,000 Hz and an amplitude of 5 mV. An equivalent Randles circuit model was fitted to the data to determine the system resistance and capacitance.
The faradaic efficiency can be calculated using the following equation under standard conditions (25 °C, 1 atm) when the volumes of H2 and O2 produced are collected simultaneously via the drainage method at a constant current density of 100 mA cm−2:
FE % = n F V m e a s u r e d Q V m × 100 %
where n is the reactive electron number, F is the Faraday constant (96,485 C mol−1), Q is the total charge passed through the electrodes, and Vm is the molar volume of gas.

3. Result and Discussion

3.1. Characterizations of Samples

We prepared sulfur-doped NiFe layered doubled hydroxides (noted as S-NiFe LDH@Cu) on a template of in situ-generated Cu nanowire arrays via the electrodeposition strategy. The overall preparation process, illustrated in Figure 1, involved several steps, including the chemical oxidization of the Cu mesh, high-temperature calcination, electroreduction, and electrochemical deposition.
The morphologies of copper nanowires (Cu NWs), NiFe LDH@Cu, and S-NiFe LDH@Cu were characterized using scanning electron microscopy (SEM). Initially, a series of treatments were applied to a copper mesh, resulting in the spontaneous growth of Cu nanowire arrays on the surface of a porous micrometer-sized pure copper substrate, significantly increasing the contact area of the deposition layer (Figure 2a). Subsequently, NiFe LDH shell layers were electrodeposited onto the surface of the Cu NWs. As depicted in Figure 2b, the surface of the nanowires became relatively rough, indicating the successful coating of NiFe LDH on the Cu core [38,39]. The morphology of S-NiFe LDH@Cu did not exhibit significant differences compared to NiFe LDH@Cu, suggesting that sulfur doping did not affect the morphology of NiFe LDH (Figure 2c). Transmission electron microscopy (TEM) images (Figure 2d,e) further demonstrated the core–shell structure of S-NiFe LDH, revealing a clear coating of rough S-NiFe LDH on the Cu nanowires. The nanowire diameter was approximately 50 nm, and the width of S-NiFe LDHs after electrochemical deposition was about 90 nm (Figure 2e). In the high-resolution TEM (HRTEM) image of S-NiFe LDH@Cu, three d-spacings were measured as 0.15 nm, 0.25 nm, and 0.21 nm. Among them, the d-spacing of 0.21 nm corresponds to the (111) plane of Cu NWs, while the d-spacings of 0.15 nm and 0.25 nm correspond to the (110) and (012) planes of S-NiFe LDH, respectively (Figure 2f). S-NiFe LDH@Cu exhibited an interplanar distance of 0.25 nm, which is larger than that of the pristine NiFe LDH (0.23 nm), indicating a strong interaction between the anions and the NiFe LDH layers (Figure S1c), confirming the successful growth of sulfur atoms in the interlayer of NiFe LDH [40]. Energy-dispersive X-ray elemental mapping analysis was further applied to confirm the spatial distribution of each element on Cu NWs. The mapping images (Figure 2g–i) demonstrated that copper, nickel, iron, and sulfur atoms were homogeneously distributed without any noticeable segregation. This suggests that the co-deposition of nickel, sulfur, and iron can be achieved in a one-step synthesis to prepare homogeneous S-NiFe LDH. Moreover, it was observed that the inclusion of sulfur (S) dopant had no discernible impact on the morphology of NiFe LDH. Even in the absence of S doping, NiFe LDH exhibited characteristic curled sheet-like structures that adhered to the surface of Cu NWs, as evidenced by the micrographs presented in Figure S1a,b. Additionally, elemental distribution analysis revealed a homogeneous dispersion of each constituent element on the Cu nanowires, forming a well-defined core–shell architecture, as depicted in Figure S1d–h. The X-ray diffraction (XRD) patterns of the investigated samples are illustrated in Figure 2m. Notably, distinct diffraction peaks were exclusively observed at 43.2°, 50.3°, and 74.0°, which can be attributed to the (111), (200), and (220) crystallographic planes of the Cu substrate, respectively (as indexed by PDF#99-0034) [41]. Conversely, no prominent diffraction peaks corresponding to LDHs were detected in the XRD patterns, regardless of the presence or absence of S doping. This observation can be ascribed to the relatively modest crystallinity of the electrodeposited S-NiFe LDH.

3.2. Electrocatalytic Performance on OER for Different Samples

The oxygen evolution reaction (OER) performance of the sample was evaluated in a 1 M KOH electrolyte at a scanning rate of 5.0 mV s−1, and the polarization curve is shown in Figure 3a. The copper nanowire array exhibited almost no OER activity. However, after electrodeposition, NiFe LDH@Cu demonstrated superior OER performance with an overpotential of only 253 mV at 50 mA cm−2, outperforming Cu NWs (517 mV) and commercial RuO2@Cu (273 mV), indicating that the layered doubled hydroxides loaded on Cu NWs primarily contribute to the OER activity in this composite material. With the doping of the S element, S-NiFe LDH@Cu achieved a further reduction in overpotential at the same current density. At a current density of 50 mA cm−2, the overpotential was only 236 mV, and at a high current density of 100 mA cm−2, the overpotential was 261 mV. This demonstrates the indispensable significance of S, which may lead to more lattice defects and synergistic effects with NiFe-LDH, effectively lowering the energy difference between intermediate species O* and OOH*, and regulating their adsorption and desorption energies on the surface, thereby enhancing OER performance [42,43]. The polarization curve presented in Figure 3c was utilized to determine the corresponding kinetic parameters, specifically, the Tafel slope. The Tafel slope value of S-NiFe LDH@Cu was calculated to be 50.64 mV dec−1, which is lower than that of NiFe LDH@Cu (61.02 mV dec−1), RuO2@Cu (82.24 mV dec−1), and Cu NWs (114.98 mV dec−1). This indicates that S-NiFe LDH@Cu exhibits a more efficient kinetic oxygen evolution reaction (OER) process, thereby enhancing its OER performance [44]. In comparison, S-NiFe LDH@Cu produced via electrodeposition, a simple and efficient preparation technique, also demonstrates excellent electrocatalytic activity among the best performing group (Figure 3d). For more detailed information, refer to Table S1, which provides comprehensive insights into the performance characteristics. The electrochemical active surface area (ECSA) is a key parameter for electrode electrochemical performance. By measuring the ECSA, the relative catalytic activities of different catalyst materials can be compared [45]. On this basis, the electrochemical active surface area (ECSA) values of the samples were calculated using the electrochemical double-layer capacitance (Cdl) equation, which can be measured using the CV (Figure 3e), and the ECSA values of S-NiFe LDH@Cu, NiFe LDH@Cu, RuO2@Cu, and Cu NWs were simulated as 18.98 mF cm−2, 16.31 mF cm−2, 5.17 mF cm−2, and 1.01 mF cm−2, respectively. The ECSA (electrochemical active surface area) of S-NiFe LDH@Cu is larger than that of NiFe LDH@Cu, indicating that the doping of S in NiFe LDH increases the electrochemical active surface area. The calculated ECSA values were also used to replace the geometric surface area, resulting in a relationship curve between current density and potential for the electrocatalyst. As shown in Figure S2, S-NiFe LDH@Cu still exhibited superior performance in achieving the maximum current density compared to the other materials. This result suggests that S-NiFe LDH@Cu enhances the intrinsic activity and improves the average activity of each active site. Therefore, the exceptional performance of this catalyst is not solely attributed to a larger active surface area. The improvement in OER performance is not only due to ECSA; other factors such as reaction kinetics or electron transport rate also contribute to the improvement of OER performance.
To elucidate the impact of electron transfer and conductivity, the electrochemical impedance spectroscopy (EIS) curve was examined at a measurement voltage of 1.55 V, as depicted in Figure 3f. In this analysis, the intersection points on the x-axis represent the solution resistance (Rs), which indicates the resistance between the working and reference electrodes [46,47]. The radius of the semicircle in the EIS curve corresponds to the charge transfer resistance (Rct), representing the rate-determining step during the OER process. Comparing S-NiFe LDH@Cu with NiFe LDH@Cu, RuO2@Cu, and Cu NWs, it can be observed that S-NiFe LDH@Cu exhibits a smaller Rct value, indicating an accelerated charge-transfer process. This phenomenon can be attributed to the intrinsic lack of catalytic activity of the copper substrate towards OER. While copper-based materials do not directly influence the electrocatalytic OER process, they possess remarkable electrical conductivity. Consequently, the incorporation of cu into the NiFe LDH catalyst effectively mitigates impedance and expedites charge transfer kinetics, fostering a synergistic interplay between the copper- and NiFe-based catalysts [48].
Motivated by the exceptional oxygen evolution reaction (OER) activities exhibited by the S-NiFe LDH@Cu catalyst, we constructed a two-electrode electrolyzer. In this setup, S-NiFe LDH@Cu was used as the anode, while commercially available Pt/C supported on a Cu mesh served as the cathode. By utilizing this experimental configuration, we were able to evaluate the performance of overall water splitting in a 1.0 M KOH electrolyte. Remarkably, the S-NiFe LDH@Cu‖Pt/C catalyst demonstrated outstanding overall water splitting performance. It achieved a current density of 100 mA cm−2 with a cell voltage of only 1.56 V (Figure 4a). This performance was comparable to that of the RuO2‖Pt/C water electrolyzer, indicating the high efficiency of the S-NiFe LDH@Cu catalyst. To assess the faradaic efficiency, we measured the amounts of H2 and O2 produced during overall water splitting over time. At a current density of 100 mA cm−2, the volumes of generated O2 and H2 closely matched the theoretical values. This suggests that the faradaic efficiency of overall water splitting using the S-NiFe LDH@Cu‖Pt/C catalyst approached 100% (Figure 4b). Furthermore, we investigated the long-term stability of the electrocatalyst for overall water splitting at high current densities. The electrolytic cell based on S-NiFe LDH@Cu‖Pt/C maintained a consistent current density of 100 mA cm−2 with negligible variation, even after continuous operation for 20 h at a specific voltage (Figure 4c). This demonstrates the stability of the S-NiFe LDH@Cu electrocatalyst for overall water splitting at high current densities. Additionally, throughout the electrolytic process, the volume ratio of H2 to O2 was consistently maintained at 2:1, further confirming the efficiency of the system. The observation of abundant gas bubbles of H2 and O2 being generated from the cathode and anode, respectively (Figure 4d), provides clear evidence of the highly efficient and durable nature of S-NiFe LDH@Cu electrocatalysts for water splitting. This observation further supports the outstanding performance and stability of the S-NiFe LDH@Cu catalyst in overall water splitting applications.

4. Conclusions

In this study, we employed a simple electrochemical deposition method to introduce sulfur into NiFe LDH, resulting in the synthesis of a S-NiFe LDH@Cu composite material. The prepared composite material exhibited excellent activity for the oxygen evolution reaction (OER), and we thoroughly investigated the influence of element doping on the catalytic performance of the OER. The self-supported electrodes of S-NiFe LDH@Cu displayed a nano-wire array structure with interlaced LDH nanosheets, forming a core–shell structure. The S-NiFe LDH@Cu material demonstrated a significantly lower overpotential of only 236 mV at a current density of 50 mA cm−2, outperforming NiFe LDH@Cu, RuO2@Cu, and Cu NWs in terms of comprehensive catalytic performance. This remarkable difference in overpotential clearly indicates the effective enhancement of OER activity achieved through sulfur doping in NiFe LDH@Cu. Another contributing factor to its excellent catalytic activity is the increase in the electrochemically active surface area resulting from the doping, as well as a reduction in electronic transfer resistance, as confirmed by the results of the electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS) tests. Furthermore, when tested using a dual-electrode electrolyzer with commercial Pt/C as the cathode, S-NiFe LDH@Cu demonstrated outstanding performance and good stability during overall water splitting. These findings suggest that the S-NiFe LDH@Cu composite material holds great promise as an efficient catalyst for water splitting applications. The results of this study provide an effective approach to enhancing the electrochemical performance of hydroxide catalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/jcs7120486/s1, Figure S1. (a,b) TEM images of NiFe LDH@Cu; (c) HRTEM of NiFe LDH@Cu; (d–h) STEM and corresponding mapping images. Figure S2. Specific current density driven from calculated ECSA versus potential. Table S1. Comparison of OER performance of S-NiFe LDH@Cu with other reported OER electrocatalysts in alkaline solution.

Author Contributions

Z.Z., J.G. and Y.S.: conceptualization, methodology, investigation, writing—original draft; Q.W. and M.L.: investigation, data curation, formal analysis; F.C. and S.H.: formal analysis, writing—review and editing, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (51971059), and the Natural Science Foundation of Liaoning Province of China (2023-MS-070 and 2022-MS-293).

Data Availability Statement

All data generated of this work are included in this article.

Acknowledgments

We gratefully acknowledge the financial aid provided by the National Natural Science Foundation of China, the Natural Science Foundation of Liaoning Province of China.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. S-NiFe LDH@Cu schematic diagram of the preparation process.
Figure 1. S-NiFe LDH@Cu schematic diagram of the preparation process.
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Figure 2. (a) SEM images of Cu NWs; (b) SEM images of NiFe LDH@Cu; (c) SEM images of S-NiFe LDH@Cu; (d,e) TEM images of S-NiFe LDH@Cu; (f) HRTEM of S-NiFe LDH@Cu; (g–l) STEM and corresponding mapping images; (m) XRD patterns of S-NiFe LDH@Cu and Cu NWs.
Figure 2. (a) SEM images of Cu NWs; (b) SEM images of NiFe LDH@Cu; (c) SEM images of S-NiFe LDH@Cu; (d,e) TEM images of S-NiFe LDH@Cu; (f) HRTEM of S-NiFe LDH@Cu; (g–l) STEM and corresponding mapping images; (m) XRD patterns of S-NiFe LDH@Cu and Cu NWs.
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Figure 3. Electrocatalytic performance of different samples. (a) OER polarization curves of different samples; (b) overpotential of different samples at 50 and 100 mA cm−2; (c) Tafel plots; (d) performance diagram of OER electrocatalysts; (e) capacitive currents as a function of scan rate; and (f) Nyquist plots for different samples.
Figure 3. Electrocatalytic performance of different samples. (a) OER polarization curves of different samples; (b) overpotential of different samples at 50 and 100 mA cm−2; (c) Tafel plots; (d) performance diagram of OER electrocatalysts; (e) capacitive currents as a function of scan rate; and (f) Nyquist plots for different samples.
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Figure 4. (a) Polarization curve of different samples in 1 M KOH for overall water splitting; (b) experimental and theoretical volumes of O2/H2; (c) stability testing; (d) photo of battery-driven overall water splitting polarization curves of the catalysts.
Figure 4. (a) Polarization curve of different samples in 1 M KOH for overall water splitting; (b) experimental and theoretical volumes of O2/H2; (c) stability testing; (d) photo of battery-driven overall water splitting polarization curves of the catalysts.
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MDPI and ACS Style

Zhang, Z.; Guo, J.; Sun, Y.; Wang, Q.; Li, M.; Cao, F.; Han, S. Sulfur-Doped Nickel–Iron LDH@Cu Core–Shell Nanoarrays on Copper Mesh as High-Performance Electrocatalysts for Oxygen Evolution Reaction. J. Compos. Sci. 2023, 7, 486. https://doi.org/10.3390/jcs7120486

AMA Style

Zhang Z, Guo J, Sun Y, Wang Q, Li M, Cao F, Han S. Sulfur-Doped Nickel–Iron LDH@Cu Core–Shell Nanoarrays on Copper Mesh as High-Performance Electrocatalysts for Oxygen Evolution Reaction. Journal of Composites Science. 2023; 7(12):486. https://doi.org/10.3390/jcs7120486

Chicago/Turabian Style

Zhang, Zhichao, Jiahao Guo, Yuhan Sun, Qianwei Wang, Mengyang Li, Feng Cao, and Shuang Han. 2023. "Sulfur-Doped Nickel–Iron LDH@Cu Core–Shell Nanoarrays on Copper Mesh as High-Performance Electrocatalysts for Oxygen Evolution Reaction" Journal of Composites Science 7, no. 12: 486. https://doi.org/10.3390/jcs7120486

APA Style

Zhang, Z., Guo, J., Sun, Y., Wang, Q., Li, M., Cao, F., & Han, S. (2023). Sulfur-Doped Nickel–Iron LDH@Cu Core–Shell Nanoarrays on Copper Mesh as High-Performance Electrocatalysts for Oxygen Evolution Reaction. Journal of Composites Science, 7(12), 486. https://doi.org/10.3390/jcs7120486

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