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Three-dimensional Fe3S4@NiS hollow nanospheres as efficient electrocatalysts for oxygen evolution reaction
Journal of Electroanalytical Chemistry 850 (2019) 113436
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Three-dimensional Fe3S4@NiS hollow nanospheres as efficient electrocatalysts for oxygen evolution reaction Zewei Hao a, Pengkun Wei a, Hongzhi Kang b, Yang Yang a, Jing Li a, Xue Chen a, Donggang Guo b,⁎, Lu Liu a,⁎ a b
Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China College of Environment and Resource, Shanxi University, Taiyuan 30006, China
a r t i c l e
i n f o
Article history: Received 28 July 2019 Received in revised form 26 August 2019 Accepted 29 August 2019 Available online 30 August 2019 Keywords: Electrocatalyst Hollow nanospheres Fe3S4@NiS Oxygen evolution reaction
a b s t r a c t Due to outstanding performance in energy conversion and storage, transition metal sulfides as well as hollow microstructure have become possible alternatives to noble metal electrocatalyst and become a hot issue. Therefore, we successfully synthesized Fe3S4@NiS hollow nanospheres (HSs) by a one-step solvothermal process, applied in oxygen evolution reactions (OER) to provide sustainable energy. Owing to particular microstructure and proper elemental composition ratio, Fe3S4@NiS HSs (partially been oxidized) show an outstanding OER performance with the overpotential and Tafel slope of only 338 mV at 10 mA cm−2 and 73 mV dec−1 in 0.1 M KOH solution, respectively. © 2019 Elsevier B.V. All rights reserved.
1. Introduction With the global environmental degradation and gradually increasing energy demands, it's essential to find environmentally friendly and cost-effective energy for the sustainable conversion and storage [1–3]. Highly efficient oxygen evolution reaction (OER) provides a promising strategy to meet the energy requirement because of the characteristic of low overpotential, high mass activity and high stability, so it has attracted great interests in recent years [4,5]. Typically, working electrode, reference electrode (Ag/AgCl) and counter electrode (Pt plate) constitute a standard three electrode system device for the OER [6]. The performance of OER is highly decided by electrocatalysts coated on the working electrode. Obviously, an efficient electrocatalyst should possess outstanding electrocatalytic ability, durable stability, and Lowcost [7]. So far, noble metal oxides like IrO2 and RuO2 are commonly used as electrocatalysts for OER, which show the excellent catalytic performance [8,9]. However, their widespread application is limited due to high cost and rare resources. Therefore, it's very crucial to develop efficient and economical electrocatalysts to substitute the expensive noble metals or metal oxides. Research of remarkable OER nanocatalysts has acquired great progresses for decades [10]. These studies have exposed that the ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Guo), [email protected] (L. Liu).
https://doi.org/10.1016/j.jelechem.2019.113436 1572-6657/© 2019 Elsevier B.V. All rights reserved.
performance of OER nanocatalysts mostly depends on their structural features and chemical compositions. In view of the properties containing large surface area, high active site, low density and multiple mesopores, nanomaterials with hollow nanostructures have attracted great attention in OER field [11,12]. Vast OER catalysts with hollow nanostructure, e.g. hollow nanocages [13,14], nanospheres [15,16] and nanotubulars [17], have been probed and exhibited superior catalytic activity. Xu and co-workers [18] developed CoFe2O4 hollow nanospheres by a facile hydrothermal method, which greatly improves the performance of OER than the corresponding solid CoFe2O4 nanospheres. Similarly, the lepidocrocite VOOH hollow nanospheres were synthesized by Shi's group [19], exhibiting relatively good OER catalytic activity compared to the VOOH solid nanospheres. Recent research has indicated that electrocatalysts with hybrid heterogeneous structure possibly show superior OER activity and stability to their single-component counterparts [20–22]. Benefiting from high catalytic activities and low cost, transition metal compounds (TMCs) with different metal cations have been made vast efforts. Especially, iron and nickel sulfides show exceptional electrochemical performances in OER field [23–26]. Ma et al. [27] used NiS-CoS nanorod arrays synthesized via simple electro-chemical plating as efficient OER catalysts, the as-prepared samples exhibit a smaller overpotential and Tafel slope than that of NiS, CoS and RuO2. Besides, the FeS2/CoS2 nanosheets were fabricated by Li and co-workers [28], exhibiting more high electrocatalytic activity and lasting stability than FeS2 and CoS2 for OER.
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Inspired by the above aspects, heterogeneous Fe3S4@NiS hollow HSs were engineered by a one-step solvothermal process, as the catalyst of OER. By contract, bimetallic Fe3S4@NiS HSs (partially been oxidized in OER process) demonstrate more remarkable electrochemical properties than the corresponding Fe3S4 and NiS, even outperforming RuO2 electrode, which benefit from a hollow structure and synergic effect between Fe3S4 and NiS. More notably, the overpotential and Tafel slope, based on Fe3S4@NiS HSs, merely reached 338 mV at 10 mA cm−2 and 73 mV dec−1, respectively, exhibiting more excellent property than the Fe3S4, NiS and RuO2 catalysts.
from 0 to 1.0 V (vs. Ag/AgCl). To keep stability of LSV values, LSV was recorded several times. The double-layer capacitance (Cdl) was accessed through the slope of the sweep rate versus current density at 1.20 V. Electrochemical impedance spectroscopy (EIS) was performed through the direct-current bias potential of 5 mV in a frequency range from 0.01 Hz to 100 kHz. The long-term stability is measured by taking continuous CV for 2000 cycles. To compare the variable of electrochemical activity, the LSV curves before and after cycling were recorded. All the polarization curves have been iR-corrected.
2. Experimental
3. Results and discussion
2.1. Materials and reagents
A facile hydrothermal process was used to prepare NiS, Fe3S4 and Fe3S4@NiS. The phase structure of the Fe3S4@NiS HSs was investigated by XRD analysis. As displayed in Fig. 1, the as-obtained NiS and Fe3S4 were ascribed to NiS (PDF: 02-1280) and Fe3S4 (PDF: 16-0713) standard cards, respectively. Compare to the XRD pattern of single NiS, there are two extra tiny peaks on XRD spectrum of Fe3S4@NiS at 29°and 52°, which could be indexed to Fe3S4 (PDF:16-0713). Due to the intervention of Fe3S4, the above two diffraction peaks of Fe3S4@NiS were shifted to lower angles compared with the single Fe3S4. Therefore, it is reasonable to indicate that the Fe3S4@NiS HSs have been synthesized successfully. The microstructure and architecture features were characterized by SEM. The morphology of NiS and Fe3S4 is shown in Fig. S1, and the solid nanospheres and nanosheets structures were easily observed, respectively. As exhibited in Fig. 2a and b, the as-constructed Fe3S4@NiS shows hollow spherical structure formed by nanoparticles. As Fig. S2 shows, nitrogen adsorption–desorption analysis of the sample displays the properties of the porous structure, which is in accordance with typical type IV sorption isotherm. According to Fig. 2c and d, the TEM image further revealed the hollow nanospheres morphology of Fe3S4@NiS HSs with an average diameter of about 300 nm. The lattice spacing of Fe3S4@ NiS HSs is gauged to be about 0.295 and 0.300 nm, which is corresponding to the (101) and (311) planes of NiS and Fe3S4in the HRTEM image from Fig. 2e, respectively. In TEM process, the elemental mappings were measured, which demonstrate the elements of Fe, Ni and S uniformly disperse in Fe3S4@NiS, as seen in Fig. 2f–i. Besides, EDS analysis also confirmed the presence of Fe, Ni and S with a molar ratio of 1.0:3.6:5.4, close to the stoichiometric ratio of the Fe3S4@NiS HSs (Fig. S3).
All reagents used in the experiments were of analytical level purity without purification. Iron (III) nitrate nonahydrate, Nickel(II) nitrate hexahydrate, and Thioacetamide were prepared from Tianjin Fengchuan Chemical Reagent Company. Benzyl alcohol (C7H8O), Ruthenium(IV) oxide (RuO2) and absolute ethanol were obtained from Aladdin. Deionized (DI) water was used in all experimental procedures. 2.2. Synthesis of Fe3S4@NiS HSs The Fe3S4@NiS HSs were fabricated by a one-step solvothermal process. In detail, 1 mmol of Iron (III) nitrate nonahydrate and 3 mmol of Nickel (II) nitrate hexahydrate were added to 40 mL of benzyl alcohol under vigorous stirring for 0.5 h. Then, 6 mmol of CH3CSNH2 was dissolved in the above clear light purple solution with agitation about 0.5 h. Finally, the dispersion was transferred to a Teflon-lined stainless steel autoclave (50 mL), heated to 220 °C and maintained at 12 h. The synthesized black samples were centrifuged, cleaned in the DI water or ethanol several times, and dried at 50 °C overnight. Furthermore, the pure Fe3S4 and NiS were prepared as contrasts. Unique iron and nickel source was used for the syntheses of Fe3S4 and NiS under the same conditions, respectively. 2.3. Material characterization The phase and crystal structures of the as-prepared products was measured by X-ray diffraction (XRD, D/max-2500, Rigaku) using CuKα radiation (λ = 1.54056 Å) from 10 to 80°. The morphology and nanostructure of the as-obtained samples were characterized through field-emission scanning electron microscopy (FE-SEM, Nanosem 430, FEI) equipped with energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy observations were given through highresolution transmission electron microscopy (HRTEM, Tecnai G2 F20, operating at 200 kV, FEI). Nitrogen adsorption and desorption measurements were recorded on Tristar 3000 nitrogen adsorption apparatus at 77.4 K. The X-ray photoelectron spectroscopy (XPS) spectra were collected with Thermo Scientific ESCALAB 250 XPS instrument. 2.4. Electrochemical measurements Electrochemical measurements were executed by using a system of three-electrode electrochemical cell on a Zahner IM6 electrochemical workstation. A thin layer of as-prepared samples was coated on the working electrode for electrochemical characterization. 5.0 mg of the samples was dissolved with 20 μL of Nafion solution as binder, 360 μL of ethanol and 220 μL of water, and agitated ultrasonically for 90 min to enable good dispersion in the solution. Then, 12.0 μL of the homogeneous slurry was extracted by using a micropipette and injected slowly on the glass carbon electrode. Then, the glass carbon was dried for 10 min under room temperature, which fabricates an as-prepared working electrode. The cyclic voltammetry (CV) curves and linear sweep voltammetry (LSV) were measured by sweeping the potential
Fig. 1. XRD patterns of Fe3S4@NiS, NiS and Fe3S4.
Z. Hao et al. / Journal of Electroanalytical Chemistry 850 (2019) 113436
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Fig. 2. (a, b) SEM images, (c, d and f) TEM images, (e) HRTEM images and (g–i) elemental maps of as-synthesized Fe3S4@NiS HSs.
Microscopic elemental compositions and valence-state information of Fe3S4@NiS HSs were further obtained by XPS. In Fig. 3a, the XPS spectra indicate the attendance of Fe, Ni, and S element in the Fe3S4@NiS HSs. The spectrum of Fe 2p reveals two binding energy peaks at 709.0 eV and 722.9 eV, which shows the presence of Fe (III) and Fe (II) species in as-prepared samples in Fig. 3b [29]. For the highresolution Ni 2p XPS of Fe3S4@NiS HSs from Fig. 3c, the peaks at around 855.8 and 873.4 eV are assigned to the Ni 2p3/2 and Ni 2p1/2, respectively [23]. Moreover, there are two shake-up satellites at 880.7 eV and 861.1 eV. Fig. 3d exhibits the peak at 159.7 eV in the S 2p spectrum is attributed to S2− [27]. Moreover, S 2p peaks for Fe3S4 and NiS at 158.8 eV and 166.2 eV further confirm the co-existence of Fe3S4 and NiS, respectively. To explore the electrochemical performance of Fe3S4@NiS HSs, the electrochemical tests were performed on the three-electrode-work station in 0.1 M KOH solution. Because of the oxidation reaction occurred in the OER process, the Fe3S4@NiS HSs contained a certain amount of oxide. Firstly, the OER catalytic characteristic of Fe3S4@NiS HSs was
assessed by typical polarization curves. The OER polarization curves were recorded by LSV curves. Fig. 4a shows the polarization curves of NiS, Fe3S4, Fe3S4@NiS HSs and RuO2. It can be found that the OER catalytic performance of Fe3S4@NiS HSs was greatly improved compared with the corresponding Fe3S4 and NiS catalysts, and the current densities rose rapidly with the enhancement of positive potential. In detail, Fe3S4@NiS HSs exhibits a low overpotential of 338 mV at a current density of 10 mA cm−2, which is lower than NiS (378 mV), Fe3S4 (385 mV) and RuO2 (418 mV). Compared to the behaviors of many similar catalysts like nanocrystalline NiS particles [30], NiO NWs [31], Ni0.4Co0.6oxide [32] and Co–FeS2 [33] in alkaline electrolytes (listed in Table. S1), this overpotential is more favorable. To calculate the OER kinetics of the as-synthesized samples, the Tafel slopes were evaluated on the basis of the corresponding LSV curves [34,35]. A smaller Tafel slope stands for more advantageous kinetic and outstanding catalytic performance of OER. As exhibited in Fig. 4b, the Tafel slope value for RuO2 is 118 mV dec−1, which is nearby with the normal reference values [36]. In addition, the Tafel slope of Fe3S4@
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Fig. 3. (a) XPS survey spectrum and high-resolution scans of (b) Fe 2p, (c) Ni 2p and (d) S 2p of Fe3S4@NiS HSs.
NiS HSs is only 73 mV dec−1, smaller than that of NiS (123 mV dec−1), Fe3S4 (139 mV dec−1), suggesting superior catalytic activity. The EIS measurements of NiS, Fe3S4 and Fe3S4@NiS HSs were performed to analyze the electron transfer rate at the catalyst/electrolyte interface [37]. Normally, the image of Nyquist plot like a semicircle as well as its diameter indicates the value of charge-transfer resistance (Rct). Typically, smaller Rct represents more favorable electrocatalytic kinetics. The Rct value of Fe3S4@NiS HSs is 0.9 Ω, which is smaller than that of NiS (2.2 Ω), Fe3S4 (2.1 Ω) and RuO2 (4.4 Ω), showing more efficient electron transfer frequency and better catalytic activity for OER, as displayed in Fig. 4c. Benefitting from unique hollow architecture and high conductivity, Fe3S4@NiS HSs improve charge transport capability and accelerate the reaction at electrolyte/electrode interface in OER process. To further compare the catalytic activities, the Cdl was assessed by utilizing cyclic voltammetry measurements at different scan rates (20, 40, 60, 80, 100 and 120 mV s−1). The slope of the plot is proportional to Cdl, which provides an indication of the relatively active surface area [38]. Fig. 4d reveals that the Cdl value of Fe3S4@NiS HSs is higher than that of NiS and Fe3S4, even slightly surpassing that RuO2. The enhancement of Cdl may be contributed to the hollow architecture and the synergistic effect between Fe3S4 and NiS, which improve the catalytic performance of OER. The stability of electrocatalysts is also a crucial index for longstanding applications [39]. To assess long-term durability, multiple CV tests were executed. Comparing the initial LSV curve, there is a slight
difference after 2000-cycle CV scanning, exhibiting impressive electrocatalytic durability for the OER, as revealed in Fig. 5. 4. Conclusion In summary, Fe3S4@NiS HSs have been fabricated and utilized as the catalysts of OER. The electrochemical measurements display that Fe3S4@NiS HSs (partially been oxidized) possess more excellent electrical conductivity and catalytic activity than the relevant Fe3S4 and NiS. Under 0.1 M KOH solution, Fe3S4@NiS HSs catalyst obtained a low overpotential of 338 mV and Tafel slope of 73 mV decade−1 to achieve 10 mA cm−2, evidently smaller than RuO2. The outstanding catalytic activity of Fe3S4@NiS HSs could be attributed to the superior hollow spheres structure and ternary chemical composition. Acknowledgments The work was supported by the National Science Fund for Distinguished Young Scholars (Grant 21425729) from the National Natural Science Foundation of China, and the Tianjin science and technology support key projects (18YFZCSF00500). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113436.
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Fig. 4. (a) LSV curves, (b) Tafel slopes, (c) Nyquist plots, (d) determined double-layer capacitance (Cdl) for NiS, Fe3S4, Fe3S4@NiS HSs and RuO2.
References
Fig. 5. Cycle stability of Fe3S4@NiS HSs before and after 2000 CV cycles.
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Three-dimensional Fe3S4@NiS hollow nanospheres as efficient electrocatalysts for oxygen evolution reaction Zewei Hao a, Pengkun Wei a, Hongzhi Kang b, Yang Yang a, Jing Li a, Xue Chen a, Donggang Guo b,⁎, Lu Liu a,⁎ a b
Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China College of Environment and Resource, Shanxi University, Taiyuan 30006, China
a r t i c l e
i n f o
Article history: Received 28 July 2019 Received in revised form 26 August 2019 Accepted 29 August 2019 Available online 30 August 2019 Keywords: Electrocatalyst Hollow nanospheres Fe3S4@NiS Oxygen evolution reaction
a b s t r a c t Due to outstanding performance in energy conversion and storage, transition metal sulfides as well as hollow microstructure have become possible alternatives to noble metal electrocatalyst and become a hot issue. Therefore, we successfully synthesized Fe3S4@NiS hollow nanospheres (HSs) by a one-step solvothermal process, applied in oxygen evolution reactions (OER) to provide sustainable energy. Owing to particular microstructure and proper elemental composition ratio, Fe3S4@NiS HSs (partially been oxidized) show an outstanding OER performance with the overpotential and Tafel slope of only 338 mV at 10 mA cm−2 and 73 mV dec−1 in 0.1 M KOH solution, respectively. © 2019 Elsevier B.V. All rights reserved.
1. Introduction With the global environmental degradation and gradually increasing energy demands, it's essential to find environmentally friendly and cost-effective energy for the sustainable conversion and storage [1–3]. Highly efficient oxygen evolution reaction (OER) provides a promising strategy to meet the energy requirement because of the characteristic of low overpotential, high mass activity and high stability, so it has attracted great interests in recent years [4,5]. Typically, working electrode, reference electrode (Ag/AgCl) and counter electrode (Pt plate) constitute a standard three electrode system device for the OER [6]. The performance of OER is highly decided by electrocatalysts coated on the working electrode. Obviously, an efficient electrocatalyst should possess outstanding electrocatalytic ability, durable stability, and Lowcost [7]. So far, noble metal oxides like IrO2 and RuO2 are commonly used as electrocatalysts for OER, which show the excellent catalytic performance [8,9]. However, their widespread application is limited due to high cost and rare resources. Therefore, it's very crucial to develop efficient and economical electrocatalysts to substitute the expensive noble metals or metal oxides. Research of remarkable OER nanocatalysts has acquired great progresses for decades [10]. These studies have exposed that the ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Guo), [email protected] (L. Liu).
https://doi.org/10.1016/j.jelechem.2019.113436 1572-6657/© 2019 Elsevier B.V. All rights reserved.
performance of OER nanocatalysts mostly depends on their structural features and chemical compositions. In view of the properties containing large surface area, high active site, low density and multiple mesopores, nanomaterials with hollow nanostructures have attracted great attention in OER field [11,12]. Vast OER catalysts with hollow nanostructure, e.g. hollow nanocages [13,14], nanospheres [15,16] and nanotubulars [17], have been probed and exhibited superior catalytic activity. Xu and co-workers [18] developed CoFe2O4 hollow nanospheres by a facile hydrothermal method, which greatly improves the performance of OER than the corresponding solid CoFe2O4 nanospheres. Similarly, the lepidocrocite VOOH hollow nanospheres were synthesized by Shi's group [19], exhibiting relatively good OER catalytic activity compared to the VOOH solid nanospheres. Recent research has indicated that electrocatalysts with hybrid heterogeneous structure possibly show superior OER activity and stability to their single-component counterparts [20–22]. Benefiting from high catalytic activities and low cost, transition metal compounds (TMCs) with different metal cations have been made vast efforts. Especially, iron and nickel sulfides show exceptional electrochemical performances in OER field [23–26]. Ma et al. [27] used NiS-CoS nanorod arrays synthesized via simple electro-chemical plating as efficient OER catalysts, the as-prepared samples exhibit a smaller overpotential and Tafel slope than that of NiS, CoS and RuO2. Besides, the FeS2/CoS2 nanosheets were fabricated by Li and co-workers [28], exhibiting more high electrocatalytic activity and lasting stability than FeS2 and CoS2 for OER.
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Inspired by the above aspects, heterogeneous Fe3S4@NiS hollow HSs were engineered by a one-step solvothermal process, as the catalyst of OER. By contract, bimetallic Fe3S4@NiS HSs (partially been oxidized in OER process) demonstrate more remarkable electrochemical properties than the corresponding Fe3S4 and NiS, even outperforming RuO2 electrode, which benefit from a hollow structure and synergic effect between Fe3S4 and NiS. More notably, the overpotential and Tafel slope, based on Fe3S4@NiS HSs, merely reached 338 mV at 10 mA cm−2 and 73 mV dec−1, respectively, exhibiting more excellent property than the Fe3S4, NiS and RuO2 catalysts.
from 0 to 1.0 V (vs. Ag/AgCl). To keep stability of LSV values, LSV was recorded several times. The double-layer capacitance (Cdl) was accessed through the slope of the sweep rate versus current density at 1.20 V. Electrochemical impedance spectroscopy (EIS) was performed through the direct-current bias potential of 5 mV in a frequency range from 0.01 Hz to 100 kHz. The long-term stability is measured by taking continuous CV for 2000 cycles. To compare the variable of electrochemical activity, the LSV curves before and after cycling were recorded. All the polarization curves have been iR-corrected.
2. Experimental
3. Results and discussion
2.1. Materials and reagents
A facile hydrothermal process was used to prepare NiS, Fe3S4 and Fe3S4@NiS. The phase structure of the Fe3S4@NiS HSs was investigated by XRD analysis. As displayed in Fig. 1, the as-obtained NiS and Fe3S4 were ascribed to NiS (PDF: 02-1280) and Fe3S4 (PDF: 16-0713) standard cards, respectively. Compare to the XRD pattern of single NiS, there are two extra tiny peaks on XRD spectrum of Fe3S4@NiS at 29°and 52°, which could be indexed to Fe3S4 (PDF:16-0713). Due to the intervention of Fe3S4, the above two diffraction peaks of Fe3S4@NiS were shifted to lower angles compared with the single Fe3S4. Therefore, it is reasonable to indicate that the Fe3S4@NiS HSs have been synthesized successfully. The microstructure and architecture features were characterized by SEM. The morphology of NiS and Fe3S4 is shown in Fig. S1, and the solid nanospheres and nanosheets structures were easily observed, respectively. As exhibited in Fig. 2a and b, the as-constructed Fe3S4@NiS shows hollow spherical structure formed by nanoparticles. As Fig. S2 shows, nitrogen adsorption–desorption analysis of the sample displays the properties of the porous structure, which is in accordance with typical type IV sorption isotherm. According to Fig. 2c and d, the TEM image further revealed the hollow nanospheres morphology of Fe3S4@NiS HSs with an average diameter of about 300 nm. The lattice spacing of Fe3S4@ NiS HSs is gauged to be about 0.295 and 0.300 nm, which is corresponding to the (101) and (311) planes of NiS and Fe3S4in the HRTEM image from Fig. 2e, respectively. In TEM process, the elemental mappings were measured, which demonstrate the elements of Fe, Ni and S uniformly disperse in Fe3S4@NiS, as seen in Fig. 2f–i. Besides, EDS analysis also confirmed the presence of Fe, Ni and S with a molar ratio of 1.0:3.6:5.4, close to the stoichiometric ratio of the Fe3S4@NiS HSs (Fig. S3).
All reagents used in the experiments were of analytical level purity without purification. Iron (III) nitrate nonahydrate, Nickel(II) nitrate hexahydrate, and Thioacetamide were prepared from Tianjin Fengchuan Chemical Reagent Company. Benzyl alcohol (C7H8O), Ruthenium(IV) oxide (RuO2) and absolute ethanol were obtained from Aladdin. Deionized (DI) water was used in all experimental procedures. 2.2. Synthesis of Fe3S4@NiS HSs The Fe3S4@NiS HSs were fabricated by a one-step solvothermal process. In detail, 1 mmol of Iron (III) nitrate nonahydrate and 3 mmol of Nickel (II) nitrate hexahydrate were added to 40 mL of benzyl alcohol under vigorous stirring for 0.5 h. Then, 6 mmol of CH3CSNH2 was dissolved in the above clear light purple solution with agitation about 0.5 h. Finally, the dispersion was transferred to a Teflon-lined stainless steel autoclave (50 mL), heated to 220 °C and maintained at 12 h. The synthesized black samples were centrifuged, cleaned in the DI water or ethanol several times, and dried at 50 °C overnight. Furthermore, the pure Fe3S4 and NiS were prepared as contrasts. Unique iron and nickel source was used for the syntheses of Fe3S4 and NiS under the same conditions, respectively. 2.3. Material characterization The phase and crystal structures of the as-prepared products was measured by X-ray diffraction (XRD, D/max-2500, Rigaku) using CuKα radiation (λ = 1.54056 Å) from 10 to 80°. The morphology and nanostructure of the as-obtained samples were characterized through field-emission scanning electron microscopy (FE-SEM, Nanosem 430, FEI) equipped with energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy observations were given through highresolution transmission electron microscopy (HRTEM, Tecnai G2 F20, operating at 200 kV, FEI). Nitrogen adsorption and desorption measurements were recorded on Tristar 3000 nitrogen adsorption apparatus at 77.4 K. The X-ray photoelectron spectroscopy (XPS) spectra were collected with Thermo Scientific ESCALAB 250 XPS instrument. 2.4. Electrochemical measurements Electrochemical measurements were executed by using a system of three-electrode electrochemical cell on a Zahner IM6 electrochemical workstation. A thin layer of as-prepared samples was coated on the working electrode for electrochemical characterization. 5.0 mg of the samples was dissolved with 20 μL of Nafion solution as binder, 360 μL of ethanol and 220 μL of water, and agitated ultrasonically for 90 min to enable good dispersion in the solution. Then, 12.0 μL of the homogeneous slurry was extracted by using a micropipette and injected slowly on the glass carbon electrode. Then, the glass carbon was dried for 10 min under room temperature, which fabricates an as-prepared working electrode. The cyclic voltammetry (CV) curves and linear sweep voltammetry (LSV) were measured by sweeping the potential
Fig. 1. XRD patterns of Fe3S4@NiS, NiS and Fe3S4.
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Fig. 2. (a, b) SEM images, (c, d and f) TEM images, (e) HRTEM images and (g–i) elemental maps of as-synthesized Fe3S4@NiS HSs.
Microscopic elemental compositions and valence-state information of Fe3S4@NiS HSs were further obtained by XPS. In Fig. 3a, the XPS spectra indicate the attendance of Fe, Ni, and S element in the Fe3S4@NiS HSs. The spectrum of Fe 2p reveals two binding energy peaks at 709.0 eV and 722.9 eV, which shows the presence of Fe (III) and Fe (II) species in as-prepared samples in Fig. 3b [29]. For the highresolution Ni 2p XPS of Fe3S4@NiS HSs from Fig. 3c, the peaks at around 855.8 and 873.4 eV are assigned to the Ni 2p3/2 and Ni 2p1/2, respectively [23]. Moreover, there are two shake-up satellites at 880.7 eV and 861.1 eV. Fig. 3d exhibits the peak at 159.7 eV in the S 2p spectrum is attributed to S2− [27]. Moreover, S 2p peaks for Fe3S4 and NiS at 158.8 eV and 166.2 eV further confirm the co-existence of Fe3S4 and NiS, respectively. To explore the electrochemical performance of Fe3S4@NiS HSs, the electrochemical tests were performed on the three-electrode-work station in 0.1 M KOH solution. Because of the oxidation reaction occurred in the OER process, the Fe3S4@NiS HSs contained a certain amount of oxide. Firstly, the OER catalytic characteristic of Fe3S4@NiS HSs was
assessed by typical polarization curves. The OER polarization curves were recorded by LSV curves. Fig. 4a shows the polarization curves of NiS, Fe3S4, Fe3S4@NiS HSs and RuO2. It can be found that the OER catalytic performance of Fe3S4@NiS HSs was greatly improved compared with the corresponding Fe3S4 and NiS catalysts, and the current densities rose rapidly with the enhancement of positive potential. In detail, Fe3S4@NiS HSs exhibits a low overpotential of 338 mV at a current density of 10 mA cm−2, which is lower than NiS (378 mV), Fe3S4 (385 mV) and RuO2 (418 mV). Compared to the behaviors of many similar catalysts like nanocrystalline NiS particles [30], NiO NWs [31], Ni0.4Co0.6oxide [32] and Co–FeS2 [33] in alkaline electrolytes (listed in Table. S1), this overpotential is more favorable. To calculate the OER kinetics of the as-synthesized samples, the Tafel slopes were evaluated on the basis of the corresponding LSV curves [34,35]. A smaller Tafel slope stands for more advantageous kinetic and outstanding catalytic performance of OER. As exhibited in Fig. 4b, the Tafel slope value for RuO2 is 118 mV dec−1, which is nearby with the normal reference values [36]. In addition, the Tafel slope of Fe3S4@
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Fig. 3. (a) XPS survey spectrum and high-resolution scans of (b) Fe 2p, (c) Ni 2p and (d) S 2p of Fe3S4@NiS HSs.
NiS HSs is only 73 mV dec−1, smaller than that of NiS (123 mV dec−1), Fe3S4 (139 mV dec−1), suggesting superior catalytic activity. The EIS measurements of NiS, Fe3S4 and Fe3S4@NiS HSs were performed to analyze the electron transfer rate at the catalyst/electrolyte interface [37]. Normally, the image of Nyquist plot like a semicircle as well as its diameter indicates the value of charge-transfer resistance (Rct). Typically, smaller Rct represents more favorable electrocatalytic kinetics. The Rct value of Fe3S4@NiS HSs is 0.9 Ω, which is smaller than that of NiS (2.2 Ω), Fe3S4 (2.1 Ω) and RuO2 (4.4 Ω), showing more efficient electron transfer frequency and better catalytic activity for OER, as displayed in Fig. 4c. Benefitting from unique hollow architecture and high conductivity, Fe3S4@NiS HSs improve charge transport capability and accelerate the reaction at electrolyte/electrode interface in OER process. To further compare the catalytic activities, the Cdl was assessed by utilizing cyclic voltammetry measurements at different scan rates (20, 40, 60, 80, 100 and 120 mV s−1). The slope of the plot is proportional to Cdl, which provides an indication of the relatively active surface area [38]. Fig. 4d reveals that the Cdl value of Fe3S4@NiS HSs is higher than that of NiS and Fe3S4, even slightly surpassing that RuO2. The enhancement of Cdl may be contributed to the hollow architecture and the synergistic effect between Fe3S4 and NiS, which improve the catalytic performance of OER. The stability of electrocatalysts is also a crucial index for longstanding applications [39]. To assess long-term durability, multiple CV tests were executed. Comparing the initial LSV curve, there is a slight
difference after 2000-cycle CV scanning, exhibiting impressive electrocatalytic durability for the OER, as revealed in Fig. 5. 4. Conclusion In summary, Fe3S4@NiS HSs have been fabricated and utilized as the catalysts of OER. The electrochemical measurements display that Fe3S4@NiS HSs (partially been oxidized) possess more excellent electrical conductivity and catalytic activity than the relevant Fe3S4 and NiS. Under 0.1 M KOH solution, Fe3S4@NiS HSs catalyst obtained a low overpotential of 338 mV and Tafel slope of 73 mV decade−1 to achieve 10 mA cm−2, evidently smaller than RuO2. The outstanding catalytic activity of Fe3S4@NiS HSs could be attributed to the superior hollow spheres structure and ternary chemical composition. Acknowledgments The work was supported by the National Science Fund for Distinguished Young Scholars (Grant 21425729) from the National Natural Science Foundation of China, and the Tianjin science and technology support key projects (18YFZCSF00500). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113436.
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Fig. 4. (a) LSV curves, (b) Tafel slopes, (c) Nyquist plots, (d) determined double-layer capacitance (Cdl) for NiS, Fe3S4, Fe3S4@NiS HSs and RuO2.
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Fig. 5. Cycle stability of Fe3S4@NiS HSs before and after 2000 CV cycles.
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