Structural, Morphological, and Electrochemical Performance of Ceo2/Nio Nanocomposite for Supercapacitor Applications

applied sciences

Article Structural, Morphological, and Electrochemical Performance of CeO2/NiO Nanocomposite for Supercapacitor Applications

Naushad Ahmad 1, Abdulaziz Ali Alghamdi 1, Hessah A. AL-Abdulkarim 1, Ghulam M. Mustafa 2, Neazar Baghdadi 3 and Fahad A. Alharthi 1,*

1 Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia; [email protected] (N.A.); [email protected] (A.A.A.); [email protected] (H.A.A.-A.) 2 Department of Physics, The University of Lahore, Lahore 54590, Pakistan; [email protected] 3 Center of Nanotechnology, King Abdulaziz University, Jeddah 80200, Saudi Arabia; [email protected] * Correspondence: [email protected]

Abstract: The composite of ceria has been widely studied as an electrode material for supercapacitors

applications due to its high energy density. Herein, we synthesize CeO2/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 Ra- man 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 den-

Citation: Ahmad, N.; Ali Alghamdi, sities are performed to probe the electrochemical response which revealed the potential of CeO2/NiO A.; AL-Abdulkarim, H.A.; Mustafa, nanocomposite as an electrode material for energy storage devices. G.M.; Baghdadi, N.; Alharthi, F.A. Structural, Morphological, and Keywords: CeO2/NiO nanocomposite; electrode materials; supercapacitor; energy density; power density Electrochemical Performance of

CeO2/NiO Nanocomposite for Supercapacitor Applications. Appl. Sci. 2021, 11, 411. https://doi.org/ 1. Introduction 10.3390/app11010411 An emerging device for storing and converting energy, the supercapacitor has been Received: 22 November 2020 used in place of batteries and conventional capacitors. They possess many advantages and Accepted: 30 December 2020 potential due to various reasons. They exhibit high power density and great efficiencies Published: 4 January 2021 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 Publisher’s Note: MDPI stays neu- portable electronic devices as well as aerospace and military fields and many more. Mainly, tral with regard to jurisdictional clai- supercapacitors are categorized into two types. First, is electric double-layer capacitors ms in published maps and institutio- (EDLCs) and the second is pseudocapacitors (PCs). The EDLCs have higher values of nal affiliations. 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, Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. their energy density is still low which limits their applications. The other form of superca- This article is an open access article pacitors, PCs, can store charge on the surface of the electrode material having fast faradaic distributed under the terms and con- redox reactions and rely on high reversibility confirming the battery-like nature [3,4]. The ditions of the Creative Commons At- electrolyte ions that cause fast faradaic reactions can be inserted and deserted on the in- tribution (CC BY) license (https:// terfaces between electrolyte and electrode. The specific capacitance of PCs is greater than creativecommons.org/licenses/by/ EDLCs. Commonly used electrode materials in PCs are oxides, sulfides, hydroxides, and 4.0/). phosphides of transition metals. We synthesized this nanocomposite using oxides of nickel

Appl. Sci. 2021, 11, 411. https://doi.org/10.3390/app11010411 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 411 2 of 12

and cerium. The properties of EDLCs and PCs can be combined in hybrid supercapacitors (HSCs) in which negative electrode materials are made up of carbon materials while positive electrode materials are composed of battery-like transition metal oxides (TMOs) [5,6]. This approach also enhances the potential window or the working voltage range of the device. This increase in the potential window is directly linked with the energy density of 1 2 the electrode material which is measured using: E = 2 CV . In the recent decade, a lot of work has been devoted to the positive electrode materials that have high energy density, specific capacitances, and are eco-friendly [7–9]. Rare earth metal oxide, cerium oxide, is an intensively studied material because of its vast applications such as in portable devices, electrical vehicles, and sustainable energy devices. The composite of cerium oxide can be used for catalytic redox reactions and to clean the exhaust of automobiles. The reversible chemical reaction depends upon the chemical potential of oxygen between the two cerium oxides that can cause a catalytic effect. These two cerium oxides are oxygen-poor Ce2O3 and oxygen-rich CeO2. To expand the catalytic process of cerium oxide it is much needed to find novel metal ions that can be doped in CeO2 which can further improve the oxygen storage capacity [10,11]. In this regard, researchers working across the globe have attempted to mix the oxides of rare-earth metals, transition metals, and alkaline metals with Ce sub-lattice. To compensate charges in the lattice, the anion vacancy sites can be created by changing Ce4+ with Ce2+ and Ce3+ cations which leads to enhance the ionic conductivity for oxygen. However, the doping is carried out under dopant selection criteria i.e., the ionic radius of dopant must be comparable to the ionic radius of the host [12]. From the literature, it is noticed that the addition of transition metal ions is suitable for doping in CeO2. Generally, the composites of CeO2 with Cu doping represent enhanced catalytic performances as well as high oxygen storage capacities and high specific surface areas [13]. Some salient features of CeO2 which make it a potential material for various industrial applications include: (i) exceptional mechanical strength and high transmission, (ii) high oxygen storage capacity and high conductivity, (iii) high specific surface area and good redox performances, (iv) thermally stable at high temperatures and have abundant active sites, and (v) have a lot of oxygen vacancies on the surface. All these properties can further be improved by making composite or doping of metals or metal oxides in ceria. Various transition metal oxides such as zinc oxide, manganese oxide, nickel oxide, titanium oxide, and copper oxide have been doped in ceria to enhance its performance [14]. The composites of cerium oxide have potential applications such as electrochemical sensors, photo-catalyst as well as in energy storage systems and supercapacitors. Ahmed et al. [15] used the solution combustion route to synthesize the nanocomposite of NiO/CeO2 where they used metal nitrates as oxidizers and glycine as fuel. Different characterization techniques including XRD, TEM, and FTIR were used to investigate structural along with morphological and surface characteristics of this nanocomposite. Their results revealed that these compositions were suitable for electrochemical systems and can be used as electrodes due to their reproducibility as well as electrocatalytic activity and stability. The same group of researchers [16] doped ZnO in cerium oxide via a microwave-assisted method and confirmed the crystalline structure of composites with a high density of parti- cles. Ansari et al. [17] investigated the physiochemical characteristics of the nanocomposite of CeO2 by doping Cu in it. By increasing the doping concentration of Cu, band gap energies were increased which is an indication of the production of oxygen vacancies. These compositions can be utilized as electrode material for fuel cells and electrochemical systems as they can be operated at low temperatures up to 162 ◦C[18]. In the present work, we synthesized nanocomposites of NiO/CeO2 via hydrothermal route and investigated their structural, microstructural, morphological, optical, and electrochemical properties in detail which witnessed the importance of nanocomposites as an electrode material for energy storage devices. Appl. Sci. 2021, 11, 411 3 of 12

2. Experimental 2.1. Synthesis of CeO2/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/CeO2 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].

2.2. 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].

2.3. 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 cm2 and a weight of about ◦ Appl. Sci. 2021, 10, x FOR PEER REVIEW1 mg. The electrode was dried at 60 C overnight before the test. 3M KOH was used4 of 12 as the

electrolyte. The synthesis summary of electrode preparation is shown in Figure1.

Figure 1. A schematic of electrode synthesis.

3. 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 devel- opment 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 2. Indexed XRD pattern of CeO2/NiO nanocomposite. Appl. Sci. 2021, 10, x FOR PEER REVIEW 4 of 12

Appl. Sci. 2021, 11, 411 4 of 12 Figure 1. A schematic of electrode synthesis.

3. Results Results and and Discussion XRD is a powerful technique that is extens extensivelyively used to study the crystal structure, lattice parameters, parameters, crystallite crystallite size, size, dislocation dislocation density density as as well well as ascrystal crystal strain. strain. The The devel- de- opmentvelopment of a ofparticular a particular crystalline crystalline phase phase was critically was critically governed governed by the calcination by the calcination process. Figureprocess. 2 Figureshows2 theshows XRD the pattern XRD pattern of CeO of2/NiO CeO nanocomposite2/NiO nanocomposite in the 2 inθ therange 2θ ofrange 20–75 of degrees.20–75 degrees The diffracted. The diffracted peaks peaks appearing appearing at 2θ at values 2θ values of 28.7° of 28.7 and◦ and 47.4° 47.4 were◦ were indexed indexed as (111)as (111) and and (220) (220) and and matched matched with with the ICSD the ICSD ref. no. ref. 00-001-0800 no. 00-001-0800 which which confirmed confirmed the pres- the ◦ ◦ ◦ ◦ encepresence of CeO of CeO2 while2 while peaks peaks appearing appearing at 33.1°, at 33.1 56.4°,, 56.4 62.3°,, 62.3and 69.8°, and were 69.8 indexedwere indexed as (200), as (311),(200), (311),(103), (103),and (440) and (440)and matched and matched with withICSD ICSD ref. no. ref. 01-073-1519 no. 01-073-1519 which which confirmed confirmed the presencethe presence of the of theNiO NiO phase phase having having an an FCC FCC structure structure [22]. [22 ].Since Since there there was was no no peak peak other than CeO2 and NiO, it confirmedconfirmed thethe formationformation ofofCeO CeO22/NiO/NiO nanocomposite. For CeO2,, the most intense peak was (111) and forfor NiO,NiO, itit waswas (200).(200). UsingUsing Scherrer’sScherrer’s formula, the crystallite size was calculated from the most intense peaks. The calculated crystallite size was ~10 nm for CeO 22 andand 15 15 nm nm for for NiO. NiO.

Figure 2. Indexed XRD pattern of CeO2/NiO nanocomposite. Figure 2. Indexed XRD pattern of CeO2/NiO nanocomposite.

Figure3 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 (Figure3a) covered more area and thus provided useful information regarding particle distribution which was uniform throughout the surface. When magnification was increased (Figure3b), 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]. Appl. Sci. 2021, 10, x FOR PEER REVIEW 5 of 12 Appl. Sci. 2021, 10, x FOR PEER REVIEW 5 of 12

Figure 3 shows the SEM images of CeO2/NiO nanocomposite at (a) 4000 and 50,000 Figure 3 shows the SEM images of CeO2/NiO nanocomposite at (a) 4000 and 50,000 magnificationsmagnifications from from where where we we can can analyze analyze the the morphology, morphology, particle particle size size distribution, distribution, and and surfacesurface texture. texture. The The image image at at low low magnification magnification (Figure (Figure 3a) 3a) covered covered more more area area and and thus thus providedprovided useful useful information information regarding regarding partic particlele distribution distribution which which was was uniform uniform through- through- outout the the surface. surface. When When magnification magnification was was incr increasedeased (Figure (Figure 3b), 3b), it it provided provided more more chance chance Appl. Sci. 2021, 11,to 411to look look into into the the minor minor details details and and thus thus revealed revealed a a cauliflower’s cauliflower’s leaf-like leaf-like morphology. morphology. This This 5 of 12 kindkind of of morphology morphology exposed exposed high high surface surface area area and and thus thus had had high high potential potential as as an an electrode electrode materialmaterial for for energy energy storage storage devices devices like like supercapacitors supercapacitors [23]. [23].

Figure 3. SEM images of CeO2/NiO nanocomposite at (a) 4000× and (b) 50,000×. Figure 3.Figure SEM 3.imagesSEM imagesof CeO2 of/NiO CeO nanocomposite2/NiO nanocomposite at (a) 4000× at ( aand) 4000 (b)× 50,000×.and (b) 50,000×.

ToTo check check the the elemental elementalTo check composition thecomposition elemental of of composition th thee prepared prepared of sample, sample, the prepared EDX EDX analysis analysis sample, was was EDX per- per- analysis was per- formedformed at at a a fixed fixedformed area area of atof SEM aSEM ﬁxed im im areaagesages ofas as SEMshown shown images in in Figure Figure as shown 4. 4. The The in purity purity Figure of of4 .the Thethe sample sample purity could ofcould the sample could bebe verified verified from frombe the veriﬁedthe EDX EDX spectra fromspectra the because because EDX spectra there there were becausewere only only there peaks peaks were of of Ce, onlyCe, Ni, Ni, peaks and and ofO O in Ce,in the the Ni, and O in the graphgraph with with wt. wt. graph% % of of 44.80, 44.80, with wt.23.08, 23.08, % and of and 44.80, 25.84, 25.84, 23.08, respectively. respectively. and 25.84, In In respectively. addition, addition, a a peak Inpeak addition, of of C C was was a also peakalso of C was also therethere in in the the low lowthere energy energy in theregion region low which energywhich came came region from from which the the environment. came environment. from the The The environment. weight weight percentages percentages The weight percentages ofof all all these these elements elementsof all thesewere were elementsprovided provided werein in the the provided tabular tabular form inform the as as tabular an an inset. inset. form as an inset.

FigureFigure 4. 4. ( a(a) )SEM SEM image image of of CeO2/NiO CeO2/NiO nanocomposite nanocomposite with with scan scan area area and and ( b(b) )Energy-dispersive Energy-dispersive X- X- Figure 4. (aray) SEM spectroscopy image of CeO (EDX)2/NiO spectrum nanocomposite with a weight with scanpercentage area and of (allb) elements. Energy-dispersive X-ray spectroscopy (EDX) spectrum withray a spectroscopy weight percentage (EDX) ofspectrum all elements. with a weight percentage of all elements. In order to reveal the minor details of morphology and crystal geometry, the HR- In order to revealIn order the tominor reveal details the minor of morphology details of morphology and crystal and geometry, crystal geometry,the HR- the HR-TEM TEM images were captured as shown in Figure 5. Figure 5a,b revealed the polycrystalline TEM images wereimages captured were captured as shown as in shown Figure in 5. Figure Figure5. 5a,b Figure revealed5a,b revealed the polycrystalline the polycrystalline nature nature of this nanocomposite with the interplanar spacing of 0.155 nm. Some of these crys- nature of this nanocompositeof this nanocomposite with the with interp thelanar interplanar spacing spacing of 0.155 ofnm. 0.155 Some nm. ofSome these ofcrys- these crystallites tallites were grown in bipyramid shape as highlighted by sketching its schematic. The tallites were grownwere grown in bipyramid in bipyramid shape shape as highlighted as highlighted by sketching by sketching its schematic. its schematic. The The presence presence of concentric circles shown in the SEAD pattern supported the polycrystalline presence of concentricof concentric circles circles shown shown in the in SEAD the SEAD pattern pattern supported supported the polycrystalline the polycrystalline nature nature (Figure 5c). Here, the estimated value of crystallite size was in good agreement nature (Figure( Figure5c). Here,5c ). Here,the estimated the estimated value valueof crystallite of crystallite size was size in was good in good agreement agreement with the with the XRD results [24]. with the XRD XRDresults results [24]. [24]. Appl. Sci. 2021, 10, x FOR PEER REVIEW 6 of 12

Appl. Sci. 2021, 11, 411 6 of 12 Appl. Sci. 2021, 10, x FOR PEER REVIEW 6 of 12

Figure 5. (a,b) HR-TEM images, (c) SEAD pattern, and (d) d-spacing of CeO2/NiO nanocomposite.

FTIR spectrum of CeO2/NiO nanocomposite in the range of 400–4400 cm−1 is shown FigureFigure 5.5.( (aa,b,b)) HR-TEMHR-TEM images,images, ((cc)) SEADSEAD pattern,pattern, and and ( d(d)) d-spacing d-spacing of of CeO CeO22/NiO/NiO nanocomposite. in Figure 6. Here we observed multiple peaks corresponding to bond stretching and bond −1 FTIRvibrations spectrum FTIRof of metal CeO spectrum2 /NiOoxides. nanocomposite of CeO2/NiO nanocomposite in the range of in 400–4400 the range cm of−1 400–4400 is shown cm is shown in Figure 6. Herein Figurewe observed6. Here multiple we observed peaks multiple corresponding peaks corresponding to bond stretching to bond and stretching bond and bond vibrations of metalvibrations oxides. of metal oxides.

CeO -NiO 60 2 1 CeO -NiO – 60 55 2 1 – 1 – 2871 cm 1 1 55 –

50 – 1 – 1060 cm 1060 1 2871 cm 1 – 1 – 1 –

50 45 – 3474 cm 1610 cm 1610 1060 cm 1060 1 1 – 1 Transmission (%) Transmission – – 541 cm 45 2370 cm 40 3474 cm 1610 cm 1610 1 – Transmission (%) Transmission 541 cm 2930 cm 40 35 2370 cm 4000 3000 2000 1000

2930 cm Wavenumber (cm–1) 35

Figure4000 6. FTIR 3000 spectrum of 2000 CeO /NiO nanocomposite. 1000 2 –1 Figure 6. FTIR spectrumWavenumber of CeO2/NiO (cm nanocomposite.) − A signiﬁcant peak centered at 404 cm 1 the characteristic peak corresponding to the Astretching significant vibration peak centered of the Ni-O at 404 bond cm in−1 the NiO characteristic present in this peak nanocomposite corresponding [25 ].to The the same Figure 6. FTIR spectrum of CeO2/NiO nanocomposite. stretchingkind vibration of observation of thewas Ni-O made bond by in KorosecNiO present and Bukovecin this nanocomposite at a slightly higher [25]. The wave same number −1 −1 A significantkind ofi.e., observation peak 443 cm centered [was26]. atmade The 404 adjacent bycm Korosec−1 the peak characteristic and centered Bukovec atpeak at 541 a corresponding cmslightlywhich higher was to wave the attributed number to the stretching vibrationcharacteristic of the Ni-O stretching bond in of NiO the present Ce-O bond in this and nanocomposite thus conﬁrmed [25]. the The presence same of CeO2 [27]. −1 −1 kind of observationThe peaks was made appearing by Korosec at 3474 and cm Bukovecand 1610 at a slightly cm originated higher wave from number the bond stretching Appl. Sci. 2021, 10, x FOR PEER REVIEW 7 of 12

Appl. Sci. 2021, 11, 411 i.e., 443 cm−1 [26]. The adjacent peak centered at 541 cm−1 which was attributed to the char-7 of 12 acteristic 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 and bond bending of H O[28]. Other absorption peaks appearing at 2345 and 1618 cm−1 presence of metal oxide bonds2 [29]. verified the presence of metal oxide bonds [29]. Figure 7 depicts the Raman scattering spectrum of the prepared nanocomposite of Figure7 depicts the Raman scattering spectrum of the prepared nanocomposite CeO2/NiO. It is clear in the figure that there are two well-defined and sharp peaks at 556 of CeO2/NiO. It is clear in the figure that there are two well-defined and sharp peaks cm−1 and 540 cm−1 in the spectra. For the fluorite cubic structure of ceria, the characteristic at 556 cm−1 and 540 cm−1 in the spectra. For the fluorite cubic structure of ceria, the −1 peakcharacteristic was observed peak at was 556 observedcm that atwas 556 sharp-intense. cm−1 that was While sharp-intense. the other low-intense While the peak other −1 observedlow-intense at 540 peak cm observed corresponded at 540 cmto −the1 corresponded NiO. These obtained to the NiO. peaks These in the obtained Raman peaksspectra in werethe Raman in accordance spectra werewith the in accordance already reported with the peaks already in the reported literature peaks [30]. in These the literature character- [30]. isticThese peaks characteristic were also peaksin accordance were also with in accordance the peaks obtained with the peaksin FTIR obtained spectra. in FTIR spectra.

50 CeO2-NiO

Raman Intensity (arb.u.) Raman Intensity 10

500 1000 1500 2000 2500 3000 3500 − Wavenumber (cm 1)

FigureFigure 7. 7. RamanRaman spectrum spectrum of of CeO CeO2/NiO2/NiO nanocomposite. nanocomposite.

TheThe XPS XPS analysis analysis could could be be used used to to determin determinee the the presence presence of of different different elements elements and and theirtheir oxidation oxidation states. states. The The XPS XPS spectrum spectrum of this of thisnanocomposite nanocomposite is shown is shown in Figure in Figure 8. Fig-8. ureFigure 8a shows8a shows the survey the survey spectrum spectrum of the of whol the wholee nanocomposite nanocomposite while while Figure Figure 8b–d 8revealsb–d re- theveals spectrum the spectrum of O1s, of Ni2p, O1s, Ni2p,and Ce3d and Ce3drespecti respectively.vely. In O1s, In O1s,three three peaks peaks were were observed observed at 528,at 528, 531, 531, and and 534 534 eV eV(Figure (Figure 8b).8b). In Inthe the oxygen oxygen profile, profile, we we had had three three overlapping overlapping peaks peaks which were due to the presence of oxygen from CeO as well as from NiO. The most intense which were due to the presence of oxygen from CeO2 2 as well as from NiO. The most in- tensepeak peak appeared appeared at 531 at eV 531 [31 eV]. [31]. Two Two peaks peaks of Ni of appeared Ni appeared at 855 at eV 855 and eV 861 and eV 861 (Figure eV (Fig-8c). ureSimilarly, 8c). Similarly, Figure8 Figured shows 8d the shows photoelectron the photoelectron peaks of peaks Ce-3d. of The Ce-3d. peaks The in thispeaks spectrum in this spectrumwere a little were ambiguous a little ambiguous not only not because only ofbeca theuse multi-oxidation of the multi-oxidation states of states Ce but of also Ce but due to the hybridization of 4f of Ce with 2p of oxygen atoms. This hybridization leads to the also due to the hybridization of 4f of Ce with 2p of oxygen atoms. This hybridization leads splitting of Ce-peaks into multiple peaks. However, there was no peak corresponding to to the splitting of Ce-peaks into multiple peaks. However, there was no peak correspond- Ce2O3 in this spectrum [32]. In order to determine the specific surface area and pore vol- ing to Ce2O3 in this spectrum [32]. In order to determine the specific surface area and pore ume, we performed the BET analysis which showed that the synthesized electrode material volume, we performed the BET analysis which showed that the synthesized electrode ma- had 21.1817 g/m2 specific surface area while BJH exposed the value of pore volume to terial had 21.1817 g/m2 specific surface area while BJH exposed the value of pore volume be 0.076 cm3/g during adsorption which increased to 0.095 cm3/g during the desorption to be 0.076 cm3/g during adsorption which increased to 0.095 cm3/g during the desorption process. process. Appl. Sci. 2021, 10Appl., x FOR Sci. PEER2021, REVIEW11, 411 8 of 12 8 of 12

FigureFigure 8. (a) Survey8. (a) Survey spectrum spectrum of CeO of2 /NiOCeO2/NiO nanocomposite, nanocomposite, XPS spectraXPS spectra of (b )of O1s, (b) O1s, (c) Ni2p, (c) Ni2p, and (andd) Ce3d. (d) Ce3d. In order to explore the electrochemical performance of this nanocomposite as an In order toelectrode explore material,the electrochemical the CV measurement performance was of this carried nanocomposite out. The CV as an loops elec- of CeO2/NiO trode materialnanocomposite, the CV measurement were at different was carried scan out. rates The i.e., CV 0.005, loops 0.01, of 0.05, CeO and2/NiO 0.1 V/snano- in the potential composite werewindow at different of 0–0.5 scan V, as rates shown i.e. in, 0.005, Figure 0.01,9a. The0.05, charge and 0.1 stored V/s inin thethe material potentia wasl measured window of 0–0.5from V, theas shown area of in CV Figure loops. 9a. A The broader charge reduction stored in peakthe material appearing was atmeasured ~0.4V was attributed from the area toof CV the loops. oxidation A broader of NiO reduction which clearly peak appearing revealed theat ~0.4V pseudocapacitive was attributed behavior to of this the oxidation nanocomposite.of NiO which clearly A sudden reveal increaseed the inpseudocapacitive current values could behavior be observed of this nano- which may be due composite. A tosudden oxidation increase of anions in current present values in the could electrolyte. be observed The samewhich kind may of be observation due to had been oxidation of anionsreported present recently in the by electrolyte. Schiavi et al.,The 2020 same [33 kind,34]. of The observation values of ha speciﬁcd been capacitances re- (Cs) could be measured using the formula ported recently by Schiavi et al., 2020 [33,34]. The values of specific capacitances (Cs) could be measured using the formula Cs = A/2∆V × ν × m (1) Cs = A/2∆V × ν × m (1) Here, A is the area enclosed by the CV curve, ∆V is the potential window, m is the 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. Table1 comprises the values of C s of this mass of the electrode,nanocomposite and ν atis differentthe scan scanrate. rates. Table It 1 is comprises obvious from the thisvalues table of thatCs of speciﬁc this capacitance 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 decreased withattributed the increase to the of decreasescan rate ofs. This contact decrease time of of charges Cs with with increasing the electrode. scan rate Figure was 9b shows the attributed to thegalvanostatic decrease of charge-discharge contact time of charges (GCD) curve with ofthe the electrode. synthesized Figure nanocomposite 9b shows at various the galvanostaticcurrent charge densities.-discharge (GCD) curve of the synthesized nanocomposite at various current densities.

Appl. Sci. 2021, 10,Appl. x FOR Sci. PEER2021 REVIEW, 11, 411 9 of 12 9 of 12

Table 1. CalculatedTable values 1. Calculated of specific values capacitance of speciﬁc of CeO capacitance2/NiO nanocomposites of CeO2/NiO at nanocomposites different scan at different scan rates. rates.

Area Scan RateArea PotentialScan Rate WindowPotential Mass Window SpecificMass CapacitanceSpeciﬁc Capacitance (VA) (V/s)(VA) (V/s)(V) (g)(V) (F/g)(g) (F/g) 8.24 × 10−6 0.0058.24 × 10−6 0.005 0.5 0.001 0.5 1.6478 0.001 1.6478 1.19 × 10−5 1.190.01× 10−5 0.01 0.5 0.001 0.5 1.1893 0.001 1.1893 −5 3.79 × 10−5 3.790.05× 10 0.05 0.5 0.001 0.5 0.7571 0.001 0.7571 × −5 6.32 × 10−5 6.320.1 10 0.1 0.5 0.001 0.5 0.6322 0.001 0.6322

By observing theBy observingdischarge time the discharge from these time GCD from curves, these the GCD Cs, curves,energy thedensity Cs, energy (E), density (E), and power densityand power(P) were density calculated (P) were using calculated the following using relations the following [35–37]. relations [35–37].

E = 1/2Cs (∆V)2 2 (2) E = 1/2Cs (∆V) (2)

P = E/(∆t) P = E/(∆t)(3) (3)

Here, ∆V is theHere, change ∆V in is potential the change and in ∆potentialt is the discharge and ∆t is the time. discharge The calculated time. The values calculated of Cs, values E, and P at different current densitiesof areCs, summarizedE, and P at different in Table 2current. densities are summarized in Table 2.

Figure 9. (a)Figure CV curves 9. (a) and CV ( curvesb) GCD and curves (b) GCD of CeO curves2/NiO of nanocomposite. CeO2/NiO nanocomposite.

Table 2. Calculation of specific capacitance, energy density, and power density of CeO2/NiO nanocomposites.Table 2. Calculation of speciﬁc capacitance, energy density, and power density of CeO2/NiO nanocomposites. Current Discharge Potential Specific Energy Power Density TimeCurrent DropDischarge CapacitancePotential SpeciﬁcDensity DensityEnergy Power Density Time Drop Capacitance Density Density (A/g) (s) (V) (mF/g) (mF/g*V2) (mF/s.g*V2) (A/g) (s) (V) (mF/g) (mF/g*V2) (mF/s.g*V2) 1 × 10−4 42.09 0.5 8.418 1.052 0.025 1 × 10−4 42.09 0.5 8.418 1.052 0.025 2 × 10−4 16.44 0.5 6.576 0.822 0.05 2 × 10−4 16.44 0.5 6.576 0.822 0.05 −4 3 × 10 12.833 × 10−4 0.5 12.83 7.698 0.5 7.698 0.962 0.075 0.962 0.075 4 × 10−4 8.874 × 10−4 0.5 8.87 7.096 0.5 7.096 0.887 0.887 0.1 0.1 6 × 10−4 5.516 × 10−4 0.5 5.51 6.612 0.5 6.612 0.826 0.826 0.15 0.15 −4 8 × 10−4 3.818 × 10 0.5 3.81 6.096 0.5 6.096 0.726 0.726 0.20 0.20 1 × 10−3 2.91 0.5 2.962 0.229 0.0786 1 × 10−3 2.91 0.5 2.962 0.229 0.0786

In order to haveIn order more to insight, have more electrochemical insight, electrochemical impedance impedance spectroscopy spectroscopy was per- was performed formed which whichcould provide could provide a quantitative a quantitative value of value resistance of resistance offered by offered the electrolyte by the electrolyte and and electrode toelectrode the charge to the transfer. charge An transfer. EIS plot An of EIS this plot nanocomposite of this nanocomposite is shown in is Figure shown in Figure 10. 10. Appl. Sci. Sci. 20212021,, 1011,, x 411 FOR PEER REVIEW 1010 of of 12 12

FigureFigure 10. 10. NyquistNyquist plot plot of of CeO CeO22/NiO/NiO nanocomposite. nanocomposite.

Generally,Generally, the EIS EIS plots plots represented represented a a slop slop line line in in low-frequency low-frequency regime regime and and semicir- semicir- cularcular arc arc in in high-frequency regime. regime. Usually, Usually, the the semicircle in the high-frequency region determines the solution/electrolyte resistance (Rs) while the arc in the low frequency helps determines the solution/electrolyte resistance (Rs) while the arc in the low frequency helps toto determine thethe capacitivecapacitive nature nature of of the the material material which which is quantiﬁedis quantified by theby Warburgthe Warburg coef- coefficientﬁcient (W). (W). In the In presentthe present study, study, we could we co seeuld the see semicircle the semicircle in the high-frequencyin the high-frequency region which is represented in the inset of Figure 10. The diameter of this semicircle was 0.5 Ω region which is represented in the inset of Figure 10. The diameter of this semicircle was which determined the resistance of electrolyte. Furthermore, the value of charge transfer 0.5 Ω which determined the resistance of electrolyte. Furthermore, the value of charge resistance could also be estimated from the semicircle in the low-frequency region. For transfer resistance could also be estimated from the semicircle in the low-frequency re- better performance of the electrode material, it should have as low resistance as possible. gion. For better performance of the electrode material, it should have as low resistance as In previous literature, the minimum reported value of charge transfer resistance is 1.2 Ω, possible. In previous literature, the minimum reported value of charge transfer resistance however, in this case, we obtained the minimum value of 0.5 Ω. The comparison of the is 1.2 Ω, however, in this case, we obtained the minimum value of 0.5 Ω. The comparison present work with the literature is given in Table3[38–41]. of the present work with the literature is given in Table 3 [38–41]. Table 3. Comparison of the present study with the literature. Table 3. Comparison of the present study with the literature. Charge Reported Composition Synthesis Method TransferredCharge Reference Year Composition Synthesis Method Reported YearResistance Transferred (Ω) Reference

RGO/β-Ni(OH)2/CeO2 Hydrothermal 2020Resistance 1.99 (Ω) [38] RGO/βYMnO-Ni(OH)2/CeO23/CeO2 Hydrothermalsonochemistry 20202020 3201.99 [38] [39] carbon/CeO2 polymer pyrolysis method 2018 1.2 [40] YMnO3/CeO2CeO2/NiO sonochemistry Hydrothermal 2018 2020 1.9 320 [[39]41] carbon/CeO2CeO2/NiO polymer Hydrothermal pyrolysis method 2018 0.4 1.2 This [40] work CeO2/NiO Hydrothermal 2018 1.9 [41] 4. ConclusionsCeO2/NiO Hydrothermal 0.4 This work In nutshell, we synthesized CeO2/NiO nanocomposite via the hydrothermal route 4.to Conclusions explore its electrochemical performance as an electrode material for supercapacitor

applications.In nutshell, First we of synthesized all, the presence CeO2/NiO of both nanocomposite CeO2 and NiO via phases the hydrothermal was identified route from to exploreits structural its electrochemical analysis. Then performance morphology as an and electrode composition material verification for supercapacitor was carried appli- out using SEM and EDX analysis. Polycrystalline nature with crystallites of bipyramid shape cations. First of all, the presence of both CeO2 and NiO phases was identified from its structuralwas examined analysis. through Then TEMmorphology images. and FTIR, composition Raman, andverification XPS analysis was carried also supported out using SEMthe formation and EDX ofanalysis. CeO2/NiO Polycrystalline nanocomposite. nature Calculatedwith crystallites values of of bipyramid specific capacitance, shape was examinedenergy density, through and TEM power images. density FTIR, exposed Raman, that and CeO XPS2/NiO analysis nanocomposite also supported has superiorthe for- electrochemical performance as compared to pure ceria or nickel oxide. mation 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. Appl. Sci. 2021, 11, 411 11 of 12

Author Contributions: Data curation, H.A.A.-A. and N.B.; Investigation, N.A.; Methodology, N.A.; Project administration, F.A.A.; Resources, A.A.A.; Software, A.A.A.; Validation, H.A.A.-A.; Writing— original draft, G.M.M.; Writing—review & editing, N.A. and F.A.A. All authors have read and agreed to the published version of the manuscript. Funding: The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through Research Group No. RGP-1438-040. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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