- Email: [email protected]
Ultrathin Ni0.85Se nanosheets supported by Ni skeleton with high performance toward hybrid supercapacitors
Journal of Energy Storage 26 (2019) 100972
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Ultrathin Ni0.85Se nanosheets supported by Ni skeleton with high performance toward hybrid supercapacitors ⁎
Jiaqin Yanga,b, , Yuanning Hua, Yuhang Fana, Xiaoran Lia, Baofeng Lia, Jiahui Wanga, Lirong Xua, a b
T ⁎
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong, 273165, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ni0.85Se Ni skeleton Composite Hybrid supercapacitors
In order to improve the cycling stability and accelerate the electrons transfer at high current density, a synthetic strategy of [email protected] composites is proposed by adjusting the molar ratio of Ni and Se elements via one-step hydrothermal method. In addition, the [email protected] composite shows a special structure with metal Ni as skeleton to support the growth of ultrathin Ni0.85Se nanosheets, which will be further enhance the conductivity and the specific surface area, respectively. Therefore, with the combination of Ni and Ni0.85Se, the [email protected] composite exhibits well long-term cycling stability and high energy/power density as investigated as electrode materials for hybrid supercapacitors: a high specific capacitance of 140.0 F g−1 is obtained after 10000 cycles at the current density of 2 A g−1.
1. Introduction With a capacitive electrode and a battery-type faradaic electrode, hybrid supercapacitors show higher energy density than conventional carbon-based electrical double-layer capacitors (EDLCs) [1–5]. Meanwhile, high-performance electrode materials are critical for meeting the demands of novel energy storage devices [6–9]. As nickel-based materials usually exhibit a pair of distinctly separated faradaic redox peaks due to its phase transition in alkaline media, they have been recently regarded as a promising battery-type material for hybrid supercapacitors [10–13]. Moreover, as there are various defects in the compounds with non-stoichiometric ratio according to the research of modern crystal structure, the compounds can exhibit excellent properties in the fields of electricity, optics and magnetism and can be applies for the preparation of functional materials [14]. Owing to the valence electron configuration of nickel (3d84s2) and the small difference of electronegativity between nickel (χ = 1.9) and selenium (χ = 2.4), nickel and selenium can form compounds with different stoichiometric ratios, showing multiple oxidation states and benefiting for energy storage [15,16]. Therefore, nickel selenide has been reported and show great prospective in varieties of application, such as supercapacitors [17–21], dye-sensitized solar cells [22], hydrogen evolution [23], fuel cell catalysts [24] and hydrogen storage materials [25], etc. In view of that the electrode materials for supercapacitors suffer poor cycling stability and low electrons transfer at high current density,
⁎
numerous research efforts have been devoted to strengthen the conductivity and improve the power/energy density by combining with other conductive substrate, such as Ni foam, carbon cloth, graphene, CNTs and so on. Take Ni0.85Se for example, Ni0.85Se@MoSe2, CoSe2/ Ni0.85Se and Ni0.85Se arrays directly on Ni foam were reported for hybrid supercapacitors, respectively [26–29]. Ye et al. prepared Ni0.85Se nanosheets array on carbon fiber cloth for a high-performance asymmetric supercapacitor [30]. In this article, [email protected] composite was prepared via a templatefree and one-step hydrothermal method. The composite exhibits a loose appearance, which is composed with numbers of ultrathin Ni0.85Se nano-sheets and Ni skeleton. Furthermore, benefiting from the largely enhanced specific surface area and conductivity with the complex composition and structure, the samples show excellent electrochemical performance. A high specific capacitance and superior cycling stability are achieved, which is 140.0 F g−1 after 10,000 cycles at the current density of 2 A g−1. Therefore, the aforementioned result indicates that the designation of composite with complex composition and structure is an effective method for the improvement of electrochemical performance.
Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Yang), [email protected] (L. Xu).
https://doi.org/10.1016/j.est.2019.100972 Received 18 June 2019; Received in revised form 3 September 2019; Accepted 18 September 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 26 (2019) 100972
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2. Experimental 2.1. Synthesis of [email protected] composite [email protected] composite was synthesized by a hydrothermal method. The typical preparation process was described as following: 0.1188 g NiCl2.6H2O was dissolved into 50 mL deionized water under stirring, and 0.0148 Se power was dissolved into 10 mL hydrazine hydrate under stirring. Subsequently, both of the aforementioned mixtures were mixed together. After stirred for another 30 min, the blended solution was transferred into 100 mL Teflon lined stainless steel autoclaves for hydrothermal reaction at 100 °C for 12 h. Finally, the precipitates were washed with deionized water and ethanol for several times and dried at 60 °C overnight with [email protected] composite obtained (S3). For comparison, the amount Se power is adjusted as 0, 0.0098 and 0.0195 g, and the corresponding sample was signed as S1, S2 and S4, respectively. Fig. 1. XRD pattern of the freshly prepared S1, S2, S3 and S4.
2.2. Characterization
E=
The crystalline structures of the as-prepared samples were characterized by X-ray diffraction (XRD) spectra (Rigaku MiniFlex600). The morphologies were detected by SEM on a Zeiss Sigma 500 VP thermal field scanning electron microscope, and TEM on a JEM-2100PLUS transmission electron microscope. XPS was carried out with thermo ESCALAB 250XI BET multifunctional imaging electron spectrometer. The specific surface area was calculated by the Brunauere–Emmette–Teller (BET) method (Micromeritics Tristar-3000 surface area). The thickness of Ni0.85Se nanosheets was tested by atomic force microscope (AFM, Seiko SPA400).
P=
Csp =
1 mv (Vf − Vi )
(1)
∫V
Vf
i
I (V ) dV
E t
(3) (4)
According to the previous reported literature [19], Ni2+ will be initially reduced by hydrazine hydrate with the formation of metal Ni, and then nickel selenide are prepared with the reaction between Ni and Se. Therefore, a synthetic strategy of [email protected] composites is proposed by adjusting the molar ratio of Ni and Se elements. As the molar ratio of NiCl2.6H2O to Se is fixed at 1:0, 4:1, 8:3 and 2:1, the obtained samples are signed as S1, S2, S3 and S4, respectively, and the corresponding XRD pattern are shown in Fig. 1. Obviously, all the diffraction peaks of S1 can be perfectly indexed to Ni (*, JCPDS NO.1-1260). Meanwhile, the diffraction peaks of Ni0.85Se (JCPDS NO.18-888) emerge as the introduction of selenium source, and the peaks intensity gradually increases along with the increasing of selenium source. As the molar ratio adjusts at 2:1, there is mainly Ni0.85Se diffraction peaks (∀) observed. The relatively low peak intensity should be due to the ultrathin structure of Ni0.85Se nanosheets. Fig. S1 and Fig. 2a show the morphology of S1, S2, S3 and S4, respectively. As shown in Fig. S1a, S1 exhibits flower architectures composed by tiny nanorods, which is confirmed by low and high magnification TEM images in Fig. S1b, c. When a small quantity of Ni0.85Se is formed for S2, flaky structure begins to be observed in Fig. S1d–f. Continue to increase the production of Ni0.85Se (S3), a loose packing structure (Fig. 2a, b) is prepared along with the increasing production of Ni0.85Se. Obviously, the existence of Ni acts as skeleton to support the growth of Ni0.85Se, showing [email protected] encapsulation structure (Fig. 2c–e). Furthermore, the constituent unit of nanosheets shows ultrathin structure with several nanometer thickness, which is confirmed by atomic force microscope (AFM) in Fig. S2. The loose internal structure and ultrathin nanosheets endow a gratifying specific surface area of 66.54 m2 g−1 (Fig. S3), which will be benefit for the development of the electrochemical performance. Moreover, the Ni@ Ni0.85Se structure is further confirmed by HRTEM observation in Fig. 2f, the lattice spacing of Ni (d(111) = 0.20 nm) and Ni0.85Se (d(101) = 0.27 nm) further verify the encapsulation structure [35,36]. As there is only Ni0.85Se formed (S4), it also composed by nanosheets in Fig. S1g, h. In Fig. S1i, the lattice spacing is 0.27 nm, which is corresponding with the (101) lattice plane of Ni0.85Se. As shown in Fig. S4 and Fig. 3a, both of Ni and Se elements can be detected by energy dispersive spectrometer (EDS). For S4, the fraction of Ni atom is 44.88 % (Fig. S4b), which is closed to theoretical fraction of Ni in Ni0.85Se
The working electrode was fabricated by mixing active material (S1, S2, S3 or S4), acetylene black and PVDF in a weight ratio of 80:10:10. The formed paste was spread on a piece of 1.0 cm × 1.0 cm nickel foam, and dried under vacuum at 80 °C overnight. Electrochemical measurements were conducted in a three-electrode arrangement in 2 M KOH electrolyte. A Pt plate and Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was conducted with an electrochemical workstation (CHI604E) at scan rates of 5, 10, 20, 30 and 50 mV s−1. The repeated charge/discharge tests were conducted on a Land battery system at the current densities of 1, 2, 3, 5, 8, 10 and 12 A g−1. The specific capacitance is calculated according to Eq. (1): C (F g−1) is the specific capacitance, I (A) represents the discharge current, and m (g), ΔV (V) and Δt (s) designate the mass of active materials, potential drop during discharge and total discharge time, respectively [31,32].
I Δt mΔV
M
3. Results and discussion
2.3. Electrochemical measurements
C=
I ∫ Vdt
(2)
Based on CV measurement, the specific capacitance Csp can be calculated according to Eq. (2): where m (g) is the mass of the active materials, v is the scan rate (V s−1), Vf (V) and Vi (V) are the integration potential limits of the voltammetric curve, and I (A) is the current. With the obtained sample as positive electrode and activated carbon (AC) as negative electrode, hybrid asymmetric supercapacitor was assembled to investigate the electrochemical performance. The energy density E (Wh kg−1) and power density P (W kg−1) were calculated according to Eqs. (3) and (4): where I, V and t are on behalf of the current density, cell voltage and discharge time of the asymmetric supercapacitors, respectively; M is the total mass of negative and positive electrodes [33,34]. 2
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Fig. 2. (a) SEM image, (b, c) TEM image and (d) HRTEM image of S3.
and O 1s detected in the [email protected] composite. As the obtained sample is exposed to air for preservation, the surface oxidation of Ni0.85Se gives rise to the existence of O element [37,38]. For the highresolution XPS spectra of Ni 2p in Fig. 3c, two peaks are mainly observed at 855.5 eV and 873.2 eV, which derives from Ni 2p3/2 and Ni 2p1/2 of Ni0.85Se, respectively [39]. Meanwhile, the peaks located at 861.2 eV and 879.6 eV can be assigned to the shake-up satellites [40].
(45.94 %), confirms the formation of Ni0.85Se pure phase. Meanwhile, the fractions of Ni atoms tested in S2 and S3 are 82.67 % (Fig. S4a) and 61.20 % (Fig. 3a), respectively, which further reveals the complicated composition composed by Ni and Ni0.85Se. Take S3 for example, X-ray photoelectron spectroscopy (XPS) is performed on to study the surface electronic states. As shown in the survey spectrum (Fig. 3b), there are four elements of Ni 2p, Se 3d, C 1s
Fig. 3. (a) EDS pattern and XPS spectra of S3: (b) full spectrum, (c) Ni 2p, (d) Se 3d. 3
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Fig. 4. (a) CV curves of S3 at different scan rates; (b) the calculated specific capacitance at different scan rate and (c) Ip vs. v1/2 plots of S1, S2, S3 and S4, respectively; (d) charge/discharge curves of S3 at different current densities; (e) cycle life of S1, S2, S3 and S4 at 2 A g−1; (f) rate performance of S1, S2, S3 and S4. 2 D = I pa /(2.69 × 105 × n3/2 × A × C × v1/2)2
As shown in the Se 3d spectrum (Fig. 3d), the characteristic peak appearing at 54.3 eV corresponds to the Se 3d, and the broad peak located at 58.8 eV confirms the oxidation state of Se (SeOx) [41]. The electrochemical performances of obtained samples are evaluate by using a three-electrode cell in 2.0 M KOH solution with a potential window ranging from 0 to 0.5 V versus Hg/HgO. Fig. 4a and Fig. S5a, c, e show the typical CV curves of S1, S2, S3 and S4 at scan rates of 5, 10, 20, 30 and 50 mV s−1, respectively. Obviously, a pair of well-defined redox peaks located at 0.35 V and 0.45 V is observed, which suggests the battery-type reversible faradaic reactions during the electrochemical process and shows as reactions (5) and (6) [17]. Ni0.85Se + OH– ↔ Ni0.85SeOH + e− –
Ni0.85SeOH + OH ↔ Ni0.85SeO + H2O + e
where Ipa represents the peak current density, n is the number of electrons in the reaction, A is the surface area of the electrode, C is the proton concentration, and ν is the scan rate. In this work, n, A and C can be considered as constant. Therefore, the diffusion coefficient ratio of three film electrodes is calculated to be D(S2):D(S3):D (S4) = (17.63:19.70:15.51)2 = 1.29:1.613:1, indicating S3 electrode material possesses the highest ion mobility at a specific scan rate of 5 mV s−1. Fig. 4d and Fig. S5b, d, f, show the galvanostatic charge/ discharge (GCD) curves of the electrodes at various current densities. The potential plateau in the GCD curves is corresponding to the redox peak position in the CV curves, which further confirms the battery-type faradaic behavior. Both of cycling stability and rate performance are two significant indicators to evaluate an electrode. As shown in Fig. 4e, repeated charge/discharge measurement is performed on at a current density of 2 A g−1 to study the cycling stability of S1–S4 electrodes. Due to the high electronic conductivity of metal Ni, S4 electrode with pure phase show well cycling stability and rate performance. Therefore, compared with the sharp capacitance fading for S4 electrode, S1–S3 electrodes show well cycling stability with the existence of Ni. Impressively, S3 electrode delivers higher specific capacitance, which maintains a specific capacitance of 835.2 F g−1 (about 78.4 % of the initial capacitance) after 2000 cycles. In addition, a capacitance growth process is observed for the initial 100 cycles due to its electrochemical activation. Fig. 4f shows the rate performance of S1–S4 at different current densities ranging from 1 to 8 A g−1. The specific capacitances of S3 electrode are 1135.3, 1008.3, 911.5 and 864.0 F g−1 at current densities of 1, 2, 5, and 8 A g−1, respectively. Meanwhile, about 76.1% of the capacitance can be retained with the increasing of the current density,
(5) −
(6)
The specific capacitances were calculated basing on the CV curves and shown in Fig. 4b. Significantly, the S3 electrode shows higher calculated capacitances than S1, S2 and S4 electrodes, showing 964.7, 870.6, 781.7, 764.4 and 641.9 F g−1 at the scan rate of 5, 10, 20, 30 and 50 mV s−1, respectively. Compared to the capacitance obtained at 10 mV s−1, the lower capacitance at 5 mV s−1 should be due to the incomplete activation of the electrode. As shown in Fig. 4c, a power-law relationship (Ip = aνb) is associated between the peak current density (Ip) and square root of sweep rate (ν1/2) from 5 to 50 mV s−1 for both cathodic and anodic peaks. Furthermore, a diffusion-controlled batterytype Faradaic process can be concluded with the b-value of 0.5 [42,43]. Meanwhile, with the space inside loose packing structure, the diffusion pathway of electrolyte can be shortened and the apparent diffusion coefficient (D) of the OH− ions can be calculated from the Randlese–Sevcik Equation as given by the following [44,45].
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Fig. 5. (a) CV curves and (b) charge/discharge curves of S3//AC hybrid supercapacitor; (c) cyclic performance and (d) Ragone plot of S1//AC, S2//AC, S3//AC and S4//AC hybrid supercapacitors.
process during the initial cycling stage and excellent cycling stabilities for the subsequent cycles. Particularly, the S3//AC hybrid supercapacitor device shows higher specific capacitance and better cycling stability. A reversible specific capacitance of 140.0 F g−1 can be maintained after 10,000 cycles. The rate performances of the hybrid supercapacitor devices are conducted and shown in Fig. S9. Compared with S1//AC, S2//AC and S4//AC hybrid supercapacitor devices, S3// AC hybrid supercapacitor device delivers better electrochemical performance. The reversible specific capacitances can reach 142.7, 141.3, 138.5, 133.8, 129.2, 125.0 and 121.2 F g−1 at the current densities of 1, 2, 3, 5, 8, 10 and 12 A g−1, respectively. Based on the results of rate performance, the energy/power densities are calculated and the Ragone plots are illustrated in Fig. 5d. Particularly, S3//AC hybrid supercapacitor device shows competitive energy/power densities with the previous reported literatures [29,50–53]. The energy density is 44.6 Wh kg−1 at a power density of 254.5 W kg−1, and it still maintains 37.9 Wh kg−1 even at a high power density of 3.3 kW kg−1. To summarize, the excellent electrochemical performance of S3//AC hybrid supercapacitor can be attributed to the following effects: (1) the introduction of Ni largely strengthen the conductivity of the electrode materials, and also acts as skeleton to support Ni0.85Se; (2) ultrathin nanosheet structure enhances the specific surface area, supplying more redox active sites; (3) the space inside loose packing structure shortens the diffusion pathway of electrolyte and relieve volume change during the repeated charge/discharge processes.
suggesting good rate performance of S3 electrode. Compared with the other three samples, the excellent electrochemical performance of S3 electrode may be attributed to the encapsulation structure and composition. Fig. S6 shows typical Nyquist plots of S1, S2, S3 and S4 electrodes, which measured by electrochemical impedance spectroscopy (EIS). The Nyquist plot exhibits a nearly negligible semicircle indicating high conductivity of the electrodes at the high-frequency range, which suggests charge transfer resistance. As for the low-frequency region, a straight long tail nearly perpendicular to the x axis representing the mass capacitance confirms the ideal capacitive behavior of the electrodes [46,47]. Both of the obtained samples and active carbon (AC), acting as positive and negative electrodes, are combined into one hybrid supercapacitors device to evaluate the practical application. As shown in Fig. S7, the electrochemical performance of AC electrode was investigated with a potential range from −1.0 to 0 V at a current density of 1 A g−1. In Fig. S7a, a specific capacitance of 125.8 F g−1 is obtained at the initial cycle, and maintains 113.0 F g−1 after 5000 cycles, indicating excellent cycling stability. Meanwhile, Fig. 5b shows the charge/discharge curves of AC electrode at current density of 1 A g−1 with typical EDLC behavior. In order to obtain the optimal performance of a hybrid supercapacitor, the mass ratio between the two electrodes m+/m− can be adjusted according to Eq. (7) [48].
m+ C × ΔE− = electrode − m− Celectrode + × ΔE+
(7) 4. Conclusion
where the m, Celectrode and ΔE represent the active mass, specific capacitance and potential window of each electrode, respectively. As shown in Fig. 5a and Fig. S8a, c, e, the CV curves are collected at various sweep rate of 5, 10, 20, and 50 mV s−1 at potential ranging from 0 to 1.5 V. Obviously, both of the battery-like and electric doublelayer capacitance characteristics are observed from the shape of CV curves [49]. In Fig. 5b and Fig. S8b, d, f, no distinct platform is observed in the corresponding charge/discharge curves, measured at different current densities ranging from 1 to 12 A g−1. As shown in Fig. 5c, the cycling stability of the hybrid supercapacitors is measured by repeated charge/discharge process at a current density of 2 A g−1. It is noted that all the hybrid supercapacitor devices (S1//AC, S2//AC, S3//AC and S4//AC) present an active
In summary, [email protected] composite has been successfully synthesized via one-step hydrothermal method. As the Ni skeleton accelerates the transmission of electrons and the ultrathin nanosheet structure enhances the specific capacitance, excellent electrochemical performances with well cycling stability and high energy/power densities are presented for [email protected] composite with the complex structure. In addition, the aforementioned results indicate that this synthetic method is an effective method to improve the electrochemical performance of electrode materials for supercapacitors.
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Acknowledgments
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Ultrathin Ni0.85Se nanosheets supported by Ni skeleton with high performance toward hybrid supercapacitors ⁎
Jiaqin Yanga,b, , Yuanning Hua, Yuhang Fana, Xiaoran Lia, Baofeng Lia, Jiahui Wanga, Lirong Xua, a b
T ⁎
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong, 273165, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ni0.85Se Ni skeleton Composite Hybrid supercapacitors
In order to improve the cycling stability and accelerate the electrons transfer at high current density, a synthetic strategy of [email protected] composites is proposed by adjusting the molar ratio of Ni and Se elements via one-step hydrothermal method. In addition, the [email protected] composite shows a special structure with metal Ni as skeleton to support the growth of ultrathin Ni0.85Se nanosheets, which will be further enhance the conductivity and the specific surface area, respectively. Therefore, with the combination of Ni and Ni0.85Se, the [email protected] composite exhibits well long-term cycling stability and high energy/power density as investigated as electrode materials for hybrid supercapacitors: a high specific capacitance of 140.0 F g−1 is obtained after 10000 cycles at the current density of 2 A g−1.
1. Introduction With a capacitive electrode and a battery-type faradaic electrode, hybrid supercapacitors show higher energy density than conventional carbon-based electrical double-layer capacitors (EDLCs) [1–5]. Meanwhile, high-performance electrode materials are critical for meeting the demands of novel energy storage devices [6–9]. As nickel-based materials usually exhibit a pair of distinctly separated faradaic redox peaks due to its phase transition in alkaline media, they have been recently regarded as a promising battery-type material for hybrid supercapacitors [10–13]. Moreover, as there are various defects in the compounds with non-stoichiometric ratio according to the research of modern crystal structure, the compounds can exhibit excellent properties in the fields of electricity, optics and magnetism and can be applies for the preparation of functional materials [14]. Owing to the valence electron configuration of nickel (3d84s2) and the small difference of electronegativity between nickel (χ = 1.9) and selenium (χ = 2.4), nickel and selenium can form compounds with different stoichiometric ratios, showing multiple oxidation states and benefiting for energy storage [15,16]. Therefore, nickel selenide has been reported and show great prospective in varieties of application, such as supercapacitors [17–21], dye-sensitized solar cells [22], hydrogen evolution [23], fuel cell catalysts [24] and hydrogen storage materials [25], etc. In view of that the electrode materials for supercapacitors suffer poor cycling stability and low electrons transfer at high current density,
⁎
numerous research efforts have been devoted to strengthen the conductivity and improve the power/energy density by combining with other conductive substrate, such as Ni foam, carbon cloth, graphene, CNTs and so on. Take Ni0.85Se for example, Ni0.85Se@MoSe2, CoSe2/ Ni0.85Se and Ni0.85Se arrays directly on Ni foam were reported for hybrid supercapacitors, respectively [26–29]. Ye et al. prepared Ni0.85Se nanosheets array on carbon fiber cloth for a high-performance asymmetric supercapacitor [30]. In this article, [email protected] composite was prepared via a templatefree and one-step hydrothermal method. The composite exhibits a loose appearance, which is composed with numbers of ultrathin Ni0.85Se nano-sheets and Ni skeleton. Furthermore, benefiting from the largely enhanced specific surface area and conductivity with the complex composition and structure, the samples show excellent electrochemical performance. A high specific capacitance and superior cycling stability are achieved, which is 140.0 F g−1 after 10,000 cycles at the current density of 2 A g−1. Therefore, the aforementioned result indicates that the designation of composite with complex composition and structure is an effective method for the improvement of electrochemical performance.
Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Yang), [email protected] (L. Xu).
https://doi.org/10.1016/j.est.2019.100972 Received 18 June 2019; Received in revised form 3 September 2019; Accepted 18 September 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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2. Experimental 2.1. Synthesis of [email protected] composite [email protected] composite was synthesized by a hydrothermal method. The typical preparation process was described as following: 0.1188 g NiCl2.6H2O was dissolved into 50 mL deionized water under stirring, and 0.0148 Se power was dissolved into 10 mL hydrazine hydrate under stirring. Subsequently, both of the aforementioned mixtures were mixed together. After stirred for another 30 min, the blended solution was transferred into 100 mL Teflon lined stainless steel autoclaves for hydrothermal reaction at 100 °C for 12 h. Finally, the precipitates were washed with deionized water and ethanol for several times and dried at 60 °C overnight with [email protected] composite obtained (S3). For comparison, the amount Se power is adjusted as 0, 0.0098 and 0.0195 g, and the corresponding sample was signed as S1, S2 and S4, respectively. Fig. 1. XRD pattern of the freshly prepared S1, S2, S3 and S4.
2.2. Characterization
E=
The crystalline structures of the as-prepared samples were characterized by X-ray diffraction (XRD) spectra (Rigaku MiniFlex600). The morphologies were detected by SEM on a Zeiss Sigma 500 VP thermal field scanning electron microscope, and TEM on a JEM-2100PLUS transmission electron microscope. XPS was carried out with thermo ESCALAB 250XI BET multifunctional imaging electron spectrometer. The specific surface area was calculated by the Brunauere–Emmette–Teller (BET) method (Micromeritics Tristar-3000 surface area). The thickness of Ni0.85Se nanosheets was tested by atomic force microscope (AFM, Seiko SPA400).
P=
Csp =
1 mv (Vf − Vi )
(1)
∫V
Vf
i
I (V ) dV
E t
(3) (4)
According to the previous reported literature [19], Ni2+ will be initially reduced by hydrazine hydrate with the formation of metal Ni, and then nickel selenide are prepared with the reaction between Ni and Se. Therefore, a synthetic strategy of [email protected] composites is proposed by adjusting the molar ratio of Ni and Se elements. As the molar ratio of NiCl2.6H2O to Se is fixed at 1:0, 4:1, 8:3 and 2:1, the obtained samples are signed as S1, S2, S3 and S4, respectively, and the corresponding XRD pattern are shown in Fig. 1. Obviously, all the diffraction peaks of S1 can be perfectly indexed to Ni (*, JCPDS NO.1-1260). Meanwhile, the diffraction peaks of Ni0.85Se (JCPDS NO.18-888) emerge as the introduction of selenium source, and the peaks intensity gradually increases along with the increasing of selenium source. As the molar ratio adjusts at 2:1, there is mainly Ni0.85Se diffraction peaks (∀) observed. The relatively low peak intensity should be due to the ultrathin structure of Ni0.85Se nanosheets. Fig. S1 and Fig. 2a show the morphology of S1, S2, S3 and S4, respectively. As shown in Fig. S1a, S1 exhibits flower architectures composed by tiny nanorods, which is confirmed by low and high magnification TEM images in Fig. S1b, c. When a small quantity of Ni0.85Se is formed for S2, flaky structure begins to be observed in Fig. S1d–f. Continue to increase the production of Ni0.85Se (S3), a loose packing structure (Fig. 2a, b) is prepared along with the increasing production of Ni0.85Se. Obviously, the existence of Ni acts as skeleton to support the growth of Ni0.85Se, showing [email protected] encapsulation structure (Fig. 2c–e). Furthermore, the constituent unit of nanosheets shows ultrathin structure with several nanometer thickness, which is confirmed by atomic force microscope (AFM) in Fig. S2. The loose internal structure and ultrathin nanosheets endow a gratifying specific surface area of 66.54 m2 g−1 (Fig. S3), which will be benefit for the development of the electrochemical performance. Moreover, the Ni@ Ni0.85Se structure is further confirmed by HRTEM observation in Fig. 2f, the lattice spacing of Ni (d(111) = 0.20 nm) and Ni0.85Se (d(101) = 0.27 nm) further verify the encapsulation structure [35,36]. As there is only Ni0.85Se formed (S4), it also composed by nanosheets in Fig. S1g, h. In Fig. S1i, the lattice spacing is 0.27 nm, which is corresponding with the (101) lattice plane of Ni0.85Se. As shown in Fig. S4 and Fig. 3a, both of Ni and Se elements can be detected by energy dispersive spectrometer (EDS). For S4, the fraction of Ni atom is 44.88 % (Fig. S4b), which is closed to theoretical fraction of Ni in Ni0.85Se
The working electrode was fabricated by mixing active material (S1, S2, S3 or S4), acetylene black and PVDF in a weight ratio of 80:10:10. The formed paste was spread on a piece of 1.0 cm × 1.0 cm nickel foam, and dried under vacuum at 80 °C overnight. Electrochemical measurements were conducted in a three-electrode arrangement in 2 M KOH electrolyte. A Pt plate and Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was conducted with an electrochemical workstation (CHI604E) at scan rates of 5, 10, 20, 30 and 50 mV s−1. The repeated charge/discharge tests were conducted on a Land battery system at the current densities of 1, 2, 3, 5, 8, 10 and 12 A g−1. The specific capacitance is calculated according to Eq. (1): C (F g−1) is the specific capacitance, I (A) represents the discharge current, and m (g), ΔV (V) and Δt (s) designate the mass of active materials, potential drop during discharge and total discharge time, respectively [31,32].
I Δt mΔV
M
3. Results and discussion
2.3. Electrochemical measurements
C=
I ∫ Vdt
(2)
Based on CV measurement, the specific capacitance Csp can be calculated according to Eq. (2): where m (g) is the mass of the active materials, v is the scan rate (V s−1), Vf (V) and Vi (V) are the integration potential limits of the voltammetric curve, and I (A) is the current. With the obtained sample as positive electrode and activated carbon (AC) as negative electrode, hybrid asymmetric supercapacitor was assembled to investigate the electrochemical performance. The energy density E (Wh kg−1) and power density P (W kg−1) were calculated according to Eqs. (3) and (4): where I, V and t are on behalf of the current density, cell voltage and discharge time of the asymmetric supercapacitors, respectively; M is the total mass of negative and positive electrodes [33,34]. 2
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Fig. 2. (a) SEM image, (b, c) TEM image and (d) HRTEM image of S3.
and O 1s detected in the [email protected] composite. As the obtained sample is exposed to air for preservation, the surface oxidation of Ni0.85Se gives rise to the existence of O element [37,38]. For the highresolution XPS spectra of Ni 2p in Fig. 3c, two peaks are mainly observed at 855.5 eV and 873.2 eV, which derives from Ni 2p3/2 and Ni 2p1/2 of Ni0.85Se, respectively [39]. Meanwhile, the peaks located at 861.2 eV and 879.6 eV can be assigned to the shake-up satellites [40].
(45.94 %), confirms the formation of Ni0.85Se pure phase. Meanwhile, the fractions of Ni atoms tested in S2 and S3 are 82.67 % (Fig. S4a) and 61.20 % (Fig. 3a), respectively, which further reveals the complicated composition composed by Ni and Ni0.85Se. Take S3 for example, X-ray photoelectron spectroscopy (XPS) is performed on to study the surface electronic states. As shown in the survey spectrum (Fig. 3b), there are four elements of Ni 2p, Se 3d, C 1s
Fig. 3. (a) EDS pattern and XPS spectra of S3: (b) full spectrum, (c) Ni 2p, (d) Se 3d. 3
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Fig. 4. (a) CV curves of S3 at different scan rates; (b) the calculated specific capacitance at different scan rate and (c) Ip vs. v1/2 plots of S1, S2, S3 and S4, respectively; (d) charge/discharge curves of S3 at different current densities; (e) cycle life of S1, S2, S3 and S4 at 2 A g−1; (f) rate performance of S1, S2, S3 and S4. 2 D = I pa /(2.69 × 105 × n3/2 × A × C × v1/2)2
As shown in the Se 3d spectrum (Fig. 3d), the characteristic peak appearing at 54.3 eV corresponds to the Se 3d, and the broad peak located at 58.8 eV confirms the oxidation state of Se (SeOx) [41]. The electrochemical performances of obtained samples are evaluate by using a three-electrode cell in 2.0 M KOH solution with a potential window ranging from 0 to 0.5 V versus Hg/HgO. Fig. 4a and Fig. S5a, c, e show the typical CV curves of S1, S2, S3 and S4 at scan rates of 5, 10, 20, 30 and 50 mV s−1, respectively. Obviously, a pair of well-defined redox peaks located at 0.35 V and 0.45 V is observed, which suggests the battery-type reversible faradaic reactions during the electrochemical process and shows as reactions (5) and (6) [17]. Ni0.85Se + OH– ↔ Ni0.85SeOH + e− –
Ni0.85SeOH + OH ↔ Ni0.85SeO + H2O + e
where Ipa represents the peak current density, n is the number of electrons in the reaction, A is the surface area of the electrode, C is the proton concentration, and ν is the scan rate. In this work, n, A and C can be considered as constant. Therefore, the diffusion coefficient ratio of three film electrodes is calculated to be D(S2):D(S3):D (S4) = (17.63:19.70:15.51)2 = 1.29:1.613:1, indicating S3 electrode material possesses the highest ion mobility at a specific scan rate of 5 mV s−1. Fig. 4d and Fig. S5b, d, f, show the galvanostatic charge/ discharge (GCD) curves of the electrodes at various current densities. The potential plateau in the GCD curves is corresponding to the redox peak position in the CV curves, which further confirms the battery-type faradaic behavior. Both of cycling stability and rate performance are two significant indicators to evaluate an electrode. As shown in Fig. 4e, repeated charge/discharge measurement is performed on at a current density of 2 A g−1 to study the cycling stability of S1–S4 electrodes. Due to the high electronic conductivity of metal Ni, S4 electrode with pure phase show well cycling stability and rate performance. Therefore, compared with the sharp capacitance fading for S4 electrode, S1–S3 electrodes show well cycling stability with the existence of Ni. Impressively, S3 electrode delivers higher specific capacitance, which maintains a specific capacitance of 835.2 F g−1 (about 78.4 % of the initial capacitance) after 2000 cycles. In addition, a capacitance growth process is observed for the initial 100 cycles due to its electrochemical activation. Fig. 4f shows the rate performance of S1–S4 at different current densities ranging from 1 to 8 A g−1. The specific capacitances of S3 electrode are 1135.3, 1008.3, 911.5 and 864.0 F g−1 at current densities of 1, 2, 5, and 8 A g−1, respectively. Meanwhile, about 76.1% of the capacitance can be retained with the increasing of the current density,
(5) −
(6)
The specific capacitances were calculated basing on the CV curves and shown in Fig. 4b. Significantly, the S3 electrode shows higher calculated capacitances than S1, S2 and S4 electrodes, showing 964.7, 870.6, 781.7, 764.4 and 641.9 F g−1 at the scan rate of 5, 10, 20, 30 and 50 mV s−1, respectively. Compared to the capacitance obtained at 10 mV s−1, the lower capacitance at 5 mV s−1 should be due to the incomplete activation of the electrode. As shown in Fig. 4c, a power-law relationship (Ip = aνb) is associated between the peak current density (Ip) and square root of sweep rate (ν1/2) from 5 to 50 mV s−1 for both cathodic and anodic peaks. Furthermore, a diffusion-controlled batterytype Faradaic process can be concluded with the b-value of 0.5 [42,43]. Meanwhile, with the space inside loose packing structure, the diffusion pathway of electrolyte can be shortened and the apparent diffusion coefficient (D) of the OH− ions can be calculated from the Randlese–Sevcik Equation as given by the following [44,45].
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Fig. 5. (a) CV curves and (b) charge/discharge curves of S3//AC hybrid supercapacitor; (c) cyclic performance and (d) Ragone plot of S1//AC, S2//AC, S3//AC and S4//AC hybrid supercapacitors.
process during the initial cycling stage and excellent cycling stabilities for the subsequent cycles. Particularly, the S3//AC hybrid supercapacitor device shows higher specific capacitance and better cycling stability. A reversible specific capacitance of 140.0 F g−1 can be maintained after 10,000 cycles. The rate performances of the hybrid supercapacitor devices are conducted and shown in Fig. S9. Compared with S1//AC, S2//AC and S4//AC hybrid supercapacitor devices, S3// AC hybrid supercapacitor device delivers better electrochemical performance. The reversible specific capacitances can reach 142.7, 141.3, 138.5, 133.8, 129.2, 125.0 and 121.2 F g−1 at the current densities of 1, 2, 3, 5, 8, 10 and 12 A g−1, respectively. Based on the results of rate performance, the energy/power densities are calculated and the Ragone plots are illustrated in Fig. 5d. Particularly, S3//AC hybrid supercapacitor device shows competitive energy/power densities with the previous reported literatures [29,50–53]. The energy density is 44.6 Wh kg−1 at a power density of 254.5 W kg−1, and it still maintains 37.9 Wh kg−1 even at a high power density of 3.3 kW kg−1. To summarize, the excellent electrochemical performance of S3//AC hybrid supercapacitor can be attributed to the following effects: (1) the introduction of Ni largely strengthen the conductivity of the electrode materials, and also acts as skeleton to support Ni0.85Se; (2) ultrathin nanosheet structure enhances the specific surface area, supplying more redox active sites; (3) the space inside loose packing structure shortens the diffusion pathway of electrolyte and relieve volume change during the repeated charge/discharge processes.
suggesting good rate performance of S3 electrode. Compared with the other three samples, the excellent electrochemical performance of S3 electrode may be attributed to the encapsulation structure and composition. Fig. S6 shows typical Nyquist plots of S1, S2, S3 and S4 electrodes, which measured by electrochemical impedance spectroscopy (EIS). The Nyquist plot exhibits a nearly negligible semicircle indicating high conductivity of the electrodes at the high-frequency range, which suggests charge transfer resistance. As for the low-frequency region, a straight long tail nearly perpendicular to the x axis representing the mass capacitance confirms the ideal capacitive behavior of the electrodes [46,47]. Both of the obtained samples and active carbon (AC), acting as positive and negative electrodes, are combined into one hybrid supercapacitors device to evaluate the practical application. As shown in Fig. S7, the electrochemical performance of AC electrode was investigated with a potential range from −1.0 to 0 V at a current density of 1 A g−1. In Fig. S7a, a specific capacitance of 125.8 F g−1 is obtained at the initial cycle, and maintains 113.0 F g−1 after 5000 cycles, indicating excellent cycling stability. Meanwhile, Fig. 5b shows the charge/discharge curves of AC electrode at current density of 1 A g−1 with typical EDLC behavior. In order to obtain the optimal performance of a hybrid supercapacitor, the mass ratio between the two electrodes m+/m− can be adjusted according to Eq. (7) [48].
m+ C × ΔE− = electrode − m− Celectrode + × ΔE+
(7) 4. Conclusion
where the m, Celectrode and ΔE represent the active mass, specific capacitance and potential window of each electrode, respectively. As shown in Fig. 5a and Fig. S8a, c, e, the CV curves are collected at various sweep rate of 5, 10, 20, and 50 mV s−1 at potential ranging from 0 to 1.5 V. Obviously, both of the battery-like and electric doublelayer capacitance characteristics are observed from the shape of CV curves [49]. In Fig. 5b and Fig. S8b, d, f, no distinct platform is observed in the corresponding charge/discharge curves, measured at different current densities ranging from 1 to 12 A g−1. As shown in Fig. 5c, the cycling stability of the hybrid supercapacitors is measured by repeated charge/discharge process at a current density of 2 A g−1. It is noted that all the hybrid supercapacitor devices (S1//AC, S2//AC, S3//AC and S4//AC) present an active
In summary, [email protected] composite has been successfully synthesized via one-step hydrothermal method. As the Ni skeleton accelerates the transmission of electrons and the ultrathin nanosheet structure enhances the specific capacitance, excellent electrochemical performances with well cycling stability and high energy/power densities are presented for [email protected] composite with the complex structure. In addition, the aforementioned results indicate that this synthetic method is an effective method to improve the electrochemical performance of electrode materials for supercapacitors.
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Acknowledgments
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