Sea urchin-like sulfur-doped Ni(OH)2 as an efficient electrocatalyst for oxygen evolution reaction

doi:10.21203/rs.3.rs-4988201/v1

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Sea urchin-like sulfur-doped Ni(OH)2 as an efficient electrocatalyst for oxygen evolution reaction

https://doi.org/10.21203/rs.3.rs-4988201/v1

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The development of high-performance and cost-effective electrocatalysts toward the oxygen evolution reaction (OER) is remarkably desirable but challenging. Herein, we design and fabricate a sea urchin-like S-doped Ni(OH)₂ electrocatalyst on nickel foam using a simple hydrothermal method, followed by treatment with Na₂S solution. The introduction of S not only modulates the electronic structure of Ni(OH)₂, but also improve the electronic conductivity, thus enhancing the OER performance of Ni(OH)₂. Owing to the free-standing feature, modified electronic structure and sea urchin-like structure, the optimized S-Ni(OH)₂-30 min delivered excellent OER performance with overpotentials of 306 and 392 mV at 10 and 100 mA cm^− 2, respectively, Tafel slope of 89.2 mV dec^− 1 and stability for 12 h at 20 mA cm^− 2. This work demonstrates the importance of incorporating S in Ni(OH)₂ to optimize the electronic structure for improving OER activity and provides a promising pathway to synthesize Ni(OH)₂-based electrocatalysts.

Water splitting

Ni(OH)2

S-doping

Electrocatalyst

Oxygen evolution reaction

The continuous depletion of fossil fuels and severe deterioration of the environment have motivated numerous efforts to exploit clean, efficient and renewable energy. Hydrogen, with high energy density, carbon-free emission and renewable traits, has been regarded as new-generation energy carrier. The production of hydrogen via electrocatalytic water splitting driven by renewable energy sources (such as wind, solar, and tidal power) has been recognized as a promising strategy [1]. However, as one of half-reactions at the anode of water splitting, the oxygen evolution reaction (OER) is considered as the bottleneck in water splitting, due to the sluggish reaction kinetics, which includes a complex proton-coupled electron transfer step. In order to break the kinetic barrier and reduce production costs, designing highly active, low-cost electrocatalysts for OER is essential. At present, RuO₂/IrO₂ are regarded as the state-of-the-art OER electrocatalysts [2, 3]. However, the scarcity, high cost and poor long-term stability prohibit their industrial applications. In this regard, a great deal of non-noble-metal-based catalysts including oxides, carbides, phosphides and sulfides have been studied as promising alternatives for OER catalysts [4].

Among the non-noble-metal-based catalysts, layered metal hydroxides, especially Ni(OH)₂ has attracted extensive attention due to their simple preparation, earth abundance and tunable composition. Nevertheless, further improvement of Ni(OH)₂ for OER is limited by their poor electrical conductivity and insufficient active sites exposure [5]. To address these issues, there are two operable approaches have been developed: (1) regulating the catalyst electronic structures to improve the intrinsic activity, such as defect engineering, doping with other elements, hybridization with other high active components and interface engineering. Among these strategies, the doping of S in hydroxides has been demonstrated to be one of the effective ways to modulate the electronic structure and thus optimize the intrinsic activity [6, 7]. (2) exposing abundant active sites and accelerating the electron transfer, such as hybridization with the conductive substrates, designing nanostructure and vacancy engineering. Among them, constructing hierarchical structure on NF has been regarded as an attractive way to facilitate the charge transport and offer abundant active sites and thus improve OER performance [8, 9].

Inspired by the above discussions, we report a simple strategy to synthesize sea urchin-like S-doped Ni(OH)₂ catalysts on nickel foam via hydrothermal reaction and immersion treatment. The S doping can regulate the electronic structure, and improve the electronic conductivity, which significantly enhance OER activity. Benefitting from sea urchin-like structure and the optimized electronic structure, the optimized S-Ni(OH)₂-30 min exhibits prominent OER activity with overpotentials of 306 and 392 mV at 10 and 100 mA cm^− 2, a low Tafel slope of 89.2 mV dec^− 1 and long-term stability over 12 h. This study not only emphasizes the importance of the introduction of S dopant but also offers a facile method for the rational design and fabrication of efficient and durable Ni(OH)₂-based electrocatalysts for OER.

2.1. Chemicals

Ni(NO₃)₂·6H₂O, urea, N,N-dimethylformamide (DMF), Na₂S·9H₂O were obtained from Aladdin Industrial Corporation. Nickel foam (NF) was obtained from Shenzhen Kejing Star Technology Co., LTD. NF was washed with 3 M HCl solution, acetone, ethanol and water under ultrasound for 5 min, respectively. Deionized water was obtained through a laboratory water purification system.

2.2. Synthesis of Ni(OH)₂

3.2 mmol Ni(NO₃)₂·6H₂O and 5 mmol urea were dissolved in 30 mL water via magnetic stirring to obtain a uniform solution. The mixed solution was transferred into a 50 mL Teflon-lined autoclave and the treated NF was submerged into the liquid. Then the autoclave was sealed and heated at 120 ^oC for 10 h. After naturally cooling to room temperature, the NF loaded with Ni(OH)₂ was taken out, washed with water three times and dried at 60 ^oC for 12 h for further use.

2.3. Synthesis of S-doped Ni(OH)₂catalysts

S-Ni(OH)₂-5 s was prepared via solution impregnation method. Typically, the above-synthesized Ni(OH)₂ was immersed in 0.7 M Na₂S solution (30 mL) for 5 seconds. The resulting product was taken out and washed with water for several times, and dried at 60 ^oC for 12 h. For comparison, other S-Ni(OH)₂-x (x = 5 s, 5 min, 30 min, 1.5 h and 12 h) were synthesized by the same method as for S-Ni(OH)₂-5 s except for change the reaction time of impregnation process to 5 min, 30 min, 1.5 h and 12 h.

2.4. Structural characterization

Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/MAX 2400 diffractometer employing a Cu Ka radiation (λ = 1.5405 Å). The scanning electron microscopy (SEM) images were carried out on a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) operating at 20 kV. and the corresponding energy dispersive X-ray spectroscopy (EDX) were conducted using the same SEM microscope. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM), EDX elemental mapping and selected-area electron diffraction (SAED) patterns were measured using a Field Emission Transmission Electron Microscope (TEM FEI, Tecnai G2 F30) working at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were collected on a Thermo Escalab 250 Xi XPS. Raman spectra were performed on an iHR550 Raman spectrophotometer (Horiba, France).

2.5. Electrochemical measurements

All electrochemical measurements were conducted on a CHI760D with a three-electrode system, in 1 M KOH solution. The self-supported catalysts, a carbon rod and Hg/HgO served as the working, counter, and reference electrodes, respectively. The electrolyte was saturated by N₂ before the electrochemical measurements, and the N₂bubbling was kept during OER testing. The potentials reported in this work were transferred to the potentials versus reversible hydrogen electrode (RHE) according to the Nernst equation: E(RHE)=E(Hg/HgO)+0.0977V+0.0596×pH. The working electrodes were activated by cyclic voltammetry (CV) for several cycles. After that, the polarization curves of the catalysts were tested at a scan rate of 1 mV s^-1 with 95% iR compensation. By plotting overpotential vs. log (current density), Tafel slopes were obtained. Electrochemical impedance spectroscopy (EIS) was tested at an overpotential of 5 mV in the frequency range of 0.01 – 100kHz. The ECSA was determined by double layer capacitance (C_dl), which was deduced from cyclic voltammetry (CV) measurements in the non-Faradaic region under different scan rates of 40, 60, 80, 100, 120 and 140 mV/s. C_dl was estimated by plotting ΔJ = (j_anodic - j_cathodic)/2 at 0 V vs. HgO/Hg against the scan rates, and the linear slope is equivalent to C_dl. The long-term stability test was measured by chronopotentiometry measurement at a constant current density of 20 mA cm^-2.

3.1. Material characterization

The sea urchin-like S-Ni(OH)₂ catalyst was fabricated through a two-step process, as shown in Scheme 1. First, sea urchin-like Ni(OH)₂ was uniformly grown on the surface of NF via a simple hydrothermal route. Then, the as-synthesized Ni(OH)₂ was soaked in Na₂S solution with different times (5 s, 5 min, 30 min, 1.5 h, 12 h) to form S-Ni(OH)₂ with sea urchin-like structure.

The morphology and structure of the catalysts were elucidated by Scanning electron microscopy (SEM) and (TEM), as shown in Fig. 1. SEM images of Ni(OH)₂ (Fig. 1) exhibit that a mass of nanowires assembled and generated sea urchin-like structure, covering the surface of the NF. After soaking in a Na₂S solution, the obtained S-Ni(OH)₂-30 min still kept sea urchin-like morphology. Energy dispersive spectroscopy (EDS) demonstrates the presence of Ni, C, O and S elements, which verified the atomic ratio of Ni and S is estimated to 36.59:7.38 (Fig. S1, Supporting Information). Transmission electron microscopy (TEM) was employed to characterize the detailed structure of S-Ni(OH)₂-30 min scrapped off from Ni foam by ultrasonication. From TEM images (Fig. 1g), we find that S-Ni(OH)₂-30 min still retains the sea urchin-like morphology, suggesting the structural stability of S-Ni(OH)₂-30 min. HRTEM images (Fig. 1h) exhibits the lattice fringes of 0.219 nm, which match well with the (103) plan of α-Ni(OH)₂, respectively. The EDS mapping exhibits homogeneous distribution of S, Ni, and O elements in the S-Ni(OH)₂-30 min (Fig. 1i).

The crystal structures of the as-fabricated S-Ni(OH)₂-30 min and Ni(OH)₂ were studied by X-ray diffraction (XRD) (Fig. 2a). The diffraction peaks located around the 44.5^o, 51.9^o and 76.4^o correspond to the (111), (200), and (220) crystal faces of metallic Ni (JCPDS No. 04-0850). Apart from the diffraction peaks of NF, the other peaks of Ni(OH)₂ and S-Ni(OH)₂-30 min are attributed to α-Ni(OH)₂ (JCPDS No. 22–0444) and β-Ni(OH)₂ (JCPDS No. 14–0117). However, relative to pristine α-Ni(OH)₂, the (003) and (006) plans of Ni(OH)₂ and S-Ni(OH)₂-30 min both shift to lower angle, due to the insertion of anions into the lattice of Ni(OH)₂ [10].

XPS was performed to study the surface composition and chemical state of Ni(OH)₂ and S-Ni(OH)₂-30 min. The XPS survey spectrum verifies the presence of Ni, C and O elements in Ni(OH)₂, Ni, C, O and S elements in S-Ni(OH)₂-30 min (Fig. S2, Supporting Information). As for the Ni 2p spectra of S-Ni(OH)₂-30 min (Fig. 2b), the two main peaks located at the binding energies of 856.7 and 874.4 eV can be assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively, and those at 862.5 and 880.5 eV corresponding to their satellite peaks [11]. The binding energies of Ni 2p_3/2 and Ni 2p_1/2 in Ni(OH)₂ are negatively shifted to 856.5 and 874.2 eV, relative to those in S-Ni(OH)₂-30 min [6]. The O 1s spectra for S-Ni(OH)₂-30min (Fig. 2c) shows three peaks located at 530, 531.5 and 533 eV, which can be ascribed to Ni-O bonds [12], Ni-OH [12] and adsorbed molecular water [13], respectively. The S 2p spectrum of S-Ni(OH)₂-30 min (Fig. 2d) presents two main characteristic peaks located at 162.6 and 164.4 eV, which can be indexed to S 2p_3/2 and S 2p_1/2 of Ni-S bonds [14], whereas the peak at 168.6 eV are ascribed to the oxidized sulfur species from surface oxidation [15]. Compared wth Ni(OH)₂-S, the main peaks of O 1s in Ni(OH)₂ are positively shifted about 0.2 eV. All the above XPS results demonstrate that the electron transfer from Ni to O and S, and strong electronic interaction between Ni and S in S-Ni(OH)₂-30 min, which could effectively modify the OER performance of S-Ni(OH)₂-30 min.

3.2. Electrocatalytic OER

To evaluate the OER electrocatalytic activities of S-Ni(OH)₂-5 s, S-Ni(OH)₂-5 min, S-Ni(OH)₂-30 min, S-Ni(OH)₂-1.5 h, S-Ni(OH)₂-12 h and NF, we performed linear-sweep voltammetry (LSV) measurements in 1 M KOH electrolyte. As shown in Fig. 3a, the S-Ni(OH)₂-30 min exhibits the best OER performance, requiring a lower overpotential of 306 mV to deliver density of 10 mA cm^− 2, while S-Ni(OH)₂-5s, S-Ni(OH)₂-5 min, S-Ni(OH)₂-1.5 h, S-Ni(OH)₂-12h, Ni(OH)₂, RuO₂ and NF require larger overpotentials of 304, 306, 295, 312, 318, 329 and 521 mV to reach the same current density, respectively. Moreover, in comparison of S-Ni(OH)₂-5 s (415 mV), S-Ni(OH)₂-5 min (418 mV), S-Ni(OH)₂-1.5 h (399 mV), S-Ni(OH)₂-12 h (424 mV), Ni(OH)₂ (455 mV) and RuO₂ (452 mV), S-Ni(OH)₂-30 min reach the current density of 100 mA cm^− 2 at the lower overpotential of 392 mV. To gain insight into the OER kinetics, Tafel slopes were analyzed (Fig. 3b). S-Ni(OH)₂-30 min shows the Tafel slope of 89.2 mV dec^− 1, which is smaller than those of S-Ni(OH)₂-5 s (93 mV dec^− 1), S-Ni(OH)₂-5 min (94.5 mV dec^− 1), S-Ni(OH)₂-1.5 h (90.8 mV dec^− 1), S-Ni(OH)₂-12 h (96.1 mV dec^− 1), and NF (138.2 mV dec^− 1). The Tafel slope of Ni(OH)₂ is slightly smaller than that of S-Ni(OH)₂-30 min. The electrochemical impedance spectroscopy (EIS) was performed to explore the electrode kinetics during the OER process. All EIS data (Fig. 3c) were fitted well with the equivalent circuit model, which was comprised of the resistance of the electrolyte solution (R_s), constant phase element (CPE) and charge-transfer impedance (R_ct). Based on Nyquist plots, S-Ni(OH)₂-30 min has the smallest semicircle diameter than S-Ni(OH)₂-5 s, S-Ni(OH)₂-5 min, S-Ni(OH)₂-1.5 h, S-Ni(OH)₂-12 h, Ni(OH)₂ and NF, indicating the fastest charge transfer kinetics for OER.

To further probe the origin of excellent performance of S-Ni(OH)₂-30 min, we performed Cdl measurements to estimate ECSA via CV method (Fig. S3). As shown in Fig. 3d, the Cdl value of S-Ni(OH)₂-30 min is 1.35 mF cm^− 2, which is close to that of S-Ni(OH)₂-5 s (1.33 mF cm^− 2), and larger than those of Ni(OH)₂ (0.99 mF cm^− 2), S-Ni(OH)₂-1.5 h (1.11 mF cm^− 2), S-Ni(OH)₂-12 h (1.23 mF cm^− 2), but smaller than that of S-Ni(OH)₂-5 min (1.57 mF cm^− 2), implying that ECSA is not the only factor for the outstanding OER activity of S-Ni(OH)2–30 min. Moreover, the long-term stability is another critical factor for broader applications. The stability test of S-Ni(OH)₂-30 min was conducted by chronopotentiometric measurements at a current density of 20 mA cm^− 2. S-Ni(OH)₂-30 min could maintain its catalytic performance for at least 12 h with slight attenuation, as shown in Fig. 3e. Subsequently, the polarization curves (Fig. 3f) show negligible change after stability test, indicating that S-Ni(OH)₂-30 min has high stability.

To explore the morphology and structure changes of S-Ni(OH)₂-30 min after the CP test, we performed a series of structural characterizations. As shown in Fig. S4, S-Ni(OH)₂-30 min maintains sea urchin-like morphology after the long-term stability test, indicating its structural stability. Meanwhile, as shown in Fig. S5, the lattice fringe of 0.209 nm and 0.219 nm are assigned to the (105) and (104) crystal plane of NiOOH (PDF No. 06–0075), respectively [16]. After OER test, the XRD pattern shows the existence of α-Ni(OH)₂ (JCPDS No. 22–0444) and β-Ni(OH)₂ (Fig. S6a). Moreover, XPS characterization was applied to investigate the chemical states information of S-Ni(OH)₂-30 min after OER test. As for Ni 2p XPS spectra (Fig. 6b), the peaks located at 855.8 and 873.5 eV as well as two satellite peaks at 861.9 and 880.1 eV correspond to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively [17]. In addition, the spin-energy separation was calculated to be 17.7 eV, indicating the presence of Ni(OH)2 species. The other two peaks (857.5 and 875.4 eV) are assigned to Ni³⁺ 2p_3/2 and Ni³⁺ 2p_1/2, which proves the formation of NiOOH species [18]. In addition, the O 1s spectra (Fig. S6c) shows the coexistence of three peaks corresponding to adsorbed water (532.6 eV), hydroxyl groups bonded with metal cations (531.3 eV), lattice oxygen (529.9 eV), implying the generation of Ni–Fe(oxy) hydroxide during water oxidation process [19, 20]. As for S 2p XPS spectra (Fig. S6d), the two major peaks ascribed to S 2p_3/2 and S 2p_1/2 of Ni-S bonds at 162.6 and 164.4 eV disappeared after the OER test, which may be due to the surface oxidation of S element. Raman characterization was carried out to further investigate the structure evolution after the OER stability test (Fig. S7). The broad bands at 474 and 554 cm^− 1 matched well with the spectral features of NiOOH [21]. Based on all the above results, we speculate that the formed nickel oxide/hydroxide converted from S-Ni(OH)₂-30 min at the surface during OER. The generated nickel oxide/hydroxide are known as the real active species, which can optimize adsorption/desorption ability of intermediates, thereby enhancing intrinsic catalytic activity.

According to the above structural analysis and electrochemical results, the improved catalytic activity of S-Ni(OH)₂-30 min could be attributed to the following points: (1) The optimal catalyst presents sea urchin-like structure can provide enough active sites, and facilitate the penetration of reactive species. (2) The self-supported S-Ni(OH)₂-30 min directly grown on conductive Ni foam eliminates using polymer binder, which not only improves the stability of catalysts due to the strong mechanical connection between the catalysts and conductive substrates, but also provides more exposed active sites and facilitates the efficient electron transport. (3) S doping can effectively adjust the electronic structure of catalysts and thus boosting intrinsic activity, which is consistent with the previous reports [7, 22].

In summary, a series of sea urchin-like S-doped Ni(OH)₂ electrocatalyst on nickel foam are synthesized through hydrothermal method and subsequent soaking process. Benefiting from the self-standing feature, sea urchin-like structure and S doping induced electronic structure tuning, the optimized S-Ni(OH)₂-30 min not only offer much accessible active sites, fast charge transfer, but also has high intrinsic activity. As expected, optimized S-Ni(OH)₂-30 min displays high OER performance with overpotentials of 306 and 392 mV 10 and 100 mA cm^− 2, respectively, a small Tafel slope of 89.2 mV dec^− 1 as well as high stability. This work not only demonstrates the important role of the introduction of S dopant for enhancing the OER activity through regulating the electronic structure and facilitating the charge transport but also paves an attractive way for the design of efficient OER catalysts.

Author Contribution

Fang Wu designed the project and wrote the manuscript. Qiu Li synthesized the materials and did the electrocatalytic performance tests. Jin-Long Ge, Yujun Zhu and Yupei Zhao convinced the experimental results and data discussion. Yuhong Jiao, Zhong Wu, Chao Feng conducted the characterizations including XRD, SEM, TEM, Raman and XPS. All authors reviewed the manuscript.

Acknowledgments

This work is supported by the National Science Foundation for Cultivation of Bengbu University (No. 2021pyxm08) and Starting Research Fund of Bengbu University (No. BBXY2021KYQD04). Natural Science Foundation of Bengbu University (2024YYX45QD). Natural Science Foundation of Universities of Anhui Province (Grant No. 2022AH040257). The excellent innovative scientific research team of silicon based materials (No. 2022AH010101). Excellent Young Talents Foundation in Universities of Anhui Province (No. gxyq2022109). Natural Science Foundation of Bengbu University (No. 2022ZR04zd).

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Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

Scheme1.tif
Scheme 1 Schematic illustration for the synthesis, taking S-Ni(OH)₂-30 min for example.
Fig.S1.tif
Fig.S2.tif
Fig.S3.tif
Fig.S4.tif
Fig.S5.tif
Fig.S6.tif
Fig.S7.tif
SupplementaryMaterial.docx

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Sea urchin-like sulfur-doped Ni(OH)2 as an efficient electrocatalyst for oxygen evolution reaction

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Abstract

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1. Introduction

2. Experimental section

3. Results and Discussion

3.1. Material characterization

3.2. Electrocatalytic OER

4. Conclusions

Declarations

Author Contribution

Acknowledgments

References

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Additional Declarations

Supplementary Files

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