Effect of Nickel doping on Cobalt Oxide Nanoparticles for energy storage applications

13 We present a comprehensive study on the utilization of Ni-doped Co 3 O 4 nanoparticles for 14 energy storage applications, particularly in supercapacitors. X-ray diffraction analysis confirms 15 the structural integrity and phase purity of the samples, exhibiting the characteristic peaks of the 16 cubic spinel structure X-ray photoelectron spectroscopy confirms the presence of Co, Ni, and O 17 elements, with different valence states observed. Scanning electron microscope images reveal 18 irregular nano-flakes with increased particle size and reduced porosity as Ni doping 19 concentration rises. Electrochemical analysis, including cyclic voltammetry and galvanostatic 20 charge-discharge tests, demonstrates promising performance. Specifically, the 3 wt% Ni-doped 21 Co 3 O 4 sample exhibits a maximum specific capacitance of 299 F/g at a scan rate of 5 mV/s. The 22 GCD profile of all the three Ni doped Co 3 O 4 Nps were carried out. All of them revealed quasi 23 triangular charge-discharge curve that are due to both pseudo capacitive and electric double layer 24 process. Moreover, the 3% Ni-doped Co 3 O 4 nanoparticles demonstrate a maximum specific 25 capacitance of 347 F/g at a scan rate of 1.5 A/g. Additionally, the 5% Ni-doped Co 3 O 4 26 nanoparticles exhibit an impressive capacity retention of 92.87% even after 1500 cycles. Our 27 findings indicate that


Introduction
Over the past three decades, the rise in industrial activities and the transformation of modern lifestyles have created a significant demand for electric energy.A majority of this demand is currently fulfilled by burning fossil fuels, such as coal, petroleum, and natural gas [1][2][3].Unfortunately, the combustion process releases greenhouse gases, including CO2, N2O, CH4, and hydrofluorocarbons, which pose severe environmental threats to ecological systems.To address the increasing energy requirements and preserve our natural ecosystems, it is crucial to explore alternative energy sources.Renewable energies like solar, wind, and tidal power have gained significant attention as viable alternatives.However, efficient storage systems are needed to harness and store these renewable energy sources effectively when they are abundantly available.Devices such as batteries, fuel cells, and supercapacitors [4][5] can provide the necessary energy storage capabilities.Among them, supercapacitors (SCs) are particularly appealing due to their high power density, rapid charge-discharge capabilities, and ability to retain high capacitance [6].Enhancing the capacitance of supercapacitors is crucial as the energy density of a capacitor is directly proportional to the square of its capacitance.Achieving higher capacitance values requires improvements in electrode and electrolyte performance [7,8].
Consequently, extensive research efforts have been focused on modifying the structure of supercapacitors to facilitate better electron transport and ionic diffusion [9].
Researchers have explored innovative approaches to enhance the electrochemical performance of pseudo capacitors.For instance, Wang and colleagues focused on developing nano wires that provide efficient pathways for ion transport and multiple hierarchical structures to enhance ion displacement [9].Another study by Liu et al. [10] demonstrated the electrochemical performance of porous WO3 with graphene, which exhibited a specific capacitance of 308.2 mF cm -2 at a scan rate of 2 mV/s.In a different direction [11][12] investigated an integrated hybrid supercapacitor utilizing aqueous Zn-ion, which showed impressive capacity retention of 93% even after 80000 cycles.Meanwhile, Cheng et al. [13] developed a nano-composite based on activated carbon, exhibiting a remarkable retention rate of approximately 91.4%.Transition metal oxides such as Co3O4, MnO2, and RuO2 have been extensively studied for their potential in pseudo capacitors.Among them, Ruthenium oxide has demonstrated high specific capacitance and excellent cyclic stability, albeit limited scalability due to its high cost.On the other hand, Cobalt Oxide (Co3O4) is a promising candidate due to its P-type semiconductor nature, antiferromagnetic properties, and theoretical specific capacitance of 3560 F/g [14][15].However, its sluggish electrical conduction behavior and limited electrochemical stability in the GCD process hinder the attainment of its theoretical specific capacitance [16].To overcome these limitations, researchers have adopted various synthetic approaches and nano-structured techniques.One promising method involves doping Co3O4 with transition metals, such as nickel.Nickel is an attractive dopant due to its cost-effectiveness and favorable redox properties, coupled with its similar atomic radius to cobalt [17,18].
Consequently, the researchers decided to incorporate nickel as a dopant for Co3O4 and synthesized Ni-doped Co3O4 using the coprecipitate method, which offers simplicity, costeffectiveness, and scalability [19,20].
In this study involved preparing Ni-doped Co3O4 with different concentrations (3 wt%, 5 wt%, and 7 wt%) and conducting structural and morphological analyses.The specific capacitance of the synthesized samples was evaluated, resulting in 347 F/g, 293 F/g, and 258 F/g for 3 wt%, 5 wt%, and 7 wt% Ni-doped Co3O4, respectively, at 1.5 A/g.Remarkably, the 3 wt% Ni-doped Co3O4 nanoparticles exhibited a superior capacity retention of 93% even after 1500 cycles.Based on these findings, it can be concluded that 3 wt% Ni-doped Co3O4 holds significant promise for enhanced pseudo capacitor applications.

Synthesis of Ni-doped Co3O4 Nanoparticles
The synthesis of Ni-doped Co3O4 nanostructures involved a simple coprecipitate route.
To begin, cobalt nitrate hexahydrate [Co(NO3)•6H2O] (0.2 M) and 3 wt% of nickel nitrate hexahydrate [Ni(NO3)•6H2O] were dissolved in a minimal amount of deionized water using a magnetic stirrer at a temperature of 27 °C for a duration of 30 minutes, making up a total volume of 100 ml.Subsequently, a 0.4 M solution of sodium hydroxide (NaOH) was prepared by mixing 100 ml of NaOH with a magnetic stirrer for another 30 minutes.The NaOH solution was then slowly added dropwise to the dopant mixture solution and stirred for 90 minutes at 27 °C.This process resulted in the formation of a dark blue precipitate, which was allowed to settle for 30 minutes.After removing the supernate, the settled precipitate was filtered and washed multiple times with deionized water and ethanol.The doped sample was then collected and dried at 80 °C in a hot air oven for duration of two hours.Finally, the obtained samples were subjected to calcination at 600 °C for three hours using a muffle furnace.This procedure was repeated to prepare samples with 5 wt% and 7 wt% of Ni-doped Co3O4 nanostructures, which were named Co3O4: 3%Ni, Co3O4: 5%Ni, and Co3O4: 7 Ni based on the respective weight percentages of the dopant precursor.

Characterization Technique
The structural properties of Ni-doped Co3O4 samples were examined using advanced analytical instruments.A Bruker Axs D8 advance instrument diffractometer, utilizing CuKα radiation as the source, was employed for the analysis.To study the FTIR transmission, a Nicolet 5DX FTIR spectrometer was utilized.The structural morphology and elemental composition of the samples were examined using field emission-scanning electron microscopy (FESEM; GEMINI SUPRA 55VP Zeiss).For investigating the elemental composition and oxidation states of various elements, an X-ray photoelectron spectrometer PHI-VERSA PROBE III (k-alpha, Thermo-Fischer Scientific) operating at 15 kV with an Al target and under ultra-high vacuum conditions was employed.The TEM studies of the Ni-doped samples were conducted using HRTEM: JOEL/JEM 2100.Furthermore, the electrochemical analysis of the samples was performed using a three-electrode cell connected to a single-channel potentiostat (PGSTAT 204-Metrohm Autolab).

Preparation and fabrication of working electrode
To begin the fabrication process, small Ni-foam pieces measuring 1 cm x 2 cm were initially obtained.These pieces were thoroughly washed multiple times with deionized water.Subsequently, they underwent a treatment involving immersion in 1M nitric acid (HNO3) followed by another round of washing with deionized water.Next, the Ni-foam pieces were immersed in 2M hydrochloric acid to eliminate any oxides present, and once again washed with deionized water.To ensure further cleaning, the collected Ni foam pieces were dipped in ethanol and subjected to ultrasonic treatment for 15 minutes.Finally, the Ni-foam pieces were dried at 90°C in a hot-air oven for 8 hours.
For the fabrication of the desired 3 wt% Ni-doped Co3O4 sample on the Ni foam, a mixture of the prepared Co3O4:3wt%Ni sample (active material), activated carbon, and polyvinylidene fluoride was prepared in a weight ratio of 7:2:1.The mixture was thoroughly ground in a mortar.N-methyl-2-pyrrolidone (NMP) was added to the mixture in small amounts, and the entire slurry was mixed well until a homogeneous consistency was achieved.The resulting slurry was then coated onto the clean Ni foam, covering an area of 1 cm x 1 cm, to create the coated active electrode.This electrode was subsequently dried in ambient air at 70°C for 1 hour.Finally, it was heated in a hot air oven at 90°C for 8 hours to eliminate any remaining water and solvents.The mass of the active material present in the electrode was determined by calculating the difference between the mass of the coated electrode and the bare Ni-foam.The aforementioned fabrication method was also employed for the preparation of the working electrodes containing 5 wt% and 7 wt% Ni-doped Co3O4 nanoparticles.

Electrochemical Cell Setup
The electrochemical analysis of 3 wt%, 5 wt% and 7wt% of Ni doped Co3O4 was carried out in three electrode cell setup.The electrochemical cell consist of Ag/AgCl as reference electrode, the Ni doped Co3O4 coated foam as working electrode and platinum coil as counter electrode.All the electrodes were kept inside 1 M of KOH (electrolyte).

X ray diffraction studies
The X-ray diffraction (XRD) analysis provides valuable insights into the structural characteristics, phase purity, and crystallite size of the Co3O4 samples doped with nickel (Ni).Furthermore, lattice distortions and chemical interactions between Ni and the host material contribute to the overall effect.To determine the average crystallite size of the doped samples, Debye-Scherrer's formula (Equation 1) is utilized.
where, K is scherrer constant (0.9),  is the wave length of x-ray (1.54 Å) β is the full width at the half maximum of the diffraction peaks (FWHM).For the prominent plane (311), the crystallite size for 3wt%, 5wt% and 7wt% Ni doped Co3O4 nanoparticles are founded as 39 nm, 45 nm and 68 nm respectively.The increase in Ni doping concentration in Co3O4 leads to an increase in the average crystallite size while simultaneously decreasing the dislocation density and microstrain.This phenomenon can be attributed to several factors.Firstly, Ni atoms, when introduced into the Co3O4 lattice, promote lattice expansion, creating more space for crystal growth.This expansion allows larger crystallites to form.Moreover, the presence of Ni atoms serves as sites for nucleation, facilitating the growth of new crystallites and consequently contributing to the overall increase in crystallite size.Furthermore, the introduction of Ni atoms enhances the mobility of atoms within the lattice, facilitating easier rearrangement and promoting crystal growth.This increased mobility allows for the development of larger crystallites.The higher concentration of Ni doping also increases the availability of Ni atoms for incorporation into the crystal lattice, leading to a greater number of Ni atoms participating in crystal growth, resulting in larger crystallites.

FTIR spectrum Analysis
Figure 2 illustrates the FTIR analysis conducted on Ni-doped Co3O4 nanoparticles, with doping levels of 3 wt%, 5 wt%, and 7 wt%.The analysis encompassed the wave number range from 400 to 4000 cm -1 .Interestingly, all the doped samples exhibited distinct absorption peaks at 575 cm -1 and 663 cm -1 , which can be ascribed to the stretching vibration of Co-O or Ni-O bands within the Co3O4 spinel structure [Fd3m group] [21].The band at 575 cm -1 corresponds to the OB3 vibration, where the symbol B denotes co3+ ions occupying the octahedral site.Likewise, the second band at 663 cm -1 corresponds to ABO3 vibrations, where A represents Co 2+ ions in the tetrahedral site.Furthermore, the peaks observed at 1034 cm -1 and 1115 cm -1 are attributed to the presence of C-O bonds, while the peak at 3430 cm -1 corresponds to O-H vibrations [22].

Surface chemical components and various valence states present in 7 wt% Ni doped
Co3O4 Nps were analyzed through X-ray photoelectron spectroscopy (XPS).The XPS survey spectrum of the nanoparticles (Figure 3  eV were associated with Ni 3+ , while those of 855.99 eV and 873.00 eV corresponded to Ni 2+ [23,24].These XPS findings demonstrate that the surface composition of Ni doped Co3O4 comprises Co 3+ , Co 2+ , Ni 3+ , and Ni 2+ .

HRSEM, HRTEM and EDAX analysis
Figure 4 displays SEM micrographs of Co3O4 nanoparticles doped with different weight percentages of Ni (3 wt%, 5 wt%, and 7 wt%).It is observed that the Figure 4a-c, the micrographs reveal an irregular nano flake-like structure with sizes ranging from 60 nm to 130 nm.The average particle sizes of Co3O4:3%Ni, Co3O4:5%Ni, and Co3O4:3%Ni were calculated to be 63 nm, 67 nm, and 74 nm, respectively.It was observed that as the doping concentration of Ni increased, the particle size of Ni-doped Co3O4 also increased.Additionally, some void spaces and pores were observed upon closer examination.These pores were present throughout the surface of both 3 wt% and 5 wt% Ni-doped Co3O4 nanoparticles, while their presence was reduced in the case of 7 wt% Ni-doped Co3O4.The presence of these pores influenced the porous structure, which played a crucial role in determining the surface area of the nanostructures.The presence of pores resulted in a higher surface area, thereby enhancing the electrochemical activity.Morphological studies of Ni-doped Co3O4 samples were conducted using highresolution transmission electron microscopy (HRTEM).Figure 5 illustrates the TEM images of 7 wt% Ni-doped Co3O4 nanostructures, which exhibited hexagonal nanostructures with sizes ranging from 41 nm to 105 nm.Some nano capsule-like structures were also observed.Hence, based on the X-ray diffraction studies, FTIR studies, SEM analysis, and TEM studies, it can be concluded that the facile coprecipitated Ni-doped Co3O4 samples possess high crystallinity.The incorporation of nickel into the cobalt oxide nanostructure was further confirmed through EDAX analysis.
Figure 6 presents the EDAX analysis spectra of Ni-doped Co3O4 nanoparticles with various weight percentages, verifying the presence of Co, O, and Ni elements.The inserted analysis in each spectrum displayed their respective atomic percentages.It is observed that the inset table, the increase in nickel atomic percentage with an increase in cobalt atomic percentage up to 5% of nickel doping can be attributed to the substitution of nickel atoms into the cobalt oxide structure.At lower doping levels, the majority of the nickel atoms replace cobalt atoms in the crystal lattice, resulting in an increase in both nickel and cobalt atomic percentages.However, beyond 5% nickel doping, the cobalt atomic percentage starts to decrease.This could be due to limited substitution sites within the crystal lattice for nickel atoms or the formation of impurities or separate nickel-rich regions within the cobalt oxide structure.These factors contribute to a decrease in the overall cobalt atomic percentage after 7% of nickel doping.

Cyclic-voltammetry analysis
The electrochemical behavior of Ni (at 3 wt%, 5 wt% and 7 wt%) doped Co3O4 nanoparticles were analysed by using Cyclic Voltammetry (CV), Galvanostatic Charge-Discharge (GCD) And Electrochemical Impedance Spectroscopy (EIS).Figure 7 illustrates the CV curves of Ni-doped Co3O4 nanoparticles obtained at different scan rates, namely 100 mV/s, 50 mV/s, 20 mV/s, 10 mV/s, and 5 mV/s.These measurements were conducted using a 1 M KOH solution within a potential range of -0.4 V to 0.8 V.The presence of a pair of redox peaks in all Ni-doped Co3O4 materials suggests the pseudo-capacitive behavior of the electrodes.In the case of 3 wt% Ni-doped Co3O4 nanostructures, an anodic peak at 0.4 V and a cathodic peak at 0.2 V were observed at a scan rate of 5 mV/s.Notably, as the scan rate increases from 5 mV/s to 100 mV/s, the anodic peaks shift towards higher potential while the cathodic peaks shift towards lower potential.These redox peaks are attributed to the faradic nature of the Ni-doped Co3O4 samples, and the peak shifts are influenced by various resistive behaviors occurring at the electrode/electrolyte interface [25][26][27].The CV curves display an asymmetric nature, indicating the irreversibility of Co3O4 during the redox process.The specific capacitance of the Ni-doped Co3O4 samples is evaluated using Equation 2.
where, SC is the specific capacitance of the active material,  represents the applied scan rate m is the active mass of the electrode material, (V 1 − V 2 ) gives the range of the potential window and ∫ IV dV is the area under the CV curve.using the above Equation 2, the specific capacitance of Ni doped Co3O4 samples is calculated at various scan rates and are listed in the following Table 1.At low scan rate of 5mV/s surprisingly, the specific capacitance (SC) of 3%, 5% and 5% Ni doped samples are calculated as 299 F/g, 244 F/g and 236 F/g respectively.It is observed that the SC decreases with the increasing in Ni concentration.The variation of SC of Ni doped Co3O4 samples with different scan rates are presented in Figure 8.
The specific capacitance (SC) value decreases linearly as the scan rate increases.A maximum specific capacitance of 299 F/g was observed for Co3O4 particles doped with 3 wt% Ni at a scan rate of 5 mV/s.An ion-exchange mechanism is indicated by Figure 8, which demonstrates a decrease in specific capacitance with an increase in scan rate for all the doped electrodes.At low scan rates, ions have enough time to diffuse into the electrode and utilize its wide surface, resulting in an increase in SC.However, at high scan rates, ions can only access the outer surface of the electrode, leading to a lower SC.

Galvanostatic Charge-Discharge Test (GCD)
The galvanostatic charge-discharge analysis was carried out for all the Ni doped Co3O4 samples.The Figure 9 showed the GCD profile of Ni doped Co3O4 Nps studied at different current densities in the range 9 Ag -1 to 1.5 Ag -1 with the presence of 1.0 M KOH electrolyte.The specific capacitance of the electrode is calculated using the Equation 3. SC= I.t / m. dV F g -1 (3) where, SC is the specific capacitance of the electrode (F g -1 ), I represents the discharge current (A), m represents the active mass of the coated material (g) and t represents the discharge time (s).The specific capacitance of the Ni-doped Co3O4 samples at various current densities is presented in Table 2, showcasing the obtained results.At a scan rate of 1.5 Ag -1 , the specific capacitance values for the 3 wt% Ni-doped Co3O4, 5 wt% Ni-doped Co3O4, and 7 wt% Ni-doped Co3O4 samples were calculated as 347 F/g, 293 F/g, and 258 F/g, respectively.Figure 9 illustrates the GCD profiles for all the investigated samples, indicating a notable dependency of the specific capacitance of Ni-doped Co3O4 Nps on the current density.A consistent decrease in specific capacitance is observed across all plots with increasing current density, attributed to the diffusion effect hindering the migration of electrolyte ions.Moreover, the specific capacitance of the doped samples diminishes as the Ni doping concentration in the Co3O4 lattice structures increases.This phenomenon may be attributed to the enlargement of grain size, as evident in the FESEM images, resulting in a reduction in the surface area of the active materials.

Electrochemical Impedance spectroscopic Analysis (EIS)
Electrochemical impedance studies were conducted on Ni-doped Co3O4 nanoparticles to explore their behavior in terms of ionic conductivity, ion diffusion, and charge transport.Figure

Supplementary Files
This is a list of supplementary les associated with this preprint.Click to download.

Figure 1
Figure 1 illustrates the XRD patterns of the 3 wt%, 5 wt%, and 7 wt% Ni-doped samples.These patterns reveal multiple diffraction peaks, which align with the cubic spinel structure of Co3O4 [JCPDS:043:1003] [20].The observed peaks correspond to the reflection planes (220), (311), (400), (511), and (440) of cubic Co3O4 nanoparticles, indicating the absence of any secondary phases involving nickel or nickel oxides.Upon increasing the Ni doping concentration, a slight shift towards the lower side is observed in the diffraction peak corresponding to the (311) phase of the Ni-doped Co3O4 samples.This shift can be attributed to the subtle differences in the ionic radii of Co 2+ and Ni 2+ ions, which replace each other within the cobalt oxide crystal structures [21].Increasing the Ni doping concentration causes a slight shift towards the lower side due to changes in the electronic band structure or lattice structure of the material.These changes alter the material's properties, such as energy levels, conductivity, or optical properties.The addition of Ni atoms introduces new energy states within the bandgap, resulting in the observed shift.
(a)) confirmed the presence of Co, Ni, and O elements.

Figure 3 (
Figure 3(b) displayed the high-resolution XPS profile of the Co 2p orbitals in Ni doped Co3O4, revealing two spin-orbit doublets peaks and a pair of satellites.The deconvoluted peaks (Figure 5.3(b)) at 779.88 and 795.02 eV corresponded to Co 2p3/2, while the peaks at 781.14 and 796.99 eV were associated with Co 2p1/2 (He et al., 2019).Deconvolution of the O 1s spectrum (Figure 3(c)) for Ni doped Co3O4 Nps resulted in two distinct peaks located at 529.89 eV (attributed to surface oxygen) and 531.23 eV (related to lattice oxygen) (Nagajothi et al., 2022).Furthermore, the Ni 2p spectrum (Figure 3(d)) exhibited two spin-orbit doublets representative of Ni 2+ and Ni 3+ species, along with two shakeup satellites.The binding energies of 854.41 eV and 872.74

Figures
Figures