Morphology-dependent binder-free CuNiO2electrode material with excellent electrochemical performances for supercapacitors

Journal of Energy Storage 26 (2019) 101037

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Morphology-dependent binder-free CuNiO2electrode material with excellent electrochemical performances for supercapacitors

T

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Chang-Seob Songa, Chandu V. V. Muralee Gopia, , Rajangam Vinodha, Sangaraju Sambasivamb, ⁎ ⁎ Reddi Mohan Naidu Kallac, Ihab M Obaidatb, , Hee-Je Kima, a

School of Electrical and Computer Engineering, Pusan National University, Geumjeong-gu, Busan 46241, South Korea Department of Physics, United Arab Emirates University, Al Ain, UAE c Department of Science and Humanities, Sri Venkateswara Engineering College, Krakambadi Road, Tirupati, Andhra Pradesh 517507, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ammonium fluoride CuNiO2 Nanosheet-like Supercapacitor Hydrothermal route

Rational design for structure and morphology of multi-component metal oxides is an efficient and promising way to enhance energy storage performance of electrode materials. In this present work, nanosheet-like CuNiO2 heterostructures are fabricated using facile one-step hydrothermal route by introducing various amounts of ammonium fluoride (NH4F) as structure-directing agent. The NH4F assisted synthesis of CuNiO2 materials on Ni foam current collector can be effectively utilized as binder-free battery-type electrode materials for supercapacitors. With an assistance of NH4F, the structural, morphological and composition evolutions of CuNiO2 electrodes are discussed effectively using X-ray diffraction, scanning electron microscopy and transmission electron microscopy and X-ray photoelectron spectroscopy characterizations. The CuNiO2 electrode material prepared with 0.4 M NH4F provides large number of active sites, superior conductivity and rapid charge transfer, which are promote fast Faradaic redox reactions. As a battery-type material, the optimized 0.4-CuNiO2 electrode material (NH4F is 0.4 M) exhibits a high specific capacity (~153.02 mA h g−1 at 2 A g−1), excellent rate capability (~87.4% retains even at 10 A g−1), and outstanding cycling stability (~94.14% at 6 A g−1 over 3000 cycles), respectively. Thereby, this study paves the path into rational design for structure and morphology of multi-component metal oxides for improving energy storage performance.

1. Introduction

capacitance with reduced material cost. At present, various pseudocapacitive (MnO2, RuO2, etc.) and battery-type (NiO, Ni(OH)2, Co3O4, Ni3S2, CuO, NiSe etc.) materials have been widely investigated as efficient electroactive materials for supercapacitors due to their outstanding advantages such as, effective faradic reaction, high availability and low-cost [10,15–18]. However, these single metal based materials deliver low-energy storage performance and have a short lifespan, which limit their use in high energy density based supercapacitor. To overcome this issue, the construction of multimetal electrode materials in-situ grown on current collectors has been confirmed to deliver more extraordinary energy storage performance [19,20]. A variety of multi-component electrode materials (including oxides, sulfides, selenides, layered double hydroxides (LDH), etc.), such as MnCo2O4, NiCo2O4, NiMoO4, FeCo2O4, CuNiO2, NiCoO2, CuCo2S4, NiCo2Se4, NiMn LDH etc., have been constructed so for and remarkably enhance the electrochemical properties, compared with the single component [21–30]. Among them, CuNiO2 is a promising electroactive material for supercapacitor applications due to it's low-cost, high-

With the rapid and sustainable development of economy, increase of pollution and depletion of fossil fuel resources, it is essential to develop high quality, renewable and promising energy storage system (ESS) [1,2]. So far, various ESSs such as, lithium-ion (Li-ion) batteries, fuel cells and supercapacitors have been developed, which are beneficial in providing energy to meet our needs [3–5]. Among various ESSs, electrochemical capacitors or supercapacitors are gained considerable attention due to their inherent advantages including high power density, long-cycling span, rapid charge-discharge process, cost-effective and environmental friendliness [6,7], resulting in potential energy storage devices for the application in hybrid vehicles, smart grid, etc. [8,9]. Generally, the performance of supercapacitor is mainly relied on the electroactive material. Hence, designing highly-efficient electroactive materials can make supercapacitors more satisfying practically applicable of future energy storage devices [10–14]. Therefore, it is crucial to design and develop new electroactive materials that couple high

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Corresponding authors E-mail addresses: [email protected] (C.V.V.M. Gopi), [email protected] (I.M. Obaidat), [email protected] (H.-J. Kim).

https://doi.org/10.1016/j.est.2019.101037 Received 24 September 2019; Received in revised form 22 October 2019; Accepted 24 October 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.

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spectroscopy (EDX, 15 kV). The crystal structure was studied by X-ray diffraction (XRD) analysis (D8 ADVANCE with a DAVINCI diffractometer (Bruker AXS)) with Cu Kα radiation operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) was carried out on a CJ111 high-resolution electron microscope with an acceleration voltage of 200 kV. The chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB 250).

energy storage performance, and ease of fabrication [23]. However, the research on CuNiO2 material for supercapacitor is quite limited and it is highly desirable to construct high-performance multicomponent CuNiO2 electroactive material with versatile morphologies for supercapacitors. To further enhance the utilization of CuNiO2 material, various nanostructures have been directly grown on current collectors to produce high surface areas, short electron- and ion-transport pathways (which are required for high capacitance), and exceptional rate capability and cycle performance. In view of above discussion, we have attempted to enhance the supercapacitor properties of CuNiO2 electroactive material using various concentrations of ammonium fluoride (NH4F). The multi-component CuNiO2 electrodes at different amounts of NH4F were prepared by facile and cost-effective hydrothermal route on Ni foam substrate. The direct growth of electroactive material on current collector excludes the conductive additives and binders during the preparation processes, resulting to the higher conductivity and enhanced ion/electron transportation [31]. Predominantly, the essential role of NH4F in an evolution of CuNiO2 electrode morphology was studied. The active material fabricated with 0.4 M of NH4F delivered the superior capacity, excellent rate-capability and outstanding cycling stability over 3000 cycles, revealing that CuNiO2 could be a promising electroactive material for energy storage applications.

2.4. Electrochemical measurements

2. Experimental

The electrochemical measurements (cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS)) of the CuNiO2 materials were examined using BioLogic Sp-150 workstation operated in three-electrode setup in 3 M KOH aqueous electrolyte. The as-fabricated CuNiO2, platinum wire and Ag/AgCl electrodes are served as the working, counter and reference electrodes, respectively. CV plots were recorded within the potential range of 0–0.6 V at different scan rates. GCD profiles were obtained in the potential window of 0–0.55 V at various current densities. Furthermore, EIS measurement was recorded in the frequency range of 0.01–100 kHz at 5 mV amplitude. Moreover, the long-term cycle stability of the CuNiO2 electrode was examined by the GCD test at a current density of 6 A g−1. The specific capacity (QSC) values of the CuNiO2 electrode are calculated from GCD plots using the following equation [32]:

2.1. Chemicals

QSC =

Nickel (II) nitrate hexahydrate (Ni(NO3)2•6H2O), cobalt (II) nitrate hexahydrate (Co(NO3)2•6H2O), ammonium fluoride (NH4F), potassium hydroxide (KOH), hydrochloric acid (HCl) and urea (CH4N2O) were received from Sigma-Aldrich Co., South Korea. All the chemicals are analytical grade and were used directly without further purification.

where I, Δt and m have their usual meanings.

2.2. Preparation of CuNiO2electroactive material on Ni foam substrate

CuNiO2 electrode is fabricated on Ni foam current collector using facile one-step hydrothermal route. Accordingly, we prepared the binder-free nanosheet-like CuNiO2 heterostructures on Ni foam substrate with an assistance of NH4F in Cu- and Ni-based salt solution. Here, the NH4F releases the F− anions in growth solution offer a large number of active sites for the nucleation and growth of nanostructures on Ni foam, leading to the high mass loading of electroactive material. In addition, the NH4F offers a compact binding between the as-grown materials and Ni foam substrate, which is greatly favorable for tuning of morphology. Hence, improved energy storage performance along with long cycling stability could be expected. The phase and crystal structure of the as-fabricated electrode was firstly examined by XRD characterization. Fig. 1 depicts the XRD pattern of the 0.4-CuNiO2 electrode. Peaks denoted with Ni are standard diffraction peaks of nickel foam substrate. XRD spectra exhibit diffraction peaks at 2θ = =37.1° (111), 41.3° (002), 43.2° (200) and 64.3° (220) are indexed to the (111), (002), (200) and (220) crystal planes, which are well matched with standard JCPDS card no-06-0720 [23,33]. The elemental composition and chemical bonding states of the asfabricated electrode was characterized using XPS analysis. As depicted in Fig. 2a, XPS survey spectrum of 0.4-CuNiO2 electrode exhibit the presence of Cu, Ni, O, F and C signals. The presence of carbon signal in the survey spectrum is due to the exposure of sample to the air. Also, the existence of F is mainly due to the NH4F. The high-resolution XPS spectrum of Cu 2p shown in Fig. 2b delivered peaks originated at 932.6, 936.6, 952.5 and 956.5 eV are attributed to Cu1+ 2p3/2, Cu2+ 2p3/2, Cu1+ 2p1/2 and Cu2+ 2p1/2 states, respectively, along with shake-up satellite peak (denoted as Sat.), which further clarifies that the electrode is composed of various states of copper [34]. The high-resolution Ni 2p spectrum (Fig. 2c) exhibited a fitting peaks at 856.6 and 874.5 eV are assigned to Ni2+ 2p3/2 and Ni2+ 2p1/2, while those at 858.4 and 876.4 eV are corresponding to Ni3+ 2p3/2 and Ni3+ 2p1/2, respectively,

I × Δt m × 3.6

(1)

3. Results and discussion 3.1. Structural and morphological characterization

At various concentrations of NH4F, a binder-free CuNiO2 electroactive material was anchored on Ni foam using a simple one-step hydrothermal reaction. Prior to the deposition, the Ni foam substrate (1 × 2 cm2) was carefully cleaned with 3 M HCl, acetone, ethanol and deionized (DI) water in an ultrasound bath for 15 min each. To synthesize CuNiO2 material on Ni foam, 0.1 M of Cu(NO3)2•6H2O, 0.1 M of Ni(NO3)2•6H2O, 0.2 M of CH4N2O and 0.4 M NH4F were dissolved into 60 mL DI water under magnetic stirring for 15 min. The resultant solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave with cleaned Ni foam and maintained at 100 °C for 6 h. After cooling, the CuNiO2 material loaded on Ni foam was removed from the autoclave and washed several times with DI water and ethanol and then finally dried in an oven at 60 °C for overnight. To examine the effect of NH4F on the morphologies of CuNiO2 electrode material, the growth solution was prepared with various amounts of NH4F (0.2 and 0.6 M) under the similar fabrication procedures. The as prepared samples are named as 0.2-CuNiO2, 0.4-CuNiO2, and 0.6-CuNiO2 based on the NH4F concentrations (0.2, 0.4 and 0.6 M) respectively. The mass loading of 0.2-CuNiO2, 0.4-CuNiO2, and 0.6-CuNiO2 active materials on Ni foam are obtained to be 2.6, 3.1 and 2.95 mg cm−2 by subtracting the mass of bare Ni foam from the mass of active material loaded on Ni foam (i.e., CuNiO2/Ni foam) with analytical balance (accuracy of 0.01 mg). In our previous work, we have prepared the bare CuNiO2 electroactive material without the addition of NH4F amount [23]. 2.3. Material characterization The surface morphology and elemental mapping of the resulting samples were characterized by field emission scanning electron microscopy (SEM, S-2400, Hitachi) with energy-dispersive X-ray 2

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greatly influenced by the concentration of NH4F. With the absence of NH4F (i.e., 0 M), the as-prepared CuNiO2 electrode delivered a dandelion flower-like morphology, which was reported in our previous work [23]. When a small amount of NH4F (i.e., 0.2 M) is introduced to growth solution, the surface morphology was transformed into uniform distribution of nanosheet-like morphology over Ni foam surface and the thickness of nanosheets were found to be in the range of 165–217 nm, respectively (Fig. 3a–c). With further increasing the NH4F amount to 0.4 M, the nanosheets are well interconnected with each other, the void between the nanosheets are reduced and also the thickness of nanosheets was reduced to 130 nm (Fig 3d–f). Upon further increasing the concentration of NH4F (0.6 M), the CuNiO2 deliver an exfoliated nanosheet-like morphology and further reducing the thickness of the nanosheets (108 nm) (Fig. 3g–i). Moreover, mass loading of CuNiO2 material on Ni foam substrate also varied due to the pre-activation of substrate with liberated F− ions. From this SEM study, it is evident that the NH4F amount is not used in tuning of surface morphology, but also affecting the mass of active material. Furthermore, TEM measurement was used to investigate the morphologies of the as-prepared CuNiO2 electrodes at various amounts of NH4F. Fig. 4 shows the low- and highmagnification TEM images of the as-prepared CuNiO2 electrodes. Based on the NH4F concentrations in the growth solutions (0.2, 0.4, 0.6 M), all the CuNiO2 electrodes exhibited the nanosheet-like morphology (Fig. 4a–f). This nanosheet-like morphology enhances the surface area and provides abundant electroactive sites for redox reactions. Moreover, the elemental mapping images further manifest the homogeneous distribution and coexistence of Cu, Ni, and O C elements in the 0.4CuNiO2 material (Fig. S1).

Fig. 1. XRD of 0.4-CuNiO2 material on Ni foam substrate.

and also exhibited the two shakeup satellites (denoted as Sat.) [35]. The high-resolution O 1s spectra (Fig. 2d) deliver a peaks at 533.5 (O1), 532.2 (O2), and 531.2 eV (O3), which are due to the chemisorbed water on or within the surface, lattice oxygen, and oxygen ions at the surface [36]. The XRD and XPS measurements confirm that the CuNiO2 is uniformly distributed over Ni foam surface without any impurities. The structure and morphology of as-prepared electrodes were characterized by SEM and TEM analysis. The morphology of the electrode materials plays a crucial role in the energy storage performance of supercapacitor. Fig. 3 depicts the SEM images of the as-prepared CuNiO2 electrodes at various concentrations of NH4F in growth solution. It is evident from Fig. 3, the surface morphology of the CuNiO2 is

3.2. Electrochemical characterization The electrochemical measurements of binder-free NH4F-based CuNiO2 electrodes were examined in a three-electrode setup using 3 M KOH aqueous electrolyte. Fig. 5a depicts the comparative CV plots of

Fig. 2. (a) XPS survey spectra of 0.4-CuNiO2 electrode and (b–d) deconvoluted spectra of Cu 2p, Ni 2p and O 1s elements. 3

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Fig. 3. Low- and high-magnification SEM images of (a–c) 0.2-CuNiO2, (d–f) 0.4-CuNiO2 and (g–i) 0.6-CuNiO2 electrodes on Ni foam substrate.

Fig. 4. TEM images of (a, b) 0.2-CuNiO2, (c, d) 0.4-CuNiO2 and (e, f) 0.6-CuNiO2 electrodes on Ni foam substrate.

redox reaction related to M-O/M-O-OH (M refers Cu or Ni) with the aid of OH− anions. In addition, 0.4-CuNiO2 electrode delivered the higher current response and larger integral CV area than that of 0.2-CuNiO2 and 0.6-CuNiO2 electrodes, respectively, owing to the high electro

0.2-CuNiO2, 0.4-CuNiO2 and 0.6-CuNiO2 electrodes at constant scan rate of 10 mV s−1. From the CV plots, it is evident that all the electrodes delivered well-defined redox peaks, denoting the battery-type behavior of material. The pair of redox peaks are mainly attributed to the Faradic 4

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Fig. 5. (a) Comparative CV plots of as-prepared electrodes at a constant scan rate of 10 mV s−1. CV profiles of (b) 0.2-CuNiO2, (c) 0.4-CuNiO2 and (d) 0.6-CuNiO2 electrodes at various scan rates.

the specific capacity values of 153.02, 149.94, 141.77, and 133.75 mA h g−1 at 2, 4, 6 and 10 A g−1, respectively, which are much higher than that of bare-CuNiO2 (111.52, 104.59, 99.86 and 90.83 mA h g−1) [23], 0.2-CuNiO2 (126.88, 122.8, 113.4, and 102.55 mA h g−1) and 0.6-CuNiO2 (93.4, 86.11, 78.41, and 65.93 mA h g−1) electrodes at same current densities. Noticeably, the 0.4-CuNiO2 electrode achieved a rate capability of 87.4%, which is superior to that of bare-CuNiO2 (81.4%) [23], 0.2-CuNiO2 (80.82%) and 0.6-CuNiO2 electrodes (70.59%) electrodes, respectively. Moreover, the cycling life-span of electroactive material is important for the real time applications of supercapacitor. Fig. 6f depicts the cycling stability of 0.4-CuNiO2 electrode examined at 6 A g−1 over 3000 cycles. It is obvious that the small increment in specific capacity values can be observed over 1500 cycles, which is ascribed to the full activation of CuNiO2 material by continuous penetration of electrolyte ions into their interior parts [38]. Over 3000 cycles, the 0.4-CuNiO2 electrode exhibits an excellent cycling life of ~94.14% retention, which is superior to that of bare-CuNiO2 electrode (~89.13%) [23], indicating the outstanding cycling stability of 0.4-CuNiO2 electrode. Furthermore, the SEM analysis of the 0.4-CuNiO2 electrode was characterized after cycling test (inset of Fig. 6f). From the SEM image, it is evident that the 0.4-CuNiO2 still exhibiting the nanosheet-like morphology and the material is still stick to the current collector, which indicating the excellent stability of 0.4-CuNiO2 material. Such specific capacity and cycling performance of 0.4-CuNiO2 electrode are impressive when compared with the many of those previously reported multi-component metal-based nanomaterials [39–45], as shown in Table 1. The superior energy-storage performance is attributed to the nanosheet-like 0.4CuNiO2 electrode possessing large number of active sites and offers the fast ion/electron transports. Furthermore, EIS measurement was carried out to examine the charge transport behavior and electrode conductivity of as-prepared electrodes. Fig. 7a depicts the Nyquist plots of 0.2-CuNiO2, 0.4-CuNiO2

activity and large surface area of nanosheet-like morphology of 0.4CuNiO2. Fig. 5b–d show the CV plots of the 0.2-CuNiO2, 0.4-CuNiO2 and 0.6-CuNiO2 electrodes at various scan rates of 2–50 mV s−1. With increasing the scan rate, a pair of redox peaks were observed with increased peak current values and the similar CV shapes were observed, revealing the good reversibility and rapid redox reactions of the electroactive material. Meanwhile, the anodic and cathodic peaks were shifted slightly to positive and negative directions with the increase of scan rate, which is due to good ion diffusion rate and lower resistance of the material during the electrochemical redox reaction [37]. The mechanism of electrochemical reactions can be ascribed to the reversible Faradaic redox reactions of Cu and Ni species based on the following equations:

CuNiO2 +2OH− ↔ CuOOH + NiOOH + 2e−

NiOOH +

OH−

↔ NiO2 + H2 O +

e−

(2) (3)

Furthermore, GCD is a useful technique to demonstrate the electrochemical performance of the as-fabricated samples. Fig. 6a depicts the comparative GCD plots of 0.2-CuNiO2, 0.4-CuNiO2 and 0.6-CuNiO2 electrodes at constant current density of 2 A g−1. All the electrodes deliver the typical battery-type behavior with nonlinear charge–discharge potential plateaus that consistent with CV studies. As expected, the discharge time of 0.4-CuNiO2 sample was evidently longer than that of the 0.2-CuNiO2 and 0.6-CuNiO2 samples, demonstrating the much higher specific capacity values for the 0.4-CuNiO2 sample. The GCD profiles of the as-prepared NH4F-based CuNiO2 electrodes were measured at various current densities (2 to 10 A g−1) and the corresponding curves are shown in Fig. 6b–d. From the GCD plateaus, as shown in Fig. 6b–d, the consistent battery-type behavior and symmetric charge-discharge time further confirm the good reversibility and Faradaic efficiency of the materials. Based on discharge times and Eq. (1), the calculated specific capacity values as a function of the current density are plotted in Fig. 6e. 0.4-CuNiO2 electrode delivered 5

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Fig. 6. (a) Comparative GCD profiles of as-fabricated electrodes at 2 A g−1. GCD curves of (b) 0.2-CuNiO2, (c) 0.4-CuNiO2 and (d) 0.6-CuNiO2 electrodes at different current densities. (e) Specific capacity values of as-prepared samples electrodes. (f) Cycling performance of the 0.4-CuNiO2 electrode examined at 6 A g−1 and inset shows the SEM image of 0.4-CuNiO2 electrode after cycling test.

firstly and successfully synthesized on Ni foam substrate using facile one-pot hydrothermal approach with an aid of NH4F. The as-fabricated NH4F-based CuNiO2 electrodes effectively served as binder-free batterytype electrode materials for supercapacitors without any further treatment. The morphology and thickness of nanosheets were effectively influenced with an assistance of NH4F. SEM, TEM, XRD and XPS characterizations were conducted to demonstrate the structural, morphological and composition evolutions. EIS study revealed that the 0.4CuNiO2 electrode exhibited lower charge transfer resistance (0.05 Ω cm2) and series resistance (0.37 Ω cm2), suggesting the higher electrical conductivity and efficient charge transfer at the interface of electrode/electrolyte. As a battery-type electrode, the optimized nanosheet-like 0.4-CuNiO2 electrode achieved the high specific capacity of ~153.02 mA h g−1 at 2 A g−1, exceptional rate capability of ~87.4% even at 10 A g−1 and superior cycling stability with a capacity retention of ~94.14% even over 3000 cycles. As consequence, this study has also been proposed an efficient approach to enhance the energy storage performance of electroactive materials by introducing structure-directing agents to develop for hierarchical multi-component metal-based nanomaterials.

and 0.6-CuNiO2 electrodes in a frequency range of 0.01–100 kHz and inset shows the high-magnified Nyquist plots. Fig. 7b presents the corresponding equivalent circuit to fit the Nyquist plots. From the equivalent circuit, RS, Rct, CPE and ZW represent the series resistance, charge transfer resistance, constant-phase element and Warburg resistance [46]. All the Nyquist plots contain a small semi-circle in the high-frequency region related to charge-transfer resistance (Rct) occurring at the interface of active materials/electrolyte; and a sloping straight line in the low-frequency region ascribed to Warburg resistance (ZW). The intercept on the real axis at high-frequency denotes the equivalent series resistance (RS). Both Rs (0.37 Ω cm2) and Rct (0.05 Ω cm2) of 0.4-CuNiO2 electrode are much lower than those of 0.2CuNiO2 (0.48 Ω cm2 and 0.13 Ω cm2) and 0.6-CuNiO2 (0.51 Ω cm2 and 0.25 Ω cm2) electrodes, respectively, demonstrating that 0.4-CuNiO2 electrode can provide the better electrical conductivity and more effective charge transfer. Besides, the 0.4-CuNiO2 electrode delivers the more vertical straight line than the 0.2-CuNiO2 and 0.6-CuNiO2 electrodes, which suggests a lower Warburg resistance and fast electrolyte diffusion. 4. Conclusions In summary, the nanosheet-like CuNiO2 nanostructures have been

Table 1 Comparison of electrochemical performance of binder-free nanosheet-like 0.4-CuNiO2 material with recently reported other multi-component metal-based nanomaterials. Electrode

Electrolyte

Specific capacity (QSC, mA h g−1)

Cycling stability (no. of cycle)

Year (Reference)

MnCo2O4 CoNiO2 NiCoO2 LiCoO2 NiCo2O4 ZnCo2O4 MgCo2O4 Bare-CuNiO2 0.4-CuNiO2

3M 2M 2M 3M 2M 6M 2M 3M 3M

75.41 at 1 A g−1 130.6 at 1 A g−1 45.62 at 0.62 A g−1 76.61 at 4.16 A g−1 100.33 at 1 mA cm−2 78.89 at 1 A g−1 70.55 at 2 A g−1 111.52 at 2 A g−1 153.02 at 2 A g−1

76% (1000) – 95.2% (1500) – 94.5% (5000) – 95.9% (2000) 89.13% (3000) 94.14% (3000)

2016 [39] 2019 [40] 2017 [41] 2018 [42] 2019 [43] 2018 [44] 2016 [45] 2019 [23] This Work

KOH KOH KOH KOH KOH KOH KOH KOH KOH

6

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Fig. 7. (a) EIS plots of as-prepared electrodes and inset shows the enlarged plot. (b) Equivalent circuit to fit the Nyquist plots.

Declaration of Competing Interest The authors declare no conflicts of interest.

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Acknowledgements

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This work was supported by BK 21 PLUS, Creative Human Resource Development Program for IT Convergence, Pusan National University, Busan, South Korea. Also, this work was supported by UAEU Program for Advanced Research (UPAR) under Grant no. 31S312.

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Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101037.

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