Facile synthesis of Ce-doped α-cobalt hydroxide nanoflakes battery type electrode with an enhanced capacitive contribution for asymmetric supercapacitors

Journal of Energy Storage 28 (2020) 101227

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Facile synthesis of Ce-doped α-cobalt hydroxide nanoflakes battery type electrode with an enhanced capacitive contribution for asymmetric supercapacitors

T

⁎

R.C. Rohita, A. Jenifera, Ajay D. Jagadalea, , Vijay S. Kumbharb,c, Hyeonkwon Leeb,c, ⁎⁎ Kiyoung Leeb, a b c

Center for Energy Storage and Conversion, School of Electrical and Electronics Engineering, SASTRA Deemed University, Thanjavur, 613401, Tamilnadu, India School of Nano & Materials Science and Engineering, Kyungpook National University, 2559 Gyeongsang-daero, Sangju, Gyeongbuk, South Korea Research Institute of Environmental Science & Technology, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, South Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: α-cobalt hydroxide Cerium Supercapacitor Step potential electrochemical spectroscopy

The nanostructure of layered cobalt hydroxide has been gaining increasing attention in the field of electrochemical energy storage due to its high surface area and compatible cages for the redox reactions. Nevertheless, its poor electrical conductivity hinders application toward rate performance. In this work, cerium (Ce)-doped αcobalt hydroxide (α-Co(OH)2) thin-film electrodes were prepared with improved rate capability via electrodeposition method. To investigate the effect of Ce doping on the structural, morphological and electrochemical properties of α-Co(OH)2, it was doped in different molar percentages ranging from 0 to 3 mol%. The α-Co(OH)2 doped with 1 mol% Ce showed an excellent charge storage performance with a specific capacity of 415 C g−1 and the relatively higher cyclic stability of 73%. The deconvolution of capacitive and diffusion-controlled contributions has been performed using step potential electrochemical spectroscopy which showed the capacitive contribution of the film increased from 38% to 45% when doped with Ce by 1 mol%. This improved capacitive contribution of α-Co(OH)2 battery-type electrode can facilitate high power performance without compromising the specific energy of the supercapacitor. Finally, this electrode was employed for the fabrication of asymmetric supercapacitor that showed excellent cyclic stability, specific energy and power.

1. Introduction Since fossil fuels are readily accessible carbon source, it is widely used for industries and other massive energy consumption systems. Owing to the depletion of fossil fuels the renewable energy sources such as solar, wind, hydroelectric, etc. have attained a greater attraction [1]. These sources mainly generate electrical energy which is easy for energy distribution and transportation. To handle this electric power efficiently, the renaissance of high-performance electrochemical energy storage systems has emerged [2]. In the present state of the art, batteries and supercapacitors are reliable and commercialized storage devices, in which supercapacitors (SCs) have attracted major attention due to their high specific power, fast charging-discharging rate and excellent cyclic stability [3]. Because of the unsatisfactory energy performance of supercapacitors, researchers have been trying to improve the specific energy by synthesizing advanced energy storage materials.

⁎

The electrode materials used for supercapacitors can be classified into three types, (i) carbonaceous materials, (ii) metal oxides/hydroxides, and (iii) conducting polymers [4]. Carbonaceous materials store the energy predominantly via electrostatic double-layer capacitive (EDLC) mechanism. The conducting polymers involve fast faradic redox reactions, whereas the metal oxides/hydroxides exhibit the property of both EDLC and redox reactions [5]. Owing to this interesting phenomenon and capability of storing high energy with excellent cyclic stability, the metal oxides have received a great impression among researchers to develop high-performance supercapacitors [6]. RuO2 is a classic example of the metal oxides with the only disadvantage of high cost which leads to the search of alternative materials with performance as much as RuO2. Recently, a variety of alternative metal oxides/hydroxides has been prepared. These includes Co3O4 [7], Co(OH)2 [8], NiO [9], Ni(OH)2 [10], MnO2 [11], Fe3O4 [12], MoO2 [13], etc. Amongst all, Co(OH)2 has attracted great attention because of its

Corresponding author. Co-corresponding author. E-mail addresses: [email protected] (A.D. Jagadale), [email protected] (K. Lee).

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https://doi.org/10.1016/j.est.2020.101227 Received 7 December 2019; Received in revised form 9 January 2020; Accepted 18 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.

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R.C. Rohit, et al.

counter, and reference electrodes, respectively. The 0.1 M solution of Co(NO3)2.6H2O was used as an electrolyte. To dope Ce into cobalt hydroxide, Ce(NO3)3.6H2O with different molar percentages (0 to 3 mol %) with respect to cobalt precursor was added to the electrolyte separately. The deposition was performed by applying the constant cathodic current of 3 mA for 5 min. After deposition, electrodes were washed with distilled water to remove the excess ions from the surface of the electrode. The coated samples were dried at 90 °C for 2 h and used for further characterizations. The un-doped and 1, 2 and 3 mol% Ce-doped samples were labeled as CHCe0, CHCe1, CHCe2, and CHCe3, respectively.

superior battery-like supercapacitive performance similar to RuO2 [14–16]. Also, it has high theoretical capacitance (1381 F g−1) and a layered structure that facilitates the fast ion insertion and desertion processes [17,18]. The chemical, physical, catalytic and electronic properties of the material can be altered by introducing the dopant in the material. Dopants either segregate on nanostructures or can be inserted into the lattices [19]. Among the metal oxide dopants, cerium has a greater platform because of its different oxidation states like Ce3+ and Ce4+ which depend upon the surrounding conditions. Recently, Anjali et al [20] have doped NiO nanoparticles with Ce3+ ions which showed a 5fold increment in the specific capacitance of the pristine nanoparticles. In another study, Gawali et al [21] concluded that the drastic improvement in the capacitance of the NiO is attributed to the change in the electronic structure that further leads to the decrement in the intrinsic resistance. On the other hand, in the battery-type electrode like α-Co(OH)2, the charge is stored via three different processes i) electrical double layer formation (due to high surface area), pseudocapacitive (surface redox reaction) and diffusion controlled kinetics (bulk redox reactions). In order to prepare high-performance energy storage materials, the deconvolution of capacitive and diffusion-controlled contributions is highly recommended. Previously, different electrochemical techniques have been reported for the deconvolution of storage contributions [22–24]. Interestingly, Forghani and Donne [25] have recently invented a rigorous method known as step potential electrochemical spectroscopy (SPECS) for the deconvolution of the aforementioned contributions. In the SPECS, the current as a function of time is recorded for every potential step that lies within the potential window. Herein, we used the SPECS method for the deconvolution of capacitive and diffusion controlled contributions of α-Co(OH)2 before and after the doping of Ce. To the best of our knowledge, there is no report available on Ce doped α-Co(OH)2 prepared via facile electrodeposition process for supercapacitor application. In the present work, Ce-doped α-Co(OH)2 thin-film electrodes were prepared via electrodeposition method. The effect of Ce doping on the structural, morphological and electrochemical properties of α-Co(OH)2 has been investigated. The α-Co(OH)2 doped with 1 mol% Ce showed a maximum specific capacity of 415 C g−1 at the scan rate of 5 mV s−1. The step potential electrochemical spectroscopy showed the capacitive contribution of α-Co(OH)2 film was increased from 38% to 45% when doped with 1 mol% Ce. The asymmetric supercapacitor was fabricated with 1 mol% Ce doped α-Co(OH)2 as a positive and activated carbon (AC) as a negative electrodes that demonstrated highest specific energy of 14 Wh kg−1 at the specific power of 500 W kg−1 with wide operating potential window of 1.6 V. These results indicate that Ce doped α-Co (OH)2 is a promising electrode material for asymmetric supercapacitors.

2.3. Characterization XRD patterns were recorded by using an X-ray diffractometer (PANalytical X'pert Pro MPD) equipped with a Cu Kα radiation source (λ = 1.5406 Å). The compositional studies of the samples were collected by energy-dispersive X-ray spectrometry (EDS) (JSM6701f, JEOL, Japan), X-ray photoelectron spectroscopy (XPS) (ESCALAB-MKII) and X-ray fluorescence spectroscopy (XRF) (Bruker S8 Tiger). The morphological structure was examined by FE-SEM (JSM6701f, JEOL, Japan) and TEM (JSM2100, JEOL, Japan). The mass of the active material on the SS substrate was measured by the weight difference method using an electronic balance (CAI-35, Contech Instruments Ltd). The electrochemical measurements were evaluated by the three-electrode system using a Biologic SP-150 electrochemical workstation. In the three-electrode system, doped/undoped α-Co(OH)2 thin films on the SS substrate, coiled platinum, and Ag/AgCl were used as a working, counter and reference electrodes, respectively. The capacities of the doped/undoped α-Co(OH)2 thin-film electrodes were calculated from the CV curves by using the following formula, v

C=

∫v1 2 I (V ) dv Ms

Cg−1

(1) −1

where ‘C’ is the capacity (C g

), ‘M’ is the mass of the active material v2

(g), ‘s’ is the scan rate (V s−1), and ∫ I (V ) dv is the area under the cyclic v1

voltammetry curve [26]. The specific capacities of the film electrodes were also calculated from the GCD curves using the following formula,

C=

I × Δt −1 Cg M

(2)

where ‘I’ is the discharging current (A), ‘Δt’ is the discharging time (s) and ‘M’ is the mass of the active material (g). The areal capacity of the electrodes was calculated from GCD curves by using the following formula,

I × Δt Ccm−2 S

2. Experimental section

CA =

2.1. Chemicals and materials

where ‘I’ is the discharging current (A), ‘Δt’ is the discharging time (s) and ‘s’ is the geometric surface area of the active electrode material (1 cm2) [27]. In order to test further applicability of the electrodes, an asymmetric supercapacitor device was fabricated using the Swagelok cell in which CHCe1 film was used as a positive electrode, AC as a negative electrode and piece of Whatman filter paper soaked in the 2 M KOH solution as a separator. The fabricated device was further subjected to electrochemical studies. The evaluation of charge balance and the calculation of specific capacitance, specific energy, and specific power were carried out using formulae stated in the supporting information.

Analytical grade cobalt nitrate hexahydrate (Co(NO3)2.6H2O), and cerous nitrate hexahydrate (Ce(NO3)3.6H2O) were purchased from Sisco Research Laboratories Pvt. Ltd. (SRL), India and potassium hydroxide (KOH) pellets were purchased from Sigma-Aldrich, USA. The stainless steel (SS) of grade 304 with a thickness of 0.005 mm was purchased from Labtronics enterprises, India. 2.2. Preparation of cerium doped cobalt hydroxide thin films The SS substrates were cut into a dimension of 1 ✗ 6 cm2 and cleaned with water and ethanol via ultrasonication. Further, these were insulated by keeping the dimension 1 ✗ 1 cm2 open for the deposition of the material. The electrodeposition was performed using a three-electrode system in which SS substrate (1 cm2 exposed area), coiled platinum and silver /silver chloride (Ag/AgCl) were used as a working,

(3)

3. Results and discussion 3.1. Formation of α-Co(OH)2 thin films The electrodeposition was carried out by applying the cathodic 2

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Fig. 1. (A) XRD patterns of (i) CHCe0, (ii) CHCe1, (iii) CHCe2 and (iv) CHCe3 samples, (B) XPS survey spectrum of CHCe1 sample, and high resolution XPS spectra of (C) Co 2p and (D) Ce 3d.

current density of 3 mA cm−2. The deposition process was based on the nitrate ion reduction reaction [28]. Initially, when the current density of 3 mA cm−2 was maintained between the working and the counter electrode, nitrate ions from the electrolyte started reducing on the surface of the working electrode (SS substrate) and the pH near the electrode surface was increased. This increased pH helped in the precipitation of Co and Ce ions from the electrolyte to form doped cobalt hydroxide films. This process can be explained by the following electrochemical reactions [17],

NO−3 + 7H2 O + 8e− → NH+4 + 10OH−

(4)

Co2 + + 2OH− → Co(OH)2

(5)

Ce3 + + 3OH− → Ce(OH)3

(6)

characteristic peaks at binding energies of 796.7 and 781.4 eV. These are attributed to the Co 2P3/2 and Co 2P1/2 atomic orbitals, respectively which confirmed the presence of Co2+ and Co3+ states in the α-Co (OH)2 [31,32]. In addition, the satellite peaks at 786.1 and 802.5 eV corresponded to the high spin state of the Co2+ [33]. In the high-resolution spectrum of the Ce 3d (Fig. 1D), no clear peak is observed because of the meager amount of Ce in the host material [34]. However, a slight elevation is observed at around 885 eV which may be due to the presence of Ce4+ 3d5/2 electron state [21,35]. In order to further confirm the existence of the Ce in the α-Co(OH)2, CHCe0 and CHCe1 samples were characterized via the XRF technique. The detailed XRF reports for CHCe0 and CHCe1 samples are given in the supporting information. From the report, it is seen that the CHCe1 depicts the presence of approximately 1% of Ce with respect to Co in the sample. Also, some additional elements such as Fe, Cr, Ni, and Mn have been predominantly observed which are attributed to the stainless steel used for the deposition. This study confirmed the successful doping of Ce into αCo(OH)2. As shown in the schematic (Fig. 2A), the incorporation of Ce transformed the microstructure of α-Co(OH)2 from vertically grown nanoflakes to horizontally grown agglomerated nanoflakes. The FESEM image of CHCe0 (Fig. 2B) shows the formation of vertically grown α-Co(OH)2 nanoflakes on the surface of the SS substrate. When dopant was introduced (Fig. 2C), the nanoflakes were grown in the horizontal direction forming a flat and microporous structure unlike the open and flaky structure of the undoped films. This kind of microstructure facilitates high surface area and porous channels compatible for rapid redox reactions at the interface between electrode and electrolyte. Further doping enhanced the agglomeration of the nanoflakes leading to the compact microstructure with reduced apparent surface area (Fig. 2D and E). These morphological changes may be attributed to the

3.2. Structural and morphological studies Fig. 1A shows the XRD patterns of CHCe0, CHCe1, CHCe2 and CHCe3 samples with the well-defined peaks at around 2θ values of 10.96°, 22.12°, 33.96°, 38.16°, and 58.86° which can be indexed to the Miller indices of (001), (002), (100), (005), (100) and (110), respectively. These results are in good agreement with the JCPDS card no. 46–0605 which reveals the formation of α-Co(OH)2 [29]. Because of the small doping percentages, the Ce doping did not influence the XRD patterns significantly. Since there is no considerable change in the position, broadening and intensity of the peaks, it indicated that there is no significant effect of Ce doping on the crystalline structure of the material [21,30]. Fig. 1B represents the XPS survey spectrum of the CHCe1 sample and clearly shows the presence of cobalt, cerium and oxygen elements. Fig. 1C shows the high-resolution spectrum of Co 2p which depicts 3

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Fig. 2. (A) Schematic of the preparation process of Ce doped α-Co(OH)2, FE-SEM images of (B) CHCe0, (C) CHCe1, (D) CHCe2, (E) CHCe3 samples, (F) high magnification image of CHCe1 and (G) higher magnification TEM image of CHCe1 (inset: low magnification TEM image of CHCe1 sample).

microstructure of the film. To understand the rate capability of the CHCe1 electrode, CV curves were recorded at different scan rates range from 5 to 100 mV s−1 which is shown in Fig. 3C (CV curves at various scan rates for CHCe0, CHCe2, and CHCe3 electrodes are shown in Figure S5). The CV curves depict that the increment in the scan rate increases the area under the curves which are in accordance with the ideal supercapacitive behavior. The values of specific capacities obtained for the scan rates of 5, 10, 20, 50 and 100 mV s−1 were calculated as 415, 396, 382, 361 and 335 C g−1, respectively. Interestingly, even at the higher scan rate of 100 mV s−1, capacity retention was 81% indicating an excellent rate capability of the electrode. Furthermore, the specific electrochemically active surface area (ECSA) for each film electrode was evaluated from the electrochemical double-layer capacitance of the film surface [36]. As shown in table S1, the specific ECSAs of CHCe0, CHCe1, CHCe2, and CHCe3 electrodes were estimated to be 0.59, 0.66, 0.65 and 0.29 m2 g−1, respectively (supporting information). The appreciable ECSA value for the CHCe1 electrode can be attributed to the high specific surface area which provides more accessible sites in contact with the electrolyte. The GCD curves for CHCe0, CHCe1, CHCe2, and CHCe3 electrodes at the current density of 3 A g−1 are given in Fig. 3D. These highly symmetric GCD curves demonstrate an insignificant value of iR drop suggesting the lower internal resistance of the electrode. The values of capacities are calculated as 271, 373, 363 and 301 C g−1, respectively. These values calculated by GCD curves are in good agreement with the values obtained by CV curves. The maximum specific capacity obtained in this work is higher than that reported in the literature (Table 1). Since in the present work Ce doped α-Co(OH)2 films were coated on the flexible SS substrate, they have great potential to be used for flexible supercapacitors. To check their applicability, the areal capacities of CHCe0, CHCe1, CHCe2, and CHCe3 electrodes were calculated as 128, 191, 186 and 137 mC cm−2, respectively at the scan rate of 5 mV s−1. Fig. 3E represents the GCD curves of CHCe1 at various current densities showing the non-linear battery-type behavior (GCD curves for CHCe0, CHCe2, and CHCe3 can be seen from Figure S4). The variation of

merging of cerium into α-Co(OH)2 that alters the microstructure in shape and size due to the aggregation of cerium within α-Co(OH)2 [19]. Such microstructure deteriorates the capacitive ability as well as the cyclic stability of the electrodes [17,34]. Fig. 2F shows the high-resolution FE-SEM image of the CHCe1 sample indicating the formation nanoflakes of the size ranges from 8 to 12 nm. Fig. 2G shows the highresolution TEM image of CHCe1 sample which shows that the nanoflakes are formed of even smaller porous nanosheets. This porous and nanostructured form of the Ce doped α-Co(OH)2 film improves the capacitive performance phenomenally. Furthermore, the EDAX study confirms the presence of Co and Ce without any impurity which supports XPS and XRF data (See supporting information Figure S2). 3.3. Electrochemical characterization Fig. 3A represents CV curves of CHCe0, CHCe1, CHCe2, and CHCe3 film electrodes at the scan rate of 5 mV s−1. For CHCe0 and CHCe3 electrodes, the CV was performed in the potential window from −0.2 to 0.4 V, whereas for CHCe1 and CHCe2, CV was performed in the potential window from −0.3 to 0.4 V. All the CV curves clearly show the oxidation and reduction peaks. The electrochemical reaction corresponds to the charging-discharging process of the CHCe1 electrode is given below [33],

Ce:Co(OH)2 + OH− ↔ Ce: CoOOH + H2 O + e−

(7)

The specific capacities of the samples were calculated using the formula given in the experimental section. The obtained capacities for the electrodes CHCe0, CHCe1, CHCe2 and CHCe3 at the scan rate of 5 mV s−1 are 277, 415, 400 and 316 C g−1, respectively. Fig. 3B shows the bar diagram indicating the values of specific capacities for different samples. The α-Co(OH)2 doped with 1 mol% Ce showed maximum specific capacity, which can be attributed to the nanoflakes-like morphology that provided a high surface area for the electrochemical reaction at the electrode-electrolyte interface. Further doping deteriorates the capacity performance due to the agglomerated compact 4

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Fig. 3. (A) CV curves of CHCe0, CHCe1, CHCe2 and CHCe3 electrodes at the scan rate of 5 mV s−1, (B) specific capacities calculated for CHCe0, CHCe1, CHCe2 and CHCe3 electrodes at the scan rate of 5 mV s−1, (C) CV curves of CHCe1 electrode at different scan rates (D) GCD curves of CHCe0, CHCe1, CHCe2 and CHCe3 electrodes at current density of 3 A g−1 (E) GCD curves of CHCe1 electrode at different current densities and (F) variation of specific capacity and current density for CHCe0, CHCe1, CHCe2 and CHCe3 electrodes.

Table 1 Capacitive performance of cobalt hydroxide-based electrodes previously reported in the literature and that of prepared in this study. S.no

Material

Potential window (V vs Ag/AgCl)

Specific capacity (C g−1)

Reference

1 2 3 4 5 6 7 8 9 10

Graphene/Co(OH)2 Co(OH)2/GO/chitosan Graphene/flower-like Co(OH)2 α-Co(OH)2/graphene nanosheets MOF-derived Co(OH)2 β-Co(OH)2/CMC Nano cobalt silicate hydroxide Carbonated Co(OH)2/AC Carbon nano fiber/Co(OH)2 1% Cerium doped α- Co(OH)2

0.5 0.7 0.4 0.6 0.5 0.6 0.55 0.45 1 0.7

237 274 192 284 303 306 130 135 157 415

[37] [38] [39] [40] [41] [42] [43] [44] [45] In this work

5

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Fig. 4. (A) Nyquist plots for CHCe0, CHCe1, CHCe2 and CHCe3 electrodes in the frequency range from 0.1 Hz–105 Hz, (B) variation of capacity retention and cycle number, and (C) Nyquist plots for CHCe1 electrode before and after 2000 GCD cycles (top left inset: magnified high frequency region, bottom right inset: fitted equivalent circuit).

of the CHCe0, CHCe1, CHCe2, and CHCe3 electrodes was examined at the high current density of 20 A g−1 for 2000 GCD cycles and the corresponding capacity retention was obtained as 65, 73, 69 and 50%, respectively (Fig. 4B). Relatively stable performance of the CHCe1 electrode is attributed to the horizontally grown porous nanoflakes like morphology which facilitates strong and robust network during charging-discharging processes. The stability performance in the present case is better than the composite materials recently reported by Yin et al. [37] and Huang et al [38]. In order to understand the degradation of stability over cycling, the cycled CHCe1 electrode was characterized for EIS, CV and FE-SEM and results are shown in Fig. 4C and figure S6 (supporting information). Fig. S6 (B) shows the microstructure of the film changed significantly with the formation of randomly distributed and loosely connected nanoflakes. The apparent surface area of the material did not change significantly. However, the CV curve depicts that the slight increment in the oxidation current whereas reduction current decreased rapidly after cycling. This kind of irreversible cycling behavior is responsible for the degradation of the battery type electrode material. Moreover, Fig. 4C shows the Nyquist plots for the CHCe1 electrode before and after 2000 cycles. The Re values of the CHCe1 electrode were obtained as 1.10 and 1.18 Ω, before and after cycling, respectively suggesting there is no considerable change in the intrinsic and the ionic resistances after 2000 GCD cycles. The estimated value of Rct for the CHCe1 electrode was increased from 0.47 to 1.72 Ω after cycling. The increased Rct value is ascribed to the change in the microstructure of the material after cycling [46]. The spike inclined to the real impedance axis in the lower

current density and the capacity is given in Fig. 3F. The capacity retention at the higher current density of 5 A g−1 for the electrodes CHCe0, CHCe1, CHCe2 and CHCe3 was obtained as 92, 96, 96 and 90% (capacity retention of different samples calculated from the CV at the scan rate of 100 mV s−1 can be seen from the Figure S5). The capacity retention for CHCe1 and CHCe2 electrodes was found to be equal which may be due to the excellent conductivity of both electrodes. Also, the capacity retentions for the CHCe0 and CHCe3 electrodes were relatively lower, this may be due to the poor electrical conductivity of the pristine and the heavily doped α-Co(OH)2 electrodes, respectively. EIS was used to estimate the electrical parameters of the electrodes. Fig. 4A shows the Nyquist plots for all the electrodes in the frequency range from 0.1 Hz to 1 MHz. From the figure, it is seen that all the samples demonstrate a spike at a lower frequency region and semicircle at the higher frequency region. The inset of Fig. 4C shows the equivalent circuit fitted for the impedance data in which Re represents the contribution of the intrinsic resistance and ionic resistance of the material whereas the charge transfer resistance (Rct) reflects the resistance associated with the electron transfer process during electrochemical reactions. The estimated values of Re for CHCe0, CHCe1, CHCe2, and CHCe3 electrodes were 0.862, 1.101, 1.455 and 1.153Ω, respectively manifesting no significant change in the ionic and materials resistances. The Rct for CHCe0, CHCe1, CHCe2, and CHCe3 electrodes can be estimated as 2.57, 0.48, 1.47 and 2.16 Ω, respectively. The lower Rct value of CHCe1 is attributed to the appropriate doping amount of Ce facilitating easy pathways for electronic diffusion during charging-discharging processes. Furthermore, the stability performance 6

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Fig. 5. (A) Current response for a SPECS experiment with potential steps of ± 25 mV for a time of 300 s each (B) SPECS current response during forward scan at the potential step of 0.1 V vs Ag/AgCl for CHCe1 electrode (inset: magnified image for first 5 s), synthetic voltammograms obtained from the SPECS data at scan rate of 0.083 mV/s for (C) CHCe0 and (D) CHCe1 electrodes.

shows the breakdown of the fitted current values correspond to the IDL1 IDL2, ID, and IR. It is observed that the currents originated due to the geometric and porous surfaces decline rapidly to zero and the diffusioncontrolled current decays slowly. In order to find out different charge storage contributions in the undoped and the doped film electrodes, SPECS study has been employed for CHCe0 and CHCe1 electrodes. Fig. 5C and D show the synthetic CV curves obtained from the SPECS data with different storage contributions for CHCe0 and CHCe1 electrodes, respectively at the scan rate of 0.083 mV s−1. The same CV curves were used to figure out the specific capacities associated with various storage contributions. The double-layer current obtained for the geometric surface was found to be negligible as compared to the current obtained for the porous surface. Therefore, in the present analysis, the current obtained due to the formation of a double layer on the porous surface has been considered. For the electrodes, CHCe0, and CHCe1, the porous and diffusion-controlled capacities were found to be 38% and 28% of the total capacity, respectively. Interestingly, when α-Co(OH)2 was doped with 1 mol% Ce, double layer capacitance associated with the porous surface increased to 45% and the diffusion-controlled capacity decreased to 16%. This clearly indicates that the doping does not directly change the crystal structure of the material but it changes the microstructure by creating the microporous channels which enhanced the double layer capacitive contribution of the electrode. The rest of the contribution in the electrodes might be due to the residual current which arises due to the incomplete redox reaction. It is noted that SPECS can guide us to fabricate high-performance battery-type

frequency region is formed by the Warburg impedance which can be ascribed to the frequency dependence of the ion diffusion in the electrolyte. Interestingly, before cycling, the inclined portion of the curve covers real resistances from 2.12 to 2.6 Ω, however, after 2000 GCD cycles, the slope of the 45° portion of the curve spanned the real resistances from 2.7 to 25.9 Ω which clearly indicates the enhancement in the diffusional resistance of the material. This can be attributed to the diffusional resistance of electrolyte among and into the porous structure of the altered microstructure of the Ce doped α-Co(OH)2 electrode. In SPECS, the current is measured with respect to time for all the potential steps over a potential window of the material (Fig. 5A). The SPECS current is due to the result of the diffusion-limited process and the electric double layer formation. The total current of SPECS at particular potential step can be calculated by using the formula [25], (8)

IT = IDL1 + IDL2 + ID + IR =

ΔE t t ⎞ + ΔE exp ⎛− ⎞ + B + IR exp ⎛− RS6 RS2 t1/2 ⎝ RS1CDL1 ⎠ ⎝ RS2CDL2 ⎠ ⎜

⎟

⎜

⎟

(9)

where, IDL1 and IDL2 are the double-layer current values associated with the geometric (CDL1) and porous (CDL2) electric double layer capacitances, respectively, ID is the diffusion-limited current and IR is the residual current. Fig. 5B shows the current versus time plot at the potential step of 0.1 V vs Ag/AgCl during the positive scan. It is seen from the plot that when the potential is applied, current rapidly decreases because of the formation of an electric double layer on the film electrode. This current response was modeled using Eq. 9 and the inset 7

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Fig. 6. (A) CV curves of CHCe1//AC asymmetric supercapacitor at different scan rates, (B) GCD curves of CHCe1//AC supercapacitor at different current densities, (C) variation of specific capacitance of device with cycle number (inset: photograph of lightening LED using two devices connected in series and (D) Nyquist plots for device before and after 5000 GCD cycles.

4. Conclusion

supercapacitor electrode materials with an improved capacitive contribution. In order to understand the applicability of Ce doped α-Co(OH)2 thin films, aqueous asymmetric supercapacitors were fabricated. Fig. 6A shows CV curves of aqueous asymmetric CHCe1//AC supercapacitor at different scan rates within the potential window of 0 to 1.6 V. CV curves show distorted rectangular shape with redox peaks correspond to oxidation and reduction of cobalt hydroxide during the charging-discharging process. Fig. 6B shows GCD curves of CHCe1//AC supercapacitor at different current densities in the potential range from 0 to 1.6 V. The specific capacitance of the device was calculated as 39.4 F g−1 at the current density of 0.5 A g−1. The maximum specific energy of the device was calculated as 14 Wh kg−1 at the specific power of 400 W kg−1, respectively. Values obtained in the present case are quite comparable with the cobalt hydroxide based asymmetric supercapacitors reported in the literature (Table S1, supporting information). As a result of the synergic effect between positive and negative electrodes, as-fabricated asymmetric CHCe1//AC supercapacitor showed impressive cyclic stability with capacitance retention of 90.4% after 5000 cycles as shown in Fig. 6C. Interestingly, these supercapacitors can power LED well after being charged at 2 mA for 5 min, as shown in the inset photograph of Fig. 6C. The Nyquist plots of the asymmetric CHCe1//AC supercapacitor before and after 5000 cycles are shown in Fig. 6D. There is no any considerable change in Nyquist plot before and after cycling but there is a small angle change in an inclined spike which shows that change in the Warburg impedance may be ascribed to the reasons such as morphological changes in the CHCe1 electrode, passive layer formation on the surface of the electrode and electrolyte decomposition.

In summary, Ce doped α-Co(OH)2 thin films were prepared successfully via electrodeposition method and the effect of doping on the structural, morphological and, electrochemical properties has been investigated. XRD results depict that the Ce doping doesn't modify the crystalline structure of the α-Co(OH)2 films. The morphological study shows that the doping significantly changes the microstructure of the αCo(OH)2 film. The α-Co(OH)2 doped with 1 mol% Ce shows the formation of porous and horizontally grown nanoflakes. From the electrochemical studies, we identified that the 1% Ce doped α-Co(OH)2 shows a higher specific capacity of 415 C g−1 with good cyclic stability of 73% even after 2000 cycles. SPECS results clearly showed that the doping enhances the capacitive contribution which further improves the rate capability of the electrode as evidenced by CV and GCD studies. Finally, asymmetric supercapacitor fabricated using Ce-doped α-Co (OH)2 and AC electrodes shows excellent specific energy of 14 Wh kg−1 at the specific power of 400 W kg−1 along with cyclic stability of 90.4% after 5000 cycles. Thus, this study clearly highlighted that Ce-doped αCo(OH)2 electrode can act as an efficient electrode material for supercapacitors. CRediT authorship contribution statement R.C. Rohit: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. A. Jenifer: Visualization. Ajay D. Jagadale: Conceptualization, Investigation, Writing - review & editing, Supervision. Vijay S. Kumbhar: Visualization, Writing - review 8

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& editing. Hyeonkwon Lee: Visualization, Writing - review & editing. Kiyoung Lee: Visualization, Writing - review & editing, Supervision.

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