Structural, Morphological, and Electrochemical Performance of CeO 2 /NiO Nanocomposite for Supercapacitor Applications

: The composite of ceria has been widely studied as an electrode material for supercapacitors applications due to its high energy density. Herein, we synthesize CeO 2 /NiO nanocomposite via a hydrothermal route and explore its different aspects using various characterization techniques. The crystal structure is investigated using X-ray diffraction, Fourier transform infrared, and Raman spectroscopy. The formation of nanoﬂakes which combine to form ﬂower-like morphology is observed from scanning electron microscope images. Selected area scans conﬁrm the presence of all elements in accordance with their stoichiometric amount and thus authenticate the elemental purity. Polycrystalline nature with crystallite size 8–10 nm having truncated octahedron shape is conﬁrmed from tunneling electron microscope images. Using X-ray photoelectron spectroscopy the different oxidation states of Ce and Ni are observed which play the role of active sites in the electrochemical performance of this nanocomposite material. Cyclic Voltammetry(CV) measurements at different scan rates and Galvanic Charge Discharge (GCD) measurements at different current densities are performed to probe the electrochemical response which revealed the potential of CeO 2 /NiO nanocomposite as an electrode material for energy storage devices. speciﬁc capacitance, energy density, and power density of CeO nanocomposites.


Introduction
An emerging device for storing and converting energy, the supercapacitor has been used in place of batteries and conventional capacitors. They possess many advantages and potential due to various reasons. They exhibit high power density and great efficiencies along with a high capability of charge/discharge and long cyclic stability [1]. There are many applications of supercapacitors in different fields that include auto-motives and portable electronic devices as well as aerospace and military fields and many more. Mainly, supercapacitors are categorized into two types. First, is electric double-layer capacitors (EDLCs) and the second is pseudocapacitors (PCs). The EDLCs have higher values of capacitance in contrast with conventional capacitors due to the short distance up to the nanometer level between the double layers [2]. The ability of charge storage in EDLCs on the surface of electrode materials depends upon the adsorption and desorption of electrolyte ions. Despite the high reversibility and perfect cyclic performance of EDLCs, their energy density is still low which limits their applications. The other form of supercapacitors, PCs, can store charge on the surface of the electrode material having fast faradaic redox reactions and rely on high reversibility confirming the battery-like nature [3,4]. The electrolyte ions that cause fast faradaic reactions can be inserted and deserted on the interfaces between electrolyte and electrode. The specific capacitance of PCs is greater than EDLCs. Commonly used electrode materials in PCs are oxides, sulfides, hydroxides, and phosphides of transition metals. We synthesized this nanocomposite using oxides of nickel

Synthesis of CeO 2 /NiO Nanocomposite
The nanocomposite of different metal oxides with ceria can be made using different chemical methods like ball milling, co-precipitation, sol-gel auto-combustion, and the hydrothermal method. Among these available methods, we adopted the hydrothermal method to synthesize the NiO/CeO 2 nanocomposite. For this purpose, the weighted amounts of nitrates of Ni and Ce were dissolved in 50 mL distilled water and then the fuel agent, glycine, was put into the solution of nitrates. In order to keep the pH at 10, 1M solution of NaOH was put drop-wise in the solution and stirred continuously. This stirring was continued for an hour. After that, this solution was shifted into a Teflon-lined autoclave made of stainless steel which was placed in an oven at 150 • C for 12 h for hydrothermal reaction to occur. Furthermore, the autoclave was cooled to room temperature (RT) and the resultant precipitates were filtered. These precipitates were washed several times with DI water and ethanol. After washing, these precipitates were dried at 80 • C for 24 h. Lastly, the obtained powder was calcined at 550 • C for 5 h [19].

Characterization Techniques
The structural investigation was carried out using Bruker D8 advanced diffractometer with Cu Kα radiation having λ = 0.1540 nm. This pattern was recorded in the 2θ range of 20 to 75 degrees. The structure was further analyzed using Raman spectroscopy. The morphological analysis was done by JOEL JSM-7600F scanning electron microscopy (SEM) and then elemental analysis was done with the help of energy-dispersive X-ray spectroscopy (EDX). An accelerating voltage of 20 kV was set for EDX. The surface chemical compositions were investigated using X-ray photoelectron spectroscopy (XPS). CS350 potentiostat was used to record the CV loops at different scan rates (0.005, 0.01, 0.05, and 0.1 mVs −1 ) in the potential window 0-0.5 V. Using the same instrument, charge/discharge curves and impedance measurements were also recorded [20,21].

Fabrication of Supercapacitor Electrode and Electrochemical Tests
All electrochemical experiments were performed on a CHI 601C Electrochemical Workstation (CH Instrument, Austin, TX, USA), using a traditional three-electrode electrochemical cell (a working volume of 5 mL) with a working electrode, an Ag/AgCl (saturated KCl) reference electrode, and a platinum wire counter electrode. The working electrode was prepared by mixing active material with carbon black (conductive agent) and polyvinylidene fluoride (PVDF) (binder) with a mass ratio of 8:1:1 using ethanol as solvent. The mixture was cast onto Ni foam with an area of 1 cm 2 and a weight of about 1 mg. The electrode was dried at 60 • C overnight before the test. 3M KOH was used as the electrolyte. The synthesis summary of electrode preparation is shown in Figure 1.

Results and Discussion
XRD is a powerful technique that is extensively used to study the crystal structure, lattice parameters, crystallite size, dislocation density as well as crystal strain. The development of a particular crystalline phase was critically governed by the calcination process. Figure 2 shows the XRD pattern of CeO2/NiO nanocomposite in the 2θ range of 20-75 degrees. The diffracted peaks appearing at 2θ values of 28.7° and 47.4° were indexed as (111) and (220) and matched with the ICSD ref. no. 00-001-0800 which confirmed the pres-

Results and Discussion
XRD is a powerful technique that is extensively used to study the crystal structure, lattice parameters, crystallite size, dislocation density as well as crystal strain. The development of a particular crystalline phase was critically governed by the calcination process. Figure [22]. Since there was no peak other than CeO 2 and NiO, it confirmed the formation of CeO 2 /NiO nanocomposite. For CeO 2 , the most intense peak was (111) and for NiO, it was (200). Using Scherrer's formula, the crystallite size was calculated from the most intense peaks. The calculated crystallite size was~10 nm for CeO 2 and 15 nm for NiO.

Results and Discussion
XRD is a powerful technique that is extensively used to study the crystal structure, lattice parameters, crystallite size, dislocation density as well as crystal strain. The development of a particular crystalline phase was critically governed by the calcination process. Figure 2 shows the XRD pattern of CeO2/NiO nanocomposite in the 2θ range of 20-75 degrees. The diffracted peaks appearing at 2θ values of 28.7° and 47.4° were indexed as (111) and (220) and matched with the ICSD ref. no. 00-001-0800 which confirmed the presence of CeO2 while peaks appearing at 33.1°, 56.4°, 62.3°, and 69.8° were indexed as (200), (311), (103), and (440) and matched with ICSD ref. no. 01-073-1519 which confirmed the presence of the NiO phase having an FCC structure [22]. Since there was no peak other than CeO2 and NiO, it confirmed the formation of CeO2/NiO nanocomposite. For CeO2, the most intense peak was (111) and for NiO, it was (200). Using Scherrer's formula, the crystallite size was calculated from the most intense peaks. The calculated crystallite size was ~10 nm for CeO2 and 15 nm for NiO.   Figure 3 shows the SEM images of CeO 2 /NiO nanocomposite at (a) 4000 and 50,000 magnifications from where we can analyze the morphology, particle size distribution, and surface texture. The image at low magnification ( Figure 3a) covered more area and thus provided useful information regarding particle distribution which was uniform throughout the surface. When magnification was increased (Figure 3b), it provided more chance to look into the minor details and thus revealed a cauliflower's leaf-like morphology. This kind of morphology exposed high surface area and thus had high potential as an electrode material for energy storage devices like supercapacitors [23]. magnifications from where we can analyze the morphology, particle size distribution, and surface texture. The image at low magnification ( Figure 3a) covered more area and thus provided useful information regarding particle distribution which was uniform throughout the surface. When magnification was increased (Figure 3b), it provided more chance to look into the minor details and thus revealed a cauliflower's leaf-like morphology. This kind of morphology exposed high surface area and thus had high potential as an electrode material for energy storage devices like supercapacitors [23]. To check the elemental composition of the prepared sample, EDX analysis was performed at a fixed area of SEM images as shown in Figure 4. The purity of the sample could be verified from the EDX spectra because there were only peaks of Ce, Ni, and O in the graph with wt. % of 44.80, 23.08, and 25.84, respectively. In addition, a peak of C was also there in the low energy region which came from the environment. The weight percentages of all these elements were provided in the tabular form as an inset. In order to reveal the minor details of morphology and crystal geometry, the HR-TEM images were captured as shown in Figure 5. Figure 5a,b revealed the polycrystalline nature of this nanocomposite with the interplanar spacing of 0.155 nm. Some of these crystallites were grown in bipyramid shape as highlighted by sketching its schematic. The presence of concentric circles shown in the SEAD pattern supported the polycrystalline nature ( Figure 5c). Here, the estimated value of crystallite size was in good agreement with the XRD results [24]. To check the elemental composition of the prepared sample, EDX analysis was performed at a fixed area of SEM images as shown in Figure 4. The purity of the sample could be verified from the EDX spectra because there were only peaks of Ce, Ni, and O in the graph with wt. % of 44.80, 23.08, and 25.84, respectively. In addition, a peak of C was also there in the low energy region which came from the environment. The weight percentages of all these elements were provided in the tabular form as an inset.
magnifications from where we can analyze the morphology, particle size distribution, and surface texture. The image at low magnification ( Figure 3a) covered more area and thus provided useful information regarding particle distribution which was uniform throughout the surface. When magnification was increased (Figure 3b), it provided more chance to look into the minor details and thus revealed a cauliflower's leaf-like morphology. This kind of morphology exposed high surface area and thus had high potential as an electrode material for energy storage devices like supercapacitors [23]. To check the elemental composition of the prepared sample, EDX analysis was performed at a fixed area of SEM images as shown in Figure 4. The purity of the sample could be verified from the EDX spectra because there were only peaks of Ce, Ni, and O in the graph with wt. % of 44.80, 23.08, and 25.84, respectively. In addition, a peak of C was also there in the low energy region which came from the environment. The weight percentages of all these elements were provided in the tabular form as an inset. In order to reveal the minor details of morphology and crystal geometry, the HR-TEM images were captured as shown in Figure 5. Figure 5a,b revealed the polycrystalline nature of this nanocomposite with the interplanar spacing of 0.155 nm. Some of these crystallites were grown in bipyramid shape as highlighted by sketching its schematic. The presence of concentric circles shown in the SEAD pattern supported the polycrystalline nature ( Figure 5c). Here, the estimated value of crystallite size was in good agreement with the XRD results [24]. In order to reveal the minor details of morphology and crystal geometry, the HR-TEM images were captured as shown in Figure 5. Figure 5a,b revealed the polycrystalline nature of this nanocomposite with the interplanar spacing of 0.155 nm. Some of these crystallites were grown in bipyramid shape as highlighted by sketching its schematic. The presence of concentric circles shown in the SEAD pattern supported the polycrystalline nature ( Figure 5c). Here, the estimated value of crystallite size was in good agreement with the XRD results [24]. i. 2021, 10, x FOR PEER REVIEW 6 of 12 FTIR spectrum of CeO2/NiO nanocomposite in the range of 400-4400 cm −1 is shown in Figure 6. Here we observed multiple peaks corresponding to bond stretching and bond vibrations of metal oxides. A significant peak centered at 404 cm −1 the characteristic peak corresponding to the stretching vibration of the Ni-O bond in NiO present in this nanocomposite [25]. The same kind of observation was made by Korosec and Bukovec at a slightly higher wave number FTIR spectrum of CeO2/NiO nanocomposite in the range of 400-4400 cm −1 is shown in Figure 6. Here we observed multiple peaks corresponding to bond stretching and bond vibrations of metal oxides. A significant peak centered at 404 cm −1 the characteristic peak corresponding to the stretching vibration of the Ni-O bond in NiO present in this nanocomposite [25]. The same kind of observation was made by Korosec and Bukovec at a slightly higher wave number A significant peak centered at 404 cm −1 the characteristic peak corresponding to the stretching vibration of the Ni-O bond in NiO present in this nanocomposite [25]. The same kind of observation was made by Korosec and Bukovec at a slightly higher wave number i.e., 443 cm −1 [26]. The adjacent peak centered at 541 cm −1 which was attributed to the characteristic stretching of the Ce-O bond and thus confirmed the presence of CeO 2 [27]. The peaks appearing at 3474 cm −1 and 1610 cm −1 originated from the bond stretching and bond bending of H 2 O [28]. Other absorption peaks appearing at 2345 and 1618 cm −1 verified the presence of metal oxide bonds [29]. Figure 7 depicts the Raman scattering spectrum of the prepared nanocomposite of CeO 2 /NiO. It is clear in the figure that there are two well-defined and sharp peaks at 556 cm −1 and 540 cm −1 in the spectra. For the fluorite cubic structure of ceria, the characteristic peak was observed at 556 cm −1 that was sharp-intense. While the other low-intense peak observed at 540 cm −1 corresponded to the NiO. These obtained peaks in the Raman spectra were in accordance with the already reported peaks in the literature [30]. These characteristic peaks were also in accordance with the peaks obtained in FTIR spectra. i.e., 443 cm −1 [26]. The adjacent peak centered at 541 cm −1 which was attributed to the characteristic stretching of the Ce-O bond and thus confirmed the presence of CeO2 [27]. The peaks appearing at 3474 cm −1 and 1610 cm −1 originated from the bond stretching and bond bending of H2O [28]. Other absorption peaks appearing at 2345 and 1618 cm −1 verified the presence of metal oxide bonds [29]. Figure 7 depicts the Raman scattering spectrum of the prepared nanocomposite of CeO2/NiO. It is clear in the figure that there are two well-defined and sharp peaks at 556 cm −1 and 540 cm −1 in the spectra. For the fluorite cubic structure of ceria, the characteristic peak was observed at 556 cm −1 that was sharp-intense. While the other low-intense peak observed at 540 cm −1 corresponded to the NiO. These obtained peaks in the Raman spectra were in accordance with the already reported peaks in the literature [30]. These characteristic peaks were also in accordance with the peaks obtained in FTIR spectra. The XPS analysis could be used to determine the presence of different elements and their oxidation states. The XPS spectrum of this nanocomposite is shown in Figure 8. Figure 8a shows the survey spectrum of the whole nanocomposite while Figure 8b-d reveals the spectrum of O1s, Ni2p, and Ce3d respectively. In O1s, three peaks were observed at 528, 531, and 534 eV (Figure 8b). In the oxygen profile, we had three overlapping peaks which were due to the presence of oxygen from CeO2 as well as from NiO. The most intense peak appeared at 531 eV [31]. Two peaks of Ni appeared at 855 eV and 861 eV (Figure 8c). Similarly, Figure 8d shows the photoelectron peaks of Ce-3d. The peaks in this spectrum were a little ambiguous not only because of the multi-oxidation states of Ce but also due to the hybridization of 4f of Ce with 2p of oxygen atoms. This hybridization leads to the splitting of Ce-peaks into multiple peaks. However, there was no peak corresponding to Ce2O3 in this spectrum [32]. In order to determine the specific surface area and pore volume, we performed the BET analysis which showed that the synthesized electrode material had 21.1817 g/m 2 specific surface area while BJH exposed the value of pore volume to be 0.076 cm 3 /g during adsorption which increased to 0.095 cm 3 /g during the desorption process. The XPS analysis could be used to determine the presence of different elements and their oxidation states. The XPS spectrum of this nanocomposite is shown in Figure 8. Figure 8a shows the survey spectrum of the whole nanocomposite while Figure 8b-d reveals the spectrum of O1s, Ni2p, and Ce3d respectively. In O1s, three peaks were observed at 528, 531, and 534 eV (Figure 8b). In the oxygen profile, we had three overlapping peaks which were due to the presence of oxygen from CeO 2 as well as from NiO. The most intense peak appeared at 531 eV [31]. Two peaks of Ni appeared at 855 eV and 861 eV (Figure 8c). Similarly, Figure 8d shows the photoelectron peaks of Ce-3d. The peaks in this spectrum were a little ambiguous not only because of the multi-oxidation states of Ce but also due to the hybridization of 4f of Ce with 2p of oxygen atoms. This hybridization leads to the splitting of Ce-peaks into multiple peaks. However, there was no peak corresponding to Ce 2 O 3 in this spectrum [32]. In order to determine the specific surface area and pore volume, we performed the BET analysis which showed that the synthesized electrode material had 21.1817 g/m 2 specific surface area while BJH exposed the value of pore volume to be 0.076 cm 3 /g during adsorption which increased to 0.095 cm 3 /g during the desorption process. ci. 2021, 10, x FOR PEER REVIEW 8 of 12 In order to explore the electrochemical performance of this nanocomposite as an electrode material, the CV measurement was carried out. The CV loops of CeO2/NiO nanocomposite were at different scan rates i.e., 0.005, 0.01, 0.05, and 0.1 V/s in the potential window of 0-0.5 V, as shown in Figure 9a. The charge stored in the material was measured from the area of CV loops. A broader reduction peak appearing at ~0.4V was attributed to the oxidation of NiO which clearly revealed the pseudocapacitive behavior of this nanocomposite. A sudden increase in current values could be observed which may be due to oxidation of anions present in the electrolyte. The same kind of observation had been reported recently by Schiavi et al., 2020 [33,34]. The values of specific capacitances (Cs) could be measured using the formula Here, A is the area enclosed by the CV curve, ∆V is the potential window, m is the mass of the electrode, and ν is the scan rate. Table 1 comprises the values of Cs of this nanocomposite at different scan rates. It is obvious from this table that specific capacitance decreased with the increase of scan rates. This decrease of Cs with increasing scan rate was attributed to the decrease of contact time of charges with the electrode. Figure 9b shows the galvanostatic charge-discharge (GCD) curve of the synthesized nanocomposite at various current densities. In order to explore the electrochemical performance of this nanocomposite as an electrode material, the CV measurement was carried out. The CV loops of CeO 2 /NiO nanocomposite were at different scan rates i.e., 0.005, 0.01, 0.05, and 0.1 V/s in the potential window of 0-0.5 V, as shown in Figure 9a. The charge stored in the material was measured from the area of CV loops. A broader reduction peak appearing at~0.4V was attributed to the oxidation of NiO which clearly revealed the pseudocapacitive behavior of this nanocomposite. A sudden increase in current values could be observed which may be due to oxidation of anions present in the electrolyte. The same kind of observation had been reported recently by Schiavi et al., 2020 [33,34]. The values of specific capacitances (C s ) could be measured using the formula Here, A is the area enclosed by the CV curve, ∆V is the potential window, m is the mass of the electrode, and ν is the scan rate. Table 1 comprises the values of C s of this nanocomposite at different scan rates. It is obvious from this table that specific capacitance decreased with the increase of scan rates. This decrease of C s with increasing scan rate was attributed to the decrease of contact time of charges with the electrode. Figure 9b shows the galvanostatic charge-discharge (GCD) curve of the synthesized nanocomposite at various current densities. By observing the discharge time from these GCD curves, the C s , energy density (E), and power density (P) were calculated using the following relations [35][36][37]. E = 1/2C s (∆V) 2 (2) Here, ∆V is the change in potential and ∆t is the discharge time. The calculated values of C s , E, and P at different current densities are summarized in Table 2.
i. 2021, 10, x FOR PEER REVIEW 9 of 12 By observing the discharge time from these GCD curves, the Cs, energy density (E), and power density (P) were calculated using the following relations [35][36][37]. E = 1/2Cs (∆V) 2 (2) Here, ∆V is the change in potential and ∆t is the discharge time. The calculated values of Cs, E, and P at different current densities are summarized in Table 2.  In order to have more insight, electrochemical impedance spectroscopy was performed which could provide a quantitative value of resistance offered by the electrolyte and electrode to the charge transfer. An EIS plot of this nanocomposite is shown in Figure  10.  In order to have more insight, electrochemical impedance spectroscopy was performed which could provide a quantitative value of resistance offered by the electrolyte and electrode to the charge transfer. An EIS plot of this nanocomposite is shown in Figure 10. Generally, the EIS plots represented a slop line in low-frequency regime and semicircular arc in high-frequency regime. Usually, the semicircle in the high-frequency region determines the solution/electrolyte resistance (Rs) while the arc in the low frequency helps to determine the capacitive nature of the material which is quantified by the Warburg coefficient (W). In the present study, we could see the semicircle in the high-frequency region which is represented in the inset of Figure 10. The diameter of this semicircle was 0.5 Ω which determined the resistance of electrolyte. Furthermore, the value of charge transfer resistance could also be estimated from the semicircle in the low-frequency region. For better performance of the electrode material, it should have as low resistance as possible. In previous literature, the minimum reported value of charge transfer resistance is 1.2 Ω, however, in this case, we obtained the minimum value of 0.5 Ω. The comparison of the present work with the literature is given in Table 3 [38][39][40][41].

Conclusions
In nutshell, we synthesized CeO2/NiO nanocomposite via the hydrothermal route to explore its electrochemical performance as an electrode material for supercapacitor applications. First of all, the presence of both CeO2 and NiO phases was identified from its structural analysis. Then morphology and composition verification was carried out using SEM and EDX analysis. Polycrystalline nature with crystallites of bipyramid shape was examined through TEM images. FTIR, Raman, and XPS analysis also supported the formation of CeO2/NiO nanocomposite. Calculated values of specific capacitance, energy density, and power density exposed that CeO2/NiO nanocomposite has superior electrochemical performance as compared to pure ceria or nickel oxide. Generally, the EIS plots represented a slop line in low-frequency regime and semicircular arc in high-frequency regime. Usually, the semicircle in the high-frequency region determines the solution/electrolyte resistance (R s ) while the arc in the low frequency helps to determine the capacitive nature of the material which is quantified by the Warburg coefficient (W). In the present study, we could see the semicircle in the high-frequency region which is represented in the inset of Figure 10. The diameter of this semicircle was 0.5 Ω which determined the resistance of electrolyte. Furthermore, the value of charge transfer resistance could also be estimated from the semicircle in the low-frequency region. For better performance of the electrode material, it should have as low resistance as possible. In previous literature, the minimum reported value of charge transfer resistance is 1.2 Ω, however, in this case, we obtained the minimum value of 0.5 Ω. The comparison of the present work with the literature is given in Table 3 [38][39][40][41].

Conclusions
In nutshell, we synthesized CeO 2 /NiO nanocomposite via the hydrothermal route to explore its electrochemical performance as an electrode material for supercapacitor applications. First of all, the presence of both CeO 2 and NiO phases was identified from its structural analysis. Then morphology and composition verification was carried out using SEM and EDX analysis. Polycrystalline nature with crystallites of bipyramid shape was examined through TEM images. FTIR, Raman, and XPS analysis also supported the formation of CeO 2 /NiO nanocomposite. Calculated values of specific capacitance, energy density, and power density exposed that CeO 2 /NiO nanocomposite has superior electrochemical performance as compared to pure ceria or nickel oxide.