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Co3O4 electrode with needle-like arrays for outstanding supercapacitors
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MWCNTs-GONRs/Co3O4 electrode with needle-like arrays for outstanding supercapacitors Hengrui Qiu, Xuejiao Sun, Yongqiang Zhang∗∗, Wenxiu He∗ School of Chemistry and Chemical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia, 014010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MWCNTs-GONRs Co3O4 array structure Electrochemical performance Supercapacitors
Multiwalled carbon nanotubes-graphene oxide nanoribbons (MWCNTs-GONRs) exhibit high specific surface area and good electroconductivity because of their unique three-dimensional cross-linking structure with the properties of both CNTs and GONRs. In this study, a hydrothermal method was employed to anchor MWCNTs–GONRs onto a Ni foam (NF) to obtain a precursor substrate. Subsequently, Co3O4 arrays were grown on the NF substrate to synthesize a MWCNTs–GONR/Co3O4 electrode. The electrode showed a capacitance of 846.2 F g−1 at 1 A g−1 and a capacitance retention of 90.1% after 3000 cycles. Furthermore, MWCNTs–GONRs/Co3O4 and active carbon (AC) were used as the positive and negative electrodes, respectively, to assemble a supercapacitor, which delivered a maximum energy density of 38.23 W h kg−1 and a high power density of 6.80 kW kg−1. In addition, the specific capacitance of the device reached a maximum of 91.5% after 9000 cycles. Thus, the MWCNTs–GONRs/Co3O4 electrode showed huge potential for supercapacitor applications.
1. Introduction With an increase in the demand for electronic equipment, the need for developing energy storage devices has increased tremendously. Among energy storage devices, supercapacitors have gained immense attention owing to their long service life and high power density [1–4]. And M oxides/hydroxides (M = Mn, Ni, Co) have been used as electrodes to improve the properties of supercapacitors [5–9]. The performance of an electrode is affected not only by its chemical composition but also its structure. The structure of an electrode affects its electrochemical properties such as capacitance and cycling stability [10]. Xie et al. obtained a three-dimensional (3D) reduced graphene oxide/Co3O4 electrode via a solvothermal approach. The electrode showed a capacitance of 660 F g−1 at 0.5 A g−1 [11]. Ma et al. reported a Co3O4/RGO nanosheet array with a capacitance of 518.8 F g−1 at 0.5 A g−1 [12]. Guan et al. prepared a Co3O4 needle-loaded GO electrode with a capacitance of 157.7 F g−1 at 0.1 A g−1 [13]. Hence, nanoarray and hierarchical structures facilitate ion diffusion in electrodes and increase their specific surface area, thus improving their electrochemical performance [14,15]. Because multiwalled carbon nanotube–graphene oxide nanoribbons (MWCNT–GONRs) exhibit a unique 3D crosslinking structure with the properties of both CNTs and GONRs [16], we used them as the carbon
∗
source to synthesize the electrode in this study. The MWCNTs–GONRs were anchored onto a nickel foam (NF) to obtain a substrate. Subsequently, Co3O4 arrays were grown on this substrate to obtain a 3D needle-like array hierarchical structure. This structure conferred the electrode many excellent electrochemical properties such as high specific capacitance and good cycling stability because of the synergistic effect of the MWCNTs–GONRs and Co3O4 needle-like arrays [17]. The 3D hierarchical structure designed in this study improved the performance of the electrode. Meanwhile, the electrode was synthesized directly via the hydrothermal method without using any adhesive, thus reducing the impedance of the electrode. The electrode showed a capacitance of 846.2 F g−1 at 1.0 A g−1 and a capacitance retention of 90.1% after 3000 cycles. Furthermore, we assembled an asymmetric supercapacitor using MWCNTs–GONRs/Co3O4 and active carbon (AC) as the positive and negative electrodes, respectively. This device delivered a maximum capacitance of 95.24 F g−1 at 1 A g−1 and energy density of 38.23 W h kg−1 at 0.85 kW kg−1. 2. Experimental 2.1. Material synthesis MWCNTs-GONRs and MWCNT-GONR/NF (labeled as MW-NF) have
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (W. He).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.116 Received 21 November 2019; Received in revised form 9 December 2019; Accepted 11 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hengrui Qiu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.116
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Fig. 1. The formation process of MW-Co.
Fig. 2. (a) XRD patterns, (b) survey scan, (c) Co 2p and (d) C 1s spectrum.
Fig. 3. (a) BET and (b) pore size curves of all samples.
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Fig. 4. The morphology of Co3O4–NF and MW-Co: (a and c) SEM, (b and d) HRSEM.
Fig. 5. Images of MW-Co: (a and b) TEM, (c) high-resolution TEM and the inset shows the SAED of the sample, and (d) EDS mapping.
AXIS Supra X-ray photoelectron spectroscopy (XPS) and LABRAM-HR spectrometer (Raman) to detect samples. Finally, the electrochemical properties of CV, GCD, and EIS were carried out in Zennium E electrochemical workstation. The MW-Co, Hg/HgO, and Pt electrodes were chosen as the working, reference, and counter electrodes, respectively. A 6 M KOH solution as the electrolyte.
been synthesized in our previous work [18]. Co(NO3)2·6H2O (0.582 g), NH4F (0.148 g), CO(NH2)2 (0.6 g) and deionized water (45 mL) were stirred for 30 min until the solution turned pink. Subsequently, the MWNF precursor was added to this solution and heated at 110 °C for 7 h. The precursor was then rinsed, dried and finally annealed at 400 °C for 2 h (at a heating rate of 1 °C min−1), after which it was cooled to room temperature at a rate of 1 °C min−1. Finally, the MWCNTs–GONRs/ Co3O4–NF electrode (MW-Co) electrode was obtained. For comparison, a Co3O4–NF electrode was also synthesized. The active material masses of the Co3O4–NF and MW-Co electrodes were 2.85 and 3.00 mg cm−2, respectively.
2.3. Supercapacitor assembly and measurements The supercapacitor was assembled using MW-Co and AC. Before assembling the supercapacitor, the AC electrode was prepared. The mass loading of the AC electrode was calculated according to the charge balance theory, using Equations (1)–(3) [19–21]:
2.2. Characterizations The crystallographic structures and morphologies of materials were obtained via Gemini 500, HT-7800 scanning/transmission electron microscope (SEM/TEM) and D8-Discover X-ray diffractometer (XRD). BET surface areas, element composition and the existence of MWCNTGONR were determined using JW-DX SSA Brunner-Emmet-Teller (BET),
C = I × t /(ΔV × m)
(1)
m+/ m− = C− × ΔV−/ C+ × ΔV+
(2)
q = C × ΔV × m
(3)
Then the active carbon, acetylene black and PTFE (mass ratio of 3
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Fig. 6. CV curves of (a) MW-Co, Co3O4–NF, MW-NF, and NF at a scanning rate of 25 mV s−1, and (b) MW-Co at 5, 10, 25, and 50 mV s−1. (c) b values of MW-Co, Co3O4–NF and (d) capacitance control ratios of MW-Co at 5, 10, 25, and 50 mV s−1.
Fig. 2b showed the XPS survey scan of MW-Co. The valence states were confirmed by analyzing the C and Co spectra (The atomic concentration of C, O and Co in MW-Co were 29.80%, 51.54% and 18.66%, respectively). The Co 2p spectrum was presented in Fig. 2c, which showed two prominent peaks at 779.9 and 795.7 eV, corresponding to the Co 2p1/2 and Co 2p3/2 of Co3O4 [26,27]. The C 1s spectrum (Fig. 2d) showed three peaks corresponding to C–C, C–O, and C]O (284.9, 286.1 and 288.8 eV), which were consistent with some previous reports [28,29]. Interestingly, the intensity of the C–C was much higher than that of C]O and C–O, which demonstrated that MWCNTs-GONRs was reduced. The Raman spectra of the samples were obtained in order to confirm the presence of MWCNTs-GONRs in MW-Co (Fig. S1). The D, G peaks and ID/IG corresponded to the atomic lattice defects, telescopic vibration and degree of graphitization, respectively [30,31]. Meanwhile, the D, G, and 2D peaks of the samples were approximately 1350, 1601, and 2693 cm−1, respectively [32]. Compared to MWCNTSs-GONRs, MW-Co showed a higher ID/IG, which proved that MWCNTSs-GONRs was reduced during the synthesis process [33]. The BET and pore size curves of MW-Co, Co3O4–NF and NF were displayed in Fig. 3. The BET surface areas of MW-Co, Co3O4–NF and NF were calculated, based on the curves (Fig. 3a), to be 20.1111, 8.8595 and 0.2528 m2 g−1, respectively. MW-Co exhibited a type-IV N2 isotherm as it showed a hysteresis loop at 0.5–1.0 P/P0 [34]. Moreover, the BJH pore size distribution of all samples were approximately 16.5 nm, indicating that a large number of mesoporous pores existed in the sample (Fig. 3b). The large BET surface area of MW-Co could be attributed to the loading of MWCNTs-GONRs onto the NF which
8:1:1) were ground in a mortar for 1 h and the resulting paste was smeared onto the NF. This substrate was then dried for 6 h to obtain the AC electrode. The energy and power density of the device were calculated using the following equations [22–24]:
E = 0.5 × Cs × ΔV 2
(4)
P = E × 3600/ Δt
(5)
where m, I, t, V, C, and q in the above equations represent the mass loading, current density, discharge time, potential range, capacitance, and quantity of electric charge, respectively. E and P represent the energy and power density of the device. The m(MW-Co)/m(AC) ratio was 0.58, as calculated using Eq. (2). 3. Results and discussion The schematic for the preparation of the MW-Co electrode was shown in Fig. 1. The crystallographic phases of the electrode were analyzed using XRD (Fig. 2a). The peaks at 11.5° and 27.1° corresponded to the (001) and (002) planes of MWCNT-GONR, respectively [25]. Unfortunately, the peaks of MWCNT-GONR in MW-Co were weak because the NF substrate exhibited very strong Ni diffraction peaks. Furthermore, the peaks at 18.9°, 31.4°, 36.7°, 44.8°, 59.5°, and 65.2°, corresponded to the (111), (220), (311), (400), (511), and (440) planes of Co3O4, respectively (JCPDS No: 43–1003) [24]. Hence, the XRD results confirmed the formation of MWCNTs-GONRs and Co3O4 compound was successful. 4
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Fig. 7. The performances of electrodes: (a) GCD curves of MW-Co and Co3O4–NF at 1.0 A g−1, (b) MW-Co at 1.0–10.0 A g−1; (c) the relationship between current density and capacitance, (d) cycling stability at 8 A g−1 of electrodes.
(Fig. 4c). Interestingly, MW-Co was significantly more compact and ordered (Fig. 4d). The structure of MW-Co was further analyzed by TEM (Fig. 5a and b). It could be observed from the images that MWCNTs–GONRs showed a tube-sheet structure. This was consistent with the XRD analysis. The measured lattice spacings of 0.201 and 0.466 nm were in line with the (400) and (111) planes of Co3O4 (Fig. 5c). The inset in Fig. 5c presented aureoles corresponding to the (111), (220), (311), (400), (511) and (440) planes of Co3O4 and demonstrated its polycrystalline nature. At last, the elemental mapping images were exhibited in Fig. 5d, which further confirmed the growth of Co3O4 needles on MWCNTSs-GONRs. The electrodes were immersed in 6 M KOH solution and a threeelectrode cell was used for the CV measurements. The CV curves of all the electrodes were shown in Fig. 6a. All the electrodes showed redox peaks, suggesting that the electrodes possessed pseudocapacitance. The relevant equations were as follows [35]:
Co3 O4 + OH− + H2 O ⇔ 3CoOOH + e−
(6)
CoOOH + OH− ⇔ CoO2 + H2 O + e−
(7)
Fig. 8. The EIS of electrodes.
produced a substrate with large BET surface area. And the surface area of the electrode was further enhanced by the 3D needle-like array hierarchical structure of the Co3O4 array. The morphologies of the electrodes were examined using SEM and TEM. Co3O4 showed a needle-like structure with a length of approximately 800 nm (Fig. 4a). The low-resolution image (Fig. 4b) showed that when Co3O4 was grown on NF, a slightly messy array structure was generated (per needle length was approximately 700 nm). After replacing the NF with a MW-NF precursor, the Co3O4 still presented a needle-like shape but was uniformly distributed on the substrate
Among all the electrodes, MW-Co showed the largest closed curve area, indicating that it possessed the highest specific capacitance as well. The capacitance of NF and MW-NF could be ignored because of their very small closed curve areas (the GCD curves of MW-NF were displayed in Fig. S2). The redox peaks were observed in the CV curves of MW-Co electrode at 5, 10, 25 and 50 mV s−1 (Fig. 6b). The redox peaks tended to move to the two ends of the X axis with an increase in the scanning rate. This was because the reaction of active substance was incomplete at high sweep speed. Nevertheless, the shape of the 50 mV s−1 curve of the MW-Co electrode was similar to that of the 5
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Fig. 9. Illustration of the device: (a and b) appearance and construction. CV curves of (c) positive and negative electrodes at 25 mV s−1 and (d) supercapacitor from 1.3 to 1.9 V.
5 mV s−1 curve, indicating its excellent reversibility [19,36]. Furthermore, the reaction kinetics of the electrodes were investigated from their CV curves. The following equations were used (8–10):
i = av b
(8)
log i = b log v + log a
(9)
i (V )/ v1/2 = k1 v1/2 + k2
(10)
679.2 F g−1, respectively. As expected, the capacitance of MW-Co was much higher than that of Co3O4–NF. This was consistent with the CV results. The result could be ascribed to the fact that the MW-Co contained MWCNTs-GONRs, which improved the charge transfer rate and ion diffusion rate. The GCD curves (Fig. 7b) described the relationship between C and I of the MW-Co electrode. Compared to Co3O4–NF electrode (the GCD curves of Co3O4–NF at different current densities were given in Fig. S4), I increased from 1 to 10 A g−1 for MW-Co (Co3O4–NF) and capacitance decreased from 846.2 (679.2) to 440.0 (304.5) F g−1, the corresponding rate capabilities were 52.0% and 44.8%, respectively (Fig. 7c). Furthermore, after 3000 cycles, the capacitance of the MW-Co (Co3O4–NF) electrode decreased from 537.4 (425.6) to 484.0 (287.0) F g−1 and the corresponding capacitance retentions were 90.1% and 67.4%, respectively (as shown in Fig. 7d). These outstanding performances could be attributed to the needle-like arrays of the MW-Co electrode and the synergistic effect of Co3O4 and MWCNTs-GONRs, which resulted in a large specific surface area, good electrical conductivity. The MWCNTs-GONRs made the Co3O4 needle more compact and ordered, thus enhancing the structure and cycling stabilities of MW-Co, and also avoiding the volume expansion of Co3O4 during the charge-discharge process (the information of XRD and SEM after 3000 cycles was shown in Fig. S5). EIS was employed to investigate the kinetic mechanisms of the electrodes. The EIS spectra and corresponding equivalent circuit (inset) were given in Fig. 8. The spectra were composed of a semicircle and a line, and the starting point and radius of arc were Rs and Rct, respectively. The slope of the line represented Warburg impedance [37]. According to the equivalent circuit, the Rs values of the MW-Co and Co3O4–NF electrodes were 0.77 and 0.97 Ω, respectively. The Rct
where i, v, a, b indicate the current, scanning rate, and the adjustable parameters, respectively. To obtain the value of b, the logarithm of both sides of equation (8) was taken, and subsequently linear fitting of log i and log v was done. Meanwhile, the value of b also indicated that the electrode possessed battery or pseudo-capacitor properties. The V, k1, k2 present the specified voltage and the adjustable parameters, respectively. In the same way, the value of k1 could be obtained by linear fitting i(V)/v1/2 and v1/2 in formula (10) under the specified voltage. Finally, k1v was the contribution of pseudocapacitor to the current at each specific voltage. The b value of MW-Co was found to be higher than that of Co3O4–NF (0.79 > 0.77), indicating its faster reaction kinetics (Fig. 6c). The percentage of diffusion/capacitance control was shown in Fig. 6d. As expected, at low scanning rates, the diffusion control was dominant. With an increase in the scanning rate, the contribution of the diffusion control began to decrease (the contribution fitting CV curves were shown in Fig. S3). The GCD curves of the samples were shown in Fig. 7a. The discharge times of the MW-Co and Co3O4–NF electrodes were 448.5 and 360.0 s, respectively, and the corresponding capacitances were 846.2 and 6
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Fig. 10. (a) CV curves of device tested at different scan rates from 25 to 100 mV s−1; (b) GCD curves of device at 1–8 A g−1; (c) Ragone plot of device and a photo of lighting a red LED (inset); (d) Cycling performance of device at 8 A g−1 and the GCD curves of supercapacitor (inset). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
values of the MW-Co and Co3O4–NF electrodes were 0.71 and 0.82 Ω, respectively. Furthermore, the MW-Co electrode exhibited a slightly smaller slant angle of the linear part than the Co3O4–NF. This indicated that the Warburg impedance of the MW-Co electrode was lower than that of the Co3O4–NF electrode. The hierarchical needle-like arrays structure of the MW-Co electrode generated a large number of exposed active sites for the Faradaic reactions and enhanced the interfacial charge transport in the electrode. In addition, the synergistic effect of the Co3O4 nanoneedles and MWCNTs-GONRs also effectively reduced the impedance of the electrode. A supercapacitor was fabricated to test the practicability of the MWCo electrode (Fig. 9a and b). Firstly, the optimal operating potential of the supercapacitor was obtained by analyzing the AC and MW-Co electrodes. The CV curves of AC and MW-Co electrodes at -1–0 V and 0–0.7 V presented a similar rectangle shapes and redox peaks (Fig. 9c), indicating the character of double-layer capacitor and pseudocapacitance, respectively (GCD curve of the AC electrode was shown in Fig. S6) [36]. Therefore, initially the operating potential range of the supercapacitor was estimated to be 0–1.7 V. Subsequent CV test of device at 0–(1.3 to 1.9) V, revealed the shapes of the curves to be comparable (Fig. 9d), demonstrating the accuracy in the selection of the device's operating potential. However, in order to ensure normal operation of the supercapacitor, 1.7 V was selected as the final operating potential. And this potential was higher than those reported previously [38,39]. Fig. 10a showed the CV curves of the device at 25, 50 and 100 mV s−1 over the potential range of 0–1.7 V. At high scan rates, the CV curves showed similar shapes, which was in good agreement with the result of the three-electrode system. Meanwhile, the ability of the MW-Co
electrode to transport ions and electrons at high rates was also confirmed [37]. The Cs of device were 95.24, 70.59, 49.41, 40.94 and 34.87 F g−1 at 1–8 A g−1 as shown in Fig. 10b. Ragone plots revealed the relationship between energy and power densities (Fig. 10c). The device possessed 38.23 (14.00) Wh kg−1 at 0.85 (6.8) kW kg−1, which exceeded the previously reported results [9,24,36,40,41]. Fig. 10d revealed the cycling stability of device and the inset describes the GCD curves of first and final cycle of device. Finally, as depicted in the inset of Fig. 10c, the device could power a red light-emitting diode, demonstrating its potential as a supercapacitor.
4. Conclusions A MW-Co electrode was fabricated via the hydrothermal method and used as positive electrode for supercapacitors. The device showed a specific capacitance of 95.24 F g−1 at 1 A g−1 and a high cycling stability of 91.5% after 9000 cycles. Furthermore, the maximum energy and power density of the device were 38.23 W h kg−1 and 6.8 kW kg−1, respectively. Its outstanding performances could be ascribed to the Co3O4 arrays and MWCNTs-GONRs resulting in increased active sites on the electrode, thus enhancing the electrochemical reaction. Moreover, the synergistic effect of the Co3O4 arrays and MWCNTs-GONRs was also an important factor, which effectively improved the cycling performance and the specific capacitance of the electrode. In the end, we are optimistic that this electrode will have great potential value for application in supercapacitors.
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Notes [18]
The authors declare no competing financial interest. Declaration of competing interest
[19]
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.
[20]
Acknowledgments
[21] [22]
This work was supported by the National Natural Science Foundation of China and Natural Science Foundation of Inner Mongolia (No.21766024, 51864039 and 2019MS02022).
[23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.116.
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MWCNTs-GONRs/Co3O4 electrode with needle-like arrays for outstanding supercapacitors Hengrui Qiu, Xuejiao Sun, Yongqiang Zhang∗∗, Wenxiu He∗ School of Chemistry and Chemical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia, 014010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MWCNTs-GONRs Co3O4 array structure Electrochemical performance Supercapacitors
Multiwalled carbon nanotubes-graphene oxide nanoribbons (MWCNTs-GONRs) exhibit high specific surface area and good electroconductivity because of their unique three-dimensional cross-linking structure with the properties of both CNTs and GONRs. In this study, a hydrothermal method was employed to anchor MWCNTs–GONRs onto a Ni foam (NF) to obtain a precursor substrate. Subsequently, Co3O4 arrays were grown on the NF substrate to synthesize a MWCNTs–GONR/Co3O4 electrode. The electrode showed a capacitance of 846.2 F g−1 at 1 A g−1 and a capacitance retention of 90.1% after 3000 cycles. Furthermore, MWCNTs–GONRs/Co3O4 and active carbon (AC) were used as the positive and negative electrodes, respectively, to assemble a supercapacitor, which delivered a maximum energy density of 38.23 W h kg−1 and a high power density of 6.80 kW kg−1. In addition, the specific capacitance of the device reached a maximum of 91.5% after 9000 cycles. Thus, the MWCNTs–GONRs/Co3O4 electrode showed huge potential for supercapacitor applications.
1. Introduction With an increase in the demand for electronic equipment, the need for developing energy storage devices has increased tremendously. Among energy storage devices, supercapacitors have gained immense attention owing to their long service life and high power density [1–4]. And M oxides/hydroxides (M = Mn, Ni, Co) have been used as electrodes to improve the properties of supercapacitors [5–9]. The performance of an electrode is affected not only by its chemical composition but also its structure. The structure of an electrode affects its electrochemical properties such as capacitance and cycling stability [10]. Xie et al. obtained a three-dimensional (3D) reduced graphene oxide/Co3O4 electrode via a solvothermal approach. The electrode showed a capacitance of 660 F g−1 at 0.5 A g−1 [11]. Ma et al. reported a Co3O4/RGO nanosheet array with a capacitance of 518.8 F g−1 at 0.5 A g−1 [12]. Guan et al. prepared a Co3O4 needle-loaded GO electrode with a capacitance of 157.7 F g−1 at 0.1 A g−1 [13]. Hence, nanoarray and hierarchical structures facilitate ion diffusion in electrodes and increase their specific surface area, thus improving their electrochemical performance [14,15]. Because multiwalled carbon nanotube–graphene oxide nanoribbons (MWCNT–GONRs) exhibit a unique 3D crosslinking structure with the properties of both CNTs and GONRs [16], we used them as the carbon
∗
source to synthesize the electrode in this study. The MWCNTs–GONRs were anchored onto a nickel foam (NF) to obtain a substrate. Subsequently, Co3O4 arrays were grown on this substrate to obtain a 3D needle-like array hierarchical structure. This structure conferred the electrode many excellent electrochemical properties such as high specific capacitance and good cycling stability because of the synergistic effect of the MWCNTs–GONRs and Co3O4 needle-like arrays [17]. The 3D hierarchical structure designed in this study improved the performance of the electrode. Meanwhile, the electrode was synthesized directly via the hydrothermal method without using any adhesive, thus reducing the impedance of the electrode. The electrode showed a capacitance of 846.2 F g−1 at 1.0 A g−1 and a capacitance retention of 90.1% after 3000 cycles. Furthermore, we assembled an asymmetric supercapacitor using MWCNTs–GONRs/Co3O4 and active carbon (AC) as the positive and negative electrodes, respectively. This device delivered a maximum capacitance of 95.24 F g−1 at 1 A g−1 and energy density of 38.23 W h kg−1 at 0.85 kW kg−1. 2. Experimental 2.1. Material synthesis MWCNTs-GONRs and MWCNT-GONR/NF (labeled as MW-NF) have
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (W. He).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.116 Received 21 November 2019; Received in revised form 9 December 2019; Accepted 11 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hengrui Qiu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.116
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Fig. 1. The formation process of MW-Co.
Fig. 2. (a) XRD patterns, (b) survey scan, (c) Co 2p and (d) C 1s spectrum.
Fig. 3. (a) BET and (b) pore size curves of all samples.
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Fig. 4. The morphology of Co3O4–NF and MW-Co: (a and c) SEM, (b and d) HRSEM.
Fig. 5. Images of MW-Co: (a and b) TEM, (c) high-resolution TEM and the inset shows the SAED of the sample, and (d) EDS mapping.
AXIS Supra X-ray photoelectron spectroscopy (XPS) and LABRAM-HR spectrometer (Raman) to detect samples. Finally, the electrochemical properties of CV, GCD, and EIS were carried out in Zennium E electrochemical workstation. The MW-Co, Hg/HgO, and Pt electrodes were chosen as the working, reference, and counter electrodes, respectively. A 6 M KOH solution as the electrolyte.
been synthesized in our previous work [18]. Co(NO3)2·6H2O (0.582 g), NH4F (0.148 g), CO(NH2)2 (0.6 g) and deionized water (45 mL) were stirred for 30 min until the solution turned pink. Subsequently, the MWNF precursor was added to this solution and heated at 110 °C for 7 h. The precursor was then rinsed, dried and finally annealed at 400 °C for 2 h (at a heating rate of 1 °C min−1), after which it was cooled to room temperature at a rate of 1 °C min−1. Finally, the MWCNTs–GONRs/ Co3O4–NF electrode (MW-Co) electrode was obtained. For comparison, a Co3O4–NF electrode was also synthesized. The active material masses of the Co3O4–NF and MW-Co electrodes were 2.85 and 3.00 mg cm−2, respectively.
2.3. Supercapacitor assembly and measurements The supercapacitor was assembled using MW-Co and AC. Before assembling the supercapacitor, the AC electrode was prepared. The mass loading of the AC electrode was calculated according to the charge balance theory, using Equations (1)–(3) [19–21]:
2.2. Characterizations The crystallographic structures and morphologies of materials were obtained via Gemini 500, HT-7800 scanning/transmission electron microscope (SEM/TEM) and D8-Discover X-ray diffractometer (XRD). BET surface areas, element composition and the existence of MWCNTGONR were determined using JW-DX SSA Brunner-Emmet-Teller (BET),
C = I × t /(ΔV × m)
(1)
m+/ m− = C− × ΔV−/ C+ × ΔV+
(2)
q = C × ΔV × m
(3)
Then the active carbon, acetylene black and PTFE (mass ratio of 3
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Fig. 6. CV curves of (a) MW-Co, Co3O4–NF, MW-NF, and NF at a scanning rate of 25 mV s−1, and (b) MW-Co at 5, 10, 25, and 50 mV s−1. (c) b values of MW-Co, Co3O4–NF and (d) capacitance control ratios of MW-Co at 5, 10, 25, and 50 mV s−1.
Fig. 2b showed the XPS survey scan of MW-Co. The valence states were confirmed by analyzing the C and Co spectra (The atomic concentration of C, O and Co in MW-Co were 29.80%, 51.54% and 18.66%, respectively). The Co 2p spectrum was presented in Fig. 2c, which showed two prominent peaks at 779.9 and 795.7 eV, corresponding to the Co 2p1/2 and Co 2p3/2 of Co3O4 [26,27]. The C 1s spectrum (Fig. 2d) showed three peaks corresponding to C–C, C–O, and C]O (284.9, 286.1 and 288.8 eV), which were consistent with some previous reports [28,29]. Interestingly, the intensity of the C–C was much higher than that of C]O and C–O, which demonstrated that MWCNTs-GONRs was reduced. The Raman spectra of the samples were obtained in order to confirm the presence of MWCNTs-GONRs in MW-Co (Fig. S1). The D, G peaks and ID/IG corresponded to the atomic lattice defects, telescopic vibration and degree of graphitization, respectively [30,31]. Meanwhile, the D, G, and 2D peaks of the samples were approximately 1350, 1601, and 2693 cm−1, respectively [32]. Compared to MWCNTSs-GONRs, MW-Co showed a higher ID/IG, which proved that MWCNTSs-GONRs was reduced during the synthesis process [33]. The BET and pore size curves of MW-Co, Co3O4–NF and NF were displayed in Fig. 3. The BET surface areas of MW-Co, Co3O4–NF and NF were calculated, based on the curves (Fig. 3a), to be 20.1111, 8.8595 and 0.2528 m2 g−1, respectively. MW-Co exhibited a type-IV N2 isotherm as it showed a hysteresis loop at 0.5–1.0 P/P0 [34]. Moreover, the BJH pore size distribution of all samples were approximately 16.5 nm, indicating that a large number of mesoporous pores existed in the sample (Fig. 3b). The large BET surface area of MW-Co could be attributed to the loading of MWCNTs-GONRs onto the NF which
8:1:1) were ground in a mortar for 1 h and the resulting paste was smeared onto the NF. This substrate was then dried for 6 h to obtain the AC electrode. The energy and power density of the device were calculated using the following equations [22–24]:
E = 0.5 × Cs × ΔV 2
(4)
P = E × 3600/ Δt
(5)
where m, I, t, V, C, and q in the above equations represent the mass loading, current density, discharge time, potential range, capacitance, and quantity of electric charge, respectively. E and P represent the energy and power density of the device. The m(MW-Co)/m(AC) ratio was 0.58, as calculated using Eq. (2). 3. Results and discussion The schematic for the preparation of the MW-Co electrode was shown in Fig. 1. The crystallographic phases of the electrode were analyzed using XRD (Fig. 2a). The peaks at 11.5° and 27.1° corresponded to the (001) and (002) planes of MWCNT-GONR, respectively [25]. Unfortunately, the peaks of MWCNT-GONR in MW-Co were weak because the NF substrate exhibited very strong Ni diffraction peaks. Furthermore, the peaks at 18.9°, 31.4°, 36.7°, 44.8°, 59.5°, and 65.2°, corresponded to the (111), (220), (311), (400), (511), and (440) planes of Co3O4, respectively (JCPDS No: 43–1003) [24]. Hence, the XRD results confirmed the formation of MWCNTs-GONRs and Co3O4 compound was successful. 4
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Fig. 7. The performances of electrodes: (a) GCD curves of MW-Co and Co3O4–NF at 1.0 A g−1, (b) MW-Co at 1.0–10.0 A g−1; (c) the relationship between current density and capacitance, (d) cycling stability at 8 A g−1 of electrodes.
(Fig. 4c). Interestingly, MW-Co was significantly more compact and ordered (Fig. 4d). The structure of MW-Co was further analyzed by TEM (Fig. 5a and b). It could be observed from the images that MWCNTs–GONRs showed a tube-sheet structure. This was consistent with the XRD analysis. The measured lattice spacings of 0.201 and 0.466 nm were in line with the (400) and (111) planes of Co3O4 (Fig. 5c). The inset in Fig. 5c presented aureoles corresponding to the (111), (220), (311), (400), (511) and (440) planes of Co3O4 and demonstrated its polycrystalline nature. At last, the elemental mapping images were exhibited in Fig. 5d, which further confirmed the growth of Co3O4 needles on MWCNTSs-GONRs. The electrodes were immersed in 6 M KOH solution and a threeelectrode cell was used for the CV measurements. The CV curves of all the electrodes were shown in Fig. 6a. All the electrodes showed redox peaks, suggesting that the electrodes possessed pseudocapacitance. The relevant equations were as follows [35]:
Co3 O4 + OH− + H2 O ⇔ 3CoOOH + e−
(6)
CoOOH + OH− ⇔ CoO2 + H2 O + e−
(7)
Fig. 8. The EIS of electrodes.
produced a substrate with large BET surface area. And the surface area of the electrode was further enhanced by the 3D needle-like array hierarchical structure of the Co3O4 array. The morphologies of the electrodes were examined using SEM and TEM. Co3O4 showed a needle-like structure with a length of approximately 800 nm (Fig. 4a). The low-resolution image (Fig. 4b) showed that when Co3O4 was grown on NF, a slightly messy array structure was generated (per needle length was approximately 700 nm). After replacing the NF with a MW-NF precursor, the Co3O4 still presented a needle-like shape but was uniformly distributed on the substrate
Among all the electrodes, MW-Co showed the largest closed curve area, indicating that it possessed the highest specific capacitance as well. The capacitance of NF and MW-NF could be ignored because of their very small closed curve areas (the GCD curves of MW-NF were displayed in Fig. S2). The redox peaks were observed in the CV curves of MW-Co electrode at 5, 10, 25 and 50 mV s−1 (Fig. 6b). The redox peaks tended to move to the two ends of the X axis with an increase in the scanning rate. This was because the reaction of active substance was incomplete at high sweep speed. Nevertheless, the shape of the 50 mV s−1 curve of the MW-Co electrode was similar to that of the 5
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Fig. 9. Illustration of the device: (a and b) appearance and construction. CV curves of (c) positive and negative electrodes at 25 mV s−1 and (d) supercapacitor from 1.3 to 1.9 V.
5 mV s−1 curve, indicating its excellent reversibility [19,36]. Furthermore, the reaction kinetics of the electrodes were investigated from their CV curves. The following equations were used (8–10):
i = av b
(8)
log i = b log v + log a
(9)
i (V )/ v1/2 = k1 v1/2 + k2
(10)
679.2 F g−1, respectively. As expected, the capacitance of MW-Co was much higher than that of Co3O4–NF. This was consistent with the CV results. The result could be ascribed to the fact that the MW-Co contained MWCNTs-GONRs, which improved the charge transfer rate and ion diffusion rate. The GCD curves (Fig. 7b) described the relationship between C and I of the MW-Co electrode. Compared to Co3O4–NF electrode (the GCD curves of Co3O4–NF at different current densities were given in Fig. S4), I increased from 1 to 10 A g−1 for MW-Co (Co3O4–NF) and capacitance decreased from 846.2 (679.2) to 440.0 (304.5) F g−1, the corresponding rate capabilities were 52.0% and 44.8%, respectively (Fig. 7c). Furthermore, after 3000 cycles, the capacitance of the MW-Co (Co3O4–NF) electrode decreased from 537.4 (425.6) to 484.0 (287.0) F g−1 and the corresponding capacitance retentions were 90.1% and 67.4%, respectively (as shown in Fig. 7d). These outstanding performances could be attributed to the needle-like arrays of the MW-Co electrode and the synergistic effect of Co3O4 and MWCNTs-GONRs, which resulted in a large specific surface area, good electrical conductivity. The MWCNTs-GONRs made the Co3O4 needle more compact and ordered, thus enhancing the structure and cycling stabilities of MW-Co, and also avoiding the volume expansion of Co3O4 during the charge-discharge process (the information of XRD and SEM after 3000 cycles was shown in Fig. S5). EIS was employed to investigate the kinetic mechanisms of the electrodes. The EIS spectra and corresponding equivalent circuit (inset) were given in Fig. 8. The spectra were composed of a semicircle and a line, and the starting point and radius of arc were Rs and Rct, respectively. The slope of the line represented Warburg impedance [37]. According to the equivalent circuit, the Rs values of the MW-Co and Co3O4–NF electrodes were 0.77 and 0.97 Ω, respectively. The Rct
where i, v, a, b indicate the current, scanning rate, and the adjustable parameters, respectively. To obtain the value of b, the logarithm of both sides of equation (8) was taken, and subsequently linear fitting of log i and log v was done. Meanwhile, the value of b also indicated that the electrode possessed battery or pseudo-capacitor properties. The V, k1, k2 present the specified voltage and the adjustable parameters, respectively. In the same way, the value of k1 could be obtained by linear fitting i(V)/v1/2 and v1/2 in formula (10) under the specified voltage. Finally, k1v was the contribution of pseudocapacitor to the current at each specific voltage. The b value of MW-Co was found to be higher than that of Co3O4–NF (0.79 > 0.77), indicating its faster reaction kinetics (Fig. 6c). The percentage of diffusion/capacitance control was shown in Fig. 6d. As expected, at low scanning rates, the diffusion control was dominant. With an increase in the scanning rate, the contribution of the diffusion control began to decrease (the contribution fitting CV curves were shown in Fig. S3). The GCD curves of the samples were shown in Fig. 7a. The discharge times of the MW-Co and Co3O4–NF electrodes were 448.5 and 360.0 s, respectively, and the corresponding capacitances were 846.2 and 6
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Fig. 10. (a) CV curves of device tested at different scan rates from 25 to 100 mV s−1; (b) GCD curves of device at 1–8 A g−1; (c) Ragone plot of device and a photo of lighting a red LED (inset); (d) Cycling performance of device at 8 A g−1 and the GCD curves of supercapacitor (inset). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
values of the MW-Co and Co3O4–NF electrodes were 0.71 and 0.82 Ω, respectively. Furthermore, the MW-Co electrode exhibited a slightly smaller slant angle of the linear part than the Co3O4–NF. This indicated that the Warburg impedance of the MW-Co electrode was lower than that of the Co3O4–NF electrode. The hierarchical needle-like arrays structure of the MW-Co electrode generated a large number of exposed active sites for the Faradaic reactions and enhanced the interfacial charge transport in the electrode. In addition, the synergistic effect of the Co3O4 nanoneedles and MWCNTs-GONRs also effectively reduced the impedance of the electrode. A supercapacitor was fabricated to test the practicability of the MWCo electrode (Fig. 9a and b). Firstly, the optimal operating potential of the supercapacitor was obtained by analyzing the AC and MW-Co electrodes. The CV curves of AC and MW-Co electrodes at -1–0 V and 0–0.7 V presented a similar rectangle shapes and redox peaks (Fig. 9c), indicating the character of double-layer capacitor and pseudocapacitance, respectively (GCD curve of the AC electrode was shown in Fig. S6) [36]. Therefore, initially the operating potential range of the supercapacitor was estimated to be 0–1.7 V. Subsequent CV test of device at 0–(1.3 to 1.9) V, revealed the shapes of the curves to be comparable (Fig. 9d), demonstrating the accuracy in the selection of the device's operating potential. However, in order to ensure normal operation of the supercapacitor, 1.7 V was selected as the final operating potential. And this potential was higher than those reported previously [38,39]. Fig. 10a showed the CV curves of the device at 25, 50 and 100 mV s−1 over the potential range of 0–1.7 V. At high scan rates, the CV curves showed similar shapes, which was in good agreement with the result of the three-electrode system. Meanwhile, the ability of the MW-Co
electrode to transport ions and electrons at high rates was also confirmed [37]. The Cs of device were 95.24, 70.59, 49.41, 40.94 and 34.87 F g−1 at 1–8 A g−1 as shown in Fig. 10b. Ragone plots revealed the relationship between energy and power densities (Fig. 10c). The device possessed 38.23 (14.00) Wh kg−1 at 0.85 (6.8) kW kg−1, which exceeded the previously reported results [9,24,36,40,41]. Fig. 10d revealed the cycling stability of device and the inset describes the GCD curves of first and final cycle of device. Finally, as depicted in the inset of Fig. 10c, the device could power a red light-emitting diode, demonstrating its potential as a supercapacitor.
4. Conclusions A MW-Co electrode was fabricated via the hydrothermal method and used as positive electrode for supercapacitors. The device showed a specific capacitance of 95.24 F g−1 at 1 A g−1 and a high cycling stability of 91.5% after 9000 cycles. Furthermore, the maximum energy and power density of the device were 38.23 W h kg−1 and 6.8 kW kg−1, respectively. Its outstanding performances could be ascribed to the Co3O4 arrays and MWCNTs-GONRs resulting in increased active sites on the electrode, thus enhancing the electrochemical reaction. Moreover, the synergistic effect of the Co3O4 arrays and MWCNTs-GONRs was also an important factor, which effectively improved the cycling performance and the specific capacitance of the electrode. In the end, we are optimistic that this electrode will have great potential value for application in supercapacitors.
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Notes [18]
The authors declare no competing financial interest. Declaration of competing interest
[19]
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.
[20]
Acknowledgments
[21] [22]
This work was supported by the National Natural Science Foundation of China and Natural Science Foundation of Inner Mongolia (No.21766024, 51864039 and 2019MS02022).
[23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.12.116.
[25]
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