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Low Ni-doped Co3O4 porous nanoplates for enhanced hydrogen and oxygen evolution reaction
Journal Pre-proof Low Ni-doped Co3O4 porous nanoplates for enhanced hydrogen and oxygen evolution reaction Li Li, Qianqian Xu, Yongxing Zhang, Jia Li, Jinhui Fang, Yongping Dai, Xingliang Cheng, Yu You, Xuanhua Li PII:
S0925-8388(20)30113-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.153750
Reference:
JALCOM 153750
To appear in:
Journal of Alloys and Compounds
Received Date: 25 November 2019 Revised Date:
5 January 2020
Accepted Date: 7 January 2020
Please cite this article as: L. Li, Q. Xu, Y. Zhang, J. Li, J. Fang, Y. Dai, X. Cheng, Y. You, X. Li, Low Ni-doped Co3O4 porous nanoplates for enhanced hydrogen and oxygen evolution reaction, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153750. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement: Yongxing Zhang and Xuanhua Li: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition and Project administration. Jia Li, Jinhui Fang and Yongping Dai: Visualization and Investigation. Xingliang Cheng and Yu You: Validation and Funding acquisition. Li Li and Qianqian Xu: Data curation, Writing- Original draft preparation, WritingReviewing and Editing.
Low Ni-doped Co3O4 Porous Nanoplates for Enhanced Hydrogen and Oxygen Evolution Reaction Li Li1, ‡, Qianqian Xu1, ‡, Yongxing Zhang1*, Jia Li1, Jinhui Fang1, Yongping Dai1, Xingliang Cheng1, Yu You1 and Xuanhua Li2*
1
Key Laboratory of Green and Precise Synthetic Chemistry and Application, Ministry of Education,
Department of Materials Science and Engineering, Huaibei Normal University, Huaibei 235000, P. R. China
2
State Key Laboratory of Solidification Processing Center of Nano Energy Materials, School of Materials
Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, P. R. China E-mail: [email protected]; E-mail: [email protected] ‡
These authors contributed equally to this work
Abstract In this paper, low Ni-doped Co3O4 (Ni: 2 wt%, 4 wt%, 6 wt% and 8 wt%) porous nanoplates are synthesized by hydrothermal method and annealing treatment. For the low Ni-doped Co3O4, X-ray diffraction (XRD) and Raman data show that the Ni ions are successfully incorporated into the Co3O4 lattices. Electrochemical measurements indicate that Co3O4 with low amount of Ni doping possesses remarkably enhanced activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. And especially, 4wt% Ni-doped Co3O4 possesses the most efficient activity with an overpotential of 240 mV at the current density of 10 mA cm-2 for OER and 120 mV for HER in 1 M KOH than the pure Co3O4 and other Ni-doped Co3O4 samples. Our study suggests that the low doping would have a great potential in rational design of spinel oxides toward highly efficient electrocatalysis.
Keywords: Ni-Doped Co3O4; Porous Nanoplates; Oxygen Evolution Reaction; Hydrogen Evolution Reaction; Hydrothermal Method 1
1. Introduction Recently, electrochemical water splitting has drawn ever-increasing attention because it is an effective way to obtain high efficiency renewable hydrogen energy. [1-6] As is known to all, two half reactions, namely, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), are involved in water splitting.[7-8] At present, the advanced electrocatalysts for water splitting are noble metal based materials (i.e. Pt for HER and IrO2/RuO2 for OER). [9-10] However, the high cost and scarcity have limited their large-scale application. In this regard, the key challenge is to develop natural abundance, low-cost and green electrocatalysts with high activity that can be comparable to those mentioned above. In the past few decades, a large number of transition metal based materials, including oxide, [11-12] chalcogenides, [13-14] nitrides, [15-16] layered double hydroxide [17-19], carbides [20-21] and phosphides, [22] have been intensively studied as bifunctional catalyst materials for water splitting. Among these materials, cobalt based materials stand out as a kind of efficient catalysts, such as NixCo3-xO4 nanowire arrays [23], NixCo3-xO4 nanoneedle arrays [24], NiCo2O4 nanoparticles [25], nickel cobalt oxide nanoplates [26] and some cobalt based composites [27-29]. Co3O4 has been studied widely in various fields, such as degradation [30], Zn-air batteries [31], supercapacitor [32] and water splitting [33] because of their prominent properties. Two strategies have been adopted to enhance the activity of Co3O4 in previous reports. The first way is to change the preparation methods. Various methods have been employed to change the morphology and structure, including direct high-temperature oxidation [34], hot plate combustion method [35] and precipitation method [36], hydrothermal reaction [37] and the like. It is no doubt that the electrocatalytic activity of nanomaterials is closely related to their morphology and structure. In typically, compared with the bulk structure, two-dimensional (2D) nanoplates with porous structure have higher specific surface area, and they can provide more active sites, leading to enhancement of the catalytic activity. The other is doping, such as carbon (C)-doping [33, 38], PdO-doping [39], La3+-doping [40], sulfur (S)-doping [41], iron (Fe)-doping [42-43], manganese (Mn)-doping [44-45], chromium (Cr)-doping [46] and so on. Furthermore, nickel (Ni)-doped Co3O4 is also reported for electrocatalytic property [47-49]. However, it is rarely reported that the Co3O4 with low nickel doping as a highly efficient electrocatalyst for both OER and HER in alkaline media. Herein, Ni-doped Co3O4 porous nanoplates are synthesised by hydrothermal method and subsequent annealing treatment. The electrocatalytic performances for both OER and HER of the as-prepared porous 2
materials have been explored in alkaline media. And from the result, it is found that the Ni-doped Co3O4 porous nanoplates have the following features: (a) they can be obtained by using nontoxic and inexpensive raw materials and a low-cost synthesis methods; (b) their unique porous structures provide a superior number of active sites exposed to surface reactions as well as excellent electrocatalytic activity; (c) the materials with low doping concentrations have remarkably enhanced activity for both OER and HER in alkaline media.
2. Results and discussion
Fig. 1 (a) XRD patterns of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 porous nanoplates; (b) The enlarged XRD patterns of the red rectangular in (a); (c) EDS spectrum of the 4wt%Ni-Co3O4 porous nanoplates; (d) Raman spectra of (1) the pure Co3O4 and (2) 4wt%Ni-Co3O4 porous nanoplates
Fig.1(a) shows the X-ray diffraction (XRD) patterns of the pure Co3O4 and other four Ni-doped Co3O4 powders after annealing in muffle furnace at 550 oC for 3 hours. All of the diffraction peaks are coincided well with the cubic spinel type structure Co3O4 (JCPDS no. 74-2120, Fd3m space group). And there are no other peaks can be observed, which indicate high-purity samples are obtained. The detailed study based on 3
the enlarged XRD pattern is shown in Fig. 1(b) (25-40°), obvious shifts of the two corresponding peaks are observed between the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplate samples. From Fig.1(c), the energy dispersive spectrometer (EDS) data of the 4wt% Ni-Co3O4 further reveals the existence of Co, O and Ni elements, while there are only Co and O elements for the pure Co3O4 in Fig. S1. Raman spectra of the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplates are shown in Fig. 1(d). Five Raman bands at 192, 473, 515, 612, and 679 cm−1 are detected for the pure Co3O4, while the bands at 194, 480, 520, 617, and 687 cm-1 for 4wt% Ni-Co3O4 shift slightly to higher wavenumbers. The results indicate the Ni ions are incorporated into the Co3O4 lattices.
Fig. 2 XPS characterization of the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplates: (a) survey spectrum; (b)Ni 2p of 4wt% Ni-Co3O4; (c) Co 2p; (d) O 1s
The components and the valence states of the pure Co3O4 and 4%wt Ni-Co3O4 porous nanoplates are investigated by the X-ray photoelectron spectroscopy (XPS). Fig. 2(a) shows the wide scan survey spectra of 4
the pure Co3O4 and 4%wt Ni-Co3O4 porous nanoplates. And the dotted rectangle of 4%wt Ni-Co3O4 in Fig. 2(a) is enlarged in Fig. 2(b), which indicates the existence of Ni element in this sample. In addition, the peaks at 855.5 eV and 872.8 eV with a peak separation of 17.3 eV in the Ni2p spectrum are typical for Ni2+ species, further confirming the efficient doping of Ni2+ in Co3O4 nanoplates. Compared to the pure Co3O4, there are obvious negative shifts of the binding energies of Co2p (0.23 eV, Fig. 2(c)) and O1s (0.22 eV, Fig. 2(d)) for the 4%wt Ni-Co3O4, respectively. These results also indicate that Ni ions are successfully doped into Co3O4.
Fig. 3 Morphological and structural characterizations of the 4wt% Ni-Co3O4 porous nanoplates: (a, b, c) FESEM images with low- and high-magnification. (d) The FESEM image and corresponding elemental mapping images. (e)The TEM images and (f) the HRTEM image of the 4wt% Ni-Co3O4 porous nanoplates
As shown in Fig. 3(a), the field emission scanning electron microscope (FESEM) image of the overall morphology for the 4wt% Ni-Co3O4 indicates that many nanoplates are obtained. Fig. 3(b) shows that there are many pores in the 4wt% Ni-Co3O4 nanoplates. And from Fig. 3(c), the thickness of the nanoplates is about 40 nm. The SEM images of Co3O4, 2wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 porous nanoplates are shown in Fig. S2. It can be seen that the morphology has not changed a lot with the Ni incorporation. Fig. 3(d) presents the FESEM image of the 4wt% Ni-Co3O4 porous nanoplates and corresponding elemental mapping images, revealing that Co, O and Ni elements are homogeneously distributed in the whole porous nanoplates. From Fig. 3(e), the 4wt% Ni-Co3O4 porous nanoplates is 5
constructed by numerous nanoplates (inset) and small pores, which can provide a large number of exposed active edge sites. And the small pores can provide an available pathway for electrolyte and electron transfer, so that can facilitate the dissociation of water. The high-resolution transmission electron microscope (HRTEM) image of the 4wt% Ni-Co3O4 porous nanoplates indicates that the lattice fringe of the Co3O4 was ~0.244 nm, which can be attributed to the (311) crystal plane (Fig. 3(f)), while the pure Co3O4 is ~0.240 nm (the inset of Fig. S2(a)).
Fig. 4 (a) LSV curves for OER of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 6
8wt% Ni-Co3O4 in 1 M KOH electrolyte with a sweep rate of 1 mV s-1; (b) Tafel plots of the catalysts; (c) Electrochemical impedance spectra; (d) Plots of the double-layer capacitances at 0.15 V (vs. Ag/AgCl); (e) Chronopotentiometry curve of the pure Co3O4 and the 4wt% Ni-Co3O4 at a constant current density of 10 mA cm-2, contact angle measurements of (1) the pure Co3O4, (2) 4wt% Ni-Co3O4 porous nanoplates (inset); (f) Cycles stability of the 4wt% Ni-Co3O4 porous nanoplates
The OER catalytic activity of the pure Co3O4 and a series of Ni-doped Co3O4 porous nanoplates are measured in 1 M KOH solution with a three electrodes system. Fig. 4(a) shows representative linear sweep voltammetry (LSV) polarization curves of the as-prepared samples without any iR-corrected. The 4wt% Ni-Co3O4 possesses the lowest overpotential of 240 mV at a current density of 10 mV cm-2 in comparison to those of the pure Co3O4 (350 mV), 2wt% Ni-Co3O4 (280 mV), 6wt% Ni-Co3O4 (330 mV) and 8wt% Ni-Co3O4 (335 mV). It is found that the OER performance is reduced greatly with excess Ni incorporation because high Ni doping concentration in Co3O4 decreases the amount of oxygen in the surface and result in a lower activity[47]. As shown in Fig. 4(b), the Tafel slope of 4wt% Ni-Co3O4 is 64.2 mV dec-1, which is lower than those of the pure Co3O4 (160.1 mV dec-1), 2wt% Ni-Co3O4 (81.4 mV dec-1), 6wt% Ni-Co3O4 (79.8 mV dec-1) and 8wt% Ni-Co3O4 (116.7 mV dec-1). The outstanding performance of the 4wt% Ni-Co3O4 porous nanoplate catalysts is also evidenced by the electrochemical impedance spectra (EIS) data (Fig. 4(c)), which shows a significantly smaller interfacial charge transfer resistance. And the result indicates that the 4wt% Ni-Co3O4 porous nanoplates possesses promoted OER kinetics comparing to those pure Co3O4 and other Ni-doped Co3O4. To explore the improved OER performance of the 4wt% Ni-Co3O4, the effective electrochemical active surface area (ECSA) is calculated by measuring the double-layer capacitance (Cdl), as shown in Fig. 4(d), which derived from cyclic voltammograms (CV) results at different scan rates (Fig. S3). The 4wt% Ni-Co3O4 yields a high capacitance value up to 12.16 mF cm-2, much larger than those of the Co3O4 (4.30 mF cm-2), 2wt% Ni-Co3O4 (6.67 mF cm-2), 6wt% Ni-Co3O4 (4.70 mF cm-2) and 8wt % Ni-Co3O4 (4.19 mF cm-2) electrodes. Furthermore, the electrochemical stability of the 4wt% Ni-Co3O4 is evaluated at 10 mA cm-2. As shown in Fig. 4(e), the potential of the 4wt% Ni-Co3O4 remains stable for a long period time (16 h) during the OER test, indicating very high catalytic stability under alkaline condition, while the overpotential of the pure Co3O4 has increased significantly. As shown in the inset of Fig. 4(e), the contact angle of the 4wt% Ni-Co3O4 porous nanoplates is 22°, which is slightly smaller than the pure Co3O4 (29°), suggesting that the 4wt% Ni-Co3O4 porous nanoplates has higher hydrophilic surface that can facilitate the electron transfer and enable facile release of evolved O2 bubbles. The result is well in 7
agreement with the electrocatalytic test. The improvement of hydrophilicity may attribute to the increased number of defective sites with Ni ion doping [50]. In addition, the LSV curve of the 4wt% Ni-Co3O4 porous nanoplates after 2000 cycles of continuous CV scanning still remains the same as the original one (Fig. 4(f)). The result indicates that the OER performance of the pure Co3O4 catalysts can be enhanced after a suitable amount of Ni ions incorporation which can be attributed to the unique synergistic effects between Ni and Co [49] and the higher hydrophilic surface of the 4wt% Ni-Co3O4 porous nanoplates.
Fig. 5 (a) LSV curves for HER of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 in 1 M KOH electrolyte with a sweep rate of 1 mV s-1; (b) Tafel plots of the catalysts; (c) Chronopotentiometry curve of the 4wt% Ni-Co3O4 at a constant current density of -10 mA cm-2
HER performances of the as-prepared samples are also investigated in alkaline solutions. Fig. 5(a) shows the LSV curves of the various samples for the HER. The 4wt% Ni-Co3O4 electrode demonstrates optimized electrocatalytic performance for HER. It possesses an overpotential of 120 mV at a current 8
density of -10 mA cm−2, which is much lower than those of the other catalysts. The Tafel slope also indicates that the catalytic kinetics of the 4wt% Ni-Co3O4 has been greatly improved. As shown in Fig. 5(b), the Tafel slope of 4wt% Ni-Co3O4 is 62 mV dec-1, which is lower than those of the pure Co3O4 (245.2 mV dec-1), 2wt% Ni-Co3O4 (99.5 mV dec-1), 6wt% Ni-Co3O4 (123.8 mV dec-1) and 8wt% Ni-Co3O4 (132.3 mV dec-1). This result can further indicate that the electrocatalytic properties of pure Co3O4 catalysts can be enhanced after a suitable amount of Ni incorporation. Furthermore, the electrochemical stability for HER of the 4wt% Ni-Co3O4 is further evaluated at -10 mA cm-2. As shown in Fig. 5(c), the potential of the 4wt% Ni-Co3O4 could maintain stable for 16 h for HER under alkaline condition.
3. Conclusion In conclusion, the Ni-doped Co3O4 porous nanoplates are prepared by hydrothermal method and annealing treatment. The 4wt%Ni-Co3O4 porous nanoplates can reach to a high current density at a low overpotential, which indicate that the activities of Co3O4 for both OER and HER in alkaline media can be remarkably enhanced with the proper nickel doping. These findings suggest that transition metal oxides with low concentration doping will inspire the rational design of other highly active bifunctional electrocatalysts.
Conflicts of Interest The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgements This research was supported by the Natural Science Foundation of Anhui Province (1708085ME96 and 1908085ME118), and the Project of Shaanxi Young Stars in Science and Technology (2017KJXX-18).
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Highlights 1. The Ni-doped Co3O4 porous nanoplates are successfully synthesized by hydrothermal method and annealing treatment just with the assistance of urea. 2. This is the first report of low-doped Ni-doped Co3O4 porous nanoplates for both OER and HER in alkaline media. 3. The 4wt % Ni-doped Co3O4 possess enhanced activity and great stability for both OER and HER than the pure Co3O4.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
None
S0925-8388(20)30113-4
DOI:
https://doi.org/10.1016/j.jallcom.2020.153750
Reference:
JALCOM 153750
To appear in:
Journal of Alloys and Compounds
Received Date: 25 November 2019 Revised Date:
5 January 2020
Accepted Date: 7 January 2020
Please cite this article as: L. Li, Q. Xu, Y. Zhang, J. Li, J. Fang, Y. Dai, X. Cheng, Y. You, X. Li, Low Ni-doped Co3O4 porous nanoplates for enhanced hydrogen and oxygen evolution reaction, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153750. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement: Yongxing Zhang and Xuanhua Li: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition and Project administration. Jia Li, Jinhui Fang and Yongping Dai: Visualization and Investigation. Xingliang Cheng and Yu You: Validation and Funding acquisition. Li Li and Qianqian Xu: Data curation, Writing- Original draft preparation, WritingReviewing and Editing.
Low Ni-doped Co3O4 Porous Nanoplates for Enhanced Hydrogen and Oxygen Evolution Reaction Li Li1, ‡, Qianqian Xu1, ‡, Yongxing Zhang1*, Jia Li1, Jinhui Fang1, Yongping Dai1, Xingliang Cheng1, Yu You1 and Xuanhua Li2*
1
Key Laboratory of Green and Precise Synthetic Chemistry and Application, Ministry of Education,
Department of Materials Science and Engineering, Huaibei Normal University, Huaibei 235000, P. R. China
2
State Key Laboratory of Solidification Processing Center of Nano Energy Materials, School of Materials
Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, P. R. China E-mail: [email protected]; E-mail: [email protected] ‡
These authors contributed equally to this work
Abstract In this paper, low Ni-doped Co3O4 (Ni: 2 wt%, 4 wt%, 6 wt% and 8 wt%) porous nanoplates are synthesized by hydrothermal method and annealing treatment. For the low Ni-doped Co3O4, X-ray diffraction (XRD) and Raman data show that the Ni ions are successfully incorporated into the Co3O4 lattices. Electrochemical measurements indicate that Co3O4 with low amount of Ni doping possesses remarkably enhanced activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. And especially, 4wt% Ni-doped Co3O4 possesses the most efficient activity with an overpotential of 240 mV at the current density of 10 mA cm-2 for OER and 120 mV for HER in 1 M KOH than the pure Co3O4 and other Ni-doped Co3O4 samples. Our study suggests that the low doping would have a great potential in rational design of spinel oxides toward highly efficient electrocatalysis.
Keywords: Ni-Doped Co3O4; Porous Nanoplates; Oxygen Evolution Reaction; Hydrogen Evolution Reaction; Hydrothermal Method 1
1. Introduction Recently, electrochemical water splitting has drawn ever-increasing attention because it is an effective way to obtain high efficiency renewable hydrogen energy. [1-6] As is known to all, two half reactions, namely, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), are involved in water splitting.[7-8] At present, the advanced electrocatalysts for water splitting are noble metal based materials (i.e. Pt for HER and IrO2/RuO2 for OER). [9-10] However, the high cost and scarcity have limited their large-scale application. In this regard, the key challenge is to develop natural abundance, low-cost and green electrocatalysts with high activity that can be comparable to those mentioned above. In the past few decades, a large number of transition metal based materials, including oxide, [11-12] chalcogenides, [13-14] nitrides, [15-16] layered double hydroxide [17-19], carbides [20-21] and phosphides, [22] have been intensively studied as bifunctional catalyst materials for water splitting. Among these materials, cobalt based materials stand out as a kind of efficient catalysts, such as NixCo3-xO4 nanowire arrays [23], NixCo3-xO4 nanoneedle arrays [24], NiCo2O4 nanoparticles [25], nickel cobalt oxide nanoplates [26] and some cobalt based composites [27-29]. Co3O4 has been studied widely in various fields, such as degradation [30], Zn-air batteries [31], supercapacitor [32] and water splitting [33] because of their prominent properties. Two strategies have been adopted to enhance the activity of Co3O4 in previous reports. The first way is to change the preparation methods. Various methods have been employed to change the morphology and structure, including direct high-temperature oxidation [34], hot plate combustion method [35] and precipitation method [36], hydrothermal reaction [37] and the like. It is no doubt that the electrocatalytic activity of nanomaterials is closely related to their morphology and structure. In typically, compared with the bulk structure, two-dimensional (2D) nanoplates with porous structure have higher specific surface area, and they can provide more active sites, leading to enhancement of the catalytic activity. The other is doping, such as carbon (C)-doping [33, 38], PdO-doping [39], La3+-doping [40], sulfur (S)-doping [41], iron (Fe)-doping [42-43], manganese (Mn)-doping [44-45], chromium (Cr)-doping [46] and so on. Furthermore, nickel (Ni)-doped Co3O4 is also reported for electrocatalytic property [47-49]. However, it is rarely reported that the Co3O4 with low nickel doping as a highly efficient electrocatalyst for both OER and HER in alkaline media. Herein, Ni-doped Co3O4 porous nanoplates are synthesised by hydrothermal method and subsequent annealing treatment. The electrocatalytic performances for both OER and HER of the as-prepared porous 2
materials have been explored in alkaline media. And from the result, it is found that the Ni-doped Co3O4 porous nanoplates have the following features: (a) they can be obtained by using nontoxic and inexpensive raw materials and a low-cost synthesis methods; (b) their unique porous structures provide a superior number of active sites exposed to surface reactions as well as excellent electrocatalytic activity; (c) the materials with low doping concentrations have remarkably enhanced activity for both OER and HER in alkaline media.
2. Results and discussion
Fig. 1 (a) XRD patterns of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 porous nanoplates; (b) The enlarged XRD patterns of the red rectangular in (a); (c) EDS spectrum of the 4wt%Ni-Co3O4 porous nanoplates; (d) Raman spectra of (1) the pure Co3O4 and (2) 4wt%Ni-Co3O4 porous nanoplates
Fig.1(a) shows the X-ray diffraction (XRD) patterns of the pure Co3O4 and other four Ni-doped Co3O4 powders after annealing in muffle furnace at 550 oC for 3 hours. All of the diffraction peaks are coincided well with the cubic spinel type structure Co3O4 (JCPDS no. 74-2120, Fd3m space group). And there are no other peaks can be observed, which indicate high-purity samples are obtained. The detailed study based on 3
the enlarged XRD pattern is shown in Fig. 1(b) (25-40°), obvious shifts of the two corresponding peaks are observed between the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplate samples. From Fig.1(c), the energy dispersive spectrometer (EDS) data of the 4wt% Ni-Co3O4 further reveals the existence of Co, O and Ni elements, while there are only Co and O elements for the pure Co3O4 in Fig. S1. Raman spectra of the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplates are shown in Fig. 1(d). Five Raman bands at 192, 473, 515, 612, and 679 cm−1 are detected for the pure Co3O4, while the bands at 194, 480, 520, 617, and 687 cm-1 for 4wt% Ni-Co3O4 shift slightly to higher wavenumbers. The results indicate the Ni ions are incorporated into the Co3O4 lattices.
Fig. 2 XPS characterization of the pure Co3O4 and 4wt% Ni-Co3O4 porous nanoplates: (a) survey spectrum; (b)Ni 2p of 4wt% Ni-Co3O4; (c) Co 2p; (d) O 1s
The components and the valence states of the pure Co3O4 and 4%wt Ni-Co3O4 porous nanoplates are investigated by the X-ray photoelectron spectroscopy (XPS). Fig. 2(a) shows the wide scan survey spectra of 4
the pure Co3O4 and 4%wt Ni-Co3O4 porous nanoplates. And the dotted rectangle of 4%wt Ni-Co3O4 in Fig. 2(a) is enlarged in Fig. 2(b), which indicates the existence of Ni element in this sample. In addition, the peaks at 855.5 eV and 872.8 eV with a peak separation of 17.3 eV in the Ni2p spectrum are typical for Ni2+ species, further confirming the efficient doping of Ni2+ in Co3O4 nanoplates. Compared to the pure Co3O4, there are obvious negative shifts of the binding energies of Co2p (0.23 eV, Fig. 2(c)) and O1s (0.22 eV, Fig. 2(d)) for the 4%wt Ni-Co3O4, respectively. These results also indicate that Ni ions are successfully doped into Co3O4.
Fig. 3 Morphological and structural characterizations of the 4wt% Ni-Co3O4 porous nanoplates: (a, b, c) FESEM images with low- and high-magnification. (d) The FESEM image and corresponding elemental mapping images. (e)The TEM images and (f) the HRTEM image of the 4wt% Ni-Co3O4 porous nanoplates
As shown in Fig. 3(a), the field emission scanning electron microscope (FESEM) image of the overall morphology for the 4wt% Ni-Co3O4 indicates that many nanoplates are obtained. Fig. 3(b) shows that there are many pores in the 4wt% Ni-Co3O4 nanoplates. And from Fig. 3(c), the thickness of the nanoplates is about 40 nm. The SEM images of Co3O4, 2wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 porous nanoplates are shown in Fig. S2. It can be seen that the morphology has not changed a lot with the Ni incorporation. Fig. 3(d) presents the FESEM image of the 4wt% Ni-Co3O4 porous nanoplates and corresponding elemental mapping images, revealing that Co, O and Ni elements are homogeneously distributed in the whole porous nanoplates. From Fig. 3(e), the 4wt% Ni-Co3O4 porous nanoplates is 5
constructed by numerous nanoplates (inset) and small pores, which can provide a large number of exposed active edge sites. And the small pores can provide an available pathway for electrolyte and electron transfer, so that can facilitate the dissociation of water. The high-resolution transmission electron microscope (HRTEM) image of the 4wt% Ni-Co3O4 porous nanoplates indicates that the lattice fringe of the Co3O4 was ~0.244 nm, which can be attributed to the (311) crystal plane (Fig. 3(f)), while the pure Co3O4 is ~0.240 nm (the inset of Fig. S2(a)).
Fig. 4 (a) LSV curves for OER of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 6
8wt% Ni-Co3O4 in 1 M KOH electrolyte with a sweep rate of 1 mV s-1; (b) Tafel plots of the catalysts; (c) Electrochemical impedance spectra; (d) Plots of the double-layer capacitances at 0.15 V (vs. Ag/AgCl); (e) Chronopotentiometry curve of the pure Co3O4 and the 4wt% Ni-Co3O4 at a constant current density of 10 mA cm-2, contact angle measurements of (1) the pure Co3O4, (2) 4wt% Ni-Co3O4 porous nanoplates (inset); (f) Cycles stability of the 4wt% Ni-Co3O4 porous nanoplates
The OER catalytic activity of the pure Co3O4 and a series of Ni-doped Co3O4 porous nanoplates are measured in 1 M KOH solution with a three electrodes system. Fig. 4(a) shows representative linear sweep voltammetry (LSV) polarization curves of the as-prepared samples without any iR-corrected. The 4wt% Ni-Co3O4 possesses the lowest overpotential of 240 mV at a current density of 10 mV cm-2 in comparison to those of the pure Co3O4 (350 mV), 2wt% Ni-Co3O4 (280 mV), 6wt% Ni-Co3O4 (330 mV) and 8wt% Ni-Co3O4 (335 mV). It is found that the OER performance is reduced greatly with excess Ni incorporation because high Ni doping concentration in Co3O4 decreases the amount of oxygen in the surface and result in a lower activity[47]. As shown in Fig. 4(b), the Tafel slope of 4wt% Ni-Co3O4 is 64.2 mV dec-1, which is lower than those of the pure Co3O4 (160.1 mV dec-1), 2wt% Ni-Co3O4 (81.4 mV dec-1), 6wt% Ni-Co3O4 (79.8 mV dec-1) and 8wt% Ni-Co3O4 (116.7 mV dec-1). The outstanding performance of the 4wt% Ni-Co3O4 porous nanoplate catalysts is also evidenced by the electrochemical impedance spectra (EIS) data (Fig. 4(c)), which shows a significantly smaller interfacial charge transfer resistance. And the result indicates that the 4wt% Ni-Co3O4 porous nanoplates possesses promoted OER kinetics comparing to those pure Co3O4 and other Ni-doped Co3O4. To explore the improved OER performance of the 4wt% Ni-Co3O4, the effective electrochemical active surface area (ECSA) is calculated by measuring the double-layer capacitance (Cdl), as shown in Fig. 4(d), which derived from cyclic voltammograms (CV) results at different scan rates (Fig. S3). The 4wt% Ni-Co3O4 yields a high capacitance value up to 12.16 mF cm-2, much larger than those of the Co3O4 (4.30 mF cm-2), 2wt% Ni-Co3O4 (6.67 mF cm-2), 6wt% Ni-Co3O4 (4.70 mF cm-2) and 8wt % Ni-Co3O4 (4.19 mF cm-2) electrodes. Furthermore, the electrochemical stability of the 4wt% Ni-Co3O4 is evaluated at 10 mA cm-2. As shown in Fig. 4(e), the potential of the 4wt% Ni-Co3O4 remains stable for a long period time (16 h) during the OER test, indicating very high catalytic stability under alkaline condition, while the overpotential of the pure Co3O4 has increased significantly. As shown in the inset of Fig. 4(e), the contact angle of the 4wt% Ni-Co3O4 porous nanoplates is 22°, which is slightly smaller than the pure Co3O4 (29°), suggesting that the 4wt% Ni-Co3O4 porous nanoplates has higher hydrophilic surface that can facilitate the electron transfer and enable facile release of evolved O2 bubbles. The result is well in 7
agreement with the electrocatalytic test. The improvement of hydrophilicity may attribute to the increased number of defective sites with Ni ion doping [50]. In addition, the LSV curve of the 4wt% Ni-Co3O4 porous nanoplates after 2000 cycles of continuous CV scanning still remains the same as the original one (Fig. 4(f)). The result indicates that the OER performance of the pure Co3O4 catalysts can be enhanced after a suitable amount of Ni ions incorporation which can be attributed to the unique synergistic effects between Ni and Co [49] and the higher hydrophilic surface of the 4wt% Ni-Co3O4 porous nanoplates.
Fig. 5 (a) LSV curves for HER of the pure Co3O4, 2wt% Ni-Co3O4, 4wt% Ni-Co3O4, 6wt% Ni-Co3O4 and 8wt% Ni-Co3O4 in 1 M KOH electrolyte with a sweep rate of 1 mV s-1; (b) Tafel plots of the catalysts; (c) Chronopotentiometry curve of the 4wt% Ni-Co3O4 at a constant current density of -10 mA cm-2
HER performances of the as-prepared samples are also investigated in alkaline solutions. Fig. 5(a) shows the LSV curves of the various samples for the HER. The 4wt% Ni-Co3O4 electrode demonstrates optimized electrocatalytic performance for HER. It possesses an overpotential of 120 mV at a current 8
density of -10 mA cm−2, which is much lower than those of the other catalysts. The Tafel slope also indicates that the catalytic kinetics of the 4wt% Ni-Co3O4 has been greatly improved. As shown in Fig. 5(b), the Tafel slope of 4wt% Ni-Co3O4 is 62 mV dec-1, which is lower than those of the pure Co3O4 (245.2 mV dec-1), 2wt% Ni-Co3O4 (99.5 mV dec-1), 6wt% Ni-Co3O4 (123.8 mV dec-1) and 8wt% Ni-Co3O4 (132.3 mV dec-1). This result can further indicate that the electrocatalytic properties of pure Co3O4 catalysts can be enhanced after a suitable amount of Ni incorporation. Furthermore, the electrochemical stability for HER of the 4wt% Ni-Co3O4 is further evaluated at -10 mA cm-2. As shown in Fig. 5(c), the potential of the 4wt% Ni-Co3O4 could maintain stable for 16 h for HER under alkaline condition.
3. Conclusion In conclusion, the Ni-doped Co3O4 porous nanoplates are prepared by hydrothermal method and annealing treatment. The 4wt%Ni-Co3O4 porous nanoplates can reach to a high current density at a low overpotential, which indicate that the activities of Co3O4 for both OER and HER in alkaline media can be remarkably enhanced with the proper nickel doping. These findings suggest that transition metal oxides with low concentration doping will inspire the rational design of other highly active bifunctional electrocatalysts.
Conflicts of Interest The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgements This research was supported by the Natural Science Foundation of Anhui Province (1708085ME96 and 1908085ME118), and the Project of Shaanxi Young Stars in Science and Technology (2017KJXX-18).
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Highlights 1. The Ni-doped Co3O4 porous nanoplates are successfully synthesized by hydrothermal method and annealing treatment just with the assistance of urea. 2. This is the first report of low-doped Ni-doped Co3O4 porous nanoplates for both OER and HER in alkaline media. 3. The 4wt % Ni-doped Co3O4 possess enhanced activity and great stability for both OER and HER than the pure Co3O4.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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