Accepted Manuscript Excellent electrochemical behavior of graphene oxide based aluminum sulfide nanowalls for supercapacitor applications Muhammad Faisal Iqbal, Muhammad Naeem Ashiq, Mahmood-Ul Hassan, Rahat Nawaz, Aneeqa Masood, Aamir Razaq PII:
S0360-5442(18)31188-5
DOI:
10.1016/j.energy.2018.06.123
Reference:
EGY 13165
To appear in:
Energy
Received Date: 8 March 2018 Revised Date:
28 May 2018
Accepted Date: 18 June 2018
Please cite this article as: Iqbal MF, Ashiq MN, Hassan M-U, Nawaz R, Masood A, Razaq A, Excellent electrochemical behavior of graphene oxide based aluminum sulfide nanowalls for supercapacitor applications, Energy (2018), doi: 10.1016/j.energy.2018.06.123. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Excellent Electrochemical Behavior of Graphene Oxide based Aluminum Sulfide Nanowalls for Supercapacitor Applications Muhammad Faisal Iqbalabc*, Muhammad Naeem Ashiqd, Mahmood-Ul-Hassana, Rahat Nawazd, Aneeqa Masoode, Aamir Razaqe* Materials Growth and Simulation Laboratory, Department of Physics, University of The
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a
Punjab, Lahore 54590, Pakistan b
International Center for Materials Nanoarchitectonics (MANA), National Institute for
c
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Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Department of Physics, Lahore Garrison University, Sector C, DHA Phase-VI Lahore
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54000, Pakistan d
Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan
e
Department of Physics, COMSATS University Islamabad, Lahore Campus 54000, Pakistan
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* = Corresponding Author
E-mail:
[email protected] Muhammad Faisal Iqbal),
[email protected] (Aamir Razaq)
Graphical Abstract
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Phone #: 0092-336-0986638, 0092-3107516125
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ACCEPTED MANUSCRIPT Abstract Graphene oxide based electrode materials show remarkable electrochemical properties due to the improved specific surface area and electrical conductivity suggesting supercapacitor applications. Hydrothermally synthesized graphene oxide based aluminum sulfide nanowalls
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on nickel foam (NF) have revealed excellent pseudocapacitive behavior with the specific capacitance 2362.15 Fg-1 at 2mVs-1 as observed through cyclic voltammetry. The galvanostatic charge-discharge measurements confirmed a specific capacitance 2373.51 Fg-1
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at 3 mAcm-2. Hexagonal phase of the graphene oxide (GO) based Al2S3 nanowalls also showed good discharge time 820 s and energy density 118.68 WhKg-1 at 3 mAcm-2.
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Moreover, the fabricated electrode material exhibited good power density 2663.58 WKg-1 at 20 mAcm-2. The impedance results also confirmed pseudocapacitive characteristics and revealed weak contact and Warburg resistances for the electrode material in half cell. Hence, GO layer made the Al2S3 nanowalls as prominent electrode material for asymmetric
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supercapacitors. Additionally, excellent symmetric behavior was also found to be exhibited that again suggested supercapacitor applications.
Supercapacitor,
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Key words: Aluminum sulfide, Energy density, Graphene oxide, Specific capacitance,
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1. Introduction
The demand of supercapacitors is growing continuously for technical applications in electrical vehicles, which is due to the growing market interests to reduce the traditional fossil fuels dependence [1-5]. Based on the charge storing procedure [6-9], supercapacitors are called as electric double layered capacitors (EDLCs) if electrostatic charge is stored, and pseudocapacitors if reversible Faradic reaction occur at the electroactive surface of the electrodes [10-12].
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ACCEPTED MANUSCRIPT Electrode materials are the fundamental component responsible to reveal the optimum characteristics of the supercapacitors. Various electrode materials being investigated for supercapacitor applications include the carbonaceous based structures, transitions metal oxides (TMOs), hydroxides, conducting polymers etc. [9, 13-17]. However, there are inherent
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deficiencies such as limited specific capacitance (Scp) of carbonaceous structures, low electrical conductivity exhibited by TMOs [18-20], poor cycling-stability and limited discharge time demonstrated by the conducting polymers (which also involve complex
power densities of the designed supercapacitors.
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growth procedures) [21]. Such a variety of limitations result in suppressed Scp, energy and
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The metal sulfides being cheaper and easy to fabricate, exhibit best electrical conductivity, are attractive for technical applications in supercapacitor. In-spite of a variety of attempts employed to optimize the electrical conductivity and enhance the mechanical stability for realizing the electrochemically active electrode materials for practical supercapacitor
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applications [4, 22], the use of graphene/graphene oxide (GO) have found to be more effective one [23-27]. Furthermore, various composites of graphene/GO and metal sulfides have been explored for electrochemical applications e.g., NiS/GO [28], nickel
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sulfide/graphene/carbon nanotube [18], hollow Co3S4 nanospheres fabricated on the reduced
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graphene oxide (rGO) [29], GO based strontium sulfide nanorods [30] and MoS2 on graphene [31]. The light-weight metal sulfides especially Al2S3 based material systems are still less investigated for electrochemical applications [32]. Aluminum sulfide appears in different phases such as cubic, hexagonal, tetragonal and rhombohedral etc. Although the electrochemical behavior of active aluminum sulfide has been found appropriate for energy storage applications, however, efficiency is still needed to be further improved [33]. In the present work, aluminum sulfide (Al2S3) nanowalls have been synthesized on nickel foam (NF) substrate with an already fabricated GO layer to employ as an electrode. Especially, the
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ACCEPTED MANUSCRIPT symmetric behavior of GO based Al2S3 nanowalls has been elucidated for exploring the supercapacitor applications. The prepared product was studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) for performing the structural and morphological analyses, respectively. While the electrical conductivity was measured using the four-probe
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set-up. The electrochemical behaviors were elucidated using cyclic voltammetry,
which have been measured using autolab PGSTAT12. 2. Experimental Section 2.1 Fabrication of Aluminum Sulfide3 Nanowalls
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galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS),
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The graphene oxide (GO) synthesized using modified Hummers and Offenmans method was deposited as thin film on the nickel foam (NF) substrate using ultra-sonication process, which already has been elaborated in our previous work [30]. For fabricating the GO based aluminum sulfide nanowalls hydrothermal technique was applied. The NF with pre-deposited
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GO layer was dipped into 0.1 M Al(NO3)3.9H2O solution in a canonical flask with magnetic stirring for 45 min. At the same time, 0.15 M K2S solution was dropped using burette that resulted Al2S3 precipitates formation. A proposed chemical reaction is given in Eq. 1.
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2 Al(NO ) . nH O() + 3 K S() Al S + 6 KNO + nH O (1)
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Cetyltrimethylammonium bromide (100 g) was also mixed as a surfactant. The prepared precipitates along with the immersed substrate were placed into the autoclave that was subjected to 120 ºC for 4 h using an electric oven. The whole employed synthesis scheme is summarized in Fig. 1. Fig. 1 illustrates that for constructing the basic formula unit, during hydrothermal process, Al(NO3)3.nH2O is dissociated for preparing two Al3+ ions, which readily react with three S2- ions dissociated from K2S (aq) to form Al2S3. Moreover, the nonuniform and random distribution of the growth centers on the surface of GO on NF substrate resulted some preferential sites for further growth that results in the nanowalls like growth of
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ACCEPTED MANUSCRIPT Al2S3. For a comparison study, NF substrates without pre-deposited GO thin film were treated in a separate autoclave at same the conditions and solution composition to synthesize individual Al2S3 on NF substrate. The final product was separated, and after cleaning using deionized water was dried at 50 ºC for 30 min. After the hydrothermal reaction, the active
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masses measured for individual Al2S3 and GO based Al2S3 were measured as 4.9 and 7.2 mg-
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cm-2, respectively.
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Figure 1: The schematic diagram showing the steps employed for the synthesis of GO based Al2S3 nanowalls on NF substrate.
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2.2 Characterization
The structural and morphological studies of the individual GO and Al2S3 thin layers, and GO based Al2S3 prepared on NF were performed with X-ray diffraction (JDX-3532-JEOL, using Cu-Kα rays) and scanning electron microscopy (JEOL SM6490), respectively. The electrical properties were measured at room temperature (RT) by employing four-probe method (ISO 9001-2000 SVS/NC). The electrochemical behaviors were explored at RT using Autolab PGSTAT12 (by employing three electrodes system). The electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) 5
ACCEPTED MANUSCRIPT analyses were done to reveal the electrochemical behavior of the fabricated electrode material in a 2 M KOH electrolyte solution. To perform electrochemical study, Ag/AgCl, platinum wire and the prepared material were employed as the reference, counter and working electrodes, respectively. Moreover, two electrode based symmetric system was also applied
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to reveal the electrochemical nature of individual Al2S3 and GO based Al2S3 fabricated on the NF substrate. 3. Results and Discussion
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The XRD line-scan, as shown in Fig. 2, confirmed that GO based of Al2S3 prepared on NF substrate exhibit hexagonal phase, which was due to the diffraction peaks appeared at 2θ
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values 22.39, 25, 30.12, 37.765, 52.71 and 77.051°, which are consistent with the standard JCPDS card: 01-084-2321 for hexagonal Al2S3. Whereas the XRD peak at 12.07° was found corresponding to GO thin film [34]. The XRD peak at 45.31° and shoulders at 52.765° and 77.051° ( see inset in Fig. 2) were found corresponding to NF substrate. The XRD line-scan
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measured for individual Al2S3 on NF substrate plotted in Fig. S1 has been found according to
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JCPDS: 01-084-2321, again confirming the hexagonal phase.
Figure 2: The XRD pattern for GO based Al2S3 nanowalls fabricated onto the NF substrate. The inset figure show shoulders of peaks representing Ni foam substrate.
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ACCEPTED MANUSCRIPT The surface morphological characteristics, in terms of size, shape, of the synthesized materials has a dominating affect over the exhibited physical and chemical natures [35]. Therefore, morphological characterization was done using scanning electron microscopy (SEM), and the measured images are given in Fig. 3. The SEM images of varying
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magnifications, as given in Fig. 3 (a-d), revealed the interconnected Al2S3 nanowalls (few nanometers wide) network formed when fabricated on the GO thin film. Approximately, similar kind of surface morphology has also been reported for ZnO, CdS and Co3O4, which
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were studied to expose the electrochemical properties [36-39]. The SEM images measured for individual Al2S3, as given in Fig. 3 (e-f), were measured at two different magnifications
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X30, 000 and X50, 000, respectively. It was evident that an incomplete reaction results partial growth of individual Al2S3 because nanowalls network was not observed at the surface. However, the use of GO under layer resulted a complete Al2S3 growth, which elucidated the importance of the application GO thin film. Hence, GO under layer and the
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ultra-sonication process provided active positions for the proper growth of Al2S3 because sticking of incoming particles occurs at some preferential site due to the increased adhesion that caused nanowalls growth. Therefore, absence of GO layer caused weakened adhesion
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between NF and Al2S3, due to which, incomplete reaction preceded improper growth that
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resulted the absence of nanowalls.
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Figure 3: SEM images measured for GO based Al2S3 nanowalls, prepared on NF substrate, at various magnifications (a) X10,000 (b) X30,000 and (c) X50,000 and (d) X100,000. The SEM images for individual Al2S3 on NF substrate at the magnifications (e) X30,000 and (f) X50,000 are also given.
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ACCEPTED MANUSCRIPT The apparent worth for measuring the electrical conductivity is due to its significant role for enhancing the electrochemical performance of the electrode materials to realize practical applications in supercapacitors [40-42]. The electrical conductivities were found using fourprobe method by observing the I-V characteristics, which are shown in Fig. 4. The electrical
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conductivities extracted for individual GO and thin films and GO based Al2S3 nanowalls has been determined as 106.62, 120.30 and 128 Scm-1, respectively. The p-type and n-type natures of GO and Al2S3 after deposition caused Fermi level to shift towards the conduction
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band, and therefore, many active cites for charge carriers were offered and resulted improved electrical conductivity of GO based Al2S3 nanowalls compared with that by the individual
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Al2S3.
Figure 4: The I-V characteristics, of GO based Al2S3 nanowalls, individual GO and Al2S3 layers fabricated on NF substrate, measured using four-probe method.
The electrochemical natures are revealed by measuring CV, EIS and the galvanostatic discharge mechanism of GO based Al2S3 nanowalls using Autolab PGSTAT12. Firstly, GO based Al2S3 nanowalls was tested as an electrode for half-cell, so first three electrode setup was employed in 2 M KOH electrolyte. The CV curves for individual Al2S3 and GO based Al2S3 nanowalls measured at 2, 5, 10, 20 and 25 mVs-1 are shown in Fig. 5(b-c), respectively. 9
ACCEPTED MANUSCRIPT A comparison of the CV curves of the thin layers of GO, individual Al2S3 and Al2S3 nanowalls on GO measured at a scan rate 10 mVs-1 are also shown in Fig. 5(a), where it is evident all three show well-defined redox peaks confirming the dominant faradic reaction that suggest pseudocapacitive nature the fabricated electrode [43]. Furthermore, a smaller
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difference in the potentials at the peaks as compared to that for the batteries, and the linearly varying current with the square root of the scan rate exhibited by Al2S3 nanowalls fabricated on GO (see Fig. 5 (d)) confirmed that the surface controlled reactions demonstrate a
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pseudocapacitors like nature [44, 45]. Fig. 5 (a) shows that current density for Al2S3 nanowalls on GO is higher than that by the individual layers of GO and Al2S3, which
S =
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illustrates the worth of the use of GO under layer. The Scp is measured using Eq. 2: IdV !
m ∗ S ∗ ∆V
(2)
The symbols I and dV show current and voltage difference, which are extracted from the
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anodic and cathodic peaks, m is active mass, S shows scan rate and ∆V is potential window. The parameters Va and Vc denote the anodic and cathode peaks potential, respectively, determined from the CV curves for finding the redox potential dV. The Scp for Al2S3
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nanowalls on GO was observed as 2362.15, 2286.33, 2075.34, 1890.30, 1745.42 and 1598.80 Fg-1 at 2, 5, 10, 15, 20 and 25 mVs-1, respectively, which are higher than those observed for
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other metal sulfides [4, 46]. The Scp was found decreasing with the scan rate, which may be due to the higher scan rates those result more polarization and make the material more resistive [47]. Furthermore, the high electrical conductivity and the nanostructured network of Al2S3 nanowalls significantly influenced Scp. The observed Scp for individual Al2S3 was found as 1097.88, 1036.78, 1027, 951.83, 909.18 and 858 Fg-1 at corresponding scan rates of 2, 3, 5, 10, 15, 20 and 25 mVs-1. The electrochemical performance of GO thin layer has been already presented in our previous reports [30]. Hence, the Scp measured for Al2S3 nanowalls on GO was found significantly higher than that by either individual GO or Al2S3 layers, 10
ACCEPTED MANUSCRIPT which was found obviously due to the use of GO thin layer. The use of GO thin layers introduced active sites for Al2S3 nanowalls making easy paths for the carrier flow that
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improved that conductivity, which actually increases Scp.
Figure 5: The CV curves for the (a) comparison of individual GO and Al2S3 thin film and GO based Al2S3 nanowalls at the scan rate 10 mVs-1. The CV curves measured at varying
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scan rates for (b) individual Al2S3 and (c) GO based Al2S3 nanowalls on the NF substrate. (d) Redox peak current plotted against square root of the scan rates for GO based Al2S3
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nanowalls.
The Galvanostatic charge/discharge (GCD) for individual Al2S3 and Al2S3 nanowalls was measured using three electrodes electrochemical set up. The GCD profile measured for Al2S3 nanowalls on GO layer and individual Al2S3, as shown in Fig. 6 (a, b), revealed GCD time 1820, 1147, 534, 338, 204 s and 733, 502, 232, 144, 101 s at 3, 5, 10, 15 and 20 mAcm-2, respectively. Furthermore, the plateau region of the GCD profile was also found wellmatched with the CV curves and confirmed pseudocapacitive behavior of electrode materials.
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ACCEPTED MANUSCRIPT The plateau region of GCD is distinct than the rectangular shape for EDLCs and the linear trend for batteries, instead its behavior was found between the both that confirmed the pseudocapacitive nature Al2S3 nanowalls suggesting technical utilities in supercapacitors [11, 44]. The Al2S3 nanowalls showed some surface controlled reaction, which is shown in
Al S + OH %& ' Al S OH + e%&
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Eq. 3, which was suggested by R. Ramachandran et al for ZnS/G composite [48]. (3)
As the GCD time was found decreasing with current density, which was justified due to the
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enhanced excitation carriers making the electrode materials more resistive. GCD time for individual Al2S3 (Fig. 6b) at 3 mAcm-2 was found exhibiting maximum of 733 s.
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The Al2S3 nanowalls fabricated on GO confirmed the large GCD time than that by the individual Al2S3 at all the similar current densities, which may due to the relatively better electrical conductivity of Al2S3 nanowalls resulted by the incorporation of GO under layer. The Scp was also measured from GCD profile using Eq. 4: I
(4)
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S =
m∗(
∆V ) dt
The applied current is represented by I, dV/dt shows slope of the discharge plateau and m
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expressed electrode active mass. The Scp for Al2S3 nanowalls measured at the current density 3, 5, 10, 15 and 20 mAcm-2, from the GCD profile, was found as 2373.51, 2159.45, 1850.40
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and 1572 Fg-1, respectively, and which have been also found well-matched with those measured from the CV curves elucidating the accuracy and precision of the presented results. Therefore, it is apparent that ions and electrons could not gain enough time for insertion/ desertion at the electrode surface under the influence of higher current densities, and such a factor develop the resistive behavior by exhibiting more polarization at the higher current density [49]. The Scp for individual Al2S3, at the applied current density 3, 5, 10, 15 and 20 mAcm-2, were found as 1108.25, 1051.35, 1021.20, 1004.34 and 981.14 Fg-1, respectively,
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ACCEPTED MANUSCRIPT which was found lower than that by the Al2S3 nanowalls. Therefore, higher Scp for Al2S3 nanowalls on GO thin film can be caused by the higher electrical conductivity and the surface area as compared to that by GO layer. The higher electrical conductivity of the Al2S3 nanowalls with unique surface characteristics were justified by the intrinsic active sites for
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free carriers those generate easy path for the conduction and resulted the suppressed tendency of the Al2S3 nanowalls on GO to get polarized causing high Scp for the electrode structure. The energy and power density of GO based Al2S3 nanowalls were also calculated at the
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similar current densities from the galvanostatic discharge time using Eq. 5 & 6, respectively [50]:
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1 E = C. (∆V) (5) 2 E P = (6) t
Where Scp is the measured capacitance, ∆V and t show potential window and the discharge
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time, respectively. The measured power densities of GO based Al2S3 was determined as 521, 774.3, 1435.66, 1965 and 2663.60 WKg-1 at the energy density 118.68, 108, 92.52, 78.60 and 74.73 WhKg-1, respectively. The energy density measured for the fabricated GO based Al2S3
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nanowalls electrode structure was found relatively higher than by the other electrodes [5154]. The energy density for individual Al2S3 was measured as 55.41, 52.57, 51.06, 50.22 and
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49.06 WhKg-1. While the delivered power density was 604.50, 996.02, 1976.50, 3171.61 and 4415.14 Wkg-1 at 3, 5, 10, 15 and 20mAcm-2, respectively. Moreover, the maximum Scp and energy density up to 1108.25 Fg-1 and 55.41 WhKg-1, respectively, have been exhibited by individual Al2S3 on NF under the applied current density 3 mAcm-2, while the maxiumum power desnity of 4415.14 WKg-1 was measured at the energy density 49.06 WhKg-1. The Al2S3 nanowalls fabricated on GO thin film illustrated excellent Scp and energy density as well as good power density. Moreover, Al2S3 nanowalls also showed comparatively higher
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ACCEPTED MANUSCRIPT Scp than many other electrode structures, as is evident in Table 1. The excellent electrochemical performance of Al2S3 nanowalls have been found mainly due to the optimum electrical conductivity and most suitable surface morphology, those were responsible to
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generate active sites and avoid the electrode from polarization for long time in half cell.
Figure 6: GCD profile for (a) Al2S3 nanowalls on GO and (b) individual Al2S3 on NF substrate for half-cell.
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ACCEPTED MANUSCRIPT Table 1: Comparison of Scp for GO based Al2S3 nanowalls with the reported literature. Material
Substrate
Electrolyte
Current density
Scp (F/g)
Ref.
505.88
[55]
2
Cobalt Sulfide
Stainless
2 M NaOH
2
Nanoparticles
steel
MnS
Stainless
PVA-KOH
1
steel
Gel
NiCo2S4 @ Co(OH)2
NF
2M KOH
4
Nanostructured
NF
1M NaOH
Ni3S2 5
GO based Strontium
NF
sulfide nanorods 6
GO based
NF
Nanorambutan 7
GO
based
[54]
1055
[53]
1293
[43]
3
1831.14
[30]
2 M KOH
3
2178.16
[56]
3
2373.51
Present
2 M KOH
work
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nanowalls
Al2S3 NF
5
747
2M KOH
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Hierarchical Al2S3
2
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3
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1
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mAcm-2
4.1 Electrochemical Stability and Impedance Spectroscopy The electrochemical stability and the impedance spectroscopy (EIS) have been also employed for observing the electrochemical nature of Al2S3 nanowalls fabricated on GO layer. EIS result presented in Fig. 7 reveals that semicircle nature is negligible at the higher frequency region because a linear variation is exhibited. The circuit shown in the inset of Fig. 7 (a) revealed various factor contributing in the surface control reaction, and both types of resistances (due to the solution (Rs) and the charge transfer (Rct)) were measured and found
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ACCEPTED MANUSCRIPT exhibiting the same value 606 mΩ. The values of both resistances have been found relatively smaller as compared that shown by cobalt sulfide in a previous report by K. Krishnamoorthy et al [55]. The double layered capacitance Cd and capacitance C is 0.2667 mF and 97.3 mF, respectively. The straight line at 45º was employed to extract a weak Warburg resistance
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0.376 Ω, which again confirmed the charge carrier flow necessary to improve the characteristics for supercapacitor applications [57]. Moreover, EIS data also showed that the studied electrode material are less reactive and exhibit limited polarization, due to which,
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Al2S3 nanowalls based electrode illustrated excellent CV and GCD results as evident from the determined EIS data.
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Electrode stability that is a necessary parameter for evaluating the supercapacitor applications was also determined for individual Al2S3 layer and Al2S3 nanowalls on GO fabricated on NF substrate using CV results measured at the scan rate 10 mVs-1 (see Fig. 7 (b)). The electrode stability of MnS has been investigated by Pujari et al using CV results for evaluating the
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potential applications in supercapacitors [54]. The Scp measured for Al2S3 nanowalls was found to retain 45% even after 1300 cycles. In contrast, the Scp measured for individual Al2S3 was found increasing for the first 25 cycles, however, it abruptly decreased to 58% after 155
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cycles. Therefore, individual Al2S3 took more time to be activated, but after approaching
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maximum specific capacity, a more abrupt loss appeared as compared to that by Al2S3 nanowalls based electrode, which may be due to suppressed tendency for exhibiting the polarization. Hence, three-electrode study revealed that Al2S3 nanowalls exhibit appropriate electrochemical behavior suggesting potential applications in asymmetric supercapacitors.
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Figure 7: (a) Nyquist plot of GO based Al2S3 Nanowalls on NF substrate in 2 M KOH electrolyte (b) Electrode stability of GO based Al2S3 Nanowalls and Individual Al2S3 on NF substrate.
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4. Symmetric behavior: Two-electrodes system As, the symmetric behavior observed using two-electrodes set-up is a more appropriate
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for elucidating optimum electrode performance to suggest supercapacitor applications, hence, measurements done using two-electrodes symmetric system for Al2S3 nanowalls and individual Al2S3 were performed using Gold deposited copper foils as the current collector in 2M KOH (5ml) and the filter paper as a separator. The construction of two-electrode symmetric system is presented in Fig. S2. The CV curves, as shown in Fig. 9 (a), measured for Al2S3 nanowalls using two-electrode setup also showed well-resolved redox peaks for the scan rates 5, 10 and 30 mVs-1. It confirmed that the surface controlled reaction exhibit a pseudocapacitive behavior in Al2S3 nanowalls. The plateau region evident from the GCD 17
ACCEPTED MANUSCRIPT profiles, as presented in Fig. 9 (b), was found consistent with the CV curves and the charge storage mechanism (as expressed in Fig. 8) evidenced the presence of pseudocapacitive behavior. Fig. 8 illustrates that the OH-1 ion moves for conduction from one electrode surface to the other electrodes by following Eq. 3 during charging and discharging process, and
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hence, a cycling process is continuously exhibited during the device operation in this manner. The Scp and energy density measured for Al2S3 nanowalls were 231.18, 113.10, 112.88, 111.61 Fg-1 and 20.55, 10.05, 10.03 and 9.92 WhKg-1, respectively, at 0.5, 1, 2 and 3 mAcm.
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The measured energy density for Al2S3 nanowalls was found good and much higher than for
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MoS2 and nitrogen-doped carbon nanofibers, as reported by Krishnamoorthy et al and Chen et al, respectively [58, 59]. The power density was determined as 133.3, 329, 384.25 and 585.50 WKg-1 at the corresponding energy density 20.55, 10.05, 10.03 and 9.92 WhKg-1. The CV and GCD profiles measured for individual Al2S3, as shown in Fig. S3, (for details see
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supporting file) revealed poor symmetric behavior as compare to that by Al2S3 nanowalls. Therefore, Al2S3 nanowalls was suggested suitable because it exhibited good electrochemical performance in both half-cell and by two electrodes symmetric system, which was justified to
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occurring due to higher electrical conductivity for facilitating the easy charge carrier transport
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and offering hindrance for the polarization losses.
Figure 8: The charge storage mechanism for GO based Al2S3 nanowalls occurred during two electrodes symmetric measurements. 18
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Figure 9: The (a) CV and (b) GCD profiles of Al2S3 nanowalls on GO measured using twoelectrodes symmetric systems.
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6. Conclusion
The GO based light metal Al2S3 nanowalls synthesized using hydrothermally technique on
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the NF substrate have shown good asymmetric behavior, when employed as the standard electrode, which is justified with the large electrical conductivity and nanostructured surface morphology. The poor electrochemical behavior exhibited by individual Al2S3 suggest that the introduction of GO under layer has been caused for high Scp 2373.51 Fg-1 and energy density 118.68 WhKg-1 at the current density 3mAcm-2, as measured using GCD test for Al2S3 nanowalls. Al2S3 nanowalls also exhibited a power density 2663.60 WKg-1 at the energy density of 74.73WhKg-1, which exposed Al2S3 nanowalls suitable candidate for
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ACCEPTED MANUSCRIPT asymmetric supercapacitor applications by forming an appropriate electrode structure. Similarly, Al2S3 nanowalls also showed good Scp (231.18 Fg-1) and energy density (20.55 WhKg-1) at 0.5 mAcm-2, as measured using two-electrode symmetric system. Moreover, Al2S3 nanowalls also delivered a power density 585.50 WhKg-1 at the energy density 9.92
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WKg-1. Hence, the present study shows that GO based Al2S3 nanowalls exhibit good electrochemical behavior, which is revealed good performance for both asymmetric as well as symmetric supercapacitor applications.
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7. Nomenclature:
(Al2S3)
Aluminum Nitrate Cyclic voltammetry Electric double layer Capacitor
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Aluminum Sulfide
(Al(NO3)3)
(CV) (EDLCs) (EIS)
Energy density
(E)
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Electrochemical Impedance Spectroscopy
(GCD)
Graphene Oxide
(GO)
Nickel Foam
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Potassium sulfide
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Galvanostatic Charge Discharge
(NF) (K2S)
Power density
(P)
Scanning Electron Microscopy
(SEM)
Specific capacitance
(SCP)
X-ray Diffraction
(XRD)
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Highlights
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ACCEPTED MANUSCRIPT
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GO based Al2S3 nanowalls showed excellent pseudo capacitance of 2362.15 Fg-1.
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GO based Al2S3 nanowalls revealed Pd of 2663.58 WKg-1 at current density 20 mAcm-2.
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GO based Al2S3 nanowalls showed high Csp (2373.51 Fg-1) and Ed (118.68 WhKg-1).
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GO based Al2S3 nanowalls showed good symmetric behavior with Ed of 20.55 WhKg-1.
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GO based Al2S3 nanowalls is suitable for symmetric and asymmetric supercapacitors.
