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Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction
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Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction Bo Xu*, Zhiming Chen, Haijing Zhang, Yiqiang Sun, Cuncheng Li** School of Chemistry and Chemical Engineering, University of Jinan, Shandong, Jinan, 250022, China
article info
abstract
Article history:
Development of efficient catalyst with low-cost, earth-abundant metals for sustainable
Received 14 September 2017
hydrogen generation is still an intriguing challenge. Doping of cation or anion is regarded
Received in revised form
as an effective approach for ingenious modulation of chemical composition related to the
12 October 2017
electrocatalysts for improving their activity. Herein, a ternary Ni(S0.49Se0.51)2 porous flakes
Accepted 14 October 2017
array on carbon fiber cloth (CFC) via simultaneous sulfuration and selenylation of Ni(OH)2
Available online xxx
flakes. Owing to the synergistic effect between Ni, S and Se atoms, high surface area, abundant active site and good conductivity of the Ni(S0.49Se0.51)2/CFC, the as-synthesized
Keywords:
catalyst requires lower overpotential (113 mV) than pure NiS2/CFC and NiSe2/CFC to
Hydrogen evolution reaction
drive 10 mA cm2 for hydrogen evolution reaction (HER) in 1.0 M KOH with good
Electrocatalysis
sustainability.
Nickel chalcogenides
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing concern over global energy crisis and environmental issues, immense research interest has been paid on the development of sustainable and clean energy sources [1e3]. Hydrogen is considered as an ideal candidate for the replacing of fossil fuels due to its great advantages as high energy density, environment friendly quality and so on [4e6]. Water electrolysis, which can be divided into two half reactions: hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode, is regarded as a promising effective approach for the generations of molecular hydrogen (H2) [7e11]. Currently, Pt represents the most efficient electrocatalysts for HER, while the high cost and scarcity severely hinder its widespread applications. Therefore, the core issue in this field is to develop low-cost catalysts from earth-abundant transition metals with tremendous activity to
overcome the slow kinetic and decrease the necessary overpotential to drive the reactions [12e15]. For this purpose, studies on transition metal oxides, nitrides, phosphides, sulfides, selenides and so on have been extensively carried out [16e19]. However, although great advance has been made in this field, further improvement of the catalytic performance including activity and stability through rational design and ingenious fabrication is still urgent and challenging. Transition-metal chalcogenides (TMCs) with unique electron states near their Fermi levels have gained attracted enormous attention [20e22]. Various kinds of TMCs including NiS [23], NiSe2 [24], WSe2 [25], MoS2 [26] and so on have been intensively studied for their satisfactory catalytic efficiency over the past years. Among these, nickel based chalcogenides is an important member of TMCs with marked catalytic activity due to the unique valence electronic configuration of Ni (3d84s2) and their good metallic conductivity [27]. Very recently, some works focusing on the synthesis and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Xu), [email protected] (C. Li). https://doi.org/10.1016/j.ijhydene.2017.10.081 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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property study of nickel based chalcogenides with different morphology are reported. Ni-S or Ni-Se catalysts with various structures including particle [28], wire [29] and sheet [30] were recently found to exhibit interesting HER performance. These results suggest that morphology have important influence on the performance of the catalyst. On the other hand, modulation of the chemical composition, such as doping or alloying of metal atoms into the crystal lattice of electrocatalysts, has been proved to be an effective method to enhance the catalytic activity of nickel based chalcogenides through ingenious regulation of the physical and chemical properties [31e33]. For example, Fe doped NiSe [34], Co doped NiSe2 [35], Mo doped NiS [36] and so on have been reported showing fascinating water splitting activity. However, even though cation-doping is now widely used to improve performance of nickel based chalcogenides, anion-doping is relatively rare. Since both nickel sulfides and selenides exhibit remarkable HER catalytic activity, it could be reasonably inferred that their ternary compounds may lead to the further improved HER performance due to the synergistic effect. Moreover, because of the electronegativity proximity of sulfur (c ¼ 2.58) and selenium (c ¼ 2.55), formation of non-stoichiometric compounds based on nickel, sulfur and selenium is feasible. Hence, in this paper, carbon fiber cloth (CFC) supported ternary Ni(S0.49Se0.51)2 porous flakes array was fabricated through a wet chemical-hydrothermal route combined with subsequent in situ simultaneous sulfuration and selenylation reaction. CFC was used as supporter for the growth of porous Ni(S0.49Se0.51)2 flakes array to expose abundant active sites to modify the catalytic activity. The ternary Ni(S0.49Se0.51)2 catalyst exhibits higher catalytic activity towards HER compared to NiS2 or NiSe2 alone due to the intrinsic synergistic effect. The design and fabrication of this dual-anionic nickel chalcogenide electrocatalyst may serve as a common strategy for the synthesis of other non-precious electrocatalysts.
and then immersed in the solution. The autoclave was then heated at 120 C for 6 h. Ni(OH)2/CFC was obtained after slowly cooled down to room temperature and washed with distilled water for several times.
Synthesis of Ni(S0.49Se0.51)2/CFC flake For the preparation of Ni-S-Se/CFC flake, the obtained Ni(OH)2/ CFC was first dried and weighed. Then the Ni(OH)2/CFC was placed at the downstream side of the furnace and a mixture of sulfur and selenium powder (total 100 mg, 1:3 in molar ratio) was placed at the upstream side of the furnace. Subsequently, the sample was heated at 450 C for 1.5 h with a heating rate of 10 C min1 under nitrogen atmosphere, and then naturally cooled to ambient temperature. For the preparation of NiS2/ CFC or NiSe2/CFC, S or Se powder was used respectively. The as-obtained Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/CFC flakes were weighed again and the loading mass was calculated to be about 1.53, 1.29 and 1.62 mg/cm2, respectively.
Characterization Powder XRD data were recorded on an X-ray diffractometer (Bruker D8 Focus 2000) using Cu Ka (l ¼ 1.5406 A) radiation at a scanning rate of 4 min1 in the 2theta region of 10e80 . Transmission electron microscopy (TEM) images were carried out on JEOL, JEM-1400. SEM images were characterized by a FEI QUANTA FEG250 SEM operating at 5 kV. High-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were investigated by a JEOL model JEM 2010 EX instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250XI XPS system equipped with Al Ka radiation (hn ¼ 1486.6 eV). Inductively coupled plasmaoptical emission spectroscope (ICP-OES) was carried on an Agilent 725 series instrument.
Experimental
Electrochemical testing
Materials
The whole catalytic measurements were performed on a CHI660D workstation (CH Instruments, Inc., Shanghai) by a standard three-electrode system, which includes a platinum wire as counter electrode, an Ag/AgCl electrode as reference electrode and the as-obtained Ni(S0.49Se0.51)2/CFC flake as working electrode in different electrolyte including 1.0 M aqueous KOH, 0.5 M H2SO4 and 1.0 M PBS. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 5 mV s1 with 90% iR compensation. The measured potentials vs Ag/AgCl were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.197).
Urea, ammonium fluoride (NH4F), S and Se powder were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. Ni(NO3)2$6H2O and KOH were provided by Aladdin Ltd. (Shanghai, China). Carbon fiber cloth (CFC) was purchased from Shandong Aikang Science and Technology Co. Ltd. All the reagents in our experiment were used as received. The water used throughout all experiments was purified through a Millipore system.
Synthesis of Ni(OH)2/CFC flake Ni(OH)2/CFC flake was prepared according to the previous reported procedure with slightly modification [37]. Typically, 0.75 mmol of Ni(NO3)2$6H2O, 1.5 mmol NH4F and 3.75 mmol urea were first dissolved into 15 ml deionized water. The above solution was then transferred into Teflon-lined stainless steel autoclave. Pieces of CFC was first ultrasonically washed with acetone, ethanol and water for 20 min each, and then dried in air. The weight of each treated CFC was recorded
Results and discussion For the preparation of Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/ CFC flakes, Ni(OH)2/CFC flakes were first synthesized through hydrothermal method. Then the Ni(OH)2/CFC flakes were directly sulfided or selenized to give the NiS2/CFC or NiSe2/CFC flakes. The ternary Ni(S0.49Se0.51)2/CFC flake was obtained by
Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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simultaneous sulfuration and selenylation of Ni(OH)2/CFC as exhibited in Scheme 1. Their morphologies were observed by SEM. As shown in Fig. S1, Ni(OH)2/CFC exhibits full coverage of the CFC with Ni(OH)2 nanoflake array. X-ray diffraction (XRD) pattern (Fig. S2) shows that peaks of the as-synthesized product can be ascribed to hexagonal Ni(OH)2 (14-0117) and Ni(OH)2$0.75H2O (38-0715). Ni(S0.49Se0.51)2/CFC show similar morphologies with that of the Ni(OH)2 nanoflake array but with rougher surface consist of small particles and more pores in the flakes as shown in Fig. 1a and b. The height of the flakes array is about 2e3 mm. SEM images of NiS2/CFC and NiSe2/CFC were shown in Fig. S3. Both of the two products hold similar morphologies with Ni(S0.49Se0.51)2/CFC. The corresponding energy-dispersive X-ray (EDX) elemental mapping images (Fig. 1c) reveal that the Ni, S and Se atoms are distributed uniformly in the Ni(S0.49Se0.51)2 nanoflake. TEM image shown in Fig. 2a reveals that size of the particles is around 100 nm. The high-resolution TEM (HRTEM) image (Fig. 2b) demonstrates the lattice fringes with the interplanar spacing of 0.239 nm, which slightly larger than the (211) plane of NiS2 (0.232 nm) but smaller than the (211) plane of NiSe2 (0.245 nm) due to the substitution of Se for S. The selective area electron diffraction (SAED) image (Fig. 2c) shows well-defined rings indexed to the (111), (200), (210), (211) and (220), indicating that the ternary Ni(S0.49Se0.51)2 compound holds cubic pyrite-phase structure of NiSe2 and solid solution phase. The powder X-ray diffraction (XRD) analysis was performed to verify the phase purity of the samples. Because the loading amount of the
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samples on CFC is very little and the diffraction intensity is weak, samples were separated from the CFC through ultrasonic treatment and collected for XRD measurement. Compared with the Ni(OH)2 precursor, only peaks corresponding to cubic NiS2 (JCPDS 11-0099) or cubic NiSe2 (JCPDS 41-1495) are observed after sulfuration or selenylation, as shown in Fig. 3. As that for the ternary Ni(S0.49Se0.51)2 compound, it can be seen that peaks show obvious shift compared to the XRD pattern of NiS2 or NiSe2, which can be ascribed to the radius difference between S and Se. Moreover, inductively coupled plasma atomic emission spectroscopy (ICP) analysis was employed for quantitative analysis of the chemical composition of Ni-S-Se compound, the result indicates that molar ratio of Ni, S and Se is 1: 0.98: 1.02. X-ray photoelectron spectroscopy (XPS) experiments were further conducted to investigate the chemical states of the elements in the Ni(S0.49Se0.51)2 compound. As shown in Fig. 4a, the XPS survey spectrum of Ni-S-Se clearly shows the peaks of Ni, S and Se with signals of C and O elements due to contamination/surface oxidation of the product. The Ni 2p3/2 and Ni 2p1/2 appear at 853.9 and 871.5 eV, respectively, as shown in Fig. 4b. Two shakeup satellites at 858.4 and 876.3 eV are observed, indicating Ni2þ oxidation state [38]. The two peaks at 162.9 and 164.0 eV shown in Fig. 4c can be attributed to S 2p3/2 and S 2p1/2 binding energies, respectively. It should be noted that two peaks at 161.2 and 169.0 eV, which can be assigned to Se 3p3/2 and Se 3p1/2 are also observed [39]. Moreover, the Se 3d region shown in Fig. 4d exhibits two peaks
Scheme 1 e Schematic diagram of the fabrication procedure for the Ni(S0.49Se0.51)2/CFC.
Fig. 1 e (a, b) SEM images and (c) Elemental mapping of Ni, S and S for the Ni(S0.49Se0.51)2/CFC. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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Fig. 2 e (a) TEM image, (b) HRTEM image and (c) SAED pattern of the Ni(S0.49Se0.51)2/CFC.
Fig. 3 e XRD patterns of the as-synthesized NiS2, NiSe2 and Ni(S0.49Se0.51)2.
at 54.9 and 55.8 eV corresponding to the binding energies of Se 3d5/2 and Se 3d3/2, respectively, confirming the existence of Se2 2 specie [40,41]. The electrocatalytic activity of Ni(S0.49Se0.51)2/CFC toward HER was evaluated and compared with NiS2/CFC, NiSe2/CFC and commercial Pt/C in N2-saturated 1.0 M KOH electrolyte using a three-electrode system. Fig. 5a shows the corresponding iR-corrected polarization curves of the as-obtained samples with current density normalized by geometric surface area of the carbon cloth. Linear sweep voltammetry plots reveal that the three electrocatalysts show great difference of HER activities. The Pt/C catalyst show best HER performance with a small overpotential of 75 mV. It's clearly observed that Ni(S0.49Se0.51)2/CFC exhibit enhanced activity with an overpotential of 113 mV at the current density of 10 mA cm2. By comparison, the overpotential of NiS2/CFC and NiSe2/CFC is located at 179 and 157 mV at the same current density of 10 mA cm2, respectively, while the CFC shows negligible HER activity under the same measured conditions. The Tafel slope, indicating the potential difference necessary to increase the current density by 10-fold, is regarded as an inherent property of the catalyst. Thus, Tafel slopes of Ni(S0.49Se0.51)2/CFC, NiS2/ CFC, NiSe2/CFC and Pt/C were further studied as shown in Fig. 5b. Fitting of the curves to Tafel equation yields Tafel slopes of 103, 122, 135 and 93 mV dec1 for Ni(S0.49Se0.51)2/CFC, NiS2/CFC, NiSe2/CFC and Pt/C, respectively. It is worth mentioning that the good activity of Ni(S0.49Se0.51)2/CFC is superior or comparable to other Ni-S or Ni-Se based electrocatalysts as listed in Table S1. The possible reason for the good HER performance can be attributed to the synergistic effect of
Fig. 4 e (a) Survey XPS spectrum of the Ni(S0.49Se0.51)2. XPS spectra of the Ni(S0.49Se0.51)2 for (b) Ni 2p, (c) S 2p and (d) Se 3d. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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Fig. 5 e (a) The HER polarization curves and (b) the corresponding Tafel plots, (c) Estimated Cdl and relative electrochemically active surface areas and (d) Nyquist plots of NiS2, NiSe2 and Ni(S0.49Se0.51)2. Ni, S and Se atoms, leading to the change of surface structure of the catalysts [42e44]. The HER performances of the Ni(S0.49Se0.51)2/CFC were also tested in 0.5 M H2SO4 and 1.0 M PBS as shown in Fig. S4. The Ni(S0.49Se0.51)2/CFC requires overpotentials of 105 and 136 mV in 0.5 M H2SO4 and 1.0 M PBS, respectively. Electrochemical double-layer capacitances (Cdl) of the Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/CFC were further measured to confirm the electrochemical active surface areas (ECSAs) of the mesoporous catalysts. Electrochemical cyclic voltammogram of the three catalysts at different scanning rates and the calculated slope from the linear relationship of the current density against the scan rate were shown in Fig. 5c and Fig. S5. The Ni(S0.49Se0.51)2/CFC shows a capacitance of 24.6 mF cm2, which is clearly higher than that of NiS2/CFC (15.3 mF cm2) and NiSe2/CFC (18.4 mF cm2), confirming the relatively larger active area for Ni(S0.49Se0.51)2/CFC. Electrochemical impedance spectroscopy (EIS) was further investigated under HER operation conditions to provide more details about the interfacial reactions and
electrode kinetics behavior of the samples. It can be conclude from the Nyquist plots shown in Fig. 5d that the observed charge transfer resistance of Ni(S0.49Se0.51)2/CFC is smaller than that of the NiS2/CFC and NiSe2/CFC, indicating a faster electron transfer rate in the hydrogen production process. Long-term stability of the Ni(S0.49Se0.51)2/CFC has also been investigated. The chronopotentiometric curves with the current density at 5, 10, and 20 mA cm2, respectively, were tested as shown in Fig. 6a. It's obviously that the Ni(S0.49Se0.51)2/CFC catalyst shows good stability with the current density keeps at three different values unchanged during the whole test process. In addition, the polarization curves of the Ni(S0.49Se0.51)2/CFC before and after operating for 10 h in 1.0 M KOH electrolyte solution were also measured. As shown in Fig. 6b, the current density of the final polarization curve only shows a negligible decrease comparing with the initial one, providing further evidence for the good stability of the Ni(S0.49Se0.51)2/CFC electrode during the continuous electrolysis reaction. SEM and XRD analysis was further carried out to investigate the structural
Fig. 6 e (a) The corresponding chronopotentiometric curve of Ni(S0.49Se0.51)2/CFC at 5, 10 and 20 mA cm¡2 and (b) polarization curves for Ni(S0.49Se0.51)2/CFC before and after 10 h for HER. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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and compositional changes of the post-HER Ni(S0.49Se0.51)2/CFC. SEM image and XRD pattern are shown in Figs. S6 and S7, which reveal that the sample is stable in alkaline solution. With such a good catalytic performance, this strategy for the synthesis of ternary catalyst gives meaningful reference for large scale hydrogen generation with minimum energy consumption [45e53].
Conclusions In summary, we report the synthesis of ternary Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth via simultaneous sulfuration and selenylation of Ni(OH)2 flakes. The assynthesized Ni(S0.49Se0.51)2/CFC shows higher HER catalytic activity compared to pure NiS2/CFC and NiSe2/CFC due to the synergistic effect between Ni, S and Se atoms. On the other hand, the unique porous flake array structure of the Ni(S0.49Se0.51)2/CFC providing high surface area and abundant active site can also improve the HER performance. Carbon fiber cloth acting as substrate not only exhibits strong interaction with Ni(S0.49Se0.51)2 but also shows good electrical conductivity that benefit the HER performance. This study provides a new route to prepare low-cost, transitional-metal catalyst with high efficiency and excellent electrocatalytic activity for HER.
Acknowledgments This work received financial support from the National Natural Science Foundation of China (21301069), the Natural Science Foundation of Shandong Province (ZR2012BQ004), the Science Foundation for Post Doctorate Research from the University of Jinan (XBH1516). C. Li, as a Taishan Scholar Endowed Professor, acknowledges the support from Shandong Province and UJN.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.10.081.
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Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
Available online at www.sciencedirect.com
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Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction Bo Xu*, Zhiming Chen, Haijing Zhang, Yiqiang Sun, Cuncheng Li** School of Chemistry and Chemical Engineering, University of Jinan, Shandong, Jinan, 250022, China
article info
abstract
Article history:
Development of efficient catalyst with low-cost, earth-abundant metals for sustainable
Received 14 September 2017
hydrogen generation is still an intriguing challenge. Doping of cation or anion is regarded
Received in revised form
as an effective approach for ingenious modulation of chemical composition related to the
12 October 2017
electrocatalysts for improving their activity. Herein, a ternary Ni(S0.49Se0.51)2 porous flakes
Accepted 14 October 2017
array on carbon fiber cloth (CFC) via simultaneous sulfuration and selenylation of Ni(OH)2
Available online xxx
flakes. Owing to the synergistic effect between Ni, S and Se atoms, high surface area, abundant active site and good conductivity of the Ni(S0.49Se0.51)2/CFC, the as-synthesized
Keywords:
catalyst requires lower overpotential (113 mV) than pure NiS2/CFC and NiSe2/CFC to
Hydrogen evolution reaction
drive 10 mA cm2 for hydrogen evolution reaction (HER) in 1.0 M KOH with good
Electrocatalysis
sustainability.
Nickel chalcogenides
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing concern over global energy crisis and environmental issues, immense research interest has been paid on the development of sustainable and clean energy sources [1e3]. Hydrogen is considered as an ideal candidate for the replacing of fossil fuels due to its great advantages as high energy density, environment friendly quality and so on [4e6]. Water electrolysis, which can be divided into two half reactions: hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode, is regarded as a promising effective approach for the generations of molecular hydrogen (H2) [7e11]. Currently, Pt represents the most efficient electrocatalysts for HER, while the high cost and scarcity severely hinder its widespread applications. Therefore, the core issue in this field is to develop low-cost catalysts from earth-abundant transition metals with tremendous activity to
overcome the slow kinetic and decrease the necessary overpotential to drive the reactions [12e15]. For this purpose, studies on transition metal oxides, nitrides, phosphides, sulfides, selenides and so on have been extensively carried out [16e19]. However, although great advance has been made in this field, further improvement of the catalytic performance including activity and stability through rational design and ingenious fabrication is still urgent and challenging. Transition-metal chalcogenides (TMCs) with unique electron states near their Fermi levels have gained attracted enormous attention [20e22]. Various kinds of TMCs including NiS [23], NiSe2 [24], WSe2 [25], MoS2 [26] and so on have been intensively studied for their satisfactory catalytic efficiency over the past years. Among these, nickel based chalcogenides is an important member of TMCs with marked catalytic activity due to the unique valence electronic configuration of Ni (3d84s2) and their good metallic conductivity [27]. Very recently, some works focusing on the synthesis and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Xu), [email protected] (C. Li). https://doi.org/10.1016/j.ijhydene.2017.10.081 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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property study of nickel based chalcogenides with different morphology are reported. Ni-S or Ni-Se catalysts with various structures including particle [28], wire [29] and sheet [30] were recently found to exhibit interesting HER performance. These results suggest that morphology have important influence on the performance of the catalyst. On the other hand, modulation of the chemical composition, such as doping or alloying of metal atoms into the crystal lattice of electrocatalysts, has been proved to be an effective method to enhance the catalytic activity of nickel based chalcogenides through ingenious regulation of the physical and chemical properties [31e33]. For example, Fe doped NiSe [34], Co doped NiSe2 [35], Mo doped NiS [36] and so on have been reported showing fascinating water splitting activity. However, even though cation-doping is now widely used to improve performance of nickel based chalcogenides, anion-doping is relatively rare. Since both nickel sulfides and selenides exhibit remarkable HER catalytic activity, it could be reasonably inferred that their ternary compounds may lead to the further improved HER performance due to the synergistic effect. Moreover, because of the electronegativity proximity of sulfur (c ¼ 2.58) and selenium (c ¼ 2.55), formation of non-stoichiometric compounds based on nickel, sulfur and selenium is feasible. Hence, in this paper, carbon fiber cloth (CFC) supported ternary Ni(S0.49Se0.51)2 porous flakes array was fabricated through a wet chemical-hydrothermal route combined with subsequent in situ simultaneous sulfuration and selenylation reaction. CFC was used as supporter for the growth of porous Ni(S0.49Se0.51)2 flakes array to expose abundant active sites to modify the catalytic activity. The ternary Ni(S0.49Se0.51)2 catalyst exhibits higher catalytic activity towards HER compared to NiS2 or NiSe2 alone due to the intrinsic synergistic effect. The design and fabrication of this dual-anionic nickel chalcogenide electrocatalyst may serve as a common strategy for the synthesis of other non-precious electrocatalysts.
and then immersed in the solution. The autoclave was then heated at 120 C for 6 h. Ni(OH)2/CFC was obtained after slowly cooled down to room temperature and washed with distilled water for several times.
Synthesis of Ni(S0.49Se0.51)2/CFC flake For the preparation of Ni-S-Se/CFC flake, the obtained Ni(OH)2/ CFC was first dried and weighed. Then the Ni(OH)2/CFC was placed at the downstream side of the furnace and a mixture of sulfur and selenium powder (total 100 mg, 1:3 in molar ratio) was placed at the upstream side of the furnace. Subsequently, the sample was heated at 450 C for 1.5 h with a heating rate of 10 C min1 under nitrogen atmosphere, and then naturally cooled to ambient temperature. For the preparation of NiS2/ CFC or NiSe2/CFC, S or Se powder was used respectively. The as-obtained Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/CFC flakes were weighed again and the loading mass was calculated to be about 1.53, 1.29 and 1.62 mg/cm2, respectively.
Characterization Powder XRD data were recorded on an X-ray diffractometer (Bruker D8 Focus 2000) using Cu Ka (l ¼ 1.5406 A) radiation at a scanning rate of 4 min1 in the 2theta region of 10e80 . Transmission electron microscopy (TEM) images were carried out on JEOL, JEM-1400. SEM images were characterized by a FEI QUANTA FEG250 SEM operating at 5 kV. High-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were investigated by a JEOL model JEM 2010 EX instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250XI XPS system equipped with Al Ka radiation (hn ¼ 1486.6 eV). Inductively coupled plasmaoptical emission spectroscope (ICP-OES) was carried on an Agilent 725 series instrument.
Experimental
Electrochemical testing
Materials
The whole catalytic measurements were performed on a CHI660D workstation (CH Instruments, Inc., Shanghai) by a standard three-electrode system, which includes a platinum wire as counter electrode, an Ag/AgCl electrode as reference electrode and the as-obtained Ni(S0.49Se0.51)2/CFC flake as working electrode in different electrolyte including 1.0 M aqueous KOH, 0.5 M H2SO4 and 1.0 M PBS. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 5 mV s1 with 90% iR compensation. The measured potentials vs Ag/AgCl were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE ¼ EAg/AgCl þ 0.059 pH þ 0.197).
Urea, ammonium fluoride (NH4F), S and Se powder were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. Ni(NO3)2$6H2O and KOH were provided by Aladdin Ltd. (Shanghai, China). Carbon fiber cloth (CFC) was purchased from Shandong Aikang Science and Technology Co. Ltd. All the reagents in our experiment were used as received. The water used throughout all experiments was purified through a Millipore system.
Synthesis of Ni(OH)2/CFC flake Ni(OH)2/CFC flake was prepared according to the previous reported procedure with slightly modification [37]. Typically, 0.75 mmol of Ni(NO3)2$6H2O, 1.5 mmol NH4F and 3.75 mmol urea were first dissolved into 15 ml deionized water. The above solution was then transferred into Teflon-lined stainless steel autoclave. Pieces of CFC was first ultrasonically washed with acetone, ethanol and water for 20 min each, and then dried in air. The weight of each treated CFC was recorded
Results and discussion For the preparation of Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/ CFC flakes, Ni(OH)2/CFC flakes were first synthesized through hydrothermal method. Then the Ni(OH)2/CFC flakes were directly sulfided or selenized to give the NiS2/CFC or NiSe2/CFC flakes. The ternary Ni(S0.49Se0.51)2/CFC flake was obtained by
Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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simultaneous sulfuration and selenylation of Ni(OH)2/CFC as exhibited in Scheme 1. Their morphologies were observed by SEM. As shown in Fig. S1, Ni(OH)2/CFC exhibits full coverage of the CFC with Ni(OH)2 nanoflake array. X-ray diffraction (XRD) pattern (Fig. S2) shows that peaks of the as-synthesized product can be ascribed to hexagonal Ni(OH)2 (14-0117) and Ni(OH)2$0.75H2O (38-0715). Ni(S0.49Se0.51)2/CFC show similar morphologies with that of the Ni(OH)2 nanoflake array but with rougher surface consist of small particles and more pores in the flakes as shown in Fig. 1a and b. The height of the flakes array is about 2e3 mm. SEM images of NiS2/CFC and NiSe2/CFC were shown in Fig. S3. Both of the two products hold similar morphologies with Ni(S0.49Se0.51)2/CFC. The corresponding energy-dispersive X-ray (EDX) elemental mapping images (Fig. 1c) reveal that the Ni, S and Se atoms are distributed uniformly in the Ni(S0.49Se0.51)2 nanoflake. TEM image shown in Fig. 2a reveals that size of the particles is around 100 nm. The high-resolution TEM (HRTEM) image (Fig. 2b) demonstrates the lattice fringes with the interplanar spacing of 0.239 nm, which slightly larger than the (211) plane of NiS2 (0.232 nm) but smaller than the (211) plane of NiSe2 (0.245 nm) due to the substitution of Se for S. The selective area electron diffraction (SAED) image (Fig. 2c) shows well-defined rings indexed to the (111), (200), (210), (211) and (220), indicating that the ternary Ni(S0.49Se0.51)2 compound holds cubic pyrite-phase structure of NiSe2 and solid solution phase. The powder X-ray diffraction (XRD) analysis was performed to verify the phase purity of the samples. Because the loading amount of the
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samples on CFC is very little and the diffraction intensity is weak, samples were separated from the CFC through ultrasonic treatment and collected for XRD measurement. Compared with the Ni(OH)2 precursor, only peaks corresponding to cubic NiS2 (JCPDS 11-0099) or cubic NiSe2 (JCPDS 41-1495) are observed after sulfuration or selenylation, as shown in Fig. 3. As that for the ternary Ni(S0.49Se0.51)2 compound, it can be seen that peaks show obvious shift compared to the XRD pattern of NiS2 or NiSe2, which can be ascribed to the radius difference between S and Se. Moreover, inductively coupled plasma atomic emission spectroscopy (ICP) analysis was employed for quantitative analysis of the chemical composition of Ni-S-Se compound, the result indicates that molar ratio of Ni, S and Se is 1: 0.98: 1.02. X-ray photoelectron spectroscopy (XPS) experiments were further conducted to investigate the chemical states of the elements in the Ni(S0.49Se0.51)2 compound. As shown in Fig. 4a, the XPS survey spectrum of Ni-S-Se clearly shows the peaks of Ni, S and Se with signals of C and O elements due to contamination/surface oxidation of the product. The Ni 2p3/2 and Ni 2p1/2 appear at 853.9 and 871.5 eV, respectively, as shown in Fig. 4b. Two shakeup satellites at 858.4 and 876.3 eV are observed, indicating Ni2þ oxidation state [38]. The two peaks at 162.9 and 164.0 eV shown in Fig. 4c can be attributed to S 2p3/2 and S 2p1/2 binding energies, respectively. It should be noted that two peaks at 161.2 and 169.0 eV, which can be assigned to Se 3p3/2 and Se 3p1/2 are also observed [39]. Moreover, the Se 3d region shown in Fig. 4d exhibits two peaks
Scheme 1 e Schematic diagram of the fabrication procedure for the Ni(S0.49Se0.51)2/CFC.
Fig. 1 e (a, b) SEM images and (c) Elemental mapping of Ni, S and S for the Ni(S0.49Se0.51)2/CFC. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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Fig. 2 e (a) TEM image, (b) HRTEM image and (c) SAED pattern of the Ni(S0.49Se0.51)2/CFC.
Fig. 3 e XRD patterns of the as-synthesized NiS2, NiSe2 and Ni(S0.49Se0.51)2.
at 54.9 and 55.8 eV corresponding to the binding energies of Se 3d5/2 and Se 3d3/2, respectively, confirming the existence of Se2 2 specie [40,41]. The electrocatalytic activity of Ni(S0.49Se0.51)2/CFC toward HER was evaluated and compared with NiS2/CFC, NiSe2/CFC and commercial Pt/C in N2-saturated 1.0 M KOH electrolyte using a three-electrode system. Fig. 5a shows the corresponding iR-corrected polarization curves of the as-obtained samples with current density normalized by geometric surface area of the carbon cloth. Linear sweep voltammetry plots reveal that the three electrocatalysts show great difference of HER activities. The Pt/C catalyst show best HER performance with a small overpotential of 75 mV. It's clearly observed that Ni(S0.49Se0.51)2/CFC exhibit enhanced activity with an overpotential of 113 mV at the current density of 10 mA cm2. By comparison, the overpotential of NiS2/CFC and NiSe2/CFC is located at 179 and 157 mV at the same current density of 10 mA cm2, respectively, while the CFC shows negligible HER activity under the same measured conditions. The Tafel slope, indicating the potential difference necessary to increase the current density by 10-fold, is regarded as an inherent property of the catalyst. Thus, Tafel slopes of Ni(S0.49Se0.51)2/CFC, NiS2/ CFC, NiSe2/CFC and Pt/C were further studied as shown in Fig. 5b. Fitting of the curves to Tafel equation yields Tafel slopes of 103, 122, 135 and 93 mV dec1 for Ni(S0.49Se0.51)2/CFC, NiS2/CFC, NiSe2/CFC and Pt/C, respectively. It is worth mentioning that the good activity of Ni(S0.49Se0.51)2/CFC is superior or comparable to other Ni-S or Ni-Se based electrocatalysts as listed in Table S1. The possible reason for the good HER performance can be attributed to the synergistic effect of
Fig. 4 e (a) Survey XPS spectrum of the Ni(S0.49Se0.51)2. XPS spectra of the Ni(S0.49Se0.51)2 for (b) Ni 2p, (c) S 2p and (d) Se 3d. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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Fig. 5 e (a) The HER polarization curves and (b) the corresponding Tafel plots, (c) Estimated Cdl and relative electrochemically active surface areas and (d) Nyquist plots of NiS2, NiSe2 and Ni(S0.49Se0.51)2. Ni, S and Se atoms, leading to the change of surface structure of the catalysts [42e44]. The HER performances of the Ni(S0.49Se0.51)2/CFC were also tested in 0.5 M H2SO4 and 1.0 M PBS as shown in Fig. S4. The Ni(S0.49Se0.51)2/CFC requires overpotentials of 105 and 136 mV in 0.5 M H2SO4 and 1.0 M PBS, respectively. Electrochemical double-layer capacitances (Cdl) of the Ni(S0.49Se0.51)2/CFC, NiS2/CFC and NiSe2/CFC were further measured to confirm the electrochemical active surface areas (ECSAs) of the mesoporous catalysts. Electrochemical cyclic voltammogram of the three catalysts at different scanning rates and the calculated slope from the linear relationship of the current density against the scan rate were shown in Fig. 5c and Fig. S5. The Ni(S0.49Se0.51)2/CFC shows a capacitance of 24.6 mF cm2, which is clearly higher than that of NiS2/CFC (15.3 mF cm2) and NiSe2/CFC (18.4 mF cm2), confirming the relatively larger active area for Ni(S0.49Se0.51)2/CFC. Electrochemical impedance spectroscopy (EIS) was further investigated under HER operation conditions to provide more details about the interfacial reactions and
electrode kinetics behavior of the samples. It can be conclude from the Nyquist plots shown in Fig. 5d that the observed charge transfer resistance of Ni(S0.49Se0.51)2/CFC is smaller than that of the NiS2/CFC and NiSe2/CFC, indicating a faster electron transfer rate in the hydrogen production process. Long-term stability of the Ni(S0.49Se0.51)2/CFC has also been investigated. The chronopotentiometric curves with the current density at 5, 10, and 20 mA cm2, respectively, were tested as shown in Fig. 6a. It's obviously that the Ni(S0.49Se0.51)2/CFC catalyst shows good stability with the current density keeps at three different values unchanged during the whole test process. In addition, the polarization curves of the Ni(S0.49Se0.51)2/CFC before and after operating for 10 h in 1.0 M KOH electrolyte solution were also measured. As shown in Fig. 6b, the current density of the final polarization curve only shows a negligible decrease comparing with the initial one, providing further evidence for the good stability of the Ni(S0.49Se0.51)2/CFC electrode during the continuous electrolysis reaction. SEM and XRD analysis was further carried out to investigate the structural
Fig. 6 e (a) The corresponding chronopotentiometric curve of Ni(S0.49Se0.51)2/CFC at 5, 10 and 20 mA cm¡2 and (b) polarization curves for Ni(S0.49Se0.51)2/CFC before and after 10 h for HER. Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081
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and compositional changes of the post-HER Ni(S0.49Se0.51)2/CFC. SEM image and XRD pattern are shown in Figs. S6 and S7, which reveal that the sample is stable in alkaline solution. With such a good catalytic performance, this strategy for the synthesis of ternary catalyst gives meaningful reference for large scale hydrogen generation with minimum energy consumption [45e53].
Conclusions In summary, we report the synthesis of ternary Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth via simultaneous sulfuration and selenylation of Ni(OH)2 flakes. The assynthesized Ni(S0.49Se0.51)2/CFC shows higher HER catalytic activity compared to pure NiS2/CFC and NiSe2/CFC due to the synergistic effect between Ni, S and Se atoms. On the other hand, the unique porous flake array structure of the Ni(S0.49Se0.51)2/CFC providing high surface area and abundant active site can also improve the HER performance. Carbon fiber cloth acting as substrate not only exhibits strong interaction with Ni(S0.49Se0.51)2 but also shows good electrical conductivity that benefit the HER performance. This study provides a new route to prepare low-cost, transitional-metal catalyst with high efficiency and excellent electrocatalytic activity for HER.
Acknowledgments This work received financial support from the National Natural Science Foundation of China (21301069), the Natural Science Foundation of Shandong Province (ZR2012BQ004), the Science Foundation for Post Doctorate Research from the University of Jinan (XBH1516). C. Li, as a Taishan Scholar Endowed Professor, acknowledges the support from Shandong Province and UJN.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.10.081.
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Please cite this article in press as: Xu B, et al., Novel Ni(S0.49Se0.51)2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.081