Capacitive Performance of Graphene-based Asymmetric Supercapacitor

Accepted Manuscript Title: Capacitive Performance of Graphene-based Asymmetric Supercapacitor Authors: C.H. Ng, H.N. Lim, S. Hayase, Z. Zainal, S. Shafie, N.M. Huang PII: DOI: Reference:

S0013-4686(17)30177-9 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.139 EA 28806

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

3-12-2016 20-1-2017 22-1-2017

Please cite this article as: C.H.Ng, H.N.Lim, S.Hayase, Z.Zainal, S.Shafie, N.M.Huang, Capacitive Performance of Graphene-based Asymmetric Supercapacitor, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.139 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.

Highlights 1. A highly crystalline flower-like reduced graphene oxide/zinc oxide/cobalt oxide nanostructure positive electrode was fabricated. 2. The asymmetric supercapacitor exhibited a high energy density of 41.8 Wh/kg. 3. The device exhibited a high specific capacitance and excellent cycling stability. 4. The asymmetric supercapacitor outperformed the KEMET commercial supercapacitor.

1

Graphical Abstract

2

Capacitive Performance of Graphene-based Asymmetric Supercapacitor C.H. Ng1,3, H.N. Lim1,2*, S. Hayase3, Z. Zainal1, S.Shafie4, N.M. Huang5 1

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM,

Serdang, Selangor, Malaysia 2

Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia,

43400 UPM, Serdang, Selangor, Malaysia 3

Graduate School of Life Science and Systems Engineering, Kyushu Institute of

Technology,2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan 4

Department of Electrical and Electronic Engineering, Universiti Putra Malaysia, 43400 UPM,

Serdang, Selangor, Malaysia 5

Centre of Printable Electronics, Deputy Vice Chancellor Office (Research & Innovation),

University of Malaya, 50603 Kuala Lumpur, Malaysia

H.N. Lim (Corresponding author*) Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia. Tel: +60 3 8946 6787 E-mail address: [email protected]

3

ABSTRACT A two-electrode asymmetric supercapacitor with positive and negative electrodes was developed. The positive electrode was composed of reduced graphene oxide/zinc oxide/cobalt oxide nanostructures (RZCo) that were prepared using a one-pot hydrothermal process. Meanwhile, polypyrrole/reduced graphene oxide (PyR) was electrodeposited on a graphite sheet as the negative electrode. These electrodes were separated by a 1.0 M Na2SO4 filter membrane. The synergistic effect between the positive and negative electrode materials widened the potential window to 1.6 V, thus contributing a high energy density of 41.8 Wh/kg at 2 mV/s, which was better than that of the KEMET commercial supercapacitor (17.7 Wh/kg). At room temperature (30 °C), the RZCo//PyR asymmetric supercapacitor exhibited a retention of 87% after 800 cycles compared to a retention of only 49% at 0 °C. Although the asymmetric supercapacitor retained 86% of its original capacitance at 60 °C, it possessed a lower specific capacitance than the asymmetric supercapacitor measured at room temperature. The RZCo//PyR asymmetric supercapacitor had a larger specific capacitance and smaller IR drop (0.09 V) than the KEMET commercial supercapacitor, which has a huge IR drop (0.22 V), providing a specific capacitance of 77.7 F/g.

KEYWORDS: Asymmetric supercapacitor, cobalt oxide, graphene, polypyrrole, zinc oxide

4

1. INTRODUCTION Supercapacitors, which are electrochemical devices that bridge the difference between batteries and conventional capacitors, have been given considerable attention because of their high power density (2–10 kW/kg), fast charging/discharging, and longer lifespans (104–106 cycles) [1-3]. Supercapacitors are classified into two classes, namely the electric double layer capacitor (EDLC) and pseudo-capacitor. The distinctive differences between EDLCs and pseudo-capacitors are solely related to their charging mechanisms. An EDLC’s electrode materials are electrochemically inactive and depend on its accumulation of charges at the electrode/electrolyte interface, whereas the occurrence of a faradaic reaction on a pseudocapacitor enables the storage of charges during the charging and discharging process [4-8]. The materials used for an EDLC, such as carbonaceous materials, have a profound effect on the establishment of its cyclic stability, whereas the materials for a pseudo-capacitor are transition metal oxides with multi-oxidative transition states and conducting polymer, which have high capacitive values in the energy storage family. Transition metal oxides have excellent oxidation and reduction reversibility characteristics over a wider potential range, which are favorable in supercapacitor applications.

Recently, there have been persistent efforts to maximize the performances of supercapacitors. The limitations of symmetric supercapacitors such as their low overall power and energy density, which have been ascribed to their narrow potential range, have prompted a switch from the symmetric configuration for a supercapacitor to an asymmetric configuration [4]. An asymmetric supercapacitor is made up of a combination of a battery-type faradaic cathode and a capacitor-type anode. These can increase the potential window range, which maximizes the operating voltage of an asymmetric supercapacitor [9-10] and increases the energy density of the energy storage device [11-12]. The occurrence of a redox reaction with or without a non-faradaic reaction and EDL (electrostatic adsorption/desorption) on either of the electrodes clearly distinguishes the asymmetric and symmetric configurations for a supercapacitor [13].

The negative electrodes or anodic materials of an asymmetric supercapacitor are mainly composed of carbonaceous materials such as carbon nanotubes (CNTs), activated carbon (AC), and graphene. A typical AC-based energy storage device with a specific surface area in the range of 1000–2000 m2/g records a gravimetric capacitance of 100–120 F/g in an organic

5

electrolyte and energy density of 4–5 Wh/kg [14], whereas a multiwalled CNT device with a specific area of 430 m2/g has a specific capacitance and energy density of 113 F/g and 0.56 Wh/kg, respectively, in a 38 wt% H2SO4 electrolyte [15]. The exceptionally high specific area of graphene (2630 m2/g) provides fertile ground for implementation in supercapacitor applications [14,16,17]. In addition, a graphene-based supercapacitor with a high specific capacitance of 154.1 F/g and high energy density of 85.6 Wh/kg in an ionic liquid electrolyte at a current density of 1 A/g has been reported [17-20]. Although monolayer graphene possesses a large specific surface area with excellent conductivity, and mechanical and electrical properties, graphene is inherently hydrophobic in nature, which leads to the nondispersion of graphene in most aqueous solvents [21-26]. The agglomeration of graphene hinders the ionic accessibility of an asymmetric supercapacitor [13], but could be solved through the use of reduced graphene oxide (rGO), which has an excellent electrical conductivity of up to 1.28 S/m at 6 wt% [27]. In addition, the remnant oxygen groups of rGO could prevent the restacking of graphitic sheets as a result of the strong Van de Waals forces. Occasionally, compositing rGO with electrochemically active materials is necessary to leverage the supercapacitor performances. Conducting polymers are capable of storing more capacitance per gram, and thereby contribute to a higher energy density and faradaic capacitance [28-30]. One of the most-sought after conducting polymers, polypyrrole (PPy), is highly conductive, has ultrahigh-flexibility, and has been reported to display a high capacitance value [31]. A hybrid of PPy and rGO significantly improved the capacitive performance of supercapacitors [28, 32-33].

Meanwhile, the positive electrodes or cathodic materials of asymmetrical supercapacitors include oxides such as cobalt oxides (Co3O4), nickel hydroxide (Ni(OH)2), manganese dioxide (MnO2), and ruthenium oxide (RuO2). Cobalt oxide (Co3O4) is a pseudo-capacitive material with high electro-activity and multi-capacitance storage ability, where Co3O4 nanostructures are able to store a charge via the double layer and pseudo-capacitive reaction. Co3O4 could theoretically deliver a specific capacitance of ~3650 F/g [2]. On the other hand, zinc oxide (ZnO) has a good carrier mobility and provides a high energy density [34-35], A one-dimensional ZnO nanostructure possesses excellent electron transport properties [36], which can enhance the capacitive performances. A composite of Co3O4@graphene nanosheet prepared using a microwave-assisted method could homogenously distribute Co3O4 nanoparticles (3–5 nm in size) on a graphene sheet [13]. The inclusion of conductive graphene sheets provided an opened pathway for rapid charge storage and delivery, which 6

resulted in a high electronic conductivity and capacitance performance, and an improved cyclic life for the device [16-17]. Thus, when transition metal oxides are included, which in our work refer to the ZnO/Co3O4 nanostructures, they behave as intercalating agents and intercalate the rGO layers to prevent the restacking of rGO sheets. This phenomenon is expected to be able to enhance the inflow and outflow of electrolyte ions, resulting in excellent electrochemical performances, in addition to increasing the porosity of the graphite sheet. The inclusion of rGO could produce better wettability and thus induce the good dispersion in water ascribed to the presence of the remaining oxygen groups, in line with the aromatic rings, which offer active sites for the π-π interaction [37]. The synergetic effects of the ternary materials (ZnO/Co3O4/rGO) hence produce excellent electrical conductivity and better porosity.

Tailoring the nanostructure of the electrode is essential to increase the surface area of the active materials, because it produces a shorter ion insertion/desertion diffusion path and enables efficient charge and mass transfer without compromising the double layer capacitance [10]. In addition, the merit of a binder-less electrode is its excellent charge transportation [10], in order to minimize the supercapacitive resistance and “dead volumes” in the electrode materials [11, 38]. We here present our work on the fabrication of an RZCo//PyR asymmetric supercapacitor on a graphite sheet via a one-step facile hydrothermal and electro-deposition process. The graphite sheet was chosen as the current collector and treated with nitric acid prior to the deposition of the active materials. This was expected to improve the functionalization between the functional groups of the active materials and the carbon surface, which would lead to an excellent electro-catalytic activity performance [3940]. To date, only a few temperature dependent studies have been reported, whereas most supercapacitor application studies have focused on the performances at room temperature. The supercapacitive performances of an asymmetric supercapacitor at different temperatures were also investigated in this study.

2. EXPERIMENTAL 2.1 Materials Graphite powder was obtained from Ashbury Graphite Mills Inc. (code no. 3061). Potassium permanganate (KMnO4, 99.9%), sulphuric acid (H2SO4, 95%–98%), phosphoric acid (H3PO4, 85%), and hydrogen peroxide (H2O2, 30%) were purchased from Systerm, Malaysia.

7

Hydrogen chloride (HCl, 37%), sodium sulphate anhydrous (Na2SO4, 70%), and a toluene-4sulfonic acid sodium salt (NapTS, 98%) were purchased from Merck. Pyrrole (99%) was purchased from Across Organic, stored at 0 °C, and distilled prior to use. The graphite sheet (natural graphite >99.5%) was purchased from Latech Singapore. The nylon membrane filter was purchased from Membrane Solutions, LLC, US. Zinc nitrate hexahydrate, (Zn(NO3)2.6H2O) and cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O) were purchased from Bendosen and R&M Chemicals, respectively. Nitric acid ACS reagent (≥65%) was obtained from Sigma-Aldrich. Urea was purchased from AnalaR, BDH laboratory supplies from England, and ammonium chloride (NH4Cl) was obtained from Merck.

2.2 Preparation of graphene oxide (GO) GO was prepared through the simplified Hummer’s method, where the oxidation reaction was carried out by mixing H2SO4:H3PO4 (360 ml:40 ml), graphite flakes, and KMnO4 using a magnetic stirrer at room temperature. The mixture was then left stirring for three days to allow the complete oxidation of the graphite. Subsequently, the H2O2 solution was added to the mixture to stop the oxidation process, and the color of the mixture turned bright yellow. Next, the graphite oxide was washed with a 1 M HCl aqueous solution three times and with deionized water ten times using the centrifugation technique until the pH reached a value of approximately 4–5 and GO was formed. The concentration of GO was 6.3 mg/ml.

2.3 Preparation of rGO/ZnO/Co3O4 (RZCo) positive electrode First, 2 mmol Co(NO3)2.6H2O, 1 mmol Zn(NO3)2.6H2O, 4 mmol NH4Cl, and 0.02 mg/ml GO precursors were dissolved in 30 ml of deionized water and stirred until homogenous. Then, 8 mmol urea was added to the purplish homogenous solution and sonicated for 15 min, followed by the immersion of the nitric acid treated graphite sheet into an autoclave for 5 h of hydrothermal reaction at 120 °C. The deposited graphite sheet was calcined at 400 °C for 2 h after the hydrothermal process. The same process was used for Co3O4/ZnO (without the inclusion of GO) as a standard comparison (denoted as ZCo for brevity).

2.4 Preparation of PPy/rGO (PyR) negative electrode PyR nanocomposite films were synthesized using a potentiostat–galvanostat at room temperature, where a platinum electrode was used as a counter-electrode, a graphite sheet was used as a working electrode, and a saturated calomel electrode was used as a reference

8

electrode. The films were electrodeposited onto the graphite sheet at a constant potential of +0.8 V for 15 min, where the deposition solution contained 0.1 M of pyrrole, 1 mg/ml of GO, and 0.1 M of NapTS. 2.5 Fabrication of RZCo//PyR asymmetric supercapacitor The fabricated positive electrode was stacked with the negative electrode, forming a sandwiched system separated by the membrane filter, where a 1 M Na2SO4 aqueous solution was employed as the redox electrolyte. The sandwich-like asymmetric supercapacitor was fit into a Swagelok for the electrochemical measurements.

2.6 Material characterizations The chemical states and compositions of the products were analyzed using X-ray photoelectron spectroscopy (XPS) by DZU Shima (Kratos Analytical Ltd.), whereas the crystalline structures of the samples were analyzed using the D8 Advance, Bruker AXS Germany (Cu, Kα one radiation) X-ray diffraction (XRD) technique. The surface morphologies of the samples were investigated using a Quanta 400F FESEM equipped with an EDX feature. The electrical conductivity test was performed by Mitsubishi Loresta-GP (MCP-T610). The Fourier transformed infrared (FTIR) spectroscopy was performed using the attenuated total reflectance of a Perkin Elmer 1650, FTIR spectrophotometer. The crystalline size of the nanomaterials was determined using the Debye–Scherrer equation, as shown in Equation (1) [41]: D

K B

cos(

(1)

)

where K is the Scherrer constant with a value of 1, refers to the x-ray wavelength, Bhkl is the full width at half maximum (FWHM) in radians, and

hkl

is the Bragg angle.

2.7 Electrochemical measurements An asymmetric supercapacitor was fabricated by employing a 1 M Na2SO4 aqueous electrolyte-soaked membrane filter as the separator and was fit into a Swagelok. The asfabricated device was subjected to cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) measurements. The applied working potential for the CV measurement was in the range of 0–1.6 V, and the measurements used scan rates of 2 to 100 mV/s. The specific capacitance of the asymmetric supercapacitor was calculated from the charge/discharge profile using Equation (2), where Cm refers to the

9

specific capacitance in farads per gram, I is the charge/discharge current, Δt is the discharging time, ΔV is the potential window, and m is the electrode mass in grams [42]: ⁄

(2)

The energy density and power density of the asymmetric supercapacitor were calculated using Equations (3) and (4), respectively [43]: ⁄

(3) ⁄

(4)

where Ecell is the energy density in Wh/kg, Ccell is the specific capacitance of an electrode obtained from CV, V is the potential window obtained from the discharge curve, Pcell is the power density in W/kg, and t refers to the discharge time.

3. RESULTS AND DISCUSSION Figure 1a shows the highly crystalline ZCo flower-like structures on the surface of a graphite sheet that acted as the current collector for the as-assembled supercapacitor. Meanwhile, Figure 1b reveals similar flower-like structures adhering on both surfaces of the translucent rGO sheets, in which the composite formed a layer on a graphite sheet. The size of the ZCo on the rGO was four times smaller than that of the neat ZCo because the growth of the flower-like structures was impeded by the presence of rGO, which acted as a support. Figure 2 depicts the postulated formation of ZCo and RZCo. In the case of ZCo, the precursors freely dispersed and adhered on the graphite sheet during the hydrothermal process, which subsequently grew and formed flower-like structures. Meanwhile, for the RZCo, the growth of the ZCo structures was hindered by the overcrowded nucleation sites on the GO. Because GO had a stronger affinity toward the graphite sheet compared to the metal ions due to the hydrophobic interaction, the GO would first adhere to the surface of the graphite sheet, followed by the metal ions. This overcrowding phenomenon restricted the growth of ZCo, resulting in a smaller particle size for the ZCo on rGO.

The XRD profile (Figure 3a) provides evidence for the embedment of the ZnO, Co3O4, and rGO on the current collector (graphite sheet). The high-intensity graphite peak shown at ~26° is ascribed to the major detection from the current collector itself as a result of the low concentration of active materials used. The GO peak was absent at ~10° and appeared with a weak peak intensity at ~24°, which showed that the graphitic carbon had been successfully

10

reduced to rGO [44]. The non-prominent rGO peak was due to the small amount (0.02 mg/ml) of GO used in this reaction. In contrast, ZnO crystalline peaks were observed at 31.35°, 37.02°, 54.73°, and 77.51°, whereas Co3O4 had peaks at 31.35°, 37.02°, 44.87°, 59.42°, and 65.22°. The Co3O4 nanoparticles were present in the cubic phase structure with the Fd-3m (227) space group. The peak for ZnO at a high intensity (54.73°) was assigned to the hkl indices of 110 [45-46]. It thus conclusively showed the complete transformation of active precursors into the oxide form. The crystalline sizes of the ZnO and Co3O4 were obtained using the Debye–Scherrer equation. The ZnO nanoparticles had an average crystalline size of 14.8 nm, whereas the Co3O4 had an average crystalline size of 13.4 nm. The FTIR spectrum could further prove the presence of Co3O4 and ZnO within the ternary system, where the peaks of Co3O4 and ZnO were indistinguishable in the region below 700 cm-1. The ZnO peaks split into two (521.94 and 643.55 cm-1) upon 400 °C calcination, as observed in Figure 3b [47]. Simultaneously, the fully transformed spinel phase of Co3O4 at 521.94 and 643.55 cm-1 indicated the OB3 vibration in the spinel lattice (B is Co3+ in an octahedral hole) at 521.94 cm-1, while the IR absorption at 643 cm-1 showed that Co2+ resided in a tetrahedral hole. It thus showed the complete formation of the Co3O4 spinel phase after heat treatment [48]. The medium intensity of the C-O peak (1069.19 cm-1) implied the remaining oxygen groups ascribed to the partial reduction of GO. In contrast, the absorption band at 1616.59 cm-1 indicated the C=C sp2 carbon [49].

The wide scan of RZCo (Figure 4a) showed the embedment of the GO, Zn, and Co elements in the form of oxide within the matrix with a binding energy in the range of ~1022 eV to ~282 eV. Generally, C1s has a binding energy of ~284 eV and O1s has a peak of 531 eV, whereas Co2p and Zn2p are located at binding energies of 743 eV and 1020 eV, respectively. The weak intensities of the wide scan analysis could be the result of employing low concentrations of precursors and active materials. The successful deposition of the Zn- and Co-based active materials on the graphite sheet was proven by the Zn2p and Co2p regions, as depicted in Figures 4b and 4c. Two strong intensities for the Zn2p peaks, which were the Zn2p1/2 and Zn2p3/2 peaks at 1043.7 eV and 1020.7 eV, respectively, showed a binding energy difference of 23 eV, and proved the formation of ZnO. In contrast, Figure 4c shows two strong deconvoluted Co2p1/2 and Co2p3/2 peaks located at ~795 eV and ~780 eV, respectively. Co possesses a huge spin-orbit splitting constant of 15 eV, and the deconvoluted Co2p3/2 peak was assigned to the oxidation state of Co3+ [50]. Figure 4d shows four deconvoluted peaks from the O1s region, where C=O bonding is located at 532.6 eV, which could be the remnant 11

oxygen groups of rGO. Weak intensities for the GO oxygen groups were observed in the C1s region (Figure 4e). This showed that the GO was successfully reduced to rGO, which was in good agreement with the XRD profile. In addition, the peak of 531.5 eV represented the OH groups adsorbed on the surface of the metal oxides and the Zn-O peak had a weak intensity at 530.3 eV, which implied the presence of ZnO materials [51]. The high-intensity C1s (Figure 4e) peak at 284.2 eV represented the sp2 hybridized carbon [52], which was expected to be attributed to GO. Thus, we deduced that the formation of ZnO and Co3O4 was accomplished within the nanocomposite upon calcination for 2 h at 400 °C. The electrical conductivity of the supercapacitor’s positive electrode was investigated to study the electrical charge flow efficiency, as depicted in Table 1. It showed that the ternary nanocomposites (83.542 S/m) had better conductivities than the binary nanocomposites (72.254 S/m). This implied an easy charge flow within the composite vicinity, which could be ascribed to the low resistivity of 1.197 × 10-2Ω. Significantly, this showed the contribution of rGO in forming a highly porous, intercalated, and less resistive pathway, consequently easing the diffusivity of the electrolyte ions, which is consistent with the FESEM image shown in Figure 1 [53].

Electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the resistivity performances such as the equivalent series resistance (ESR) and charge transfer resistance (Rct) of the electrodes from the starting frequency of 100 kHz to an ending frequency of 0.01 Hz, as shown in Figure 5a. The ESR values were obtainable at the first intercepting point on the x-axis, whereas the Rct values could be obtained from the diameter of the semicircle formed [54]. The ESR includes the electrolyte and electrode resistances, and Rct is the rate of charge transfer at the electrode and electrolyte interface. The inset shows that the magnified semicircle (Rct) of the RZCo//PyR asymmetric supercapacitor is larger than that for ZCo//PyR, which denote values of 16.519 Ω and 9.395 Ω, respectively. The high resistivity of the RZCo//PyR asymmetric supercapacitor could be attributed to the diffusion of Na+ between the electrolyte and bulky electrode interface, which produced a high series interfacial resistance and faradaic reaction [55]. Nevertheless, the limitation of rGO could be mitigated by the incorporation of pseudo-capacitive materials, where the synergistic effect between the rGO and transition metal oxides rendered a low ESR of 0.9254 Ω, which was approximately half that of the ZCo//PyR asymmetric supercapacitor (1.9331 Ω). This phenomenon could be ascribed to the inclusion of rGO and a binder-less feature, which

12

enhanced the charge transport ability and thus reduced the total resistance of the entire asymmetric supercapacitor.

The Ragone plot (Figure 5b) reflects the efficiencies of storing and delivering energy from the fabricated asymmetric supercapacitor device. The inclusion of rGO and the symbiosis effect between the positive and negative electrodes of the RZCo//PyR asymmetric supercapacitor contributed to an energy density of 15.8 Wh/kg and a power density of 3575.3 W/kg at a scan rate of 100 mV/s. Meanwhile, the amount of energy stored increased to 41.8 Wh/kg, with a attainable power density value of 188.3 W/kg at a scan rate of 2 mV/s, as opposed to the energy (14.6 Wh/kg) and power density (65.9 W/kg) of the ZCo//PyR. The RZCo//PyR asymmetric supercapacitor possessed twice the energy density and eight times the power density of the KEMET commercial supercapacitor at a scan rate of 2 mV/s (Figure 5b and Table 2). In addition, the electrochemical performances of the RZCo//PyR asymmetric supercapacitor were comparable to those of others, as depicted in Table S1. The higher energy density was achievable at the lower scan rate ascribed to the highly efficient intercalation and de-intercalation of electrolyte ions within the open pore system of the asymmetric supercapacitor matrix [24, 30, 56]. The lower energy density and inefficient energy transfer rate of ZCo//PyR could be attributed to the highly resistive environment and undesirable pore size distribution within the matrix with the absence of rGO on the positive electrode [11]. In addition, the rigid and open structure of the nanocomposite matrix served as an electrolyte ion diffusivity valley. This enhanced the energy transfer rate, which is in good agreement with the low ESR value reported in Figure 5a. The cyclic retention and stability were determined under a current density of 2 mA/cm2, as depicted in Figure 5c. The cyclic retention, which was effectively improved upon the incorporation of rGO, gave rise to a specific capacitive retention of 87% relative to an asymmetric supercapacitor without rGO, which had a retentive specific capacitance of only 61.75%. A minor degradation occurred upon the inclusion of rGO because of the EDLC nature of the graphitic materials, which could provide cyclic stability [28] without compromising the highly conductive and large surface active area properties of rGO. The pseudo-capacitive materials (Co3O4 and ZnO), which acted as spacers in preventing the restacking of rGO layers, were also essential contributors to the capacitive enhancement, because of the resistive-less electrolyte ion flow, which enhanced the charge storage reactions [57]. Nevertheless, the gradual declines in the cyclic retention observed in both plots were due to the deterioration of the active materials and polymer chain degradation after an 13

excessive swelling and shrinking process [31]. As a comparison, the ZCo//PyR asymmetric supercapacitor lost its capacitive retention ability of ~7.35 F/g after 100 cycles of the charging/discharging process, which implies an unsustainable electrocatalytic activity performance for the asymmetric supercapacitor without the inclusion of rGO. A comparison was made between the specific capacitance values of the RZCo//PyR and ZCo//PyR asymmetric supercapacitors, and the effects of the GO concentration on the specific capacitance are included in the supplementary information section. The concentration of GO played a major role in rendering a greater capacitance per gram because a higher concentration of GO could lead to the aggregation of graphitic sheets, which could impede the penetration of electrolyte ions and thereby reduce the redox-active area (Figure S1). The specific capacitance of the ZCo//PyR asymmetric supercapacitor (Figure S2) was merely 51.88 F/g at a current density of 2 mA/cm2 compared to the 139.81 F/g capacitive performance of the RZCo//PyR asymmetric supercapacitor. This phenomenon could be due to the slightly bulky matrix of the asymmetric supercapacitor without rGO, which impeded the diffusion of the electrolyte ions in and out of the matrix. In addition, it was suspected that the lower specific capacitance of the ZCo//PyR asymmetric supercapacitor could be ascribed to the solely pseudo-capacitive behavior on the positive electrode, which rapidly deteriorated the stability of the asymmetric supercapacitor.

The performances of the asymmetric supercapacitor at different operating temperature were studied (as shown in Figure 6) to investigate the ability, sustainability, and versatility of the as-fabricated energy storage device under any conditions, in either cold or hot weather, for future practical applications. The asymmetric supercapacitor was tested under 0 °C, 30 °C (room temperature), and 60 °C. Figure 6a shows that under high temperatures (30 °C and 60 °C), the asymmetric supercapacitor had a smaller ESR than under a cold condition (0 °C). The higher ESR under a cold condition could have been due to freezing or the recrystallization of the Na2SO4, which had an adverse effect on the diffusivity of the electrolyte ions. On the other hand, the ESR value of the asymmetric supercapacitor at 60 °C was comparatively higher than that under room temperature. The results are tabulated in Table 3. This phenomenon could be attributed to the vibration of the ions and electrons when subjected to a high temperature. Theoretically, the Na2SO4 electrolyte might still be stable at room temperature. However, as reported, Na2SO4 has a transition temperature at ~32 °C, where its transited phase or morphology could hinder the facilitation of electrolyte ions, thereby exaggerating the resistivity of the asymmetric supercapacitor at a higher temperature 14

[58]. In contrast, the electrolyte might start to crystalline below its stable temperature (~30 °C), which would lead to the high ESR and Rct values. The life cycle profile explicitly reflected the best performing asymmetric supercapacitor in terms of its specific capacitance and life retention (87%) at room temperature. The asymmetric supercapacitor at an operating temperature of 60 °C had a comparable life retention of 86% but a lower specific capacitance of ~50 F/g, which is about half the specific capacitance of the asymmetric supercapacitor at 30 °C. The reversibility of the asymmetric supercapacitor after being heated and cooled was investigated to study the performance stability of the energy device under the influence of temperature variations. As depicted in Figure S3a, when the temperature was increased to 30 °C from 0 °C (0 °C  30 °C), the specific capacitance of the device increased 1.7 fold compared to that of the device tested at 0 °C. However, the device tested at 30 °C still had the best specific capacitance. Meanwhile, when the temperature was decreased to 30 °C from 60 °C (60 °C  30 °C), the device could only retain 52% of its initial capacitance, which was worse than the device tested at 30 °C (87%). The performance deterioration could have been due to the partial deformation of the active materials. The charge storage matrix was destroyed by the freezing and heating processes, which was ascribed to the material contraction and expansion, and this compromised the diffusive capability of the electrolyte ions. In addition, the performances of the asymmetric supercapacitor at different defrosting times are depicted in Figure S3b. The asymmetric supercapacitor with a longer defrosting time (120 min) possessed a longer discharging time, which implies that the 120 min defrosted asymmetric supercapacitor had a better storage capability and could retain more charge than the 60 min defrosted asymmetric supercapacitor. As observed from the morphology of the RZCo materials (Figure S4), the severe cracking and inhomogeneity of the active surface conclusively evidenced the performance deterioration of the asymmetric supercapacitor after the heating and cooling processes.

The supercapacitive properties of the as-fabricated asymmetric supercapacitor were investigated through a series of electrochemical measurements (as shown in Figure 7) and were compared to those of a commercial supercapacitor to gauge the applicability, compatibility, and potential of our fabricated asymmetric supercapacitor for future commercialization. The general information and specifications of the commercial supercapacitor are listed in Table S2. As depicted in Figure 7a and 7b, the RZCo//PyR asfabricated supercapacitor exhibited a roughly two-fold higher specific capacitance than the KEMET commercial supercapacitor, which implies that our fabricated device had a better 15

capacitance performance. The KEMET commercial supercapacitor had a more obvious IR drop than the as-fabricated supercapacitor, which led to a high equivalent series resistance (ESR), as shown in Figure 7c. The ESR is closely correlated to the IR drop and discharge current density [59]. Figure 7c shows that the KEMET supercapacitor possesses a high total resistance (ESR) of 34 Ω without any signs of a semicircle detected. The ultra-high ESR could have been due to the lower conductivity of the carbon materials. In comparison, the asfabricated asymmetric supercapacitor had a lower ESR of merely 0.92 Ω, and its straight line at the low-frequency region leaned more toward the Z” axis, which implied that the device had a better capacitive behavior [60]. Life retention tests were performed (Figure 7d) for both energy storage devices, and the KEMET supercapacitor had a better cycling life, with 98% of the capacitance being retained after 800 continuous charge/discharge cycles, compared to the as-fabricated asymmetric supercapacitor (87%). The excellent reversibility of the commercial supercapacitor is ascribed to the absence of the faradaic reaction (as proven by the square voltammogram in Figure S5) and polymeric materials, which could cause electrode deterioration after the extensive swelling and contraction of the electrodes during cyclic measurements, as evidenced by the performance of the as-fabricated asymmetric supercapacitor. Despite the fact that the as-fabricated asymmetric supercapacitor had a higher power output, its cyclic sustainability would be an issue and limitation for industrial applications. Thus, relentless efforts to improve the as-fabricated device are needed for industrial realization.

CONCLUSION An asymmetric supercapacitor was successfully fabricated via a facile one-step hydrothermal and electro-deposition method. The highly crystalline flower-like RZCo nanostructure positive electrode provided excellent electrochemical performances compared to the ZCo electrode. The high potential window of 1.6 V and symbiosis effects of the positive and negative electrodes contributed to an asymmetric supercapacitor with high energy (41.8 Wh/kg) and power densities (188.3 W/kg). The calculated energy and power densities of the RZCo//PyR asymmetric supercapacitor are comparable to those recently reported in the literature. In addition, the incorporation of 0.02 mg/ml rGO on the positive electrode significantly enhanced the retention ability of the as-fabricated asymmetric supercapacitor, thus rendering a cyclic retention of 87% after being subjected to 800 charge/discharge cycles, which was 1.4-fold more stable than the ZCo//PyR asymmetric supercapacitor (61.75%). 16

Despite the ability of the KEMET commercial supercapacitor to retain 98% of its capacitance, its power output (77.7 F/g) did not outperform our as-fabricated supercapacitor. When operated at the temperature of 30 °C the asymmetric supercapacitor had better electrochemical performances than when operated at 0 °C and 60 °C. In summary, the binderless as-fabricated asymmetric supercapacitor device has the potential for wide application in the supercapacitor industry.

ACKNOWLEDGEMENT This work was supported by a Newton-Ungku Omar Fund (6386300-13501) from the British Council and MIGHT. REFERENCES [1] L.J. Xie, J.F. Wu, C.M. Chen, C.M. Zhang, L. Wan, J.L. Wang, Q.Q. Kong, C.X. Lv, K.X. Li, G.H. Sun, A novel asymmetric supercapacitor with an activated carbon cathode and a reduced graphene oxide–cobalt oxide nanocomposite anode, Journal of Power Sources, 242 (2013) 148-156. [2] B. Vidyadharan, R.A. Aziz, I.I. Misnon, G.M.A. Kumar, J. Ismail, M.M. Yusoff, R. Jose, High energy and power density asymmetric supercapacitors using electrospun cobalt oxide nanowire anode, Journal of Power Sources, 270 (2014) 526-535. [3] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X.b. Zhao, H.J. Fan, High-quality metal oxide core/shell nanowire arrays on conductive substrates for electrochemical energy storage, ACS Nano, 6 (2012) 5531-5538. [4] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chemical Society Reviews, 41 (2012) 797-828. [5] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, M.A. Yarmo, C.Y. Yin, Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors, Ceramics International, 40 (2014) 3855-3864. [6] S.M. Chen, R. Ramachandran, V. Mani, R. Saraswathi, Recent advancements in electrode materials for the high-performance electrochemical supercapacitors: areview, International Journal Electrochemical Science, 9 (2014) 4072-4085. [7] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, Preparation and characterization of polypyrrole/graphene nanocomposite films and their electrochemical performance, J Polym Res, 20 (2013) 1-10.

17

[8] Y. Cheng, H. Zhang, S. Lu, C.V. Varanasi, J. Liu, Flexible asymmetric supercapacitors with high energy and high power density in aqueous electrolytes, Nanoscale, 5 (2013) 10671073. [9] C.h. Tang, X. Yin, H. Gong, Superior performance asymmetric supercapacitors based on a directly grown commercial mass 3D Co3O4@Ni(OH)2core–shell electrode, ACS Applied Materials & Interfaces, 5 (2013) 10574-10582. [10] F.Luan, G. Wang, Y. Ling, X. Lu, H. Wang, Y. Tong, X.X. Liu, Y. Li, High energy density asymmetric supercapacitors with a nickel oxide nanoflake cathode and a 3D reduced graphene oxide anode, Nanoscale, 5 (2013) 7984-7990. [11] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Advanced Functional Materials, 21 (2011) 2366-2375. [12] T.W. Lin, C.S. Dai, K.C. Hung, High energy density asymmetric supercapacitor based on NiOOH/Ni3S2/3D graphene and Fe3O4/graphene composite electrodes, Scientific Reports, 4 (2014) 7274. [13] F. Wang, S. Xiao, Y. Hou, C. Hu, L. Liu, Y. Wu, Electrode materials for aqueous asymmetric supercapacitors, RSC Advances, 3 (2013) 13059-13084. [14] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbon-based supercapacitors produced by activation of graphene, Science, 332 (2011) 1537-1541. [15] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Electrochemical properties of high-power supercapacitors using single-walled carbon nanotube electrodes, Advanced functional materials, 11 (2001) 387-392. [16] J. Tang, Y. Yamauchi, Carbon materials: MOF morphologies in control, Nature Chemistry, 8 (2016) 638-639. [17] R.R. Salunkhe, Y.H. Lee, K.H. Chang, J.M. Li, P. Simon, J. Tang, N.L. Torad, C.C. Hu, Y. Yamauchi, Nanoarchitectured graphene-based supercapacitors for next-generation energystorage applications, Chemistry – A European Journal, 20 (2014) 13838-13852. [18] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor with an ultrahigh energy density, Nano letters, 10 (2010) 4863-4868. [19] N.L. Torad, R.R. Salunkhe, Y. Li, H. Hamoudi, M. Imura, Y. Sakka, C.C. Hu, Y. Yamauchi, Electric double-layer capacitors based on highly graphitized nanoporous carbons derived from ZIF-67, Chemistry – A European Journal, 20 (2014) 7895-7900.

18

[20] J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, Thermal conversion of core–shell metal–organic frameworks: anew method for selectively functionalized nanoporous hybrid carbon, Journal of the American Chemical Society, 137 (2015) 1572-1580. [21] Y.S. Lim, Y.P. Tan, H.N. Lim, W.T. Tan, M.A. Mahnaz, Z.A. Talib, N.M. Huang, A. Kassim, M.A. Yarmo, Polypyrrole/graphene composite films synthesized via potentiostatic deposition, Journal of Applied Polymer Science, 128 (2013) 224-229. [22] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chemistry Society Reviews, 39 (2010) 228-240. [23] J. Basu, J.K. Basu, T.K. Bhattacharyya, The evolution of graphene-based electronic devices, International Journal of Smart and Nano Materials, 1 (2010) 201-223. [24] W.K. Chee, H.N. Lim, I. Harrison, K.F. Chong, Z. Zainal, C.H. Ng, N.M. Huang, Performance of flexible and binderless polypyrrole/graphene oxide/zinc oxide supercapacitor electrode in a symmetrical two-electrode configuration, Electrochimica Acta, 157 (2015) 8894. [25] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbon-based supercapacitors produced bya of graphene, Science, 332 (2011) 1537-1541. [26] B. Ou, R. Huang, W. Wang, H. Zhou, C. He, Preparation of conductive polyaniline grafted graphene hybrid composites via graft polymerization at room temperature, RSC Advances, 4 (2014) 43212-43219. [27] X. Wang, H. Bai, Z. Yao, A. Liu, G. Shi, Electrically conductive and mechanically strong biomimetic chitosan/reduced graphene oxide composite films, Journal of Materials Chemistry, 20 (2010) 9032-9036. [28] C.H. Ng, H.N. Lim, Y.S. Lim, W.K. Chee, N.M. Huang, Fabrication of flexible polypyrrole/graphene oxide/manganese oxide supercapacitor, International Journal of Energy Research, 39 (2015) 344-355. [29] L. Yu, G. Zhang, C. Yuan, X.W.D. Lou, Hierarchical NiCo2O4@MnO2 core–shell heterostructured nanowire arrays on Ni foam as high-performance supercapacitor electrodes, Chemical Communications, 49 (2013) 137-139. [30] D.P. Dubal, R. Holze, All-solid-state flexible thin film supercapacitor based on Mn3O4 stacked nanosheets with gel electrolyte, Energy, 51 (2013) 407-412.

19

[31] Y.C. Eeu, H.N. Lim, Y.S. Lim, S.A. Zakarya, N.M. Huang, Electrodeposition of polypyrrole/reduced graphene oxide/iron oxide nanocomposite as supercapacitor electrode material, Journal of Nanomaterials, 2013 (2013) 6. [32] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, Journal of Power Sources, 196 (2011) 1-12. [33] Y.S. Lim, H.N. Lim, S.P. Lim, N.M. Huang, Catalyst-assisted electrochemical deposition of graphene decorated polypyrrole nanoparticles film for high-performance supercapacitor, RSC Advances, 4 (2014) 56445-56454. [34] Q. Chang, Z. Ma, J. Wang, Y. Yan, W. Shi, Q. Chen, Y. Huang, Q. Yu, L. Huang, Graphene nanosheets@ZnO nanorods as three-dimensional high efficient counter electrodes for dye sensitized solar cells, Electrochimica Acta, 151 (2015) 459-466. [35] J. Wang, Z. Gao, Z. Li, B. Wang, Y. Yan, Q. Liu, T. Mann, M. Zhang, Z. Jiang, Green synthesis of graphene nanosheets/ZnO composites and electrochemical properties, Journal of Solid State Chemistry, 184 (2011) 1421-1427. [36] J. Zhang, S. Wang, M. Xu, Y. Wang, B. Zhu, S. Zhang, W. Huang, S. Wu, Hierarchically porous ZnO architectures for gas sensor application, Crystal Growth & Design, 9 (2009) 3532-3537. [37] L. Qiu, X. Yang, X. Gou, W. Yang, Z.F. Ma, G.G. Wallace, D. Li, Dispersing carbon nanotubes with graphene oxide in water and synergistic effects between graphene derivatives, Chemistry-A European Journal, 16 (2010) 10653-10658. [38] J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong, H.J. Fan, Co 3O4 Nanowire@MnO2ultrathin nanosheet core/shell arrays: anew class of high-performance pseudocapacitive materials, Advanced Materials, 23 (2011) 2076-2081. [39] H. Wang, Y.H. Hu, Graphene as a counter electrode material for dye-sensitized solar cells, Energy & Environmental Science, 5 (2012) 8182-8188. [40] J. Collins, T. Ngo, D. Qu, M. Foster, Spectroscopic investigations of sequential nitric acid treatments on granulated activated carbon: Effects of surface oxygen groups on π density, Carbon, 57 (2013) 174-183. [41] D.M. Smilgies, Scherrer grain-size analysis adapted to grazing-incidence scattering with area detectors, Journal of Applied Crystallography, 42 (2009) 1030-1034. [42] M. Jin, G. Han, Y. Chang, H. Zhao, H. Zhang, Flexible electrodes based on polypyrrole/manganese

dioxide/polypropylene

fibrous

membrane

composite

for

supercapacitor, Electrochimica Acta, 56 (2011) 9838-9845.

20

[43] Y.J. Kang, S.J. Chun, S.S. Lee, B.Y. Kim, J.H. Kim, H. Chung, S.Y. Lee, W. Kim, Allsolid-state flexible supercapacitors fabricated with bacterial nanocellulose papers, carbon nanotubes, and triblock-copolymer ion gels, ACS Nano, 6 (2012) 6400-6406. [44] S. Pei, J. Zhao, J. Du, W. Ren, H.M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon, 48 (2010) 4466-4474. [45] M.M. Rahman, S.B. Khan, A.M. Asiri, K.A. Alamry, A.A.P. Khan, A. Khan, M.A. Rub, N. Azum, Acetone sensor based on solvothermally prepared ZnO doped with Co3O4 nanorods, Mikrochimica Acta, 180 (2013) 675-685. [46] M.A. Subhan, T. Ahmed, Synthesis, characterization and spectroscopic investigations of novel nano multi-metal oxide Co3O4·CeO2·ZnO, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 129 (2014) 377-381. [47] K. Sowri Babu, A. Ramachandra Reddy, C. Sujatha, K. Venugopal Reddy, A.N. Mallika, Synthesis and optical characterization of porous ZnO, Journal of Advanced Ceramics, 2 (2013) 260-265. [48] J. Xu, P. Gao, T.S. Zhao, Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells, Energy & Environmental Science, 5 (2012) 5333-5339. [49] P. Wang, J. Wang, X. Wang, H. Yu, J. Yu, M. Lei, Y. Wang, One-step synthesis of easyrecycling TiO2-rGO nanocomposite photocatalysts with enhanced photocatalytic activity, Applied Catalysis B: Environmental, 132–133 (2013) 452-459. [50] J. Baek, J. Park, A. Hwang, Y. Kang, Spectroscopic and morphological investigation of Co3O4microfibers produced by electrospinning process, Bulletin of the Korean Chemical Society, 33 (2012) 1242-1246. [51] R. Al-Gaashani, S. Radiman, A.R. Daud, N. Tabet, Y. Al-Douri, XPS and optical studies of different morphologies of ZnO nanostructures prepared by microwave methods, Ceramics International, 39 (2013) 2283-2292. [52] S.C. Ray, A. Saha, S.K. Basiruddin, S.S. Roy, N.R. Jana, Polyacrylate-coated grapheneoxide and graphene solution via chemical route for various biological application, Diamond and Related Materials, 20 (2011) 449-453. [53] S.Y. Kim, K. Yang, B.H. Kim, Enhanced electrical capacitance of heteroatom-decorated nanoporous carbon nanofiber composites containing graphene, Electrochimica Acta, 137 (2014) 781-788.

21

[54] M.D. Stoller, R.S. Ruoff, Best practice methods for determining an electrode material's performance for ultracapacitors, Energy & Environmental Science, 3 (2010) 1294-1301. [55] S.K. Meher, P. Justin, G.R. Rao, Pine-cone morphology and pseudocapacitive behavior of nanoporous nickel oxide, Electrochimica Acta, 55 (2010) 8388-8396. [56] X. Lu, D. Zheng, T. Zhai, Z. Liu, Y. Huang, S. Xie, Y. Tong, Facile synthesis of largearea manganese oxide nanorod arrays as a high-performance electrochemical supercapacitor, Energy & Environmental Science, 4 (2011) 2915-2921. [57] W. Chen, C. Xia, H.N. Alshareef, One-step electrodeposited nickel cobalt sulfide nanosheet arrays for high-performance asymmetric supercapacitors, ACS Nano, 8 (2014) 9531-9541. [58]

M.

Kenisarin,

K.

Mahkamov,

Salt

hydrates

as

latent

heat

storage

materials:thermophysical properties and costs, Solar Energy Materials and Solar Cells, 145, Part 3 (2016) 255-286. [59] Z. Lei, N. Christov, X. Zhao, Intercalation of mesoporous carbon spheres between reduced graphene oxide sheets for preparing high-rate supercapacitor electrodes, Energy & Environmental Science, 4 (2011) 1866-1873. [60] N. Basri, B. Dolah, Physical and electrochemical properties of supercapacitor electrodes derived from carbon nanotube and biomass carbon, International Journal of Electrochemical Science, 8 (2013) 257-273.

22

Figures

Figure 1. FESEM images of (a) ZCo positive electrode and (b) RZCo positive electrode.

Figure 2. Growth of (a) ZCo and (b) RZCo on graphite sheet. The eventual particle size of ZCo was dependent on the presence of GO. The graphite sheet acted as a current collector for the as-assembled supercapacitor.

23

Figure 3. (a) XRD profile and (b) FTIR spectrum of RZCo positive electrode. Inset shows the magnified XRD peaks of the RZCo positive electrode.

Figure 4. XPS analysis of (a) wide scan, (b) Zn2p, (c) Co2p, (d) O1s, and (e) C1s of RZCo positive electrode.

24

Figure 5. (a) Nyquist plot, (b) ragone plot, and (c) life retention of ZCo//PyR and RZCo//PyR asymmetric supercapacitor. Inset shows the magnified high-frequency region, where the life cycle was measured at a current density of 2 mA/cm2. The energy and power densities of the as-fabricated asymmetric supercapacitor compared to those of the KEMET commercial supercapacitor.

Figure 6. (a) Nyquist plot and (b) life cycles of asymmetric supercapacitor at different operating temperatures (0 °C, 30 °C, and 60 °C).

25

Figure 7. Specific capacitance values of KEMET commercial supercapacitor and RZCo//PyR supercapacitor from (a) cyclic voltammetry at scan rates of 100 mV/s to 2 mV/s, (b) galvanostatic charge/discharge at current density of 2 mA/cm2 to 20 mA/cm2, (c) EIS, and (d) life cycle at current density of 2 mA/cm2. Inset shows the tabulated IR drops of the KEMET commercial supercapacitor and RZCo//PyR supercapacitor.

26

Tables

Table 1. Results of electrical conductivity measurement performed on (a) ZCo and (b) RZCo graphite sheet positive electrodes. Resistivity, x10-2 (Ω) 1.384 1.197

Positive Electrode (a) ZCo (b) RZCo

Electrical conductivity (S/m) 72.254 83.542

Table 2. Energy density and power density of as-fabricated asymmetric supercapacitor against KEMET commercial supercapacitor at scan rate of 2 mV/s. Asymmetric supercapacitor

Energy density (Wh/kg)

Power density (W/kg)

ZCo//PyR RZCo//PyR KEMET

14.6 41.8 17.7

65.9 188.3 22.1

Table 3. EIS and life cycle values of RZCo//PyR asymmetric supercapacitor at 0 °C, 30 °C, and 60 °C. EIS was performed from a starting frequency of 100 kHz to an ending frequency of 0.01 Hz, while the life cycle test was performed at a current density of 2 mA/cm2. Temperature (°C)

0

30

60

ESR (Ω)

5.6

0.9

1.1

Rct (Ω)

66.2

16.5

4.8

Retention (%)

49

87

86

27