mesoporous carbon nanocomposite synthesis, morphology and electrochemical properties for gel polymer-based asymmetric supercapacitors

Journal of Electroanalytical Chemistry 852 (2019) 113504

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Copper oxide/mesoporous carbon nanocomposite synthesis, morphology and electrochemical properties for gel polymer-based asymmetric supercapacitors Balakrishnan Saravanakumar a, b, Chandran Radhakrishnan b, c, Murugan Ramasamy d, Rajendran Kaliaperumal b, Allen J. Britten b, Martin Mkandawire b, * a

Department of Physics, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu, 642 003, India Department of Chemistry, Cape Breton University, 1250 Grand Lake Rd., Sydney, Nova Scotia, B1P 6L2, Canada c Department of Automobile Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu, 642 003, India d National Centre for Earth Science Studies, Ministry of Earth Sciences, Thiruvananthapuram, India, 695011 b

A R T I C L E I N F O

A B S T R A C T

Keywords: Copper oxide Gel polymer electrolyte Mesoporous carbon Asymmetric supercapacitor

A novel nanocomposite was synthesized by combining copper oxide with mesoporous carbon (CuO/MPC) and tested for potential applications as a supercapacitor electrode. Due to the presence of highly conductive carbon, this CuO/MPC nanocomposite is expected to provide additional pathways and more electro-active sites for ion diffusion than a metal oxide electrode alone. Indeed, the CuO/MPC electrode exhibits superior supercapacitive features including the specific capacitance of 616 F g
1 at a current density of 1 A g
1, and better rate capacity and cyclic stability. Further, an asymmetric supercapacitor cell was assembled using the CuO/MPC composite with activated carbon as electrodes, and gel polymer (PVA-KOH) as electrolyte. This asymmetric device displays higher electrochemical performance with an energy density of 26.6 W h kg¡1 and a power density of 438 W kg¡1. This device holds 69% of initial capacitance after 5000 cycles. These results provide a foundation for innovation of next generation energy storage device to fulfil the future demand.

1. Introduction The increasing demand for advanced energy storage systems has kindled extensive research into supercapacitors (SCs). SCs are emerging electrochemical energy storage system holding a number of superior features compared to batteries. For instance, the hallmark features of SCs include, inter alia, rapid power discharge in a short span of time during a high power surge and long cycle life at higher current densities [1]. In SCs, the energy storage occurs by formation of double layer at the electrode/electrolyte interface. The SCs have swift electrolyte ion transport kinetics, which makes them good candidates for use in high power applications like a power station, hybrid electric vehicles and portable electronic gadgets [2]. Despite these superior features, SCs still lag behind both regular capacitors and batteries in actual applications. This is due in part to SCs usually being more expensive per unit than batteries, which is attributed to the cost of the material and developing effective electrodes. The discovery of functional electrode materials is crucial to cost-effectiveness, improved performance and mass production

in this technology. For more than a decade decade, varieties of metal oxide nanostructures have been exploited for applications as electrodes in SCs. Among them, copper oxide (CuO) is highly appealing for the SCs electrode applications due to its ease of preparation, abundance in the earth crust and its robust electrochemical performance [3]. However, CuO has poor electronic conductivity and sluggish ion migration, which are critical in the development of effective electrode. To mitigate these concerns, composite materials of comprising conductive carbon and metal oxides promise better electronic properties than bare metal oxides electrode. The presence of conductive carbon in the composite improves the surface area, electronic conductivity and reduces the volumetric strain during cycling [4]. Consequently, considerable research efforts have been invested in the past few years in developing SC electrodes based on conductive carbon-CuO composite material,. This achieve higher energy density by increasing the potential window and capacitance [3,5–8]. Some of the efforts worth mentioning include the work of Moosavifard et al. [5], who

* Corresponding author. E-mail address: [email protected] (M. Mkandawire). https://doi.org/10.1016/j.jelechem.2019.113504 Received 29 May 2019; Received in revised form 14 September 2019; Accepted 17 September 2019 Available online 5 October 2019 1572-6657/© 2019 Published by Elsevier B.V.

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reducing the expansion of CuO under charge –discharge conditions. The natural occurrence of lithium (Li) salt is limited and slowly getting depleted. It is essential to explore alternate working ions (e.g. OH
, Kþ, Naþ) for energy storage applications. Envisaged are neutral electrolytes, which are safe, having higher electronic conductivity and costeffective. Generally, solid electrolytes are used in SCs to avoid leakage and corrosion problems, but the poor interfacial characteristics leading to low ionic conductivities detracting from the aforesaid advantages. Alternatively, the liquid electrolytes have promising interfacial properties, thereby exhibiting advantages including good ionic conductivity in addition to being cost effective. However, SCs with liquid electrolytes are susceptible to corrosion of metal electrodes and the risk of electrolyte leakage [15]. Consequently, there is a need to develop electrolyte with properties between liquid and solid phase. Thus, development of a polymer gel electrolytes would provide a good alternative electrolyte with properties between liquid and solid phase. They possess good ionic conductivities, provide good mechanical integrity and reduce the corrosion/leakage issues. An example is a polyhydroxy material, poly vinyl alcohol (PVA), which has a few attractive characteristics for supercapacitor manufacturing, including biodegradability, easy preparation procedures, hydrophilic and favourable cross-linking properties [16]. Further, it has been used as a polymer matrix for producing films with higher mechanical strength [17,18]. In this work, we report our novel fabrication of an asymmetric type SC device using a combination of CuO/MPC composite and PVA/KOH gel polymer electrolyte. The CuO/MPC composite was fabricated using a simple solution-based precipitation technique. Blending of MPC with CuO nanostructures significantly upgraded the electrochemical performance of the CuO. This CuO/MPC electrode exhibits higher specific capacitance, good performance rate and better cyclic stability when it is tested for supercapacitor electrode applications. In the assembled asymmetric SC device, the CuO/MPC composite is the positrode, activated carbon is the negatrode and PVA/KOH gel polymer electrolyte is the separator cum electrolyte. Thus, the developed SC device demonstrates very high electrochemical performance.

fabricated composite electrodes of CuO nanorod arrays over carbon nanofibers. Their electrodes had specific capacitance of 398 F g¡1 at a current density of 1 mA cm¡1 in 3M KOH electrolyte and an energy density of 10 kW kg¡1 [5]. In a related work, Moosavifard et al. later hydrothermally fabricated nanoporous CuO, which achieved a specific capacitance of 431 F g¡1 at a current density of 3.5 mA cm¡1 in 3M KOH electrolyte [3]. Ki et al. [6] fabricated CuO–activated carbon nanocomposite using a plasma approach. The activated carbon acted as host material and was supposed to improve the electrochemical properties of the composite. This material possess low charge transfer resistance (1–3 Ω), but exhibits the poor specific capacitance [6]. Some of the authors tried the morphology tuning of CuO nanostructures using structure directing agents. Jagadale et al. synthesized CuO nanosheets, which they tuned morphology with surfactants, dibenzothiazolyl dibenzo-18-crown-6 ether (DDCE) and exhibited a specific capacitance of 130.6 F g¡1 [7]. Most recently, a hybrid composite of CuO/Co2O4/nitrogen enriched MWCNT has been reported by Ramesh et al., which exhibits capacitance of 279 F g¡1 with an energy density of 7.544 W h kg¡1 [8]. Liet al synthesized CuO anchored nitrogen-doped reduced graphene oxide (N-doped RGO) nanosheets and it exhibited specific capacitance of 340 F g
1 at 0.5 A g
1, and 80% of capacity retention after 500 charge and discharge cycles [9]. CuO nanospheres coated with carbon prepared by Zhao et al. had specific capacitance of 335 F g
1 and high cyclic stability of 8000 cycles [10]. Wang et al. prepared Cu2O/CuO/RGO nanocomposite, which exhibited specific capacitance of 173.4 F g
1 with superior cyclic stability (i.e. 100, 000 cycles at 10 A g
1) [11]. Cu2O–CuO/RGO nanocomposite synthesized by Nathan et al. showed the specific capacitance of 436.6 F g
1 with better capacitance retention [12]. In brief, most of these works are focusing on finding the most effective CuO-conductive carbon system and optimizing fabrication techniques. This is mainly due to most of the electrode systems are still lack optimal properties and remain expensive to fabricate. In view of this, we hypothesised that CuO-mesoporous carbon (MPC) nanocomposite would be better electrode alternative, with superior electrochemical properties as well as cheaper and easier to make than other carbon-based materials like RGO, CNT, or Graphene. The MPC has high porosity with pores sizes between 2–50 nm, which can allow guest moieties (electrolyte ions) on their outer, inner surfaces and pores [13]. Higher specific surface area of MPC enhances the electrochemical properties, when it is combined with metal oxide nanostructures [14]. Hence, it is reasonable to anticipate that the combination of MPC and CuO may improve electrochemical performance of the electrode, improving the electronic conductivity by reducing the electrolyte ion contact resistance with active material as well as

2. Experimental 2.1. Fabrication of CuO/MPC nanocomposite Sodium dodecyl sulphate (SDS), Copper nitrate (Cu (NO3)2⋅3H2O), mesoporous carbon (specific surface area >200 m2/g, BET), potassium hydroxide (KOH), Sodium hydroxide (NaOH) and HCl (30%) were purchased from Sigma Aldrich, India. Poly vinyl alcohol (PVA) was procured from Himedia Chemicals, India. All items were utilized as received

Scheme 1. Illustration of the fabrication process of CuO/MPC composite electrodes. 2

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Fig. 1. (a) XRD spectrum of CuO and CuO/MPC. (b) FT-IR spectrum of CuO and CuO/MPC.

Fig. 2. (a) SEM image of MPC (b, c) TEM images of MPC (d) SEM image of CuO (e, f) TEM image of CuO.

dispersion. These samples were named CuO/MPC and CuO respectively.

without further purification. All the solutions were made with Millipore Milli-Q water (18 M cm). An aqueous solution of Cu (NO3)2⋅3H2O (0.82 g in 100 mL of H2O) was blended with a solution containing SDS (0.090 g in 25 mL of H2O) by slow dropping followed by the addition of 2 M NaOH to adjust to the pH ~ 10. Further, MPC dispersion (0.090 g in 25 mL of H2O) was added to the solution mixture and magnetically stirred for 6 h to get homogeneous solution. The solution was aged for 72 h. The samples were collected through centrifugation and dried overnight at 80  C to get the final product (CuO/MPC). The same methodology was adopted to prepare bare CuO nanostructures except the addition of MPC

2.2. Material characterizations The crystallographic information of samples was acquired using PANalyticalX’pert- PRO X-ray diffractometer equipped with CuKα radiation (λ ¼ 1.5406 Å) from 10 to 80 . The Brunauer-Emmett-Teller (BET) method was utilized to determine the surface area of the samples. The nitrogen adsorption-desorption analyses were performed using Micromeritics ASAP 2020 analyzer. The Barrett-Joyner-Halenda (BJH) method 3

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Fig. 3. (a) SEM image of CuO/MPC (b, c, d) TEM images of CuO/MPC.

was used to derive porosity distribution from the desorption branch of the isotherm. The FT-IR spectrum of the samples was recorded using NICOLET 6700 spectrometer. FT-Raman spectrum was recorded using FEKI-Japan (STR-500 mm focal length) laser Raman spectrometer. The morphological investigations were performed using scanning electron microscopy (TESCAN VEGA 3 LMU scanning electron microscope) and transmission electron microscopy (JEOL JEM2100, 200 KeV). EDS elemental analysis and elemental mapping were carried out using Bruker Quantax 200AS system attached with the SEM.

(GCD), electrochemical impedance spectroscopy (EIS) in 2 M KOH basic electrolyte solution. All supercapacitive features were determined based on the mass of the CuO/MPC electrode active material (0.8 mg) in the electrode. The same kind of electrode preparation technique is used for preparation of bare CuO electrodes. 2.4. Fabrication of asymmetric supercapacitor device The asymmetric type SC device was constructed using CuO/MPC as a positive electrode and activated carbon (AC) as negative electrode. Both the electrodes have the geometrical area of 1 cm2. The PVA/KOH polymer gel electrolyte was used as electrolyte/separator. The PVA/KOH gel was prepared by first adding 1 g of PVA in 20 mL of H2O at 70  C under stirring condition. The PVA solution was kept undisturbed for 2 h to get the transparent solution. Then, 10 mL of 2 M KOH solution was slowly added to PVA solution and stirred for 2 h to get homogeneous gel mixture. This gel polymer electrolyte was painted on the electrodes and dried. The formulas used to evaluate electrochemical parameters are given in the supporting information. To calculate the mass balance for anode and cathode material of the device, the following formula (1) was used:

2.3. Electrochemical characterization Electrochemical measurements were performed using electrochemical workstation (CHI 660 C, CH instruments Inc., USA) with three electrode configurations. The Ag/AgCl and a platinum wire were used as reference and counter electrodes. The working electrode (CuO/MPC) comprises of 85 wt% CuO/MPC, 10 wt % Carbon black Super-P, and 5 wt % polytetrafluoroethylene binds with few drops of ethanol. This slurry was pasted onto a current collector (nickel foam, 1 cm2) and dried at 70  C for 4 h. Prior to coating, the Ni foam was immersed into HCl (37 wt %) for 5 min to remove the oxide layer. Similarly, the Ni foam was also cleaned using ethanol and DI water. The prepared electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge discharge

m
Csp þ ΔVþ ¼ mþ Csp
ΔV
4

(1)

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Fig. 4. EDS spectrum and elemental mapping images of CuO/MPC.

3. Results and discussion Scheme 1 illustrates the fabrication process of leaf-like structure CuO and CuO/MPC composites. In sol-gel process, the formation of solid phase is mainly due to the change from supersaturated into the saturated state. In this synthesis process, the anionic surfactant, SDS (CH3 (CH2)11OSO3Na), acted as structure directing agent and linkage to the copper ions. Further, it controls the rate of reaction in the solution mixture, influences size, structure and agglomeration of the particles. The SDS forms micelles with water, when the concentration of the solution exceeds critical micelle concentration (CMC) [19]. Addition of SDS-H2O solution to copper nitrate solution results to the hydrophilic end of the SDS attaching with Cuþ 2 ions. The addition of NaOH to the solution significantly initiates the hydrolysis reaction of the positively charged ion [20]. The intermediate SDS–Cu(OH)2 was formed and acting as a nuclei for further growth [21]. It is valid to mention here, the colour of the solution gradually changed to blue following slow addition of NaOH to copper nitrate solution owing to quick formation of Cu(OH)2nuclei. These nuclei started growing by the aggregation of particles in different orientations during the aging process and form CuO leaf like structure. Here, the CuO nucleates were occurred in the space between SDS micelles and size of the CuO was determined by the SDS aggregation [22]. Further, these micelles leave the CuO nanostructure when it reaches the surfactant breaking point [19]. The surface of MPC particles were activated by ultrasonication process. These CuOleaves were attached with the small sized mesoporous carbon particles. The crystallographic structure of CuO and CuO/MPC samples were revealed using X-ray diffraction analysis. Fig. 1a displays the XRD pattern of CuO and CuO/MPC. Presence of sharp peaks in both samples

Fig. 5. FT-Raman spectra of CuO and CuO/MPC.

where m is the mass of the active material, Csp is the specific capacitance and ΔV is the potential window. The calculated mass balance ratio of anode and cathode was 1: 1.28. In addition, we have extended the potential window from 1 to 1.4 V based on the trail basis.

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Fig. 6. Nitrogen adsorption-desorption isotherms of CuO (a) and CuO/MPC (b). The insets in the figures show the pore-size distribution of samples respectively.

electronic conductivity of the composite material. It is important to note that the sides of CuOleaves are well attached with the MPC. In addition, we have recorded EDS elemental mapping for CuO/MPC nanocomposite and presented in Fig. 4. The EDS spectrum shows the existence of copper (53.6 wt%), oxygen (38.11 wt%) and carbon (8.3 wt %). Most importantly, the EDS elemental mapping images of CuO/MPC indicate the uniform distribution of carbon throughout the composite. The Raman spectroscopy is viewed to be an attractive analytical tool for analysis of carbon-based hybrid materials. Fig. 5 shows the Raman spectra for CuO and CuO/MPC samples. Additionally, the spectrum for MPC is presented in Fig. S5. It possesses the bands at 1353 and 1592 cm
1 indicating the presence D and G bands of carbon. The spectrum of CuO exhibits peaks at 336 and 648 cm
1 corresponds to characteristic peaks of CuO.These peak values deviate from the other values reported in literature for CuO (e.g. 321, 628 cm
1) [33],which suggests that CuO obtain in this study are smaller - increment in peak values suggesting the reduction in grain size [34]. The spectrum of CuO/MPC shows the peaks at 1350 and 1595 cm
1 corresponds to the D and G band respectively. Occurrence of D and G bands in the spectrum suggest the presence of carbon in the composite. The G-band is attributed to the vibration of sp2-bonded carbon and D-band is due to the presence of structural disorder of sp2 carbon [35]. In addition, CuO/MPC shows ID/IG value of 0.82 indicates higher degree of graphitization in the composite [36]. To gain the better insight about the porosity and textural features of CuO and CuO/MPC samples, N2 adsorption-desorption measurements were performed. The BJH model was utilized to derive the pore size distribution profiles of both samples. Generally, pores with size between 0 and 2 nm is known as micro pores and the size between 2 and 50 nm is considered as mesopores [37]. The isotherm of pristine CuO (Fig. 6a) having capillary uptake with less steep suggesting the presence of small pores [38]. The pore size distribution curve of CuO (Inset of Fig. 6a) possesses peaks at 2.1 and 2.4 nm. Fig. 6b shows the isotherm of CuO/MPC with better capillary uptake at higher pressure. Further the pore size distribution of CuO/MPC (inset of Fig. 6b) indicates the presence of many mesopores with diameter 2.4, 3, 4.2, 6.4 and 11 nm. The BET surface area of MPC is 239.7 m2/g. The bare CuO sample possess lower specific surface area (3.29 m2/g) when compared to CuO/MPC composite sample (25.33 m2/g). The addition of MPC with bare CuO significantly improved surface area of the composite material. This enhanced surface area of CuO/MPC with mesopores is expected to provide better electrochemical performance when employed as an electrode active material for SCs.

indicating its crystalline nature. The prominent diffraction peaks matched well with (110), (111), (111), (211), (010), (202), (113), (311), (220), (311) and (222) planes of monoclinic CuO (JCPDS Card No. 050661). In addition, the samples possess C2/C space group with lattice constants of a ¼ 0.4684 nm, b ¼ 0.3425 nm and c ¼ 0.5129 nm: β ¼ 99.47 . The XRD spectrum of MPC (Fig. S1, Supplementary information) possesses strong peaks at 25.1 and 43 , corresponding to (002) and (101) planes of carbon [23]. There were no other peaks, which were observed related to carbon in XRD pattern of CuO/MPC. It is due to the very low weight percentage of MPC in the mixture with CuO [24,25]. It is noted that, the CuO/MPC possesses little broader diffraction peaks and few low intense peaks were not present compared to bare CuO. It is attributed to the presence functional groups of oxygen (carboxylic/hydroxyl) in the surface of MPC. It may disturb the growth and crystallization process of the CuO nuclei [26,27]. In addition, there were no other impurity peaks detected in the spectrum, indicating the existence of pure phase of CuO. Further, FT-IR measurements were performed to identify the functional groups present in the samples. Fig. 1b shows the FT-IR spectrum of CuO and CuO/MPC samples. The bands at 435, 532, 620 cm
1 corresponds to Cu–O stretching vibrations [28]. The band at 1072 cm
1 is attributed to C–O stretching [29]. This suggests the presence of carbon in the composite. The absorptions centred at 1110 and 1191 cm
1 are bending vibrations of Cu–O [28]. The absorption at 1624 cm
1 indicating – O stretching mode [30]. The peaks at 2912, 2920 and the presence of C– 3452 cm
1 corresponding to the C–H and O–H stretching vibration modes [31]. Due to its high degree of carbonization, the spectrum of pristine MPC (Fig. S2) possesses only a few weak absorption bands [28]. Fig. 2a shows the SEM image of MPC and Fig. 2b and c presents the TEM images of MPC. These images indicating the MPC possess sphere morphology with sizes ranges from 20 to 25 nm (Fig. S3). The bare CuO (Fig. 2d–f) exhibiting leaf like morphology with length ranges from ~300 to 800 nm and breath ranges as ~150–300 nm (Fig. S4). The formation of leaf like structure may initiated during nucleation process by the surfactant (SDS). Further, SEM (Fig. 3a) and TEM images of CuO/MPC (Fig. 3b–d) suggesting the attachment of leaf like CuO and porous MPC. The attachment of MPC particles with the surface of the CuO leaves is probably due to the very small size of MPC and its higher surface energies [32]. The MPC particles cover the CuO surface and formed a conductive carbon network like structure. This kind of attachment of carbon with CuO is expected to improve the

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Fig. 7. (a) CV curves at a scan rate of 5 mV s
1 for CuO and CuO/MPC, (b) GCD profiles of CuO and CuO/MPC at the current density of 1 A g
1 (vs Ag/AgCl), (c) Variation of specific capacitance with current density, and (d) Cyclic stability test of CuO and CuO/MPC.

nature of the electrodes. The estimated specific capacitances from GCD measurements are 383 and 616 F g
1 for CuO and CuO/MPC samples respectively. It is valuable to mention that the CuO/MPC electrode shows 38% of enhanced capacitance compared to bare CuO. This enhanced specific capacitance is due to the presence of MPC in the composite. The presence of MPC may create the additional ion buffering reservoirs for energy storage. Further, we have determined the internal resistance (R) of 2.05Ω and 1 Ω forCuO and CuO/MPC with the help of enlarged GCD curves of both electrodes presented in Fig. S7. The lower inter internal resistance for CuO/MPC is attributed to the presence of MPC with CuO. This highly conductive porous carbon effectively reduces the internal resistance. The higher electrochemical performance (i.e. enhanced specific capacitance and lower value of internal resistance) emphasises the need of adding MPC with CuO in the view of achieving. Fig. S6c and d displayed the GCD profiles of CuO and CuO/MPC electrodes at different current densities (1-10 A g
1). In addition, the variations of specific capacitance with various current densities are shown in Fig. 7c. The CuO/MPC electrode retains 229 F g
1 at a higher current density of 10 A g
1, which is higher than many recent reports including Pendashteh

The supercapacitive features of CuO and CuO/MPC samples are revealed using cyclic voltammetric (CV) and galvanostatic chargedischarge (GCD) measurements. Fig. 7a displays the CV curves of CuO and CuO/MPC with a potential window from
0.2 to 0.6 V at a scan rate of 5 mV s
1. The CV curves of the both samples exhibiting the strong cathodic and anodic peaks between 0.1 and 0.4 V which corresponds to the redox reactions of Cuþ/Cu2þ due to the intercalation of OH
electrolyte ions [39,40]. This behaviour of samples evidenced its pseudocapacitive nature of the electrodes. Fig. S6a and b shows the CV curves of CuO and CuO/MPC at different scan rates from 5 to 100 mV s
1. It is noted that the increment of scan rate initiated the shifting of cathodic peak towards negative potential side and anodic peak towards the positive potential. This trend in CV curves indicating the control over redox reaction by diffusion of electrolyte ions and the reversible nature of the electrode active material. To further evaluate the supercapacitive features of CuO and CuO/ MPC samples GCD measurements were utilized. Fig. 7b shows GCD profiles of CuO and CuO/MPC electrodes measured at a current density of 1 A g
1. This non-linear GCD profile evidence the pseudo capacitive

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KOH polymer gel electrolyte (Fig. 9a). The direct blending of viscous PVA and KOH solution forms the PVA/KOH gel. During alkali blending the KOH molecules are taken into polymer matrix by water and initiates dipole-dipole interaction between C–O and OH functional groups leads the formation of the PVA/KOH gel electrolyte [47,48]. The asymmetric type SC devices may be expected to show better performance due to combination of battery type (Carbon) and SC type (metal oxide) electrodes in a single package. For fabrication of SC device, we have utilized PVA-KOH polymer gel as the electrolyte. This polymer gel electrolyte provides a few advantages: (1) The leakage of liquid electrolyte is avoided (2) Gel polymer electrolyte itself acting as separator (3) Operational safety [49]. To estimate balanced capacitance of this asymmetric SC device, the mass of electrode active materials was estimated by mass balancing. A fabrication schematic of CuO-MPC//AC asymmetric SC is presented in Fig. 9a. We have carried out optimization of the potential window by performing electrochemical measurements in different voltage windows. An optimized potential window of 0–1.4 V was identified to further determine the device performance. The CV curves of CuO-MPC//AC asymmetric SC at various scan rates from 10 to 100 mV s¡1are presented in Fig. 9b. CV curves shows better capacitive behaviour and combined effort of double layer capacitance of AC and pseudo capacitance of CuO/MPC. Fig. 9c presents the GCD profiles of CuO-MPC//AC asymmetric SC device at different current densities from 1 to 10 A g¡1. The rectangular shape of the GCD profiles indicates the influence of the pseudocapacitive behaviour of CuO-MPC and rapid ion intercalation with PVA/KOH gel electrolyte. In addition, Fig. S10 presents the variation of specific capacitance with different current densities. The calculated specific capacitance value of 98 F g¡1 at current density of 1 A g¡1. However, it’s noted that the CuO-MPC//AC asymmetric SC shows lower capacitance compared to CuO-MPC electrode. It is due to the increase of mass of the electrode active material. Both positive and negative electrodes of the two-electrode system contributing to energy storage. Here the addition of AC electrode to assemble the SC device notably increases the mass of the SC device and leads the reduction in capacitance [50,51]. Further cycle life test was carried out at the current density of 5 A g
1 (Fig. 9d). This device retains 69% of initial capacitance after 5000 continuous charge discharge cycles. It is noted that the retention of capacitance for this device is higher than the stability of the CuO/MPC electrode measured in three electrode set-up. It is attributed to the presence of high surface area AC electrode in the device, which helps to enhance the stability of the device. The EIS measurements were carried out and respective Nyquist plot is shown in Fig. 9e to analyse the resistive properties of the device. The charge transfer resistance of 78 Ω was observed for CuO-MPC//AC device. It is more appropriate and meaningful to estimate energy and power densities for two electrode configurations compared to three electrode set-up. This device exhibits energy density of 26.6 W h kg¡1 and power density of 436 W kg¡1. These values are estimated using the GCD profile at a current density of 1 A g¡1. The electrochemical performance is comparable with other recent literatures listed in Table S1. The variation of energy density with power density is presented in Fig. 9f. It is valuable to mention here that the power density of the SC system is predominantly governed by energy that is delivered from the SC device per unit time. Also, the reduced specific capacitance due to poor access of electro active material at higher current densities harms the energy density of the SC device. Due to these reasons, there is a dropin energy density at high power densities [52].

Fig. 8. Nyquist plot of CuO and CuO/MPC electrodes.

et al., (CuO/Graphene nanosheet, 110 F g
1 @ 2 A g
1) [41], Li et al., (CuO/Nitrogen doped RGO, 170 F g
1 @ 5 A g
1), [42] Saraf et al., (CuO-Microspheres/RGO, 200 @ 0.5 A g
1), [43] Wang et al., (Cu2O/CuO/RGO nanocomposite, 136.3 @ 10 A g
1) [11]. Cyclic stability of a SC electrode material is an essential parameter towards its commercial exploitation. The cyclic stability of CuO and CuO/MPC electrodes are measured using charge-discharge tests at a current density of 10 A g
1 up to 5000 cycles and displayed in Fig. 7d. The CuO/MPC electrode retains 57% of initial capacitance after 5000 repeated charge-discharge cycles. It is higher than the cyclic stability of bare CuO. Here MPC acted as a physical support for CuO nanostructures and reduces the capacitance fading. In addition, the reduction in the capacitance is attributed to the swelling and structural deformation of the electro active material during charge-discharge process [44]. The morphology of electrode material was significantly altered by the intercalation of electrolyte ion. Even though the capacitance retention of CuO/MPC is higher than bare CuO, the composite material lost 43% of the capacitance after 5000 cycles. Further improvement in stability may attain by forming ternary composite material using some other carbon-based materials (graphene, carbon nanotubes, and etc.). Another way to get good performance in CuO/MPC electrode-based SC device is the selection of good electrode material (negative electrode) in terms of stability. Here, activated carbon (AC) may consider as a electrode material due to its higher specific surface area and mechanical stability. Electrochemical impedance measurements (EIS) were utilized for probing resistive characteristics of CuO and CuO/MPC electrodes. The Nyquist plot of both sample electrodes and equivalent circuit used to fit the EIS data are presented in Figs. 8 and S9 respectively. There are three different regions in the Nyquist plot, including depressed semicircle at high frequency region represents charge transfer resistance (Rct) [45,46]. It is due to the reaction of Cuþ/Cu2þ redox couples. It is observed that CuO and CuO/MPC electrodes show 28 Ω and 11 Ω respectively. This lower value of RctforCuO/MPC electrode is one among the reasons for its higher specific capacitance. The mid frequency region is known as Warburg element. The steeper line at low frequency region denotes capacitive nature of the electrodes. (Perpendicular line for an ideal capacitor) [4]. Generally, a three-electrode system having only one electrode contains active material. Owing to this, complete electrochemical performance of an electrode active material could not be explored. To further test the performance of CuO/MPC, we have assembled asymmetric type SC device using CuO/MPC and activated carbon as electrodes and PVA/

4. Conclusions CuO/MPC nanocomposite can be produced using simple and costeffective procedures and has potential to be a very effective and efficient electrode for supercapacitors. The co-existence of CuO with MPC is 8

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Fig. 9. (a) Fabrication schematic of CuO-MPC//AC, (b) CV curves of CuO-MPC//AC asymmetric supercapacitor at different scan rates, (c) GCD C//AC, (e) Nyquist plot of CuO-MPC//AC, and (f) Ragone plot of CuO-MPC//AC asymmetric device.

asymmetric supercapacitor devices, using PVA/KOH gel polymer as electrolyte. Our device delivers high energy and power densities with good cyclic stability. The developments of this novel CuO/MPC nanocomposite with significantly improved capacitive properties, and PVA/ KOH gel polymer as electrolyte and their proven high performance in the assembled asymmetric supercapacitor device is likely to open new horizons in designing high performance supercapacitors. This is new and

the CuO/MPC nanocomposite effectively enhances the electrochemical properties, exhibiting multi-fold improvement in capacitance and capacitance retention cycles compared to bare CuO. Secondly, it is possible to replace the traditional liquid or solid electrolyte with a gel polymer, which does neither facilitate electrode oxidation nor have risk of leakage. PVA/KOH gel polymer has performed well in the current study as electrolyte. Finally, it is possible to assemble CuO-MPC//AC

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has not been reported elsewhere.

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Conflicts of interest There are no conflicts to declare. Acknowledgements This work was done within the scope of collaboration agreement between Cape Breton University, Nova Scotia Canada and Dr. Mahalingam College of Engineering and Technology (MCET), Tamil Nadu, India. The authors acknowledge the support from Nachimuthu Industrial Association, and its Secretary Dr. C. Ramaswamy. The authors are also indebted to Ms Judy MacInnis for technical support in the lab, especially on TEM. This work was partially financed through ECBC support for the Industrial Research Chair of Mine Water Management at CBU, ACOA, NSERC Discovery Grant, CFI, NSRIT, MITACS, and NRC-IRAP grants. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2019.113504. References [1] K.S. Kumar, N. Choudhary, Y. Jung, J. Thomas, Recent advances in two-dimensional nanomaterials for supercapacitor electrode applications, ACS Energy Lett 3 (2018) 482–495. [2] T. Purkait, G. Singh, D. Kumar, M. Singh, R. Sundar De, High-performance flexible supercapacitors based on electrochemically tailored three-dimensional reduced graphene oxide networks, Sci. Rep. 8 (2018) 640–648. [3] S.E. Moosavifard, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi, Designing 3D Highly ordered nanoporousCuO electrodes for high-performance asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 4851–4860. [4] B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, V2O5/functionalized MWCNT hybrid nanocomposite: the fabrication and its enhanced supercapacitive performance, RSC Adv. 4 (2014) 37437–37445. [5] S.E. Moosavifard, J. Shamsi, S. Fani, S. Kadkhodazade, Facile synthesis of hierarchical CuO nanorod arrays on carbon nanofibers for high-performance supercapacitors, Ceram. Int. 40 (2014) 15973–15979. [6] S.J. Ki, H. Lee, Y.K. Park, S.J. Kim, K.H. An, S.C. Jung, Assessing the electrochemical performance of a supercapacitor electrode made of copper oxide and activated carbon using liquid phase plasma, Appl. Surf. Sci. 446 (2018) 243–249. [7] S.D. Jagadale, A.M. Teli, S.V. Kalake, A.D. Sawant, A.A. Yadav, P.S. Patil, Functionalized crown ether assisted morphological tuning of CuO nanosheets for electrochemical supercapacitors, J. Electroanal. Chem. 816 (2018) 99–106. [8] S. Ramesh, A. Kathalingam, K. Karuppasamy, H.S. Kim, H.S. Kim, Nanostructured CuO/Co2O4@ nitrogen doped MWCNT hybrid composite electrode for highperformance supercapacitors, Composites Part B 166 (2019) 74–85. [9] Y. Li, K. Ye, K. Cheng, D. Cao, Y. Pan, S. Kong, X. Zhang, G. Wang, Anchoring CuO nanoparticles on nitrogen-doped reduced graphene oxide nanosheets as electrode material for supercapacitor, J. Electroanal. Chem. 727 (1) (2014) 154–162. [10] T.K. Zhao, W. Yang, X. Ji, W. Jin, J.T. Hu, T. Li, In-situ synthesis of expanded graphite embedded with CuO nanospheres coated with carbon for supercapacitors, Appl. Surf. Sci. (2017), https://doi.org/10.1016/j.apsusc.2017.09.046. [11] K. Wang, X. Dong, C. Zhao, X. Qian, Y. Xu, Facile synthesis of Cu2O/CuO/RGO nanocomposite and its superior cyclability in supercapacitor, Electrochim. Acta 152 (2015) 433–442. [12] M.G.T. Nathan, J.M. Boby, R. Mahesh, S. Harish, S. Joseph, P. Sathyaraj, One-pot hydrothermal preparation of Cu2O-CuO/rGO nanocomposites with enhanced electrochemical performance for supercapacitor applications, Appl. Surf. Sci. (2018), https://doi.org/10.1016/j.apsusc.2017.12.199. [13] A.G. Slater, A.I. Cooper, Function-led design of new porous materials, Science 348 (2015), https://doi.org/10.1126/science.aaa8075. [14] B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, V2O5/nitrogen enriched mesoporous carbon spheres nanocomposite as supercapacitor electrode, Microporous Mesoporous Mater. 258 (2018) 83–94. [15] G. Ma, M. Dong, K. Sun, E. Feng, H. Peng, Z. Lei, A redox mediator doped gel polymer as an electrolyte and separator for a high performance solid state supercapacitor, J. Mater. Chem. 3 (2015) 4035–4041. [16] Q.G. Zhang, Q.L. Liu, A.M. Zhu, Y. Xiong, L. Ren, Pervaporation performance of quaternizedpoly(vinyl alcohol) and its cross linked membranes for the dehydration of ethanol, J. Membr. Sci. 335 (2009) 68–75. [17] L. Lebrun, N. Follain, M. Metayer, Elaboration of a new anion-exchange membrane with semi-interpenetrating polymer networks and characterisation, Electrochim. Acta 50 (2004) 985–993.

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