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Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes
Journal Pre-proof Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes Ibrahim M.A. Mohamed, Ahmed S. Yasin, Changkun Liu PII:
S0272-8842(19)32982-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.119
Reference:
CERI 23187
To appear in:
Ceramics International
Received Date: 31 August 2019 Revised Date:
8 October 2019
Accepted Date: 13 October 2019
Please cite this article as: I.M.A. Mohamed, A.S. Yasin, C. Liu, Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.10.119. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes Ibrahim M. A. Mohamed a,b,c,, Ahmed S. Yasin b, Changkun Liu a,d,* a
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, PR China
b
Department of Bionanosystem Engineering, Jeonbuk National University, Jeonju, Jeonbuk 561756, Republic of Korea c
d
Department of Chemistry, Faculty of Science, Sohag university, Sohag 82524, Egypt
Shenzhen Key Laboratory of Environmental Chemistry and Ecological Remediation, Shenzhen University, Shenzhen 518060, PR China
Corresponding author: *Email: [email protected] (Changkun Liu).
1
Abstract The chemical and surface modification of activated carbon (AC) through the incorporation of nanoparticles (NPs) is a simple and efficient strategy to enhance the charge transfer as well as the capacitance behavior of AC. Herein, ZnO-NPs were successfully attached at the surface of AC via one-step hydrothermal process to form ZnO-NPs@AC (ZAC). The introduced ZACmaterial was studied by surface characterization techniques including transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) to investigate the morphological and chemical characteristics of ZAC. After that, ZAC was utilized as a working electrode (WE) material for supercapacitor in alkaline and acidic electrolytes. The specific capacitance (Cs) values of ZAC showed an enhanced performance as compared with that of the pristine AC. Moreover, the low series resistance and high Warburg co-efficient of ZAC as compared to those of the AC indicated the faster -OH diffusion inside the ZAC working electrode. Additionally, the phase angle of ZAC is closer to the ideal capacitor than that of AC. Overall, this study proved that ZnO-NPs can enhance the AC capacitance behavior through the easy chemical modification of AC.
Keywords: ZnO-NPs, Activated Carbon, Supercapacitor; Electrode Material
2
1. Introduction Supercapacitors are one of the favorable devices for energy storage, especially for medical and electrical applications which need high power and prolonged life [1]. These devices bridge between devices having power output and batteries storing energy [2]. However, the development of the energy demand stimulates researchers to fabricate novel supercapacitor electrodes having both improved cycle life and output power. Basically, supercapacitors have two types based on the charge character, namely: electrochemical double-layer (EDLC) and pseudocapacitive devices. EDLC electrodes are mainly C-based materials and the storage of energy is based on the ion adsorption [3]. On the other side, pseudocapacitive devices are composed of materials having reversible and fast faradic redox behavior such as transition-metal oxides [4]. Most of the literature focused on C-based materials including carbon nanotubes and activated carbon (AC) as an efficient supercapacitor which may be attributed to better conductivity, active surface area, porosity, and electrochemical stability [5-8]. Metal-oxides at the surface of C-based materials are more promising and attractive including RuO2 [9], IrO2 [10], MnOx [11], CoOx [12], Fe3O4 [13] and ZnO [14] which are introduced with good capacitive behaviors. These metal oxides enhance the capacitance behavior of C-based materials due to the increase of the wettability of the C-materials surface in the utilized electrolyte which enhances the interaction between the oxide ion and the electrolyte [15]. Among metal-oxides, ZnO shows excellent electrical and optical characteristics and hence has been applied for photovoltaics [16, 17], gas sensing [18], biocompatible corrosion inhibitor [19], photocatalysis [20] as well as supercapacitor application [21]. Theoretically, ZnO has a high capacitance performance. However, the insufficient conductivity at high current retards the ZnO3
supercapacitors [21, 22]. Therefore, the ZnO nanoparticles (ZNPs) have been introduced in combination with the C-substrates. Recently, many techniques including green synthesis [14], MOF-strategy [23] and electrospray [24] were reported for the fabrication of ZnO/C-composite electrodes towards supercapacitor applications. However, the residual impurities in the designed ZnO might decrease or decline the electrochemical performance and/or stability. Therefore, it is important to explore simple and one-step strategies to fabricate ZnO/C-composite electrode having high capacitive efficiency. Herein, ZnO/activated carbon (AC) composite was chemically fabricated via the easy and one-step methodology and investigated as the supercapacitor electrode. The material fabrication was based on the hydrothermal treatment of AC in the presence of Zn source. The morphology, crystallinity and chemical properties of the composite were studied through FESEM, TEM, XRD, TEM-EDX, and XPS. The fabricated composite was studied for the supercapacitor application in acidic (H2SO4) and alkaline (KOH) media. The electrochemical performance was investigated through cyclic voltammetry (CV) at different conditions including scan-rate, charge-discharge curves and electrochemical impedance spectroscopy (EIS). The introduced results indicated that the fabricated ZnO/AC material has a high performance towards supercapacitor application in both acidic and alkaline media. 2. Experimental 2.1. Materials and methods Zinc acetate, commercial activated carbon (CEP-21K, PCT Co., Korea), H2SO4, and KOH flake (Alfa Aesar) were utilized without any treatment except the preparation of H2SO4 and KOH solutions through the dilution by use of de-ionized water. Nafion solution was used 4
through electrochemical study and purchased from Aldrich Co. Ltd., USA. A glassy carbon electrode (GCE) owning an area of 0.071 cm2 was purchased from CH Instruments, Inc., USA. 2.2. Synthesis of ZnO NPs @activated carbon (ZAC) nanocomposite The ZnO NPs @ AC (ZAC) were chemically fabricated via a simple hydrothermal method in the presence of deionized water. Typically, 0.5 g of AC and 45 mg of Zn-acetate were added to 50 mL of deionized water and sonicated for 2 h. Thereafter, the obtained mixture was treated hydrothermally in a Teflon-lined autoclave for 12 h at 180 °C. The hydrothermal solution was filtered and washed 3 times by H2O until the filtrate became clear. The obtained solid material was dried at 60 °C under vacuum for 48 h. The fabricated ZAC was utilized as a supercapacitor electrode after the deposition at the glassy-carbon electrode (GCE). 2.3. Surface characterization The surface characterization of the fabricated ZAC was carried out to describe the morphology, crystallinity and physicochemical properties through the utilization of transmission electron microscope (TEM, JEOL JEM-2010, Japan) and field-emission scanning electron microscope (FESEM) (Hitachi S-7400, Japan). The crystallinity of ZAC material was tested via a Rigaku Xray diffractometer (XRD) (Rigaku Co., Japan) with Cu Kα (λ = 1.540 Å) radiation over a 2θ in the range of from 5° to 90°. The chemical content of ZAC nanocomposite was investigated by Xray photoelectron spectroscopy with Al Kα irradiation (XPS, AXIS-NOVA, Kratos, Inc.). The XPS wide scan was investigated in the range of 0 - 1300 eV and the XPS core-level spectra of Zn, C and O were also derived. 2.4. Supercapacitor electrochemical performance
5
The electrochemical performance of the designed ZAC was studied using a 3-electrode (working, counter and reference electrodes) system at room temperature. The deposition of ZAC at GCE to design the utilized working electrode (WE) was carried out through the suspension solution of the investigated material in iso-propanol [25]. A Pt-wire was used as a counter or auxiliary electrode and Ag/AgCl as a reference electrode (RE) to fill the electrode system for checking the electrochemical performance. For ZAC-deposition on GCE, 2 mg of ZAC was suspended in 400 µL of 2-propanol and 20 µL of Nafion aqueous solution and then sonicated for 30 min. After that, 5 µL of the suspension was deposited onto the surface of WE. The deposition step was repeated 3 times to fill most of the electrode area (3 mm diameter, 0.071 cm2). The asdesigned WE electrode was dried gradually from room temperature to 70°C and kept at this temperature for 30 min. The as-prepared WE electrode was used to investigate the electrochemical performance of ZAC towards supercapacitor application in acid and alkaline media. Cyclic voltammetry (CV) survey scans of all experiments were taken in the range of - 0.2 V to 0.6 V. The specific capacity was estimated by use of CV and galvanostatic charge-discharge (GCD) curves in acid and alkaline media via the following relationships, respectively [26, 27]:
C =
C =
I dV (1) 2 υ m ΔV 2
V dt
(2)
m V l where Cs is the specific capacitance, ʃ I dV is the area of CV, υ is the scan rate of CV, m is the mass of the electroactive material and ∆V is the difference between the end and the start
6
potential for Equation (1). In addition, I-current is the discharge current in GCD-window and ʃ Vdt is the area under the discharge current for Equation (2). 3. Results and discussion 3.1. Surface analyses of ZAC material The morphology of ZAC material was investigated via FESEM and TEM as shown in Fig. 1. Fig. 1A and 1B display the 5K-X and 10K-X magnification of FESEM analyses of the prepared nanoparticles at the AC-surface. The surface morphology of ZAC is micro rock shape, with smooth, porous and homogeneous nature. This morphological character was proved through TEM-analyses (Fig. 1C). Also, the TEM-mapping indicated the presence of C, O, and Zn with different densities (C > O > Zn). Logically, the main chemical content of AC would be C which was easily observed according to the density of C in TEM-mapping. AC surface was known to have some oxygenated function groups which gave AC acid-basic behavior [28]. Thus, the O content was higher than Zn percentage. However, Zn-mapping was included in the mapping of O which indicated the presence of two types of O: one connected to Zn and the other one from the AC-function groups. The ZnO NPs were distributed in the form of ultrafine particles at the surface of AC without aggregation. The EDX of ZAC was studied via TEM as shown in Fig. S1 which confirmed that the C, Zn, and O element contents of ZAC were 96.04, 0.45, and 3.51, respectively. The surface chemistry of ZAC was investigated via XPS (Fig. 2 and Table (1)). The wide scan displays the presence of Zn, O, and C-elements at 1022.1, 532.25 and 284.49 eV, respectively. The carbon was the skeleton of AC, and hence the atomic percentage of the prepared ZAC was 94.95 % which was important for applications as the supercapacitor electrode 7
material because C-materials have an acceptable conductivity to be the electrode material beside its capacitive and electrocatalytic performance. The core-level XPS spectra for C, Zn and O elements were shown in Figs. 2B, 2C and 2D, respectively. The C 1s de-convolution showed three peaks at 284.08, 285.28 and 288.38 eV which were attributed to C-C, C-O and C=O bonds, respectively [29]. The ZnO formation at the AC surface was proved through the Zn 2p XPS spectra, as displayed in Fig. 2C. The two peaks were found and located at 1022.08 eV and 1045.38 eV which were assigned to Zn 2p3/2 and 2p1/2, respectively. The binding energy separation between Zn 2p3/2 and 2p1/2 peaks was 23.3 eV which indicated the +2 valence of Zn and proved that ZnO nanostructures were attached on the AC surface [30]. As shown in Fig. 2D, the O 1s deconvolution showed 3-different sub-peaks at 530.48, 532.18, 534.58 eV which were attributed to the O in ZnO, defective oxygen and chemisorbed oxygen, respectively [31]. To study the difference in chemistry between the bulk and the surface of the designed ZAC, the comparison between TEM-EDX (providing bulk concentration) and XPS (providing surface concentrations of elements) analysis was displayed in Table (S1). The contents of C, Zn and O were 96.04, 0.45 and 3.51 % respectively according to the TEM-EDX analyses. In comparison, the Zn and O contents increased to 0.58 and 4.47 % respectively with XPS analyses. The contents of Zn and O quantitatively increased at the surface of AC which confirmed the presence of more ZnO-NPs at the surface of AC. This can be attributed to the increasing rate of the nucleation step which was more than the rate in the growth step. Basically, the formation of crystals includes 2 major steps: nucleation and growth [32]. In the designed material, the nucleation rate was higher than the growth rate. As a result, the ZnO was formed at the surface of AC in the form of ultrafine particles. In short, the designed ZAC material has the chemistry of C attached to ZnO-NPs. 8
The successful design of ZnO-NPs at the AC-surface was also studied via the XRD analyses (Fig. 3 and Table (2)) to describe the crystallinity. Firstly, there were two broad peaks at 23.39o and 42.75o which corresponded to 200 and 100 planes of graphite (JCPDS 00-041-1487). The broad character of peaks indicated the amorphous structure of AC because X-rays could be scattered in different directions which led to the large bump located in the wide range of 2θ. Secondly, ZACXRD has crystalline peaks at 31.72°, 34.37, 36.22, 47.46, 56.56, 62.79, 67.86 and 68.92 which could be assigned to (100), (002), (101), (102), (110), (103), (112) and (201) planes, respectively. These peaks were exactly in the same 2θ position of ZnO hexagonal crystals and the space group was P63mc (00-036-1451) [33]. Furthermore, the hexagonal parameters for ZnO-NPs were calculated which were equal to 3.257 Å and 5.205 Å for a-parameter or b-parameter (a=b) and c crystal parameters, respectively. These parameters were calculated via the basic equation of hexagonal structure: 1/d2 = [(4/3) (h2+hk+k2) / a2] + l2/c2. The d-spacing of (hkl 100) and (hkl 101) can be utilized to estimate a, b and c parameters. Additionally, the ZnO crystallite size in ZAC was calculated via a Scherrer equation (D = (0.9λ)/(β cos θ) [34] where “λ” is X-rays wavelength (1.541874 Å), “β” is the full width at half maximum (FWHM) of the high-intensity peak (101-peak) and θ is the Bragg's angle of the radians and found at 18.40 nm (exactly 184.01 Å). Therefore, XRD suggested that hydrothermal treatment can be a successful strategy to design ZAC material. Previously, the hydrothermal method was proved as a simple and productive strategy for the design of metal-oxides @ AC materials [35]. Zn ions can be easily dissolved in aqueous solutions and re-deposited as oxide-NPs on the AC surface during the hydrothermal process. The functional groups at the surface of ZAC were studied via FT-IR as displayed in Fig. 4. The analyses showed 3 peaks at 1112, 1552 and 3421 cm-1 which were attributed to C-O, C=O and O-H bonds of AC, respectively [36]. The O-H peak was clearer and broader than the 9
expected because of the synthesis steps of ZAC which included only hydrothermal treatment without further calcination or carbonization. The FT-IR for ZnO region showed a small peak at 489 cm-1 in addition to the many peaks at 420 - 690 cm-1 which proved the presence of the Zn-O linkage [37]. The lower intensity of the Zn-O peaks could be attributed to the small content of ZnO NPs in ZAC as reported before in XPS and TEM-EDX analyses. Briefly, the FESEM, TEM, XRD, XPS, and FT-IR confirmed the successful fabrication of ZnO-NPs at the surface of AC through hydrothermal treatment. 3.2. Electrochemical performance in an alkaline electrolyte The designed ZAC material was used to fabricate the working electrode (WE) for the investigation of electrochemical performance as the supercapacitor electrode material. The electrochemical performance was tested via CV, CD, and EIS in alkaline using KOH and H2SO4 aqueous media. The CV curves were carried out with a range of potential from -0.2 to 0.6 V by use of pristine AC and ZAC at different scan rates including 10 mV/s (Fig. 5A), 20 mV/s (Fig. S2), 50 mV/s (Fig. 5B) and 70 mV/s (Fig. S3). The CV analyses of both AC and ZAC showed no redox peaks. Additionally, the shape of CV in the case of ZAC was rectangular which could be considered as close to an ideal supercapacitor which displayed its capacity for storage of valuable amounts of charges. The redox peaks could be attributed to pseudo-capacitance which was not shown in the case of the designed ZAC material. Basically, the CV area was related to the capacitance performance of the studied electrode material. The area of the ZAC curve was 8.01 x 10-4 which was higher than an AC curve (4.06 x10-4) at 10 mV/s. The enhanced CV area was seen in Fig. 5 which proved that ZAC showed higher performance for supercapacitor than the pristine AC without ZnO-NPs. The GCD curves were shown in Fig. 5C and 5D for both AC
10
and ZAC at different I/m values started from 2 A/g (Fig. 5C), 4 A/g (Fig. S4), 6 A/g (Fig. S5) and 8 A/g (Fig. 5D). The GCD plots presented the symmetric character with nearly linear behavior, which proved the enhanced capacitance of ZAC as a supercapacitor electrode material. The discharge time as an indicator for capacitance character was enhanced from 25.1 s to 98.7 s at 2 A/g for AC and ZAC, respectively. The improved performance was observed at all studied I/m: 4, 6 and 8 A/g. Fig. 6A showed the CV of ZAC in an alkaline electrolyte at different scan rates (from 10 to 70 mV/s). As the scan rate increased, the CV area was enhanced at all investigated scan rates. The specific capacitance was estimated via Equation (1) using the CV data and their integration (Fig. 6B). The specific capacitance (Cs) values of ZAC showed an enhanced performance if compared with the pristine AC. At 10 mV/s, the Cs increased from 355.05 F/g for AC to 667.18 F/g for the prepared ZAC material. This improvement was easily observed at all the investigated scan rates. Additionally, the Cs value of ZAC at 10 mV/s retained nearly 43.5 % (290.12 F/g) of its initial value after increasing the scan rate up to 70 mV/s, which demonstrated the improved capacitance behavior of ZAC-material. Fig. 6C showed the GCD of ZAC in an alkaline electrolyte at different I/m values (from 2 to 8 A/g). As the current decreased, the discharge time increased at all the investigated currents. The specific capacitance was estimated by Equation (2) using the GCD data and the discharge curve integration (Fig. 6D). The Cs values of ZAC showed an improved capacitance performance if compared with the pristine AC. At 2 A/g, the Cs of ZAC was enhanced from 94.4 F/g to 349.6 F/g. This capacitance enhancement was easily observed at all the investigated currents (2-20 A/g). Additionally, the Cs of ZAC at 2 A/g retained nearly 57.12 % (199.72 F/g) of its value after increasing the current up to 20 A/g. The enhanced Cs values in an alkaline medium demonstrated the high capacitance performance of the ZAC-material. 11
3.3. Electrochemical performance in an acidic electrolyte The introduced ZAC material was evaluated as a WE material for supercapacitor in an acidic medium beside in an alkaline medium. The electrochemical including CV and GCD were investigated as displayed in Fig. 7 and 8 by use of H2SO4 aqueous solution as the acid source of electrolyte. The CV studies were carried out for AC and ZAC material at various scan rates: 10 mV/s (Fig. S6), 20 mV/s (Fig. S7), 50 mV/s (Fig. 7A) and 70 mV/s (Fig. 7B). The CV of both AC and ZAC in an acidic electrolyte showed no redox peaks in addition to the rectangular shape of the CV of ZAC material at high scan rates (50 mV/s and 70 mV/s) which indicated the high performance of ZAC as a supercapacitor electrode material. Based on the CV data, the area under ZAC curve was 4.92 x 10-4 which was better than what was obtained from AC curve (4.05 x10-4) at the same scan rate of 10 mV/s. The increase of the CV area or ʃ I dV was easily seen in Fig. 7A and 7B at high scan rates (50 and 70 mV/s) which proved that the incorporation of ZnO NPs at AC was a successful strategy to enhance the capacitance character. The GCD data were shown in Fig. 7C and 7D for both AC and ZAC at different current (I/m) values including 2 A/g (Fig. 7C), 4 A/g (Fig. S8), 6 A/g (Fig. 7D) and 8 A/g (Fig. S9). The presented GCD plots showed symmetric character with nearly triangular character, reflecting the fast charging process. The discharge time which had a direct relation to the capacitance performance was enhanced from 26.78 to 150.40 s at 2 A/g for AC and ZAC, respectively. The improved performance was observed in CV analyses as well as GCD plots at different currents: 4, 6 and 8 A/g. Fig. 8A showed the CV of ZAC in an acidic electrolyte at different scan rates (from 10 to 100 mV/s). As the scan rate increased, the CV area or ʃ I dV was enhanced at all the presented scan rates. The Cs values were calculated via Equation (1) using the CV data and shown in Fig. 8B. The Cs values of ZAC were better than the values of the pristine AC. At 10 mV/s, the Cs increased from 12
354.19 F/g for AC to 430.54 F/g for ZAC material. This enhancement was easily seen at all the presented scan rates from 10 to 100 mV/s. Additionally, the Cs value of ZAC at 10 mV/s retained nearly 49.3 % (212.19 F/g) of its initial value after increasing the scan rate up to 100 mV/s, which proved the improved capacitance behavior of AC after modification by ZnO NPs. Fig. 8C showed the GCD of ZAC in an acidic electrolyte at different current values (from 2 to 8 A/g). As expected, the discharge time decreased as I/m values increased. The specific capacitance was calculated by Equation (2) using the integration of the discharge curve (Fig. 8D). The Cs values of AC were improved after hydrothermal modification by ZnO-NPs to form ZAC material. At 2 A/g, the Cs of ZAC was enhanced to 404.73 F/g from 102.38 F/g. This capacitance enhancement was also seen at current values from 2 to 20 A/g. The Cs of ZAC retained only 24.83 % (100.49 F/g) of its initial value after increasing the current from 2 to 20 A/g. The CV and GCD demonstrated the enhanced capacitance performance of AC after hydrothermal treatment to design ZAC-material. 3.4. EIS and stability analyses EIS is one of the most attractive and powerful techniques to prove and understand the electrochemical performance of novel electrode materials such as the prepared ZAC. Fig. 9A and 9B showed the Nyquist and bode-phase plots from 0.1 to 104 Hz in alkaline electrolytes. The inset equivalent circuit was utilized to fit the data and compared with the pristine impedance data. The resistance and Warburg parameters (Rh, Rct1, Rct2, and W) of the introduced circuit were shown in Table (3). The Rh, Rct1 and Rct2 are corresponding to the electrolyte resistance, ionic charge-transfer resistance and electron-transfer resistance, respectively [38]. The difference between the fitted and the pristine Nyquist curves was very small which could be easily
13
neglected, indicating the successful fitting of the impedance data. Additionally, the relative errors of the Rh and Warburg were small (< 2%). ZAC and AC Nyquist plots could be classified into three regions based on the frequency level (Rh, Rct1/CPE1, Rct2/CPE2-W). The left one was related to the ohmic resistance in the utilized electrolyte and the right one (long tail) which was assigned to the ion diffusion from the electrolyte to electrode [39, 40]. The intermediate one (Rct1/CPE1) was the charge transfer resistance between the used WE and the electrolyte. Both investigated electrodes showed the same behavior in the charge transfer which may be due to the use of the same electrolyte (KOH medium) owning high electrical conductivity. The Rh and Rct1 values of ZAC were slightly lower than the values of AC which indicated the improved internal conductivity of WE. The long tail or ion diffusion impedance could be represented via the Warburg element. The higher Warburg co-efficient of the ZAC WE as compared to that of AC WE indicated the faster -OH movement to form the electrical double layer [41]. Additionally, the Bode plots were studied for AC and ZAC WE as shown in Fig. 9D. The ZAC phase angle was found at 55.13o which was better than that of AC (43.41o). Basically, the known ideal behavior of any capacitor should have a 90o phase angle which was not realized in both AC and ZAC in this study [42]. ZAC angle was probably closer to the ideal than the AC one. The introduced EIS study indicated the enhancement of the capacitance behavior of AC after ZnO-NPs incorporation. Cycling stability is considered as one of the critical factors for promoting the commercial application of supercapacitors. To further study the electrochemical performance of the prepared material, cycling performance of ZAC material was evaluated at a current density of 10 A g−1 for 3000 successive charge-discharge cycles in alkaline and acidic media, and the results were displayed in Fig. 9E and 9F. The capacitance retention % was calculated from the capacitance of 14
the cycles by use of the ratio between the initial and following cycles. The first cycle was considered as 100 % and then the capacitive retention % was calculated as a percentage of the first cycle. The as-prepared ZAC retained about 89.17 % of the capacity after 3000 cycles in KOH medium, and 90.15 % of the capacity was retained after 3000 cycles in H2SO4 medium. These results further confirmed that the ZAC electrode possessed exceptional cycling stability which might be derived from the incorporation of zinc oxide into the activated carbon.
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Table (1): XPS analyses of the synthesized ZAC-material Chemical element C 1s
O 1s
Zn 2p
Start BE
Peak BE
End BE
Height K CPS
FWHM, eV
%
298.08
284.49
277.08
217.009
2.08
94.95
544.08
532.25
526.58
24.169
4.26
4.47
1050.58
1022.08
1013.08
12.895
4.18
0.58
16
Table (2): XRD analyses of the designed ZAC material (ZnO-peaks) No.
1 2 3 4 5 6 7 8
2θ°
FWHM
Area [cts*2θ°]
d-spacing [Å]
Height [cts]
Rel. Int. [%]
31.7216
0.2273
420.03
2.82083
1873.25
70.80
34.3663
0.1948
200.77
2.60957
1044.64
39.48
36.2212
0.4546
1186.48
2.48008
2645.74
100.00
47.4571
0.1299
73.69
1.91583
575.13
21.74
56.5554
0.2598
192.62
1.62732
751.67
28.41
62.7848
0.1948
116.19
1.48002
604.53
22.85
67.8577
0.1624
82.18
1.3812
513.11
19.39
68.9181
0.396
132.11
1.36139
250.21
9.46
17
hkl
100 002 101 102 110 103 112 201
Table (3): Electrochemical impedance parameters of ZAC and AC as estimated from the fitted data of impedance spectra.
WE
Rh, Ω.cm2
Rct1, Ω.cm2
Rct2, Ω.cm2
W1-R
AC
1.43
2.09
0.024
452
(error, %)
(1.2)
(5.27)
(72.82)
(2.15)
ZAC
1.14
0.15
0.39
1051
(error, %)
(1.97)
(10.5)
(4.44)
(1.75)
18
4. Conclusion ZnO-nanoparticles at the surface of the activated carbon (ZAC) as an efficient rockshape material was chemically fabricated via the hydrothermal treatment and evaluated as the novel supercapacitor electrode material in acidic and alkaline electrolytes using KOH and H2SO4 as electrolyte media, respectively. The designed ZAC was fabricated via the one-step methodology, i.e., hydrothermal treatment in the aqueous medium, AC and Zn-salt followed by drying without calcination. The introduced ZAC was studied in terms of surface characterizations including FESEM, XPS, TEM-mapping, XRD, EDX, and FT-IR. Zn-2p and ZnO crystalline peaks were observed in XPS and XRD, respectively, in addition to the Zn-O bond in FT-IR analyses. XRD peaks exactly were in the same 2θ position of ZnO hexagonal crystals (P63mc). Furthermore, the hexagonal parameters for ZnO-NPs were found at 3.257 and 5.205 Å for a or b-parameter (a=b) and c crystal parameters, respectively. Additionally, the grain size of ZnO in ZAC was calculated via a Scherrer equation and found to be 18.41 nm. Therefore, surface characterizations suggested the successful incorporation of ZnO-NPs at AC to form simply-design ZAC material. The electrochemical performance including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and impedance plots (EIS) was studied to evaluate the ZAC material as supercapacitor electrode material. The specific capacitance (Cs) values of ZAC showed an improved capacitance performance if compared with the pristine AC at all the investigated scan rates and currents for CV and GCD, respectively. Furthermore, the EIS study reported the enhanced charge transfer, ion diffusion and phase angle of ZAC as compared to AC. In short, this study presented a simple strategy to improve the capacitance characteristics of AC based on the incorporation of ZnO-NPs.
19
Acknowledgement This work was financially supported by the National Natural Science Foundation of China
(21777105)
and
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Fig. 1. FESEM images of the prepared ZAC after the hydrothermal treatment at two different magnifications: (A) 5.0 KX and (B) 10 KX. (C) TEM-image and elemental mapping of C, O and Zn
Fig. 2. XRD analyses of the prepared ZAC at the range of 5o-90o
Fig. 3. XPS wide scan analyses of the prepared ZAC and the XPS core-level spectra of C 1s, Zn 2p and O 1s.
Fig. 4. FT-IR analyses of the prepared ZAC-material in the range of 400-4100 cm-1
Fig. 5. Cyclic voltammetry results for the pristine activated carbon (AC) and ZnO@AC (ZAC) at the scan rates of (A) 10 mV/s and (B)50 mV/s; Charge-discharge curves at I/m equal to (C)2 A/g and (D) 8 A/g for both AC and ZAC in alkaline media
Fig. 6. (A) Cyclic voltammograms (CV) at different scan rates (from 10 to 70 mV/s) by use of ZnO@AC as the electrode material in alkaline electrolytes; (B) Specific capacitance (Cs) values which were estimated via Equation (1) and CV-measurements; (C) Charge-discharge (GCD) curves at different I/m from 2 A/g to 10 A/g by use of ZnO @ AC as the electrode material in alkaline electrolytes; (D) Cs values estimated via Equation (2) and GCD-measurements.
Fig. 7. Cyclic voltammetry results for the pristine AC and ZnO@AC (ZAC) at the scan rates of (A) 50 mV/s and (B) 70 mV/s; Charge-discharge curves at I/m equal to (C) 2 A/g and(D) 6 A/g for both AC and ZAC in alkaline media.
Fig. 8. (A) Cyclic voltammograms (CV) at different scan rates (from 10 to 100 mV/s) by use of ZnO-NPs@AC as the electrode material in acidic electrolytes; (B) Specific capacitance (Cs) values which were estimated via Equation (1) and CV-measurements; (C) Charge-discharge (GCD) curves at different I/m from 2 A/g to 10 A/g by use of ZnO-NPs@AC as the electrode material in acidic electrolytes; (D) Cs values estimated via Equation (2) and GCD-measurements.
Fig. 9. Nyquist plots of (A) AC and (B) ZAC with and without fitting by use of the inset equivalent circuit; (C) Nyquist plots of AC and ZAC in acidic electrolyte; (D)
Bode phase analyses of AC and ZAC in alkaline electrolyte; and (E) and (F) cycling performance of the ZAC electrode at a current density of 10 A g−1 in alkaline and acidic media, respectively.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)32982-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.119
Reference:
CERI 23187
To appear in:
Ceramics International
Received Date: 31 August 2019 Revised Date:
8 October 2019
Accepted Date: 13 October 2019
Please cite this article as: I.M.A. Mohamed, A.S. Yasin, C. Liu, Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes, Ceramics International (2019), doi: https://doi.org/10.1016/ j.ceramint.2019.10.119. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Synthesis, surface characterization and electrochemical performance of ZnO @ activated carbon as a supercapacitor electrode material in acidic and alkaline electrolytes Ibrahim M. A. Mohamed a,b,c,, Ahmed S. Yasin b, Changkun Liu a,d,* a
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, PR China
b
Department of Bionanosystem Engineering, Jeonbuk National University, Jeonju, Jeonbuk 561756, Republic of Korea c
d
Department of Chemistry, Faculty of Science, Sohag university, Sohag 82524, Egypt
Shenzhen Key Laboratory of Environmental Chemistry and Ecological Remediation, Shenzhen University, Shenzhen 518060, PR China
Corresponding author: *Email: [email protected] (Changkun Liu).
1
Abstract The chemical and surface modification of activated carbon (AC) through the incorporation of nanoparticles (NPs) is a simple and efficient strategy to enhance the charge transfer as well as the capacitance behavior of AC. Herein, ZnO-NPs were successfully attached at the surface of AC via one-step hydrothermal process to form ZnO-NPs@AC (ZAC). The introduced ZACmaterial was studied by surface characterization techniques including transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) to investigate the morphological and chemical characteristics of ZAC. After that, ZAC was utilized as a working electrode (WE) material for supercapacitor in alkaline and acidic electrolytes. The specific capacitance (Cs) values of ZAC showed an enhanced performance as compared with that of the pristine AC. Moreover, the low series resistance and high Warburg co-efficient of ZAC as compared to those of the AC indicated the faster -OH diffusion inside the ZAC working electrode. Additionally, the phase angle of ZAC is closer to the ideal capacitor than that of AC. Overall, this study proved that ZnO-NPs can enhance the AC capacitance behavior through the easy chemical modification of AC.
Keywords: ZnO-NPs, Activated Carbon, Supercapacitor; Electrode Material
2
1. Introduction Supercapacitors are one of the favorable devices for energy storage, especially for medical and electrical applications which need high power and prolonged life [1]. These devices bridge between devices having power output and batteries storing energy [2]. However, the development of the energy demand stimulates researchers to fabricate novel supercapacitor electrodes having both improved cycle life and output power. Basically, supercapacitors have two types based on the charge character, namely: electrochemical double-layer (EDLC) and pseudocapacitive devices. EDLC electrodes are mainly C-based materials and the storage of energy is based on the ion adsorption [3]. On the other side, pseudocapacitive devices are composed of materials having reversible and fast faradic redox behavior such as transition-metal oxides [4]. Most of the literature focused on C-based materials including carbon nanotubes and activated carbon (AC) as an efficient supercapacitor which may be attributed to better conductivity, active surface area, porosity, and electrochemical stability [5-8]. Metal-oxides at the surface of C-based materials are more promising and attractive including RuO2 [9], IrO2 [10], MnOx [11], CoOx [12], Fe3O4 [13] and ZnO [14] which are introduced with good capacitive behaviors. These metal oxides enhance the capacitance behavior of C-based materials due to the increase of the wettability of the C-materials surface in the utilized electrolyte which enhances the interaction between the oxide ion and the electrolyte [15]. Among metal-oxides, ZnO shows excellent electrical and optical characteristics and hence has been applied for photovoltaics [16, 17], gas sensing [18], biocompatible corrosion inhibitor [19], photocatalysis [20] as well as supercapacitor application [21]. Theoretically, ZnO has a high capacitance performance. However, the insufficient conductivity at high current retards the ZnO3
supercapacitors [21, 22]. Therefore, the ZnO nanoparticles (ZNPs) have been introduced in combination with the C-substrates. Recently, many techniques including green synthesis [14], MOF-strategy [23] and electrospray [24] were reported for the fabrication of ZnO/C-composite electrodes towards supercapacitor applications. However, the residual impurities in the designed ZnO might decrease or decline the electrochemical performance and/or stability. Therefore, it is important to explore simple and one-step strategies to fabricate ZnO/C-composite electrode having high capacitive efficiency. Herein, ZnO/activated carbon (AC) composite was chemically fabricated via the easy and one-step methodology and investigated as the supercapacitor electrode. The material fabrication was based on the hydrothermal treatment of AC in the presence of Zn source. The morphology, crystallinity and chemical properties of the composite were studied through FESEM, TEM, XRD, TEM-EDX, and XPS. The fabricated composite was studied for the supercapacitor application in acidic (H2SO4) and alkaline (KOH) media. The electrochemical performance was investigated through cyclic voltammetry (CV) at different conditions including scan-rate, charge-discharge curves and electrochemical impedance spectroscopy (EIS). The introduced results indicated that the fabricated ZnO/AC material has a high performance towards supercapacitor application in both acidic and alkaline media. 2. Experimental 2.1. Materials and methods Zinc acetate, commercial activated carbon (CEP-21K, PCT Co., Korea), H2SO4, and KOH flake (Alfa Aesar) were utilized without any treatment except the preparation of H2SO4 and KOH solutions through the dilution by use of de-ionized water. Nafion solution was used 4
through electrochemical study and purchased from Aldrich Co. Ltd., USA. A glassy carbon electrode (GCE) owning an area of 0.071 cm2 was purchased from CH Instruments, Inc., USA. 2.2. Synthesis of ZnO NPs @activated carbon (ZAC) nanocomposite The ZnO NPs @ AC (ZAC) were chemically fabricated via a simple hydrothermal method in the presence of deionized water. Typically, 0.5 g of AC and 45 mg of Zn-acetate were added to 50 mL of deionized water and sonicated for 2 h. Thereafter, the obtained mixture was treated hydrothermally in a Teflon-lined autoclave for 12 h at 180 °C. The hydrothermal solution was filtered and washed 3 times by H2O until the filtrate became clear. The obtained solid material was dried at 60 °C under vacuum for 48 h. The fabricated ZAC was utilized as a supercapacitor electrode after the deposition at the glassy-carbon electrode (GCE). 2.3. Surface characterization The surface characterization of the fabricated ZAC was carried out to describe the morphology, crystallinity and physicochemical properties through the utilization of transmission electron microscope (TEM, JEOL JEM-2010, Japan) and field-emission scanning electron microscope (FESEM) (Hitachi S-7400, Japan). The crystallinity of ZAC material was tested via a Rigaku Xray diffractometer (XRD) (Rigaku Co., Japan) with Cu Kα (λ = 1.540 Å) radiation over a 2θ in the range of from 5° to 90°. The chemical content of ZAC nanocomposite was investigated by Xray photoelectron spectroscopy with Al Kα irradiation (XPS, AXIS-NOVA, Kratos, Inc.). The XPS wide scan was investigated in the range of 0 - 1300 eV and the XPS core-level spectra of Zn, C and O were also derived. 2.4. Supercapacitor electrochemical performance
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The electrochemical performance of the designed ZAC was studied using a 3-electrode (working, counter and reference electrodes) system at room temperature. The deposition of ZAC at GCE to design the utilized working electrode (WE) was carried out through the suspension solution of the investigated material in iso-propanol [25]. A Pt-wire was used as a counter or auxiliary electrode and Ag/AgCl as a reference electrode (RE) to fill the electrode system for checking the electrochemical performance. For ZAC-deposition on GCE, 2 mg of ZAC was suspended in 400 µL of 2-propanol and 20 µL of Nafion aqueous solution and then sonicated for 30 min. After that, 5 µL of the suspension was deposited onto the surface of WE. The deposition step was repeated 3 times to fill most of the electrode area (3 mm diameter, 0.071 cm2). The asdesigned WE electrode was dried gradually from room temperature to 70°C and kept at this temperature for 30 min. The as-prepared WE electrode was used to investigate the electrochemical performance of ZAC towards supercapacitor application in acid and alkaline media. Cyclic voltammetry (CV) survey scans of all experiments were taken in the range of - 0.2 V to 0.6 V. The specific capacity was estimated by use of CV and galvanostatic charge-discharge (GCD) curves in acid and alkaline media via the following relationships, respectively [26, 27]:
C =
C =
I dV (1) 2 υ m ΔV 2
V dt
(2)
m V l where Cs is the specific capacitance, ʃ I dV is the area of CV, υ is the scan rate of CV, m is the mass of the electroactive material and ∆V is the difference between the end and the start
6
potential for Equation (1). In addition, I-current is the discharge current in GCD-window and ʃ Vdt is the area under the discharge current for Equation (2). 3. Results and discussion 3.1. Surface analyses of ZAC material The morphology of ZAC material was investigated via FESEM and TEM as shown in Fig. 1. Fig. 1A and 1B display the 5K-X and 10K-X magnification of FESEM analyses of the prepared nanoparticles at the AC-surface. The surface morphology of ZAC is micro rock shape, with smooth, porous and homogeneous nature. This morphological character was proved through TEM-analyses (Fig. 1C). Also, the TEM-mapping indicated the presence of C, O, and Zn with different densities (C > O > Zn). Logically, the main chemical content of AC would be C which was easily observed according to the density of C in TEM-mapping. AC surface was known to have some oxygenated function groups which gave AC acid-basic behavior [28]. Thus, the O content was higher than Zn percentage. However, Zn-mapping was included in the mapping of O which indicated the presence of two types of O: one connected to Zn and the other one from the AC-function groups. The ZnO NPs were distributed in the form of ultrafine particles at the surface of AC without aggregation. The EDX of ZAC was studied via TEM as shown in Fig. S1 which confirmed that the C, Zn, and O element contents of ZAC were 96.04, 0.45, and 3.51, respectively. The surface chemistry of ZAC was investigated via XPS (Fig. 2 and Table (1)). The wide scan displays the presence of Zn, O, and C-elements at 1022.1, 532.25 and 284.49 eV, respectively. The carbon was the skeleton of AC, and hence the atomic percentage of the prepared ZAC was 94.95 % which was important for applications as the supercapacitor electrode 7
material because C-materials have an acceptable conductivity to be the electrode material beside its capacitive and electrocatalytic performance. The core-level XPS spectra for C, Zn and O elements were shown in Figs. 2B, 2C and 2D, respectively. The C 1s de-convolution showed three peaks at 284.08, 285.28 and 288.38 eV which were attributed to C-C, C-O and C=O bonds, respectively [29]. The ZnO formation at the AC surface was proved through the Zn 2p XPS spectra, as displayed in Fig. 2C. The two peaks were found and located at 1022.08 eV and 1045.38 eV which were assigned to Zn 2p3/2 and 2p1/2, respectively. The binding energy separation between Zn 2p3/2 and 2p1/2 peaks was 23.3 eV which indicated the +2 valence of Zn and proved that ZnO nanostructures were attached on the AC surface [30]. As shown in Fig. 2D, the O 1s deconvolution showed 3-different sub-peaks at 530.48, 532.18, 534.58 eV which were attributed to the O in ZnO, defective oxygen and chemisorbed oxygen, respectively [31]. To study the difference in chemistry between the bulk and the surface of the designed ZAC, the comparison between TEM-EDX (providing bulk concentration) and XPS (providing surface concentrations of elements) analysis was displayed in Table (S1). The contents of C, Zn and O were 96.04, 0.45 and 3.51 % respectively according to the TEM-EDX analyses. In comparison, the Zn and O contents increased to 0.58 and 4.47 % respectively with XPS analyses. The contents of Zn and O quantitatively increased at the surface of AC which confirmed the presence of more ZnO-NPs at the surface of AC. This can be attributed to the increasing rate of the nucleation step which was more than the rate in the growth step. Basically, the formation of crystals includes 2 major steps: nucleation and growth [32]. In the designed material, the nucleation rate was higher than the growth rate. As a result, the ZnO was formed at the surface of AC in the form of ultrafine particles. In short, the designed ZAC material has the chemistry of C attached to ZnO-NPs. 8
The successful design of ZnO-NPs at the AC-surface was also studied via the XRD analyses (Fig. 3 and Table (2)) to describe the crystallinity. Firstly, there were two broad peaks at 23.39o and 42.75o which corresponded to 200 and 100 planes of graphite (JCPDS 00-041-1487). The broad character of peaks indicated the amorphous structure of AC because X-rays could be scattered in different directions which led to the large bump located in the wide range of 2θ. Secondly, ZACXRD has crystalline peaks at 31.72°, 34.37, 36.22, 47.46, 56.56, 62.79, 67.86 and 68.92 which could be assigned to (100), (002), (101), (102), (110), (103), (112) and (201) planes, respectively. These peaks were exactly in the same 2θ position of ZnO hexagonal crystals and the space group was P63mc (00-036-1451) [33]. Furthermore, the hexagonal parameters for ZnO-NPs were calculated which were equal to 3.257 Å and 5.205 Å for a-parameter or b-parameter (a=b) and c crystal parameters, respectively. These parameters were calculated via the basic equation of hexagonal structure: 1/d2 = [(4/3) (h2+hk+k2) / a2] + l2/c2. The d-spacing of (hkl 100) and (hkl 101) can be utilized to estimate a, b and c parameters. Additionally, the ZnO crystallite size in ZAC was calculated via a Scherrer equation (D = (0.9λ)/(β cos θ) [34] where “λ” is X-rays wavelength (1.541874 Å), “β” is the full width at half maximum (FWHM) of the high-intensity peak (101-peak) and θ is the Bragg's angle of the radians and found at 18.40 nm (exactly 184.01 Å). Therefore, XRD suggested that hydrothermal treatment can be a successful strategy to design ZAC material. Previously, the hydrothermal method was proved as a simple and productive strategy for the design of metal-oxides @ AC materials [35]. Zn ions can be easily dissolved in aqueous solutions and re-deposited as oxide-NPs on the AC surface during the hydrothermal process. The functional groups at the surface of ZAC were studied via FT-IR as displayed in Fig. 4. The analyses showed 3 peaks at 1112, 1552 and 3421 cm-1 which were attributed to C-O, C=O and O-H bonds of AC, respectively [36]. The O-H peak was clearer and broader than the 9
expected because of the synthesis steps of ZAC which included only hydrothermal treatment without further calcination or carbonization. The FT-IR for ZnO region showed a small peak at 489 cm-1 in addition to the many peaks at 420 - 690 cm-1 which proved the presence of the Zn-O linkage [37]. The lower intensity of the Zn-O peaks could be attributed to the small content of ZnO NPs in ZAC as reported before in XPS and TEM-EDX analyses. Briefly, the FESEM, TEM, XRD, XPS, and FT-IR confirmed the successful fabrication of ZnO-NPs at the surface of AC through hydrothermal treatment. 3.2. Electrochemical performance in an alkaline electrolyte The designed ZAC material was used to fabricate the working electrode (WE) for the investigation of electrochemical performance as the supercapacitor electrode material. The electrochemical performance was tested via CV, CD, and EIS in alkaline using KOH and H2SO4 aqueous media. The CV curves were carried out with a range of potential from -0.2 to 0.6 V by use of pristine AC and ZAC at different scan rates including 10 mV/s (Fig. 5A), 20 mV/s (Fig. S2), 50 mV/s (Fig. 5B) and 70 mV/s (Fig. S3). The CV analyses of both AC and ZAC showed no redox peaks. Additionally, the shape of CV in the case of ZAC was rectangular which could be considered as close to an ideal supercapacitor which displayed its capacity for storage of valuable amounts of charges. The redox peaks could be attributed to pseudo-capacitance which was not shown in the case of the designed ZAC material. Basically, the CV area was related to the capacitance performance of the studied electrode material. The area of the ZAC curve was 8.01 x 10-4 which was higher than an AC curve (4.06 x10-4) at 10 mV/s. The enhanced CV area was seen in Fig. 5 which proved that ZAC showed higher performance for supercapacitor than the pristine AC without ZnO-NPs. The GCD curves were shown in Fig. 5C and 5D for both AC
10
and ZAC at different I/m values started from 2 A/g (Fig. 5C), 4 A/g (Fig. S4), 6 A/g (Fig. S5) and 8 A/g (Fig. 5D). The GCD plots presented the symmetric character with nearly linear behavior, which proved the enhanced capacitance of ZAC as a supercapacitor electrode material. The discharge time as an indicator for capacitance character was enhanced from 25.1 s to 98.7 s at 2 A/g for AC and ZAC, respectively. The improved performance was observed at all studied I/m: 4, 6 and 8 A/g. Fig. 6A showed the CV of ZAC in an alkaline electrolyte at different scan rates (from 10 to 70 mV/s). As the scan rate increased, the CV area was enhanced at all investigated scan rates. The specific capacitance was estimated via Equation (1) using the CV data and their integration (Fig. 6B). The specific capacitance (Cs) values of ZAC showed an enhanced performance if compared with the pristine AC. At 10 mV/s, the Cs increased from 355.05 F/g for AC to 667.18 F/g for the prepared ZAC material. This improvement was easily observed at all the investigated scan rates. Additionally, the Cs value of ZAC at 10 mV/s retained nearly 43.5 % (290.12 F/g) of its initial value after increasing the scan rate up to 70 mV/s, which demonstrated the improved capacitance behavior of ZAC-material. Fig. 6C showed the GCD of ZAC in an alkaline electrolyte at different I/m values (from 2 to 8 A/g). As the current decreased, the discharge time increased at all the investigated currents. The specific capacitance was estimated by Equation (2) using the GCD data and the discharge curve integration (Fig. 6D). The Cs values of ZAC showed an improved capacitance performance if compared with the pristine AC. At 2 A/g, the Cs of ZAC was enhanced from 94.4 F/g to 349.6 F/g. This capacitance enhancement was easily observed at all the investigated currents (2-20 A/g). Additionally, the Cs of ZAC at 2 A/g retained nearly 57.12 % (199.72 F/g) of its value after increasing the current up to 20 A/g. The enhanced Cs values in an alkaline medium demonstrated the high capacitance performance of the ZAC-material. 11
3.3. Electrochemical performance in an acidic electrolyte The introduced ZAC material was evaluated as a WE material for supercapacitor in an acidic medium beside in an alkaline medium. The electrochemical including CV and GCD were investigated as displayed in Fig. 7 and 8 by use of H2SO4 aqueous solution as the acid source of electrolyte. The CV studies were carried out for AC and ZAC material at various scan rates: 10 mV/s (Fig. S6), 20 mV/s (Fig. S7), 50 mV/s (Fig. 7A) and 70 mV/s (Fig. 7B). The CV of both AC and ZAC in an acidic electrolyte showed no redox peaks in addition to the rectangular shape of the CV of ZAC material at high scan rates (50 mV/s and 70 mV/s) which indicated the high performance of ZAC as a supercapacitor electrode material. Based on the CV data, the area under ZAC curve was 4.92 x 10-4 which was better than what was obtained from AC curve (4.05 x10-4) at the same scan rate of 10 mV/s. The increase of the CV area or ʃ I dV was easily seen in Fig. 7A and 7B at high scan rates (50 and 70 mV/s) which proved that the incorporation of ZnO NPs at AC was a successful strategy to enhance the capacitance character. The GCD data were shown in Fig. 7C and 7D for both AC and ZAC at different current (I/m) values including 2 A/g (Fig. 7C), 4 A/g (Fig. S8), 6 A/g (Fig. 7D) and 8 A/g (Fig. S9). The presented GCD plots showed symmetric character with nearly triangular character, reflecting the fast charging process. The discharge time which had a direct relation to the capacitance performance was enhanced from 26.78 to 150.40 s at 2 A/g for AC and ZAC, respectively. The improved performance was observed in CV analyses as well as GCD plots at different currents: 4, 6 and 8 A/g. Fig. 8A showed the CV of ZAC in an acidic electrolyte at different scan rates (from 10 to 100 mV/s). As the scan rate increased, the CV area or ʃ I dV was enhanced at all the presented scan rates. The Cs values were calculated via Equation (1) using the CV data and shown in Fig. 8B. The Cs values of ZAC were better than the values of the pristine AC. At 10 mV/s, the Cs increased from 12
354.19 F/g for AC to 430.54 F/g for ZAC material. This enhancement was easily seen at all the presented scan rates from 10 to 100 mV/s. Additionally, the Cs value of ZAC at 10 mV/s retained nearly 49.3 % (212.19 F/g) of its initial value after increasing the scan rate up to 100 mV/s, which proved the improved capacitance behavior of AC after modification by ZnO NPs. Fig. 8C showed the GCD of ZAC in an acidic electrolyte at different current values (from 2 to 8 A/g). As expected, the discharge time decreased as I/m values increased. The specific capacitance was calculated by Equation (2) using the integration of the discharge curve (Fig. 8D). The Cs values of AC were improved after hydrothermal modification by ZnO-NPs to form ZAC material. At 2 A/g, the Cs of ZAC was enhanced to 404.73 F/g from 102.38 F/g. This capacitance enhancement was also seen at current values from 2 to 20 A/g. The Cs of ZAC retained only 24.83 % (100.49 F/g) of its initial value after increasing the current from 2 to 20 A/g. The CV and GCD demonstrated the enhanced capacitance performance of AC after hydrothermal treatment to design ZAC-material. 3.4. EIS and stability analyses EIS is one of the most attractive and powerful techniques to prove and understand the electrochemical performance of novel electrode materials such as the prepared ZAC. Fig. 9A and 9B showed the Nyquist and bode-phase plots from 0.1 to 104 Hz in alkaline electrolytes. The inset equivalent circuit was utilized to fit the data and compared with the pristine impedance data. The resistance and Warburg parameters (Rh, Rct1, Rct2, and W) of the introduced circuit were shown in Table (3). The Rh, Rct1 and Rct2 are corresponding to the electrolyte resistance, ionic charge-transfer resistance and electron-transfer resistance, respectively [38]. The difference between the fitted and the pristine Nyquist curves was very small which could be easily
13
neglected, indicating the successful fitting of the impedance data. Additionally, the relative errors of the Rh and Warburg were small (< 2%). ZAC and AC Nyquist plots could be classified into three regions based on the frequency level (Rh, Rct1/CPE1, Rct2/CPE2-W). The left one was related to the ohmic resistance in the utilized electrolyte and the right one (long tail) which was assigned to the ion diffusion from the electrolyte to electrode [39, 40]. The intermediate one (Rct1/CPE1) was the charge transfer resistance between the used WE and the electrolyte. Both investigated electrodes showed the same behavior in the charge transfer which may be due to the use of the same electrolyte (KOH medium) owning high electrical conductivity. The Rh and Rct1 values of ZAC were slightly lower than the values of AC which indicated the improved internal conductivity of WE. The long tail or ion diffusion impedance could be represented via the Warburg element. The higher Warburg co-efficient of the ZAC WE as compared to that of AC WE indicated the faster -OH movement to form the electrical double layer [41]. Additionally, the Bode plots were studied for AC and ZAC WE as shown in Fig. 9D. The ZAC phase angle was found at 55.13o which was better than that of AC (43.41o). Basically, the known ideal behavior of any capacitor should have a 90o phase angle which was not realized in both AC and ZAC in this study [42]. ZAC angle was probably closer to the ideal than the AC one. The introduced EIS study indicated the enhancement of the capacitance behavior of AC after ZnO-NPs incorporation. Cycling stability is considered as one of the critical factors for promoting the commercial application of supercapacitors. To further study the electrochemical performance of the prepared material, cycling performance of ZAC material was evaluated at a current density of 10 A g−1 for 3000 successive charge-discharge cycles in alkaline and acidic media, and the results were displayed in Fig. 9E and 9F. The capacitance retention % was calculated from the capacitance of 14
the cycles by use of the ratio between the initial and following cycles. The first cycle was considered as 100 % and then the capacitive retention % was calculated as a percentage of the first cycle. The as-prepared ZAC retained about 89.17 % of the capacity after 3000 cycles in KOH medium, and 90.15 % of the capacity was retained after 3000 cycles in H2SO4 medium. These results further confirmed that the ZAC electrode possessed exceptional cycling stability which might be derived from the incorporation of zinc oxide into the activated carbon.
15
Table (1): XPS analyses of the synthesized ZAC-material Chemical element C 1s
O 1s
Zn 2p
Start BE
Peak BE
End BE
Height K CPS
FWHM, eV
%
298.08
284.49
277.08
217.009
2.08
94.95
544.08
532.25
526.58
24.169
4.26
4.47
1050.58
1022.08
1013.08
12.895
4.18
0.58
16
Table (2): XRD analyses of the designed ZAC material (ZnO-peaks) No.
1 2 3 4 5 6 7 8
2θ°
FWHM
Area [cts*2θ°]
d-spacing [Å]
Height [cts]
Rel. Int. [%]
31.7216
0.2273
420.03
2.82083
1873.25
70.80
34.3663
0.1948
200.77
2.60957
1044.64
39.48
36.2212
0.4546
1186.48
2.48008
2645.74
100.00
47.4571
0.1299
73.69
1.91583
575.13
21.74
56.5554
0.2598
192.62
1.62732
751.67
28.41
62.7848
0.1948
116.19
1.48002
604.53
22.85
67.8577
0.1624
82.18
1.3812
513.11
19.39
68.9181
0.396
132.11
1.36139
250.21
9.46
17
hkl
100 002 101 102 110 103 112 201
Table (3): Electrochemical impedance parameters of ZAC and AC as estimated from the fitted data of impedance spectra.
WE
Rh, Ω.cm2
Rct1, Ω.cm2
Rct2, Ω.cm2
W1-R
AC
1.43
2.09
0.024
452
(error, %)
(1.2)
(5.27)
(72.82)
(2.15)
ZAC
1.14
0.15
0.39
1051
(error, %)
(1.97)
(10.5)
(4.44)
(1.75)
18
4. Conclusion ZnO-nanoparticles at the surface of the activated carbon (ZAC) as an efficient rockshape material was chemically fabricated via the hydrothermal treatment and evaluated as the novel supercapacitor electrode material in acidic and alkaline electrolytes using KOH and H2SO4 as electrolyte media, respectively. The designed ZAC was fabricated via the one-step methodology, i.e., hydrothermal treatment in the aqueous medium, AC and Zn-salt followed by drying without calcination. The introduced ZAC was studied in terms of surface characterizations including FESEM, XPS, TEM-mapping, XRD, EDX, and FT-IR. Zn-2p and ZnO crystalline peaks were observed in XPS and XRD, respectively, in addition to the Zn-O bond in FT-IR analyses. XRD peaks exactly were in the same 2θ position of ZnO hexagonal crystals (P63mc). Furthermore, the hexagonal parameters for ZnO-NPs were found at 3.257 and 5.205 Å for a or b-parameter (a=b) and c crystal parameters, respectively. Additionally, the grain size of ZnO in ZAC was calculated via a Scherrer equation and found to be 18.41 nm. Therefore, surface characterizations suggested the successful incorporation of ZnO-NPs at AC to form simply-design ZAC material. The electrochemical performance including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and impedance plots (EIS) was studied to evaluate the ZAC material as supercapacitor electrode material. The specific capacitance (Cs) values of ZAC showed an improved capacitance performance if compared with the pristine AC at all the investigated scan rates and currents for CV and GCD, respectively. Furthermore, the EIS study reported the enhanced charge transfer, ion diffusion and phase angle of ZAC as compared to AC. In short, this study presented a simple strategy to improve the capacitance characteristics of AC based on the incorporation of ZnO-NPs.
19
Acknowledgement This work was financially supported by the National Natural Science Foundation of China
(21777105)
and
the
Shenzhen
Science
and
Technology
Foundations
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24
Fig. 1. FESEM images of the prepared ZAC after the hydrothermal treatment at two different magnifications: (A) 5.0 KX and (B) 10 KX. (C) TEM-image and elemental mapping of C, O and Zn
Fig. 2. XRD analyses of the prepared ZAC at the range of 5o-90o
Fig. 3. XPS wide scan analyses of the prepared ZAC and the XPS core-level spectra of C 1s, Zn 2p and O 1s.
Fig. 4. FT-IR analyses of the prepared ZAC-material in the range of 400-4100 cm-1
Fig. 5. Cyclic voltammetry results for the pristine activated carbon (AC) and ZnO@AC (ZAC) at the scan rates of (A) 10 mV/s and (B)50 mV/s; Charge-discharge curves at I/m equal to (C)2 A/g and (D) 8 A/g for both AC and ZAC in alkaline media
Fig. 6. (A) Cyclic voltammograms (CV) at different scan rates (from 10 to 70 mV/s) by use of ZnO@AC as the electrode material in alkaline electrolytes; (B) Specific capacitance (Cs) values which were estimated via Equation (1) and CV-measurements; (C) Charge-discharge (GCD) curves at different I/m from 2 A/g to 10 A/g by use of ZnO @ AC as the electrode material in alkaline electrolytes; (D) Cs values estimated via Equation (2) and GCD-measurements.
Fig. 7. Cyclic voltammetry results for the pristine AC and ZnO@AC (ZAC) at the scan rates of (A) 50 mV/s and (B) 70 mV/s; Charge-discharge curves at I/m equal to (C) 2 A/g and(D) 6 A/g for both AC and ZAC in alkaline media.
Fig. 8. (A) Cyclic voltammograms (CV) at different scan rates (from 10 to 100 mV/s) by use of ZnO-NPs@AC as the electrode material in acidic electrolytes; (B) Specific capacitance (Cs) values which were estimated via Equation (1) and CV-measurements; (C) Charge-discharge (GCD) curves at different I/m from 2 A/g to 10 A/g by use of ZnO-NPs@AC as the electrode material in acidic electrolytes; (D) Cs values estimated via Equation (2) and GCD-measurements.
Fig. 9. Nyquist plots of (A) AC and (B) ZAC with and without fitting by use of the inset equivalent circuit; (C) Nyquist plots of AC and ZAC in acidic electrolyte; (D)
Bode phase analyses of AC and ZAC in alkaline electrolyte; and (E) and (F) cycling performance of the ZAC electrode at a current density of 10 A g−1 in alkaline and acidic media, respectively.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: