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Article

Expediting Corrosion Engineering for Sulfur-Doped, Self-Supporting Ni-Fe Layered Dihydroxide in Efficient Aqueous Oxygen Evolution

by
Yingjun Ma
1,2,
Jie Wang
1,2,3,*,
Hangning Liu
2,
Lin Wang
2,
Changhui Sun
2,
Liangyu Gong
2,
Xiaogang Zhang
1,* and
Jiefang Zhu
3,*
1
Jiangsu Key Laboratory of Electrochemical Energy Storage Technology, College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
2
Qingdao Engineering Research Center of Agricultural Recycling Economy Materials, College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China
3
Department of Chemistry, Ångström Laboratory, Uppsala University, 75121 Uppsala, Sweden
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(7), 394; https://doi.org/10.3390/catal14070394
Submission received: 14 March 2024 / Revised: 9 June 2024 / Accepted: 18 June 2024 / Published: 21 June 2024
(This article belongs to the Special Issue Electrocatalysis for Hydrogen/Oxygen Evolution Reactions)
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Abstract

:
Electrochemical water-splitting is widely acknowledged as a renewable strategy for hydrogen production, but it is primarily constrained by the sluggish reaction kinetics of the anode oxygen evolution reaction (OER). In our study, we employ a fast room-temperature corrosion engineering strategy for the construction of a sulfur-doped Ni-Fe layered dihydroxide catalyst (S-NiFe LDH). With the assistance of a sulfur source, microsphere morphology with an ultra-thin lamellar surface cross-arrangement can be rapidly grown on the surface of an iron foam substrate, ensuring a substantial electrochemical interface. The composition of Ni species in the catalysts can be regulated by simply adjusting the amount of Ni2+ and reaction time. Functioning as an OER catalyst, the S-NiFe LDH demonstrates high activity and reaction kinetics, featuring a minimal overpotential of 120.0 mV to deliver a current density of 10 mA cm−2, a small Tafel slope of 39.5 mV dec−1 and a notable electrical double-layer capacitance (Cdl) of 31.3 mF cm−2. The remarkable electrocatalytic performance can be attributed to its distinctive three-dimensional (3D) structure and sulfur dopants, which effectively regulate the electrochemical interface and electronic structure of NiFe LDH. This work provides valuable insights for expeditious materials design.

1. Introduction

The excessive consumption of fossil fuels has precipitated severe environmental crises and energy shortages [1,2]. In response, renewable energy resources have garnered widespread attention, with hydrogen emerging as a promising alternative. When hydrogen reacts with oxygen, it produces water, making it a clean energy option. However, the conventional method of obtaining hydrogen through the steam reforming process is energy-intensive and often results in a carbon monoxide byproduct. This limitation restricts its further application and favors the use of renewable energy devices instead. As an alternative, electrocatalytic water decomposition has become a pivotal technology for hydrogen and oxygen production. Unlike the steam reforming process, which demands a high energy input, electrocatalytic decomposition offers a more moderate approach. Yet, initiating water electrolysis through this method requires a voltage exceeding 1.23 V due to the sluggish kinetics of the reaction, resulting in substantial polarization [3,4,5]. Compared to the hydrogen evolution reaction (HER) at the cathode, the oxygen evolution reaction (OER) at the anode plays a crucial role, particularly because it involves multi-step electron transfers [6,7,8,9,10,11,12,13]. To facilitate this process, highly efficient electrocatalysts are essential to lower the energy barrier for breaking the O-H bond and forming the O-O bond [14]. Given the practical implications and feasibility of these advancements, reducing energy consumption and processing time have emerged as key objectives in the water-splitting industry [15,16,17,18].
At present, RuO2, IrO2 and other noble metal catalysts are commonly recognized to be commercially used in electrochemical water-splitting applications, but their scarcity and high price presented an obstacle to scale-up [19,20,21,22,23]. Comparatively, a large number of non-noble-metal-based catalysts, especially transition metal sulfides [24,25,26] phosphates [27], nitrides [28,29,30] and hydroxide [31,32], have been extensively studied due to their abundant earth reserves, low price and tunable electronic structure. Considering that the price of Fe is a hundred times lower than that of Ni and Co, the combination of Fe with other metal species for the construction of binary and ternary catalysts has been widely investigated [33,34,35,36,37,38,39]. Typically, Ni-Fe layered dihydroxide (NiFe LDH) is considered as an effective alternative material for noble metal catalysts because of its unique electronic properties. Moreover, the electronic structure and electrochemical interfaces can be further regulated by optimizing the ionic components. Typically, corrosion has been considered as a mild and controllable top-down strategy for the construction of NiFe LDH with a specific structure. For example, Li et al. have adopted the corrosion strategy to convert substrates into highly active and stable OER catalysts under ambient pressure. The electronic structure and chemical kinetics of the catalyst were improved by controlling the metal content and element doping [40]. Fu et al. reported a porous NiFe LDH catalyst with rich edge/surface-Fe defects on a nickel foam substrate via an elaborate atmosphere corrosion strategy. The Fe defects were derived from the local unequal-stoichiometric ratio of Fe/Ni in the nanometer or sub-nanometer region, which could be regulated by two aspects: (i) the unbalanced permeating of the acid vapor; (ii) the confined reaction of local Fe and Ni species ionized by the acid vapor. Also benefiting from the abundant defects, NiFe LDH demonstrated superior OER performance relative to the catalyst synthesized via conventional process. It should be noted that the atmosphere corrosion strategy is green and economic, which is suitable for large-scale applications [41]. Accordingly, to further enhance the OER properties of NiFe LDH, heteroatom doping would be a potential method, but there are still few reports related to the anion doping.
In this study, we have introduced a rapid room-temperature corrosion engineering approach to fabricate a sulfur-doped NiFe LDH catalyst (S-NiFe LDH) using iron foam (IF) as the substrate. The resulting catalyst exhibits a unique cross-arrangement of ultra-thin lamellar microstructures self-assembling into three-dimensional (3D) microspheres. This distinctive morphology is advantageous as it is beneficial for enlarging the electrochemical interfaces, crucial for efficient catalytic performance. The composition of Ni species within the NiFe LDH structure was easily modulated by adjusting the amount of Ni2+ ions and the reaction time during synthesis. The incorporation of sulfur sources in the synthesis process played three crucial roles: (i) promoting the surface corrosion rate on IF substrate; (ii) accelerating the formation of NiFe LDH; (iii) resulting in S dopants in the NiFe LDH structure. Therefore, the S-NiFe LDH with optimized electronic structure and morphology improved the OER performance, especially the reaction activities, reaction kinetics and long-term durability. This work offers valuable insights into the fabrication of anion-doped catalysts using a rapid corrosion strategy, presenting a promising catalyst for electrocatalytic applications.

2. Results and Discussion

2.1. Structural Characterization

As demonstrated from the catalyst construction schematic in Figure 1, a spontaneous corrosion engineering strategy was adopted to construct a S-NiFe LDH catalyst in the natural dissolved oxygen environment for the electrocatalytic OER application. During the synthetic process, the IF served as both a skeleton and Fe source, which are beneficial for accelerating electron transfer and providing a large corrosion interface. In detail, the IF is first corroded via oxygen corrosion (Equations (1) and (2)), in which the oxygen comes from the air dissolved in the solution. The slight reducibility of S2O32− in the solution will assist in stabilizing the metal ions at the IF surface. Subsequently, the corroded Fe ions combine with free sulfur species−, Ni2+, and OH− to form S-NiFe LDH. The corrosion process is as follows:
Anode reaction: Fe ↔ Fe2+ + 2e−, Fe2+ ↔ Fe3+ + e−
Cathode reaction: O2 + 2H2O + 4e− ↔ 4OH−
Precipitation reaction: Fe3+ + Ni2+ + xOH− + xS2− ↔ xS-NiFe(OH)x
During the corrosion process, different corrosion products were obtained by tuning the Ni2+ concentrations (Table S1). X-ray diffraction (XRD) was performed to characterize the phase information of the corrosion products. As shown in Figure 2a, the XRD pattern of S-NiFe LDH reacted at different Ni2+ concentrations showed two crystalline phases, in which the diffraction peaks at 44.6°, 65.1° and 82.3° correspond to the crystalline phases of Fe (PDF card: 06-0696) and the diffraction peaks at 11.5°, 23.3° and 34.6° correspond to the NiFe LDH phase (PDF card: 51-0463). It should be noted that there is negligible crystal information associated with S species, indicating that the S component in the products may exist as amorphous S species or that trace amounts of S species remained after the reaction.
The morphology and microstructure of the as-prepared S-NiFe LDH were meticulously characterized using advanced techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). The resulting analysis unveiled intriguing details about the structure of the catalyst. As shown in Figure 2b,c, SEM images depicted the presence of three-dimensional (3D) structured microspheres prominently growing on the surface of the IF substrate. These microspheres exhibit a well-organized arrangement of ultra-thin lamellae, with a measured lamellar thickness of approximately 10 nm as determined from the enlarged SEM image of the S-NiFe LDH. The 3D structure is composed of a well-cross-arranged ultra-thin lamellar structure, which is conducive to increasing the contact area between the electrolyte and catalyst, which is essential for improving the utilization of active sites within the catalyst material, thereby boosting its efficiency in catalytic reactions. Furthermore, the TEM image in Figure 2d provides additional insights into the nanosheet morphology of the S-NiFe LDH. The nanosheets appear somewhat randomly distributed, likely due to the vigorous ultrasonic dispersion with ethanol solvent before the TEM operation. Upon closer inspection at higher magnification (Figure 2e), distinct lattice fringes become apparent, indicating the excellent crystal structure of the S-NiFe LDH catalyst. Moreover, the width of two adjacent fringes was measured to be 0.26 nm, corresponding to the (0 1 2) crystal plane of the S-NiFe LDH. To further confirm the elemental composition and distribution within the S-NiFe LDH, EDS characterization of a selected sample was performed as shown in Figure 2f and Figure S1. The EDS analysis demonstrated a uniform distribution of Ni, Fe, O and S species throughout the sample. Notably, the atomic percentage of S was found to be 4.2% (as shown in Table S2), indicating the successful doping of sulfur in the S-NiFe LDH structure. This sulfur doping is significant as it may optimize the electronic properties of the NiFe LDH and enhance the catalytic performance.
Additionally, the effects of Ni2+ concentration and reaction time for the structure regulation of S-NiFe LDH were investigated in detail. When the amount of Ni2+ was tuned to 2 mmol, the surface of the IF formed a corrosion product that features a lamellar cross-arranged sphere morphology, but the boundary between spheres was difficult to clarify (Figure 3a). When the amount of Ni2+ was increased to 5 mmol, the lamellar structure gradually coalesced (Figure 3b), and the boundary between the spheres became much clearer. When the amount of Ni2+ was increased to 10 mmol, the 3D microsphere structure could be seen obviously (Figure 3c). Excessive Ni2+ concentration (20 mmol) resulted in severe destruction of the sphere structure (Figure 3d). Accordingly, the S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20 have an average lamellar thickness of 161.3 nm, 80.0 nm, 58.8 nm and 60.1 nm, respectively. Therefore, with the increase in the Ni2+ ratio, the thickness of the lamellar thickness decreases first and then increases, demonstrating that the Ni2+ ratio plays an important role in regulating the thickness of the lamellar structure. The same phenomenon was also found when gradually increasing the reaction time. At the reaction time of 2 h, microspheres began to form (Figure S2a). The diameter of the microspheres gradually increased as the reaction time extended from 2 h to 6 h (Figure S2b,c). After a 6 h reaction, the spheres began to aggregate (Figure S2d) and even collapse (Figure S2e). The results demonstrated that the Ni2+ concentration of 10 mmol and reaction time of 6 h would result in a sphere structure with a large size and lamellar micromorphology.
The surface information of NiFe LDH obtained at different Ni2+ concentrations was characterized by the XPS technique. As shown from the full-range XPS spectra in Figure 4a, the binding energies located at near 162.5, 286.7, 530.0, 855.0 and 712.5 eV correspond to S 2p, C 1s, O 1s, Ni 2p and Fe 2p, respectively. The atomic S percentages in S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20 were 3.7%, 5.0%, 3.9% and 4.0%. The sulfur content showed a volcanic trend with the increasing concentration of Ni2+, which may be ascribed to the morphology evolution and change in the Ni/Fe ratio. The presence of the C 1s signal can be attributed to the use of a conductive binder which was applied to fix the sample before XPS measurements. To further investigate the valence state of the samples, high-resolution XPS spectra were obtained. As shown in Figure 4b, the high-resolution XPS spectra showed typical peaks at 855.05 eV corresponding to the 2p3/2 binding energies of Ni 2p, demonstrating the existing valence of Ni2+. Peaks located at 861.3 eV corresponded to the satellite oscillation peaks of Ni 2p [42,43]. In addition, the S-NiFe LDH-2 sample showed a Ni0 peak at a binding energy of 854.9 eV, which may be due to the existence of a displacement reaction between IF and Ni2+. The Fe 2p spectrum (Figure 4c) was deconvoluted into six peaks, in which peaks at binding energies of 710.7 and 724.1 eV represent Fe2+, peaks at 713.5 and 725.1 eV represent Fe3+, and the remaining two peaks are attributed to oscillatory satellite peaks [44,45]. It should be noted that the peak density of Fe3+ for S-NiFe LDH-10 was much stronger than that for S-NiFe LDH-2, S-NiFe LDH-5 and S-NiFe LDH-20, suggesting that as the concentration of the sulfur source increased, the production of NiFe LDH increased, but an excessive sulfur source concentration also resulted in an inhibition of Fe3+ formation. As a result, the atomic percentage of Ni to Ni + Fe in the catalyst gradually increased (29%, 55%, 60%, 81%) with the increase in Ni2+ concentration from 2 mmol to 20 mmol (Figure S3), while the atomic percentage of Fe correspondingly decreased (71%, 45%, 40%, 19%). It is speculated that an increasing amount of Ni2+ will inhibit the reaction of Fe and then influence the electrocatalytic performance. The fitted high-resolution O 1s spectra (Figure S4) showed obvious peaks at binding energies of 529.0, 530.5 and 532.2 eV, representing M-O bonds, M-OH bonds and H2O, respectively [46,47]. As for the S 2p spectrum, weak peak density can be clarified in the four samples (Figure S5a–d), indicating the successful doping of S on the surface of NiFe LDH [14]. The addition of sulfur sources plays three main roles: (i) promoting the surface corrosion on IF substrate; (ii) accelerating the formation of NiFe LDH; (iii) resulting in a S dopant in NiFe LDH structure. Therefore, the S-NiFe LDH with an optimized electronic structure and morphology is beneficial for improving the OER performance, especially the reaction activities and the reaction kinetics.

2.2. Electrochemical Performance

A three-electrode configuration was adopted to study the electrochemical OER properties. CV curves with a wide potential window were applied to explore the electrochemical process of the as-prepared catalysts. As shown in Figure 5a, the S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20 showed distinct redox peaks, in which the oxidation peak of S-NiFe LDH-10 at approximately 0.40 V corresponds to the valence transition of Fe(II)/Fe(III) [48]. A much higher reduction peak current density of S-NiFe LDH-10 can be seen at a potential of approximately 1.0 V relative to the other three S-NiFe LDH catalysts, which corresponds to the valence transition process of Ni(II)/Ni(III) [49]. It should be noted that the oxidation peaks of the S-NiFe LDH catalysts were difficult to clarify and may be overlapped by the OER current. The surface valence transition may optimize the surface properties of the catalysts, which is beneficial for promoting the OER performance. Notably, the peak current density of S-NiFe LDH was significantly higher than that of the other three catalysts, indicating the presence of a rich electrochemical interface. Subsequent linear scanning voltammetry (LSV) was conducted, and all four catalysts exhibited higher OER activity than commercial RuO2 (Figure 5b). Moreover, S-NiFe LDH-10 has much superior OER activity compared to S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-20 and IF (Figure S6a). To accurately measure OER activity, a histogram of the five catalysts (Figure 5c) at a current density of 10 mA cm−2 was drawn from polarization curves (Figure 5b). The overpotential of S-NiFe LDH-10 was only 120.0 mV, much smaller than that of S-NiFe LDH-2 (130.0 mV), S-NiFe LDH-5 (150.0 mV), S-NiFe LDH-20 (128.0 mV) and RuO2 (310.0 mV). Tafel plots derived from the polarization curves in Figure 5b are shown in Figure 5d. The corresponding Tafel slopes were calculated to be 63.2 mV dec−1, 61.8 mV dec−1, 39.5 mV dec−1, 61.2 mV dec−1 and 30.9 mV dec−1 for S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10, S-NiFe LDH-20 and RuO2/IF, respectively. According to the Tafel slope values, S-NiFe LDH-10 corresponds to the Tafel process, and S-NiFe LDH-2, S-NiFe LDH-5 and S-NiFe LDH-20 follow a Volmer process [50]. The small Tafel slope value of S-NiFe LDH suggested a faster OER kinetics. Moreover, the OER activities of the catalysts obtained with different reaction times were also studied. It can be seen from Figure S6 that the catalysts synthesized at 6 h (NiFe LDH) have better OER activity than those prepared at other times.
Considering that electrochemical activity surface area (ECSA) is a key parameter for determining the activity and kinetics of catalysts and taking account of the principle that the electrical double-layer capacitor (Cdl) is proportional to the ECSA, the Cdl values were calculated by measuring the CV curves at different scanning rates of the catalysts at a nonpseudocapacitance potential window of 0.5–0.6 V. As shown in Figure 5e and Figures S7–S10, the Cdl values of S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20 were 6.3, 7.5, 16.2 and 11.6 mF cm−2, respectively, demonstrating the excellent electrochemical kinetics of S-NiFe LDH for the OER by exposing abundant electrochemical interfaces. The intrinsic electrical conductivity of the catalysts also plays a significant role in affecting the OER performance. As shown in Figure 5f, the S-NiFe LDH showed the smallest semicircle diameter compared to the other three catalysts, demonstrating the excellent electrical conductivity properties of S-NiFe LDH. Moreover, the catalysts obtained at different reaction times also demonstrated excellent conductivity with a corrosion time of 6 h (Figure S11 and Table S3).
Electrochemical stability is also significant for practical water-splitting applications. Therefore, the electrochemical stability of S-NiFe LDH and commercial RuO2 catalysts was tested via the chronopotentiometry strategy at a constant current density of 10 mA cm−2. It can be seen from Figure 5g that S-NiFe LDH showed excellent stability after continuous working of 250 ks (~68 h). It should be noted that the potential showed an obvious decreasing tendency from ~70 ks to 150 ks, which may be ascribed to the activation of the S-NiFe LDH at the surface of the catalyst. In contrast, the potential of the RuO2 catalyst gradually increased and failed at 60 ks (~17 h), indicating the excellent electrochemical stability of S-NiFe LDH for the OER. The structural integrity of the S-NiFe LDH was characterized by Raman spectroscopy, SEM and corresponding EDS maps after a stability test. As shown from the Raman spectroscopy of Figure 6a, although the relative intensity of S-NiFe LDH decreased after the stability test compared to the pristine S-NiFe LDH, the main composition showed negligible change, demonstrating the excellent electrochemical stability of S-NiFe LDH. In addition, the ultra-thin lamellar assembled morphology was still maintained after a ~68 h stability test demonstrated, as demonstrated by an SEM image (Figure 6b) with typical EDS spectra peaks (Figure 6c). The enlarged magnification of the SEM image and corresponding EDS maps demonstrated a uniform distribution of Fe and O but an inverse distribution of Ni and S, demonstrating a surface reconstruction of Ni and Fe species, in which the Ni and S were covered by the Fe and O after the stability test.

3. Materials and Methods

3.1. Materials and Agents

IF was bought from Cyber Electrochemical Materials Network Co., Ltd. (Tokyo, Japan), with a thickness of 1 mm; HCl and ethanol were purchased from Sinopharm Chemical Re-agent Co., Ltd. (Shanghai, China), at analytical grade; RuO2 was purchased from Shanghai Maclin Bio-chemical Technology Co., Ltd. (Shanghai, China), at ≥99.9% grade; NiCl2 6H2O was purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China), at ≥98% grade, and used without further purification; KOH and Na2S2O3 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), at analytical grade.

3.2. Preparation of Sulfur-Doped NiFe LDH Catalysts

The IF substrate was initially cut into a size of 2 × 2 cm and then submerged in a beaker containing 0.1 M HCl solution for 10 min to eliminate surface impurities. Following this step, the IF was rinsed three times, alternating between deionized water and ethanol, to ensure thorough cleaning. Subsequently, the cleaned IF was dried in a vacuum oven at 60 °C.
A fast room-temperature corrosion engineering strategy was employed for the synthesis of S-NiFe LDH. In detail, 10 mmol of NiCl2 6H2O and 0.3 mmol of Na2S2O3 were dissolved in a beaker containing 10 mL of deionized water under continuous magnetic stirring. The prepared IF substrate was then immersed in this solution and allowed to react for 6 h at room temperature. After the reaction period, the IF substrate was carefully removed from the solution and washed repeatedly with deionized water and ethanol alternately to remove any residual chemicals or byproducts. Finally, the washed substrate was dried at 60 °C for 10 h to obtain the desired S-NiFe LDH material.
To study the effect of etching time on the physical properties of the S-NiFe LDH catalysts, the reaction time was controlled to 2 h, 4 h, 6 h, 8 h and 10 h, and the corresponding resulting catalysts were denoted as S-NiFe LDH-2h, S-NiFe LDH-4h, S-NiFe LDH-6h, S-NiFe LDH-8h and S-NiFe LDH-10h, respectively. Moreover, to study the concentration effect of metal salt, the reaction was carried out with Ni2+ concentrations of 2 mmol, 5 mmol, 10 mmol and 20 mmol, and the corresponding resulting catalysts were denoted as S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20, respectively. Notably, the concentration of Na2S2O3 was kept constant for the construction of different catalysts. According to the preparation steps, S-NiFe LDH-6h and S-NiFe LDH-10 refer to the same catalyst as S-NiFe LDH.

3.3. Material Characterization

XRD patterns were obtained via a TD-3700 instrument with Cu Kα radiation (λ = 1.5406 Å); the scanning speed was 8° min−1. Scanning electron microscopy (SEM (Tokyo, Japan), JSM-IT500) was adopted to characterize the morphology and microstructure evolution of the as-prepared catalysts. Transmission electron microscopy (TEM, JEOL (Tokyo, Japan) JEM-2100F) was adopted to characterize the microstructure of the prepared catalysts. X-ray photoelectron spectroscopy (XPS (Falconbridge, ON, Canada), Axis-Ultra DLD-600W) was adopted to analyze the surface properties of the catalysts. Raman spectroscopy was performed using a Raman spectrometer (DXR (Carbondale, IL, USA) 2xi Raman microscope with excitation laser beam wavelength of 532.17 nm).

3.4. Electrochemical Measurements

All electrochemical tests were conducted using a three-electrode system with a CHI 660E electrochemical workstation (CH instruments). In this setup, a 1 M KOH solution served as the electrolyte, a graphite rod acted as the counter electrode, a reversible hydrogen electrode (RHE) functioned as the reference electrode, and the catalyst-loaded IF was employed as the working electrode with an impregnation area of 1 cm2, and the electrochemical results were normalized to the surface area, which was assumed to be 1 × 1 cm = 1 cm2. Cyclic voltammetry (CV) measurements were performed to assess the electrochemical properties of the catalyst. The scanning rate was set at 50 mV s−1 with a potential window ranging from 0 to 2.0 V. To determine the double-layer capacitance (Cdl), CV measurements were carried out within a nonpseudocapacitance potential window of 0.5 to 0.6 V, and the scanning rate was varied from 10 mV s−1 to 200 mV s−1. For linear scanning voltammetry (LSV), the potential window was set between 0.8 and 1.8 V, with a scanning rate of 5 mV s−1. EIS was performed at a potential of 1.5 V (vs. RHE); the rate ranged from 0.01 to 100,000 Hz, and the amplitude values were set to 5 mV. The EIS spectra were interpreted by fitting them to equivalent circuits.
For comparison, the RuO2 catalyst was prepared by weighing 5 mg of RuO2, dispersing it into 1 mL of a 0.1% concentration Nafion/iso-propyl alcohol solution, and subjecting the mixture to ultrasonication for 15 min. Then, 100 μL of the dispersion was taken, added to the surface of the IF substrate drop by drop, and dried naturally.

4. Conclusions

In summary, we have successfully synthesized a S-NiFe LDH with an ultra-thin lamellar self-assembled microsphere structure directly on an IF substrate using a rapid room-temperature corrosion engineering strategy. This innovative approach allowed us to tailor the yield of the NiFe LDH by precisely adjusting the amount of Ni2+ and the reaction time. The introduction of sulfur sources in the corrosion medium played a pivotal role in three main aspects: Firstly, it facilitated the surface corrosion of the IF substrate, creating an ideal foundation for the subsequent deposition of the NiFe LDH. Secondly, it accelerated the formation of the NiFe LDH phase. Lastly, the sulfur sources acted as dopants within the NiFe LDH structure, enhancing its catalytic properties. The resulting S-NiFe LDH, with its 3D microsphere structure and sulfur dopants, exhibited exceptional performance for the OER. Notably, the S-NiFe LDH catalyst demonstrated remarkable activity with an overpotential of 120.0 mV at a current density of 10 mV cm−2, showcasing its efficiency in initiating the OER process. Such activity also outperformed most of the recently published OER catalysts (Table S4). Additionally, the catalyst displayed favorable reaction kinetics, as indicated by a Tafel slope of 39.5 mV dec−1 and a double-layer capacitance (Cdl) value of 31.3 mF cm−2. These results highlight the effectiveness of our expedited NiFe LDH construction strategy in producing a high-performance OER catalyst. Consequently, further investigation into optimizing the S-NiFe LDH catalyst will involve precisely controlling the sulfur doping levels to enhance the OER performance. Additionally, we plan to implement a practical water-splitting setup to evaluate the high-current production of hydrogen and oxygen, utilizing the S-NiFe LDH catalyst as the anode, in order to fulfill the needs of the green hydrogen industry.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14070394/s1: Figure S1: EDS spectrum of NiFe LDH characterized via SEM technique; Figure S2: SEM images related to S-NiFe LDH reacted at different times; Figure S3: Atomic contents of Ni and Fe obtained from XPS survey spectra of Figure 4a; Figure S4: High-resolution O 1s XPS spectra of S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH and S-NiFe LDH-20; Figure S5: High-resolution S 2p XPS spectra of S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH, and S-NiFe LDH-20; Figure S6: Polarization curves of S-NiFe LDH-2 h, S-NiFe LDH-4 h, S-NiFe LDH, S-NiFe LDH-8 h, S-NiFe LDH-10 h and corresponding overpotential histograms; Figures S7–S10: CV curves of pure S-NiFe LDH-2, S-NiFe LDH-5 and S-NiFe LDH-20; Figure S11: EIS spectrum of S-NiFe LDH catalysts obtained at different reaction times; Table S1: Synthesis conditions of the S-NiFe-LDH catalysts; Table S2: Elemental composition of S-NiFe LDH characterized by EDS; Table S3: Rct values of eight prepared catalysts derived from the EIS spectrum; Table S4: Comparative electrochemical OER performances of different electrocatalytic materials in alkaline medium. References [51,52,53,54,55,56,57,58,59,60] are cited in the Supplementary Materials.

Author Contributions

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

Funding

This work was funded by the China Postdoctoral Science Foundation (2021M701694), Postdoctoral Science Foundation of Jiangsu Province (1006-YBA21038), National Natural Science Foundation of China (22202114), Swedish Energy Agency (50674-1) and Experimental Technology Research Project of Qingdao Agricultural University (SYJS202217).

Data Availability Statement

Original data can be obtained from the corresponding authors.

Acknowledgments

The Central Laboratory of Qingdao Agriculture University and Qingdao Engineering Research Center of Agricultural Recycling Economy Materials are acknowledged for assistance with physical and electrochemical characterizations.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration for the preparation of the S-NiFe LDH electrode and its application in the electrochemical water-splitting process.
Figure 1. Schematic illustration for the preparation of the S-NiFe LDH electrode and its application in the electrochemical water-splitting process.
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Figure 2. (a) XRD pattern of S-NiFe LDH reacted at different Ni2+ concentrations; (b) SEM image and enlarged magnification SEM image (c) of S-NiFe LDH; (d) TEM image of S-NiFe LDH; (e) high-resolution TEM (HRTEM) image of S-NiFe LDH; (f) SEM image of S-NiFe LDH and corresponding EDS maps of Fe, O and S.
Figure 2. (a) XRD pattern of S-NiFe LDH reacted at different Ni2+ concentrations; (b) SEM image and enlarged magnification SEM image (c) of S-NiFe LDH; (d) TEM image of S-NiFe LDH; (e) high-resolution TEM (HRTEM) image of S-NiFe LDH; (f) SEM image of S-NiFe LDH and corresponding EDS maps of Fe, O and S.
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Figure 3. SEM image and enlarged magnification SEM image of S-NiFe LDH-2 (a,a1); SEM image and enlarged magnification SEM image of S-NiFe LDH-5 (b,b1); SEM image and enlarged magnification SEM image of S-NiFe LDH-10 (c,c1); SEM image and enlarged magnification SEM image of S-NiFe LDH-20 (d,d1).
Figure 3. SEM image and enlarged magnification SEM image of S-NiFe LDH-2 (a,a1); SEM image and enlarged magnification SEM image of S-NiFe LDH-5 (b,b1); SEM image and enlarged magnification SEM image of S-NiFe LDH-10 (c,c1); SEM image and enlarged magnification SEM image of S-NiFe LDH-20 (d,d1).
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Figure 4. (a) XPS survey spectra of S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20; (b) high-resolution XPS spectra of Ni 2p and (c) high-resolution XPS spectra of Fe 2p for the S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH and S-NiFe LDH-20 catalysts.
Figure 4. (a) XPS survey spectra of S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH-10 and S-NiFe LDH-20; (b) high-resolution XPS spectra of Ni 2p and (c) high-resolution XPS spectra of Fe 2p for the S-NiFe LDH-2, S-NiFe LDH-5, S-NiFe LDH and S-NiFe LDH-20 catalysts.
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Figure 5. (a) CV curves of S-NiFe LDH catalysts with a potential window of 0.0–2.0 V vs. RHE; (b) OER polarization curves of the S-NiFe LDH catalysts and RuO2/IF; (c) corresponding histograms at a current density of 10 mA cm−2 from (b); (d) corresponding Tafel plots derived from (b); (e) corresponding linear plots of the scan rates versus current density variation at 0.55 V for the S-NiFe LDH catalysts; (f) EIS spectrum of S-NiFe LDH catalysts with different Ni2+ concentrations and the equivalent circuit (inset); (g) chronopotentiometry test of S-NiFe LDH and RuO2 at a constant current density of 10 mA cm−2.
Figure 5. (a) CV curves of S-NiFe LDH catalysts with a potential window of 0.0–2.0 V vs. RHE; (b) OER polarization curves of the S-NiFe LDH catalysts and RuO2/IF; (c) corresponding histograms at a current density of 10 mA cm−2 from (b); (d) corresponding Tafel plots derived from (b); (e) corresponding linear plots of the scan rates versus current density variation at 0.55 V for the S-NiFe LDH catalysts; (f) EIS spectrum of S-NiFe LDH catalysts with different Ni2+ concentrations and the equivalent circuit (inset); (g) chronopotentiometry test of S-NiFe LDH and RuO2 at a constant current density of 10 mA cm−2.
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Figure 6. (a) Raman spectroscopy of S-NiFe LDH before and after stability tests; SEM image (b) and EDS spectrum (c) of S-NiFe LDH after stability test; (d) SEM image and corresponding EDS maps of S-NiFe LDH after stability test.
Figure 6. (a) Raman spectroscopy of S-NiFe LDH before and after stability tests; SEM image (b) and EDS spectrum (c) of S-NiFe LDH after stability test; (d) SEM image and corresponding EDS maps of S-NiFe LDH after stability test.
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Ma, Y.; Wang, J.; Liu, H.; Wang, L.; Sun, C.; Gong, L.; Zhang, X.; Zhu, J. Expediting Corrosion Engineering for Sulfur-Doped, Self-Supporting Ni-Fe Layered Dihydroxide in Efficient Aqueous Oxygen Evolution. Catalysts 2024, 14, 394. https://doi.org/10.3390/catal14070394

AMA Style

Ma Y, Wang J, Liu H, Wang L, Sun C, Gong L, Zhang X, Zhu J. Expediting Corrosion Engineering for Sulfur-Doped, Self-Supporting Ni-Fe Layered Dihydroxide in Efficient Aqueous Oxygen Evolution. Catalysts. 2024; 14(7):394. https://doi.org/10.3390/catal14070394

Chicago/Turabian Style

Ma, Yingjun, Jie Wang, Hangning Liu, Lin Wang, Changhui Sun, Liangyu Gong, Xiaogang Zhang, and Jiefang Zhu. 2024. "Expediting Corrosion Engineering for Sulfur-Doped, Self-Supporting Ni-Fe Layered Dihydroxide in Efficient Aqueous Oxygen Evolution" Catalysts 14, no. 7: 394. https://doi.org/10.3390/catal14070394

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