Tuning the electrical, structural, and electrochemical properties of Sm/Ni Co-doped BiFeO₃ nanostructures for supercapacitor applications

1. Introduction

Because of the depletion of fossil fuel and growing environmental problems, the research has been done extensively to find better energy storage systems to address the needs of the future energy markets (Beka et al., 2019; Islam et al., 2023). The popularity of smart portable devices indicates innovation that comes as a result of increased scientific and technological development. Furthermore, it should be noted that rechargeable energy storage is still considered to be at relatively early stages of its technological advancement (He et al., 2018). Energy storage systems constructed to satisfy these requirements surface forever persisting issues in satisfying the need for high power density, long cycle life, and reasonable cost (Manzoor et al., 2021; Naveed ur Rehman et al., 2021; Rashid et al., 2021). As the demand for renewable energy sources is growing and the natural resources are depleted, the scientists are concentrating on new technologies of energy storage, such as batteries and super capacitors. Among the available energy storage systems, the supercapacitor can be classified as one of the systems with the most spectacular potential (Hu et al., 2021; Mali & Tripathi, 2021; Yang et al., 2021) (Hu et al., 2021; Mali & Tripathi, 2021; Yang et al., 2021). The supercapacitor, also referred to as ultracapacitor or electrochemical capacitor, acts as an intermediate between capacitors with respect to power density, and batteries with respect to energy density (Bhujun et al., 2017). Supercapacitors are extensively employed in a variety of fields such as electric automobiles, electric tools, missile technology, and medical devices (Chen et al., 2014). Thus, an asymmetric supercapacitor can gain higher capacitance and energy density by increasing its operating cell voltage (Sarma et al., 2013).

Electrochemical capacitors are classified into three varieties according to their charge storage capacity: faradaic (Sarma et al., 2013), non-faradaic (Rathinamala et al., 2021), and hybrid capacitors (Liu et al., 2021). The specific capacitance of pseudo capacitors is greater than that of electric double-layer capacitors (EDLCs). A double-layer capacitor stores energy by separating particles at the interface of electrode-electrolyte in a non-faradic manner, while the faradaic redox interactions that pseudo capacitors rely on for energy storage are managed by conducting polymers (Sahoo et al., 2018) and metal oxides (Xia et al., 2016). Compared with conducting polymer materials, transition metallic oxides have attracted lots of attention due to their high pseudo capacitance, such as RuO₂, MnO₂, and V₂O₅ (M. Li et al., 2013; Rakhi et al., 2014; Tahir Malik et al., 2024; Wei et al., 2011). The magnetic properties concerned with BiFeO₃ nanoparticles are quite specific and these properties are influenced by conditions like the configuration of the particles, and the size. However, there are some problems with this material of choice for example BiFeO₃, high leakage current. Nonetheless, this matter could be overcome by doping with various compounds such as Co³⁺, Mn²⁺, Nd³⁺, Sm³⁺_, and La³⁺ in the BFO crystal structure (Barman & Kaur, 2017; Noviastuti et al., 2021; Rakhi et al., 2014; Thang et al., 2020; Wang et al., 2019; Wei et al., 2011; H. M. Xu et al., 2016).

Perovskite materials, which are based on binary transition metals, namely ABO₃ structured compounds (where A and B represents as transition metal cations), are regarded as attractive possibilities for electrodes in electrochemical energy storage (Wang et al., 2019; H. M. Xu et al., 2016). These materials often exhibit superior electrical conductivity and more diverse oxidation-reduction states compared to single-component transition metal complexes (Kang et al., 2020a; Tomar et al., 2020). Jadhav et al. (Jadhav et al., 2016) conducted a study on nanoflakes of BiFeO₃ as an electrode material for supercapacitors, reporting as a specific capacitance for 72 F/g. Yin et al. (Kang et al., 2020b) have showed a successful hydrothermal synthesizing of BiFeO₃ nanoplate, demonstrating enhanced supercapacitor properties with an estimated C_sp value of 254 F/g at 1 mV/s. Botha et al. (Matinise et al., 2023) reported the fabrication of BFO nanoparticles by the green technique, producing C_sp value of 105 F/g. The Sm/Ni co-doped BiFeO₃ nanoparticles exhibit a remarkable capacitance of 270 F/g for Bi_0.91Sm_0.09Ni_0.09Fe_0.91O₃. Nonetheless, certain issues have been identified with BiFeO3, including its comparatively poor resistivity, subsequent formation of a secondary phase, and current leakage.

In this study, Bi_1-xSm_xFe_1-xNi_xO₃ (x = 0, 0.03, 0.06, 0.09) nanoparticles were synthesized via sol-gel method. The effects of Sm and Ni on the BFO nanoparticles were examined using different analytical techniques and their performance in supercapacitors was evaluated. The addition of Sm and Ni to the materials improved the conducting routes due to their conductive properties resulting in better electrochemical performance.

2. Materials and Methods

The Bi_1−xSm_xFe_1−xNi_xO₃ (x= 0, 0.03, 0.06 and 0.09) nano-powder were synthesized using the sol-gel method. Bismuth nitrate in the pentahydrate form [Bi(NO₃)₃.5H₂O] into nitric acid, iron nitrate nonahydrate [Fe(NO₃)₃.9H₂O], nickel nitrate hexahydrate [Ni(NO₃)₃.6H₂O], samarium nitrate in the hexahydrate form [Sm(NO₃)₃.6H₂O], and citric acid as fuel agent were used as starting materials. Firstly, the iron nitrate, nickel nitrate, samarium nitrate, and citric acid solutions were dissolved in deionized water, and bismuth nitrate was dissolved into nitric acid individually. All solutions were then mixed in single beaker. The resultant mixture was ultrasonically processed for 5 min to homogenize the solution. Ammonia solution was added dropwise into the reaction to keep the overall pH of the reaction mixture at 7. After the mixture was put on a hot plate, it was constantly stirred at 70-80°C until a fluffy gel was obtained. Finally, the mixture was heated at 120°C to allow for combustion process to obtain the final product. The dark brownish resultant powder was collected, ground, and calcined, while consecutively heating at 550°C for 3 h to release large number of gases as shown in Fig. 1.

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Fig. 1.

Schematic diagram of the sol-gel process for the synthesis of Bi_1-xSm_xFe_1-xNi_xO₃ (x = 0.00, 0.03, 0.06, 0.09) nanocomposites.

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3. Synthesis of Electrode for Super Capacitive Performance

For supercapacitor applications, Bi_1-xSm_xFe_1-xNi_xO₃ for (x = 0.00, 0.03, 0.06, 0.09) was deposited over a nickel foam electrode via a conventional slurry technique. After being cleaned with 1M hydrochloric acid, ethanol, and deionized water, the nickel electrode was then dried for a full night to prevent surface reoxidation. This was done before the loading process. A homogenous dark slurry was created by combining 0.2 mL of N-methyl-2-pyrrolidone with 7 mg of the target material, 2 mg of carbon black, and 1 mg of poly(vinylidene fluoride), and the process was repeated as needed. The slurry was deposited on the nickeled-foam electrode and allowed to dry at ambient temperature for 48 h. A potentiostat featuring a three-electrode configuration, consisting of a Ag/AgCl (reference electrode), platinum sheet (counter electrode), and the manufactured electrode (working electrode), were used to assess the characteristics of electrochemical of both pure and doped bismuth ferrite. Then, the slurry was drop-casted onto the nickel foam electrode and put to dry at room temperature 48 h. A potentiostat with a three-electrode configuration was utilized to evaluate the electrochemical properties of pure and doped bismuth ferrite. The counter electrode consisted of a platinum sheet, the reference electrode was composed of Ag/AgCl, and the working electrode was the manufactured electrode.

4. Results

There are some useful characterizations for super capacitor applications.

4.1 X-ray diffraction

The X-ray diffraction (XRD) analysis of pure phase BiFeO₃ and Sm/Ni co-doped BiFeO₃, (Bi₀.₉₇Sm₀.₀₃Fe₀.₉₇Ni₀.₀₃O₃, Bi₀.₉₄Sm₀.₀₆Fe₀.₉₄Ni₀.₀₆O₃, and Bi₀.₉₁Sm₀.₀₉Fe₀.₉₁Ni₀.₀₉O₃), has been represented as M1, M2, M3, and M4, respectively, as shown in Fig. 2., which exhibits a perovskite structure (R3c space group) in rhombohedral distorted, as corroborated by the diffraction peaks at (012), (104), (110), (113), (024), and (116). XRD was conducted using Copper Kα radiation (λ = 1.54 Å), with a step increase of 0.02° and a duration of per 0.5 seconds. The diffraction peaks corresponded with JCPDS card no. 01-086-1518, validating the effective synthesis of a rhombohedral phase, whereas secondary phases presumably resulted from the substitution of Ni and Sm ions in the BiFeO₃ lattice (Godara et al., 2015; Puli et al., 2014; Tahir Malik et al., 2024). The substitution of Bi³⁺ (1.03 Å) for Sm³⁺ (0.96 Å) resulted in peak modifications, specifically an interaction of the (104) and (110) planes at 32°, indicating lattice deformation (Arya & Negi, 2013; Dai et al., 2012). Doping BiFeO₃ with rare earth metals, such as Sm, reduces oxygen vacancies and prevents Fe³⁺ from oxidizing, making impurity phases simpler to suppress (X. Xu et al., 2013).

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Fig. 2.

XRD patterns of Bi_1-xSm_xFe_1-xNi_xO₃ for (x=0.00, 0.03, 0.06, 0.09).

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The crystallite size ranged from 56.15 to 67.37 nm showing good nanomaterial property, and all the major and minor peaks are obtained at the same 2θ values, which shows that doped metal ions are successfully replaced with parent metal ions. From XRD data analysis, the crystallite size, full width half maximum (FWHM), and d-spacing were established and shown in Table 1. The interplanar spacing (d) was determined using the Bragg’s law:

$2d\sin \theta =n\lambda$

Table 1. The structural parameters of Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) nanoparticles.

Composition	Max peak 2θ (degree)	hkl	d-spacing (Å)	FWHM (degree)	Crystallite size (nm)
BiFeO3	31.7412	104	2.8191	0.15350	56.1524
Bi0.97Sm0.03Ni0.03Fe0.97O3	31.7272	104	2.8203	0.15350	56.1505
Bi0.94Sm0.06Ni0.06Fe0.94O3	31.7124	104	2.8216	0.1279	67.3781
Bi0.91Sm0.09Ni0.09Fe0.91O3	31.7138	104	2.821	0.15350	56.1486

where n=1 for the first order diffraction and λ is wavelength and d is d-spacing. The mean crystallite size (D) was determined from the patterns shown by XRD employing Scherer’s formula:

$D=\frac{\kappa \lambda }{\beta cos\theta }$

In this context, κ represents the form factor, assigned a value of 0.94, λ denotes the wavelength of X-ray, quantified as 0.15418 nm; β signifies the complete breadth at half maximum; and θ indicates the angle of deviation (Gaber et al., 2014; Kumar & Chand, 2018; Veluswamy et al., 2018). The Fig. 3 depicted d-spacing, FWHM and crystallite size against samples. However, during the synthesis, the kinetics of formation also lead to the synthesis of the impurity phase in addition of the primary phase BiFeO₃, which alters the lattice characteristics.

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Fig. 3.

Crystallite size, d-spacing and FWHM versus samples for Bi_1-xSm_xFe_1-xNi_xO₃ for (x=0.00, 0.03, 0.06, 0.09).

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4.2 Scanning electron microscopy

As presented in Fig. 4 (BFO & BSFNO-9), the synthesized samples are at x = 0.00 and x = 0.09 with respect to. From the scanning electron microscopy (SEM) micrographs, noticeable differences in the morphology, size and texture of pure and doped BiFeO₃ (BFO) can be observed. The synthesis procedure may have caused uncontrolled grain formation, leading to where the particles of pure BFO are wide, amorphous and heavily agglomerated. This eliminates the homogeneity of the product; it also shows that the density is not consistent throughout the structure and seems to have an uneven porosity; this may affect the material’s performance in use. On the other hand, the doped BsFNO-9 material presents itself with smaller particle sizes and better dispersion along with smoother surface of the particles, thus indicating that the dopant elements, including but not limited to Sm or Ni, are vital in managing the particle growths and the enhancement of the crystalline structure (Tahir et al., 2024). Due to decreased particle aggregation in the doped sample, the dispersed particle size increased. This improvement in stability and electrical characteristics makes the doped sample more appropriate for energy storage systems. These changes in doped BFO signify how doping enhances not only the structural characteristic but also functional performance of BFO nanomaterials.

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Fig. 4.

SEM images of BFO, and BSFNO-9 nanoparticles.

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4.3 Fourier transform infrared spectroscopy

As seen in Fig. 5, the 400-4000 cm^–1 range of the nanocrystalline Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) namely BFO, BSFNO-3, BSFNO-6, BSFNO-9, respectively. The Fourier transform infrared (FTIR) band was recorded at ambient temperature. There were two types of absorption bands found: octahedral (v₁) and tetrahedral (v₂). The Fig. 5 displays the FTIR spectra of the BiFeO3 powders that have been substituted with (Ni, Sm) ions for x = 0.00, 0.03, 0.06, and 0.09. The stretching and bending of O–Fe–O and Fe–O bonds during vibrations was responsible for the absorption peaks at 616, 637, and 624 cm-1, which showed the formation of the FeO₆ octahedral structure (Arun Kumar et al., 2018; Asif et al., 2024; Fakhar-e-Alam et al., 2024). The metal-oxygen connection validated the establishment of a perovskite form. In addition, further bands were seen to emerge at x = 0.00, 0.00, 0.03, 0.06, and 0.09, respectively, at 1109 cm^-1, 1122 cm^-1, and 1112 cm^-1, characterizing the metal-oxygen band. Furthermore, for x = 0.03, 0.06, and 0.09, respectively, a third band emerged at approximately 1427 cm^-1, 1435 cm^-1 and 1449 cm^-1 arises due to the presence of nitrates. The emergence of a highly crystalline phase R3c in BFO was confirmed by these studies.

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Fig. 5.

FT-IR Spectrum for Bi_1-xSm_xFe_1-xNi_xO₃ for (x=0.00, 0.03, 0.06, 0.09).

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4.4 Ultra-violet visible spectroscopy analysis

The optical band gap is crucial for optoelectronic devices, since it dictates their optical and electrical characteristics. Fig. 6 illustrates the UV-Visible spectra of Bi_1−xSm_xNi_xFe_1-xO₃ nanoparticles, with x values of 0.00, 0.03, 0.06, and 0.09, measured for both indirect and direct band gaps. The observed optical band gap of Sm/Ni: BFO nanoparticles varied slightly depending on the number of Sm³⁺ and Ni²⁺ dopants. The optical absorption characteristics of a photocatalyst are intimately linked to its optical energy gap (Ali et al., 2024; Asif et al., 2024; J. Li et al., 2014). The absorption wavelength is inversely correlated with the size of the nanoparticle and the energy band gap. Where E_g represents the energy band gap, α denotes the coefficient of absorption, and h signifies Planck’s constant, which having value equal to 6.626 x 10^-34 Js.

${\left(\alpha hv\right)}^n=A{(hv-E_g)}^n$

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Fig. 6.

Plot (αhv)² vs. energy band gap (eV) for Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) nanoparticles

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Here, E_g denotes the energy gap, A is a constant that changes with various transitions, and n is an index that may assume values of 1/2, 3/2, 2, or 3, contingent upon the nature of the electronic transition responsible for the reflection (Serhan et al., 2019). Direct transition measured using the gap in total energy. The band gaps (E_g) of BFO, BSFNO-1, BSFNO-2, and BSFNO-3 are around 3.99, 3.96, 3.65, and 3.56 eV, respectively. The band gap of BFO-based materials indicates the energy difference between the peak of the Fe 3d mixed-valence band and O 2p, whereas the conduction band contains states associated with iron (4s) and bismuth (6p). The decrease in the band gap of co-doped BiFeO₃ is ascribed to the incorporation of Sm and Ni into the material. The substitution of Sm ions for Bi³⁺ ions at the A-site and Ni ions for Fe at the B-sites results in the formation of impurity energy levels within the energy band gap. The band gap of BSFNO samples was reduced by increasing the Sm-Ni substitution in BFO because Sm³⁺ results in greater lattice distortion than Bi³⁺ because of its smaller ionic radius (0.95 Å) than Bi³⁺ (1.03 Å), hence enhancing its photonic applications (Chakrabarti et al., 2011). The absorbance offers information about sunlight and is suitable for solar cell applications, Bi_0.91Sm_0.09Fe_0.91Ni_0.09O₃ is ideal for solar cell manufacture and energy storage devices.

4.5 Analysis of electrical conductivity

The conductivity measurements provide an effective means to elucidate the electrical behavior of BiFeO₃ and Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.03,0.06, 0.09), as evident from Fig. 7. The conductivity of materials is one of the parameters that defines the types of charge carriers and the mobility under the influence of external driving frequency (Dhahri et al., 2018; Munir et al., 2023). The influence of relative grain density and grain size is most important, as these have been widely recognized in terms of electrical properties. By lowering the electrical resistivity of a material at temperatures close to room temperature, a higher concentration of co-substituted (Ni, Sm) ions often improves the electrical conductivity of the samples obtained. The observed decrease in resistivity and rise in activated drift mobility of charge carriers verify the semiconducting nature of these materials. The dominant conduction mechanism is explained by Verwey’s electron hopping process between Fe³⁺ and Fe²⁺ ions randomly distributed over the lattice sites (Munir et al., 2023). The materials exhibit conductivity values ranging from 1.65×10^-8 to 4.95×10^-8 ohm^-1 cm^-1, as shown in Fig. 7. Substituting Ni and Sm into BiFeO₃ increases the conduction rate, with the Bi_0.91Sm_0.09Fe_0.91Ni_0.09O₃ composition being the most suitable for electrical device applications. Particularly, it is noted that major contribution of the conductivity increase can be assigned to the intrinsic conductivity of BiFeO₃, stemming from the electron transfer between Fe³⁺ and Fe²⁺ at the B-site. The discovered electronic response yields intriguing findings that may increase the features of the ionic composite system and may have practical applications in electronic devices.

$\begin{array}{c}F{e}^{\text{2}+}\to F{e}^{\text{3+}}+e^{–}\\ F{e}^{\text{3}+}+e^{–}\to F{e}^{\text{2+}}\end{array}$

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Fig. 7.

Conductivity behavior for Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) nanoparticles

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4.6 Analysis of dielectric parameters

The dielectric constant, dielectric loss and the tangent loss for Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) nanoparticles were done at frequencies ranging from 1 kHz to 1 MHz using an LCR meter. As seen in Fig. 8, the dielectric loss and dielectric constant values are found to be frequency dependent and decrease sharply with frequency rise and have a relatively constant value at higher frequencies. This observed trend was analyzed using the Maxwell-Wagner two-layer model and Koop’s theory. The Maxwell Wagner model described that the dielectric materials have irregular structures made of conductive grains surrounded by thin resistive layers. The voltage applied is mainly dissipated across these grain boundaries where formation of space charges and hence the frequency dispersion takes place. This leads to high dielectric constant values in the low frequencies region as depicted in the graph Fig. 8. However, as the frequency increases, the space charges effect for polarization at the grain boundaries reduces, making the dielectric constant less frequency dependent (Ulutas et al., 2013). The incorporation of Ni and Sm into the BiFeO₃ matrix was observed to alter the dielectric constants of the new materials. The specific compositions, Bi_0.91Sm_0.09Fe_0.91Ni_0.09O₃ demonstrated good dielectric properties, thus enabling their use in electrical devices. This knowledge of the frequency dependence of dielectric constants of these Bi-based perovskite materials gives useful information about its electrical characteristics and possible uses in electronic and energy conversion devices.

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Fig. 8.

Frequency versus dielectric constant, dielectric,c loss and tangent loss for Bi_1-xSm_xFe_1-xNi_xO₃ for (x=0.00, 0.03, 0.06, 0.09) nanoparticles.

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4.7 Cyclic voltammetry

The electrochemical analysis was done at room temperature and the scan rate was 1 mVsec^-1, all graphically represented in Fig. 9. The BFO, BiSFNO-1, BSFNO-2, and BSFNO-3 nanoparticles were placed onto nickel foam electrodes using a slurry method for use in super capacitor applications. To study electrochemical analysis, a potentiostat with three electrodes configuration poised electrode of platinum counter, an Ag/AgCl as electrode for orientation/constant and the specie as operational electrode, which is loaded on nickel foam. Fig. 9 presents the CV curves of BFO and B_1-xS_xN_xFe_1-xO₃, where x attained 0 to 0.7 V for a possible range from 0 to 0.03, 0.06, 0.09 in relation to mass and surface area correspondingly. The accumulated CV region of BSFNO-1, BSFNO-2, and BSFNO-3 is significantly greater than that of BiFeO₃, representing advanced electrolytic capacitance. Using the formula C_sp=I×Δt/m×3600, where Δt is the discharge time, I ampere is the ejection current, and m (g) is the mass of the active specimen, the specific capacitance (C_sp) was computed. The specific capacitance of all synthesized samples was found at 217.50, 237.08, 255.41, and 270.10 F/g.

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Fig. 9.

CV curves for Bi_1-xSm_xFe_1-xNi_xO₃ (x=0.00, 0.03, 0.06, 0.09) nanoparticles in 1 M of KOH.

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The BSFNO-3 sample exhibits exceptional electrochemical enactment demonstrated by the deep and improved assembled the redox peaks, demonstrating a higher electrochemical reversibility compared with that of BiFeO₃. The redox peak shapes of Ni/Sm substituted BiFeO₃ nanoparticles remained unchanged up to a scan rate of 1 mV s⁻¹, indicating the precise electrochemical activity of the electrode. Such a strong dependence is attributed to the fact that the improvement of the electrochemical activity from the Bi sites was accompanied by a considerable increase in capacitance. It is clear from the CV graph that Bi site’s participation in charge formation and redox process has greatly enhanced. Hence, the inherent electrochemical activity of transition metal sites may be significantly amplified by creating anion vacancies, potentially providing a promising material solution to future energy storage demands as shown in Table 2.

Table 2 Comparison among manifold types of electrodes materials used for energy storage applications.

Electrode materials	Specific capacitance (Csp)	Electrolyte	References
BiFeO3 nanoflakes	72.2 F/g	2M NaOH	31
BiFeO3 nanoplates	254.6 F/g	1M NaOH	32
pure-phase BiFeO3	253 F/g	3M KOH	48
Bi0.9Sm0.1FeO3 films	184 F/g	0.5M Na2SO4	49
B0.90L0.10FO3 nanoparticles	219 F/g	0.02M NaOH	50
Bi0.94Sm0.06Fe0.94Ni0.06O3	255 F/g	1M KOH	This work
Bi0.91Sm0.09Fe0.91Ni0.09O3	270 F/g	1M KOH	This work

5. Discussions

BiFeO₃-based materials have attracted increasing attention for their potential as energy storage materials owing to their exceptional dielectric and ferroelectric properties. This work emphasizes the morphological, structural, and electrochemical characteristics of sol-gel produced Bi_1-xSm_xFe_1-xNi_xO₃ nanoparticles (x = 0.00, 0.03, 0.06, 0.09). According to structural examinations, co-doped samples exhibit rhombohedral phases with grain sizes between 56 and 67 nm and the R3c space group. The XRD patterns show a pronounced (110) preferred orientation, with no splitting seen in the (110) peak findings. The enlarged XRD patterns Fig. 2 reveal a minor shift at 2θ of 32° (110) in the co-doped nanostructures, indicating an increase in lattice spacing. Table 1 presents the improved values of structural parameters, including crystallite size, surface integrity characteristics, and d-spacing of the samples. As the doping concentration of Sm³⁺ increases, it approaches the grain boundary, and the excess Sm at this border contributes to the formation of the SmFeO₃ phase. SEM morphological examination revealed irregular shapes, aggregation, and uneven surfaces characterized by a porous structure with voids and agglomeration. The holes and gaps in the structure of the resulting powders are developed by the removal of large amount of gas during the combustion process. The FT-IR spectra of all metal-oxide bonds have been shown in Fig. 5, depicted within the frequency range as shown ranging from 440-640 cm^-1. The stretching modes of the FeO₆ group and the bending vibrations of Fe-O-Fe are responsible for the maximum absorption at 480 and 620 cm⁻¹, respectively. The average a.m of B-site ions in BFO diminishes with increasing co-substitution measurements, resulting in a shift of the peak location towards higher frequencies, since it is inversely proportional to the decreased mass. All samples have analogous absorption peaks at between 384 and 480 nm. The optical bandgap (Eg) of all samples was determined from the hv and (αhν)² plot, indicating a reduction in energy bandgap values after the substitution of Sm-Ni in the host BiFeO₃, decreasing from 3.99 eV to 3.96 eV for BiSFNO-1, 3.65 eV for BSFNO-2, and 3.56 eV for BSFNO-3, as seen in Figs. 6 and 7 illustrates the conductivity variations for pure BFO and co-doped BFO. The electrical resistance of the BFO and co-substituted Sm/Ni materials is reduced at room temperature due to their conductivity values that ranges from 1.65 to 4.95×10-8 ohm^-1 cm^-1. The hopping frequency is the rate at which the dispersion area is detected. Fig. 8 illustrates the frequency reliance of the dielectric constant of Bi_1−xSm_xFe_1−xNi_xO₃ nanoparticles at room temperature, throughout the frequency range of 1 kHz to 1 MHz. Both imaginary, and the real components of dielectric constants shows a decline with rising frequency. The drop is quick and steady at higher and lower frequencies, respectively. The elevated dielectric constants in the low-frequency domain may result from several types of polarization, including electronic, atomic, orientation, and space charges. The dielectric structure has conductive granules that are separated by insulating barriers. At lower frequencies, the impact of grain boundaries prevails over that of the grains. The development of SmFeO₃ secondary phases in Bi_1−xSm_xFe_1−xNi_xO₃ leads to the substitution of Sm³⁺ ions with Fe³⁺ ions at octahedral sites, which subsequently reduces the electron hopping rate between Fe²⁺ and Fe³⁺, resulting in a diminished real dielectric constant value. BSFNO-2 nanoparticles demonstrate increased dielectric loss in comparison to pure BFO, BSFNO-1, and BFSNO-3. Consequently, it can now be inferred that at the concentrations of x = 0, 0.03, and 0.09, the nanoparticles achieved the greatest dielectric constant with lowest loss tangent. The CV data for the samples BFO, BSFNO-1, BSFNO-2, and BSFNO-3 at a scan rate of 1 mV/s has been shown in Fig. 9. Electrochemical investigations revealed that BFO and doped nanostructures were examined as electrode materials in supercapacitors, resulting in an increase in specific capacitance of nanoparticles from 217 Fg^-1 to 270 Fg^-1. The tests show that BF nanoparticles, which possess a broad spectrum of functional characteristics, may be further increased by samarium and nickel doping for use in energy storage devices. Consequently, all characterization data indicate that Bi_0.91Sm_0.09 Ni_0.09 Fe_0.91O₃ is the optimal material for energy storage applications.

6. Conclusion

To summarize, the morphological, optical and electrochemical properties of BiFeO3 and Bi1-xSmxFe1-xNixO3 nanoparticles with x = 0.00, 0.03, 0.06, 0.09, which prepared via sol-gel process, were investigated. The structural and morphological analysis validating the formation of rhombohedral phase, whereas secondary phases presumably resulted from the substitution of Ni and Sm ions in the BiFeO₃ lattice with grain size around 56-67nm. The Sm and Ni co-doped BiFeO3 nanoparticles exhibit enhancements of electrical and dielectric properties. The electrical resistance of the BFO and co-substituted Sm/Ni materials is reduced at room temperature due to their conductivity values that ranges from 1.65 to 4.95×10-8 ohm-1 cm-1. Through electrochemical studies, the BiFeO3 and Sm/Ni co-doped BiFeO3 nanoparticles, where studied for electrode material in super capacitors, the specific capacitance of nanoparticles increased from 217 Fg-1 to 270 Fg-1. Through all these measurements, it can be confirmed that BFO nanoparticles which have wide range of functional properties can be further enhanced through samarium and nickel doping without any structural transition for use in energy storage devices.