Fabrication of an Advanced Asymmetric Supercapacitor based on

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Fabrication of an Advanced Asymmetric Supercapacitor based on Three-Dimensional Copper-Nickel-Cerium-Cobalt Quaternary Oxide and GNP for Energy Storage Application Lopamudra Halder, Anirban Maitra, Amit Kumar Das, Ranadip Bera, Sumanta Kumar Karan, Sarbaranjan Paria, Aswini Bera, Suman Kumar Si, and Bhanu Bhusan Khatua ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00038 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Applied Electronic Materials

Fabrication of an Advanced Asymmetric Supercapacitor based on Three-Dimensional Copper-Nickel-Cerium-Cobalt Quaternary Oxide and GNP for Energy Storage Application Lopamudra Halder, Anirban Maitra, Amit Kumar Das, Ranadip Bera, Sumanta Kumar Karan, Sarbaranjan Paria, Aswini Bera, Suman Kumar Si, and Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur- 721302, West Bengal, India.

*Corresponding Author Dr. B. B. Khatua (Email: [email protected]). Materials Science Centre, Indian Institute of Technology, Kharagpur – 721302, India. Tel.:+91-3222-283982

ACS Paragon Plus Environment

ACS Applied Electronic Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In the present work, we demonstrate a cost-effective synthesis of 3D quaternary coppernickel-cerium-cobalt oxide (Cu-Ni-Ce-Co oxide) through a one-step hydrothermal protocol followed by a heat treatment process. The mesoporous Cu-Ni-Ce-Co oxide (with pore diameter 4.34 nm) shows a higher specific surface area (86.9 m2 g−1). The as-synthesized quaternary oxide provides an ultrahigh specific capacitance of 2696 F g−1 at 1 A g−1 along with a moderate cycle stability of 86.5 % after 3000 charge-discharge cycles. Furthermore, an asymmetric supercapacitor (ASC) was established by assembling Cu-Ni-Ce-Co oxide and graphene nano-platelets (GNP) as positive and negative electrode materials respectively and the supercapacitor performances were executed thoroughly. The ASC delivers a remarkable energy density of  51 Wh kg −1 at a power density of 581.9 W kg −1 together with long-term cyclic stability (92% specific capacitance retention after 3000 cycles). The compositional and morphological features together with superior electrochemical properties can make it advantageous for practical use in energy and power applications.

Keywords: Quaternary oxide, specific capacitance, energy density, mesoporous, crosslinked.

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1. INTRODUCTION The ever-rising energy crisis and depletion of fossil fuel have instigated the researchers for the exploration of an efficient energy storage system. Nowadays, supercapacitor or ultracapacitors have emerged as an eminent energy storage device to accomplish the demand of energy scarcity. Owing to rapid inflation of portable electronics, hybrid vehicles, fabrication of such device has drawn considerable attention by researchers.1,2 High output performance (higher power density, good rate capability, prolong cycle life) accompanied by excellent cyclic stability and environment-friendliness are the salient features of a supercapacitor.3,4,5,6 On the basis of energy storage mechanism, supercapacitors can be classified in two classes: electric double layer capacitors (EDLCs) and pseudocapacitors.7,8 Usually EDLCs involve porous, carbonaceous materials like (graphene,9 heteroatom (N2,10 B,11 Fe,12 S13) doped graphene, carbon nanotubes, carbon nanohorns) while, pseudocapacitors

solely employ binary, ternary or even quaternary metal oxides, hydroxides. From the device fabrication approach, supercapacitors are categorized into two parts. (a) symmetric (with sandwiching two similar type supercapacitor electrodes)14 and (b) asymmetric (sandwiching two dissimilar type electrode materials).15 Asymmetric supercapacitors (ASC) based on a battery type positive electrode and capacitor type negative electrode material have drawn considerable interest due to its enlarged potential window, which eventually improves the energy (E) and power density (P).16 Ternary or higher order metal oxides are being widely employed as positive electrode material in supercapacitor application owing to their high specific capacitance, composition tunability and low cost. The rationale behind working on the quaternary metal oxide is as follows: these mix oxides through their mixed and various valence states impart greater electrochemical conductivity and enhance the specific capacitance (Cs) compared to their corresponding monometallic and bimetallic oxides.

17,18

The prominent morphological features (nanostructure and three-dimensional (3D)

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architecture) and large specific surface area promote the rate of ion-diffusion resulting greater electrochemical activity19,20,21. However many studies have been reported related to ternary metal oxides but very limited work has been found on quaternary metal oxides. For instances, Tu et al. reported the synthesis of Mn-Ni-Co ternary oxide nanowires revealing a specific capacitance (Cs) of 638 F g−1 at 1 A g−1.22 Zhang et al. acquired a Cs value of 2481.8 F g−1 at 1 A g−1 for Zn-Ni-Co nanowire arrays.23 Ryu et al. reported a specific capacity of 178 C g−1 at 1 A g−1 specific current for Cu-Zn-Co oxide nanoflakes grown on Ni- foam.24 Yao et al. reported an ultrahigh specific capacitance of 490.7 F/cm3 at 1 mA/cm2 current density for hierarchical molybdenum-nickel-cobalt ternary oxide.25 Wei et al. synthesized Ce-Ni-Mn-Co oxide through solvothermal method, which results Cs value of 1840.6 F g−1 at 1 A g−1.26 Another quaternary Zn-Ni-Al-Co oxide has been reported by Zhang et al., exhibiting a specific capacity of 839.2 C g−1 at 1 A g−1.27 However, studies related to electrochemical activity of Cu-Ni-Ce-Co oxide is rare. The electrochemical contribution of the quaternary mix oxide (with Cu, Ni, Ce and Co ions) can provide rich redox reactions compared to their mono or bi-metallic oxides due to synergistic effect and enhanced oxidation states, which improves the capacitance property. Cu possesses ultrahigh electrical and thermal conductivity.28

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Ce is a rare earth element of 4f series with high electrical conductivity.30,31

Ni is the most abundant element on earth. It increases capacitive behaviour and active site density.32,11,21 Co is beneficial in increasing electronic conductivity. The electrochemical activity of the metal oxides still suffers from low conductivity and structural degradation6, resulting lower energy density. However, a decent energy density along with a remarkable power density can be achieved by building up hybrid asymmetric supercapacitor in an aqueous electrolyte. These hybrid supercapacitors comprise of a battery type electrode (as source of energy) and a capacitor type electrode (as source of power). Therefore, use of the two different potential windows from the two electrodes can provide maximum voltage

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window.

33,34,35,36

. Furthermore, synthesizing the nanostructured, freestanding material of

high electrochemical activity without any conductive substrate (nickel foam37, carbon cloth/ fiber38, copper foam39) is a great challenge. In this work, a multicomponent electrode material based on Cu-Ni-Ce-Co oxide has been developed through a facile and substrate-free (Ni-foam, Cu-foam, carbon fabric/ cloth) hydrothermal route followed by a post-annealing treatment. The precursor shows a 3D crosslinked, nanoflake structure, which on annealing collapses and numerous nanoparticles (68−73 nm) were grown on the surface of nanoflakes. The increased porosity (pore diameter 4.34 nm) and ultrahigh surface area (86.9 m2 g−1) of the electrode material further boosts the ion transportation phenomenon and expedites electrode–electrolyte interaction. The electrode material in two-electrode configuration, exhibits Cs of 2696 F g−1 at 1 A g−1 which shows 86.5 % Cs retention after 3000 charge –discharge cycles. Furthermore, GNP gives Cs of 163 F g−1 at 1 A g−1 current density. To meet the requirement of higher energy density together with wider potential window, an ASC (asymmetric supercapacitor) has been fabricated using assynthesized quaternary oxide as positive and GNP as negative electrode materials including 1 M aqueous KOH soaked Whatman filter paper serving as a separator. The ASC can reversibly perform in the potential window of 0 to 1.4 V. The ASC reveals ultrahigh Cs of 183.3 F g−1 at 1 A g−1 with remarkable energy density of  51 Wh kg 581.9 W kg

−1

−1

at power density of

. It reveals significant cycle stability of 92 % Cs retention even after 3000

charge-discharge cycles. 2. EXPERIMENTAL 2.1. Materials Details Cobalt chloride hexahydrate, nickel chloride hexahydrate, copper chloride hexahydrate, cerium sulphate hexahydrate, urea, and ammonium fluoride were obtained from Merck Chemicals, India. Polyvinylpyrrolidone (PVP) and potassium hydroxide (KOH) were

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purchased from Loba Chemie. Graphene nano platelets (multilayered, carbon purity >99.5%, thickness: 8- 10 nm) were obtained from J. K Impex, Mumbai. Deionized water (D.I) used throughout the work was collected from JL-RO100 Millipore-Q Plus water purifier. All the analytical grade reagents were used without any further purification. Commercially available stainless steel (SS) fabric was used as a conducting current collector. It was rubbed with emery polish paper followed by cleaning with distilled water and ethanol by ultrasonication. Flexibility of the SS fabric is an added advantage, which includes flexibility in the asprepared ASC device. 2.2. Synthesis of quaternary oxide The hybrid material was synthesized through a simple, cost-effective one-step hydrothermal procedure followed by a post-annealing treatment. In the typical process, 2 mM CoCl2, 6H2O, 0.2 mM NiCl2, 6H2O, 0.5 mM CuCl2, 6H2O and 0.3 mM Ce2(SO4)3, 6H2O were dissolved in 50 ml DI water along with addition of 1− 2 g of PVP. The solution was stirred for half an hour to form a clear solution. Afterwards, 1 mM of Co(NH2)2 and 0.3 mM of NH4F were added into the above reaction mixture. It was then transferred into a 50 ml Teflon lined stainless autoclave and heated in a muffle furnace at temperature of 150oC for 8 h. After cooling to room temperature, the resultant product was rinsed with DI water and ethanol eventually. Finally, the product was dried in hot oven at ~50oC for 12 h. The asobtained product was calcined at 300C for 4 h. Finally, a black product was achieved and termed as Cu-Ni-Ce-Co oxide. Prior to compare the electrochemical properties Co oxide, CuCo oxide, Cu-Ni-Co oxide were prepared following the same procedure using respective materials.

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2.3. CHARACTERIZATIONS The crystallographic information of as synthesized electrode material was identified through X-ray diffraction (XRD) analysis using X-Pert PRO diffractometer [PANalytical, Netherland] with monochromatic Cu Kα radiation (λ=0.15418 nm), at an accelerating voltage and current of 40 kV and 30 mA, respectively. The measurement was done over the range (2) of 10° to 80° at a scan rate of 0.5oC/ min. Field emission scanning electron microscopic (FESEM) analysis was performed to observe the morphology of the prepared materials, conducted in Carl Zeiss-SUPRA 40 (Oberkochen, Germany) at an working voltage of 5 kV. Microstructure of the electrode materials was identified through high-resolution transmission electron microscopic (HRTEM) analysis (HRTEM: JEM-2100, JEOL, Tokyo, Japan), operated at an accelerating voltage of 200 kV. The TEM-EDX mapping was also performed during TEM analysis. Nitrogen adsorption isotherm was conducted using a Quantachrome ChemBET TPR/TPD analyzer at 77 K. The active surface area was measured by introducing Brunauer–Emmett–Teller (BET) model while the pore size and volume were calculated by Barrett–Joyner–Halenda (BJH) method. The electrochemical measurements i.e. cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS) were performed on Biologic (SP-150) electrochemical workstation to evaluate the supercapacitive performances of the synthesized materials. The preparation and successive fabrication of the corresponding ASC device is presented in Figure 1. 2.4. Preparation of working electrodes. Electrochemical measurements of the prepared electrode and GNP (active materials) were carried out in three-electrode set up. The working electrodes were prepared in the following method: the active materials were mixed with acetylene black (AB) and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1 (w/w/w) and were dispersed in N-methyl-2-pyrrolidone (NMP) solvent to make a uniform slurry. The slurry has been coated on a piece of stainless

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steel (SS) mesh fabric of dimension 1 1 cm2 (SS fabric was used as current-collector here,) and dried for 8 h at 60−65° C. The electroactive materials coated SS fabric was used as working electrode, while platinum and saturated calomel electrodes (SCE) were employed as a counter and reference electrodes, respectively. An ASC device has been fabricated employing Cu-Ni-Ce-Co oxide coated SS as positive and GNP coated SS as negative electrodes material, using Whatman 40 filter paper soaked in 1 M KOH as a separator. The porosity of the filter paper accelerates the diffusion phenomena of the electrolyte. The mass ratio (m+: m-) of positive to negative electrode (Cu-Ni-Ce-Co oxide: GNP) was calculated to be 0.1818.

Figure 1. Schematic illustration for the synthesis of electrode material and assembly of ASC device. 3. RESULTS AND DISCUSSION The structure and chemical composition of the calcined electrode material were investigated by XRD analysis (Figure 2). The as-obtained peaks of the calcined electrode material confirm the development of Cu-Ni-Ce-Co oxide, since the peaks match well with

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cubic phase of Cu0.48Ni0.16Co2.36O4 of JCPDS card number 054-0844 of space group Fd3m and Ce7O12 phase of JCPDS card number.071-0567. The major diffraction peaks appeared at 2 values 18.8, 31.1, 36.6, 38.3°, 44.1, 55.4, 59.03, 64.9 were indexed to (111), (220), (311), (222), (400), (422), (511), (440) crystal planes of Cu0.48Ni0.16Co2.36O4, respectively. Additionally, the sharp and intense peak at 27.8 along with a tiny peak at 32.4 reveal (211) and (113) crystal planes of Ce7O12 (JCPDS card number.071-0567). The diffraction peaks of the quaternary oxide shows a slight change in its peak position due to the differences in ionic radius of Co, Cu, Ni and Ce.30

Figure 2. XRD patterns of (a) Cu-Ni-Ce-Co oxide (b) JCPDS card number 054-0844 and (c) JCPDS card number 071-0567. The surface morphology of the precursor and the hybrid material was evaluated through FESEM analysis. The FESEM images (Figure 3a−c) of the precursor at lower magnification (2 and 1 µm) revealed the formation of interconnected 3D nanoflake-based nanoflower morphology. The high magnification FESEM images exhibit highly ordered nanoflakes with smoother surfaces. The interconnected nanoflakes with an average width of

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42−52 nm were found to cross-link with each other to form a three-dimensional interconnected network structure. A minute amount of nanoparticles (size of 48− 60 nm) were scattered on the smooth surface of the nanoflakes. After annealing, the cross-linked network structure of the precursor composite was well retained but the Cu-Ni-Ce-Co precursor got dehydrated to form the oxide with collapsing the smooth nanoflake architecture.40 Each nanoflake was found to split into few layers finally transforming into thinner and looser nanostructure that probably provide more open space and large numbers of active sites for redox reaction to take place. The surface became rough as numerous nanoparticles (size of  68−73 nm) were anchored on the surface of the nanoflake layers (Figure 3d−f). This surface- roughness can increase the active surface area through which electrolyte can penetrate into the deeper pores of the electrode, facilitating easy electrolyte access to boost the capacitive performance41. The microstructure of the Cu-Ni-Ce-Co oxide material after annealing was further confirmed by HRTEM analysis. Figure 3g displays dense nanoflake morphology with transparent outer regions, which eventually signifies the porous nature of the material. The appearance of numerous nanoparticles is observed on the surface of the nanoflakes in the HRTEM image, consistent with the FESEM analysis. These large numbers of nanoparticles produce many redox active sites, which promotes the electrochemical reaction. The nanoparticles show a uniform distribution with size ranging from 33−41 nm. This mesopores help in electrolyte penetration into electrodes and fast charge transfer reaction. The HRTEM image (Figure 3h) reveals the presence of different lattice fringes of the quaternary oxide. The interplanar spacing of 0.24, 0.23, 0.20 and 0.15 nm were measured corresponding to (311), (222), (400) and (511) lattice planes of Cu-Ni-CeCo oxide. The selected area diffraction pattern (SAED) as represented in Figure 3i shows well-defined diffraction rings indicating polycrystalline nature of Cu-Ni-Ce-Co oxide. The

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diffraction rings can be indexed to (111), (311), (400) and (511) planes which agrees well with the XRD result.

Figure 3. (a−c) FESEM images of Cu-Ni-Ce-Co precursor, (d−f) FESEM images of Cu-NiCe-Co oxide, (g, h) HRTEM images and (i) SAED pattern of Cu-Ni-Ce-Co oxide. Figure 4 represents electrochemical measurements of all prepared electrode materials in three-electrode configuration utilizing 1 M aqueous KOH solution. Before performing the electrochemical characterizations, the electrodes were charged /discharged for ~100 recurring cycles at constant current density (1 A g−1) to obtain a stable Cs value,26 that enables adequate wetting of the surface of the electrodes by aqueous KOH electrolyte. CV analysis was carried out to examine the redox reaction occurring between the metal ions and the alkaline electrolyte. For comparison of electrochemical properties, CV analysis of Co

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oxide, Cu-Co oxide, Cu-Ni-Co oxide and Cu-Ni-Ce-Co oxide (as presented in Figure 4a) have been performed at 2 mVs–1 within the potential window of 0 to 0.4 V. Figure 4b reveals the CV plot of the hybrid electrode (Cu-Ni-Ce-Co oxide) material between a voltage window of 0 to 0.4 V (vs. Hg/Hg2Cl2) at different scan rates of 2, 5, 10, 30, 50, 80 and 100 mV s–1. A couple of well-defined strong redox peaks (anodic peak at 0.338 V and cathodic peak at 0.215 V) were observed in the CV curve of hybrid electrode material, demonstrating the faradaic capacitive behaviour. The redox peaks are associated with the following probable quasi-reversible redox reaction of Cu-Ni-Ce-Co oxide mediated by the OH− ion from KOH electrolyte. (Cu Ce)NiCoO +H2O+OH− CoOOH+ OH−

CoOOH+ NiOOH+ e−

CoO2+H2O+e−

(I) (II)

The shape of the CV curve remains analogous even at higher scan rates revealing excellent electrochemical reversibility and rate capability of the electrode material.42 The peak current density increases linearly with corresponding scan rates implying rate of electron transport is rapid enough even at much higher scan rates.43Cu-Ni-Ce-Co oxide electrode shows larger integrated area under the CV curve than Cu-Ni-Co oxide, Cu-Co oxide, Co oxide, suggesting better electrochemical performance of the quaternary oxide system which is attributed to fast charge transport and better electronic conductivity.44 Figure 4c represents discharge curves of electrode materials Co oxide, Cu-Co oxide, Cu-Ni-Co oxide and Cu-NiCe-Co oxide at 1 Ag−1 current density within a potential window of 0 to 0.4 V. The longer discharge curve of Cu-Ni-Ce-Co oxide demonstrates the higher specific capacitance and higher electrochemical performance. Figure 4d depicts GCD profile of Cu-Ni-Ce-Co oxide at different current densities of 1, 2, 4, 8, 10, 20 A g

−1

. The typical nonlinear discharge

curve with a voltage plateau between ~0.4–0.15 V validates pseudocapacitive nature of electrode material signifying the quasi-reversible redox reaction occurring at the electrode-

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electrolyte interface. The Cs value was calculated using the following relation based on the discharge current densities of the electrode material:

Cs =

i  t m  V

(1)

Where Cs is the specific capacitance in F g−1, (i/m) represents current density in A g−1, V and t denote potential window in volt and discharge time in second respectively. The Cs value (2696 F g −1) obtained for Cu-Ni-Ce-Co oxide (active mass of 0.008 g) at 1 A g−1 is higher than other as-synthesized electrode materials, i.e. for Cu-Ni-Co oxide 2180 F g−1 (active mass: 0.008 g), Cu-Co oxide 800 F g −1 (active mass: 0.009 g) and Co oxide 570 Fg

−1

(active mass:0.006 g). The Cs values for Cu-Ni-Ce-Co oxide were found to be 2696,

2685, 2649, 2587, 2525 and 2412 F g

−1

at 1, 2, 4, 8, 10 and 20 A g−1 current densities,

respectively. The calculated Cs of individual oxide materials as a function of discharge current density has been shown in Figure 4e. The Cs value decreased with enhanced current density because of unavailability of redox active sites for further participation in redox reaction and increased voltage drop. 45 The hybrid quaternary oxide electrode reveals a good rate capability with retention of Cs of 89.4% even at higher current density of 20 A g–1. Apart from CV, GCD test cycling stability is a significant parameter to evaluate the stability of the electrode material. Cycling performance was evaluated at a constant current density of 1 A g

–1

for 3000 GCD cycles. It reveals a retention of 86.5% of initial Cs after 3000

consecutive GCD cycles (as in Figure 4f).

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Figure 4. (a) CV plots of four different electrode materials at 2 mV s−1 scan rate, (b) CV plots of Cu-Ni-Ce-Co oxide.at various scan rates, (c) GCD plots of all electrode materials at 1 A g −1 current density, (d) GCD plot of Cu-Ni-Ce-Co oxide.at various current densities, (e) Variation of Cs of electrode materials with applied current density, (f) cyclic stability of CuNi-Ce-Co oxide. The resistive behavior of Cu-Ni-Ce-Co oxide, Cu-Ni-Co oxide, Cu-Co oxide and Co oxide was investigated through EIS analysis (as presented in Figure 5), carried out in a frequency range of 1 MHz–100 mHz. The corresponding equivalent circuit diagram has

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been provided on the inset of the Figure 5. As perceived from the Nyquist plot, it is observed that each of the impedance spectrums has two major parts, a small arc in the high-frequency region followed by a linear part in the low-frequency region. The semi-circular part denotes the charge transfer resistance (Rct), while the steeper portion at the lower frequency region signified presence of Warburg impedance (W) which demonstrates the diffusion of electrolyte within the electrode material. The initial intersection point of the semicircular region with the real impedance axis at high frequency region denotes the solution resistance (Rs). The Nyquist plot of the hybrid electrode material does not indicate an ideal capacitive behaviour so a best-fitted curve i.e. constant phase element (CPE) has been incorporated along with the equivalent circuit diagram. The fitted circuit comprise of several resistances (Rs, Rct,

W, CPE) together with double layer capacitance (Cdl) arranged in a parallel series.

Therefore, the circuit resistance has been expresses as: Rs+Q/(Rct + W) + Cdl/ Rct. The charge transfer resistance and solution resistance for Cu-Ni-Ce-Co oxide were found to be 1.171  and 0.287 , respectively, which are lower than Cu-Ni-Co oxide (Rct 1.251 , Rs. 0.4197 ), Cu-Co oxide (Rct. 1.38 , Rs. 0.825 ).and Co oxide (Rct. 1.63 , Rs. 1.098 ).

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Figure 5. Nyquist plot of Cu-Ni-Ce-Co oxide, Cu-Ni-Co oxide and Cu-Co oxide. (in the inset Co oxide). The equivalent fitted circuit diagram is presented in the upper portion of the graph. In order to explore practical application of the prepared electrode material in supercapacitor energy storage device, an ASC device was fabricated utilizing Cu-Ni-Ce-Co oxide coated stainless-steel as positive (Cu-Ni-Ce-Co oxide@SS) and GNP coated stainless steel (GNP@SS) as negative electrodes including 1 M KOH soaked laboratory Whatman filter paper. To evaluate a stable voltage–window of the ASC, CV analysis was conducted for both the positive and negative electrodes at 2 mV s–1 scan rate. The CV curve of Cu-NiCe-Co oxide@SS reveals pseudocapacitive characteristic with a pair of prominent redox peaks ranging from 0 to 0.4 V. On the other hand, the CV curve of GNP@SS is nearly rectangular-shaped, indicating EDLC behaviour in a potential window of −1 to 0 V (as shown in Figure 6a), and the overall capacitance is joint contribution from EDLC and pseudo capacitor property. Therefore, the total operating potential window was estimated as the sum of potential windows of the positive and negative electrodes. Thus, a potential window of 0 to 1.4 V has been fixed for the electrochemical characterization of ASC. The

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charge-balance equation (q+≈ q–) was employed to achieve the mass ratio of Cu-Ni-Ce-Co oxide@SS to GNP@SS. The total charge stored in electrode material has been calculated from the equation: q = C  V  m..........(2)

Where, C, ΔV and m denote specific capacitance (F g−1), working potential (V) and mass of the active material coated on SS, respectively. The mass ratio of the positive and negative electrode materials was calculated by following equation, where m+ and m− are the effective masses, C+ and C− are specific capacitance, ΔV+ and ΔV− are working potentials of positive and negative electrodes, respectively: m+ C−  V− = ...........(3) m− C+  V+

Using the charge balance equation, the obtained mass ratio for positive to negative electrode was ≈ 0.1818. Figure 6b exhibits the typical CV curves of as–fabricated Cu-Ni-Ce-Co oxide@SS//GNP@SS ASC in a potential window of 0 to 1.4 V. The appearance of welldefined redox peaks in between 0.55 V to 1.4 V depicts pseudocapacitive characteristic and the almost rectangular shape from 0 to 0.55 indicates the existence of EDLC features in the ASC. The corresponding GCD curves are presented in Figure 6c at the varying current densities of 1, 2, 4, 8, 10 and 20 A g

−1

. The discharge curve shows slight nonlinearity,

which is in accordance with CV results. The Cs values were obtained to be 183.3, 171.7, 155.8, 138.4, 129.6, 117.2 F g−1 at 1, 2, 4, 8, 10 and 20 A g

−1

current density (based on

active mass of ASC). The cycle stability test (represented in Figure 6f) of ASC was carried out over 3000 charge-discharge cycles at 1 A g−1 current density. Interestingly, the device exhibited 92% retention of initial Cs even after 3000 charge –discharge cycles, depicting a long-term cyclic stability. Energy and power density calculation is a key parameter in

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evaluating electrochemical performance of ASC. The energy and power density were calculated according to following equations, E ASC =

1  C ASC  (V ) 2 2

PASC = E ASC / T

(4) (5)

Here, EASC indicates energy density (Wh kg−1), ΔV symbolizes voltage drop during discharge process, CASC is the total specific capacitance (F g−1), PASC implies power density in W kg−1 and T represents discharge time. In the Ragone plot (represented in Figure. 6e) the maximum obtained energy density was  51 Wh kg−1 at a power density of 581.9 W kg−1. Even at high power density of 9849.5 W kg−1 the energy density was attained 31.9 Wh kg−1, which is competitive with those reported earlier. The electrochemical performance of our asfabricated ASC device with previously reported other ASC devices has been displayed in Table 1. ASC based materials

Operating Voltage range (V)

Cs ( F g–1 )

CuCo2O4@ CuCo2O4//AC ZNCO nanowires arrays//AC Ni-Co-Fe-S@NCAsNP Flower like Ni-Zn-Co oxide //AC CuCo2S4@Ni-Mn LDH//AC ZICO//NG

1.5

57.6 F g−1 at 2 mA/cm2 113.9 F g−1 at 1 A g –1 114 F g−1 at 0.2 A g–1 −

Cu-Ni-Ce-Co oxide@SS//GNP@SS

1.4

1.5 1.5 1.6 1.5 1.5

146.7 F g−1 at 2 A g –1 129.74 F g−1 at 1 A g−1 183.3 F g−1 1 A g−1

Energy density (Wh kg−1) 18

Power density (Wh kg−1) 125

References

35.6

187.6

44

35.9

375

45

44.5

880

46

45.8

1499

47

40.5

750

30

51

581.9

This work

43

Table 1: Comparison of electrochemical performances of our as prepared ASC device with other reported asymmetric supercapacitors.

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Moreover, a practical application of the ASC has been demonstrated by illuminating one red light-emitting diode (LED). Two ASC devices were connected serially and charged for 30 s. Afterwards, during discharging; the discharge current from the ASCs flows through the red LED and lightens the red LED bulb. The variation of intensity of the single red LED light just after connection, after 5 min, 7 min and 10 min intervals respectively are presented graphically in Figure 7a–d.

Figure 6. (a) CV profiles of Cu-Ni-Ce-Co oxide and GNP at a scan rate of 2 mV s−1, (b) CV plots of ASC device at various scan rates, (c) GCD plots of ASC at different current density,

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(d) variation of Cs with various current density, (e) Ragone plot of ASC device, (f) Cs retention after 3000 consecutive charge-discharge cycle

Figure 7. Photograph of the ASC device lightening the red LED light (a) after connection and at different time intervals (b) after 5 minutes, (c) after 7 minutes, and (d) after 10 minutes

The superior electrochemical performance of the ASC can be attributed to the various reasons. Firstly, the complex configuration with variable mixed valence states and strong synergy property of different metal ions (Cu, Co, Ni, Ce ions) enhances the electrochemical performances. Secondly, the three-dimensional (3D) network architecture with distinct mesoporous feature and large open spaces allow the complete utilization of the electrode material by electrolyte resulting large number of electro-active sites for the redox reaction to take place hence improved the specific capacitance and cyclic stability. The rough surface feature increases the active charge-density; therefore easy penetration of the electrolyte into the inner pores of the electrode material improves the charge transfer process

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and rate capability. Moreover, the electronic conductivity of metal ions, ultrahigh surface area and porous structure are the added pluses for achievement of better electrochemical outcome. 4. CONCLUSION Based on above discussion, it can be demonstrated that 3D quaternary mesoporous Cu-NiCe-Co oxide nanoflakes has been synthesized through a facile and cost effective hydrothermal protocol followed by subsequent heat treatment process. The self-supported 3D nanoflakes endowed with mesoporous feature and rough surface escalate the electrochemical properties. An outstanding specific capacitance value of 2696 F g−1 was obtained from the quaternary oxide material. The high-performance ASC device based on Cu-Ni-Ce-Co oxide@SS and GNP@SS (as positive and negative electrode materials respectively) exhibits a specific capacitance value of 183.3 F g−1 at 1 A g−1 current density. Moreover it delivered a remarkable energy density of  51 Wh kg−1at a power density of 581.9 W kg−1 maintaining superior cyclic stability (≈ 92 % after 3000 charge-discharge cycles). Furthermore, two serially connected ASC devices Cu-Ni-Ce-Co oxide //GNP can successfully power up one red LED bulb. From the resultant features, it can be envisioned that the Cu-Ni-Ce-Co oxide can serve as self-supported electrode material contributing an ultrahigh specific capacitance value and the as–fabricated ASC device can perform as an eminent candidate for energy storage applications.

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ASSOCIATED CONTENT Supporting Information TEM-EDS mapping analysis, surface area and pore size distribution analysis, electrochemical performance of GNP as negative electrode, Electrochemical performances of prepared electrode materials, Comparison of ASC device performance employing other different mixed oxides as positive electrodes. AUTHOR INFORMATION Corresponding Author *(B. B. Khatua) E-mail [email protected]; Tel +91-3222-283982 ORCID Bhanu Bhusan Khatua: 0000-0002-1277-0091 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We are extremely thankful to Indian Institute of Technology Kharagpur for financial support.

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Table of Content 82x33mm (300 x 300 DPI)


Fabrication of an Advanced Asymmetric Supercapacitor based on

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